This article has been accepted for publication in a future issue of this journal, but has not been

fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JSTSP.2016.2520901, IEEE Journal
of Selected Topics in Signal Processing
1

Feasibility of Mobile Cellular Communications

at Millimeter Wave Frequency
Yungsoo Kim, Hyun-Yong Lee, Member, IEEE, Philyeong Hwang, Ranjeet Kumar Patro, Member,
IEEE, Jaekon Lee, Wonil Roh, and Kyungwhoon Cheun, Senior Member, IEEE

 advanced Multi-Input and Multi-Output (MIMO) or multi

Abstract— High data rate at high mobile speed will still be an antenna techniques, Carrier Aggregation (CA) technique to
essential requirement for the future 5G mobile cellular system. provide wider bandwidth, and Heterogeneous Network (HetNet)
High frequency bands above 6 GHz are particularly promising for technique of deploying small cells in addition to the macro cells
the 5G system because of large signal bandwidths such high
at the same or different carrier frequencies [3]-[4].
frequencies can offer. By using high gain beamforming antennas,
the problem of high propagation loss at high frequencies can be Recently, such efforts have been extended to considering
overcome. However, the use of beamforming antennas at such very high frequency bands, such as the millimeter wave
high frequencies requires a significant change in the design of a (mmWave) frequency bands between 30 and 300 GHz, for
cellular system. In particular, it requires a significant change in future wireless communications because of a very large amount
key functions such as cell search, random access, measurement of of spectrum such high frequencies can offer [5]-[27]. The high
beams for fast beam adaptation, and various physical control and
frequency band with a large amount of spectrum is ideally
data channels. In this paper, we propose a new radio frame
structure for the future mobile cellular communications system at suited for use in small cells to complement macro cells in a
millimeter wave frequency that addresses such challenges. A HetNet environment and in the wireless backhaul [13]-[21].
testbed was built at Samsung Electronics, Korea, based on the Also, there has been a growing interest in using the high
proposed frame structure at 28 GHz with bandwidth of 800 MHz. frequency band for mobile cellular communications [5]-[12].
It attained the downlink (DL) data rate of 7.5 Gbps by delivering One of major challenges in using the mmWave frequency
four streams of 64 QAM data with code rate of 3/4 to two mobile
band for cellular communications is that it suffers from a severe
stations (MSs) located in a close distance to the base station
antennas at fixed positions. It also achieved the DL data rate of 1.2 propagation loss compared with the lower frequencies used for
Gbps by delivering single stream of 16 QAM data with code rate current cellular systems. However, because of the short
of 3/4 to an MS moving at 110 km/h in a single cell of up to 800 m wavelength of the mmWave frequency, it is possible to build an
in a line-of-sight environment. Finally, it implemented handover antenna array with a large number of antennas in a given
and achieved an average handover interruption time of 21 ms in a physical size, and to form a highly directional beam pattern
three-cell environment, and demonstrated feasibility of mobile
with a very large antenna gain. By having such high gain
cellular communications at millimeter wave frequency.
antennas at both transmitter and receiver sides, and steering
Index Terms—5G, millimeter wave communications, testbed. their beams to their best communication path, the loss due to
the high frequency can be almost completely compensated
[5]-[12]. The performance of a beamforming system can be
I. INTRODUCTION further improved if multiple beams formed using multiple
antenna arrays can deliver multiple streams of data to support
R ECENTproliferation of smart devices such as smart phones
and tablet PCs has increased demand for data traffic in
wireless communications [1]-[2]. In order to meet such a
either multi-user (MU) MIMO or single-user (SU) MIMO [22].
In particular, the hybrid digital precoding and analog
growing demand for wireless data traffic, various advanced beamforming technique promises a performance approaching
wireless technologies have been developed that would greatly that of the full digital beamforming technique with a much
increase the data rate and areal capacity of the wireless lower hardware complexity [23]-[25].
communications system. Among the most notable ones are The mmWave beamforming technique is well suited to
communications in a stationary environment in a point-to-point
manner, and is already used in the latest air interface for the
Copyright (c) 2014 IEEE. Personal use of this material is permitted.
However, permission to use this material for any other purposes must be wireless personal area network (WPAN) [26] and the wireless
obtained from the IEEE by sending a request to pubs-permissions@ieee.org. local area network (WLAN) [27]. However, to enable mobile
This work was presented in part at the IEEE Global Communications cellular communications based on the mmWave beamforming
Conference (GLOBECOM), San Diego, CA, USA, December 6-10, 2015.
Y. Kim, H. Y. Lee, P. Hwang, J. Lee, W. Roh and K. Cheun are with technique, there are many challenges and research areas that
Communications Research Team, Mobile Communications Business, Samsung need to be addressed [10]-[12]. The current cellular system is
Electronics Co., Ltd., Suwon, Korea (e-mail: y2.kim@samsung.com; designed based on the base station (BS) and the mobile station
hy526.lee@samsung.com; pyhwang@samsung.com; today@samsung.com;
wonil.roh@samsung.com; kw.cheun@samsung.com).
(MS) with sector wide antennas and omni-directional antennas,
R. K. Patro is with Samsung R&D Institute - Bangalore, Samsung respectively, at frequencies below 6 GHz [29], so its
Electronics Co., Ltd., Karnataka, India (e-mail: rkp.atd@samsung.com).

