ANDREWS LAYOUT 11/16/07 1:37 PM Page 84 AD HOC AND SENSOR NETWORKS Ad Hoc Networks: To Spread or Not to Spread? Jeffrey G. Andrews, University of Texas at Austin Steven Weber, Drexel University Martin Haenggi, University of Notre Dame ABSTRACT such networks will often have at least some con- nections to wired infrastructure, but we neglect Spread spectrum communication — often this for simplicity of discussion. called code-division multiple access — has been Spread spectrum transmission has long been widely adopted over the years for many types of considered attractive for ad hoc networking for a interference-challenged wireless communication number of reasons, including security and inter- systems including cellular and cordless tele- ference robustness [2, 3]. In this article we care- phones, wireless LANs and PANs, military appli- fully scrutinize the supposed advantages of using cations, and global positioning systems. In this spread spectrum — also known as code-division article we explore whether CDMA, in either its multiple access (CDMA) — in ad hoc networks. frequency hopping (FH) or direct sequence (DS) Similar to the highly contentious CDMA vs. time- form, is an appropriate design approach for division multiple access (TDMA) debate for cel- wireless ad hoc, or mesh, networks. One goal of lular systems, the considerations for ad hoc this article is to help provoke a debate by networks are also laden with subtleties. In cellular explaining the main advantages and disadvan- networks, despite CDMA’s apparent inferiority tages of CDMA in the context of ad hoc net- due to intentional self-interference, the exploita- works as exposed by recent research. We argue tion of voice activity, frequency reuse, and fast that CDMA does not inherently improve the power control were central to the ultimate success spectral efficiency of ad hoc networks; on the of CDMA. Analogously for ad hoc networks, it is contrary, its valued interference averaging effect crucial to adopt a network-level point of view that is not appreciable in ad hoc networks due to the includes considerations such as network capacity, irregular distribution of both the transmitters end-to-end delay, energy efficiency, channel and receivers. On the positive side, both types access, and routing. However, the key traits of (FH and DS) of spread spectrum allow for CDMA in an ad hoc network are very different longer hop distances and a reversal of the usual than in cellular networks with centralized trans- relationship where the desired transmitter must mitters (downlink) and receivers (uplink). We be closer to the receiver than interfering trans- now summarize the key traits of CDMA in ad hoc mitters. These two facts allow for significant networks in terms of pros and cons, which are jus- advantages over narrowband systems in terms of tified in detail in the body of the article. energy efficiency and end-to-end delay. THE ADVANTAGES OF CDMA IN INTRODUCTION AD HOC NETWORKS Applications of wireless ad hoc networks have The advantages of CDMA in ad hoc networks expanded in recent years to include not only are quite different than in cellular networks, and numerous military applications and emerging can also be distinct for the two different types of wireless sensor networks, but also many other CDMA, frequency hopping (FH) and direct exciting and commercially viable applications sequence (DS). FH and DS are described in including wireless community broadband access, more detail later. backhaul for wireless LAN access points, and Longer hops. DS and FH both allow for range extension for cell-based networks [1]. longer hops to be undertaken for a given net- This research was sup- Despite this high level of interest and commer- work density. This allows more direct routing, ported by NSF grant no. cial potential, many basic ad hoc network design reduced end-to-end delay, and perhaps counter- 0635003 (Weber), no. principles are still not well understood, and one intuitively, reduced energy consumption. 0634979 (Andrews), and important design question is the focus of this Capacity enhancements. Neither FH nor DS the DARPA IT-MANET article: Does it make sense to use spread spec- increases the overall ad hoc network capacity on its program, grant no. trum in ad hoc networks? In this article an ad own, in fact the opposite is true for DS. However, W911NF-07-1-0028 (all hoc network implies communication without the DS allows for the possibility of capacity-increasing authors). assistance of wired infrastructure. Naturally, interference canceling receivers, which are ineffec- 84 0163-6804/07/$20.00 © 2007 IEEE IEEE Communications Magazine • December 2007 ANDREWS LAYOUT 11/16/07 1:37 PM Page 85 Original signal Received signal Spread spectrum Interference P/No w/prob. (M – 1)/M uses noise-like code B SINR = P/(No + Z) w/prob. 1/M P sequences to effectively increase Z Randomly the bandwidth to be hop er Freq. hopping between far greater than the w er M freq. o signal bandwidth. w P slots o P When