OVSF-CDMA Code Assignment in Wireless Ad Hoc Networks

CDMA (code division multiple access) provides higher capacity, flexibility, scalability, reliability and security than conventional FDMA (frequency division multiple access) and TDMA (time division multiple access). It has already been widely deployed in the second generation cellular communication systems and was proposed for the emerging and future wireless systems, including the third generation cellular systems, wireless local area networks, and wireless ad hoc networks. In a CDMA system, the communication channels are defined by the pseudo-random codewords, which are carefully designed to cancel each other out as far as possible. Each communication utilizes the entire available spectrum, and every bit of data is multiplied by the codeword used by the communication channel. Thus, many duplicates of the same information is transmitted and received to ensure that at least one gets through. The number of duplicates, which is equal to the length of the codeword, is know as the spreading factor. The inverse to the length of the codeword is known as the rate of the codeword. There is a trade-off on the length of the codewords. On one hand, longer codewords can increase the number of channels and the robustness of the communications. On the other hand, longer codewords would result in lower date rate of the communication channels since the raw rate seen by the user is inverse to the codeword length. The Walsh code, used by the cdmaOne cellular system, consists of 64 codewords, each 64-bits long.