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Nima Khajehnouri 2006
Abstract of PhD Dissertation
Electrical Engineering Department, UCLA, June 2006
Advisor: Prof. Ali H. Sayed

Signal Processing for Wireless Relay and Broadcasting Networks

Nima Khajehnouri, UCLA

Spatial diversity techniques provide the receiver with multiple replicas of the transmitted signal and help combat fading effects over wireless links. This dissertation contributes to two types of diversity approaches. One type is based on using precoding schemes for multi-user wireless downlink channels when multiple antennas are employed at both the transmitter and receiver. The second type is based on using relay strategies and cooperative communication.

The proposed MIMO precoding schemes differ in several respects from previous work. First, each user can have multiple receive antennas and the transmitter uses multiple beams instead of a single beam in order to transmit multiple symbols simultaneously to each user. This generalization is useful for space-time coding schemes. We select the beamforming weights from the channel space of all users excluding the desired user, while at the same time maximizing the link capacity. Second, several power allocation schemes are derived for the cases of a homogeneous network (where all users have the same transmit power), a non-homogeneous network (where users can have different transmit powers), as well as for networks with a uniform quality of service (i.e., same throughput for all users). Third, we design precoders that minimize the interference power caused by a user on all other users, as opposed to forcefully nulling the interference. The resulting schemes relax the traditional constraint on the number of transmit and receive antennas.

We also propose and study two-hop multi-sensor relay strategies that achieve minimum mean-square-error estimation between the transmitted signal and the uncorrupted received signal with both local and global power constraints. The capacity of the resulting relay link, as well as its diversity order, are studied. The effect of channel uncertainties on system performance is examined and a modified relay scheme is proposed. We further study a multi-destination relay network, and we propose both power constrained and non-power constrained relay schemes that achieve the minimum mean-square estimate of the transmitted signal at the destination node.

Finally, we propose a globally distributed synchronization algorithm for wireless sensor networks. In the proposed scheme, there is a master node that controls the network synchronization and the other sensors periodically broadcast synchronization pulses; they also monitor different frequency channels in an effort to overcome the effect of channel fading.

Acknowledgment This work was supported in part by the National Science Foundation under grants CCF-0208573, ECS-0401188, and ECS-0601266. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.