Iterative Detection and Channel Estimation for WCDMA Systems Employing Non-Uniform QAM Constellations Nuno Souto * , Rui Dinis ** , João Carlos Silva * * Instituto Superior Técnico-Instituto de Telecomunicações/ ** ISR-IST Torre Norte 11-11, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal, nuno.souto@lx.it.pt, rdinis@ist.utl.pt, joao.carlos.silva@lx.it.pt Francisco Cercas, Americo Correia ISCTE / Instituto de Telecomunicações/ADETTI Torre Norte 11-08, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal francisco.cercas@lx.it.pt, americo.correia@iscte.pt Abstract—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. Keywords- Non-uniform QAM constellations, channel estimation, iterative interference cancellation, , WCDMA, broadcast and multicast services. I. INTRODUCTION In wireless communication networks it is often necessary to transmit the same information to all the users (broadcast transmission) or to a selected group of users (multicast transmission) in the cells. Usually different users will have different propagation conditions and thus different capacities. Cover [1] showed that in broadcast transmissions it is possible to exchange some of the capacity of the good communication links to the poor ones and the tradeoff can be worthwhile. A very simple method to accomplish this is to employ non- uniform signal constellations (also called hierarchical constellations) which are able to provide unequal bit error protection. With this type of constellations there can be several classes of bits with different error protection, to which different streams of information can be mapped. Depending on the propagation conditions, a given user can attempt to demodulate only the more protected bits or also the bits that carry the additional information. A possible application of these techniques is in the transmission of coded voice or video signals, as studied in [1]-[3]. Non-uniform 16-QAM and 64- QAM constellations have already been incorporated in the DVB-T (Digital Video Broadcasting - Terrestrial) standard [4]. In wideband code division multi-access (WCDMA) systems, the mobile propagation conditions result in frequency selective channels that produce multipath interference due to arriving replicas with relative delays superior to one chip period. Although the downlink connection uses orthogonal spreading codes for transmitting several physical channels in parallel, the presence of other multipath replicas destroys this orthogonality, leading to interference effects. This has an important impact on the link performance, especially for M- QAM (M>4) modulations. These modulations are very sensitive to interference and their performance is severely degraded in frequency selective fading channels. A possible low complexity solution to decrease the performance degradation, when using these higher order modulations, is the implementation of a RAKE receiver with an interference canceller. In [5] and [6] the use of a multipath interference canceller (MPIC) was proposed for High Speed Downlink Packet Access (HSDPA) transmissions [7] that use 16-QAM modulation. These MPICs are based on the concepts of the sub-optimal interference cancellation performed by subtractive multiuser detector schemes usually employed in the reverse link. In [8], an iterative receiver that uses feedback information from the turbo-decoder for estimating and removing the inter- path interference was proposed for WCDMA broadcast transmissions using non-uniform QAM constellations. It was shown that this turbo-MPIC performed better than the MPIC with no feedback information at the cost of a small complexity increase. Only perfect channel estimation was admitted in the paper and therefore in here we extend the work and incorporate a channel estimation block in the iterative receiver. Considering that a pilot channel is transmitted simultaneously with the data channels, it is possible to improve the channel