spread spectrum is used to support multiple Frequency Slot 1 Slot 2 Slot m Slot M users, it is called Frequency CDMA. I Figure 1. Frequency hopping works by randomly picking one of M frequency slots. A narrowband signal of similar bandwidth is usually avoided as M increases. tive in ad hoc networks unless DS is used. Both FH CDMA’s viability in ad hoc networks with mobili- and DS also provide considerable frequency diversi- ty or bursty traffic, since these scenarios require ty, which helps overcome narrowband fading. frequent code acquisition and synchronization. Network efficiency. The ability of DS-CDMA systems to successfully operate under low signal- CDMA: A MODERN OVERVIEW to-interference-plus-noise ratio (SINR) permits communication with a larger number of potential INTERFERENCE-LIMITED NETWORKS AND THE interferers, which simplifies network coordination. RECENT SUCCESS OF CDMA Security. Spread spectrum radios have innate security features: they are harder to jam, they Dense wireless networks are by nature interference- make eavesdropping more difficult, and their limited, which means that increasing the transmit presence is more difficult to detect. Although power of all nodes in the network simultaneously important for some applications, these topics are will not substantially increase the overall through- beyond the scope of this article. put of the network. Ad hoc networks pose a partic- ularly challenging interference environment because THE DISADVANTAGES OF CDMA IN the lack of agreed upon centralized transceivers AD HOC NETWORKS means that each receiver in the network must bound the level of interference in its vicinity to suc- A common drawback of CDMA in cellular and cessfully receive the desired transmission. ad hoc networks is that the system bandwidth Spread spectrum uses noise-like code needs to be considerably larger than the (per sequences to effectively increase the bandwidth to user) symbol rates. In ad hoc networks CDMA be far greater than the signal bandwidth. When has two other important drawbacks: spread spectrum is used to support multiple users, Interference averaging is ineffective. Interfer- it is called CDMA. The central tenet of CDMA is ence averaging, the hallmark of both DS- and that designing for time or frequency orthogonality FH-CDMA in cellular networks, does not pay (as in TDMA or FDMA) is not appropriate, since off in ad hoc networks. The key reason is the neighboring (i.e., intercell) interference and other lack of a centralized receiver and the associated imperfections would compromise the orthogonali- power control to that receiver. Global power ty anyway. On the other hand, CDMA tolerates control is impossible in ad hoc networks; instead, all sources of interference within bounds deter- usually just one or perhaps two interfering nodes mined by the spreading gain. Due to its robust- dominate the interference power, which makes ness, system capacity, and other implementation interference avoidance (via scheduling or slow and political/economic factors, CDMA overcame FH) far more effective than interference averag- extreme levels of early skepticism to become the ing (using DS or fast FH to proportionally underlying physical layer technology and multiple reduce the interference level). access scheme for all three important third-gener- Considerable setup costs. CDMA requires ation cellular standards: cdma2000, wideband that both the transmitter and receiver have CDMA (WCDMA), and TD-SCDMA. Based on knowledge of: this success, it is natural to seriously consider the • Agreed upon spreading (DS) or hopping viability of CDMA for the emerging class of ad (FH) sequences hoc and mesh networks. • The current position in the sequence (i.e., time synchronization) FREQUENCY HOPPING AND Acquisition of these is a nontrivial resource-con- DIRECT SEQUENCE CDMA suming process, and unless the cost of code acqui- sition and synchronization is amortized over time, CDMA techniques have historically been divided the above “pros” of CDMA may not justify this into two very different types of modulation: fre- overhead. In practice this raises questions about quency hopping and direct sequence. In this arti- IEEE Communications Magazine • December 2007 85 ANDREWS LAYOUT 11/16/07 1:37 PM Page 86 Original signal Received signal Processed received signal B P Interference P SINR = P/No + Z/M um r Z ct Spreading Despreading Interference pe s er w o W = M B P P/M Z/M Frequency Frequency Frequency I Figure 2. Direct sequence works by spreading the signal over a larger bandwidth. After processing, the desired narrowband signal re- emerges while other interference is attenuated by a factor of M. cle CDMA without further qualification refers col- is its reliance on accurate power control. In ad lectively to both of these techniques. FH is depict- hoc networks, however, equal received powers ed in Fig. 1. The total bandwidth W is divided into are impossible due to the random node positions M frequency bands of bandwidth B = W/M.