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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JSTSP.2016.2520901, IEEE Journal
of Selected Topics in Signal Processing
2

specification cannot be directly used [11]-[12]. The new frame

structure should be designed considering various characteristics
of the radio channel and devices at mmWave frequencies
[9]-[12]. Also, it must provide all the basic functions needed for
mobile communications [30] and meet the requirements such as
[28], [34] using the mmWave beamforming technique. Some of
the important functions that need to be addressed are cell search,
random access, measurement of multi beams/cells, and
physical channels and signals. Various frame structures have
been proposed until now [5], [13], [14], [17], but they are at
very early stages of development and do not include any details
on how the various functions for mobile cellular
communications can be supported.
In this paper, we propose a new radio frame structure that Fig. 1. Beam patterns for antenna array with 8 elements and 0.65 spacing.
addresses such challenges. The proposed frame structure
defines signal structures and beam operations for all the key where AE() denotes the radiation pattern of the individual
functions mentioned above for mobile cellular communications. array element at angle , NA is the number of the antenna
The proposed frame structure is flexible and scalable to support elements of the array, An is the complex amplitude of the n-th
various numbers of beams/antennas, users, or traffic conditions. element, d is the inter-element spacing,  is the wavelength of
It can support various multi-antenna techniques of the 3GPP the signal, k is the beam pointing direction, and  is the angle
LTE [31] using multiple antenna arrays. A testbed based on the under consideration. The angle  is defined with respect to bore
frame structure was built at Samsung Electronics, Korea, to test sight direction of the antenna pattern radiation.
feasibility of mobile cellular communications at the millimeter The direction of the beam can be adjusted continuously or in
wave frequency. The testbed was built at 28 GHz with the small discrete steps. Fig. 1 illustrates an example of beam
bandwidth of 800 MHz and the time division duplex (TDD) patterns generated by a linear array of 8 antenna elements with
mode. The testbed achieved 7.5 Gbps by transmitting four data 0.65 inter-element spacing and a constant amplitude An, such
streams to two MSs with four BS antenna arrays, in which 2x2 that the direction of each beam is spaced at 10 o apart from its
MIMO is supported for each MS. In an adaptive beamforming adjacent beam directions and the patterns of two adjacent
test in a single cell environment, the testbed achieved 1.2 Gbps beams meet with one another at approximately 2 to 3 dB from
by transmitting single data stream to an MS moving at 110 their peaks. Fig. 2 illustrates a transmitter and receiver structure
km/h with the BS – MS distance up to 800 m. Finally, the using multiple antenna arrays (AA). In Fig. 2, the transmitter
testbed performed handover tests in a three-cell environment, transmits Mt independent signal streams by using a baseband
and attained an average handover interruption time of 21 ms, unit with Mt digital-to-analog converter (DAC) units, Mt RF
satisfying the IMT-Advanced requirement of 27.5 ms [28]. units (RF1 ~ RFMt), Mt antenna arrays (AA1 ~ AAMt), and Mt
In Section II, a principle of beamforming for an antenna transmit beams (B1 ~ BMt). The phase shifter values of each RF
array is briefly described, and a requirement for beam unit determine the direction of the beam generated at an
adaptation to support high speed mobile communications in antenna array, independent of other antenna arrays. Likewise,
line-of-sight (LOS) environment is provided. In Section III, a the receiver generates Mr receive beams (B1 ~ BMr), and
radio frame structure is proposed for a future mobile cellular receives Mr independent signal streams by using the baseband
communications system with mmWave beamforming antennas. unit with Mr analog-to-digital converter (ADC) units, Mr RF
In Section IV, handover algorithms used in the testbed are units (RF1 ~ RFMr) and Mr antenna arrays (AA1 ~ AAMr).
described. In Section V, the testbed that was built at Samsung One important consideration to make for designing a mobile
Electronics is described, and in Section VI, the test results are communications system based on analog beamforming is how
provided. Finally, the conclusion of the paper is provided in quickly the beam should be adapted in response to changing
Section VII. TRANSMITTER RF1 AA1
...
P/S1

IFFT 1 DAC 1
... B1
...

...
Transmitter

...

...

...
...
...

II. ADAPTIVE BEAMFORMING REQUIREMENT fc ...

MIMO

...
...

... P/SMt

...

RFMt AAMt
An array of two or more antenna elements can yield a ...

IFFT Mt DAC Mt
... BMt
directional radiation pattern. The width and the direction of the
...

...

...

...

...
...
...

...
fc
radiation pattern, or the beam, depend upon the geometry of the
RECEIVER RF1 AA1
array pattern, the amplitude and phase of the signal transmitted ...
S/P1

...
by or received from each element of the array. The far-field FFT1 ADC 1
...

...

B1
...

...

...
...
...
Receiver

...
radiation pattern of an array antenna at an angle  for a beam
fc
MIMO

...
...

... S/PMr

...

RFMr AAMr
pointing towards k can be represented as [35] ...
...
N A 1 FFTMr ADC Mr
...

...

BMr
...

...

...
...
...

Pk ( )  AE ( )  An exp[( j 2d /  )n(sin   sin k )] ,

...
(1) fc

n0
Fig. 2. Transmitter and receiver with beamforming antennas.

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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JSTSP.2016.2520901, IEEE Journal
of Selected Topics in Signal Processing
3

