Received August 11, 2020, accepted August 13, 2020, date of publication August 20, 2020, date of current

version September 1, 2020.

Digital Object Identifier 10.1109/ACCESS.2020.3018140

A Comprehensive Survey of Enabling and

Emerging Technologies for Social
Distancing—Part I: Fundamentals
and Enabling Technologies
CONG T. NGUYEN 1,2 , YURIS MULYA SAPUTRA 3,5 , (Graduate Student Member, IEEE),
NGUYEN VAN HUYNH 3 , (Graduate Student Member, IEEE),
NGOC-TAN NGUYEN 3,6 , (Graduate Student Member, IEEE), TRAN VIET KHOA 6 ,
BUI MINH TUAN 6 , DIEP N. NGUYEN 3 , (Senior Member, IEEE),
DINH THAI HOANG 3 , (Member, IEEE), THANG X. VU 4 , (Member, IEEE),
ERYK DUTKIEWICZ 3 , (Senior Member, IEEE),
SYMEON CHATZINOTAS 4 , (Senior Member, IEEE),
AND BJÖRN OTTERSTEN 4 , (Fellow, IEEE)
1 Department of Computer Science and Engineering, Ho Chi Minh City University of Technology, Ho Chi MInh City 700000, Vietnam
2 Department of Computer Science and Engineering, Vietnam National University-Ho Chi Minh City, Ho Chi MInh City 700000, Vietnam
3 School of Electrical and Data Engineering, University of Technology Sydney, Sydney, NSW 2007, Australia
4 Interdisciplinary Centre for Security, Reliability and Trust, University of Luxembourg, 4365 Luxembourg City, Luxembourg
5 Department of Electrical Engineering and Informatics, Vocational College, Universitas Gadjah Mada, Yogyakarta 55281, Indonesia
6 VNU University of Engineering and Technology, Vietnam National University, Hanoi 711000, Vietnam

Corresponding author: Cong T. Nguyen (ntcong2710@gmail.com)

This work was supported in part by the Joint Technology and Innovation Research Centre—a partnership between the University of
Technology Sydney (UTS), the Vietnam National University, University of Engineering and Technology Hanoi (VNU UET), and the
Vietnam National University, Ho Chi Minh City University of Technology (VNU HCMUT).

ABSTRACT Social distancing plays a pivotal role in preventing the spread of viral diseases illnesses such as
COVID-19. By minimizing the close physical contact among people, we can reduce the chances of catching
the virus and spreading it across the community. This two-part paper aims to provide a comprehensive
survey on how emerging technologies, e.g., wireless and networking, artificial intelligence (AI) can enable,
encourage, and even enforce social distancing practice. In this Part I, we provide a comprehensive back-
ground of social distancing including basic concepts, measurements, models, and propose various practical
social distancing scenarios. We then discuss enabling wireless technologies which are especially effect-
in social distancing, e.g., symptom prediction, detection and monitoring quarantined people, and contact
tracing. The companion paper Part II surveys other emerging and related technologies, such as machine
learning, computer vision, thermal, ultrasound, etc., and discusses open issues and challenges (e.g., privacy-
preserving, scheduling, and incentive mechanisms) in implementing social distancing in practice.

INDEX TERMS Social distancing, pandemic, COVID-19, wireless, networking, positioning systems, AI,
machine learning, data analytics, localization, privacy-preserving, scheduling, incentive mechanism.

I. INTRODUCTION economy. Started in Wuhan, China [2], within only six

COVID-19 has completely changed the world’s view on months (from January to June 2020), 210 countries and terri-
pandemics with dire consequences to global health and tories around the world have reported more than ten million
infected people including more than five hundred thousand
The associate editor coordinating the review of this manuscript and deaths [3]. Besides the global health crisis, COVID-19 has
approving it for publication was Derek Abbott . also been causing massive economic losses (e.g., a possible

This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/
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25% unemployment rate in the U.S. [4], one million people ensure that the number of patients does not exceed the public
lost their jobs in Canada during March 2020 [5], 1.4 million healthcare capacity. Moreover, social distancing also delays
jobs lost in Australia [6], and a projected global 3% GDP the outbreak peak [32] so that there is more time to implement
loss [7]), resulting in a global recession as predicted by many countermeasures. Furthermore, social distancing can reduce
experts [7]–[9]. In such context, there is an urgent need for the final number of infected cases [32], and the earlier social
solutions to contain the disease spread, thereby reducing its distancing is implemented, the stronger the effects will be as
negative impacts and buying more time for pharmaceutical illustrated in Fig. 1(b) [12].
solution development. During the ongoing COVID-19 pandemic, many govern-
In the presence of contagious diseases such as SARS, ments have implemented various social distancing measures
H1N1, and COVID-19, social distancing is an effective such as travel restrictions, border control, closing public
non-pharmaceutical approach to limit the disease transmis- places, and warning their citizens to keep a 1.5–2 meters dis-
sion [10], [25], [32]. Social distancing refers to measures tance from each other when they have to go outside [13]–[15].
that minimize the disease spread by reducing the frequency Nevertheless, such aggressive and large-scale measures are
and closeness of human physical contacts, such as closing not easy to implement, e.g., not all public spaces can be
public places (e.g., schools and workplaces), avoiding mass closed, and people still have to go outside for food, health-
gatherings, and keeping a sufficient distance amongst peo- care, or essential work. In such context, technologies play
ple [10], [16]. By reducing the probability that the disease a key role in facilitating social distancing measures. For
can be transmitted from an infected person to a healthy one, example, wireless positioning systems can effectively help
social distancing can significantly reduce the disease’s spread people to keep a safe distance by measuring the distances
and severity. If implemented properly at the early stages of among people and alerting them when they are too close to
a pandemic, social distancing measures can play a key role each other. Moreover, other technologies such as Artificial
in reducing the infection rate and delay the disease’s peak, Intelligence (AI) technologies can be used to facilitate or even
thereby reducing the burden on the healthcare systems and enforce social distancing.
lowering death rates [10], [25], [32]. Fig. 1 illustrates the In this two-part paper, we present a comprehensive survey
effects of social distancing measures on the daily number of on enabling and emerging technologies for social distancing.
cases [12]. As can be observed in Fig. 1(a), social distanc- The main aims are to provide a comprehensive background
ing can reduce the peak number of infected cases [32] to on social distancing as well as effective technologies that can
be used to facilitate the social distancing practice. In Part I,
we first present basic concepts of social distancing together
with its measurements, models, effectiveness, and practical
scenarios. After that, we review enabling wireless technolo-
gies which are especially effective in monitoring and keeping
distance amongst people. In Part II [1], we survey other
emerging technologies, e.g., AI, thermal, computer vision,
ultrasound, and visible light, and discuss open issues and
challenges (e.g., privacy-preserving, scheduling, and incen-
tive mechanisms) of implementing technologies for social
distancing.
There are several surveys of enabling technologies for
the current COVID-19 pandemic, such as [18], [19], with
different focuses. Particularly, [18] surveys the application of
AI technologies and data-sharing methods for urban health
monitoring, and [19] focuses on emerging technologies such
as AI, 3D printing, blockchain, etc., and their applications
for social distancing. Different from these surveys, our paper
focuses on both the newly emerged technologies and the
readily available wireless technologies such as Wi-Fi, Blue-
tooth, Cellular, etc. Moreover, although there are few sur-
veys related to localization and positioning systems using
those wireless technologies, e.g., [20]–[23], to the best of our
knowledge, this is the first survey in the literature discussing
the applications of those technologies for social distancing.
It is worth noting that, due to the increasingly complex devel-
FIGURE 1. The effects of social distancing on an infected disease opment of many types of viruses as well as the rapid growth of
outbreak. (a) Social distancing can delay and reduce the outbreak peak.
(b) Social distancing can reduce the total number of cases. The earlier social interaction and globalization, the concept of social dis-
social distancing is applied, the stronger its effect will be [12]. tancing is not as simple as physical distancing. In fact, it also

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FIGURE 2. The organization of this (Part I) paper.

includes many non-pharmaceutical interventions or measures determined by

taken to prevent the spread of contagious diseases, such as Z ∞
monitoring, detection, and warning people (as we identify and Ro = b(a)F(a)da, (1)
0
propose in Table 1). Thanks to the significant development
where b(a) is the average number of new cases an infec-
of emerging technologies, e.g., future wireless systems, AI,
tious person will infect per unit of time during the infectious
and data analytics, many new solutions have been introduced
period a, and F(a) is the probability that the individual will
recently which can create favorable conditions for practicing
remain infectious during the period a [24].
social distancing.
Besides showing the transmissibility of a disease, Ro also
As illustrated in Fig. 2, the rest of this paper is organized as
gives some intuitive ideas on how to limit the disease spread.
follows. We first provide a brief overview of social distanc-
As observed from (1), Ro can be reduced in different ways,
ing and distance measurement methods in Section II. Then,
i.e., to decrease b(a) or F(a). To reduce b(a), there are several
Section III discusses enabling wireless technologies for social
approaches such as to lower the number of contacts the
distancing, and conclusions are given in Section IV.
infected individuals make per unit of time (e.g., avoid mass
gatherings and public places closures) or to reduce the proba-
II. SOCIAL DISTANCING: A FUNDAMENTAL bility that a contact will infect a new person (e.g., by wearing
BACKGROUND masks). To reduce F(a), the infected person needs to be cured
A. SOCIAL DISTANCING or completely avoid contacts with the non-infected (e.g.,
1) DEFINITION AND CLASSIFICATIONS isolation and quarantine).
Social distancing refers to the non-pharmaceutical measures For an infectious disease, the most common type of models
to reduce the frequency of physical contacts and the con- is the compartmental mathematical model MSEIR [26]. This
tact distances between people during an infectious disease type of model consists of five main classes M, S, E, I, R which
outbreak [11]. Social distancing methods can be classified represent different types of individuals in a community as
into public and individual measures. Public measures include follows:
closing or reducing access to educational institutions and • M: This class includes the infants with passive immunity
workplaces, canceling mass gatherings, travel restrictions, passed down from their mothers.
border control, and quarantining buildings. Individual mea- • S: This class represents the susceptible individuals,
sures consist of isolation, quarantine, and encouragement to i.e., people who can become infected.
keep physical distances between people [16]. Although these • E: When an individual of class S is infected, that indi-
measures can cause some negative impacts on the economy vidual enters a latent period during which it is infected
and individual freedom, they play a crucial role in reducing but not yet infectious, i.e., cannot transmit the disease to
the severity of a pandemic [11]. another individual. The individuals who are in the latent
period constitute class E.
• I: After the latent period is over, an individual can trans-
2) MEASUREMENTS AND MODELS mit the disease to others and is classified by class I.
The evaluation of social distancing measures is often based on • R: This class represents the individuals that have recov-
several standardized approaches. One of the main criteria for ered from the disease and have infection-acquired
social distancing measures selection is the basic reproduction immunity.
number Ro which represents on average how many people a Different combinations of these classes result in different
case (i.e., an infectious person) will infect during its entire types of models, such as MSEIR, MSEIRS, SEIR, SEIRS,
infectious period [24]. For example, Ro < 1 indicates that SIR, and SIRS. The acronyms of these models represent
every case will infect fewer than 1 person, and thus the disease the classes that they take into account and the transition
is declining in the considered population. Since the value of of individuals between these classes. The reason for these
Ro represents how quickly the disease is spreading, Ro has variations is that some classes are not needed in certain cases,
been one of the most important indicators for social distanc- e.g., birth immunity might not exist for a novel strain of virus.
ing measures selection [25], [32]. Mathematically, Ro can be For example, the MSEIRS model is similar to the MSEIR

