[CPCC] Talk June 30 11AM Jibing Wang, UCLA

[Ender Ayanoglu]

				  TALK

     Space-Time Coding for Multiple Antennas Wireless Communications:
		     Performance Analysis and Code Design

                               Jibing Wang
                      Electrical Engineering Deparment
                   University of California, Los Angeles


			     June 30, Monday
				  11 AM
			  Engineering Tower 331

                                 Abstract

The next generation of broadband wireless communications systems is
expected to provide users with wireless multimedia services such as
high-speed Internet access, wireless television and mobile computing.
The rapidly growing demand for these services is driving the
communication technology towards higher data rates and higher mobility
transmissions over mobile radio channels. However, reliable
communications are challenged by the physical limitation of the
wireless channel. An effective approach to increasing the data rate as
well as the power efficiency over wireless channels consists of
introducing temporal and spatial correlation into signals transmitted
from different antennas, namely, space-time coding.

Most work up to this point assumes the idealistic case of independent
and identically distributed (i.i.d.) channels, i.e., the spatial
fading is uncorrelated. However, in reality, the individual antennas
could be correlated due to insufficient antenna spacing and lack of
scattering. In the first part of the presentation, we propose a new
viewpoint to characterize the effect of spatial correlation on the
performance of space-time coding. We derive the exact pairwise error
probability (PEP) for space-time coding over quasi-static or fast
Rayleigh fading channels in the presence of spatial fading
correlation. We show that receive correlation always degrades the PEP
for all SNRs. We quantify the effect of receive correlation by
employing the notion of "majorization". We show that the stronger the
receive correlation, the worse the PEP for all SNRs. Unlike receive
correlation, the effect of transmit correlation depends on the
specific space-time code employed. We show that at low SNR, transmit
correlation can either improve or degrade the PEP performance. By
analyzing the worst case scenario for asymptotically high or low SNR,
we show that to guarantee robust performance for arbitrary transmit
correlation, the minimum eigenvalue of the codeword pair difference
matrix should be maximized among all codeword pairs. For orthogonally
designed space-time blocks (ODSTBC), we analyze the effects of
transmit and receive correlation on the performance in terms of PEP
and symbol error probability. We show that transmit correlation also
deteriorates the performance of ODSTBC. We demonstrate a duality
between transmit correlation and receive correlation for the ODSTBC.

The decoding of the space-time trellis coding and space-time block
coding assumes that channel state information is available at the
receiver. However, in some applications channel estimation is either
costly or even impossible. In the second part of the talk, we propose
a new criterion to design differential unitary space-time (DUST)
codes. Based on the exact pairwise error probability, we derive the
union bound on the symbol error probability (SEP) of the DUST
modulation. Instead of using the rank-and-determinant or Euclidean
distance criteria, we optimize the codes such that the union bound on
the SEP is minimized for a predetermined scenario taking into account
the number of transmit and receive antennas and the operating SNR. Our
simulation results show that for a wide range of SNRs, the codes with
the minimum union bound for a particular SNR outperform the codes
designed based on rank-and-determinant or Euclidean distance criteria.

In broadband wireless communications, frequency selective channels are
often encountered. Concatenation of space-time coding with orthogonal
frequency-division multiplexing (OFDM) has gained much interest
recently. For asymptotically high SNRs, the design criteria for
space-time codes over MIMO OFDM channels differ significantly from
those for space-time codes over flat fading channels, which complicate
the codes design. We show that when the number of receive antennas is
large, the minimum Euclidean distance among code words dominates the
performance. Therefore, the codes could be optimized by using the
Euclidean-distance criterion valid for AWGN channels. Simulation
results showed that the results valid for a number of receive antennas
tending to infinity still provide correct indications when the number
of antennas is small. For the case that channel state information is
not available at the receiver, we develop a differential unitary
space-time-frequency (DUSTF) coding scheme for MIMO OFDM systems over
frequency selective fading channels. We also propose a point of view
such that the codes optimized for frequency flat fading channels could
be employed to enjoy full space-frequency diversity and the optimum
coding gain.

Directions to ET331 on UCI campus are available from
http://www.eng.uci.edu/cpcc/?page=directions