Workshop on Telecommunications: From Research to Standards

IEEE 802.11ah: Advantages in Standards and Further Challenges for Sub 1 GHz Wi-Fi
Stefan Aust
NEC Communication Systems, Ltd., 1753 Shimonumabe, Nakahara-ku, Kawasaki, Kanagawa 211-8666, Japan Email: aust.st@ncos.nec.co.jp

R. Venkatesha Prasad, Ignas G. M. M. Niemegeers

EEMCS, Delft University of Technology, P.O. Box 5031, 2600 GA Delft, The Netherlands Email: R.R.VenkateshaPrasad@tudelft.nl; I.G.M.M.Niemegeers@tudelft.nl devices in the 2.4 GHz/5 GHz frequency band, the design of RFID (Radio Frequency IDentication) devices and other IEEE 802.15.4-based sensor networks operating in the ultra high frequency (UHF) spectrum below 1 GHz took a niche in the deployment of personal area networks. With the rapid demand for unlicensed, ubiquitous access in less-interfered frequency bands, the 900 MHz ISM bands in particular have kindled new attraction not only in the research domain but also in standardization. The target is to dene a global WLAN standard that operates at ISM frequencies below 1 GHz. For instance, in the United States, Europe and Japan, such frequencies are available, however a standard has not been available yet that utilizes such frequencies. A new IEEE 802 standardization task group (TG) aims to create a WLAN standard for PHY and MAC that operates at frequencies below 1 GHz. The so-called sub 1 GHz WLAN system is under current standardization in the IEEE 802.11ah group. In the following, we are going to reect on the advantages and challenges of a sub 1 GHz WLAN system. To the best of our knowledge there has been less focus and studies on low-frequency WLANs. In [1] the authors argue that the scope of IEEE 802.11ah is to enhance the MAC and PHY design to operate in the license-exempt bands below 1 GHz. For smart grid and smart utility communications the authors discuss the advantages of access in TV frequency bands. However the access to the so-called TV white-space is outside of the scope of IEEE 802.11ah (the operation of WLAN in the TV white space is being standardized in the IEEE 802.11af group). We then refer to our earlier work in sub 1 GHz WLAN in [2] and [3]. Use cases and scenarios for using sub 1 GHz frequency bands have been discussed in [2] and we identied smart grid, surveillance and smart farming as main applications in outdoor areas. In addition, link budget and outage probabilities were also discussed in [2]. In [3] we outlined the required PHY design of sub 1 GHz systems and argued that the proposed IEEE 802.11ah pathloss design for outdoor sub 1 GHz is somewhat too optimistic and may require a larger number access points (APs) in an outdoor deployment scenario when using WLANs operating at carrier frequencies around 900 MHz. We refer the reader to the ofcial project site of IEEE 802.11ah [4] for further information about the IEEE 802.11ah TG. We will give further

AbstractThe rapid developments in Internet-of-Things (IoT) and Machine-to-Machine (M2M) communication make it necessary to design communication systems operating in different wireless spectrum as an alternative to highly congested wireless access systems. In addition, the deployment of wireless smart meter devices is ramping up and it is expected that such devices will ood the market in the near future competing for the same wireless spectrum. The IEEE 802.11ah standardization task group is developing a global Wireless LAN (WLAN) standard that will allow wireless access using carrier frequencies below 1 GHz in the ISM (Industrial, Scientic, and Medical) band and will help Wi-Fi-enabled devices to get guaranteed access for short-burst data transmissions, such as meter data. In addition to exploiting the underutilized sub 1 GHz spectrum the improved coverage range allows new applications to emerge such as wide area based sensor networks, sensor backhaul systems and potential Wi-Fi off-loading functions. This paper summarizes the IEEE 802.11ah standardization activities in progress and discusses advantages and challenges in the design of physical layer (PHY) and media access control (MAC) schemes in the sub 1 GHz band. Index TermsSub 1 GHz, IEEE 802.11ah, long-range Wi-Fi, smart grid, IoT, M2M.

