Impacting Of Different Modulations On The Performance Of Cooperative Communication Over Time-varying Channel

In order to combat multi-path fading and improve performance of the communication link, a variety of diversity techniques are used in the modern wireless communication system. The principle of classical diversity and combination is that, two or more channels carry copies of the signal which are to be combined in the receiver to increase the overall received SNR (signal-noise ratio). But there are so many limits in the actual communication system, as the size, consumption and cost, it's hard to apply the space diversity. As the result, the cooperative diversity technique is investigated.Cooperative diversity technique is a new kind of space diversity, two mobile terminals with single or more antennas relay information for each other. Thus, cooperative diversity technique has been widely discussed as a way to form virtual arrays without using multiple antennas. Various results show that, cooperative diversity can increase the capacity of communication system, higher the data rate, combating the fading effectively. At the same time, this technique can lower the outage probability of service, and increase the quality of service and reliability too.Firstly, modulation is a key technology in modern digital communication system. Until now, many kinds of modulations have been discussed, just like binary PSK (phase shift keying), coherent FSK (frequency shifting keying), and differential PSK. These modulations will be used in different environments because of their own characteristics. This paper investigates the performance of four basic modulation schemes in rapidly time-varying channels. We get the first order statistics of time-varying Rayleigh channels by utilizing the AR (auto-regressive) model. After the theoretical analysis and derivation, we summarize the bit error rate formulas which present their relationship with the channel varied rate, coding length and SNR. Besides, we study the factors which can produce the variation of channel.Then, we discuss the improvement of system performance when we use cooperative diversity to combat Rayleigh fading, by taking AF (amplifying forward) mode for example. After the research, we know that the performance of every kind of modulations will get better.Finally, we study the performance of incremental relaying cooperative diversity with different modulations and different thresholds over independent non-identical Rayleigh fading channels. For every kind of modulation, mathematical equation for bit error rate and the SNR has been constructed. In the last place, this paper explores the effect of threshold to the incremental relaying cooperative diversity.