UC Irvine

Time asynchrony inherently exists in many wireless communication systems, especially in

multiuser scenarios, where the users are located in various locations. Different locations

and paths impose different delays on the received signals, resulting in asynchronous reception

at the receiver. In most of the works in the literature, perfect synchronization among

received signals is a common presumption. However, it might be impossible to synchronize

signals at all the nodes in the network even if an ideal infrastructure using control

signals is considered. For example, assume that the receiver encompasses multiple receive

antennas or multiple distributed base stations. Then, although the synchronization can

be realized at one of them, realizing synchronization at all of the base stations/antennas

might be impossible. Thus, it is essential to investigate the effect of the time asynchrony

in the wireless systems, particularly multiuser systems.

Asynchrony naturally imposes some performance degradation in a system designed optimally

based on having synchronized incoming signals. One immediate solution is to

eliminate the time asynchrony and achieve almost perfect synchronization among the received

signals. This question is analyzed under the notion of time synchronization. There

are many methods proposed and analyzed in literature aiming for that goal, and one of the common ones is to use control signals to achieve synchronization among different users.

However, apart from this approach's feasibility, the questions which are atypical but of

importance to answer are: Can a system be designed such that it provides performance

improvement under an asynchronous condition? Can a system, which is designed based

on the asynchronous assumption, outperform its counterpart system, which is designed based

on the assumption of perfect synchronization?

We thoroughly investigate these questions in this thesis. First, we theoretically analyze

the performance bounds of multi-antenna, multiuser systems under the asynchrony assumptions.

We show that by exploiting inherent time delays between different users in

a multiuser/multi-antenna scenario, we can improve the performance. Besides, we propose

to intentionally add timing osets in the systems that are not inherently impaired

by time asynchrony. We introduce the optimal transceiver designs under asynchronous

assumptions and analyze the performance improvement provided. We consider various

multiuser networks, including broadcasting networks, multiple access networks, and cooperative

networks, and examine the advantages and disadvantages of having asynchrony

in such multiuser networks.