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model, except that the recovered individuals can be infected measures at workplaces is evaluated by an agent-based simu-
again, and thus the MSEIRS is only used for the cases where lation approach. In particular, six different workplace strate-
immunity is not permanent. Among these models, the classic gies that reduce the number of workdays are simulated.
SIR model is the most common. Let S(t) be the number of The results show that, for seasonal influenza (Ro = 1.4),
susceptible, I (t) be the number of infected, and R(t) be the reducing the number of workdays can effectively reduce the
number of recovered individuals in a population at time t. final attack rate (e.g., up to 82% if three consecutive work-
Moreover, let β be the average number of adequate contacts days are reduced). Nevertheless, in pandemic-level influenza
(i.e., contacts that infect a new case) and γ1 be the average (Ro = 2.0), reducing the number of workdays has a signif-
infectious period. Then, the SIR model is defined by icantly weaker impact, i.e., 3% (one extra day off) to 21%
dS −βIS decrease (three extra consecutive days off). Several other
= , studies present similar results. In [28], it is shown that work-
dt N
dI βIS γI place social distancing can reduce the final attack rate by up
= − , to 39% in a Ro = 1.4 setting. Similarly, [29] shows that
dt N N
dR γI different types of measures can reduce the attack rate from
= , (2) 11% to 20% depending on the frequency of contacts among
dt N
the employees.
where N is the total population. In (2), since β is the num- For school closure measures, studies also show positive
ber of adequate contacts an infected case made and S is effects. In [30], a modeling technique is employed to examine
the number of infected cases, βIS N is the rate at which new the effects of four different social distancing measures under
susceptible individuals are infected. Moreover, the recovery three varying Ro settings. Among different types of measures,
rate is inversely proportional to the infectious period γ1 , and the school closure measure is shown to be able to reduce the
the total infection rate is the infectious rate minus the recovery final attack rate by 20%, 10%, and 5%, and the peak attack
rate. It is also worth noting that since β is the number of rate by 77%, 47%, and 32% in the cases where Ro < 1.9,
adequate contacts per unit time and γ1 is the infection time, 2.0 ≤ R0 ≤ 2.4, and Ro > 2.5, respectively. Similarly, it is
β
γ is the total number of newly infected cases caused by a shown in [31] that prolonged school closure in a pandemic
typical infected individual. This is what Ro represents, and context can reduce the final attack rate by up to 17% and the
thus γβ = Ro . peak attack rate by up to 45%.
The abovementioned SIR models neglect several important Another common social distancing measure is the isola-
aspects of a disease such as people with passive immunity and tion of confirmed cases and cases with similar symptoms.
vital dynamics (birth and deaths). Consequently, the model In [32], large-scale epidemic simulations are performed to
is only effective for modeling a novel strain of an infectious evaluate different strategies for influenza pandemic mitiga-
disease (so there is no passive immunity) over a short period tion. Among the simulated strategies, the results show that
of time (birth and death can be neglected). On the other hand, the proper implementation (such that an isolated individual
the simplicity of the model ensures that it is well-posed. As a reduces 90% of its contact rate) of isolation can reduce the
result, the classic SIR model is used in many simulations to final attack rate by 7% in a Ro = 2 setting. Similarly, it is
predict the infection rate of many infectious diseases. shown in [30] that isolation can reduce the final attack rate
by 27%, 7%, and 5%, and the peak attack rate by 89%, 72%,
3) EFFECTIVENESS and 53% in the cases where Ro < 1.9, 2.0 ≤ R0 ≤ 2.4, and
To evaluate the effectiveness of social distancing, a common Ro > 2.5, respectively.
approach is to measure the attack rate which is the percentage For household quarantines, studies have shown that this
of infected people in a susceptible population (where no one measure can be effective if the compliance level is sufficient.
is immune at the beginning of the disease) at the time of In [32], the effects of voluntary quarantine of household for
measurement [27]. The attack rate reflects the severity of a a duration of 14 days are examined. Simulations are carried
disease at a given time, and thus it has different values during out with the assumption that 50% of households will comply,
the disease outbreak. Among these values, the peak attack rate which leads to a 75% reduction of external contact rates,
is often considered and compared to the current healthcare while the internal contact rate will increase by 100%. The
capacity (e.g., intensive care unit capacity) to see the current results show that this measure can reduce the final attack rate
system’s ability to handle the peak number of patients. After by up to 6% and the peak attack rate by up to 40%. Similarly,
the outbreak is over, data is often collected to determine the in [33], simulations are performed to examine the impacts
final attack rate which is the total number of infected cases of different measures. For household quarantines, the result
over the entire course of the outbreak divided by the total shows that this measure can reduce the final attack rate by
population. 31% and the peak attack rate by 68% with Ro = 1.8 and a
Social distancing measures are proven to be effective when compliance rate of 50%.
implemented properly [27]–[33]. Different types of social Apart from the abovementioned measures, the effective-
distancing measures may have diverse levels of effectiveness ness of the other social distancing measures either received
on the disease spread. In [27], the effect of social distancing limited attention or was often considered in combination with

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another approach. In [32], the effectiveness of travel restric- Control and Prevention (CDC), U. S. Food and Drug Admin-
tions and border control measures are examined. However, istration (FDA), Australian Department of Health, and Pub-
the results only show that different levels of travel restrictions lic Health England have announced various protocols and
(from 90% to 99.9%) can delay the peak attack rate by up guidelines [13]–[17] during the current COVID-19 pandemic.
to six weeks, while how travel restrictions affect the attack Although they are proposed by different organizations, these
rate is not examined. Another type of measure that does guidelines and protocols share the same objective to limit
not receive much attention is community contact reduction the spread of the virus and many similar methods such as
measures (e.g., avoiding crowds and mass gatherings can- keeping physical distance, avoiding mass gatherings, reduc-
cellation). In [30], it is shown that this type of measure can ing unnecessary physical contact, practicing good hygiene,
reduce the final attack rate by 17%, 14%, and 10%, and the etc. Generally, these protocols and guidelines vary with the
peak attack rate by 72%, 49%, and 38% in the cases where severity of the pandemic at each particular location, e.g.,
Ro < 1.9, 2.0 ≤ Ro ≤ 2.4, and Ro > 2.5, respectively. stricter and more detailed protocols are proposed for the
When combined together, social distancing measures are places where the pandemic is more severe.
proven to be even more effective [30], [32], [34]. It is shown
in [30] that when all four measures, i.e., school closure, b: EFFECTIVENESS
isolation, workplace nonattendance, and community contact In the current COVID-19 pandemic, The World Health Orga-
reduction, are in effect, they can drastically reduce the attack nization (WHO) estimates that the value of Ro would be in
rates in all the considered Ro settings. In particular, the final the range of 2-2.5 [46]. As can be seen from the abovemen-
attack rate can decrease from 65% to only 3% and the peak tioned studies, social distancing measures can play a vital
attack rate from 474 cases per 10 thousand to only five role in mitigating this pandemic with such Ro values. For
cases, in the highest Ro setting. Similarly, [32] examines example, Fig. 3 illustrates the rolling 3-day average of daily
the effects when household quarantines, workplace closures, new confirmed COVID-19 cases in several countries [37].
border control, and travel restrictions are combined. The Generally, after a country began implementing social dis-
results show that the final and peak attack rates are three tancing (e.g., lockdown at different levels) for 13-23 days,
times and six times, respectively, lower than when no policy is the daily number of new cases begins to drop. As can also be
implemented. Moreover, the peak attack rate can be delayed
by nearly three months in a Ro = 1.7 setting. In [34], it is also
shown that when four types of measures (i.e., school closure,
household quarantines, workplace nonattendance, and com-
munity contact reduction) are in effect, the final attack rate
can be reduced 3-4 times depending on Ro .
There are several studies focusing on the negative impacts
of social distancing. In [35], simulations are performed based
on a standard SIR model to evaluate the benefit and cost of
different social distancing strategies. In this study, simula-
tions are carried out without and with social distancing under
different caution levels settings. Simulation results are evalu-
ated based on the benefits of the reduced infection rate and the
economic cost of reducing contacts. The main finding of this
work is that a favorable result can only be obtained by imple-
menting social distancing measures with a high caution level.
Since the economic cost is also considered, it is shown that
implementing social distancing with an insufficient caution
level gives worse results than that of the case without social
distancing. In [36], a game theoretical approach based on the
classic SIR model is proposed to evaluate the benefits and
costs of social distancing measures. Interestingly, the results
show that in the case where Ro < 1, the equilibrium behaviors
include no social distancing measures. Moreover, social dis-
tancing measures are shown to achieve the highest economic
benefit when Ro ≈ 2.

4) SOCIAL DISTANCING IN COVID-19 FIGURE 3. The effects of social distancing in the current
a: PROTOCOLS AND GUIDELINES COVID-19 pandemic. (a) In several countries, the daily number of cases
started to reduce approximately 2–3 weeks after the implementation of
Organizations such as European Centre for Disease social distancing [37]. (b) The impact of social distancing on the total
Prevention and Control (ECDC), U. S. Centers for Disease number of cases (flattening the curve) [38].

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seen from the second graph, the curves representing the total not always easy to do so. For example, it is hard to
number of cases become less steep after social distancing is always keep a safe distance between people (estimating
implemented (i.e., flattening the curve). and maintaining a 1.5–2 m distance all the time is not
Modeling and simulation approaches also predict positive easy), people still have to go outside for healthcare or
effects of social distancing on the current pandemic. It is food, and it is not always possible to work from home
shown in [40] that different combinations of several social (essential workers).
distancing measures, including public place closure, self- • Difficulties when many people stay at home: With the
isolation, household quarantine, and isolation of elderly peo- closure of schools and workplaces, many people will
ple, have different effects on the number of cases in ICU. have to work or study remotely, which leads to an over-
Among them, the combination of all four measures achieves whelming increase in Internet traffic and online service
the best effects, i.e., the peak number of cases is nearly demands, e.g., newly registered users of Zoom [91] and
4 times lower and the peak is delayed for three months com- Microsoft Teams [92] have increased 1270% and 775%,
pared to those of the no social distancing scenario. Moreover, respectively, since the lockdown begins.
the model also predicts that a second wave will occur in the
United Kingdom if social distancing measures are lifted. d: SECOND WAVES
Similarly, a prediction modeling approach based on the These challenges often lead to the premature termination of
SEIR model is presented in [41], which shows the effects social distancing measures by the authorities (e.g., lifting
of different social distancing strategies, including no social restriction) or people (e.g., do not comply with social distanc-
distancing, intermittent and continuous social distancing ing or resuming normal behaviors too soon) [11]. However,
implementation in 16 countries. The simulation predicts such improper implementation of social distancing may lead
that continuous implementation of social distancing achieves to dire consequences such as a second wave of the pandemic
the lowest mortality rates in all countries, although the (i.e., the attack rate rises sharply again).
authors suggest that such strategies are not sustainable for As an example, in the previous 1918 influenza pandemic
low-income countries. (the Spanish flu), social distancing measures, after their initial
In [42], the authors develop a neural network to study success in the first wave, were reduced or ended prema-
and predict the effects of strict social distancing measures turely by the authorities. Moreover, once the first wave was
(e.g., quarantine) on the pandemic mitigation in four different over, the perceived risk reduced, and people resumed normal
regions, namely Wuhan, Italy, South Korea, and the United behaviors although there had not been an effective pharma-
States. Particularly, the proposed neural network is used to ceutical solution yet. These are the main reasons that lead to
determine the parameters of the SIR and SEIR model, such the second wave [43], [44]. This second wave can be even
as β and γ in (2). The results show that a stricter social worse than the first wave, as evidenced by the 1918 pandemic
distancing strategy has a stronger impact on reducing Ro , data collected at various geographical locations. For example,
thereby reducing the pandemic severity. in Sydney, the second wave of the Spanish flu pandemic had a
slightly higher attack rate but nearly double the mortality rate
c: CHALLENGES compared to those of the first wave [43]. Another example
Despite its significant potential, it can be observed that social is presented in [44], where the mortality data of 16 United
distancing is very effective only when applied properly. Nev- States cities were collected during the 1918 influenza pan-
ertheless, it is not easy to implement because of many chal- demic. Among the cities, second waves occurred in 8 cities.
lenges such as: Compared to the first waves, the second waves in these cities
• Negative economic impacts: Many social distancing caused higher death rates in 4 cities and lower death rates in
measures, especially travel restriction, border control, the others.
and public places closure have negative impacts on the In the current COVID-19 pandemic, several countries,
economy. This may lead to premature lifting of restric- e.g., Iran, South Korea, the United States, and Singapore,
tions by the authorities, e.g., Iran, South Korea, China, have suffered from a second wave as illustrated in Fig. 4.
Germany lifted restrictions too early and had to reimpose As observed in Fig. 4(a), when the number of new cases
restrictions [39], [71]. began decreasing, the authorities of Iran started to lift and
• Personal rights violation: Restriction measures such as reduce several social distancing measures (e.g., the grad-
quarantines, canceling mass gatherings, and isolation ual reopening of government offices, shopping malls, etc.),
may conflict with ethical and religious principles, e.g., which led to the second wave after three weeks. Similarly,
Iran closed religious facilities during lockdown [45]. South Korea and the United States lifted restrictions in early
Moreover, contact tracing and tracking the movement of May (e.g., partially reopen bars, restaurants, schools, etc.),
infected people, e.g., contact tracing in Singapore [105], and a second wave occurred in both countries shortly after,
also violate people’s privacy. Consequently, people as shown in Fig. 4(b) and Fig. 4(c), respectively. In the
might not comply with these measures. case of Singapore, the authorities did not lift the restric-
• Difficulties in changing people’s behavior: Even when tions prematurely. However, only a partial lockdown was
a person wants to comply with social distancing, it is implemented, e.g., schools and businesses were not closed

