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Series in
Broadband Communications
VOLUME 18
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
Series in Broadband Communications, all titles
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