TABLE I One radio frame, Tframe = 6144000Ts = 5 ms

TRAVELLING DISTANCE AND TIME FOR 1O MOVEMENT BY MS
MS – BS Distances [m] 0 1 2 3 4
1o Travel
30 50 100 150 200 250 300 One subframe, Tsubframe = 1228800Ts = 1 ms
Distance [m] 0.52 0.87 1.75 2.62 3.49 4.36 5.24
0 1 2 3 ... 39
Time [ms] 15.7 26.2 52.4 78.5 104.7 130.9 157.1
One slot, Tslot = 30720Ts = 25 s
wireless channel conditions. For example, when the BS has an
antenna array with the beam pattern of Fig. 1, and the MS
0 1 2 3 4 5
travels in LOS environment, the BS should use beam 4 when 4.17 s 0.83 s 3.33 s
the MS is located between -10o and 0o, and beam 5 between 0o Fig. 3. Frame/Subframe/Slot structures.
and 10o. If, however, the BS is late by about 1.1o in updating its TABLE II
beam, a loss of about 3.0 dB will incur with respect to the best PARAMETERS FOR FRAME STRUCTURE
Parameters Values
beam performance for this beam pattern. The loss will increase
Sampling Rate, (1/Ts) 1.2288 GHz
to 6.0 dB for delay of about 2.2o. For an array with different
Baseline FFT/IFFT Size 4096, 3.33 s
number of elements, the values will be different. However, it
Baseline Cyclic Prefix Size 1024, 833 ns
would be helpful to gain an insight into how quickly the beam
Subcarrier Spacing 300 kHz
should be adapted to support mobile communications. Table I Signal Bandwidth 800 MHz
shows the travelling distances and the times for an MS to move Number of Used Subcarriers 2640
by 1o with respect to the BS when the MS travels at 120 km/h at
various BS-MS distances. A simpler number 1o was chosen clocks with reasonable accuracy are readily available at low
instead of 1.1o to provide a rule of thumb. The travelling cost. Each slot consists of various numbers of signal blocks or
distance and the time are 5.24 m and 157.1 ms, respectively, at orthogonal frequency division multiplexing (OFDM) symbols,
the BS-MS distance of 300 m, while they reduce to 0.52 m and but the baseline structure consists of six OFDM symbols of
15.7 ms, respectively, at 30 m. Therefore, roughly speaking, we length 4.17 s, in which the inverse Fast Fourier transform
can say that the beam adaptation time should be about 15.7 ms (IFFT) block length is 3.33 s and the cyclic prefix (CP) length
or less to support reliable communications with user mobility is 833 ns. According to the measurement taken using narrow
up to 120 km/h in LOS environment. beam antennas at 28 GHz in New York City [36], the maximum
There are several factors that determine how quickly the root-mean-square (RMS) delay spread of power delay profiles
beam can be adapted or switched in the beamforming system. (PDPs) for LOS environment was 153.6 ns, while maximum
Beam adaptation requires measurement of beams at the excess delay (MED) of PDPs 20 dB down from the maximum
receiver side, and feeding back the measurement information to peak had the maximum value of 405.7 ns. Therefore, the CP
the transmitter. Assuming that the feedback delay is very short, size of 833 ns gives sufficient margin in accommodating the
the beam measurement time determines the beam adaptation delay spread in LOS environment and the guard time for beam
speed, and should be short enough to support the user mobility. switching. With the FFT size of 4096 for the baseline OFDM
Therefore, referring back to Table I, for example, the beam symbol, the subcarrier spacing is 300 kHz. The subcarrier
measurement period should be as small as ten to twenty spacing is much smaller than the coherence bandwidth of the
milliseconds to support mobile communications in LOS LOS channel. On the other hand, larger subcarrier spacing is
environment. On the other hand, beam switching involved a preferred when considering the impairments such as frequency
series of interactions by various components in the testbed. At offset, phase noise, and Doppler shift. The subcarrier spacing of
first, the phase shifters in the RF unit were pre-loaded with the 300 kHz is larger than, for example, the maximum clock drift of
next beam values in advance, and then beam switching was a clock with 10 ppm accuracy, which is 280 kHz, and helps
triggered by the modem. The time taken to pre-load the phase simplify implementation of synchronization and acquisition.
shifters was about several microseconds in the testbed. Then, The Doppler shift for the mobility of 120 km/h at 28 GHz is
when the beam switch was triggered, the beam switch started about 3.1 kHz, and the coherence time is about 320 s. The slot
after some delay and then completed after a settling time. The size of 25 s is much less than the coherence time, so that static
total time taken to complete the beam switch after triggering channel assumption holds within a slot.
was several hundred nanoseconds in the testbed. As a result, a Each frame contains a time period in which synchronization
guard time of several hundred nanoseconds is needed for beam signal and broadcast control information are transmitted for cell
switching in the frame structure. search and acquisition by the MS, and the subframe represents
the scheduling interval for the data and control. The subframe
III. FRAME STRUCTURE length of 1 ms is the same as that of the 3GPP LTE [31], but can
Fig. 3 and Table II illustrate the proposed frame structure be changed, if necessary, to better meet the requirements of the
with important parameters, and Fig. 4 shows an example frame future mobile communications [34]. Each slot is dedicated to a
structure implemented in the testbed. The time durations of a specific task or function for mobile cellular communications,
radio frame, a subframe, and a slot are chosen as 5 ms, 1 ms, and the signal structure and beam operations are designed best
and 25 s, respectively. The sampling rate (1/Ts) is chosen as suited to the function. The slot types defined are the downlink
40 times of 30.72 MHz, the clock rate of 3GPP LTE, for which (DL)/uplink (UL) Control Slot, the DL Synchronization Signal

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of Selected Topics in Signal Processing
4

