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Similarly to the Multiple Input Multiple Output (MIMO) technique with a character of spatial diversity, the cooperative communication system improves the system performance and becomes an important issue. (Astely, Dahlman, Furuskar, Jading, Lindstrom & Parkvall, 2009; Li, Li, Lee, Lee, Mazzarese, Clerckx, & Li, 2010; Sugumaran, Raghunathan & Vivekanandan, 2009; Laneman, Tse & Wornell, 2004; Sendonaris, Erkip, & Aazhang, 2003; Nosratinia, Hunter & Hedayat, 2004; Ibrahim, Sadek, Su & Liu, 2008; Nosratinia & Hunter, 2007) Comparing with multiple carrier modulation schemes and MIMO schemes, the cooperative transmission is with a low implementation cost and a less complexity of the device and provides a high throughput performance. The relays station in the cooperative communication system could provide the spatially diversity gain by forwarding the source’s message to the destination similar to the multiple input multiple output (MIMO) technique (Sreng, Yanikomeroglu & Falconer, 2003; Sadek, Su & Liu, 2007). Although MIMO could also provide spatially diversity gain, the implementation cost and complexity of the device increase with the number of antennas. At the environment without multiple antennas, the cooperative communication system could work with spatial diversity. It increases the transmission data rate and provides a reliable channel capacity by employing the relay station. (Hong, Huang, Chiu, & Kuo, 2007; Bletsas, Shin & Win, 2006; Shi, Yu, Zhang, Chen & Qiu, 2007) The cooperative communication system could be the tendency for the future communications under the low cost consideration.
Basically, there are three cooperative strategies used for the cooperative communications. (Yang, Lin, & Wen, 2014; Wen, Chiang, Lin, & Yang, 2014; Bletsas, Khisti, Reed & Lippman, 2006) One is Amplify-and-Forward (AF) mode, another is Decode-and-Forward (DF) mode and the other is Compress and Forward (CF) mode. The transmitted signal could be amplified and retransmit to destination with AF mode (Bletsas, Shin & Win, 2007). It is easy to be implemented with a low complexity. It is good to be used to those short distance transmissions without a serious degrading factor. On the other hand, the relay station decodes and demodulates the received signal and, then, recodes and modulates the signal to retransmit to the destination with DF mode (Laneman & Wornell, 2003; Janani, Hedayat, Hunter & Nosratinia, 2004). The relay station in CF mode does not have to decode the compressed signal. It uses the coding schemes to compress the received signal and retransmit to the destination. Generally, these three modes could effectively improve the performance of system. Among those three modes employed in the cooperative communication systems, AF mode could be implemented with a low complexity (Sendonaris, Erkip, & Aazhang, 2003; Yang, Lin, & Wen, 2014). The relay station forwards and augments the attenuated received signal. Under the high signal to noise ratio (SNR) environments, DF mode could be more effective. The relay station decodes the received signal and, retransmits the coded signal. The transmitted signal from the relay station could be the same signal as it is sent from the source station. The cooperative communication could obtain the coding gain because of the error coding strategy. With consideration of the bandwidth efficiency, CF mode could be employed by data compressing technique. It is suitable to the band limited transmission channel (Kramer, Gastpar, & Gupta, 2005).