Coded Random CDMA with Partitioned Spreading

A Code Division Multiple Access (CDMA) system is considered, where a number of concurrent users, distinguished by random spreading waveforms, access the common additive white Gaussian noise (AWGN) channel. All users employ a regular low-density parity-check (LDPC) error control code (ECC). Additionally, a method, called Partitioned Spreading (PS) is used. Each user’s spreading waveform is divided into M partitions which are interleaved and spread in time. Asymptotic performance of the proposed system is studied for two different decoding schedules. In the first schedule, called two-stage schedule, iterative multiple access (MA) detection and LDPC decoding are executed separately. The second schedule, named full decoding allows to exchange information between MA and LDPC sides. Supportable system loads are derived as functions of the users’ signal-to-noise ratio (SNR). It is shown that significantly higher system loads can be achieved compared to the traditional LDPC coded random CDMA.