A smart scheme for relay selection in cooperative wireless communication systems
Ivo Sousa
0
Maria Paula Queluz
0
Antnio Rodrigues
0
0
Instituto de Telecomunicaes / Instituto Superior Tcnico, Technical University of Lisbon
, Lisbon 1049-001,
Portugal
The performance of multiple-input multiple-output systems using spatial multiplexing can be degraded when the spatial information channels are correlated. This work proposes a solution to this problem, which is based on a cooperative wireless communication system. Within the cooperative system, the relay is selected either randomly or using the smart selection scheme, a simple and distributed approach proposed herein. These relay selection schemes are evaluated for several situations within a small cell environment, using a simulator that generates frequency-selective channel realizations. The simulation results show that the smart selection scheme yields high capacity gains close to the theoretical maximum gain.
1 Introduction
In wireless communication systems, the wavelength and
the distance between transmitter and receiver terminals
are not the only factors that characterize the radio wave
propagation phenomenon. All the interacting objects
present in the surrounding environment where waves
bounce, as well as their dimension and composition, also
contribute to the propagation phenomenon. These
interacting objects are usually grouped into clusters and induce
scattering, where one or more non-uniformities in the
medium force radio waves to deviate from a straight
trajectory. Scattering leads to a propagation phenomenon
known as multipath propagation, where the transmitted
data reach the receiver multiple times by two or more
paths and/or at different time instants. These different
copies of the transmitted signal, having each one a
different attenuation, delay, and phase shift, create an
amplified or attenuated received signal power, depending on
whether the interference is constructive or destructive,
respectively. This is a random process designated as
multipath fading, which may vary according to time, space,
and/or frequency. Another type of fading is the shadow
fading, also a random process due to shadowing from
obstacles affecting the wave propagation.
Fading can be very harmful for any wireless
communication system as it can cause a strong destructive
interference resulting in a deep loss of signal, which in turn
can lead to data transmission failure. One way to cope
with this issue is to use multiple-input multiple-output
(MIMO) systems. These systems can be defined, in a
simple way, as wireless communication systems equipped
with multiple antenna elements at the transmitter and at
the receiver. MIMO systems exploit multipath and
fading propagation phenomena so as to achieve high spectral
efficiencies without requiring extra frequency spectrum
and transmission power [1,2]. However, the real benefit of
MIMO systems does not come from multiple antennas by
themselves, but from the way these systems process the
antennas signals using, e.g., spatial diversity and spatial
multiplexing [3]:
Spatial diversity is a powerful technique to mitigate
fading and increase link reliability; it combines, in the
receiver, different signals from the radio channel,
originated by multipath propagation, in order to
obtain the sources stream in better conditions. With
this MIMO technique, receiver antennas can provide
power gain and, if space-time codes are used, spatial
diversity transmission gain can also be achieved. All
of this can be reached without requiring channel
knowledge at the transmitter, i.e., without channel
state information (CSI) prior to any data transmission.
Spatial multiplexing is a technique that exploits
differences in the spatial signatures (e.g., caused by
rich scattering) of multiplexed data streams onto the
wireless channel so as to separate the different
signals, i.e., orthogonal information channels are
created when there is significant spatial
decorrelation. This can be seen as an additional
spatial dimension for communication that yields a
degree-of-freedom gain without additional power,
time, or bandwidth. Hence, the system capacity can
be increased linearly by a factor n, where n is the
minimum number of transmit and receive antennas.
This MIMO technique can be used with or without
CSI at the transmitter (a system with full CSI can lead
to higher spectral efficiencies than a system where
CSI is only available at the receiver).
The use of MIMO systems in the spatial multiplexing
mode may bring improvements in terms of spectral
efficiency. However, since the spectral efficiency gain lies on
the fact that the user is in the presence of rich
multipath, the MIMO spectral efficiency gain will decrease for
spatially correlated channels. One possible way to
circumvent this problem is to increase the separation among the
antennas at a communication end, resulting in a higher
antenna decorrelation. For the base station (BS) side,
increasing the antenna array size might not be a problem,
but for the mobile station (MS) side, (...truncated)