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This paper develops a receiver for a multiple access scenario obtained by simply overlaying transmitted signals from different users, with (essentially) the same carrier, same modulation format, etc., without the benefit of conventional multiple access signaling. A joint a posteriori probability detector is derived for co-received signals. The receiver structure follows the familiar general model for turbo processing, employing a soft-in-soft-out joint detector followed by error correction decoding, which feeds back soft information in the form of extrinsic probabilities. Since the receiver is intended to function with an existing transmission system with no modifications to the transmitter, channelization methods and/or interleavers which would normally be designed into a multi-access system are absent. The general structure is demonstrated using a system employing LDPC decoding and simulated performance results are presented. [ABSTRACT FROM PUBLISHER]

The equations for iteratively decoding low-density parity-check (LDPC) codes are generalized to compute joint probabilities of arbitrary sets of codeword bits and parity checks. The standard iterative LDPC decoder, which computes single variable probabilities, is realized as a special case. Another specialization allows pairwise joint posterior probabilities of pairs of codeword bits to be computed. These pairwise joint probabilities are used in an expectation-maximization (EM) based blind channel estimator that is ignorant of the code constraints. Channel estimates are input to a turbo equalizer that exploits the structure of the LDPC code. Feeding pairwise posterior probabilities back to the channel estimation, eliminates the need to average across time for channel estimator. Therefore, this scheme can be used to equalize very long codewords, even when the channel is time varying. This blind turbo equalizer is evaluated through computer simulations and found to perform as well as a channel-informed turbo equalizer but with approximately twice the number of turbo iterations. [ABSTRACT FROM AUTHOR]