A detailed study of a CDMA based approach to enhance ad hoc network performance

Random access schemes operate typically on a contention based common channel, which brings problems with increasing traffic load. These problems are emphasized in a multi-hop wireless ad hoc network environment. Efficient collision avoidance methods are needed, but they also tend to decrease the spatial capacity of the network. With CDMA (Code Division Multiple Access), it is possible to make multiple simultaneous co-located successful transmissions and thus increase network capacity within the limits of multiple access interference (MAI). Bi-code channel access (BCCA) is a method to apply CDMA in ad hoc networking, providing also a common access channel for network connectivity maintenance and self-configuration. With BCCA, collisions are rare, and thus, more straightforward medium access control (MAC) methods can be used. A MAC solution designed especially for BCCA (BC-MAC) and ad hoc networking increases the efficiency of the channel usage. As BCCA is based on receiver code CDMA, the spreading code of the next hop node is needed. A novel network layer spreading code distribution (NSCD) method is proposed for this purpose. The spreading codes are distributed within routing protocol control packets at the route establishment phase.A detailed study on the performance behavior of different methods is provided. Also, interesting aspects considering the ad hoc network operation are discussed. Instead of the widely used unrealistic cut propagation model, a propagation model with realistic MAI calculation is used in this study. It is shown that the use of NSCD increases the network control load only slightly, and that the performance is practically the same as it is with the assumption of known spreading codes. BCCA with its dedicated MAC and NSCD clearly outperforms commonly used methods (like IEEE 802.11) and yet is easy to implement, robust for design parameters under different conditions while maintaining the original idea of ad hoc networking.