Adaptation Techniques and Enabling Parameter Estimation Algorithms for Wireless Communications Systems - Adaptive Signal Processing in Wireless Communications - page 12

Remote
reflections

Transmitter

Receiver-2

Receiver-1

Angle Spread
(Degrees)

Angle spread
(Degrees)

FIgure 1.4 Illustration of the effect of different local scattering in angle of arrivals. Receiver-1
observes less angle spread than Receiver-2. Therefore, receiver antennas in Receiver-1 will have
more correlations.

is expected that the need for understanding spatial selectivity and related parameter
estimation techniques will gain momentum. Spatial selectivity will especially be useful
when the requirement for placing antennas close to each other increases, as in the case
of multiple antennas at the mobile units.
Spatial correlation between multiple-antenna elements is related to the spatial selec-
tivity, antenna distance, mutual coupling between antenna elements, antenna patterns,
etc. [35, 36]. Spatial correlation has significant effects on multiantenna systems. Full
capacity and performance gains of multiantenna systems can only be achieved with low
antenna correlation values. However, when this is not possible, maximum capacity can
be achieved by employing efficient adaptation techniques. Adaptive power allocation is
one way to exploit the knowledge of the spatial correlation to improve the performance
of multiantenna systems [37]. Similarly, adaptive modulation and coding, which employs
different modulation and coding schemes across multiantenna elements depending on
the channel correlation, is possible [38, 39]. In MIMO systems, adaptive power alloca-
tion has been studied by using the knowledge of channel matrix estimate and eigenvalue
analysis [40, 41].

1.3.2 Channel Quality Measurements
Channel quality estimation is by far the most important measurement that can be used
in adaptive receivers and transmitters [3]. Different ways of measuring the quality of
radio channel are possible, and many of these measurements are done in the physical
layer using baseband signal processing techniques. In most of the adaptation algorithms,