Innovative coding scheme for spread-spectrum communications

It is known that spread-spectrum (SS) communications in general, and code-division multiple access (CDMA) in particular, gain certain advantages in performance, if powerful low-rate error-correcting codes are used for error-protection in such systems. We propose an innovative coding scheme for SS systems based on Gallager (low-density parity-check) codes that can be iteratively decoded with relatively low complexity. Consequently, this scheme is readily implementable in both base station and subscriber side receivers. We focus our attention on Gallager codes with parameters compatible with the IS-95 cellular radio standard. We discuss soft- and hardware implementations of iterative decoding for Gallager codes. We estimate that by using Gallager codes, a factor of five improvement in CDMA cell capacity relative to the uncoded system can be achieved, equivalent to a factor of two improvement relative to the state-of-the art orthogonal convolutional codes