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.