Forward error correcting for CDMA systems

Spread spectrum systems especially code division multiple access systems (CDMA) match very well with channel coding employing forward error correcting (FEC) methods. We will cover in this overview article some aspects of FEC channel coding in CDMA systems: CDMA/FEC systems can be viewed as concatenated schemes with inner and outer decoding. We will show how such a system can be improved by using a soft-in/soft-out decoder which transfers a soft decision from the inner to the outer decoder. Further improvement is gained by using another soft-in/soft-out decoder for the outer code. In an iteration the soft values of the outer code bits are then used as a priori values for the inner CDMA decoder/despreader. This can he supplemented by soft decoded bits from the other users' outer decoder similar to an interference cancellation scheme. Applications to the IS 95 system with Hadamard codes show a remarkable gain from the soft-output and from the iterations. 'Turbo' decoding of two and more dimensional product-like codes with iterative decoding using 'soft-in/soft-out' decoders achieves almost channel capacity. We extend this concept to CDMA systems with PN spreading sequences where FEC coding in one dimension is performed in the time direction and FEC coding in the other dimension is performed by a systematic code in the CDMA code direction. The parity bits of this code are transmitted by extra CDMA codes. The additional interference degradation caused hereby is outweighted by the extra FEC coding gain achieved by iterative decoding.