Subject: [NEXTEL-1] Invited Contribution to XXVII URSI General Assembly, August 17-24, 2002, Maastricht, The Netherlands

From: [email-masked]
Date: Tue, 15 Jun 2004 14:55:19 -0400

Invited Contribution to XXVII URSI General Assembly, August 17-24, 2002,
Maastricht, The Netherlands

Link-Specific, Propagation-Based Wireless MIMO System Deployment
by Ernst Bonek, Technische Universitaet Wien, Vienna, Austria
[email-masked]

Setting the Scene

Propagation is at the heart of wireless communications. It sets the
ultimate limits for the transmission speed and throughput of any system
built upon radio. Given the obvious attractiveness of wireless on one
hand, and the complexity of the radio channel on the other, it is clear
that the deployment of the novel MIMO systems must consider and exploit
the radio channel in full. MIMO systems, which have multiple antennas at
each end of the link, i.e. a number of NR at the receive and NT at the
transmit sides, offer huge transmission capacity by attempting to do
just this. Ideally, independent, orthogonal data transmission channels
are set up between the individual antenna elements to maximize
transmission capacity. MIMO systems can be set-up in fixed wireless
access (FWA) systems, in wireless local area networks (WLANs), and on
top of emerging mobile communications standards, such as the Universal
Mobile Telecommunications System (UMTS).

With many users interfering with each other in limited frequency bands,
another issue comes into play, well known from public mobile
communications networks: interference limitation.  The anticipated
proliferation of WLANs will become a victim of its own success by
eventually slowing down transmission speed considerably. In the 2.45 GHz
ISM band, IEEE 802.11b will not only be subject to microwave oven
interference, but to Bluetooth devices as well. It is expected that, in
the near future, laptop computers will be furnished with antenna arrays,
e.g. on the back of the display cover. Together with smart antenna
technology at the access point (AP), MIMO systems will come quite
naturally. Besides smart reception techniques, WLAN connections, being
set-up in an ad-hoc fashion, will require some selfregulating mechanism
to curb unwanted radiation into directions where the signal cannot be
captured anyway by an intended user. Although WLAN standards according
to IEEE 802.11 and to HIPERLAN/2 have in-built mechanism to shun
occupied channels, techniques to minimize interference are essential for
the sake of transmission speed.  As a third issue, the power consumption
of high-data-rate transmission will be addressed by the topic of this
paper. It would be highly desirable to save transmitter power, expensive
to amplify to high levels, by transmitting into a few directions only.
With adaptive antenna arrays this can be done by either exploiting
directive antenna gain or by directing the available watts to a few
dedicated directions.

MIMO Deployment Based on Link-Specific Measurement

The presentation will discuss measurements by various groups [1,2,3,4]
demonstrating that MIMO capacity has a large local variation, depending
on the position of the antenna arrays and on the environment. The
immediate question that follows: how, then, to deploy MIMO systems to
maximize capacity practically?

A comparison will show the differences between two approaches: first, to
identify the channel matrix H for the particular transmit and receive
arrays with which the measurements are made, and use that to calculate
channel capacity [3]; or, second, to identify from the measurements the
actual multipath components, synthesize H from these and then calculate
capacity [5]. The former is limited by the fact that any measurement
taken is a function of the antenna arrays used for measurement. So, the
impact of variants of the antenna structure upon capacity cannot be
assessed.

The clue to the latter is given by double-directional channel
measurements, pioneered by Martin Steinbauer of Technische Universität
Wien of Vienna, Austria [6]. By measuring the transfer functions or the
impulse responses between each antenna element of the arrays at both
ends of the radio link, and by sophisticated signal processing he was
able to determine the directions of departure (DoDs) as well as the
directions of arrival (DoAs).

Common to two different forms of Steinbauer´s method is their ability to
actually track a wave carrying significant power from the transmit array
to the receive array via several (up to three) scattering points. They
rely on the shift invariance principle of the superresolution algorithm
ESPRIT [7]. To obtain an estimate of the number, strength and angles of
departing and impinging waves quite some computation is necessary. As
the computing power of laptop computers lies idle most of the time
anyway, the evaluation procedure is not too big a burden for WLANs, not
to speak of FWA where the link environment changes rather slowly if at
all.

