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Asynchronous Transmission in Multiuser Networks


Time asynchrony inherently exists in many wireless communication systems, specially in multiuser scenarios where the signals come from different 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 is a com- mon presumption. However, it might be impossible to synchronize all the nodes even if an ideal infrastructure with signal overheads is considered. For example, if the receiver encom- passes multiple receive antennas or there are multiple distributed base stations, then, the synchronization can be realized at one of them at most. Thus, it is of great importance to investigate the effect of the time asynchrony in the wireless systems. One natural question is that how to eliminate the time asynchrony and make all the received signals aligned at the receiver. This question is analyzed under the notion of time synchronization. There are many methods in literature trying to achieve this goal. However, the other question which is atypical but even more important is that, is synchronizing the received signals necessary? does the time asynchrony degrade the performance?

When the receiver is designed with the presumption of having perfect synchronization, YES, the time asynchrony will degrade the performance. Nevertheless, what if we design the system and the receiver structure with TIME ASYNCHRONY in mind. Does the system that is designed based on the time asynchrony provide worse performance compared to the synchronous one? We will thoroughly investigate this question in this thesis. In a nutshell, we show that by investigating inherent time delays between different users in a multiuser/multi- antenna scenario, we are able to improve the performance. By using proper transmission and receiver design, time asynchrony provides additional degrees of freedom in a time limited communication settings which can be exploited to improve the performance.

In Chapter one, more details about time synchronization and the problem of time asynchrony are presented. Then, we introduce the proper structure for exploiting time asynchrony and present the resulting system model. We also explain the reason behind possible advantages of time asynchrony. In Chapter two, we show implementation of different detection methods based on the asynchronous system model. We include different methods like maximum likelihood sequence detection (MLSD), successive interference cancellation (SIC) and Zero Forcing (ZF). In Chapter three, we analytically analyze the achievable performance by the asynchronous transmission and compare it with the conventional synchronous transmission. Two performance criteria are considered. One is the bit error rate (BER) in a fading channel. The other one is the achievable rate in an additive white Gaussian noise (AWGN) channel. The diversity gain of the BER performance and the multiplexing gain of the achievable rates are also derived. Finally, in Chapter 4, simulation results, some discussions and future work are presented.

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