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FIGURE 4. Second waves of COVID-19 in (a) Iran [38], (b) South Korea [38], (c) United States [38], and (d) Singapore [38]. In these countries,
the government started easing restrictions too soon, which results in a second wave.

at the beginning. Since social distancing measures were not limited to certain circumstances, and hard to implement on
strictly enforced, its effectiveness relies on people’s per- a large scale. Moreover, technologies can be used in con-
ceived risk which decreased as the number of daily new junction with these methods, e.g., uses technologies to detect
cases decreased. Consequently, the second wave occurred as crowds and inform law enforcement. Therefore, technologies
shown in Fig. 4(d). We can observe from the figures that the can play a vital role in facilitating social distancing.
pandemic’s severity in the second wave can be much more
devastating than that in the first wave, e.g., in the United e: PRACTICAL SCENARIOS
States and Singapore. Consequently, the authorities have to The practical social distancing scenarios identified/proposed
impose restrictions again, e.g., Iran, South Korea, China, in this survey are categorized and illustrated in Fig. 5. More
Germany, etc. [39], [71]. specific scenarios are summarized in Table 1. The scenarios
Until effective pharmaceutical solutions (e.g., vaccines) are can be briefly classified as follows:
successfully developed and widely available, social distanc- • Keeping distance: In these scenarios, various positioning
ing remains the best type of measures available to mitigate and AI technologies can assist in keeping sufficient
the pandemic [11]. Therefore, social distancing still plays distance (e.g., 1.5m apart) between people. Based on
a vital role in pandemic mitigation, for both the current that, when a person gets too close to another or a crowd,
COVID-19 and future pandemics. In that context, technolo- the person can be alerted (e.g., by smartphones).
gies can be leveraged to reduce social distancing negative • Real-time monitoring: Many wireless and related tech-
effects and ensure social distancing proper implementation. nologies can be utilized to monitor people and public
Besides technologies, other methods such as creating phys- places in real-time (without compromising citizens’ pri-
ical obstacles between people (e.g., plastic dividers, plastic vacy). The purposes of such monitoring are to gather
shields, etc. [227]), markings on pavements and roads [228], meaningful data (e.g., numbers of people inside build-
social distancing awareness signs [228], and law enforcement ings, contacts, symptoms, crowds, and social distanc-
involvement [228] have been applied to facilitate social dis- ing measures violations) to facilitate social distancing.
tancing. However, these methods are not always available, Based on these data, appropriate measures can be carried

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FIGURE 5. Illustrations of the practical social distancing scenarios identified/proposed in this survey. These scenarios can be categorized into seven
main groups: keeping distance, real-time monitoring, information system, incentive, scheduling, AI, and automation.

out (e.g., limit access to buildings when there are too (RFID), and Zigbee can be applied for building access
many people inside, avoid crowds, and alert/penalize scheduling.
violations). • Automation: In the social distancing context,
• Information system: Technologies such as Bluetooth, autonomous vehicles such as medical robots and
Ultra-wideband, Global Navigation Satellite Systems unmanned aerial vehicle (UAV) can be utilized to reduce
(GNSS), and thermal sensors can be employed to collect the need for human presence in essential tasks, e.g., med-
the trajectory data of the infected individuals and the ical procedures and delivery services. Technologies such
contacts that these individuals made. Based on this infor- as ultra-wideband, GPS, ultrasound, and inertial sensors
mation, susceptible people who were at the same place can be leveraged for the positioning and navigation of
or had contacts with the infected ones can take cautious these autonomous vehicles.
actions (e.g., self-isolation, and test for the disease). • Modeling and Prediction: AI technologies can be
• Incentive: Social distancing has negative impacts on employed for pandemic data mining. The results can
personal freedom and the economy. Therefore, incentive help to predict the future trends and movement of
mechanisms are needed to encourage people to comply the infected and susceptible individuals. Moreover,
with social distancing measures (e.g., incentivize people AI-based classification algorithms can be leveraged to
to share their movement data and self-isolate). Opti- detect disease symptoms in public places.
mization techniques and technologies such as Bluetooth, The applications of technologies to specific social distancing
Wi-Fi, and cellular together with economics tools like scenarios are illustrated in Fig. 6.
game theory, auctioning, and contract theory can facili-
tate those incentive mechanisms. B. POSITIONING TECHNOLOGIES
• Scheduling: Various scheduling techniques can be Since the main principle of social distancing is to increase
employed to increase the efficiency of workforce and the distances of human contacts, approaches to determine
home healthcare service scheduling, thereby decreasing the positions and measure the distance between people can
the number of employees at workplaces and patients at play a vital role in facilitating social distancing measures.
hospitals. Moreover, scheduling techniques can also be Using ubiquitous technologies, such as Wi-Fi, cellular, and
applied for traffic control to reduce the number of vehi- GNSS, positioning (localization) systems are crucial to many
cles and pedestrians on the street. Furthermore, tech- practical social distancing scenarios such as distance keeping,
nologies such as Wi-Fi, Radio frequency identification public places monitoring, contact tracing, and automation.

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TABLE 1. Practical social distancing scenarios.

1) OVERVIEW OF POSITIONING SYSTEMS a positioning system aims to continuously track the position
Fig. 7 illustrates the general process and several pop- of an object in real-time [23]. To achieve this goal, firstly,
ular methods of a positioning system [47]. Generally, signals are transmitted from the target to the receiving nodes

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FIGURE 6. Application of technologies to different social distancing scenarios. Some technologies, e.g., Cellular and GNSS, can be applied
to many scenarios, whereas technologies such as Zigbee and RFID are applicable to fewer scenarios due to their limited communication
ranges. Scenarios from the same group have the same color. The arrows that show the links from one technology to different scenarios
have the same color.

FIGURE 7. General principle of positioning systems. Signals from sensors are measured using different methods, e.g., time-based, AOA, and RSS, to derive
the corresponding signal properties such as traveling times and angles. Based on these measurements, the position of the object can be determined by
position calculation techniques such as trilateration, triangulation, and MLE.

(e.g., sensors). From the received signals, useful properties determine the distance between the receiving nodes and the
such as arrival time, signal direction, and signal strength target [47]. Time-based methods can be further classified as
(depending on the measurement methods) are extracted in the follows:
signal measurement phase. Based on these features, the posi- • Time-of-Arrival (TOA) [50]: This method determines
tion of the target can be calculated using various methods the distance D between the receiving node and the target
in the position calculation phase [47]. Several effective sig- based on the time it takes for the signal to travel from
nal measurements and position calculation methods are pre- the target to the node, i.e.,
sented in the rest of this section.
D = ct, (3)
2) SIGNAL MEASUREMENTS where c is the speed of the signal transmission and t is
Typical signal measurement methods can be classified based the time for the signal to reach the receiving node.
on the extracted property of the received signal. Among them, • Time Difference-of-Arrival (TDOA) [50]: This method
time-based methods use the arrival time of the signal to uses two kinds of signal with different speeds and

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calculates D based on the difference between them, i.e., reference nodes and the corresponding measured distances
D D D1 , D2 , and D3 , the coordinate (x, y) of the target can be
− = t1 − t2 , (4) determined by
c1 c2
p
p(x1 − x) + (y1 − y) = D1 ,
where c1 , c2 , t1 , and t2 are the speeds and arrival time of  2 2
the two signals, respectively.
(x2 − x)2 + (y2 − y)2 = D2 , (7)
• Round Trip Time (RTT) [47]: The RTT method measures p
(x3 − x) + (y3 − y) = D3 .

2 2
the duration in which the signal travels to the targets and
comes back, i.e., Instead of using distances, the Triangulation method uses
tRT − 1t the angles of the signal (from the AOA method) to deter-
D= , (5)
2 mine the target’s position. As illustrated in Fig. 7, if the
where tRT is the time of the whole round trip, and 1t coordinates of two reference nodes and the corresponding
is the predetermined delay between when the target measured angles α1 , α2 are known, the target’s position can
receives the signal and when the target starts sending be geometrically determined [47].
back. To address the uncertainty in measurements, the Maximum
A common disadvantage of the TOA and TDOA methods Likelihood Estimation (MLE) method is often employed.
is that they require synchronized clocks at the node and the This method utilizes the signal measurements from a num-
target to determine t, t1 and t2 . That may be costly to be ber of reference nodes (usually three or more) and applies
implemented as it requires frequent calibrations to maintain some statistical approaches such as the minimum variance
accuracy. Although the RTT method does not require clock estimation method [49] to calculate the target’s position while
synchronization, it needs to acquire the delay 1t which can- minimizing the impact of noises in the environment [47].
not be predicted in many circumstances [48]. Consequently,
extra efforts are needed to determine 1t. III. WIRELESS TECHNOLOGIES FOR SOCIAL DISTANCING
Unlike the time-based methods, the Angle-of-Arrival To enable social distancing, many wireless technologies
(AOA) method determines D by measuring the angle of can be adopted such as Wi-Fi, Cellular, Bluetooth, Ultra-
the incoming signals by using directional antennas or array wideband, GNSS, Zigbee, and RFID. In this section, we first
of antennas. The measured angles can then be used in the briefly provide the fundamentals of these technologies and
triangulation method to geometrically determine the target then explain how they can enable, encourage, and enforce
position. However, the main disadvantage of this method is people to practice social distancing. After that, we discuss the
that it requires extra directional antennas which are costly to potential applications, advantages, limitations, and feasibility
implement [47]. of these technologies.
The Received Signal Strength Indication (RSSI) method
measures the attenuation of the signals to determine the A. Wi-Fi
distance. Typically, the relationship between the RSSI and Due to the fact that Wi-Fi technology is widely deployed
distance can be formulated as follows [53]: in indoor environments, this technology can be considered
a promising solution to practice social distancing inside
PR = α − 10n log10 (d) + X , (6) multi-story buildings, airports, alleys, parking garages, and
where PR is the RSSI value at the receiver (e.g., access point), underground locations where GPS and other satellite tech-
d represents the distance from the user device to the access nologies may not be available or provide low accuracy [20].
point, X is a random variable (caused by the shadowing In a Wi-Fi system, a wireless transmitter, known as a wireless
effect) which follows the Gaussian distribution with zero access point (AP), is required to transmit radio signals to
mean. α is a constant value which can be known in advance communicate with user devices in its coverage area. Cur-
and depends on fading, antennas gain, and emitted power of rently, Wi-Fi enabled wireless devices are working according
the user device. n is the path loss exponent which depends on to the IEEE 802.11 standards. Wi-Fi 6 (based on 802.11ax
the channel environment between each user device and the technology) is the latest version of the Wi-Fi standards which
access point. Thus, based on the RSSI level of the received provides high-throughput and reliable communications [51].
signals, the access point can estimate the position of the user We discuss a few example scenarios of social distancing that
device in indoor environments. can be enabled by Wi-Fi as follows.

