IEEE JOURNAL ON SELECT AREAS IN COMMUNICATIONS, VOL. 16, NO. 8, OCTOBER 1998 1451
A Simple Transmit Diversity Technique
for Wireless Communications
Siavash M. Alamouti
Abstract— This paper presents a simple two-branch trans-
mit diversity scheme. Using two transmit antennas and one
receive antenna the scheme provides the same diversity order
as maximal-ratio receiver combining (MRRC) with one transmit
antenna, and two receive antennas. It is also shown that the
scheme may easily be generalized to two transmit antennas and
MMM
receive antennas to provide a diversity order of 2
MMM
. The
new scheme does not require any bandwidth expansion any
feedback from the receiver to the transmitter and its computation
complexity is similar to MRRC.
Index Terms—Antenna array processing, baseband processing,
diversity, estimation and detection, fade mitigation, maximal-
ratio combining, Rayleigh fading, smart antennas, space block
coding, space–time coding, transmit diversity, wireless commu-
nications.
I. INTRODUCTION
T
HE NEXT-generation wireless systems are required to
have high voice quality as compared to current cellular
mobile radio standards and provide high bit rate data ser-
vices (up to 2 Mbits/s). At the same time, the remote units
are supposed to be small lightweight pocket communicators.
Furthermore, they are to operate reliably in different types of
environments: macro, micro, and picocellular; urban, subur-
ban, and rural; indoor and outdoor. In other words, the next
generation systems are supposed to have better quality and
coverage, be more power and bandwidth efficient, and be
deployed in diverse environments. Yet the services must re-
main affordable for widespread market acceptance. Inevitably,
the new pocket communicators must remain relatively simple.
Fortunately, however, the economy of scale may allow more
complex base stations. In fact, it appears that base station
complexity may be the only plausible trade space for achieving
the requirements of next generation wireless systems.
The fundamental phenomenon which makes reliable wire-
less transmission difficult is time-varying multipath fading [1].
It is this phenomenon which makes tetherless transmission a
challenge when compared to fiber, coaxial cable, line-of-sight
microwave or even satellite transmissions.
Increasing the quality or reducing the effective error rate in
a multipath fading channel is extremely difficult. In additive
white Gaussian noise (AWGN), using typical modulation and
coding schemes, reducing the effective bit error rate (BER)
from 10
to 10 may require only 1- or 2-dB higher signal-
to-noise ratio (SNR). Achieving the same in a multipath fading
Manuscript received September 1, 1997; revised February 1, 1998.
The author was with AT&T Wireless Services, Redmond, WA, USA. He is
currently with Cadence Design Systems, Alta Business Unit, Bellevue, WA
98005-3016 USA (e-mail: siavash@cadence.com).
Publisher Item Identifier S 0733-8716(98)07885-8.
environment, however, may require up to 10 dB improvement
in SNR. The improvement in SNR may not be achieved by
higher transmit power or additional bandwidth, as it is contrary
to the requirements of next generation systems. It is therefore
crucial to effectively combat or reduce the effect of fading at
both the remote units and the base stations, without additional
power or any sacrifice in bandwidth.
Theoretically, the most effective technique to mitigate mul-
tipath fading in a wireless channel is transmitter power control.
If channel conditions as experienced by the receiver on one
side of the link are known at the transmitter on the other side,
the transmitter can predistort the signal in order to overcome
the effect of the channel at the receiver. There are two
fundamental problems with this approach. The major problem
is the required transmitter dynamic range. For the transmitter
to overcome a certain level of fading, it must increase its power
by that same level, which in most cases is not practical because
of radiation power limitations and the size and cost of the
amplifiers. The second problem is that the transmitter does
not have any knowledge of the channel experienced by the
receiver except in systems where the uplink (remote to base)
and downlink (base to remote) transmissions are carried over
the same frequency. Hence, the channel information has to be
fed back from the receiver to the transmitter, which results
in throughput degradation and considerable added complexity
to both the transmitter and the receiver. Moreover, in some
applications there may not be a link to feed back the channel
information.
Other effective techniques are time and frequency diversity.
Time interleaving, together with error correction coding, can
provide diversity improvement. The same holds for spread
spectrum. However, time interleaving results in large delays
when the channel is slowly varying. Equivalently, spread spec-
trum techniques are ineffective when the coherence bandwidth
of the channel is larger than the spreading bandwidth or,
equivalently, where there is relatively small delay spread in
the channel.
In most scattering environments, antenna diversity is a
practical, effective and, hence, a widely applied technique
for reducing the effect of multipath fading [1]. The classical
approach is to use multiple antennas at the receiver and
perform combining or selection and switching in order to
improve the quality of the received signal. The major problem
with using the receive diversity approach is the cost, size,
and power of the remote units. The use of multiple antennas
and radio frequency (RF) chains (or selection and switching
circuits) makes the remote units larger and more expensive.
As a result, diversity techniques have almost exclusively been
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