Article Title: Multi-antenna downlink channels with
limited feedback and user selection Authors:
Yoo,
T;Jindal, N;Goldsmith, A
Journal: IEEE J SEL AREA COMMUN
Volume: 25
Issue: 7
Page: 1478-1491
Year: SEP 2007
* Stanford Univ, Dept Elect Engn, Stanford, CA 94305
USA.
* Stanford Univ, Dept Elect Engn, Stanford, CA 94305
USA.
* Univ Minnesota, Dept Elect & Comp Engn, Minneapolis,
MN 55455 US.
(addresses have been truncated)
Why do you think your paper is highly
cited?
Our paper has both theoretical and practical significance. In the
communications and information theory community, multiple-input
multiple-output (MIMO) systems and multiuser diversity have been among
major research topics which have drawn lots of attention and been
extensively developed during the past decade. Those are two among several
key concepts that will enable higher data rates and/or reliability in the
near future for wireless communication systems.
From the theoretical side, information theorists have successfully
characterized performance limits of multiuser MIMO channels. In particular,
they have shown that the sum capacity of the channel grows in proportion to
the number of antennas (multiplexing gain) and double-logarithmically in
the number of users (multiuser diversity gain).
"To our knowledge, ours was the
first paper to address and successfully
analyze the sum-capacity of large-user MIMO
channels with limited
feedback."
Our paper lays the foundation for bringing these theoretical results into
practice by showing that the full multiplexing and multiuser diversity
gains can still be realized under practical constraints, i.e., with partial
channel knowledge at the transmitter due to limited feedback of channel
state information.
Our paper points out the importance of channel quality feedback in the form
of "Signal to Interference-plus-Noise Ratio" (SINR), and goes on to derive
the tradeoffs between such quantities as the number of feedback bits, the
number of users in the system, and the "Signal-to-Noise Ratio" (SNR). The
results in our paper open the door for subsequent research in the area of
multiuser MIMO techniques, facilitating the adoption of multiuser MIMO
technologies into, among others, the upcoming 4th-generation
cellular systems.
Does it describe a new discovery, methodology, or
synthesis of knowledge?
To our knowledge, ours was the first paper to address and successfully
analyze the sum-capacity of large-user MIMO channels with limited feedback.
Our paper put together such previous discoveries as MIMO broadcast channel
capacity, multiuser diversity, and limited feedback MIMO systems, into a
unified mathematical framework.
Would you summarize the significance of your paper in
layman's terms?
Over the past several decades, researchers have learned that by deploying
multiple antennas at both the transmitter and receiver sides, we can
greatly increase the achievable data rate over the wireless link.
Furthermore, by appropriately grouping and serving multiple users in a
single time/frequency resource, we can further increase the data rate.
These gains are significant in the sense that they come free of additional
spectrum and battery power, which are the two fundamental factors that
limit the achievable data rate on any given wireless link.
However, the biggest hurdle, in our view, in bringing this theoretical
result into practice is the limited availability of channel information at
the transmitter. Our paper attacks this problem and shows that given
sufficient number of users in the system; nearly all the gains promised by
the theory can in fact be achieved by employing the right feedback and
scheduling methods.
How did you become involved in this research, and were
there any problems along the way?
My research at Stanford has focused on multiuser MIMO systems and
information theory. Having some industry background, I have also been
interested in bringing some of the information theoretic results into
somewhat more practical settings.
Our paper is built on two previous works of ours; one is a previous work by
Prof. Andrea Goldsmith of the Department of Electrical Engineering at
Stanford University and me on zero-forcing beamforming in large-user MIMO
broadcast channels, and the other is on the subject of "MIMO broadcast
channels with limited feedback" by Prof. Nihar Jindal of the University of
Minnesota. I had a hunch that my conjecture in the paper was correct, but
the hardest part was to actually prove it by coming up with the right
combination of algorithms and mathematical tools.
Where do you see your research leading in the
future?
I am interested in continued research on this very topic, as well as
related areas such as network MIMO and relay networks.
Do you foresee any social or political implications for
your research?
I believe our paper is one big step forward toward implementing multiuser
MIMO technologies, which will enable faster wireless connection and better
user experience.
Taesang Yoo, Ph.D.
Staff Engineer
Qualcomm, Inc.
San Diego, CA, USA
KEYWORDS: MIMO; QUANTIZED FEEDBACK; LIMITED FEEDBACK;
ZERO-FORCING BEAMFORMING; MULTIUSER DIVERSITY; BROADCAST CHANNEL;
SCHEDULING; SEMI-ORTHOGONAL USER SELECTION; RANDOM BEAMFORMING.