Application of cooperative relaying schemes in downlink/uplink communications in cellular networks
George, Joseph Sanju
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In this thesis we have compared the Bit Error Rate (BER) performance of Amplify and Forward (AF), Ideal Decode and Forward (DF) cooperative communication system with conventional system where no cooperation is used. The BER performance of AF, ideal DF cooperative system and conventional system with no cooperation using BPSK is plotted against P/No. An interesting observed made is that both AF and ideal DF cooperative system are comparable in performance when the relay is selected at a faster rate and the channel is estimated at a faster rate for a wider range of P/No values. However, the ideal DF cooperative system shows better BER performance than AF cooperative system when the relay is selected at a slower rate and channel is estimated at a slower rate. At a faster relay selection and channel estimation rate AF cooperative system is better than ideal DF cooperative system since BER performance is comparable and complexity of AF is much less than DF cooperative system. One disadvantage of increasing the relay selection and channel estimation rate is that the overhead time used to select the relay increases and the throughput efficiency is reduced. It can be concluded that for a system using AF there is a tradeoff between the BER performance and throughput efficiency. The second interesting observation is that when the relay is selected and channel is estimated at a much slower rate BER performance of both AF and ideal DF cooperative system saturates for higher values of P/No. No improvement in BER performance of both AF and ideal DF cooperative system with increase in P/No was observed for slower relay selection and channel estimation rate after a certain value of P/No. We have also proposed a relay selection protocol for cooperative systems using AF cooperation protocol. The proposed relay selection protocol in this thesis has some advantages over the existing protocols proposed for cellular communication. In our protocol the relay selection is always done at the Base Station for both Uplink and Downlink transmission. This allows us to have a system in which the hardware and software needed for relay selection to be present only at the base station. Also the cooperation threshold value need not be known to all the mobile phones in the cellular network since only the Base Station will use it.