在無線通訊中，合作式通訊是一種可以提供空間多樣性的新技術。利用中繼點的幫助，可以讓傳輸資料更為可靠，並且相較於多輸入多輸出系統，更能夠降低硬體設計上的複雜度，並且降低硬體成本花費。因此，如何在網路中選擇一個最佳的中繼點成為了重要的一個課題。近來，研究者提出了許多中繼點選擇法。其中有些方法，需藉由大量的運算以及通道估測來達到較好的效能、較高的多樣性，然而，這些複雜度以及運算量，對於一個能量限制的系統會是一個很大的負擔，例如：感測網路。所以漸漸有一些方法的目的是將整體系統複雜度以及運算量降低，但是不可避免的，同時也降低了許多效能。因此，如何在系統效能以及複雜度之中取捨，亦是一個值得研究的議題。
在本篇研究中，我們提出兩個新的中繼點選擇法。這兩個方法皆需要部分的通道狀態資訊以及距離資訊來協助選擇最佳的中繼點，藉由這兩個方法，可以保持一定的效能，並且降低系統的複雜度。為了驗證效能，我們提供了數個電腦模擬結果，其顯示我們的方法能夠有效的降低錯誤率，而且其效能優於根據距離資訊的中繼點選擇法，而且運算量以及複雜度低於根據瞬間通道資訊的中繼點選擇法。此外，我們也模擬了所提出兩個中繼點選擇法之間效能的比較，其模擬結果顯示，這兩個中繼點選擇法的效能會根據中繼點的擺放位置不同而相異。

In wireless networks, cooperative relaying is an emerging technique that provides spatial diversity in an alternative way. With the help of relays, the source has more opportunities to transmit its data to the destination; in other words, the increased degrees of freedom due to relays can be utilized to combat channel impairments effectively. Therefore, how to choose the optimal relay among a set of available relays for meeting some requirements naturally becomes a critical issue. Recently, various relay selection schemes being proposed aim at choosing one and the best relay in cooperative wireless networks. Some of them require a lot of channel estimations to achieve full diversity order; some of them, on the other hand, avoid high computational complexity as well as frequent channel estimations, but the diversity order is reduced to 2 only. It is the trade-off between the system performance and complexity.
In this thesis, we propose two relay selection schemes which need the partial channel state information (CSI), including instantaneous channel power and distance information, to improve the system performance and reduce the system complexity, compared to the existing work. Simulation results demonstrate that the proposed relay selection schemes can achieve better SER performance than the geographic-based scheme and worse SER performance than the instantaneous channel-gain-based scheme.

[1] A. Sendonaris, E. Erkip, and B. Aazhang, “User cooperation diversity-Part I: System description and user cooperation diversity,”IEEE Trans. Commun., vol. 51, no. 11, pp. 1927-1938, Nov. 2003.
[2] A. Sendonaris, E. Erkip, and B. Aazhang, “User cooperation diversity-Part II: Implementation aspects and performance analysis,”IEEE Trans. Commun., vol. 51, no. 11, pp. 1938-1948, Nov. 2003.
[3] T. Cover and A. E. Gamal, “Capacity theorems for the relay channel,” IEEE Trans. Inform. Theory, vol. 25, no. 5, pp. 572-584, Sep. 1979.
[4] J. N. Laneman, D. N. C. Tse, and G. W. Wornell, “Cooperative diversity in wireless networks: efficient protocols and outage behaviour,”IEEE Trans. Inform. Theory, vol. 50, pp. 3062-3080, Dec. 2004.
[5] M. Janani, A. Hedayat, T. E. Hunter, and A. Nosratinia,“Coded cooperation in wireless communications: Space-time transmission and iterative decoding,” IEEE Trans. Signal Processing, vol. 52, no. 2, pp. 362-380, Feb. 2004.
[6] W. Su, A. S. Ibrahim, A. K.Sadek, and K. J. R. Liu, “Cooperative Communications with Relay Selection: When to Cooperate and Whom to Cooperate With?” IEEE Trans. Wireless Commun., vol. 7, no. 7, pp. 2814-2827, July 2008.
