TEL AVIV UNIVERSITY Gaddi Blumrosen

4. Combining STC with BF
4.1 Introduction
Information about the channel realization (CSI), if it is available, could be utilized to
maximize the performance. As described in chapter 3, two main techniques, BF and
STC, exist. In BF, a certain level of CSI, channel realization or statistics is a must in
the transmitter and receiver for spatial pre-coding before transmission and spatial
post-coding in reception. This family mainly works on a close loop. BF exploits space
selectivity some times with time and frequency selectivity, optimal only with perfect
CSI and the performance degraded significantly as CSI quality decreases. In STC
family, explicit CSI knowledge does not have to be known to the transmitter but just a
general statistical model of the channel is assumed in STC design, mostly
uncorrelated channel as in rich scattering environment. STC mainly works on open
loop. STC family exploits the space selectivity and time selectivity by means of the
diversity order of the system. The performance has a degradation given by (3.22), for
low rank channels (highly correlated sub-channels from channel conditions or due to
closely spaced antennas constrained by operators' space limitations) in coding gain
and consequently to performance loss to the over all gain. Thus, STC is only optimal
when the statistical channel model, usually uncorrelated fading due to wide antenna
spacing, many scatterers, is realistic.
For integrating the benefits of those two methods, and for achieving maximum
spectral efficiency, some new approaches for combining these two families of
techniques were recently developed and investigated. These different methods for
combining STC and BF, differ in optimization criterion-maximization of received
SNR, minimizing of Symbol Error Rate (SER), or maximization the transmission rate,
and also in the kind of CSI available at transmitter and correspondingly, the way CSI
is being exploited.
The first and simplest optimization criterion is maximum received SNR, e.g. third
generation standards as in [35], and also in [36] and [35]. In [35], the transmit BF
weights are calculated by the mobile in such a way they maximize the received SNR
and then fed back to the base station. [35] uses channel absolute mean value feedback
for maximizing the received SNR. It suffers from lack of channel phase exploitation
and thus loss in lack of directivity of the antenna pattern. [36] uses received SNR
criterion for the case of two transmit antennas and one receive antenna is deployed
with several known multi-paths with delay spread shorter than the symbol duration. It
results in multi-beam BF in the multi-paths direction. In general, the criterion of
received SNR maximization is inferior to other optimization criteria such as
minimizing SER or rate maximization, which include in their optimization criteria
channel statistics of higher order than the SNR. Only in the special case, where CSI is
perfectly known at the transmitter, minimizing the SER is equivalent to maximizing
the average SNR, and the optimal solution is BF only in the directions of the strongest
multi-path [47].
An optimization criterion based on rate maximization was used e.g. in [31], [44]. [31],
showed expressions for the achievable transmission rate as a function of CSI
knowledge at the transmitter. [44] derives spatial linear pre-coding (BF antenna
weights) maximizing information transfer rate for the two extreme cases of feedback.
The first one mean feedback is when channel side information resides in the mean of
the distribution and the covariance modeled as white. This kind of feedback can