This paper develops a system design for clustered wireless ad hoc networks, combining CSMA and CDMA to enable spatial reuse and simultaneous transmissions. Typically, CDMA networks require fine-tuned power control, but here that requirement is eliminated through a combination of open loop power control, user ordering, and successive interference cancellation (SIC). A network topology with high network awareness via a broadcast CSMA channel is developed. The resulting system increases network throughput and overcomes existing problems with IEEE 802.1 1.
I N T R O D U C T I O NWireless ad hoc networks, characterized by the lack of wired infrastructure, pose unique challenges in the multiple access design. Extensive research exists on code division multiple access (CDMA) for cellular systems. The multiple access capability, capture, and anti-multipath capability of spread spectrum are the main reasons that CDMA is the dominant choice for the physical layer of third generation cellular systems. Despite notable recent work [l] [2], stringent power control requirements in the absence of central infrastructure are still a primary reason why CDMA has not been exploited at the physical layer of ad hoc networks. The physical layer in IEEE 802.11 uses carrier sense multiple access with collision avoidance (CSMNCA) and supports only one transmission at a time within a transmission range. A successful packet transmission happens only when one packet appears in a time slot. In a dense network. this results in low soectral efficiencv it communicates with different nodes that may have dramatically different channels. These highly variable channels result from three primary causes (51: obstructing objects, distancedependent path loss, and small-scale Rayleigh fading. In this paper, we propose a simple open-loop method to compensate for the first two effects, and the rapid Rayleigh fading is compensated for by ordering users and using SIC 161 at the receiver. Preliminary work done in [7] shows that under fading, performance of a CDMA system incorporating SIC is comparable to that of a conventional CDMA receiver using ideal power control. However, their approach assumes perfect channel estimates. In this paper, we present an approach which incorporates SIC for ad hoc networks in the presence of imperfect channel estimates [SI, and adapt it to an ad hoc network.SIC is a nonlinear type of multiuser dztection (MUD) shown in Fig. 1 in which users are decoded successively [61,[71, [SI. The approach successively cancels the strongest remaining user, IC, by re-encoding (with PN sequence ck) the decoded bits, d k and after making an estimate of the channel, the interfering signal, zk is generated at the receiver. This is then subtracted from the received wavefonn, y ( t ) . In this manner later users do not encounter multiple access interference (MAI) caused by previous users. SIC improves performance for all users: initial users improve because the later users require less power which means less MA1 for the initial users, and later use...