Frame beams and (n % NB_MS) denotes the remainder after dividing n

Subframe[0] Subframe[1] ~ Subframe[4]
DL UL DL UL by NB_MS. During the SS/BCH Slot(s) of even numbered frames,
2n, n=0, 1, 2, …, the MS uses the best beam for its serving cell
0 1 2 3 4 5 ... 32 33 34 35 36 37 38 39 0 1 2 3 4 5 ... 32 33 34 35 36 37 38 39
and synchronizes to the cell. Also, during the Control or Data
C C S S D D ... D D G C R R R G C C B B D D ... D D G C C D B G Slots, the MS uses the best beam for its serving cell for
DL/UL Control DL SS/BCH UL RACH DL/UL BM DL/UL Data Guard communication with the serving cell. Since the MS changes its
Slots (C) Slots (S) Slots (R) Slots (B) Slots (D) Time (G)
beam only during the SS/BCH Slot(s) of odd numbered frames,
communication with the serving cell is not affected by the cell
Fig. 4. Frame structure implemented in the testbed.
search operation. Also, for example, for an MS with eight
and Broadcast Channel (SS/BCH) Slot, the UL Random Access
beams, cell search using all the MS beams takes 80 ms for the
Channel (RACH) Slot, the DL/UL Beam Measurement (BM)
above frame structure, which is fast enough for identifying
Slot, and the DL/UL Data Slot.
neighbor cells to support mobile cellular communications.
The number of the SS/BCH Slots, the RACH Slots, or the
Therefore, the proposed cell search method is well suited to
BM Slots in a subframe/frame can be optimally selected and
mobile cellular communications. Various cell discovery
changed considering various aspects of beamforming system,
methods have been proposed in the past [37]-[39], but they are
such as the number of BS or MS beams, the number of BS
best suited to WLAN or WPAN, and not suitable for mobile
antenna arrays, the cell size, and the mobile speed to support.
cellular communications. However, the proposed cell search
The number of the Control Slots and the Data Slots can be
method assumes that the radio network is synchronized, and
adapted to changing traffic conditions at each subframe as in
additional steps are needed when the network is not ideally
LTE [31]. However, the signal structure of each slot type is
synchronized. For example, if the signal from a neighbor cell
unaffected by the adaptation. This greatly simplifies the
arrives at the MS late by about 8.3 s with respect to that from
implementation of the modem.
the serving cell, the last symbol pair in the SS/BCH Slot(s)
The frame structure shown in Fig. 4 is an example frame
from the neighbor cell is not received in the cell search
structure that was implemented in the testbed. In each subframe,
operation by the MS. Then the MS may fail to detect the
slots from 0 to 33 were assigned as the DL, slots from 35 to 38
neighbor cell if the beam for the last pair was the only one that
as the UL, and slots 34 and 39 as the guard times for DL-UL
could have resulted in successful cell detection. One solution is
switching. Slots 0 and 1 were assigned as the DL Control Slots,
that the BS reverses the order of the beams in the SS/BCH
and slots from 4 to 33 as the DL Data Slots. Slots 2 and 3 were
Slot(s) at different cell search periods, so that the MS can detect
assigned as the DL SS/BCH Slots in subframe 0, and as the DL
the neighbor cell at least once in two cell-search periods. This
BM Slots in subframes from 1 to 4. In subframe 0, slot 35 was
in effect doubles the cell search period, which is now 160 ms
assigned as the UL Control Slot, and slots from 36 to 38 as the
for the MS with 8 beams.
UL RACH Slots. In subframes from 1 to 4, slots 35 and 36 were
The Beam Measurement (BM) Slot is dedicated to the
assigned as the UL Control Slots, slot 37 as the UL Data Slot,
measurement of transmit and receive beams in the DL and the
and slot 38 as the UL BM Slot.
UL, and to the measurement for handover. In a BM Slot, the
The Synchronization Signal and Broadcast Channel
transmitter switches its beam at the start of the slot, and
(SS/BCH) Slot is dedicated to cell search and synchronization
transmits the entire slot with the same beam, whereas the
operation of the beamforming system. In the SS/BCH Slot, two
receiver receives the signal while switching its beam as many
OFDM symbols are transmitted using a same beam as a pair in
times as possible in the slot. More than one BM Slot can be
which one of the symbols is dedicated to the SS and the other to
transmitted using the same transmit beam. In the BM Slot(s), a
the BCH, and then the next two pairs are transmitted using
sequence of an identical IFFT signal block without the CP is
different beams. More than one SS/BCH Slot can be allocated
repeated. The IFFT size of 1024, 833 ns was used for the BM
in a frame, but the position of the first SS/BCH Slot is assumed
signal in the testbed. The receiver switches the receive beam
to be fixed and its overhead should be minimized if possible.
with a time period larger than the IFFT period to accommodate
The BCH contains information about the position of each pair
a guard time for beam switching. The BM signal for each
from the start of the SS/BCH Slot(s), and the MS detects only
antenna array and cell is assigned with unique set of subcarriers
the best pair to acquire synchronization and the broadcast
that do not overlap with those of other antennas/cells to prevent
control information of the cell. Detection of neighbor cells as
interference. When the signals from neighbor cells are not
well as synchronization to the serving cell is supported for the
ideally frame synchronized to the serving cell, interference can
MS with beamforming antennas. Since the neighbor BSs can be
occur either at the start or at the end of the BM Slot(s).
located in any directions from the MS, the MS has to switch the
Therefore, parts of the slot(s) at the start and the end are not
receive beam for all directions to search the neighbor BSs.
used in the measurement by the receiver as guard times. As a
However, when the MS switches its beam to a direction other
result, interference free measurement is supported in multi-cell
than the best direction to its serving BS, its communication with
environments. Also, beam measurement of neighbor cells as
the serving BS will be hindered. One solution to this problem is
well as of the serving cell is supported without interrupting
that during the SS/BCH Slot(s) of odd numbered frames, 2n+1,
communication with the serving cell. The beam measurement is
n=0, 1, 2, …, the MS changes its beam and detects neighbor
supported in the UL as well as in the DL. However, since the
cells with beam (n % NB_MS), where NB_MS is the number of MS
number of subcarriers used for beam measurement is limited,

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5

the same set of subcarriers used in a cell must be reused in other

cells in the cellular network, creating interference. So a careful MS
UE Source BS Target BS Gateway
cell planning is needed to minimize the interference. The best 1. Measurement Configuration
(RRCConnectionReconfiguration)
transmit - receive beam pair for a BS and an MS can be Packet data Packet data
determined as the one for which the received power of the BM Measurement
2. Measurement Report
signal is the highest. The serving cell of an MS can be 3. HO Decision
4. Handover Request
determined as the one for which the received power of the BM 4. Handover Request
signal for the best beam pair is the highest. Therefore, the 5. Admission Control
received power of the BM signal for the best beam pair of a cell 6. Handover Response
7. Handover Command 6. Handover Response
can be used as a measure of handover in place of the reference 8. SN Status Transfer 8. Data Transfer
signal received power (RSRP) used in the 3GPP LTE [32]. For

HO Interruption Time
8. Data Transfer
Detach from serving cell and
the above frame structure of Fig. 4, the DL beam measurement synchronize to new cell

