Adaptive Equalization for Fading Mobile Radio Channels
Report UPTEC 92124R, 125pp, November. 1992.
The receiver in a digital mobile radio system has to detect and adjust
for time-variations in the dispersive channel. In some standards,
such as D-AMPS in North America, the time-variability
is rapid and provides
severe challenges for existing adaptation algorithms, such as
LMS and RLS. The thesis presents novel algorithms, one of
which has subsequently been patented.
The development of
indirect methods for adaptive equalization, applied to fading digital
mobile radio channels encountered in the proposed
North American Digital Cellular system, is the main
subject of this thesis.
New algorithms for channel estimation in severe Rayleigh fading
environments are presented, They are based on the principles of
stochastic and deterministic internal modelling of time-varying
coefficients of a FIR channel model.
In the stochastic case,
channel estimators are based on simplified second order
autoregressive models and low-complexity approximation of a
A novel averaging approach is used to replace the on-line update
of a Riccati equation with the determination of a constant matrix.
When using a simplified second order AR model of the
time-varying parameters, the resulting adaptation gain filters
can be expressed in analytical form.
This results in algorithms with
high performance at LMS computational complexity.
In the deterministic case, channel coefficients are
parameterized by first order Fourier series expansions, with
unknown fundamental frequencies.
A simplified prediction error identification algorithm is derived
to estimate the Fourier coefficients and the fundamental frequencies,
Several combinations of equalizers/Viterbi schemes with different
channel estimators were studied on simulated data, generated from a
Rayleigh fading channel model.
As compared to channel estimation with LMS and RLS algorithms,
the new channel estimators provide higher performance.
Channel estimators based on second order internal stochastic models,
used in conjunction with decision feedback equalizers (DFE's),
provide adaptive equalization with high performance,
at a low computational complexity.
Indirect adaptation of equalizer parameters, based on
is also compared with the conventional direct approach to
adaptive equalization. The indirect method offers superior
performance, mainly since Rayleigh fading
channel coefficients change in a regular way.
Direct adjustment of the equalizer coefficients would instead
have to track optimal adjusted values
which drift with strongly time-varying rates of change.
- Related publications:
- A series of four papers outlining the later development of a
complete design methodology, based on stochastic models
of time-varying parameters:
of general constant-gain adaptation algorithms.
Part II: Analysis
of stability and performance, for slow and fast variations.
The Wiener LMS
adaptation algorithm, a special case with low complexity.
A Case Study on IS-136 1900MHz channels.
- PhD Thesis
by L Lindbom 1995, presenting the general design
(IEEE ICASSP'93) summarizing the proposed KLMS algorithm.
of time-varying channel coefficients in IS-136 800 MHz systems.