3) POSITION CALCULATION 1) CROWD DETECTION

Based on the measured signal properties, different methods One potential application of Wi-Fi technology in social dis-
are employed to calculate the target’s position. Among them, tancing is positioning [53], [70]. Based on the location of
Trilateration is a common method which uses three reference users, the authorities can detect crowds inside a building and
nodes and the distances between them to the target to calcu- force them to maintain a safe distance. This is an essential fac-
late the position [47], as illustrated in Fig. 7. More specif- tor to practice social distancing during a pandemic outbreak
ically, using the coordinates (x1 , y1 ), (x2 , y2 ), (x3 , y3 ) of the in indoor public places such as train stations and airports.

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There are two main reasons making Wi-Fi technology pos- To address these problems, several solutions [55]–[59]
sible in social distancing. First, due to the convenience of are proposed to enable indoor localization in dynamic and
hardware facilities, we can quickly deploy Wi-Fi systems for complicated areas such as airports and train stations. With
user positioning with very low cost and efforts [52]. Sec- these solutions, the authorities can detect crowds and force
ond, with recent advances in Wi-Fi-based indoor positioning, people to leave to enable social distancing during pandemic
Wi-Fi can provide reliable and precise location services to outbreaks. Specifically, in [55], the authors show that when
enable social distancing. The most common and easiest way the environment changes, e.g., the presence of people in the
for indoor positioning is to calculate the user’s location based line of sight between the user device and the access point,
on the RSSI of the received signals from the user device [53], the performance of conventional RSSI-based localization
[54]. However, the accuracy of this solution much depends techniques is greatly decreased. Thus, the authors propose
on the propagation model. Thus, in [53], the authors present an adaptive signal model fingerprinting algorithm to adapt to
a new method to dynamically estimate the channel model the dynamic of the environment by detecting users’ positions
from the user device to the access point. The key idea of and updating the database simultaneously. In [59], the authors
this solution is continuously determining the RSSI values in propose a new localization technique to locate multiple users
real-time to obtain the estimated channel model that is close in different areas by performing a fine-grained localization.
to the real channel model. Once the propagation is estimated, In addition, the authors introduce a transfer mechanism to
the distance between the access point and the user device can adjust the fingerprint database over multiple areas to mini-
be accurately determined. After that, the user’s location will mize human intervention.
be derived by using the trilateration mechanism. An interesting design is proposed in [60] to locate and track
Differently, the authors in [54] propose to adopt the inertial people by using Wi-Fi technology, namely Wi-Vi (stands for
navigation system (INS) to significantly increase the accu- Wi-Fi Vision). This technology allows the authorities to track
racy of conventional RSSI-based methods. The key idea of people in indoor environments and detect potential crowds,
this solution is using a Kalman filter to combine and fill so that they can take appropriate actions to enable social dis-
the signal database with the INS data. As such, the authors tancing, e.g., inform people not to go to potentially crowded
can obtain the average distance error as small as 0.6 m. The places. In particular, Wi-Vi uses a MIMO interference nulling
above RSSI-based solutions can be easily adopted to detect to remove reflections from static objects and only focuses on
crowds in indoor environments. Then, the local authorities moving objects, e.g., a user. Moreover, the authors propose
can take appropriate actions to disperse the crowds or suggest to consider the movement of a user as an antenna array and
other people not to go to the place. For example, if there then track the user by observing its RF beams. If there are
are too many people in a supermarket, the authorities can many people having the same direction, e.g., going to the
notify and recommend new coming customers to go to other same place, the authorities can notify them to avoid forming
supermarkets or come in another time so that they can avoid crowds. Thus, Wi-Vi can be considered a promising technol-
crowds. ogy to enable social distancing.
However, to efficiently detect crowds, Wi-Fi-based local-
2) CROWD DETECTION IN DYNAMIC ENVIRONMENTS ization systems may require several transceivers attached to
Although the RSSI-based solution can detect the user’s loca- each access point to obtain high accuracy. Another problem
tion with sufficient accuracy, it may not be effective in is to differentiate between human and machine terminals.
dynamic and complicated indoor environments such as air- To address this problem, fingerprint databases can be used to
ports or train stations [55]–[57]. This is due to the effects of detect machine terminals which are usually placed at known
non-line of sight (NLOS) propagation on the wireless signals locations. Nevertheless, this solution may not be feasible if
between the user’s device and the access point, especially in we consider autonomous robots in the environments, and thus
dynamic and complicated environments in which the wireless can be a potential research direction.
signals are greatly scattered by obstacle shadows or peo-
ple (e.g., running and walking) [55]. Another RSSI-based 3) PUBLIC PLACE MONITORING AND ACCESS SCHEDULING
indoor localization technique is the fingerprinting approach Another way to apply Wi-Fi technology in social distancing
(or radio map) that locates devices based on a previously is by controlling the number of people inside a building, e.g.,
built database. In particular, this database contains the sig- supermarket, shopping mall, and university. Specifically, with
nal fingerprints corresponding to several access points in various Wi-Fi access points implemented inside the building,
a specific area. Nevertheless, collecting fingerprint data is the number of people currently inside the building can be
time-consuming and laborious [58], especially in large areas estimated based on the number of connections from user
such as airports or train stations. In addition, it is infeasi- devices to the access points. For example, the authors in [61]
ble to directly apply the pre-obtained fingerprint database propose a low-cost cyber physical social sensing system
to new areas for localization [59]. The main reason is that which tracks the Wi-Fi messages between the devices, e.g.
the adjustment process to apply the fingerprint database of smartphones, and the access points. Based on these messages,
an area to another is time-consuming and usually requires meaningful information such as the number of people within
human intervention. the coverage area of the access points can be extracted. Using

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FIGURE 8. Cellular technology can support different social distancing scenarios. In real-time monitoring
and infected movement data scenarios, cellular can help to determine people’s location. Based on these
locations, people and traffic density can also be predicted. Cellular can also support Internet-based
services, thereby encourage people to stay at home.

this information, several actions can be taken, such as forcing be feasible for outdoor environments. For outdoor environ-
people to queue before entering the building to maintain a ments, other wireless technologies, e.g., Bluetooth, GPS, and
safe number of people inside the facilities at the same time. cellular technologies, can be considered.
Another application is notifying people who want to go to a
building. Specifically, based on the number of people inside B. CELLULAR
the building, the authorities can encourage/force them to Over the past four decades, cellular networks have seen
stay home or come at a different time if the place is too tremendous growth throughout four generations and become
crowded. However, the accuracy of this approach depends the primary way of digital communications. The fifth genera-
on many factors such as the number of smart devices one tion (5G) of cellular networks is coming around 2020 with the
person possesses, how many devices can be connected to a first standard. According to the Cisco mobile traffic forecast,
network simultaneously, and whether the user connects to the there will be more than 13 billion mobile devices connected
access point as many people completely rely on their cellular to the Internet by 2023 [72]. That positions the cellular
connections. technology at the center to enable social distancing in many
circumstances including real-time monitoring, people density
4) STAY-AT-HOME ENCOURAGEMENT prediction and encouraging stay-at-home by enabling 5G live
Wi-Fi technology can also be used to encourage people to stay broadcasting, as illustrated in Fig. 8.
at home by detecting the frequency of moving outside their
houses for a particular time, e.g., a day. Specifically, when 1) REAL-TIME MONITORING
user devices move far away from the access point inside their Individual tracking and mobility pattern monitoring are
houses, the connection between them will be weak or lost. potential approaches using cellular technology to practice
Based on this information, the access points can estimate the social distancing as shown in Fig. 8(a). According to the
frequency of moving out of their house and then notify the 3GPP standard, the current cellular networks, i.e., LTE and
users to encourage them to stay at home as much as possible. LTE-A, are employing various localization methods such as
Summary: Wi-Fi technology is a prominent solution to Assisted-GNSS (A-GNSS), Enhanced Cell-ID (E-CID), and
quickly and effectively enable, encourage, and force people Observed TDoA (O-TDoA) as specified in the Release 9;
to practice social distancing. With the current advances of Uplink-TDoA (U-TDoA) included in the Release 11; and
Wi-Fi, the accuracy of localization systems can be signifi- with the aids of other technologies like Wi-Fi, Bluetooth,
cantly improved, resulting in effective and precise applica- and Terrestrial Beacon System (TBS) as stated in the
tions for social distancing. However, Wi-Fi-based technology Release 13 [73], [74]. Cellphone location data collected by
is mainly used for indoor environments as this technology the current cellular network is normally used for network
requires several access points for localization which may not operations and managers [74] such as network planning and

