Cognitive radio on TV bands a new approa

CHEN LAYOUT 6/5/08 1:24 PM Page 16 W I R E L E S S T E C H N O L O G I E S A D VA N C E S F O R E M E R G E N C Y A N D R U R A L C O M M U N I C AT I O N S COGNITIVE RADIO ON TV BANDS: A NEW APPROACH TO PROVIDE WIRELESS CONNECTIVITY FOR RURAL AREAS YING-CHANG LIANG AND ANH TUAN HOANG, INSTITUTE FOR INFOCOMM RESEARCH HSIAO-HWA CHEN, NATIONAL CHENG KUNG UNIVERSITY ABSTRACT WRAN BS Employing wireless technologies to provide connectivity for rural areas is an active topic in Microphone protectionthe academic and industrial communities In this area Wireless microphones We discuss the challenges of rural communications and reviewing existing wireless technologies that have been proposed or implemented for this market We then focus on an emerging technology, cognitive radio, that promises to be a viable solution for rural communications 16 article we begin by discussing the challenges of rural communications and reviewing existing wireless technologies that have been proposed or implemented for this market We then focus on an emerging technology, cognitive radio, that promises to be a viable solution for rural communications The most notable candidate for rural cognitive radio technology is the IEEE 802.22 standard that is currently being developed and is based on time division duplexing, orthogonal frequency division multiple access, and opportunistic use of the VHF/UHF TV bands We address two important issues that can affect the success of IEEE 802.22 technology in rural deployments, namely, to: • Provide suitable service models • Overcome the problem of long TDD turnaround time in large rural cells For the first issue, we introduce a service model that combines TV broadcasting and data services to facilitate service adoption For the second issue, we propose an adaptive TDD approach that effectively eliminates the requirement for long TDD turn-around time and thus, increases the efficiency of large-coverage rural networks INTRODUCTION According to a report published by the United Nations, more than three billion people are currently living in rural areas [1] In developing countries like China and India, about 70 percent of the total population live in rural communities, which are spread far and wide over large geographic areas For these communities, it is believed that providing communications services is an important step to facilitate development and social equity [2] Apart from that, rural com- 1536-1284/08/$25.00 © 2008 IEEE munications networks are crucial in disaster/emergency response scenarios However, providing rural communications is often challenging due to the mismatch between costs and demand Most rural areas have low population density and the demand for services per individual or household can be much lower in rural areas if compared to urban areas To create a viable business, operators must aim for low-cost solutions However, the deployment and maintenance of rural communications networks can be costly due to large areas requiring coverage, lack of transportation, and difficult terrains This is particularly true for wired networks because wires or cables must be laid all the way to the destinations As a result, wireless technologies usually are preferred for rural connectivity In fact, there are various approaches that consider wireless technologies for rural communications [3–7] Proposals that make use of cellular or satellite technologies can be found in [3, 4] Recently, with the proliferation of Wi-Fi, there have been proposals to extend this shortrange/local-area-network technology for rural coverage [5–7] Nevertheless, the challenges of providing low-cost services to a low-demand market still remain Cognitive radio is an emerging technology that promises to overcome one of the most challenging problems of modern wireless communications, namely, spectrum scarcity Access to radio spectrum today is based largely on fixed allocation, that is, different frequency bands are allocated to different services With the proliferation of wireless applications and services in many countries, most of the available spectrum has been allocated On the other hand, careful studies reveal that most of the allocated spectrum experiences low utilization [8] By intelligently detecting and making use of the allocated but under-utilized spectrum, cognitive radio enables wireless networks to operate without requiring dedicated spectrum This, in the con- IEEE Wireless Communications • June 2008 Authorized licensed use limited to: Huazhong University of Science and Technology Downloaded on December 