HO Delay
period is 10 ms and 20 ms when the number of BS beams is 8 9. RACH procedure

and 16, respectively. When even more beams are used at the BS, 10. UL allocation + UL Timing Adjustment
11. Handover Complete
the beam measurement period will increase, or more slots must
Packet data Packet data
be allocated as the BM Slot.
The random access channel (RACH) Slot is dedicated to the Fig. 5. BS initiated HO procedure.
UL RACH operation and the UL synchronization. In the RACH This means that hundreds of multiple access users can be
Slot, the MS transmits the RACH signal with a fixed beam, and supported in each subframe. Various multi antenna techniques
the BS receives the signal with the beam switched to more than of the 3GPP LTE [31] can be supported in the frame structure
one direction. The MS may choose candidate transmit beam(s) when multiple antenna arrays are used [22]-[25]. In particular,
for the RACH Slot from the DL beam measurement. An IFFT digital precoding in addition to analog beamforming is possible.
signal block without the CP is repeatedly transmitted during the Reference signal structure corresponding to each antenna array
time period allocated as the RACH Slot, and the IFFT size of or signal layer for the multi antenna techniques can be defined
8192, 6.66 s was used in the testbed to support the RACH for the Data Slot as in the 3GPP LTE [31].
coverage of up to 1 km. The BS has to switch the receive beam
to receive the RACH signal from more than one direction. This IV. HANDOVER ALGORITHM
is necessary because the RACH Slot can be transmitted from In order to better support user mobility in multi-cell
any location within the cell area, but the best BS beams to environment, we considered two types of handover (HO)
receive the signal are usually unknown to the BS at that time. procedures: the BS initiated HO and the MS initiated HO.
The received signal power can change quite dramatically as the
receive beam is switched. As a result, automatic gain control A. BS Initiated HO
(AGC) was implemented at each beam switching in the testbed. The BS initiated HO procedure is based on and modified
The Control Slot is dedicated to short bursts of information from the 3GPP Intra-LTE HO procedure [33] and is shown in
such as the physical control channel in the DL and the UL. In Fig. 5. In the BS initiated HO, the MS sends measurement
the Control Slot, the BS can switch its beam at the start of each reports to the source BS, which is the serving cell before HO,
OFDM symbol to the direction of the MS to transmit or receive and the source BS makes HO decision based on the reports
the signal. Each OFDM symbol has reference signals at from the MS and sends HO request message to the target BS via
predefined subcarrier positions, and channel estimation at the the gateway. The target BS performs admission control and
receiver is done at each symbol. Transmission of broadcast sends HO response message to the source BS via the gateway.
information can be supported by repeating transmission of the If the HO response is affirmative, the source BS issues HO
same broadcast information with different beams. command to the MS which then performs random access
The Data Slot is dedicated to a large burst of data such as the procedure with the target BS and establishes UL
user data in the DL and the UL. In the Data Slot, the BS beam synchronization. When the UL synchronization is successfully
can be switched only at the start of the slot and is fixed within established, the MS sends HO complete message to the target
the slot, so the radio channel is static in the Data Slot, and the BS. The target BS sends MS context release message to the
resource block (RB) and reference signals similar to those source BS via the gateway, confirming a successful handover
defined in the 3GPP LTE [31] can be designed. By scheduling and enabling the source BS resources to be released.
the Data Slots to MSs, time division multiplexing/multiple
B. MS Initiated HO
access (TDM/TDMA) is supported, which means at least 10
users can be supported by TDM/TDMA in a subframe. Also, by In the MS initiated HO, the source BS configures the HO
scheduling RBs of a Data Slot, frequency division multiplexing event conditions, but the MS makes HO decision and selects a
target BS, as illustrated in Fig. 6. Some of the previous works
/multiple access (FDM/FDMA) can be supported for the MSs
on the MS initiated HO procedure can be found in [40]-[41].
for which the best BS beams are the same. For example, when
When the HO decision is made, the MS goes to the
the signal bandwidth is 800 MHz with 2640 subcarriers, if the non-connected state with the source BS, and starts the HO
RB size is 60 subcarriers in the frequency domain, then up to 44 RACH procedure with the target BS. A group of RACH
(=2640/60) FDM/FDMA users can be supported in a Data Slot.

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6

MS
UE Source BS Target BS Gateway
1. Measurement Configuration
Packet data Packet data
Measurement

2. HO Decision (HO_d)
Detach from serving cell
and synchronize to new cell

3. RACH procedure Fig. 7. BS RF transceiver and antenna.

HO Delay

HO RACH (MSG1)
HO Interruption Time

MSG2
MSG3
4. MS-Reg-Request
5. MS-Reg-Response
MSG4

6. MS-Connection-Release
7. RLC-State-Notify-Request
8. NotifyGW-Request
9. NotifyGW-Response
10. RLC-State-Notify-ACK (HO Complete)
Packet data Packet data

Fig. 6. MS initiated HO procedure. Fig. 8. MS RF transceiver and antenna.

The antenna array of Fig. 7 for the BS side consisted of 48
sequences are reserved for the HO RACH procedure. The
antenna elements; 8 horizontal elements by 6 vertical elements.
source BS will also detect the HO RACH from the MS and
change the data state of the MS to IDLE. Upon receiving HO This would have required 48 sets of phase shifters, mixers and
MSG3, the target BS registers the MS with the gateway at steps RF paths. But, in order to reduce the hardware complexity of
4 and 5, and transmits HO MSG4 to the MS, confirming a the RF unit, three antenna elements in the vertical direction
successful completion of the HO RACH procedure. The were grouped as a sub-array such that all the elements in a
gateway sends MS-Connection-Release message to the source sub-array were connected to the same phase shifter, mixer, and
BS. In the MS initiated HO procedure, the target BS does not RF path. As a result, only 16 sets of phase shifters, mixers and
receive any messages from the source BS by which the MS RF paths were constructed instead of 48 for each antenna array.
context information can be transferred. Hence, it is necessary A price to pay for adopting this sub-array structure was a
for the MS to provide its context to the target BS to resume data reduction in the beam scanning range and an increase in the side
transmission. The MS transmits the RLC-State-Notify-Request lobe levels of the beam patterns in the vertical direction, which,
(NotifyREQ) message with its RLC state values to the target however, was small enough to meet our needs. The RF/antenna
BS, and the target BS, in turn, delivers the information to the unit of Fig. 7 contained two antenna arrays with an identical
gateway. The gateway acknowledges the target BS with array pattern, and the two antenna arrays were positioned to
NotifyGW-Response message and forwards the DL data for the face the same direction to provide the same coverage. The
MS to the target BS. The target BS acknowledges the MS with distance between the center positions of the two arrays was
the RLC-State- Notify-ACK (NotifyACK) message and starts about 17 cm.
transmission of DL data. In the MS initiated HO procedure, the As for the MS, two sets of RF and antenna were developed,
communication between the MS and the source BS is
and Fig. 8 shows one of them. The MS RF/antenna unit
completely eliminated to reduce HO failure due to a sudden
consisted of two antenna arrays; each array consisted of 4
radio link failure between the MS and the source BS.
elements and was positioned at one of the edges of the RF board
to provide horizontal beam coverage of either 90o or 180o.
V. ADAPTIVE BEAMFORMING TESTBED
Therefore, the MS RF/antenna unit provided total coverage of
In this section, the RF, antenna and modem of the testbed either 180o or 360o. Table III lists key parameters of the
developed at Samsung Electronics are described. RF/antenna units of the testbed, and the transmit powers were
A. RF and Antenna chosen similar to those of the current system. The power
Figs. 7 and 8 show the RF and antenna units of the testbed at TABLE III
PARAMETERS FOR BS AND MS RF/ANTENNAS
the BS and the MS, respectively. Each unit housed two antenna Parameters BS MS
arrays and related RF components, generating two beams at the
Carrier Frequency 27.925 GHz
same time for signal transmission or reception. The RF
Bandwidth 800 MHz
transceiver consisted of a set of power amplifiers (PAs), phase
Max. Transmit Power 37 dBm 23 dBm
shifters, mixers, and related RF circuitry. The phase shifter
Number of RF Paths 16 4
changed the phase of the signal transmitted by or received from
each antenna element of the array, and determined the beam Array Antenna Configuration 8x6 Planar 4 x1 Linear
pattern formed by the antenna array. A set of beam patterns was Max. Antenna Gain 21 dBi 7 dBi
Half-Power Beam Widths
generated in advance, and each beam pattern was assigned with 10 (H), 10 (V) 20 (H), 60o (V)
o o o
of the Center Beam
a unique beam identifier (ID). The phase shifter values for each 6 dB Antenna Coverage (Horizontal) 110o 90o or 180o
beam pattern were stored either in the modem or in the RF unit. Effective Isotropic Radiated Power 58 dBm 30 dBm

