  This paper investigates a channel-coded multiuser system operated with
orthogonal frequency-division multiplexing (OFDM) and interleaved division
multiple-access (IDMA). To realize the potential advantage of OFDM-IDMA, two
challenges must be addressed. The first challenge is the estimation of multiple
channel parameters. An issue is how to contain the estimation errors of the
channel parameters of the multiple users, considering that the overall
estimation errors may increase with the number of users because the estimations
of their channel parameters are intertwined with each other. The second
challenge is that the transmitters of the multiple users may be driven by
different RF oscillators. The associated frequency asynchrony may cause
multiple CFOs at the receiver. Compared with a single-user receiver where the
single CFO can be compensated away, a particular difficulty for a multiuser
receiver is that it is not possible to compensate for all the multiple CFOs
simultaneously. To tackle the two challenges, we put forth a framework to solve
the problems of multiuser channel-parameter estimation, CFO compensation, and
channel decoding jointly and iteratively. The framework employs the space
alternating generalized expectation-maximization (SAGE) algortihm to decompose
the multisuser problem into multiple single-user problems, and the
expectation-conditional maximization (ECM) algorithm to tackle each of the
single-user subproblems. Iterative executions of SAGE and ECM in the framework
allow the two aforementioned challenges to be tackled in an optimal manner.
Simulations and real experiments based on software-defined radio indicate that,
compared with other approaches, our approach can achieve significant
performance gains.