I. I NTRODUCTION The rise in license-exempt wireless technologies, such as Wi-Fi and ZigBee, indicates that Internet access and data transmission are becoming increasingly wireless. For instance, the IEEE 802.11 standard and its amendments a/b/g/n dene wireless transmissions at 2.4 GHz/5 GHz in the ISM frequency band and make it simple for Internet user to build up their own WLANs, e.g., such as wireless indoor networks. The widespread deployment of IEEE 802.11-based wireless networks in indoor environments and also in urban areas has led to steep increase in mutual interference problems resulting in signicant performance loss due to data packet collisions and retransmissions where Internet users suffer from low data rate or even complete network disruption. Furthermore, Wi-Fi enabled devices can be found in various mobile devices, such as tablet PCs and smart phones, thus increasing even more the competition about unlicensed wireless access in the ISM band. Smart grid applications, IoT and M2M communication will further result in saturated spectrum when the same frequencies at 2.4 GHz/5 GHz are being shared. Beside the successful deployment of IEEE 802.11

978-1-4577-2053-6/12/$31.00 2012 IEEE

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Fig. 1.

Advantages of a standardized sub 1 GHz system

insight into the development of a global sub 1 GHz WLAN system and also what lies ahead and enlist many challenges and issue thereof. This paper is organized as follows: In Section II the advantages of using sub 1 GHz frequency band is discussed. IEEE 802.11ah use cases are explained in Section III. IEEE 802.11ah standardization challenges are dealt with in Section IV. We conclude in Section V. II. A DVANTAGES OF S TANDARDIZED S UB 1 GH Z WLAN S Even though Wi-Fi is standardized for the 2.4 GHz/5 GHz ISM frequency band, there is already non-standard modied Wi-Fi equipment available that operates in the 900 MHz ISM band. As for now, various vendors take the core technology, e.g., IEEE 802.11a, and change the frequency. The demand for a standardized low-frequency WLAN comes, in part, from the smart grid community, who like lower frequencies for linking to smart meters of the larger wireless coverage and lower obstruction losses when using lower UHF spectrum for wireless data transmission. A problem, though, has been the lack of interoperability between such sub 1 GHz devices. Each vendor has its own implementation, and smart grid customers do not want to be tied to one single vendor. IEEE 802.11ah will offer a variety of advantages, such as simple to use in outdoor environments in addition to excellent propagation characteristics at low frequencies [5] and different levels of installation scenarios (license-exempt, light licensing, professional and interference reduced). High sensitivity and link margin increase the reliability of IEEE 802.11ah. In addition, energy saving strategies will be integral part of the IEEE 802.11ah standard. Details are shown in Fig. 1. In the following we outline the main advantages of a standardized sub 1 GHz WLAN: Longer range and less power consumed due to optimal propagation characteristics below 1 GHz. No licensing and regulatory issues (ISM band). License-exempt in various different countries. Almost clear co-existence issues. Easy to understand, follow and to implement for network device manufacturers. Enrichment of current wireless communication devices, e.g., IEEE 802.11a/b/g/n. Next, we will motivate the IEEE 802.11ah use cases to discuss the required PHY and MAC design. The IEEE Standards

Fig. 2.

Adopted IEEE 802.11ah use case: smart grid

Association Standards Board approved a request by the IEEE 802.11 to start a project that will amend the IEEE 802.11 standard to include sub 1 GHz operation. This project, under new IEEE 802.11ah TG, does not include TV white space frequencies, which is being handled at the IEEE 802.11af TG. The most important contribution of this amendment will be standard RF channel width and center frequencies. Because IEEE 802.11 is an international standard, global frequency allocation schemes will be considered. III. IEEE 802.11 AH U SE C ASES We provide an overview of the use cases that have been adopted by the IEEE 802.11ah TG. We also refer to additional use cases to further motivate the usability of the future IEEE 802.11ah standard. The following discussion on use cases provides comprehensive view of the advantages of using sub 1 GHz bands in various domains and scenarios. A. Sensor networks IEEE 802.11ah includes sensor networks as one of three adopted use cases. Sensing can be executed as short-burst data transmissions and covers smart metering such as gas, water and power consumption [6]. Wireless controlled power distribution systems are also considered. Due to the increased penetration through walls at lower frequencies, a higher number of sensors can be covered in one-hop fashion. Fig. 2 shows a simple smart grid scenario where IEEE 802.11ah is applied (IEEE 802.11ah AP and wireless meter stations). This use case denition was contributed by the authors of this paper and adopted by the IEEE 802.11ah TG [4]. B. Backhaul networks for sensors The second adopted use case covers the backhaul connection between sensors and/or data collectors and remote servers. The large coverage of sub 1 GHz allows a simple network design to link sub 1 GHz APs together, e.g., as wireless mesh networks. Fig. 3 shows a backhaul sensor network, including IEEE 802.11ah APs and router/gateways to connect sensor networks, e.g., IEEE 802.15.4g.