9, 2008 at 20:43 from IEEE Xplore Restrictions apply CHEN LAYOUT 6/5/08 1:24 PM Page 17 text of rural communications, means cognitive radio networks can be deployed at lower costs The most notable example of cognitive radio technologies for rural communications is the IEEE 802.22 wireless regional area network (WRAN) standard that is currently being developed, which is based on time division duplexing (TDD), orthogonal frequency division multiple access (OFDMA), and opportunistic use of very high frequency/ultra high frequency (VHF/UHF) TV bands [9] Apart from the fact that 802.22 technology does not require dedicated spectrum, which significantly saves deployment costs, the large network coverage makes this technology particularly suitable for rural deployment Due to the favorable propagation condition in the VHF/UHF bands, an 802.22 WRAN signal can reach a much longer distance, compared to WiFi and WiMAX signals transmitted on frequencies above GHz In fact, 802.22 WRAN is designed to provide wireless broadband access to rural and suburban areas, with an average coverage radius of 33 km that can increase to 100 km To realize the advantages of 802.22 technology, there are many technical challenges that must be overcome For example, to avoid causing interference to incumbent users, namely, TV receivers and FCC Part 74 wireless microphones, WRAN systems must be able to perform spectrum sensing and detect these incumbents at very low signal strength [9] Another challenge is how multiple WRANs can coexist and interact with networks of other types Whereas these challenges are general in any 802.22 deployment, for rural scenarios the success of 802.22 technology and the like depends on two important factors, namely: • Whether service providers can offer attractive content models for the rural market • How to ensure the efficiency of operation in large coverage scenarios In terms of service content, it should be noted that customers in rural communities are more familiar with and have higher demand for traditional applications such as telephony and TV broadcasting, than for the Internet, data, or other multimedia applications [7] To account for this, we propose a service model that combines TV broadcasting and data services to facilitate the growth of rural demand for connectivity To reduce costs and increase customer population, rural wireless networks must be deployed in large coverage areas In such cases, if TDD is used as in 802.22 WRAN, the system efficiency will be seriously affected by the TDD turn-around time, that is, the time a system must stay idle for nearby and faraway subscribers to synchronize their uplink transmission To address this problem, we propose an adaptive TDD technology that effectively eliminates the requirement for long TDD turn-around time and thus increases the efficiency of large coverage rural networks RURAL CONNECTIVITY: CHALLENGES AND EXISTING TECHNOLOGIES In this section, we highlight the challenges faced by service providers in the rural information and communications technology (ICT) market This is followed by a discussion of existing technologies that were proposed or deployed for rural communications CHALLENGES OF HIGH COSTS AND LOW DEMAND Compared to the urban ICT market, the rural market exhibits a significant mismatch between costs and demand In particular, the rural ICT market can be characterized as follows • High deployment/maintenance cost: Deploying and maintaining a communications network in rural areas often incurs higher costs, compared to doing so in urban areas This is due to large coverage areas, difficult terrains, lack of transportation, and often a shortage of local trained staff • Low customer density and demand: Most rural areas are sparsely populated Moreover, customers in the rural market often have lower incomes than those in urban areas and therefore, have less to spend on ICT services The low service demand also is due to low customer awareness and expertise This is particularly true in developing countries • Slow service adoption rate: It has been observed that it takes longer for consumers in rural areas to adopt new services compared to those in urban areas [7] With the previously mentioned characteristics of the rural ICT market, service providers tend to face the following chicken and egg problem To reduce the service costs, network providers require a large or fast-growing customer base; however, in rural markets, the customer demand can only be increased if services are offered at sufficiently low rates To overcome this problem, rural network providers should aim for