Differential MMSE: A Framework for Robust Adaptive Interference Suppression for Direct-Sequence CDMA Over Fading Channels

Abstract

In this paper, we investigate the performance of a generic fil- terbank transceiver in the presence of a carrier frequency offset. We pro- pose a theoretical model to compute the error probability in the case of a generic frequency-selective channel. Moreover, the proposed method is also extended to deal with nonuniform, i.e., wavelet, transceivers. The ac- curacy of the model is evaluated by means of computer simulations, using several types of filterbank transceivers. Systems based on cyclic prefix (CP) and zero padding (ZP) ar considered to avoid interblock interference (IBI) in frequency-selective channels. The analytical results obtained with the proposed method allow us to quickly compare different systems, charac- terized by a different filterbank selectivity, as well as by different methods to combat IBI (ZP or CP). Abstract—Various log-likelihood-ratio-based belief-propagation (LLR- BP) decoding algorithms and their reduced-complexity derivatives for low- density parity-check (LDPC) codes are presented. Numerically accurate representations of the check-node update computation used in LLR-BP decoding are described. Furthermore, approximate representation of the decoding computations are shown to achieve a reduction in complexity, by simplifying the check-node update or symbol-node update, or both. In par- ticular, two main approaches for simplified check-node updates are pre- sented that are based on the so-called min-sum approximation coupled with either a normalization term or an additive offset term. Density evolution is used to analyze the performance of these decoding algorithms, to determine the optimum values of the key parameters, and to evaluate finite quanti- zation effects. Simulation results show that these reduced-complexity de- coding algorithms for LDPC codes achieve a performance very close to that of the BP algorithm. The unified treatment of decoding techniques for LDPC codes presented here provides flexibility in selecting the appro- priate scheme from a performance, latency, computational complexity, and memory-requirement perspective.