Review on codes for multiple access communications

The radio spectrum is a finite resource and it is important that it is to be utilized efficiently by all users. Steps are required to be taken to deploy modulation and multiple access techniques that improve spectrum utilization. Code division multiple access (CDMA) is a multiple access technique based on spread spectrum (SS) techniques. CDMA users are uniquely identified by a code sequence embedded as an address within the carrier waveform. To recover the signal, the receiver cross correlates the sum of incoming signals against an address sequence. Recovering the desired data, requires the received signal to have properties that minimize the received signal to have properties that minimize the interference resulting from the presence of other signals. CDMA systems are dependent on the design of codes that are orthogonal to one another to minimize interference resulting from the presence of other signals. A goal of SS system designer for multiple access communications is to find a set of spreading codes (or) sequences such that as many users as possible can use a band of frequencies with as little interference as possible. This paper reviews the codes used for CDMA applications. A detailed description has been given on the generation of codes, its properties and its applications based on its properties along with the example system.