Abstract
The problem of channel estimation and in-phase/quadrature-phase (IQ) imbalance is one of the main challenges that has to be faced by uplink transmission in orthogonal frequency division multiple access (OFDMA) systems. The previously reported investigations for such systems are limited to synchronous and uncoded transmission. Furthermore, in these previous works, equalization of the IQ imbalance should be performed after the estimation process in order to properly detect data symbols. In this contribution, the transmit and receive IQ imbalance for uplink OFDMA systems is investigated in the context of asynchronous and bit-interleaved coded modulation (BICM) transmission. We propose a novel data detector exploiting the unwanted IQ as a beneficial resource to achieve a diversity gain, without the necessity of an additional equalizer. In addition, we develop a novel code-aided algorithm to estimate the propagation delays and the overall channel impulse responses (CIRs), which include the physical CIRs and IQ imbalance occurring at both the transmitters and base station. Since the exact maximum-likelihood (ML) solution to this problem turns out to be too complex for practical purposes, we resort to the space alternating generalized expectation-maximization (SAGE) algorithm as a low complexity method to perform parameter estimation. The proposed estimation algorithm operates in an iterative way, in which the soft information provided by the channel decoder is utilized as a priori information to refine the estimates. The computational complexity analysis and simulation results confirm the effectiveness of the proposed estimation algorithm and the proposed detector for practical application.