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Fig. 3.

Adopted IEEE 802.11ah use case: sensor backhaul network

Fig. 4.

Proposed IEEE 802.11ah channelization for US

C. Extended Wi-Fi range for cellular trafc off-loading The third adopted use case considers technical requirements for a Wi-Fi based cellular trafc off-loading in IEEE 802.11ah. It is important that the technology used for off-loading has at least comparable performance to the cellular system being offloaded both from the user as well as the operator perspectives. Therefore, it is essential to consider what kind of spectral efciency, user throughput, and system load the current and future cellular networks may support. Based on that, we need to consider the performance requirements for IEEE 802.11ah. Although it can be easily understood that a large coverage may be utilized for off-loading, some may argue that user expectations can easily be covered by existing wireless standards such as IEEE 802.11n. IEEE 802.11ah off-loading feature are expected to provide real additional value for the end user in the US market where up to 16 MHz bandwidth is available (see Section IV, B). Next, we outline additional (not adopted) use cases and scenarios for IEEE 802.11ah. D. Machine-to-Machine (M2M) communication The future IEEE 802.11ah standard has been found as an optimal candidate as wireless communication system for M2M communication. Wireless M2M communication allows data transfer for direct machine-to-machine communication with little or no human interaction [7]. Whereas current systems are optimized more for human-to-human (H2H) communications, IEEE 802.11ah standard will mainly consider sensing applications. Due to different M2M standards activities happening in various standardization organizations, IEEE 802.11ah could play an important role in providing a base for a global M2M wireless standard, which some entities consider as a precursor of cloud computing [8]. This includes smart metering, eet management, security sensing, and on-demand business charging applications. IEEE 802.11ah will address required functions such as low power consumption, large number of devices, long-range and short-burst data transmissions. E. Rural communication (connecting the unconnected) Wireless communication in rural areas, such as in outback areas, has lead to some effort that is also titled as connecting the unconnected. Large potential is given by sub 1 GHz due

to the wider range [9]. E-health and e-learning would be main applications in such environments and it has been argued that a positive impact on social economics including the Gross Domestic Product (GDP) growth can occur [10]. IV. IEEE 802.11 AH S TANDARDIZATION C HALLENGES A. IEEE 802.11ah project target and time line The IEEE Standards Board approved the sub 1 GHz IEEE 802.11 (Wi-Fi) Project in November 2010. The sponsor ballot is expected for spring 2013. The project target is specied in the PAR (Project Authorization Request) as listed under [4]. Since November 2010 the time line has been extended several times mainly to allow consensus within the standardization group, e.g., for use case identication, optimal PHY channelization, and requirements on the MAC design. B. Requirements on IEEE 802.11ah PHY design Future IEEE 802.11ah standardized devices will operate as a Multi Input Multi Output Orthogonal Frequency Division Multiplexing (MIMO-OFDM) wireless system at different sub 1 GHz ISM bands which are available in various countries, including United States, South Korea, China, Europe, Japan, Singapore. In the following we outline the adopted and proposed channelization for IEEE 802.11ah. The US channelization has been one of the most discussed channelization for IEEE 802.11ah. This is due to the fact that the United States regularory allows up to 16 MHz bandwidth between 902 MHz and 928 MHz. Fig. 4 depicts the proposed US bandwidth, including 1, 2, 4, 8, and 16 MHz. Thus, it is less attractive for vendors having small bandwidth supported at 1 MHz. However, 1 MHz and 2 MHz have been adopted as common channel bandwidth for IEEE 802.11ah (Japan only allows 1 MHz bandwidth). The standardization group supports 1 MHz for the US and all other countries which are under consideration in IEEE 802.11ah. A compromise would be a link adaptation scheme which allows 1 MHz/2 MHz operation when STAs are out of AP coverage (see Section IV, E). In Fig. 5 the adopted channelization for South Korea is shown, starting from 917.5 MHz and ends at 923.5 MHz. 6 channels with 1 MHz bandwidth are adopted. In addition, 3 channels at 2 MHz and 1 channel at 4 MHz bandwidth will be standardized in IEEE 802.11ah. The reason for the 0.5 MHz

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Fig. 5.