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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JSTSP.2016.2520901, IEEE Journal
of Selected Topics in Signal Processing
7

Fig. 10. High data rate test with two BS – MS pairs.

Fig. 9. The Modem hardware.
rate (BLER) of about 0.017%, so that the net data rate of 7.5
consumption of the RF units due to analog beamforming, Gbps was achieved by the two MSs. SU-MIMO in LOS
except for the power amplifiers, was insignificant. environment supporting multiple data streams is not always
B. Modem Hardware possible in general, but can be achieved when the BS – MS
distance is small, as discussed in [42]. For the antenna
Fig. 9 shows the modem hardware for the BS and the MS. It
configuration of the testbed, 2x2 MIMO in LOS environment
consisted of an analog front end (AFE) daughter board, seven
was supported for the BS – MS distance of up to about 20 m.
Virtex-7 FPGAs, a CPU module with a processor from Tilera
and a GPS module. The AFE board consisted of two ADCs, B. Adaptive Beamforming Test at 110 km/h in Single Cell
two DACs and two Virtex-6 FPGAs to process two streams of Adaptive beamforming in a single cell environment was
signals. The AFE board converted the digital baseband I and Q tested in a motorsports park, in Korea, in which the MS moved
signals at 1.2288 Giga sample per second (Gsps) to the digital at the speed of 110 km/h as shown in Fig. 11. A BS was located
IF signals at 2.4576 Gsps, and vice versa, in digital domain.
at the roof of a 3 story building at the height of about 15 - 20 m
The seven FPGAs implemented Cell Search/AGC, IFFT/FFT,
from the ground. The BS generated 8 beams in horizontal
BM, Channel Estimation, QAM/Soft-Bit generation, LDPC
direction. An MS was installed in a minivan and its RF/antenna
encoding/decoding. Power consumption was high due to high
bandwidth processing, but not due to analog beamforming. The unit was mounted on top of the vehicle. The MS had two
number of ADCs/DACs required for analog beamforming is antenna arrays that provided combined coverage of about 180o
much smaller than for the full digital method, and thus the in horizontal direction; each array generated 8 beams and
power consumption of the modem is greatly reduced. provided half of the 180o coverage. Fig. 11 shows the
movement of the vehicle along the track during 30 seconds of
VI. TEST RESULTS testing, with the positions marked as 0 s up to 30 s and its
distance to the BS ranged from 130 m to 800 m. The vehicle
A. High Data Rate Test for Two Stationary MSs speed was about 110 km/h approximately from the position 15 s
Transmitting multiple streams of data to multiple users at the
same time using multiple beams is an effective way of
increasing the data rate, and was tested as shown in Fig. 10.
Two BSs were positioned in a near open space environment in a
close proximity to one another. Each BS generated two beams
and transmitted two streams of data simultaneously to an MS. MS
The locations of the MSs and the directions of the beams from 0s 30s
0 7
the BSs were chosen such that the interference between the Ant.#1
10s 15s 20s 800m
MSs was minimized, and yet the angle between the directions 7
0
100km/h
110km/h 500m
7 50m
BS
0
towards the MSs from the BSs was less than 60o. The Data Slot Ant.#0

in the testbed supported reference signals for four data streams

with four transmit antenna arrays; each antenna array was 5
BS
BestBeamID

MS
assigned with unique reference signals different from those of 0
5 10 15 20 25 30
other antenna arrays. This is equivalent to the case in which one
BS with 4 antenna arrays transmits 4 data streams to two MSs. 1
Antenna
BestMS

The Data Slot of the proposed frame structure can support 0

5 10 15 20 25 30
various multi antenna techniques of the 3GPP LTE [31].
However, digital precoding was not yet implemented in the test 1500
Throughput

1000
[Mbps]

of Fig. 10. In the test of Fig. 10, each BS-MS path achieved the 500
0
5 10 15 20 25 30
data rate of 3.77 Gbps by 2x2 MIMO, supporting two data Time[s]
streams with 64 QAM and 3/4 code rate with the block error
Fig.11. Adaptive beamforming test at 110 km/h in a single cell.