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optimization to enhance the Quality of Service (QoS) rather concerns compared with individual-level tracking (i.e., it sat-
than user applications due to privacy and network resource isfies the EU privacy rules [81]). The metadata can be used
concerns. However, in the context of social distancing, user to obtain the mobility patterns, and thus the governments can
tracking based on data of user movement history can be monitor whether people are complying with the lock-down
very effective, e.g., for quarantined people detection, and rules or not. It can also be employed to model the spread of
infected people tracing. The authorities can check whether the virus to aid the governments in analyzing and evaluating
infected people are violating quarantine requirements or not. the effectiveness of ongoing quarantine measures during the
In cases they do not follow the requirements, the authorities outbreak.
can send warning messages or even perform some aggressive
measures, e.g., fines and arrests, to force them to self-isolate. 2) PEOPLE DENSITY PREDICTION
Moreover, when a user has been exposed to the virus, In addition to the real-time crowd monitoring and modeling
the user’s mobility history can be extracted to investigate the spread of the virus, the movement history data can be
the spread of the virus. In these cases, the cellular technol- utilized to predict the network traffic due to the large-scale
ogy can outperform other wireless technologies in term of location data provided by carriers and the recent advances of
availability and popularity. For example, localization services machine learning. There are various works on network traffic
relying on wireless technologies such as GPS always need prediction proposed in [86]–[90] using the history of users’
to be run in the foreground application (i.e., the availabil- movements. Furthermore, the number of users in a specific
ity), while this service is a part of cellular network opera- area can also be estimated from the network traffic of that area
tions. In addition, Ultra-wideband and Zigbee technologies as illustrated in Fig. 8(b). Thus, the authorities can predict
require additional hardware [122], [161] (i.e., the popularity). the crowd gathering in public places (e.g., shopping malls,
Incoming 5G networks with the presence of key technolo- airports, and train stations) relying on the corresponding
gies such as mm-Wave communications, D2D communica- forecasted network traffic. Then, appropriate actions can be
tions, and Ultra-dense networks (UDNs) [75] are capable performed by the authorities to prevent crowd gathering in
of performing a high precision localization. Two position- these places. For example, if the predicted number of people
ing schemes exploiting the mm-Wave communications are entering a shopping mall exceeds a threshold, the authorities
proposed in [76] based on the validation of triangulation can notify customers to avoid coming to this place at this time
measurements and angle of differences of arrival (ADoA). or recommend them to go to other shopping malls having
The simulation results show that the triangulate-validate and lower densities. In addition, this method can also be applied
ADoA methods can obtain a sub-meter accuracy level with a in residential areas to study how often people stay home as
probability of 85% and 70%, respectively in a 18 m × 16 m well as predict when they go out or the places they come
indoor area. The authors in [77] propose a positioning scheme to. This can provide significant data input for network traffic
in UDNs using a cascaded Extended Kalman filter (EKF) forecasts in public places. In addition, if they regularly go to
structure to fuse the DoA and ToA estimations from the non-essential places, the authorities can warn or force them
reference nodes. The proposed scheme can localize a moving to stay at home as much as possible.
target at speed 50 km/h with a sub-meter level accuracy in an
outdoor environment. It can be used for tracking vehicles and 3) STAY-AT-HOME ENCOURAGEMENT
monitoring the traffic density. To implement social distancing, many people must do their
Recently, some governments have required telecom com- daily activities remotely from their home such as working,
panies to share cellphone location data to implement social studying, and entertainment. Therefore, some video confer-
distancing to deal with COVID-19. For instance, Taiwan ence applications used to work from home or study online
deployed an ‘‘electronic fence’’ exploiting the cellular-based have witnessed an explosion of downloads. For example,
triangulation methods to ensure that the quarantined cases the Zoom application has achieved an increase by 1,270%
stay in their homes [78]. The local officials call them twice from 22 Feb to 22 Mar in 2020 [91] and the number
a day to ensure they do not leave their phones at home and of newly registered users of Microsoft Teams has also
visit them within 15 minutes after their phones are turned risen 775% monthly in Italy after the full lock-down was
off or if they move away from their homes. The Moscow started [92]. As a result, 5G live broadcasting technology
government is also said to be planning to use SIM card can be used to encourage people to stay at home while
data for tracking foreigners and residents who have close minimizing the impact on their work, or study (Fig. 8(c)).
contacts with foreigners when the border closure order is Especially, this is probably applicable to cases where land-
lifted [79]. However, individual tracking using cellular tech- line Internet is not available. There are many works to
nology has raised concerns about privacy [80], [81]. Instead, enhance the quality of video multicast/broadcast applications
group/crowd detecting and monitoring based on shared loca- by utilizing the advances of 5G networks [93]–[97]. Video
tion data which is anonymous and aggregated from carriers multicast/broadcast services are defined as an ultra-high def-
become the key approach utilized by several governments inition slice in a MIMO system [93]. To improve the spectral
such as Italy, Germany, Austria, the UK, Korea, and Aus- efficiency for video multicast/broadcast in the proposed sys-
tralia [82]–[85]. This approach is intended to alleviate privacy tem, the authors introduce a hybrid digital-analog scheme to

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FIGURE 9. Contact tracing application based on Bluetooth technology [103]. The application will record the event when
two people have close contact with each other. Later, when one of them is tested positive for an infectious disease,
the application can notify the other person.

tackle channel condition and antenna heterogeneity. Another forecasted network traffic. The low latency feature of 5G net-
possible solution that can significantly improve qualities for works in data processing using edge/fog computing enables
video multicasting/broadcasting is data caching. A novel quick responses of the authorities (e.g., send notifications
caching paradigm proposed in [94] is applied for multi- instantly), for example, to prevent close contact. However,
cast services in heterogeneous networks. With the awareness the use of subscriber’s location data for social distancing
of multicast files, the proposed caching policy can select measures is subject to great privacy concerns from citizens.
files efficiently for the caches. Studies in [95], [96] propose
using NOMA techniques to support multicast/broadcast by C. BLUETOOTH
increasing the spectrum efficiency in multi-user environ- With the explosive growth of Bluetooth-enabled devices,
ments. Finally, the authors in [97] propose a video multi- Bluetooth technology is another solution for social distancing
cast orchestration scheme for 5G UDNs which can help to in both indoor and outdoor environments. In particular, Blue-
improve the spectrum efficiency. tooth is a wireless technology used for short-range wireless
communications in the range from 2.4 to 2.485 GHz [98],
4) INFECTED MOVEMENT DATA [99]. Bluetooth devices can automatically detect and connect
Due to the omnipresence of mobile phones and the near to other devices nearby, forming a kind of ad-hoc called
world-wide coverage of cellular signals, cellular technology piconet [99]. Recently, Bluetooth Low Energy (BLE) has
can be an effective tool to track the movement of people. been introduced as an extended version of the classic Blue-
Unlike in the quarantined people detection scenarios where tooth to reduce the energy usage of devices and improve the
these people may deliberately leave their phones at home, communication performance [99]. Given the above, the BLE
people do not have any reason to do so in the infected move- localization technology possesses several advantages com-
ment data scenario. Therefore, cellular can be an effective pared with those of the Wi-Fi localization. First, the BLE sig-
technology in this scenario. The authors of [190] summarize nals have a higher sample rate than that of the Wi-Fi signals
the methods to trace human position in outdoor environments (i.e., 0.25 Hz ∼ 2 Hz) [100]. Second, the BLE technology
using base stations and indoor environments using access consumes less power than that of the Wi-Fi technology, and
points. However, the positioning accuracy for outdoor envi- thus it can be implemented widely in handheld devices. Third,
ronments still needs to be improved because a small error by the BLE signals can be obtained from most smart devices,
using the cellular network technology can cause a big error in while Wi-Fi signals can be obtained from only access points.
the distance measurement. Finally, BLE beacons are usually powered by battery, and
Summary: Cellular technology can be considered one of thereby they are more flexible and easier to deploy than
the most important approaches to assist social distancing. Wi-Fi. It is worth noting that Bluetooth is mainly used for
It can be deployed on a large scale due to its convenience infrastructureless adhoc communications in contrast to other
and omnipresence compared to other wireless technologies. technologies.
It can be used to track quarantined or infected individuals.
Furthermore, it can provide a unique solution to not only 1) CONTACT TRACING
monitor crowds in real-time, but also allow the local author- One application of Bluetooth in social distancing is contact
ities to predict the forming of crowds in public areas (e.g., tracing [101], [102] as illustrated in Fig. 9. The key idea
airports, train stations, and shopping malls) based on the is using Bluetooth to detect other users in close proximity

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with their information (e.g., identifier) stored in a person’s which deploy direct-sequence spread spectrum and orthogo-
Bluetooth device, e.g., a mobile phone. When there is an nal frequency-division multiplexing signaling methods. Sim-
infected case, the authorities can ask people to share these ilarly, Bluetooth devices can avoid interference from other
records as a part of a contact tracing investigation. Thereby, wireless devices, e.g., Wi-Fi enabled devices, by using the
the authorities can detect people who may have close contact spread-spectrum frequency hopping technique to randomly
with the infected one and notify them promptly to prevent the use one of 79 different frequencies in Bluetooth bands.
spreading of diseases. Several attempts to use Bluetooth in As such, the interference from other devices is significantly
contact tracing have been reported. Apple and Google have reduced, thereby improving the accuracy of localization
recently introduced a mobile application (running on both systems.
iPhone and Android devices) that can detect other smart-
phones nearby using Bluetooth technology [103]. If a person 3) DISTANCE BETWEEN TWO PEOPLE
is tested positive for a disease, he/she will enter the result in Bluetooth can also be used to determine the distance between
the app to inform others about that. Then, people who may two persons by using their Bluetooth-enabled devices, e.g.,
have close contact with the positive case will be notified and smartphone or smartwatch, as depicted in Fig. 10. Specifi-
instructed about what to do next. Note that a Wi-Fi or cellular cally, similar to the Wi-Fi technology, based on RSSI levels,
connection would also be required to enable the app. Sim- a device can calculate the distances between itself and other
ilar apps have been recently launched in Singapore [105], nearby devices [113]. It is worth noting that Bluetooth tech-
Europe [107], and India [108]. nology can allow a device to connect to multiple devices at the
same time [98]. Thus, the device can simultaneously detect
2) CROWD DETECTION distances to multiple devices in its coverage. If the distance
Bluetooth technology can be used to detect crowds in indoor is less than a given threshold, e.g. 1.5 meters [13], the devices
environments to practice social distancing with the latest can warn and/or encourage users to practice social distancing.
advances in Bluetooth localization techniques [111], [113].
In particular, based on signals received from users’ Bluetooth
devices, a central controller can calculate the positions of
users and detect/predict crowds in indoor environments. If a
crowd is detected, the local manager can force people to leave
to practice social distancing. In addition, they can advise
people who want to go to the place to come at a different time
if the place is too crowded at the moment. In [111], the authors
point out that with the development of BLE, Bluetooth-based
indoor localization can be considered a practical method to FIGURE 10. Keeping distance between any two persons using Bluetooth
technology. A Bluetooth-enabled device such as a smartphone can
locate Bluetooth devices in indoor environments due to its calculate the distances between itself and other nearby
low battery cost and high communication performance. The Bluetooth-enabled devices. When another device comes into close
proximity, a warning notification can be sent to the user.
authors then propose indoor localization schemes that collect
RSSI measurements to detect the user’s location by using the
triangulation mechanism. Summary: Bluetooth technology is a very promising solu-
In [112], the authors show that the BLE technology is tion to enable social distancing. However, the privacy of users
strongly affected by the fast fading and interference, result- needs to be taken into account as the applications require
ing in a low accuracy when detecting the user’s device. users to share information with the authorities and third par-
To improve the accuracy of the BLE positioning, the authors ties. This can be a research direction to ensure privacy and
run several experimental tests to choose the optimal parame- encourage people to share their information to prevent the
ters to set up BLE localization systems. The authors demon- spreading of diseases. In addition, there are several draw-
strate that the BLE-based indoor localization can achieve a backs of Bluetooth technology in social distancing which
better performance than that of Wi-Fi localization systems. need to be considered such as the accuracy of localization
The authors of [114] point out that the accuracy of BLE-based techniques when the users’ devices are located inside the
localization is strongly affected by advertising channels, pockets or bags and their devices always need to turn on
human movements, and human obstacles. To address these the Bluetooth mode. Furthermore, combining Bluetooth and
problems, they propose a dynamic AI model that can detect other technologies (e.g., Wi-Fi [117]) to improve the local-
human obstacles by using three BLE advertising channels. ization accuracy is also an open research direction.
Then, the RSSI values will be compensated accordingly.
In [115] and [116], the authors show that Wi-Fi-based D. ULTRA-WIDEBAND
and Bluetooth based localization systems can be strongly Ultra-WideBand (UWB) technology has been deemed to be
affected by the interference from other wireless devices a promising candidate for precise Indoor Positioning Sys-
operating at 2.4 GHz bands. To mitigate the interference, tems (IPSs) that can sustain an accuracy at the centime-
Wi-Fi devices can use 802.11b and 802.11g/n standards ter level in the ranges from short to medium. This is due