Adpoted IEEE 802.11ah channelization for Korea

Fig. 8.

Adpoted IEEE 802.11ah channelization for China

Fig. 6.

Adpoted IEEE 802.11ah channelization for Japan

Fig. 9.

Adpoted IEEE 802.11ah channelization for Singapore

C. Sub 1 GHz related regulatory requirements and IEEE 802.15.4d co-existence in the case of Japan
Fig. 7. Adpoted IEEE 802.11ah channelization for Europe

frequency off-set in the channelization for Korea is to reduce possible mutual interference with wireless legacy systems at lower frequencies. In Fig. 6 the channelization for Japan is presented, starting at 916.5 MHz and ends at 927.5 MHz. The channelization starts with 0.5 MHz off-set, because the Japanese spectrum regulations specify center frequencies instead of start/stop bands [11]. This channelization was contributed by the authors of this paper and adopted by the IEEE 802.11ah TG [4]. Fig. 7 shows the adopted channelization for Europe, starting at 863 MHz and ends at 868 MHz. 5 channels with 1 MHz bandwidth are available. In addition 2 channels with 2 MHz bandwidth have been adopted. The adopted channelization for China is shown in Fig. 8, starting at 755 MHz and ends at 787 MHz. Frequencies between 755 MHz to 779 MHz will allow a max. sending power at 5mW. Between 779 MHz and 787 MHz, max. 10 mW are allowed. In the higher frequency regime 4 channels with 2 MHz, 2 channels with 4 MHz and 1 channel with 8 MHz are approved. It is worth to note that most of the sub 1 GHz frequencies in China are used by TV broadcast stations, thus it makes it inappropriate to adopt such frequencies for IEEE 802.11ah. In Fig. 9 the proposed channelization for Singapore is shown. It starts from 920 MHz and ends at 925 MHz including 5 channels with 1 MHz bandwidth.

The use of the 950 MHz band (950 MHz to 958 MHz) for LR-WPAN (Low Rate Wireless Personal Area Networks) has only been recently allocated by the Japanese Regulatory committee. The Japanese regulation includes requirements to address co-existence for devices operating in the sub 1 GHz band, e.g., Listen Before Talk (LBT), transmission control and duty cycle restrictions. However, two PHYs specied for use in the 950 MHz band, such as IEEE 802.15.4d and IEEE 802.11ah can potentially cause mutual interference. Together with the short duration (burst nature) of 802.15.4 packets and the use of CSMA-CA, co-existence is not considered to be a problem for the two PHYs when they share a common channel [11]. The regulation in [11] requires that a device uses LBT schemes prior to transmission if the duty cycle of transmission exceeds 0.1 and that a device does not continuously transmit. The maximum continuous transmission time and the duty cycle of transmission are dependent on the LBT duration. For instance, the parameters macTxControlActiveDuration and macTxControlPauseDuration permit a higher layer to control both the duration for which a device may transmit and the duration of the pause period, i.e., the time during which the MAC must pause to allow other devices access to the channel. These values are dependent on the transmission power and channel and will be further discussed in the IEEE 802.11ah standardization process. D. IEEE 802.11ah path loss models IEEE 802.11ah path loss models for indoor have been discussed in [3]. The basic assumption is a path loss model for AP-to-STA communication for both, indoor and outdoor locations [12]. In addition multi-oor and STA-to-STA path

