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。利用中繼站的幫助，可以讓資料在傳輸過程更為可靠，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。因此，如何在網路中選擇一個最佳的中繼站成為了重要的一個課題。
近年來有關合作式通訊的研究持續的發表，為了使得資料能夠有效的傳送到距離較遠的目的地，有許多作者提出，使用多中繼站同時傳送，利用合作式波束的方法，使得傳送功率能夠更有效傳送至目的地。要形成一個好的合作式波束，將取決於被使用的系統是固定式網路(主波寬度要窄)或行動式網路(主波寬度要寬)，但不管是何種系統，都需要有越低的旁波功率越好。而在多使用者的系統之下，旁波功率越低，其他使用者所受到的干擾功率也就越小。且中繼站的分佈，將會獨立決定一個合作式波束的分佈。然而，目前大多數所發表的研究，都是針對所有可以利用的中繼站，全部都使用，並未對於中繼站的分佈，來做一個傳送決策，因為有些中繼站的傳送是會造成旁波功率上升，而導致干擾功率上升，使得多使用者系統的訊號對干擾比變差。所以我們在此提出一個選擇的方式，在可以降低平均干擾功率之下，及減少接收端，須回傳過多的資料給中繼站，仍然可以維持一個好的低干擾波束。
本論文中，我們採用降低平均干擾功率為基礎，透過最佳化方法的處理，設計多中繼站選擇演算法。為了採用傳統式波束成型技術以及降低接收端回傳的位元數，導致本論文所提出的多中繼站選擇演算法達到次最佳化的效能，但模擬圖將會顯現出，透過我們的選擇法，仍會相當靠近最佳的解。
在本篇研究中，可以透過系統所設定的中繼站個數，從我們所提出的多中繼站選擇法中，挑出較適合的中繼站，最後由接收端，回傳訊號，通知該加入最後傳送機制的中繼站，此挑選可以降低平均干擾功率，也可以降低接收端回傳的位元數。

Cooperative beamforming or collaborative beamforming is an energy-efficient cooperative diversity technique that cophases multiple received signals at the receiver to compensate the phase differences due to defferent propagation delays. With perfect synchronization, a deterministic mainlobe power level can be guaranteed at the inteneded direction for a given network topology. However, the sidelobes, depending on the relays’ positions as well as the weighting vectors, have probabilistic power levels over the angle domain. Moreover, a high power level sidelobe may cause an unacceptable interference level at an unintended receiver. Thus, limiting the sidelobes to meet some requirements is of importance in cooperative beamforming. In this thesis, we develop a two-step multi-relay selection scheme for cooperative conventional beamforming based on a selection vector design. We formulate an optimization problem that minimizes the average power over all unintended directions with a constraint of unit power gain pointing to the desired user. Using the Lagrange multiplier, we further reduce the optimization problem as an eigenvalue decomposition problem. The solution is then used to form two selection criteria, where the first one is for finding the optimal reference relay and the second one is for selecting the optimal relaying set for cooperative conventional beamforming according to the reference relay. The proposed relay selection method provides a reduced-complexity, suboptimal solution for cooperative beamforming, where the receiver selects a set of cooperating relays by broadcasting a binary feedback message. Simulation results show that the proposed multi-relay selection scheme provides lower average power over all unintended directions than both the near and far cases, where the performance is close to that of the exhaustive search approach.

[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] H. Ochiai, P. Mitran, H. V. Poor, and V. Tarokh, “Collaborative beamforming for distributed wireless ad hoc sensor networks,” IEEE Trans. Signal Processing, vol. 53, no. 11, pp. 4110-4124, Nov. 2005.
[4] M. F. A. Ahmed and S. A. Vorobyov, “Collaborative beamforming for wireless sensor networks with Gaussian distributed sensor nodes,” IEEE Trans. Wireless Commun. , vol. 8, no. 2, pp. 638-643, Feb. 2009
[5] K. Yao, R. E. Hudson, C. W. Reed, D. Chen, and F. Lorenzelli, “Blind beamforming on randomly distributed sensor array system,” IEEE J. Sel. Areas Commun., vol. 16, no. 10, pp. 1555-1567, Oct. 1998.
[6] K. Zarifi, S. Affes, and A. Ghraybe, “Collaborative null-steering beamforming for uniform distributed wireless sensor networks,” IEEE Trans. Signal Processing, vol.58, no.3, pp. 1889-1903, Mar. 2010.
[7] L. Dong, A. P. Petropulu, and H. V. Poor, “A cross-layer approach to collaborative beamforming for wireless ad hoc networks,” IEEE Trans. On Signal Processing, vol. 56, no. 7, pp. 2981-2993, Jul. 2008.