Iterative Detection and Channel Estimation for WCDMA Systems Employing Non-Uniform QAM Constellations

In this paper we consider the use of non-uniform QAM constellations (Quadrature Amplitude Modulation) for broadcast or multicast transmissions within WCDMA-based systems (Wideband Code Division Multiple Access). These constellations are employed so as to increase the transmission efficiency, since they are able to provide unequal error protection. This allows the transmission of several simultaneous bit streams with different error protection depending on the associated importance. With this strategy, the most important information streams can be received by all users while the less relevant information will only be extracted by users with good propagation conditions. However, these constellations are very sensitive to inter-symbolic interference introduced by multipath propagation, especially for large constellations. Moreover, large non-uniform constellations are also very sensitive to channel estimation errors. For this reason, we propose an iterative receiver for joint detection and channel estimation. The proposed receiver takes advantage of the presence of a turbo-code in the transmission system, and uses feedback information from the turbo-decoder to estimate and suppress the interference, as well as to provide enhanced channel estimates. KeywordsNon-uniform QAM constellations, channel estimation, iterative interference cancellation, , WCDMA, broadcast and multicast services.