solutions that incur low costs and offer large coverage The rural ICT market exhibits a significant mismatch between costs and demand To overcome this problem, rural network providers should aim for solutions that incur low costs and offer large coverage EXISTING TECHNOLOGIES In the following, we discuss the technologies that were proposed or deployed for rural communications Cellular/Wireless Local Loop/Satellite — Fixed cellular and wireless local loop (WLL) technologies have been proposed for rural communications, for example, in [4] Their advantage is the relatively short deployment time Moreover, with the proliferation of cellular technologies, the cost of portable devices has decreased significantly Nevertheless, these technologies still require a large user base to offset their costs Small satellite earth stations are widely used in developing regions, usually for distribution of TV signals and interactive voice/data Examples include the bank networks in remote parts of Brazil and the India National Information Center Network for government data services [2] However, the hardware costs and service charges of satellite communications are relatively high for the rural market [7] Wi-Fi and WiMAX — With the success of Wi-Fi technology for short-distance, local area network applications, there has been significant interest in using this technology for rural connectivity An example is the Digital Gangetic Plain project, developed jointly by IIT Kanpur, India and Media Lab Asia, where IEEE 802.11 technolo- IEEE Wireless Communications • June 2008 Authorized licensed use limited to: Huazhong University of Science and Technology Downloaded on December 9, 2008 at 20:43 from IEEE Xplore Restrictions apply 17 CHEN LAYOUT 6/5/08 1:24 PM WRAN systems will operate on the VHF/UHF TV bands, that is, from 54 MHz to 862 MHz, by opportunistically making use of the unused TV channels While doing so, it must ensure that no harmful interference is caused to the incumbent users Page 18 gies are used to provide long-haul access links This is achieved by using highly directional antennas mounted on tall structures and tuning 802.11 protocols to obtain a much longer coverage (more than 30 km) [5] Compared to cellular and satellite technologies, the advantages of WiFi include ease of set-up and maintenance, relatively high bandwidth, and low costs for both users and providers Another candidate technology is WiMAX, which is based on the IEEE 802.16 standards WiMAX is described as an enabling technology that provides last mile wireless broadband access as an alternative to cable and DSL Currently, WiMAX trials are being performed in several countries DakNet — Asynchronous Service Network — DakNet is a network architecture based on store-and-forward Instead of real-time services, DakNet provides remote communities with useful asynchronous Internet access The rationale are [7]: • Real-time communications are generally too expensive as a widespread investment for the nascent rural ICT market • Asynchronous ICT services appear to be sufficient to meet most of the needs of rural communities • Local information caches can be used to provide local users with immediate access to commonly requested information without the need for real-time Internet access The three major entities in DakNet are the hub, the mobile access point (MAP), and Info Kiosk A physical transport vehicle, for example, a car or motorbike, carries a MAP through a rural area where the DakNet service is provided When a MAP reaches within the communication range of an Internet hub or an Info Kiosk, it uploads or downloads e-mail, voice mail, and other offline data content [7] However, due to its asynchronous nature, DakNet is not suitable for disaster/emergency relief situations COGNITIVE RADIO AS AN EMERGING TECHNOLOGY In this section, we introduce the concepts of opportunistic spectrum access and cognitive radio After that, we describe how the IEEE 802.22 cognitive radio technology can be employed in rural communications and highlight challenges that must be overcome OPPORTUNISTIC SPECTRUM ACCESS Traditionally, the U.S Federal Communications Commission (FCC) regulates the radio spectrum resource by allocating separate frequency bands for different purposes Today, nearly 75 percent of the UHF band (300 MHz–3 GHz) has been allocated in this command and control manner Such a rigid and long-term allocation approach leads to spectrum under-utilization Recent measurements by the FCC show that 70 percent of the allocated spectrum in the United States is not utilized [11] This motivates the concept