Ad Hoc Networks: To Spread or Not to Spread?
Jeffrey G. Andrews, Martin Haenggi · 2007
Spread spectrum communication – often called Code Division Multiple Access (CDMA) – has been widely adopted over the years for many types interference-challenged wireless communication systems including cellular and cordless telephones, wireless LANs and PANs, military applications, and global positioning systems. In this paper, we explore whether CDMA – in either its frequency hopping (FH) or direct sequence (DS) forms – is an appropriate design approach for wireless ad hoc, or mesh, networks. CDMA’s merits for centralized cellular networks were widely debated in the late 1980s through the 1990s, and greatly increased the understanding of CDMA in particular and interference-limited cellular networks in general. Such a discussion has not occurred for decentralized (ad hoc) networks, and one goal of this paper is to help provoke this debate by explaining the main advantages and disadvantages of CDMA in the context of ad hoc networks as exposed by recent research. We will argue that CDMA does not inherently improve the spectral efficiency of ad hoc networks; on the contrary, its valued interference averaging effect is not appreciable in ad hoc networks due to the irregular distribution of both the transmitters and receivers. On the positive side, both types (FH and DS) of spread spectrum allow for (i) longer hop distances and (ii) a reversal of the usual relationship that the desired transmitter must be closer to the receiver than interfering transmitters. These two facts allow for significant advantages over narrowband (NB) systems in terms of energy efficiency and end-to-end delay. We also examine the considerable effect that a spread spectrum physical layer has on MAC protocols. 1