IEEE Communications Magazine • October 2004
60
0163-6804/04/$20.00 © 2004 IEEE
ADAPTIVE ANTENNAS AND MIMO
SYSTEMS FOR WIRELESS COMMUNICATIONS
INTRODUCTION
The attention multiple-input multiple-output
(MIMO) communication systems have attracted
in recent years has motivated work by numerous
companies on commercial products. For exam-
ple, during the past year, Airgo Networks (www.
airgonetworks.com), ArrayComm (www.array-
comm.com), and Vivato (www.vivato.net) have
developed multiple-antenna technologies for
802.11 wireless networks. Such multiple-antenna
access points potentially allow higher through
-
put, increased diversity, and reduced interfer
-
ence as they communicate with multiple wireless
users. Other multi-user applications that have
recently drawn attention include cooperation
among several cellular base stations when trans-
mitting to multiple mobiles and downlink pro-
cessing on a digital subscriber line (DSL).
Among the important questions to be addressed
for these scenarios are “What is the highest total
throughput for one of these multiple-output
transmitters when multiple users are present?”
and “How can a multi-user system achieve this
rate?” We address these questions in this article
while giving an overview of multiple-user MIMO
systems.
MIMO techniques were first investigated in
single-user scenarios; an excellent overview of
this problem is given in [1]. It is well known that
in a MIMO system with
n
T
transmit and n
R
receive antennas, capacity grows linearly with
min(
n
T
,n
R
). Current interest in the multiple-user
case is motivated by recent results indicating that
similar capacity scaling applies when an
n
T
-
antenna access point communicates with
n
R
users (e.g., see [2]). The capacity for the MIMO
multiple-user channel has been analyzed using
coding techniques referred to as “writing on
dirty paper.” This technique was developed in
[3] with interference cancellation in mind; it was
shown that the capacity of a channel where the
transmitter knows the interfering signal is the
same as if there were no interference. The dirty
paper analogy comes from comparing the inter-
ference in a communications channel to dirt that
is present on a piece of paper. The signal is the
ink, which is chosen based on the interference
(dirt) that is present.
In addition to wireless networks with a multi-
antenna base station, the multi-user MIMO
downlink model also applies to many other sys
-
tems. The downlink of a DSL system with
crosstalk between the wires for each user is one
scenario where the transmitter terminals can
cooperate, but the far end of the MIMO channel
cannot. Other examples include multiple-cell
multiple access channels with cooperation among
Quentin H. Spencer, Distribution Control Systems, Inc.
Christian B. Peel, Swiss Federal Institute of Technology
A. Lee Swindlehurst, Brigham Young University
Martin Haardt, Ilmenau University of Technology
ABSTRACT
Multiple-input multiple-output (MIMO)
communication techniques have been an
important area of focus for next-generation
wireless systems because of their potential for
high capacity, increased diversity, and interfer-
ence suppression. For applications such as
wireless LANs and cellular telephony, MIMO
systems will likely be deployed in environ-
ments where a single base must communicate
with many users simultaneously. As a result,
the study of multi-user MIMO systems has
emerged recently as an important research
topic. Such systems have the potential to com-
bine the high capacity achievable with MIMO
processing with the benefits of space-division
multiple access. In this article we review sever
-
al algorithms that have been proposed with
this goal in mind. We describe two classes of
solutions. The first uses a signal processing
approach with various types of transmitter
beamforming. The second uses “dirty paper”
coding to overcome the interference a user
sees from signals intended for other users. We
conclude by describing future areas of research
in multi-user MIMO communications.
An Introduction to the
Multi-User MIMO Downlink