PHD Synchronization and Channel Estimation in OFDM Systems.pdf

Synchronization
and
Channel Estimation
in OFDM Systems
Jan-Jaap van de Beek
Lule
a University of Technology
Division of Signal Processing
Lule
a, Sweden
September 1998
Supervisor
Professor Per Ola B
orjesson, Lule
a University of Technology.
ISSN: 1402 – 1544
ISRN: LTU - DT - - 98/32--SE
Published 1998
Reproduced by Universitetstryckeriet, Lule
a.
Abstract
This thesis consists of a prologue and six parts that address two aspects of orthogonal
frequency-division multiplexing (OFDM) communication systems: synchronization and
channel estimation. For the synchronization aspect, focus is on estimators of time and
frequency o
sets that exploit the cyclic pre
x. For the channel estimation aspect, a
concept is presented and analyzed that exploits the frequency correlation of the channel
bines plexity with high performance.
Synchronization errors in OFDM can cause intersymbol and intercarrier interference.
One way to suppress these interferences in OFDM systems is to track the carrier frequency
of the received signal and the proper start of the OFDM symbols. The joint maximum
likelihood estimator of symbol time and carrier frequency o
sets for OFDM systems using
a cyclic pre
x is presented. The maximum likelihood estimator, derived for an additive
white Gaussian noise channel, exploits the redundancy introduced by the pre
x and is
independent of how the subcarriers are modulated. Therefore, it does not require extra
pilot information.
Some properties of practical OFDM systems change the premises of this estimator.
In particular, in systems with pulse shaping, channel dispersion, or slowly changing
synchronization errors, estimator performance can gain from more suitably chosen signal
models. Three extended signal models, the associated maximum likelihood estimators,
and structures for their implementation are presented.
Furthermore, tracking synchronization of a multiuser OFDM system, which often has
been questioned, is feasib