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F. Requirements on IEEE 802.11ah MAC The IEEE 802.11ah MAC design considers alternative Distributed Coordination Function (DCF) methods. In addition, a contention-free MAC design is considered supporting a large number of stations, e.g., thousands of STAs [2], which are required for M2M and IoT applications. Power efciency for sensor devices are proposed and are required to support the adopted sensor network use case and may include the adoption of modied IEEE 802.11v power saving features and enhanced ultra-low power consumption strategies, such as Radio-on-Demand (ROD) for IEEE 802.11ah as proposed by the authors of this paper [4]. V. C ONCLUSIONS With the rapid deployment of smart meter systems, IoT and M2M applications, the demand for short-burst data transmission becomes a challenging task when using current WiFi frequency bands. The IEEE 802.11ah standard will dene wireless access in the sub 1 GHz ISM band in various frequency domains, including US, Europe, Japan, China and Korea and will exploit a larger coverage area and high penetration. To this effect, this paper presents the advances of the standardization of sub 1 GHz WLANs and at the same time it discusses the challenges that need to be addressed so that the emerging IEEE 802.11ah standard will efciently dene a PHY and MAC for Wi-Fi communication in sub 1 GHz. R EFERENCES loss models are proposed. Fig. 10 shows a STA-to-STA path loss scenario. Two stations operate in AP-to-STA communication, thus mutual interference between the STAs occurs when transmitting at same locations. E. Enhanced coverage extension through repetition Due to the excellent propagation characteristic in lower UHF bands, the IEEE 802.11ah link margin will be 6 dB higher at 900 MHz compared to wireless transmissions at 2.4 GHz [3]. Further improvement of coverage and link budget performance can be achieved when using repetition. Different Modulation and Coding Schemes (MCS) are proposed in the following. MCS0-Rep (1 MHz) is 6 dB higher than MCS0 (2 MHz) and provides a larger coverage compared to nonrepetition based schemes. Based on the IEEE 802.11ah path loss model [3], the range covered by MCS0- Rep2 (1 MHz) is higher than MCS0 for 2 MHz bandwidth, as shown in Fig. 11. Fig. 11 also shows an indoor coverage example of 1 MHz Basic Service Set (BSS) over 2 MHz BSS. A path loss exponent with 36.7 is assumed. The sensitivity improvement is given at 6 dB. The assumed indoor coverage leads to 1.45 times higher coverage when using MCS0-Rep2 at 1 MHz. Fig. 11 shows outdoor coverage of 1 MHz BSS over 2 MHz BSS. A path loss exponent at 37.6 is assumed and sensitivity improvement is given at 6 dB. The assumed coverage leads to 1.74 times higher coverage when using MCS-Rep2.
[1] C.-S. Sum, H. Harada, F. Kojima, Z. Lan, and R. Funada, Smart utility networks in tv white space, IEEE Communications Magazine, vol. 49, issue 7, pp. 132-139, July 2011. [2] S. Aust, T. Ito, Sub 1 GHz Wireless LAN Deployment Scenarios and Design Implications in Rural Areas, the Global Communications Conference, Exhibition and Industry Forum (GLOBECOM) Workshop on Rural Communications: Technologies, Applications, Strategies and Policies (RuralComm 2011), Houston, Texas, USA, 5-9 December 2011. [3] S. Aust, T. Ito, Sub 1 GHz Wireless LAN Propagation Path Loss Models for Urban Smart Grid Applications, the International Conference on Computing, Networking and Communication (ICNC) Workshop on Communication Technologies Support to the Smart Grid, Maui, Hawaii, USA, January 30 - February 2, 2012. [4] Status of Project IEEE 802.11ah, IEEE P802.11 - Task Group AHMeeting Update, http://www.ieee802.org/11/Reports/tgah udate.html. [5] J. S. Seybold, Introduction to RF Propagation, Wiley, 2005. [6] NIST Priority Action Plan 2, Guidelines for Assessing Wireless Standards for Smart Grid Applications, ver. 1.0, December 31, 2010. [7] K. Chang, A. Soong, M. Tseng, and Z. Ziang, Global Wireless Machine-to-Machine Standardization, IEEE Internet Computing, pp. 6469, March/April 2011. [8] N. Nikaein, S. Krco, Latency for Real-Time Machine-to-Machine Communication in LTE-based System Architecture, in the Proceedings of the European Wireless 2011, Vienna, Austria, April 27-29, 2011. [9] V. M. Rohokale, et al., A Cooperative Internet of Things (IoT) for Rural Healthcare Monitoring and Control, in 2nd International Conference on Wireless Communication, Vehicular Technology, Information Theory and Electronics Systems, March 2011, pp. 1-6. [10] Y. Su, et al., Building an Information Quality Lab on E-health in the Rural Areas for Healthcare Education, International Conference on Ehealth, Ecosystems and Technologies, April 2010, pp. 332-335. [11] Association of Radio Industries and Business (ARIB), 950 MHz-Band Telemeter, Telecontrol and Data Transmission Radio Equipment for Specied Low Power Radio Station, English translation, ARIB STD-T96 Ver. 1.0, June 6, 2008. [12] V. Ponampalam, J. Wang, R. Porat, TGah Outdoor Channel Models Revised Text, May 2011.

Fig. 10.

IEEE 802.11ah STA-2-STA scenario

Fig. 11.

Proposed IEEE 802.11ah repetition schemes

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