The Role of Coding in CDMA Systems With Multiuser Detection

In a direct sequence code division multiple access (CDMA) system, a xed bandwidth expansion could be allocated to either error correcting coding or modulation signal spreading. The role of coding in CDMA systems that employ multiuser detection is explored here. A comparison is made between the maximum throughputs (in bits/chip or bits/s/Hz) achievable using multiuser detection and single-user detection, for both \single-cell" and cellular systems. For the single-cell scenario, our results indicate that asyn-chronous multiuser detection systems employing long (random) spreading sequences and low rate coding, can provide considerable improvements in spectral eeciency over single-user detection systems. However, for cellular systems with universal frequency reuse, the improvement is spectral ee-ciency (per cell) ooered by multiuser detection may not be as signiicant. I INTRODUCTION In code-division multiple-access (CDMA) systems, redundancy via bandwidth expansion is used to counter multiple-access interference from other users, and other sources of interference 1]. In practical CDMA systems, this redundancy may be introduced in two ways: spreading and error-control coding. Spreading can be considered to be a trivial form of coding, with zero coding gain, that is used purely for signal separation in the demodulation process 2]. Spreading may be used to orthogonalize users to mitigate multiple-access interference in synchronous systems; and knowledge of spreading sequences of other users may be used for multiuser detection in asynchronous systems. Error control coding, on the other hand, is used to correct errors caused by residual multiple access interference (treated as random noise) and other sources of interference. Practical considerations dictate that the error-control codes are encoded and decoded separately for each of the users. The decoder for a particular user, receives soft or hard decision outputs, relevant to that user, from the demodulator. The decoder uses neither the information about the inter-ferers codes nor their spreading sequences. Spreading and coding both play useful roles in CDMA systems; thus a natural question to ask is how a given band-width expansion factor should be allocated between coding and spreading. An appropriate goal might be to choose the coding and spreading factors to maximize the spectral ee-ciency of the system. A useful measure of spectral eeciency is the system throughput 3], deened to be the total number of bits (summed over all users) that can be transmitted reliably on the channel in one chip interval using ideal error control coding/decoding for all users 1. The coding-spreading tradeoo for the various users may …