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to its unique characteristics (e.g., high time-domain res- Recently, device-free localization (or passive positioning)
olution, immunity of multipath, low-cost implementation, techniques have witnessed significant interest. This is due to
low power consumption, and good penetration) [118]. Due the capability to tackle inherent problems of aforementioned
to the wide bandwidth nature of UWB signals (at least communication-based localization approaches: (i) privacy
500 MHz as specified by FCC [119]), the impulse radio (IR) issues (e.g., tracking targets do not need to communicate
UWB technology has the capability of generating a series of with an access point/network coordinator, and thus it can
very short duration Gaussian pulses in time-domain which protect private information of the target), and (ii) physical
enables its advantages compared with other RF technology. obstacles (e.g., LOS communications have significant impact
Pulse position modulation with time hopping (TH-PPM) by obstacles) [126]. The high time-domain resolution feature
is the most popular modulation scheme exploited in the of the IR-UWB technology enables the device-free localiza-
impulse radio based UWB [120]. This pulse can directly tion methods relying on the changes of very short pulses
propagate in the radio channel without requiring additional properties between two transceivers because of absorption,
carrier modulation. The baseband-like architecture of the scattering, diffraction, reflection, and refraction [127], [128].
IR-UWB facilitates extremely simple and low-power trans- In particular, the authors in [127] use monostatic radar mod-
mitters. Thus, the advantages of the IR-UWB technology can ules (i.e., P410 platform) equipped with one transmitter and
greatly support social distancing, even better than other wire- one receiver for multi-target tracking based on Gaussian
less technologies (e.g., higher accuracy in indoor positioning mixture probability hypothesis density (GM-PHD) filters.
applications) or provide exclusive solutions (e.g., device-free Information (including raw signal, bandpass signal, motion
tracking/counting) for some scenarios, as discussed below. filtered signal, and detection list) extracted from the reflected
signals is used to estimate the locations of targets with an
1) REAL-TIME MONITORING accuracy at the decimeter level. To improve the accuracy,
In this section, we review some social distancing scenarios a multi-static is deployed in [128] to track a person in
using Ultra-wideband technology for real-time monitoring real-time by determining the difference between the channel
such as crowd detection (e.g., tracking users’ location), public impulse response with the presence of a new object and that of
place monitoring and access scheduling (e.g., counting the the previous one without the object. The location of the object
number of people in a specific area). can be found with the mean error of only 3 cm by applying
a leading edge detection algorithm on the difference between
a: CROWD DETECTION the two measurements. However, the limitation of this work
One of the major solutions for crowd detection is track- is that it can track only one target at a time. Motivated by the
ing locations of people in public areas. There are many above works, we can easily deploy device-free localization
commercial products exploiting the IR-UWB technology techniques for crowd detection in public areas without reveal-
for real-time localization in both daily life and factories ing any personal information and hardware requirements on
such as DecaWave [121], BeSpoon [122], Zebra [123], target objects. Thereby, the authorities can locate the exact
Ubisense [124]. DecaWave and BeSpoon claim their products locations of crowds and have appropriate actions to disperse
based on ranging measurements can offer an accuracy under crowds or force them to practice social distancing.
10 cm [121], [122]. Furthermore, Ubisense and Zebra provide
industrial products which can obtain a high accuracy even b: PUBLIC PLACE MONITORING AND ACCESS SCHEDULING
in cluttered, indoor factory environments [123], [124]. All A simple solution for public place monitoring is referred
of them support real-time positioning for multiple mobile to as device (or tag)-free counting techniques [129], [130].
tags by using the triangulation techniques based on the abso- Specifically, the authors in [129] propose an advanced peo-
lute locations of reference nodes or anchors (e.g., UWB ple counting algorithm using the revelation of the received
transceivers). Especially, the Dimension4 sensor invented signal pattern according to the number of people as illus-
by Ubisense can be integrated with a built-in GPS module trated in Fig. 11(a). This method enables people counting
for outdoor tracking purposes. Experiments conducted to even with the presence of dense multipath signals in the
evaluate holistically the performance of three commercial environment which is not able to be performed by counting
products (i.e., DecaWave, BeSpoon, and Ubisense) under techniques based on detecting single signals corresponding to
indoor industrial environment setting (with the presence of individual persons. For example, other counting approaches
NLOS) can be found in [125]. The availability of commercial using Wi-Fi and Zigbee rely on the number of connec-
UWB-based localization systems enables real-time people tions from users to an access point (i.e., Wi-Fi) or a net-
tracking in public places by localizing their UWB-supported work coordinator (i.e., Zigbee). Major clusters are picked up
phones, or personal belongings equipped with tags (e.g., keys to find main pulses having maximum amplitudes. A joint
and shoes). Thus, the authorities can detect the crowd to probability density function derived from these main pulses
notify them and other people in the area, disperse the crowd is utilized to derive the maximum likelihood (ML) equa-
or even predict and prevent the forming of the crowd by tion. Then, the estimated number of people is determined
using AI/Machine learning algorithms based on the previ- to be the figure having the maximum likelihood as shown
ously collected data. in Fig. 11(b). Similarly, the solution in [130] also provides

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or phones. However, these products can be employed to detect

close proximity between users in public places. Thanks to
the IR-UWB technology, they can frequently broadcast pilot
messages containing some information (e.g., their specific
IDs, timestamp, etc.) to nearby devices for ranging measure-
ments with extremely low energy consumption. Then, sur-
rounding devices can utilize the information of the received
messages to estimate the distance from the source device and
warn the users if they are too close to each other (e.g., less
than 1.5m in a pandemic situation [13]). In addition, these
devices can also store other information like who had close
contacts with them along with the distances and duration
periods. This information is very important because it can
be used to trace close contacts in the future (e.g., investigate
the spread of the virus in a pandemic) with minimal privacy
violation.
In order to help people to avoid crowds, especially vul-
nerable or at-risk groups, in indoor environments such as
shopping malls, hospitals, and office buildings, BeSpoon
introduces a commercial product that allows moving targets
to self-localize their positions very accurately (i.e., less than
10 cm over 600 m in LOS environments) in a short time
by using the IR-UWB technology [122]. This product pro-
vides both evaluation kits and an ultra-compact UWB module
which can be easily integrated into off-the-shelf products
(e.g., shopping trolleys or baskets) for localization and nav-
FIGURE 11. Tag-less counting technique using the UWB technology. igation purposes. A SnapLoc platform proposed in [132]
(a) The UWB module emits an impulse signal and receives different allows an unlimited number of tags to self-estimate their
signals reflected back from people and other objects. (b) Signals reflected
from people (red) and objects (blue) have different amplitudes. Using a locations at position update rates up to 2.3 kHz. It uses
maximum likelihood approach, the number of people can be determined. the TDoA technique based on all simultaneous responded
information from reference nodes integrated into one single
channel impulse response. By combining with the positioning
a counting approach without positioning targets by using service (i.e., to provide locations of other people in a specific
the crowd-centric method based on energy detection. With- area), a navigation application exploiting commercial prod-
out requiring hardware deployment like Wi-Fi and Zigbee, ucts like BeSpoon can be developed to assist people (e.g.,
the approaches proposed in [129], [130] can provide a customers) for self-detecting their current locations as well
low-cost and high-privacy solution to detect the number of as crowds’ locations along the way, thereby assisting them to
people in public areas. Further actions can be conducted plan their moves and navigate to stay away from crowds.
by the local manager to maintain social distancing such as Summary: With the aforementioned potential applications,
scheduling people to enter the place based on the count- the IR-UWB systems can be considered an outstanding solu-
ing information or giving advice to other people who are tion to handle social distancing in both indoor and outdoor
planning to go to the crowded place to come at a different environments. The IR-UWB based localization systems dis-
time. cussed in [121]–[124] can be employed for detecting and
monitoring crowds in public places with a low-cost deploy-
2) KEEPING DISTANCE AND CONTACT TRACING ment. Although this technology can also be used to monitor
Similar to Bluetooth, the IR-UWB technology can also be the positions of self-isolated people to check whether they
applied to maintain the distances between people as well may violate the quarantine requirements or not, it is less
as close physical contact tracing by using ranging methods attractive than other RF technologies like Wi-Fi or cellu-
with high precision in both indoor and outdoor environ- lar which do not require to install additional hardware for
ments [121], [131]. While DecaWave provides a ranging tracking purposes. In addition, UWB-enabled phones like
measurement using sensors and tags [121], Apple has already iPhone 11 series can assist users in practicing social distanc-
brought this feature to their phones (e.g., Iphone 11 series) ing without localization and navigation services. However,
for their primitive location-based services (e.g., finding this solution only works with a modern iPhone equipped with
objects and improving AirDrop) [131]. These approaches use a UWB chip. Last but not least, the device-free technology
time-based ranging techniques like ToF, TDoA or combined presented in [127]–[130] is a great advantage of the IR-UWB
ToF and AoA to measure the distances to nearby sensors, tags, technology compared with other wireless technologies for

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the crowd detecting and monitoring in public places with is especially important for navigation in autonomous sys-
acceptable accuracy at the decimeter level [127]. tems, such as robots, UAVs, and self-driving cars. Thus,
in a pandemic outbreak when people are required to stay
E. GLOBAL NAVIGATION SATELLITE SYSTEMS (GNSS) at home, GNSS-based autonomous services play a key role
The GNSS has been being the most widely used for position- to minimize physical contact between people. For example,
ing purposes in the outdoor environment nowadays. GNSS customers can shop online and receive their items with drone
satellites orbit the Earth and continuously broadcast navi- delivery services. Such kind of services has been introduced
gation messages. When a receiver receives the navigation recently by some large retail corporations such as Ama-
messages from the satellites, it calculates the distances from zon and DHL. Similarly, robotaxi services have been intro-
its location to the satellites based on the transmitting time in duced recently in some countries to deal with COVID-19
the messages. Basically, to calculate the current location of a outbreak [143], [149]. It can be clearly seen that these
user, it requires at least three different navigation messages GNSS-based autonomous services can contribute a signifi-
from three different satellites (based on the Trilateration cant part in implementing social distancing in practice by
mechanism in Section II). However, in practice, to achieve minimizing the required human presence for delivery and
high accuracy in calculating the location of a user, at least transportation.
four different messages from four satellites are required (the
fourth one is to address the time synchronization problem 3) KEEPING DISTANCE AND CROWD DETECTION
at the receiver) [133]. Currently, some GNSS systems (e.g., In [142], the authors introduce a GNSS service which can be
Galileo [134]) can achieve an accuracy of less than 1 m. As a used to determine the locations of users, thereby being able to
result, GNSS systems can be considered a very promising warn them if they violate the social distancing requirements.
solution to enable social distancing practice. In particular, in this service, mobile users are required to
install a mobile application which can track the location of
1) REAL-TIME MONITORING the users based on GPS technology. Then, the users’ locations
Due to the outstanding features of GNSS technology in will be updated constantly to the service provider. Thus, based
locating people, especially in outdoor environments, this on the users’ locations, the service provider can determine
technology is very useful for tracking people to practice whether the user violates the social distancing requirements
social distancing. Specifically, most smartphones are cur- or not. For example, if there are more than two users located
rently equipped with GPS devices which can be used to too close to each other (e.g., less than two meters), the service
track locations of mobile users when needed. In the context provider can send warning messages to remind the users.
of a pandemic outbreak, e.g., COVID-19, people suspected Furthermore, in the cases where a user goes to restricted
of being infected, for example, returning from an infected areas, e.g., isolated areas, they will receive warning messages
area, will be required to be self-isolated. Thus, to monitor to be aware of using protection measures.
these people, the authorities can ask them to wear GPS-based
positioning devices to make sure that they do not leave 4) INFECTED MOVEMENT DATA
their residences during the quarantine [146], [147]. The main In the infected movement data scenario, GNSS can be a very
advantage of using GNSS technology compared to Wi-Fi effective technology because of its world-wide coverage and
or Infrared-based solutions for people tracking is that this positioning accuracy is not the main concern. For the outdoor
technology allows to monitor people anywhere and anytime environment, using GNSS alone can be sufficient for tracking
globally, and thus even if the suspects move from one city to the location of infected people. With the omnipresence of
another city, the authorities still can track and monitor them. smartphones with built-in GPS feature, the movement path
However, one of the major disadvantages of this technology of the infected people can be easily determined. However,
is that it depends on the satellite signals. Thus, in some areas the main concern in this scenario is that people have to
with weak or high interference signals (e.g., inside a building turn on GPS service on their smartphones, which necessitate
or in crowded areas), the location accuracy is very low [140], mechanisms to incentivize people to share their movement
[144], [148]. To overcome this limitation, pseudolites have information.
been proposed. Pseudolites are ground-based transceivers Summary: Although this GNSS-based service has many
that can act as an alternative for satellites to transmit GNSS advantages in practicing social distancing, e.g., tracking
signals. These pseudolites can be installed in the areas where users, keeping distance, and group monitoring, it has some
satellite signals are weak to enhance the positioning accuracy shortcomings which limit its applications in practice. Specif-
of the GPS. Nevertheless, pseudolites have not been widely ically, this service requires tracking locations of users based
deployed because of their high price and strict time synchro- on GPS in a real-time manner, which may cause some extra
nization requirement [145]. implementation costs and privacy issues for users. Further-
more, in terms of determining the distance between two
2) AUTOMATION people, the accuracy of GNSS services is not high in general,
Another useful application of GNSS to practice social dis- especially for distances less than two meters. Thus, some
tancing is automation. It comes from the fact that GNSS recent advanced GNSS technologies like [136], [138], [139],