of opportunistic spectrum access that allows secondary networks to borrow unused radio spectrum from primary licensed networks [14] 18 The core technology behind opportunistic spectrum access is cognitive radio ([12]), which consists of the following components: • Spectrum sensing: cognitive radio devices can sense the spectrum environment to identify frequency bands that are not occupied by primary users • Dynamic spectrum management: cognitive radio networks can dynamically select the best available frequency bands for communications and monitor the spectrum environment to protect primary users • Adaptive communications: Cognitive radio devices can configure their transmission parameters to opportunistically make the best use of the ever-changing available spectrum Cognitive radio targets spatial and temporal spectrum white space by allowing secondary users to identify and exploit local and instantaneous spectrum availability in non-intrusive manners IEEE 802.22 — PROVIDING RURAL WIRELESS BROADBAND The IEEE 802.22 Working Group was formed in 2004 to develop a standard for wireless regional area networks (WRANs), based on cognitive radio technology [10] WRAN systems will operate on the VHF/UHF TV bands, that is, from 54 MHz to 862 MHz, by opportunistically making use of the unused TV channels While doing so, it must ensure that no harmful interference is caused to the incumbent users, which include TV receivers and FCC Part 74 wireless microphones [10] Figure illustrates a typical WRAN deployment that consists of a WRAN base station (WRAN BS) serving multiple fixed-location wireless customer premise equipment (CPE) Figure also shows a TV station and a wireless microphone system The WRAN must ensure that interference caused to all TV receivers within the TV protection contour (around 150 km from the TV station) is below a predefined threshold Similarly, there is a protection area for the microphone system, but with a much smaller size (a few hundred meters) For rural communications, 802.22 technology offers two main advantages, that is, no dedicated spectrum is required, and the coverage is large These two advantages help service providers address the cost-demand mismatch discussed earlier In particular, as no dedicated spectrum is required, service providers can save the cost of obtaining a spectrum license In addition, wide coverage is essential to reach a large customer base in rural areas 802.22 technology is designed to provide the average coverage of 33 km and can increase to 100 km Although most of the 802.22 related work has focused on various technical challenges, such as designing advanced spectrum sensing to detect weak TV and microphone signals or providing coexistence for multiple WRANs, we contend that these issues may not be critical for remote, rural WRAN deployments In particular, in remote areas, there may be TV channels that are always available for 802.22 access, that is, without any need of spectrum sensing Also, it is not IEEE Wireless Communications • June 2008 Authorized licensed use limited to: Huazhong University of Science and Technology Downloaded on December 9, 2008 at 20:43 from IEEE Xplore Restrictions apply 6/5/08 1:24 PM Page 19 likely that multiple WRANs will be deployed in a sparsely populated region Based on the previously mentioned arguments, we believe that the success of 802.22 as a technology for rural communications, significantly depends on whether a scalable service model can be introduced for the rural market, and whether the technology can be deployed in an efficient way A scalable service model that is tailored for rural customer demand and awareness is discussed later Regarding system efficiency, a major concern is that when TDD is used in WRANs with large coverage areas, the long TDD turn-around time will significantly reduce the system throughput We will discuss an approach to overcome this problem TV protection contour TV station COMBINING TV AND DATA SERVICES FOR RURAL AREAS Two important factors that determine the growth of the rural ICT market are cost and service content In terms of service content, as pointed out in [7] and other surveys, although there are many potential applications, in the short-term, only email, scan-mail, voice-over-e-mail, and chat are likely to be revenue-generating applications for the rural market We further contend that to increase the demand in the initial phase of network deployment, service providers should focus on traditional applications such as telephone and TV broadcasting Data services such as e-mail, Internet access, and video streaming, should