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|Title: ||Robust transmit beamforming design using outage probability specification|
|Authors: ||Du, Huiqin|
|Supervisor(s): ||Chung, Pei-Jung|
|Issue Date: ||2010|
|Publisher: ||The University of Edinburgh|
|Abstract: ||Transmit beamforming (precoding) is a powerful technique for enhancing the channel capacity
and reliability of multiple-input and multiple-output (MIMO) wireless systems. The optimum
exploitation of the benefits provided by MIMO systems can be achieved when a perfect channel
state information at transmitter (CSIT) is available. In practices, however, the channel knowledge
is generally imperfect at transmitter because of the inevitable errors induced by finite
feedback channel capacity, quantization and other physical constraints. Such errors degrade the
system performance severely. Hence, robustness has become a crucial issue.
Current robust designs address the channel imperfections with the worst-case and stochastic approaches.
In worst-case analysis, the channel uncertainties are considered as deterministic and
norm-bounded, and the resulting design is a conservative optimization that guarantees a certain
quality of service (QoS) for every allowable perturbation. The latter approach focuses on the
average performance under the assumption of channel statistics, such as mean and covariance.
The system performance could break down when persistent extreme errors occur. Thus, an
outage probability-based approach is developed by keeping a low probability that channel condition
falls below an acceptable level. Compared to the aforementioned methods, this approach
can optimize the average performance as well as consider the extreme scenarios proportionally.
This thesis implements the outage-probability specification into transmit beamforming design
for three scenarios: the single-user MIMO system and the corresponding adaptive modulation
scheme as well as the multi-user MIMO system. In a single-user MIMO system, the transmit
beamformer provides the maximum average received SNR and ensures the robustness to the
CSIT errors by introducing probabilistic constraint on the instantaneous SNR. Beside the robustness
against channel imperfections, the outage probability-based approach also provides a
tight BER bound for adaptive modulation scheme, so that the maximum transmission rate can
be achieved by taking advantage of transmit beamforming. Moreover, in multi-user MIMO
(MU-MIMO) systems, the leakage power is accounted by probability measurement. The resulting
transmit beamformer is designed based on signal-to-leakage-plus-noise ratio (SLNR)
criteria, which maximizes the average received SNR and guarantees the least leakage energy
from the desired user. In such a setting, an outstanding BER performance can be achieved as
well as high reliability of signal-to-interference-plus-noise ratio (SINR).
Given the superior overall performances and significantly improved robustness, the probabilistic
approach provides an attractive alternative to existing robust techniques under imperfect
channel information at transmitter.|
|Keywords: ||transmit beamforming|
|Appears in Collections:||Engineering thesis and dissertation collection|
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