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[145] can be used to improve the accuracy of the GPS. How- obtain Wi-Fi fingerprints by using ZigBee interference sig-
ever, these technologies are still quite expensive and have natures. The key idea of this work is using ZigBee interfaces
not been widely deployed for public services, and thus more to detect Wi-Fi access points which can significantly save
research in this direction should be further explored. Privacy energy compared with using Wi-Fi interfaces. Furthermore,
issues will be discussed in Part II [1] with several solutions a K-nearest neighbor method with the Manhattan distance is
such as location information protection and personal identity introduced to increase the accuracy of the localization system.
protection. The experimental results show that the proposed solution can
save 68% of energy compared with the method using Wi-Fi
F. ZIGBEE interfaces. The accuracy is also improved by 87% compared
Zigbee is also a potential technology that can help to state-of-the-art Wi-Fi fingerprint-based approaches.
to enable social distancing. In particular, Zigbee is a
standard-based wireless communication technology for 2) PUBLIC PLACE MONITORING AND ACCESS SCHEDULING
low-cost and low-power wireless networks such as wire- In a Zigbee system, there is a central hub, known as the
less sensor networks. A Zigbee system consists of a central network coordinator, to control other connected devices in the
hub, e.g., network coordinator, and Zigbee-enabled devices. network. Thus, Zigbee can be used to control the number of
Zigbee-enabled devices can communicate with each other people in indoor environments. Specifically, when a person
at the range of up to 65 feet (20 meters) with an unlimited equipped with a Zigbee-enabled device (e.g., ID card or
number of hops. Compared with Wi-Fi and Bluetooth tech- member card) enters the place, the device will connect to
nologies, Zigbee is designed to be cheaper and simpler, mak- the Zigbee central hub. As such, the central hub is able to
ing it possible for low-cost and low-power communications calculate the total number of people inside the place at a given
for smart devices [159], [160]. Moreover, Zigbee can oper- time. Based on this information, the local manager can ask
ate at several frequencies, such as 2.4 GHz, 868 MHz, and people to queue before entering the place if it is too crowded.
915 MHz. Given the above, Zigbee is ideal for constructing Summary: Zigbee technology can play an important role
mesh networks with long battery life and reliable communica- in enabling social distancing during pandemic outbreaks.
tions [160]. As a result, Zigbee can be considered a promising However, Zigbee is a relatively new technology and has
candidate in several applications that enable social distancing not been widely adopted in our daily life, and thus limit-
during a pandemic outbreak. ing its practical applications. Nevertheless, with the support
from leading companies such as Amazon, Google, Apple,
1) CROWD DETECTION and Texas Instruments [160], the number of Zigbee-enable
One promising application of Zigbee is detecting and tracking devices is expected to explosively increase in the near
users’ location in indoor environments. The key idea is that future. Furthermore, combining Zigbee with other technolo-
based on the RSSI level of the received signals from the user’s gies (e.g., Wi-Fi [163]) is also a promising research direction
Zigbee-enabled device, the Zigbee control hub can determine to improve the performance of localization systems in terms
the location of the user. Several research works report that of accuracy and robustness.
Zigbee localization systems can achieve high accuracy with
low-power and low-cost devices [159]. Based on the location G. RFID
of users, the central hub can detect crowds, i.e., many users in RFID plays a key role in real-time object localizing and
the same area, and notify the local manager to ask people to tracking [150]. An RFID localization system usually consists
practice social distancing during a pandemic outbreak. With of three main components: (i) RFID readers, (ii) RFID tags,
the state-of-the-art mechanisms in the literature, the accu- and (iii) a data processing system [151]. Typically, RFID tags
racy of Zigbee localization systems is significantly improved, can be categorized into two types: (i) active tags and (ii) pas-
making it feasible for social distancing. In [161], the authors sive tags. A passive RFID tag can operate without requiring
propose a novel framework to enhance the localization accu- any power source, and it is powered by the electromagnetic
racy of Zigbee devices by considering the effect of ‘‘drift field generated by the RFID reader. In contrast, an active
phenomenon’’ when users move from one place to another RFID tag has its own power source, e.g., a battery, and
place in indoor environments. The authors then demonstrate continuously broadcasts its own signals. Active RFID tags are
that the proposed framework can increase the accuracy by up usually used in localization systems. Thus, RFID technology
to 60% compared with conventional solutions. can be considered a potential technology to practice social
Differently, in [162], the authors introduce an ensem- distancing.
ble mechanism to further improve the localization accuracy.
In particular, instead of using the RSSI level, the proposed 1) CROWD DETECTION
solution combines the gradient-based search, the linear least One potential application of RFID technology is locating
square approximation, and multidimensional scaling methods users in the indoor environment based on recent RFID-based
together with spatial dependent weights of the environment to localization solutions [150], [154]. To that end, each user
approximate the target’s location. In [163], the authors pro- is equipped with an RFID tag, e.g., the staff ID or member
pose an energy-efficient indoor localization system that can cards. Based on the backscattered signals from the RFID

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tag, the RFID reader can determine the location of the user. tag by modeling the spline structure. Based on the users’ loca-
If there are too many people in the same area, the system tions, Remix can detect crowds in hospitals and advice the
can notify the authorities to take appropriate actions, e.g., authorities to take appropriate actions to practice social dis-
force people to leave the area to practice social distancing. tancing. Note that this solution can also be deployed to detect
Several recent mechanisms in the literature can be adopted crowds in other places such as workplaces, schools, and
to make this application possible during pandemic outbreaks. supermarkets where backscatter tags can be easily attached
In [152], the authors propose an RFID-based localization sys- to users/customers’ cards, e.g., staff cards, student cards, and
tem for indoor environments with high localization granular- member cards.
ity and accuracy. The key idea of this solution is reducing the
RSSI shifts, localization error, and computational complex- 2) PUBLIC PLACE MONITORING AND ACCESS SCHEDULING
ity by using Heron-bilateration estimation and Kalman-filter Another application of RFID in social distancing is monitor-
drift removal. In [153], the authors propose to use a moving ing the number of people inside a place, e.g., a building or
robot to enhance the accuracy of a real-time RFID-based supermarket. In particular, an RFID reader will be deployed
localization system. In particular, the robot is able to perform at the main gate of a place, and users are equipped with RFID
Simultaneous Localization and Mapping (SLAM), and thus tags (can be either active and passive tags). The users’ tags
it can continuously interrogate all RFID tags in its area. can broadcast their ID (active) or send their ID upon receiving
Then, based on passive RFID tags at known locations, we can RF signals from the RFID reader (passive). When a user
estimate the location of target tags by properly manipulating enters the place, the RFID reader can receive the user’s ID
the measured backscattered power. Alternatively, in [150], and increase the counter value. As such, the RFID reader can
the authors propose to equipped two RFID tags at the target calculate the number of people inside the place. If there are
instead of only one as in conventional solutions to improve the too many people, the system can notify the local manager to
accuracy of localization techniques. Adding one more RFID force people to queue before entering the place to practice
tag possesses several advantages: (i) easy to implement and social distancing. This solution can be deployed in supermar-
adjust the RFID reader’s antenna, (ii) enabling fine-grained kets or workplaces where the customers/staff usually have
calculation, and (iii) enabling accurate calibration. The exper- member/staff ID cards which can be equipped with RFID
imental results then show that equipping two tags at the user tags.
can greatly increase the localization accuracy of the system. Summary: RFID technology is a potential solution to
However, the RFID technology has several limitations enable social distancing. However, unlike other wireless tech-
due to the fact that both the receiver and the RF source nologies, RFID technology has not been widely adopted in
are in the RFID reader. Specifically, the modulated sig- practice due to its complexity in implementation. Specifi-
nals backscattered from the RFID tag are strongly affected cally, to be able to detect the location of people by using RFID
by the round-trip path loss from the receiver and the RF technology, they need to be equipped with RFID tags. How-
source. In addition, the RFID system can also be affected ever, RFID tags are not readily available likes Wi-Fi access
by the near-far problem [155]. To address these problems, points or Bluetooth. Thus, applications of RFID technology
a few recent works propose to use bistatic and ambient for social distancing are still limited in practice.
backscattered communication technologies (extended ver- Table 2 summarizes the technologies discussed in this
sion of RFID) for localization [156], [157]. The key idea is Section. Technologies that have a wide communication range
separating the RF source from the receiver. The RF source such as cellular and GNSS are effective solutions for the sce-
now can be a dedicated carrier emitter or an ambient RF narios where it is necessary to track people’s location over a
source. The tag can then transmit data to the receiver by large area (e.g., the infected movement data scenario). On the
backscattering the RF signals generated by the RF source. other hand, technologies with a shorter communication range
Based on the received signals, the receiver can estimate the (e.g., Wi-Fi, Bluetooth, Zigbee, and RFID) are more suitable
location of the tag. In [157], the authors propose a localiza- for scenarios that involve indoor environments such as public
tion system based on backscatter communications to locate place monitoring. Moreover, technologies that can achieve
patients in a hospital. In particular, each patient is equipped a high positioning accuracy (e.g., Ultra-wideband and Blue-
with a backscatter tag which can backscatter signals broad- tooth) can be employed to keep a safe distance between any
cast by an RF source. Then, the location of a patient can be two people, except for GNSS since it requires a high cost
detected by a localization algorithm, namely Remix, based to maintain sufficient accuracy. Furthermore, most of these
on the backscattered signals from the backscatter tags. Remix technologies are ready to be implemented and integrated with
consists of two processes. First, the algorithm approximates existing systems such as smartphones. However, user privacy
the distance from the tag to the receiver based on the backscat- is an open issue for most wireless technologies. Furthermore,
tered signals. Second, the signal paths are modeled with linear other emerging wireless technologies such as LoRaWAN,
splines. Then, an optimization problem is solved to find the Z-Wave, and NFC [158] have not been well investigated in
effective distances corresponding to the paths that are close the literature for positioning systems, and thus they could
to the actual paths from the tag to the receiver. As a result, be potential research directions for social distancing in the
Remix can accurately estimate the position of the backscatter future.

