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On Information Theoretic Aspects of Multi-Cell Wireless Systems
Shlomo Shamai (Shitz),
Professor of Electrical Engineering, Technion--Israel Institute of Technology:
Thursday October 3, 2002 4:45 pm, Princeton University, Friend 101:
Some information theoretic aspects of cellular communication systems
are addressed, focusing on a simple model suggested by Wyner (1994), of
a linear cell array. Accordingly, it is assumed that the system
cells are ordered in an infinite linear array, and that only adjacent
cell interference is present, characterized by a single parameter alpha.
Starting with the downlink channel, it is assumed that only a single
user is to be served in each cell (which is equivalent to intra-cell
TDMA, or orthogonal DS-CDMA, and is in fact optimal in the non-fading
environment).
The channels are assumed to be either non-fading, or slowly changing
ergodic flat fading channels. A linear pre-processing plus encoding
scheme is proposed, which significantly enhances cellular downlink
performance, while putting the complexity burden on the transmitting
end. The approach incorporates the ``writing on dirty paper' result
(Costa (1983)) and some variants for eliminating the effect of
uncorrelated interference, being fully known at the transmitter but
unknown at the receiver.
The attainable average throughput of the proposed scheme is optimal
and approaches that associated with optimum joint processing at the high
SNR region.
Turning to the uplink channel, and assuming Rayleigh flat-fading,
we examine attainable rates with various single and multiple
cell processing strategies. We proceed then to investigate an
optimally coded randomly spread DS-CDMA system with single-cell site,
multiuser detection. The discussion is confined to asymptotic analysis
where both the number of users per cell and the processing gain go to
infinity, while their ratio goes to some finite constant. The spectral
efficiency of various multiuser detection strategies is evaluated
assuming single cell-site processing, and equal transmit powers for all
users in all cells.
Comparative results demonstrate how performance is affected by the
introduction of inter-cell interference (with and without fading),
and what is the penalty associated with the randomly spread coded
DS-CDMA strategy.
Extensions, and related problems deserving further attention will also
be shortly discussed.
The talk is based on joint works with Benjamin Zaidel, EE Dept. Technion,
Haifa Israel and Sergio Verdu, EE. Dept. Princeton University
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