Introduction

In Chapter 4, we basically studied how to assign time slots to links such that the simultaneous transmissions will be interference-free. In Chapter 5, we studied how to assign frequency channels to wireless terminals such that (1) they are interference-free; i.e., the spectrums assigned to nearby terminals will be disjoint; or (2) the links formed by the assigned channels will form a network with certain networking properties such as being connected. In this chapter, we study the channel assignment to wireless networks when the channels are defined by CDMA codes.

Code-division multiple access (CDMA) provides a higher capacity, flexibility, scalability, reliability, and security than conventional frequency-division multiple access (FDMA) and time-division multiple access (TDMA). It has already been widely deployed in 2G cellular communication systems and was proposed for the emerging and future wireless systems, including WLANs and wireless ad hoc networks. In a CDMA system, the communication channels are defined by 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 are transmitted to ensure that at least one gets through. The number of duplicates, which is equal to the length of the codeword, is known as the spreading factor. The inverse of the length of the codeword is known as the rate of the codeword.