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Series in Broadband Communications

Mauro Pesce

Recursive Demodulation Schemes for the
WCDMA Downlink in the UMTS Terrestrial
Radio Access

1. edition 2005; VIII, 250 pages/Seiten, € 64,00. ISBN 3-86628-018-7

This thesis addresses the design of different recursive and non-recursive demodulation schemes for the wideband code-division multiple access (WCDMA) downlink in the Universal Mobile Telecommunication System (UMTS) Terrestrial Radio Access (UTRA).

We consider a simplified---with respect to the UTRA specifications---WCDMA downlink system, where there are K simultaneously active users to be served in the specific cell and both base station and mobile-terminal-of-interest (MOI) are each equipped with one antenna. The received signal at the MOI is the sum of an information bearing signal and an additive thermal noise and is described by a time-discrete model. The information bearing signal results from the sum of a common pilot channel (CPICH) and K user signals at the output of a time-varying multipath channel.

Applying specific approximations for the coefficients of the channel impulse response (CIR) and the downlink signals, we derive different state-space models for the received signal, where the models serve as a basis for the development of recursive schemes performing channel estimation, equalization for given CIRs, or joint channel estimation and equalization. While Kalman filters are adopted in the first two cases, their use is ruled out in the last case due to nonlinearities in the observation equations of the corresponding state-space representations. Hence, we derive recursive schemes that employ first- or second-order expansions of these nonlinearities and, furthermore, can cope with the effects of nonminimum-phase channels, as encountered in typical mobile communications environments.

In a further step, the developed recursive equalization schemes are extended in order to incorporate a CPICH-based constraint that exploits the orthogonality among the transmit signals. In most cases, the constrained versions of the recursive equalizers lead to performance improvements with respect to the corresponding unconstrained schemes. The CPICH-based constraint as well as additional constraints involving the combined spreading and scrambling code of the MOI are also incorporated into non-recursive block-equalizers applying the least-squares (LS) or minimum mean-square error (MMSE) criteria which turn out to be rather robust with respect to nonminimum-phase channel characteristics.

In order to improve the quality of the channel estimates being used by the non-recursive block-equalizers, novel concepts for adaptive block-equalization using modified LS and MMSE methods are presented. The gradient-based adaptation, relying on a cost function involving the CPICH, is carried out directly with respect to the CIR coefficients required to compute the block-equalizer matrices. Therefore, in contrast to classical adaptive filter techniques, the number of coefficients to be adapted corresponds to the number of propagation paths and is thus independent of the chosen block size. The adaptive non-recursive block-equalizers yield remarkable performance improvements with respect to the corresponding non-adaptive schemes. Still, the best bit-error rate performance is obtained using recursive receiver schemes.
Keywords: UMTS, UTRA-FDD, WCDMA downlink, recursive equalization, recursive channel estimation, adaptive least-squares and minimum mean-square error block-equalization, constrained equalization

Schlagwörter: UMTS, UTRA-FDD, WCDMA-Abwärtsstrecke, rekursive Entzerrung, rekursive Kanalschätzung, adaptive Block-Entzerrung basierend auf den Methoden der kleinsten Quadrate oder des minimalen mittleren Fehlerquadrats, Entzerrung mit Randbedingungen

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