be introduced gradually, in accordance with the adoption and awareness of rural customers Based on the above arguments, we propose that service providers start with a TV broadcasting service, as the demand and awareness already exist in rural communities This TV broadcasting can be combined with delay-tolerant, asynchronous data applications such as e-mail and voice mail These data applications can be scaled up in accordance with demand Furthermore, when the need arises, such as in emergency/disaster responses, TV broadcasting can be switched off to deliver command, control, and rescue information With this, we can eventually create a widespread wireless infrastructure that grows seamlessly with the rural communications market Figure illustrates how digital TV and data payloads can be multiplexed into the downlink (DL) channel, in both time and frequency WRAN BS Microphone protection area WRAN coverage ■ Figure IEEE 802.22 WRAN deployment Each WRAN system consists of a BS serving fixed wireless subscribers (CPE) Incumbent users are TV receivers and FCC's Part 74 wireless microphones domains, for a system that employs TDD and OFDMA, that is, similar to IEEE 802.22 In scenario (a), a digital TV payload is transmitted on the earlier time slots of the downlink subframe This is followed by downlink and uplink (UL) data payloads In scenario (b), both TV and DL data payloads can be transmitted simultaneously, but on orthogonal frequency subchannels These two configurations provide the flexibility to support different amounts of data payload, for example, e-mail, file transfer, and different latency requirements We note that there are existing technologies that support digital TV broadcasting, such as the European Digital Video Broadcasting standards (DVB-T for terrestrial and DVB-H for handhelds), the North America Advanced Television DL subframe (a) UL subframe Control and signaling Frequency UL data payload DL data payload Time TV payload Control and signaling Time Frequency Wireless microphones WRAN CPE TV payload DL data payload DL subframe (b) UL data payload CHEN LAYOUT UL subframe ■ Figure Different frame configurations that support combined digital TV and data services for a system that employs TDD and OFDMA (e.g., IEEE 802.22) IEEE Wireless Communications • June 2008 Authorized licensed use limited to: Huazhong University of Science and Technology Downloaded on December 9, 2008 at 20:43 from IEEE Xplore Restrictions apply 19 CHEN LAYOUT 6/5/08 1:24 PM Page 20 Systems Committee (ATSC) standards, and multichannel multipoint distribution service (MMDS) DVB and MMDS are also based on OFDM DVB and ATSC technologies only focus on TV broadcasting, not general data services MMDS can be used for both video and data services, but the high operating frequency (above GHz) makes it unsuitable for large rural coverage In our opinion, 802.22 technology that operates on VHF/UHF bands and supports the flexible frame structures in Fig can be a good solution to provide both TV and data services to the rural market ADAPTIVE TDD FOR LARGE RURAL COVERAGE TDD TURN-AROUND TIME The operation of a point-to-multipoint system employing TDD and OFDMA can be illustrated in Fig Time is divided into fixed-length frames; each consists of a DL subframe and an Time DL subframe OFDMA symbol UL subframe Distance BS DL subframe UL subframe CPE1 DL subframe UL subframe CPE2 Propagation delay from BS to CPE2 Switch from receiving to transmitting mode ■ Figure Structure of conventional TDD Uplink transmission from nearby CPE is delayed to align with uplink transmission from faraway CPE Time UL subframe DL subframe Distance BS Extra symbols gained by ATDD CPE1 CPE2 Propagation delay from BS to CPE2 Switch from receiving to transmitting mode ■ Figure Structure of proposed adaptive TDD Uplink transmission from nearby CPE1 arrives at the BS earlier than that from faraway CPE The OFDMA symbol boundaries of CPE and CPE uplink transmissions are synchronized at the BS 20 UL subframe that are used respectively for downlink and uplink transmissions In the time domain, DL and UL subframes are further divided into multiple OFDMA symbols; each consists of a set of orthogonal subchannels in the frequency domain Due to the difference in propagation delay from BS to the CPE, different CPE finish the downlink reception at different time instances Specifically, a nearby CPE can finish its DL reception long before a faraway CPE does This also means that a nearby CPE is ready to start uplink transmission before a faraway CPE is However, to guarantee reliable reception at the