Uppsala universitet

Tracking of Time-varying Mobile Radio Channels,
Part II: A Case Study.

Lars Lindbom , Ericsson Infotech
Anders Ahlén , Uppsala University
Mikael Sternad , Uppsala University and
Magnus Falkenström , Ericsson Infotech.

IEEE Transactions on Communication, Vol. 50, January 2002, pp. 156-167.

A brief version of the paper,
"Tracking of Time-varying Mobile Radio Channels with WLMS Algorithms:
A Case Study on D-AMPS 1900 Channels"

by M. Sternad, L. Lindbom and A. Ahlén, is published at
IEEE Vehicular Technology Conference - VTC200-Spring Tokyo, Japan, May 15-18, 2000, pp 2507-2511. © IEEE

The IS-136 digital mobile TDMA systems are widely used on the American continents and a similar standard exists in Japan. Their relatively low symbol rate and long data slots (6.67 ms) cause severe fading. In such 1900 MHz systems, one or two fading dips can be expected within each data slot. Large variations in fading rates and frequency selectivity are encountered.

Well designed channel estimators are therefore crucial for obtaining adequate performance. Estimates obtained from training sequences (synchronization words) cannot be used within the whole slot and interpolation of channel estimates between training sequences provides inadequate performance. The same is true for the decision-directed LMS and RLS adaptation algorithms. We will here investigate the application of WLMS algorithms to the estimation of the fading channels.

Low-complexity WLMS adaptation algorithms, of use for channel estimation, have been derived in a companion paper. Their use and design is here evaluated on the fast fading radio channels encountered in IS-136 TDMA systems, with the aim of clarifying several issues: How much can channel estimation performance be improved with these tools, as compared to LMS adaptation? When can an improved tracking MSE be expected to result in a meaningful reduction of the bit error rate? Will optimal prediction of future channel estimates significantly improve the equalization in fast fading environments? Can one single tracker with fixed gain be used for all encountered Doppler frequencies and SNRs, or must a more elaborate scheme be adopted?

These questions are here investigated both analytically and by simulation. An exact analytical expression for the tracking MSE on two-tap FIR channels is presented and utilized. With this tool, the MSE performance and robustness of WLMS algorithms based on different statistical models can be investigated. A simulation study then compares the uncoded bit error rate of detectors, where channel trackers are used in decision directed mode in conjunction with Viterbi algorithms. A Viterbi detector combined with WLMS, based on second order autoregressive fading models possibly combined with integration, provides good performance and robustness at a reasonable complexity.

Related publications:
Part I: The Wiener LMS adaptation algorithm.
Design of the general constant-gain adaptation algorithms.
Analysis of stability and performance, for slow and fast variations.

PhD Thesis by Lars Lindbom.
Licenciate Thesis by Lars Lindbom, on averaged Kalman designs (KLMS)
and on deterministic sinusoid modelling of fading channels.
Master Thesis by Magnus Falkenström, on the grid approach to robust design of channel estimators.
Conference paper (IFAC Como 2001) on averaged robust design for uncertain fading models.

Conference version, VTC2000S Tokyo: : Postscript, 238K ; Pdf, 425K
Conference poster , VTC2000S Tokyo: Postscript, 200K ; Pdf, 62K

Paper version (IEEE-COM): Postscript, 391K ; Pdf, 265K

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