Multiuser Serially Concatenated Continuous Phase Modulation
A multiuser communication system using serially concatenated and randomly interleaved continuous phase modulation (SCCPM) over the additive white Gaussian noise (AWGN) channel is investigated. The users, which may be asynchronous, are allowed to have individual energy levels as well as carrier frequencies and phases. This model incorporates multiple-access signaling similar to direct-sequence code division multiple-access (DS-CDMA), trellis-coded multiple-access (TCMA), and frequency division multiple-access (FDMA) with arbitrary spectral overlap, as well as non-intentional co-channel or adjacent channel interference of the same signaling type. First, the system is analyzed through analytical upper bounds on the average bit error probability for a given user under maximum-likelihood (ML) detection, where the R> average is over the ensemble of systems over all sets of interleavers. It is shown that in a properly designed system, the bit error probability vanishes for infinite interleaver sizes and a sufficiently large channel signal-to-noise ratio (SNR), regardless of the signal correlation between the users. Thus, even with equal modulation, energy levels, and carrier frequencies and phases, the users can be detected adequately provided they employ random interleaving. The second part of the analysis concerns iterative decoding of multiuser SCCPM. A convergence analysis based on EXIT charts is presented, along with decoding threshold estimates. It is observed that in systems with no frequency offset, the performance of ML detection does not always carry over to iterative decoding. On the other hand, for many other systems excellent performance can be obtained both in terms of power efficiency (bit error rate as a function of SNR) and spectral efficiency (bandwidth). In particular, systems are demonstrated with performance within 1 dB of the average-power limited Shannon capacity at 1 bit/s/Hz, and within 2.3 dB at 2 bits/s/Hz.