[7] C.-L. Wang and S.-J. Syue, “A Geographic-Based Approach to Relay Selection for Wireless Ad Hoc Relay Networks,” in Proc. 2009 IEEE Veh. Technol. Conf. - Spring (VTC 2009-Spring), April 2009, pp. 1-5.
[8] D. G. Brennan, “Linear diversity combining techniques,” Proc. IEEE, vol. 91, pp. 331-356, Feb. 2003.
[9] W. Su, A. S. Ibrahim, A. K. Sadek, and K. J. R. Liu, “Cooperative Communication Protocols in Wireless Networks: Performance analysis and Optimal Power Allocation,”Wireless Personal Communication., vol. 44, pp. 181-217, Jan. 2008.
[10] A. K. Sadek, W. Su, and K. J. R. Liu,“Multinode Cooperative Communications in Wireless Networks”IEEE Trans. Signal Processing, vol. 55, pp. 341-355, Jan. 2007.
[11] J. Boyer, D. D. Falconer, and H. Yanikomeroglu,“Multihop diversity in wireless relaying channels,”IEEE. Trans. Commun., vol. 52, pp. 1820-1830, Oct. 2004.
[12] J. Luo, R. Blum, L. Cimini, L. Greenstein, and A. Haimovich, “Decoded-and-forward cooperative diversity with power allocation in wireless networks,”in Proc. 2005 IEEE Global Telecommun., Conf. (GLOBECOM 2005), vol. 5, Nov. 2005, pp. 3048-3052.
[13] J. L. Vicario, A. Morell, A. Bel, and G. Seco-Granados, “Optimal power allocation in opportunistic relaying with outdated CSI,” in Proc. 2008 IEEE Sensor Array and Multichannel Signal Processing Workshop (SAM), July 2008, pp. 49-53.
[14] J. N. Laneman and G. W. Wornell, “Distributed space-time-coded protocols for exploiting cooperative diversity in wireless networks,”IEEE Trans. Inform. Theory, vol. 49, pp. 2415-2425, Oct. 2003.
[15] C. L. Wang and S. J. Syue, “An efficient relay selection protocol for cooperative wireless sensor networks,” in Proc. 2009 IEEE Wireless Commun. and Netw. Conf. (WCNC 2009), April 2009, pp. 1-5.
[16] N. Seunghoon, V. Mai, and V. Tarokh, “Relay selection methods for wireless cooperative communications,” in Proc. 2008 Inform. Sciences and Systems (CISS 2008), Mar. 2008, pp. 859-864.
[17] K. S. Hwang and T. C. Ko, “An efficient relay selection algorithm for cooperative networks,” in Proc. 2007 IEEE Veh. Technol. Conf. - (VTC 2007-Fall), Nov. 2007, pp. 81-85.
[18] H. C. Yang and M. S. Alouini, “Improvement the performance of switched diversity with post-examining selection,”IEEE Trans. Wireless Commun., vol. 5, pp. 67-71, Jan. 2006.
[19] A. Bletsas, A. Khisti, D. P. Reed, and A. Lippman,“A simple cooperative diversity method based on network path selection,” IEEE J. Sel. Areas. Commun., vol. 24, pp. 659-672, Mar. 2006.
[20] S. Phaiboon, “An empirically based path loss model for indoor wireless channels in laboratory building,” in Proc. 2002 IEEE Region Ten Conf. (TENCON 2002), vol. 2, Oct. 2002, pp. 1020-1023.
[21] F. A. Onat, A. Adinoyi, Y. Fan, H. Yanikomeroglu, and J. S. Thompson, “Optimum threshold for SNR-based selective digital relaying schemes in cooperative wireless networks,” in Proc. 2007 IEEE Wireless Commun. and Netw. Conf. (WCNC 2007), Mar. 2007, pp. 969-974.