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TABLE 2. Summary of wireless technologies applications to social distancing.

IV. CONCLUSION [4] E. Lempinen. COVID-19: Economic Impact, Human Solutions. Berkeley
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prevent the spread of contagious diseases such as COVID-19. human-solutions/
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on how technologies can enable, encourage, and enforce 19 Struck. CBC News, Apr. 9, 2020. Accessed: Apr. 20, 2020. [Online].
Available: https://www.cbc.ca/news/business/canada-jobs-march-covid-
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[223] F. Tian, B. Liu, X. Sun, X. Zhang, G. Cao, and L. Gui, ‘‘Movement-based NGOC-TAN NGUYEN (Graduate Student
incentive for crowdsourcing,’’ IEEE Trans. Veh. Technol., vol. 66, no. 8, Member, IEEE) received the B.Sc. degree in
pp. 7223–7233, Aug. 2017. mechatronics and the M.Sc. degree in electrical
[224] J.-X. Liu, Y.-D. Ji, W.-F. Lv, and K. Xu, ‘‘Budget-aware dynamic incen- engineering from the University of Engineering
tive mechanism in spatial crowdsourcing,’’ J. Comput. Sci. Technol., and Technology (UET), Vietnam National Univer-
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degree (dual doctoral program) with the University
IEEE Commun. Mag., vol. 57, no. 3, pp. 132–137, Mar. 2019.
of Technology Sydney (UTS) and VNU-UET.
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[227] S. Ankel, Business Insider. (May 10, 2020). Photos Show How also a Lecturer with Thang Long University. His main interests include
the World is Readapting to Socially Distanced Life During the wireless communications, visible light communications, energy harvesting,
Coronavirus Pandemic, From Plastic Table Barriers to Taped- backscatter communications, and the IoT.
Up Urinals. Accessed: Jun. 29, 2020. [Online]. Available:
https://www.businessinsider.com/photos-show-world-adapting-to-
social-distancing-guidelines-in-pandemic-2020-5
[228] Ministry of Housing, Communities & Local Government. (Jun. 26, 2020).
Coronavirus (COVID-19): Safer Public Places—Urban Centres
and Green Spaces. Accessed: Jun. 29, 2020. [Online]. Available: TRAN VIET KHOA received the B.Sc. degree
https://www.gov.uk/guidance/safer-public-places-urban-centres-and- in electronics and telecommunications from the
green-spaces-covid-19 University of Engineering and Technology (UET),
Vietnam National University (VNU), in 2008,
and the M.Sc. degree from Paris-Sud 11, France,
in 2010. He is currently pursuing the Ph.D.
CONG T. NGUYEN received the B.E. degree
degree with the Joint Technology and Innovation
in electrical engineering and information from
Research Center between VNU and the University
the Frankfurt University of Applied Sciences,
of Technology Sydney (UTS). He was a Network
in 2014, and the M.Sc. degree in global production
Engineer at Viettel Network Corporation, from
engineering and management from the Technical
2012 to 2018. His research interests include the IoT, deep learning, and
University of Berlin, in 2016. He is currently pur-
cyberattack detection.
suing the Ph.D. degree with the UTS-HCMUT
Joint Technology and Innovation Research Centre
between the Ho Chi Minh City University of Tech-
nology and the University of Technology Sydney
(UTS). His research interests include operations research, blockchain tech-
nology, game theory, and optimizations. BUI MINH TUAN received the B.Sc. and M.Sc.
degrees from Le Quy Don Technical University,
Hanoi, Vietnam, in 2008 and 2013, respectively.
YURIS MULYA SAPUTRA (Graduate Student He is currently pursuing the Ph.D. degree with
Member, IEEE) received the B.E. degree in the VNU-UTS Joint Technology and Innovation
telecommunication engineering from the Institut Research Center between Vietnam National Uni-
Teknologi Bandung (ITB), Indonesia, in 2010, and versity and the University of Technology Sydney.
the M.Sc. degree in electrical and information He was a Researcher at The Military Institute of
engineering from the Seoul National University Science and Technology. His major is electron-
of Science and Technology (SeoulTech), South ics and communications. His research interests
Korea, in 2014. He is currently pursuing the Ph.D. include the IoT, physical layer security, and computer vision.
degree with the University of Technology Sydney
(UTS), Australia, and a full-time Lecturer with
Universitas Gadjah Mada (UGM), Indonesia. He was a Researcher with the
Intelligent Systems Research Group, SeoulTech, from 2014 to 2015, and
an Application Software Developer with the Digital Appliance Division,
Samsung Electronics, Indonesia, from 2010 to 2012. His research interests DIEP N. NGUYEN (Senior Member, IEEE)
include mobile computing, energy efficiency, and optimization problems for received the M.E. degree in electrical and com-
wireless communication networks. puter engineering from the University of Califor-
nia at San Diego (UCSD) and the Ph.D. degree
in electrical and computer engineering from The
University of Arizona (UA). He was a DECRA
NGUYEN VAN HUYNH (Graduate Student Research Fellow with Macquarie University and
Member, IEEE) received the B.E. degree in elec- a Member of Technical Staff with Broadcom,
tronics and telecommunications engineering from CA, USA, ARCON Corporation, Boston, and con-
the Hanoi University of Science and Technol- sulting the Federal Administration of Aviation,
ogy (HUST), Vietnam, in 2016. He is currently on turning detection of UAVs and aircraft, and the U.S. Air Force Research
pursuing the Ph.D. degree with the University Laboratory, on antijamming. He is currently a Faculty Member with the Fac-
of Technology Sydney (UTS), Australia. From ulty of Engineering and Information Technology, University of Technology
2017 to 2018, he was a Researcher with Nanyang Sydney (UTS). His recent research interests include computer networking,
Technological University (NTU), Singapore. His wireless communications, and machine learning application, with an empha-
research interests include wireless-powered com- sis on systems’ performance and security/privacy. He received several awards
munications, green communications, and applications of machine learning from LG Electronics, UCSD, The University of Arizona, the U.S. National
in wireless communications. Science Foundation, and the Australian Research Council.

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DINH THAI HOANG (Member, IEEE) received SYMEON CHATZINOTAS (Senior Member,
the Ph.D. degree in computer science and engi- IEEE) received the M.Eng. degree in telecommu-
neering from Nanyang Technological University, nications from the Aristotle University of Thes-
Singapore, in 2016. He is currently a Faculty saloniki, Thessaloniki, Greece, in 2003, and the
Member at the School of Electrical and Data M.Sc. and Ph.D. degrees in electronic engineer-
Engineering, University of Technology Sydney, ing from the University of Surrey, Surrey, U.K.,
Australia. His research interests include emerging in 2006 and 2009, respectively. He has been a
topics in wireless communications and network- Visiting Professor at the University of Parma,
ing, such as ambient backscatter communications, Italy. He was involved in numerous research and
vehicular communications, cybersecurity, the IoT, development projects for the National Center for
and 5G networks. He is an Exemplary Reviewer of the IEEE TRANSACTIONS Scientific Research Demokritos, the Center of Research and Technology
ON COMMUNICATIONS, in 2018, and the IEEE TRANSACTIONS ON WIRELESS Hellas, and the Center of Communication Systems Research, University of
COMMUNICATIONS, in 2017 and 2018. He is currently an Editor of the IEEE Surrey. He is currently a Full Professor/Chief Scientist I and the Co-Head of
WIRELESS COMMUNICATIONS LETTERS and the IEEE TRANSACTIONS ON COGNITIVE the SIGCOM Research Group at SnT, University of Luxembourg. He has
COMMUNICATIONS AND NETWORKING. (co-)authored more than 400 technical articles in refereed international
journals, conferences, and scientific books. He was a co-recipient of the
2014 IEEE Distinguished Contributions to Satellite Communications Award,
the CROWNCOM 2015 Best Paper Award, and the 2018 EURASIC JWCN
Best Paper Award. He is currently in the Editorial Board of the IEEE
THANG X. VU (Member, IEEE) was born in Hai
OPEN JOURNAL OF VEHICULAR TECHNOLOGY and the INTERNATIONAL JOURNAL OF
Duong, Vietnam. He received the B.S. and M.Sc.
SATELLITE COMMUNICATIONS AND NETWORKING.
degrees in electronics and telecommunications
engineering from the VNU University of Engi-
neering and Technology, Vietnam, in 2007 and
2009, respectively, and the Ph.D. degree in electri-
cal engineering from University Paris-Sud, France,
BJÖRN OTTERSTEN (Fellow, IEEE) was born
in 2014.
in Stockholm, Sweden, in 1961. He received the
In 2010, he received the Allocation de
M.S. degree in electrical engineering and applied
Recherche Fellowship to study Ph.D. degree in
physics from Linköping University, Linköping,
France. From September 2010 to May 2014, he was with the Laboratory of
Sweden, in 1986, and the Ph.D. degree in electrical
Signals and Systems (LSS), a joint laboratory of CNRS, CentraleSupelec,
engineering from Stanford University, Stanford,
and University Paris-Sud XI, France. From July 2014 to January 2016,
CA, USA, in 1990.
he was a Postdoctoral Researcher with the Information Systems Technology
He has held research positions with the Depart-
and Design (ISTD) Pillar, Singapore University of Technology and Design
ment of Electrical Engineering, Linköping Univer-
(SUTD), Singapore. He is currently a Research Associate at the Interdis-
sity, the Information Systems Laboratory, Stanford
ciplinary Centre for Security, Reliability and Trust (SnT), University of
University, the Katholieke Universiteit Leuven, Leuven, Belgium, and the
Luxembourg. His research interests include wireless communications, with
University of Luxembourg, Luxembourg. From 1996 to 1997, he was the
particular interests of wireless edge caching, cloud radio access networks,
Director of Research with ArrayComm, Inc., a start-up in San Jose, CA,
machine learning for communications, and cross-layer resources optimiza-
USA, based on his patented technology. In 1991, he was appointed as a
tion. He was a recipient of the SigTelCom 2019 Best Paper Award.
Professor of signal processing with the Royal Institute of Technology (KTH),
Stockholm. From 1992 to 2004, he was the Head of the Department for Sig-
nals, Sensors, and Systems, KTH, and from 2004 to 2008, he was the Dean
of the School of Electrical Engineering, KTH. He is currently the Director
ERYK DUTKIEWICZ (Senior Member, IEEE) for the Interdisciplinary Centre for Security, Reliability and Trust, University
received the B.E. degree in electrical and elec- of Luxembourg. He is a Fellow of the EURASIP. He was a recipient of the
tronic engineering and the M.Sc. degree in applied IEEE Signal Processing Society Technical Achievement Award, in 2011, and
mathematics from The University of Adelaide, the European Research Council advanced research grant twice, from 2009 to
in 1988 and 1992, respectively, and the Ph.D. 2013 and from 2017 to 2022. He has coauthored journal articles that received
degree in telecommunications from the University the IEEE Signal Processing Society Best Paper Award, in 1993, 2001, 2006,
of Wollongong, in 1996. His industry experience and 2013, and the seven IEEE conference best paper awards. He has served as
includes the management of the Wireless Research an Associate Editor for the IEEE TRANSACTIONS ON SIGNAL PROCESSING and in
Laboratory, Motorola, in 2000. He is currently the Editorial Board of the IEEE Signal Processing Magazine. He is currently
the Head of the School of Electrical and Data a member of the editorial boards of the EURASIP Signal Processing Journal,
Engineering, University of Technology Sydney, Australia. He also holds a the EURASIP Journal of Advances Signal Processing, and Foundations and
professorial appointment at Hokkaido University, Japan. His current research Trends of Signal Processing.
interests include 5G/6G and the Internet-of-Things networks.

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