BS, the UL transmissions from different CPE must be scheduled in a way such that the OFDMA symbol boundaries are aligned at the BS The existing approach is to schedule the UL transmissions of all CPE based on the farthest one Specifically, even when nearby CPE finish DL reception and are ready for UL transmission, it is delayed so that UL transmissions reach the BS at the same time as those from the farthest CPE This can be illustrated in Fig 3, where the UL transmission of nearby CPE is delayed to align (at the BS) with that of the faraway CPE The delay CPE endures is equal to the difference in round-trip propagation delays between CPE and CPE As was mentioned, 802.22 rural networks should target large service areas, for example, up to 100 km in a coverage radius This means that the difference in the round-trip propagation delay between nearby and edge CPE can be significant For example, if the coverage radius is 100 km, then the difference in round-trip propagation delay will be {2×100×103/3×108} = 660 × 10 –6 s = 660 µs This delay, when compared to the typical frame duration of to 10 ms is significant and can seriously reduce the operation efficiency of 802.22 WRANs ADAPTIVE TDD Our proposed scheme, termed adaptive TDD (ATTD), allows the transition gap between DL and UL subframes to be CPE-dependant Specifically, after finishing their downlink reception, nearby CPE is scheduled to start uplink transmission first, and far-away CPE start uplink transmission later While doing so, the OFDMA symbol boundaries of all CPE are synchronized at the BS for reliable communications Adaptive TDD is illustrated in Fig When the difference between the round-trip propagation delays of nearby CPE and faraway CPE is comparable to the OFDMA symbol duration, CPE is allowed to start UL transmission right after finishing DL reception Faraway CPE is scheduled to start UL transmission later such that the OFDMA symbol boundaries of all CPE are aligned at the BS In Fig 4, at the BS, the first OFDMA symbol of CPE is aligned with the fourth OFDMA symbol of CPE THE IMPACT OF ADAPTIVE TDD When adaptive TDD allows nearby CPE to start uplink transmission early and gain extra OFDMA symbols, this throughput gain can be beneficial to all users in the system and for both IEEE Wireless Communications • June 2008 Authorized licensed use limited to: Huazhong University of Science and Technology Downloaded on December 9, 2008 at 20:43 from IEEE Xplore Restrictions apply 1:24 PM Page 21 OFDMA symbols DL for user DL for user Freqency subchannels Control and signaling Freqency subchannels OFDMA symbols UL for user UL for user DL for user DL for user DL subframe UL for user UL for user UL subframe UL for user DL for user DL for user DL for user UL for user DL for user 6/5/08 Control and signaling CHEN LAYOUT UL for user UL for user DL for user DL subframe Frame structure for conventional TDD UL for user UL subframe Frame structure for conventional adaptive TDD ■ Figure Exploiting the gain of adaptive TDD Two OFDMA symbols are gained by adaptive TDD, and the BS can use this gain to support one extra user (user 5) in both DL and UL NUMERICAL RESULTS To demonstrate the performance gain of adaptive TDD, we consider a 802.22 deployment scenario with the following parameters The cell radius is 50 km; all CPE located inside a km inner disk from the BS is regarded as nearby, and all CPE locating outside this inner disk is regarded as far away We vary the percentage of nearby CPE from percent to 90 percent, where the low and high percentages of nearby CPE, respectively, represent the case of uniform and center-concentrated CPE distributions As nearby CPE usually experience good channel conditions, they can transmit at higher rates compared to faraway CPE Here, we assume that all CPE inside the inner disk can transmit using 64-QAM and 3/4 code rate and that all CPE outside the inner disk can transmit using quadrature phase shift keying (QPSK) and 1/2 code rate When the number of extra OFDMA symbols gained is fixed, the percentage gain in uplink capacity depends on the frame size The shorter the frame size, the higher the percentage gain in uplink capacity We consider the frame sizes of 5, 10, and 20 ms The TV channel bandwidth is MHz, and OFDMA is based on 2048 fast Fouri- 60 Percentage increase in UL throughput uplink and downlink usage This is because the BS can exploit the throughput gain of adaptive TDD in different ways, for example, to give some users extra uplink bandwidth or to support more users in the uplink, or even to keep the uplink load unchanged and schedule more downlink