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S

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 3866280187
This thesis addresses the design of different recursive and
nonrecursive demodulation schemes for the wideband codedivision multiple
access (WCDMA) downlink in the Universal Mobile Telecommunication System (UMTS)
Terrestrial Radio Access (UTRA).
We consider a simplifiedwith respect to the UTRA specificationsWCDMA
downlink system, where there are K simultaneously active users to be served in
the specific cell and both base station and mobileterminalofinterest (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 timediscrete model. The
information bearing signal results from the sum of a common pilot channel
(CPICH) and K user signals at the output of a timevarying multipath channel.
Applying specific approximations for the coefficients of the channel impulse
response (CIR) and the downlink signals, we derive different statespace 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
statespace representations. Hence, we derive recursive schemes that employ
first or secondorder expansions of these nonlinearities and, furthermore, can
cope with the effects of nonminimumphase channels,
as encountered in typical mobile communications environments.
In a further step, the developed recursive equalization schemes are extended in
order to incorporate a CPICHbased 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 CPICHbased constraint as well as
additional constraints involving the combined spreading and scrambling code of
the MOI are also incorporated into nonrecursive blockequalizers applying the
leastsquares (LS) or minimum meansquare error (MMSE) criteria which turn out
to be rather robust with respect to nonminimumphase
channel characteristics.
In order to improve the quality of the channel estimates being used by the
nonrecursive blockequalizers, novel concepts for adaptive blockequalization
using modified LS and MMSE methods are presented. The gradientbased adaptation,
relying on a cost function involving the CPICH, is carried out directly with
respect to the CIR coefficients required to compute the blockequalizer
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
nonrecursive blockequalizers yield remarkable performance improvements with
respect to the corresponding nonadaptive schemes. Still, the best biterror
rate performance is obtained using recursive receiver schemes.
Keywords: UMTS, UTRAFDD, WCDMA downlink, recursive equalization, recursive channel estimation, adaptive leastsquares and minimum meansquare error blockequalization, constrained equalization
Schlagwörter: UMTS, UTRAFDD, WCDMAAbwärtsstrecke,
rekursive Entzerrung, rekursive Kanalschätzung, adaptive BlockEntzerrung
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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