Interference Reduction in CDMA Mobile Radio Systems
One of the most important performance limiting factors in CDMA mobile radio systems is the multiple access interference. Consequently, some of the proposals for third generation mobile radio systems as for instance the Chinese TD-SCDMA already incorporate simple joint detection techniques like linear zero forcing estimation. However, these linear joint detection techniques are rather complex and can only be applied in systems with low numbers of simultaneously active users, i.e., CDMA systems with an additional TDMA component. Furthermore, the performance of linear joint detection especially in scenarios with high system loads, i.e., numbers of users close to the spreading factor, is unsatisfactory. At the Research Group for Radio Communications advanced joint detection techniques which combine low complexity and superior performance are developed. These advanced joint detection techniques rely on the turbo principle. The FEC code typically used in mobile radio systems and the CDMA spreading are considered as a serially concatenated code. A low complexity high performance decoder for such serially concatenated codes consists of two decoders alternately decoding the two codes and exchanging extrinsic information between the two decoders. A generalized detector architecture, which includes turbo detectors as well as conventional joint detectors like the zero forcing estimator or parallel interference cancellation is designed. By using sliding window techniques the above mentioned advanced joint detection techniques can also be applied in CDMA systems without a TDMA component, e.g. W-CDMA.