transmission The above arguments are illustrated in Fig 5, where example bandwidth allocations are shown for the conventional TDD and the adaptive TDD approaches When conventional TDD is used, the BS can schedule DL and UL traffic for four users When adaptive TDD is used, two nearby users, namely, user and can transmit UL data two symbols earlier than the rest Due to this gain of two OFDMA symbols, the BS can schedule the DL and UL traffic for one extra user, namely, user 5, whereas the bandwidth allocated to the existing four users remains unchanged Frame size = ms, CP = 1/4 Frame size = ms, CP = 1/8 Frame size = 10 ms, CP = 1/4 Frame size = 10 ms, CP = 1/8 Frame size = 20 ms, CP = 1/4 Frame size = 20 ms, CP = 1/8 50 40 30 20 10 10 15 20 25 30 35 40 45 50 55 60 Percentage of nearby CPEs ■ Figure Throughput gain of adaptive TDD er transform (FFT) size with cyclic prefix set at 1/4 and 1/8 In Fig 6, we plot the percentage gain in the uplink throughput versus the percentage of nearby CPE With the chosen parameters, a CPE located inside the inner km disk can transmit UL data at one OFDMA symbol earlier than CPE located outside the inner disk As can be seen, the gain in average UL capacity when employing the proposed adaptive TDD scheme is significant The highest gain is around 30 percent, and the lowest gain is around percent The gain decreases when the percentage of nearby CPE increases This trend can be explained as follows The absolute gain, in terms of UL throughput, is almost constant (due to the fixed one OFDMA symbol gain) On the other hand, the absolute average throughput increases with the percentage of nearby CPE As a result, the percentage gain, which is equivalent to absolute IEEE Wireless Communications • June 2008 Authorized licensed use limited to: Huazhong University of Science and Technology Downloaded on December 9, 2008 at 20:43 from IEEE Xplore Restrictions apply 21 CHEN LAYOUT 6/5/08 1:24 PM We discussed two important issues that can affect the success of IEEE 802.22 technology for rural applications: providing a suitable rural service model; overcoming the problem of long TDD turn-around time in large rural cells Page 22 gain divided by the absolute throughput, will decrease as the percentage of nearby CPE increases CONCLUSIONS In this article, we began with a discussion on the challenges of rural communications and then reviewed existing wireless technologies that were implemented or proposed for this market We then focused on an emerging technology, cognitive radio, that promises to be a viable solution for rural communications The most notable example of a rural cognitive radio system is the IEEE 802.22 standard that currently is being developed, which is based on TDD, OFDMA, and opportunistic use of VHF/UHF TV bands We discussed two important issues that can affect the success of IEEE 802.22 technology for rural applications: • Providing a suitable rural service model • Overcoming the problem of long TDD turnaround time in large rural cells For the first issue, we introduced a service model that combines TV broadcasting and data services to facilitate the growth of rural demand for connectivity For the second issue, we proposed adaptive TDD technology that effectively eliminates the requirement for long TDD turnaround time and thus, increases the efficiency of large coverage, rural networks REFERENCES [1] Department of Economic and Social Affairs, United Nations, “World Urbanization Prospects: The 2005 Revision,” Oct 2006; http://www.un.org/esa/population/ publications/WUP2005/2005wup.htm [2] E Hudson, “Economic and Social Benefits of Rural Telecommunications: A Report to the World Bank,” June 1995 [3] M D Farrimond, “PCN and Other Radio-Based Telecommunications Technologies for Rural Regions of the World,” Proc 2nd Int’l Conf Rural Telecommun., London, U.K., 1990, pp 99–104 [4] R Westerveld and R Prasad, “Rural Communication in India Using Fixed Cellular Radio Systems,” IEEE Commun Mag., Oct 1994, pp 70–74 [5] RuralNet 802.11-Based Low-Cost Networking for Rural India; http://www.cse.iitk.ac.in/users/braman/dgp.html [6] Y Kawasumi, “Deployment of WiFi for Rural Communities in Japan and ITU’s Initiative for Pilot Projects,” Proc 6th Int’l Wksp Enterprise Networking and Computing in Healthcare Industry, 2004, HEALTHCOM 2004, June 2004 [7] A Pentland, R Fletcher, and A Hasson, “DakNet: Rethinking Connectivity in Developing Nations,” IEEE Computer, vol 37, no 1, Jan 2004, pp 78–83 [8] FCC, “Facilitating Opportunities for Flexible, Efficient, and Reliable Spectrum Use Employing Cognitive Radio Technologies, Notice of Proposed Rule Making and Order, FCC 03-322,” Dec 2003 [9] IEEE 802.22 WG Web site; http://www.ieee802.org/22/ 22 [10] IEEE 802.22 Wireless RAN, “Functional Requirements for the 802.22 WRAN Standard, IEEE 802.2205/0007r46,” Oct 2005 [11] Federal Communications Commission, “Spectrum Policy Task Force Report, FCC 02-155,” Nov 2002 [12] J Mitola, “Cognitive Radio for Flexible Mobile Multimedia Communications,” Proc IEEE Int’l Wksp Mobile Multimedia Commun., 1999, pp 3–10 [13] Y.