Low-Complexity Localized Walsh Decoding For CDMA Systems

As software radio becomes increasingly popular for wireless infrastructure, intelligent signal processing algorithms are required to make the most efficient use of the computational cycles available on general-purpose processors. In particular, the design of CDMA receiver algorithms for software radio is challenging because signals must be processed at rates much higher than other wireless standards. While traditional hardware implementations of the CDMA standard IS-95 despread signals received at the base station prior to Walsh decoding, in this paper we take advantage of the flexibility afforded by software implementations and develop three classes of Walsh decoding algorithms that do not require full despreading of incoming signals at the base station. Two proposed classes of algorithms exploit the fact that Walsh codes are locally decodable codes, which have the surprising property that any bit of the message can be recovered (with some probability) by examining only a small number of symbols of the codeword. We also describe a third class of algorithms based on code puncturing. All these algorithms enable trading off computation for performance, and we use them to dynamically minimize the computational requirements of the despreader and subsequent Walsh decoder such that a target bit-error rate is maintained with changing channel conditions. These algorithms are applicable to other CDMA-based systems that use Walsh codes for orthogonal modulation, and the third class is also applicable to CDMA-based systems that use codes for channelization, including lxRTT, EV-DO, and W-CDMA