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until the end of the test. The test was logged at every 100 ms by
the MS. The beam measurement period was 10 ms, and the MS
determined the best DL beam pair and fed back the best beam
information to the BS. Also, the MS reported the channel
quality information (CQI) to the BS at a regular interval. Based
on the feedback information, the BS updated the transmit beam
and the modulation and coding scheme (MCS).
Fig. 11 shows plots of the beam IDs of the BS and the MS,
the best antenna of the MS and the throughput during the test.
The best BS beam ID changed from 1 to 7 one by one as the
vehicle moved. The best beam and antenna of the MS also
changed similarly during the test, except at about 12 s. This
sudden change at 12 s was due to a guard post that blocked the
LOS path between the BS and the MS. The testbed achieved the
DL data rate of over 1.2 Gbps with 16 QAM and code rate of
3/4 during the test when the LOS path was available, but the Fig.12. Handover field test scenario.
data rate fell to about 500 Mbps with QPSK and code rate of 3/4 after the HO decision (HO_d). The average HO delay was
when the LOS path was blocked. In this test, MIMO was not about 12 ms, and the average HO interruption time was about
supported, and only the best antenna was selected at the BS and 21 ms, satisfying the HO interruption time requirement of 27.5
the MS to deliver single data stream. In the testbed, the DL ms of the IMT-Advanced [28]. UHD video ran smoothly
transmit power was fixed, whereas the UL transmit power was without interruption at 98% of the HO locations.
updated based on the DL received power without the control of The MS kept track of the received power of the serving cell
the BS. and that of the best target cell for the MS initiated HO
procedure. HO decision was made at the MS when the average
C. Handover Test in Three-Cell Environment at 20 km/h target cell received power was consistently higher than the
Extensive multi-cell handover tests for 5G testbed were average serving cell power by a predefined threshold. Fig. 13
conducted at Samsung Electronics, Suwon, Korea facility. shows plots of the received powers of the serving cell and the
Example HO scenario used for field test is shown in Fig. 12. best target cell, the serving cell ID, the best beam ID of the
Three BSs were installed at different locations having cell IDs 0, serving cell, the best beam ID of the best MS antenna and the
2 and 4, respectively. Each BS generated 16 beams in total; 8 timing advance (TA) values during 10 seconds of MS travel
horizontal beams and 2 vertical beams. The coverage for each from location 1a to 1b (marked in Fig. 12). Similarly, Fig. 14
cell is illustrated in the Fig. 12. These three BSs were connected
to a common gateway with S1 interface. An MS was installed
in a minivan and the MS RF and antenna unit were mounted on
top of the vehicle. The MS had two antenna arrays that
provided the coverage of about 360o in horizontal direction;
each array generating 8 beams with the coverage of 180o. The
path taken by the vehicle for these tests is illustrated in the
figure by colored directed arrows. The colors of the arrows are
chosen to match those of the serving cells in the test route.
Throughout the tests, the vehicle moved at about 20 km/h.
Initially, HO tests were performed using the BS initiated HO
procedure of Fig. 5. Handover failure rates using this procedure
were very high, especially at location 2 (star-marked in Fig. 12).
This high HO failure rate was mainly attributed to the UL radio
link failure with the serving BS before completing the HO
procedure. Even for successful HO cases, around 30% of trials
incurred more than 500 ms of HO delay. This was mainly due
to the large delay incurred in steps 7 and 8 (Fig. 5) caused by
high interference from target cell and high DL and UL packet
losses. To address the high HO failure rate and large HO delay
issues of the BS initiated HO procedure, the MS initiated HO
procedure of Fig. 6 was implemented and tested.
HO tests were repeated using the MS initiated HO procedure
in the same test environment as before. The HO delay in Fig. 6
is defined as the time taken for the MS to successfully receive
MSG4 from the target BS after the HO decision (HO_d). The
HO interruption time is defined as the time taken for the MS to Fig.13. Received powers of cells, Serving cell ID, Best serving cell beam,
Best MS beam, and Timing advance values at HO location 1.
successfully receive the DL data packets from the target BS

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of Selected Topics in Signal Processing
9

directional beamforming antennas. In the proposed frame

structure, slots dedicated to various functions of mobile cellular
communications are defined such that the signal structure and
beam operations of a slot are designed optimally for its function,
and are different from those for other functions. The number of
slots corresponding to different functions can be flexibly and
optimally selected for beamforming systems with different
number of beams/antenna arrays or adapted to changing traffic
conditions in the frame structure. The proposed frame structure
supports fast cell search and measurement of beams of multi
cells to support mobile cellular communications. Resource
block and reference signal structure similar to those of the
3GPP LTE [31] can be defined for the Data Slot of the proposed
frame structure. By scheduling the Data Slots, TDM/TDMA is
supported. Also, by scheduling resource blocks of a Data Slot,
FDM/FDMA can be supported for the MSs for which the best
BS beams are the same. As a result, hundreds of multiple access
users can be flexibly supported in the proposed frame structure.
Various multi antenna techniques of the 3GPP LTE and digital
precoding in addition to analog beamforming can be supported
using multiple antenna arrays in the proposed frame structure.
A testbed was built at 28 GHz with bandwidth of 800 MHz and
TDD mode based on the proposed frame structure at Samsung
Electronics, Korea. The testbed achieved the aggregate data
rate of 7.5 Gbps for two MSs in the downlink by supporting 2x2
MIMO to each MS in stationary. In an adaptive beamforming
Fig. 14. Received powers of cells, Serving cell ID, Best serving cell beam, test in a single cell, the testbed achieved the data rate of 1.2
Best MS beam, and Timing advance values at HO location 2. Gbps for an MS moving at the speed of 110 km/h. Finally, the
shows those values from location 2a to 2b (marked in Fig. 12). testbed successfully implemented and tested handover in a
Ping-pong effects during the HO in Fig. 14 was minimized by three-cell environment, attained an average handover delay of
taking local average of the received power, and applying 3 dB 12 ms and an average handover interruption time of 21 ms,
offset and hysteresis in the HO decision. satisfying the handover interruption time requirement of 27.5
During the test of Fig. 13, the antennas of BS 4 were not ms of the IMT-Advanced [28], and successfully demonstrated
adequately down tilted to cover the test route very close to BS 4 feasibility of mobile cellular communications at millimeter
(test route from 1a to 1b in Fig. 12). In this test, as the MS wave frequency with beam forming antennas.
moved from 1a towards 1b, the received power from BS 4
decreased by almost 15 dB until time 4 s, while at the same time ACKNOWLEDGMENT
the TA value decreased, although very slightly. This is against The authors would like to thank their colleagues at Samsung
our intuition that when the TA value is decreased, the received Electronics who have made valuable contributions to this work
power should be increased in LOS environment. This strange but have not been included as co-authors in this paper.
phenomenon happened because the MS was so close to the BS
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[23] O. El Ayach, S. Rajagopal, S. Abu-Surra, Z. Pi, and R. W. Heath, Jr., Madison in 1989, and the Ph.D. degree
“Spatially Sparse Precoding in Millimeter Wave MIMO Systems,” IEEE
Trans. Wireless Commun., vol.13, no.3, pp. 1499-1513, Mar. 2014.
from the Korea Advanced Institute of
[24] A. Alkhateeb, O. El Ayach, G. Leus, R. W. Heath Jr, “Channel Estimation
Science and Technology (KAIST) in 2000. He joined Samsung
and Hybrid Precoding for Millimeter Wave Cellular Systems,” IEEE J. Advanced Institute of Technology (SAIT), Korea in 1991, and
Sel. Topics in Signal Processing, vol.8, no.5, pp.831-846, Oct. 2014. then transferred to DMC R&D Center at Samsung Electronics,
[25] J. Singh, S. Ramakrishna, “On the Feasibility of Codebook-Based Suwon, Korea in 2006. He contributed to the research and
Beamforming in Millimeter Wave Systems With Multiple Antenna development of 4G physical layer and MAC layer algorithms as
Arrays,” IEEE Trans. Wireless Commun., vol. 14, no. 5, pp. 2670-2683,
May 2015. a project leader at SAIT and Samsung Electronics. He also
[26] IEEE 802.15 WPAN Millimeter Wave Alternative PHY Task Group 3c contributed to the design and implementation of 5G millimeter
(TG3c). [Online]. Available: http://www.ieee802. org/15/pub/TG3c.html. wave communications system. His current research activities
are in 5G mobile communications system.

