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Great Expectations: the Value of Spatial Diversity in Wireless Networks

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Great Expectations: the Value of Spatial Diversity in Wireless Networks Great Expectations: The Value of Spatial Diversity in Wireless Networks SUHAS N. DIGGAVI, MEMBER, IEEE, NAOFAL AL-DHAHIR, SENIOR MEMBER, IEEE, A. STAMOULIS, MEMBER, IEEE, AND A. R. CALDERBANK, FELLOW, IEEE Invited Paper In this paper, the effect of spatial diversity on the throughput The challenge here is that Moore’s Law does not seem to and reliability of wireless networks is examined. Spatial diversity apply to rechargeable battery capacity, and though the den- is realized through multiple independently fading transmit/re- sity of transistors on a chip has consistently doubled every ceive antenna paths in single-user communication and through independently fading links in multiuser communication. Adopting 18 mo, the energy density of batteries only seems to double spatial diversity as a central theme, we start by studying its every 10 years. This need to conserve energy (see [2] and ref- information-theoretic foundations, then we illustrate its benefits erences therein) leads us to focus on what is possible when across the physical (signal transmission/coding and receiver signal signal processing at the terminal is limited. Throughout this processing) and networking (resource allocation, routing, and paper, we use the cost and complexity of the receiver to applications) layers. Throughout the paper, we discuss engineering intuition and tradeoffs, emphasizing the strong interactions be- bound the resources available for signal processing. Wireless tween the various network functionalities. spectrum itself is a valuable resource that also needs to be conserved given the economic imperative of return on multi- Keywords—Ad hoc networks, channel estimation, diversity, fading channels, hybrid networks, information theory for wireless billion-dollar investments by wireless carriers [1]. communications, multiple-antenna communications, multiuser The fundamental distinction of wireless communication diversity, real-time applications, resource allocation, scheduling, is the ability to communicate on the move [71]. This rep- signal processing for communications, space–time coding (STC), resents both a freedom to the end user and also a challenge transport protocols, wireless networks. to the system designer. Wireless communication hinges on transmitting information riding on radio (electromagnetic) I. INTRODUCTION waves and, hence, the information undergoes attenuation ef- fects (fading) of radio waves (see Section II for more details). Great expectations start with the mobile terminal and the These attenuation effects could also vary with time due to promise of 3G1 data rates, real-time internet protocol (IP) user mobility, making wireless a challenging communication services2 (including voice), and hours of useful battery life. medium. The distinction from wired (twisted-pair, coaxial cable, Manuscript received October 16, 2003; revised November 5, 2003. optic fiber) communication is that the transmission channel is S. N. Diggavi was with the AT&TShannon Labs, Florham Park, NJ 07932 unpredictable (random) and could vary over very short time USA. He is now with Swiss Federal Institute of Technology, Lausanne, CH 1015 Switzerland (e-mail: [email protected]). scales (order of microseconds). Given the limited power re- N. Al-Dhahir was with the AT&T Shannon Labs, Florham Park, NJ 07932 sources, this leads to new challenges in signal transmission USA. He is now with University of Texas at Dallas, Richardson, TX 75083 (see, for example, [207]). Furthermore, since the medium is USA (e-mail: [email protected]). A. Stamoulis is now with Qualcomm, San Diego, CA 92121 USA (e-mail: inherently shared by several users, the equitable sharing of [email protected]). limited resources is an important challenge. In addition, the A. R. Calderbank was with the AT&T Shannon Labs, Florham Park, NJ freedom of mobility also implies that end users need to be 07932 USA. He is now with the Program in Applied and Computational Mathematics, Princeton University, Princeton, NJ 08544 USA (e-mail: located in order to deliver information to them. This leads [email protected]). to a challenge in dealing with the changing topology of the Digital Object Identifier 10.1109/JPROC.2003.821914 wireless network. Finally, the wireless device would typi- 1G: Generation. Current wireless systems are 2G or 2.5G, where rudimen- cally connect to the wired Internet infrastructure and a major tary data services are being deployed. challenge is the interoperability of these disparate media. 