-C Liang et al., “System Description and Operation Principles for IEEE 802.22 WRANs,” http://www.ieee802 org/22/, Nov 2005 [14] Y.-C Liang et al., “Sensing-Throughput Tradeoff for Cognitive Radio Networks,” IEEE Trans Wireless Commun., vol 7, no 4, Apr 2008, pp 1326–37 BIOGRAPHIES YING-CHANG LIANG [SM’00] (ycliang@i2r.a-star.edu.sg) is currently a senior scientist at the Institute for Infocomm Research (I2R), Singapore He also holds adjunct associate professorship positions in Nanyang Technological University and National University of Singapore His research interests include cognitive radio, reconfigurable signal processing systems for broadband communications, space-time wireless communications, and information theory From December 2002 to December 2003, he was a visiting scholar with the Department of Electrical Engineering, Stanford University At I2R he has been leading the research activities in cognitive radio and standardization activities in IEEE 802.22 WRANs He received Best Paper Awards from IEEE VTC-Fall 1999 and IEEE PIMRC 2005 ANH TUAN HOANG [M] (athoang@i2r.a-star.edu.sg) received a Bachelor’s degree (with First Class Honors) in telecommunications engineering from the University of Sydney in 2000 He completed his Ph.D degree in electrical engineering at the National University of Singapore in 2005 He is currently a research fellow in the Department of Networking Protocols, I2R His research focuses on design/optimization of wireless communication networks Specific areas of interest include cross-layer design, dynamic spectrum access, and cooperative communications HSIAO-HWA CHEN [SM’00] (hshwchen@ieee.org) is currently a full professor in the Department of Engineering Science, National Cheng Kung University, Taiwan, and he was the founding director of the Institute of Communications Engineering of the National Sun Yat-Sen University, Taiwan he received B.Sc and M.Sc degrees from Zhejiang University, China, and a Ph.D degree from the University of Oulu, Finland, in 1982, 1985, and 1990, respectively, all in electrical engineering He has authored or co-authored over 200 technical papers in major international journals and conferences, five books, and several book chapters in the areas of communications, including the books entitled Next Generation Wireless Systems and Networks and The Next Generation CDMA Technologies (Wiley, 2005 and 2007) He has been an active volunteer for various IEEE technical activities for over 20 years Currently, he is serving as chair of the IEEE ComSoc Radio Communications Committee and vice chair of the IEEE ComSoc Communications & Information Security Technical Committee He served or is serving as symposium chair/co-chair of many major IEEE conferences, including VTC, ICC, GLOBECOM, and WCNC, and so on He served or is serving as associate editor and/or guest editor of numerous important technical journals in communications He is serving as Chief Editor (Asia and Pacific) for Wiley's Wireless Communications and Mobile Computing Journal and Wiley's International Journal of Communication Systems Currently, he is Editor-in-Chief of Wiley's Security and Communication Networks Journal (http://www.interscience wiley.com/journal/security) He is also an adjunct professor at Zhejiang University, China, and Shanghai Jiao Tong University, China IEEE Wireless Communications • June 2008 Authorized licensed use limited to: Huazhong University of Science and Technology Downloaded on December 9, 2008 at 20:43 from IEEE Xplore Restrictions apply  ... a TV broadcasting service, as the demand and awareness already exist in rural communities This TV broadcasting can be combined with delay-tolerant, asynchronous data applications such as e-mail... payload DL data payload Time TV payload Control and signaling Time Frequency Wireless microphones WRAN CPE TV payload DL data payload DL subframe (b) UL data payload CHEN LAYOUT UL subframe ■... nearby CPE can finish its DL reception long before a faraway CPE does This also means that a nearby CPE is ready to start uplink transmission before a faraway CPE is However, to guarantee reliable