Performance Evaluation of Asynchronous Distributed Space Time Block Coded System for Cooperative Communication

Abstract

Cooperative communication has attracted much attention of the researchers for being one of the potential candidates for the 5G and future generations of wireless communications. It forms a virtual MIMO antenna array by utilizing a third terminal called relay which assists the direct communication. Benefits of multiple inputs multiple outputs (MIMO) designed specifically are reaped by the cooperative communication using spatially distributed antennas. The exorbitant cost of multiple antenna installation and system complexity are thus avoided in cooperative communications. There are added advantages of increased data rates, network capacity, reliability, network range, etc at the cost of system and computational complexity. The distributed space time block codes (DSTBC) is a scheme where the signals arriving from the relays are stacked as individual rows of STBC at the receiver. Due to the random spatial positioning and different times of transmission from the relays, the DSTBC system is prone to timing delays, thereby introducing asynchronicity to it. Thus cooperative communications are inherently asynchronous in nature. Several delay tolerant techniques which maintain diversity benefits in asynchronous cooperative communication have been presented in the existing literature. The objectives formulated and achieved in this thesis are based upon an asynchronous DSTBC (ADSTBC) system which yields diversity and capacity gains in terms of Average Bit error Rate (ABER) and Ergodic capacity respectively, in delay prone environment. The ADSTBC system is designed using Optimized Asynchronous Linear Dispersion (OALD) matrices. The basic system model is improved by employing Optimal Relay Select ion (ORS). Results show that the dual relay selection provides significant performance gains in terms of increased diversity and reduced complexity as compared to ADSTBC systems. Also from the results, it is concluded that as the number of candidate relays increase, the performance increase along with. The error performance of the system improves significantly by concatenating it with powerful error correcting codes such as Low Density Parity Check (LDPC) codes in this work. The concatenated LDPC-ADSTBC system shows enhanced reliability in terms of Outage Probability and Pair wise Error Probability (PEP). The LDPC-ADSTBC system overrides the ADSTBC system in terms of power saving for a given value of performance index/value/criteria. The error performance of the proposed concatenated DSTBC system is compared against another time domain technique TR-STBC designed for asynchronous scenario. The results confirm the better results of the former against the latter. The performance of LDPC-ADSTBC system can further be increased by joint relay and transmitreceive antenna selection (RTRAS) as RTRAS-LDPC-ADSTBC system. The ADSTBC as well as the concatenated LDPC-ADSTBC systems are analyzed in different fading scenarios such as Rayleigh and Rician. The simulated res ults show conformity with their analytical versions.