One of the surprises of evaluating double-directional measurements was
that these directions may be more or less discrete, particularly when
several reflections or scatterings are involved before a departing wave
reached a receive antenna. This latter condition certainly can be the
case in non-line-of-sight situations (NLOS). An interesting consequence
of discrete DODs and DOAs is that such situations lend themselves for
deterministic instead of stochastic sitespecific channel modeling.
Deterministic channel modeling has previously mostly been associated
with ray-tracing or similar methods using databases about the
propagation environment [8]. In contrast to ray tracing, the procedure
favored in the present paper relies on measurement of an actual
propagation environment. Examples of measured office environments that
will be presented will clearly demonstrate that there exist at least
large angular sectors, either from which no significant power was
received or into which no power is transmitted that would eventually
reach an intended receive antenna.  It is also interesting to
distinguish between situations in which the incoming multipath huddles
around a narrow angular range, or conversely, where the major multipath
is widely separated. Having observed either situation in a given
environment would lead straightforward to a proper selection of an
optimum smart antenna algorithm for the receiver following the receive
array. In the first case, one would probably opt for an exploitation of
diversity, in the second case beamforming will be more appropriate.  The
procedure envisaged to deploy a MIMO system in a given environment is
this.

Before data transmission,

First, a test phase in which the transfer functions between NT antenna
elements on the transmit and NR antenna elements on the receive sides
are measured;

Then, an evaluation of the DoAs and DoDs by the double-directional
procedure via one of two superresolution evaluation methods (Alternating
Estimation and Beamforming, and Joint Parameter Estimation and Pairing
[9]). Reference [5] gives the details to calculate MIMO capacity from
these directions and the paths linking them;

Next, antenna weights wT and wR are calculated and set either to
maximize link throughput or to minimize the angular transmit range to
those DoDs for which corresponding DoAs exist (“valid DoDs”).
Transmitting in valid DoDs only will keep transmit power to the
necessary minimum and will help in keeping the system interference low.
This step requires a certain, but modest exchange of control data
between mobile terminal and access point (or two or more mobile
terminals);

Finally, the communication link being established, data transfer can
commence.

Summary and Conclusions

The tools and procedures either to optimize link capacity in a given
environment;
to minimize interference; or to minimize transmit power in the
deployment of MIMO systems will be explained and discussed. A key
enabler is the identification and tracking of individual multipath
components, a result of the so-called double-directional measurement
method. The determination of DoAs and DoDs is largely independent of the
arrays used for the preparatory measurement. A convenient side result
will be guidelines where to exploit beamforming and where diversity in a
given smart antenna installation. Discussion will also include
techniques to operate MIMO antennas with variable data rates [10].
References

[1] M. Steinbauer et al., “Array measurement of the double-directional
mobile radio channel,” in Proc. of IEEE Vehicular Technology Conf. 2000
(VTC’2000) – Spring, (Tokyo, Japan), May 2000.
[2] J. W. Wallace and M. A. Jensen, “Measured Characteristics of the
Wireless MIMO Channel”, Proc. IEEE Vehicular Technology Conf. Fall 2001
(VTC Fall 2001), Atlantic City, N.J., USA, 7-11 Oct. 2001.
[3] J.-P. Kermoal et al., “Experimental investigation of multipath
richness for multielement transmit and receive antenna arrays,” in Proc.
of IEEE Vehicular Technology Conf. 2000 (VTC’2000) –Spring, (Tokyo,
Japan), May 2000.  [4] D. Gesbert et al., ”Performance evaluation for
scattering MIMO channel models,” Asilomar Conf. 2001, Pacific Grove
(CA), Oct. 2001.
[5] A. F. Molisch et al., “ Capacity of MIMO Systems Based on Measured
Wireless Channels”, to appear in IEEE J. Selected Areas in
Communications, Special Issue on Channel and Propagation Models for
Wireless System Design, 2002 [6] M. Steinbauer et al., “ The
Double-Directional Radio Propagation Channel” IEEE Antennas &
Propagation Magazine, Vol. 43, No. 4, August 2001, pp. 51-63.  M.
Steinbauer, “A comprehensive transmission and channel model for
directional radio channels”, COST 259 Temporary Document TD(98)027,
Berne, Switzerland, February 1998 [7] M. Haardt, “Efficient One-, Two-,
and Multidimensional High-Resolution Array Signal Processing, PhD
Dissertation, Technische Universität München, Germany, August 1996 [8]
L. Correia, ed., “Wireless Flexible Personalized Communications,” J.
Wiley, 2001.
[9] M. Steinbauer, “The Radio Propagation Channel – A Non-Directional,
Directional and
Double-Directional Point-of-View”, PhD Dissertation, Technische
Universitaet Wien,
November 2001, Vienna, Austria
[10] Lucent Technologies, “Increasing MIMO throughput with per-antenna
rate control”,
3GPP TSG RAN WG1, Document TSG-R1 (01) 0879, September 2001

http://hawk.iszf.irk.ru/URSI2002/GAabstracts/papers/p2258.pdf

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