2See [65] for a discussion of architecture and philosophy. These constitute formidable challenges to the designer and 0018-9219/04$20.00 © 2004 IEEE PROCEEDINGS OF THE IEEE, VOL. 92, NO. 2, FEBRUARY 2004 219 Fig. 1. Roadmap to the paper. this paper explores some modern analysis and design tools Most of our emphasis is on delay-bounded services, hence, that can help surmount these challenges to meet the great ex- on outage capacity, where the usual assumption is that fading pectations. is quasi-static, that is constant over a frame of data, and then The main characteristic of wireless communication is changing in an independent manner from frame to frame. the uncertainty (randomness): randomness in users’ trans- Recall that a 3% outage capacity (a typical assumption for mission channels and randomness in users’ geographical voice services) is the transmission rate that can be achieved locations. These, in turn, lead to random signal attenuation 97% of the time. Moreover, multiple-user environments independently across users (see Section II). This random- bring to fore fundamental questions of resource sharing. We ness can also be used as a tool to enhance performance examine how diversity impacts per-user performance also through diversity. We broadly define diversity as the method in Section III. of conveying information through multiple independent Two antennas at the wireless access point (base station) instantiations of these random attenuations. The diversity provide two independent paths from the base station to the tool we utilize is that of spatial diversity, through multiple mobile. By distributing information across the two paths, and antennas and multiple users. Spatial diversity has emerged by appropriate signal processing at the receiver, we in effect as the single most important tool for reliable wireless com- construct a single channel that is better than either path. Su- munication. The focus in this paper is on spatial diversity, perposition of fading on these two paths at the receiver re- through spatially separate antennas, which in their simplest duces the variation in received signal strength at the mobile. instantiation are used to provide replicas of the transmitted Reduced channel fluctuations allows smoother and more ef- signal to the receiver. The independent links (due to inde- ficient power control, since the base station is continually ad- pendent locations) of different users can also be used as a justing transmit power on the basis of reported signal strength form of multiuser diversity. In addition, diversity also can at the mobile. At a systems level, this means that the base be utilized through temporal and frequency domains. The station requires less power to support existing users, or that topic of this paper is the various methods by which spatial more users can be supported for a given constraint on radi- diversity can contribute to reliable wireless communication ated signal power at the base station. The gains are signif- and is summarized in Fig. 1. icant; for example, system studies by Parkvall et al. [199] Information theory [70], [112] provides tools to estab- show up to 100% increase in the number of users that can lish fundamental limits on information transmission. In be supported on the wideband code-division multiple access Section III we review such fundamental limits on spatial di- (WCDMA) downlink from the base station to the mobile ter- versity, both in single-user and multiple-user environments. minal. We review the results on single-user multiple-antenna The earliest form of spatial transmit diversity is the delay capacity established by Telatar [245] and by Foschini [103]. diversity scheme proposed by Wittneben [282] where a 220 PROCEEDINGS OF THE IEEE, VOL. 92, NO. 2, FEBRUARY 2004 signal is transmitted from the second antenna, then de- functional separation and partition of the traditional network layed one time slot, and transmitted from the first antenna. protocol stack [65], [159]. We discuss this in Section V Signal processing is used at the receiver to decode the in the context of spatial diversity in wireless networks. superposition of the original and time-delayed codewords. Section V also addresses ad hoc wireless networks where Viewing multiple-antenna diversity as independent conduits there might be multiple wireless hops, and where mobility of information, more sophisticated transmission (coding) has a significant impact on capacity [81], [126]. Here, the schemes can be designed to get closer to the fundamental definition of cross-layer protocols, specifically the balance limits discussed in Section III. Using this approach, we focus between transmission to update routing information and on space–time codes defined by Tarokh et al. [242] and transmission of information, is central to design. Alamouti [14] that introduce correlation of signals across the The main issue to note in investigating these interlayer different transmit antennas. Space–time codes can provide interactions is that of the time
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