Coded Spreading with m-sequences

Abstract

Maximum length shift register (MLSR) sequences are utilized for joint channel coding and data spreading. Compared to uncoded spreading, more than 5 dB coding gain is obtained at BER of 10/sup -3/ in additive white Gaussian noise (AWGN) channels. The coding gain increases up to 22 dB in Rayleigh fading channels. In the proposed algorithm, information bits are segmented into blocks and coded by cyclic MLSR coding. The code word output of the systematic MLSR encoder (m-sequence) is then used for spreading the uncoded information bits in each data block. At the receiver, the autocorrelation property of the m-sequences is used for mutual despreading and decoding of the received signal. An optimum soft decision decoder is implemented by a parallel bank of correlators, which are matched to each spreading sequence. Despreading and channel decoding operations are performed concurrently. The ingenuity of the algorithm lies in the fact that all the redundant channel coding bits are carried by spreading sequences, and thus avoids energy per information bit loss due to channel coding. Utilizing the same spreading code for all the data bits in the block provides diversity, which improves the performance substantially in fading channels.