1932-4553 (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JSTSP.2016.2520901, IEEE Journal
of Selected Topics in Signal Processing
11

Wonil Roh is currently a Vice President

Hyun-Yong Lee (S’01-M’08) received the and Head of Advanced Communications
B.S., M.S. and Ph.D. degrees in electrical Lab at Samsung Electronics Corp in Korea,
engineering from Korea Advanced responsible for research of next generation
Institute of Science and Technology mobile communications technologies. He
(KAIST), Daejeon, Korea in 2001, 2003 started working at Samsung Electronics in
and 2008, respectively. From 2008 to 2011, 2003 in research and development of
he served as a postdoctoral associate at the CDMA and Mobile WiMAX base-stations
School of Electrical Engineering, KAIST. with the main focus on multi-antenna
Since May 2011, he has been working with algorithms and system analysis. Then he led overall WiMAX
Samsung Electronics Co., Ltd., Suwon, Korea. He has standard activities and strategy in Samsung including IEEE, the
contributed to the development of the modem of 5G mobile WiMAX Forum and ITU-R, and served as Chair of Technical
communications system. His current fields of interest include Working Group (TWG) of the WiMAX Forum from 2006 to
research and development of 5G communications system. 2011. Since 2011, he has been leading research efforts for the
next generation cellular (Beyond 4G or 5G) technologies at
Samsung with a focus on development of disruptive
Philyeong Hwang is a senior engineer at technologies and feasibility studies. He holds a Doctorate
the Communications Research Team, degree in Electrical Engineering at Stanford University in USA.
Samsung electronics, Suwon, Korea. He
received his Master’s degree in Computer
Engineering from Kwangwoon University,
Seoul, Korea in 1997. He has contributed Kyungwhoon Cheun (S’88-M’90) is
to the development of communications currently an Executive Vice President and
protocols for Home network, Cellular and Head of the Communications Research
WLAN (e.g. IEEE 802.11n). His current Team at Samsung Electronics, Suwon,
research activities are in the area of 5G radio resource Korea. He received his B.S. in Electronics
management and scheduling. Engineering from Seoul National
University in 1985. He earned is M.S. and
Ph.D. degrees from the University of
Ranjeet Kumar Patro is a senior chief Michigan, Ann Arbor in 1987 and 1989, respectively. He was a
engineer at the Advanced Research Team, professor at University of Delaware from 1989 to 1991 and
Samsung R&D Institute India, Bangalore. with the Pohang University of Science and Technology
He received his Master’s degree in (POSTECH) from 1991 to 2014. At POSTECH, he headed the
Electrical and Communication national ITRC center for Broadband OFDM Multiple Access
Engineering from Indian Institute of (BrOMA), an 8 year research program supported by the Korean
Science (IISc), Bangalore, in the year 2003. Ministry of Knowledge and Economy. He has also served as an
He has contributed to standards and papers engineering consultant to numerous industry and was on leave
in the Cellular, WLAN (e.g. IEEE at Witechs and NSystems in San Diego where he developed
802.11n), and WPAN (e.g. Zigbee, BAN) fields. His research efficient receiver algorithms for the IEEE802.11 based WLANs
activities include 5G wireless networks, energy-efficient and WCDMA. He also served as the Chief Technical Officer
wireless communications, radio resource management and (CTO) for Pulsus Technologies Inc. during 2004 to 2012,
scheduling. where he was in change or developing sound processing
algorithms and sigma-delta modulation based full digital audio
amplifier SoCs. Since 2012 he has been with Samsung
Jaekon Lee is a principal engineer at the Electronics and currently leads the Communications Research
Communications Research Team, Team in the area of next generation cellular and Wi-Fi
Samsung Electronics. He received the B.S. networks.
degree in Electrical and Communication
Engineering from Hanyang University,
Korea in 1991. He has contributed to the
research and development of wireless
communication systems including
DAMPS, GSM, 3G, 4G and Digital
Multimedia Broadcasting. His current research activities are in
5G communications system.

1932-4553 (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.