Abstract:
This dissertation is concerned with the use of current Smart Antenna Systems
(SAS) for utilizing the spatial dimensions of the wireless channel to improve upon the
capacity of wireless systems. This enhancement is achieved for downlink Multiple
Input Single Output (MISO) wireless channels by augmenting link quality by the
provision of beamforming, diversity and interference cancellation gain in SAS. One
such system consists of a Transmit Beamformer (TB) at the base Station (BS) which
provides beamforming gain, interference cancellation gain and, on provision of
feedback from the Mobile Station (MS), gives beamforming gain even in channels
undergoing independent fading at the antennas of BS. The beamforming gain of the
TB is reduced in those fast fading channels that are uncorrelated across the antennas
of the Transmit Antenna Array (TAA).
Currently researchers are actively pursuing TB techniques based on element
space beamforming. In an Element Space Transmit Beamformer (ES TB), each
antenna weight of the TAA is individually adaptive to the varying channel conditions.
This thesis, in contrast, presents an Adaptive Transmit Beamspace Beamformer
(ATBBF) that uses beamspace beamforming to provide beamforming gain. An
ATBBF consists of several orthogonal Transmit Beamformers (TBs) on the TAA. In
beamspace beamforming a specific set of antenna weights determines an orthogonal
direction of the TB. This weight vector (corresponding to each orthogonal beam) is
further weighted by an adaptable weight. This weight, called the beamspace weight,
thus adaptively weighs one orthogonal beam and is updated based on the changing
channel conditions. An ATBBF deploys this set of orthogonal beams in space to
transmit signals to a MS.
The superposition of adaptively weighted orthogonal beams in space forms the
beam pattern of an ATBBF directed in the desired direction. Simulations in single,
double and triple multipath static wireless channels demonstrate the unique evolution
and convergence of the ATBBF's beam pattern in relation to the convergence of its
beamspace weights. The relationship between the magnitude and convergence time of
a beamspace weight upon the multipath wireless channel is also elaborated.
Furthermore, for the above channels, performance curves of both implementations of
an ATBBF i.e. Full Dimension Adaptive Transmit Beamspace Beamformer (FD
14ATBBF) and Reduced Dimension Adaptive Transmit Beamspace Beamformer (RD
ATBBF), along with that of an ES TB (having similar feedback mechanism) are
simulated with uniform initial adaptable weights. Their analyses reveal that contrary
to the RD ATBBF and ES TB, the FD ATBBF converges to optimum performance in
all channels. The FD ATBBF also has a lesser convergence time than the TB in most
double and triple multipath channels.
The performance metric of an ATBBF is derived and analyzed in a dynamic
channel undergoing Rayleigh fading independently at the antennas. A performance
comparison between a FD ATBBF, RD ATBBF and an ES TB is made in terms of
convergence and tracking of various slow and fast fading channels by simulations.
Comparisons show that the FD ATBBF gives a performance equivalent to that of an
ES TB and outperforms the RD ATBBF. Thus the FD ATBBF can provide
beamforming gain and fading diversity similar to that of an ES TB. The performance
of the FD ATBBF improves on increasing the number of antenna elements of the
TAA from two to four i.e. from 2.91 dB in slow fading to 0.01 dB in fastest fading
dynamic channel.
Bit Error Rate (BER) studies for the above dynamic channels in the presence
of noise are carried out to compare the BER curves of the FD ATBBF, ES TB and RD
ATBBF with non beamforming techniques like Space Time Codes (STC). The studies
confirm the performance analysis above by noting that the BER curves of the FD
ATBBF and ES TB are similar in slow and fast fading channels and both outperform
the RD ATBBF. The FD ATBBF also gives a gain of 2.2 dB and 4.8 dB over STC in
slow fading for two and four antennas respectively. Furthermore, for four antennas,
the FD ATBBF and RD ATBBF offer double data rate than the best performing STC
technique, under constrained bandwidth requirements.
In order to improve the performance of an ATBBF in fast fading channels for
two antennas, a simple combination of an ATBBF with STC called the Space Time
Coded Adaptive Transmit Beamspace Beamformer (STCATBBF) is proposed. This
technique is made possible by utilizing the orthogonal beams of an ATBBF. The BER
curves of the STCATBBF are simulated and compared with the FD ATBBF and STC
in fast and slow flat fading channels. It is shown that in fast fading channels the
STCATBBF gives a gain of 2.2 dB to 5.4 dB over the FD ATBBF, while in slow
fading it outperforms STC .