在傳統的轉送解碼的合作式網路下, 中繼點的選擇方法主要遵循最大−最
小策略, 也就是說被選來幫忙傳送資料封包的中繼點與其最大-最小的通道增
益相關。然而, 最大−最小策略有可能對主要系統(primary system) 造成多餘的干擾導致所謂的轉換比(transfer ratio) 變得較差。值得注意的是我們把轉換比定義為次要系統(secondary system) 的容量增益與主要系統的容量損失之比值, 而主要系統的損失乃次要系統的通訊活動所造成的。在本篇論文, 我們針對重疊式(underlay) 的感知無線電網路提出一種名為最大頻譜使用效率之中繼選擇的中繼點選擇方法。我們的選擇方法主要的概念在於選擇能使主要系統與次要系統的容量相加最大化的中繼點, 因此我們的選擇方法確保了主要系統的容量損失不會被浪費。最後, 經由模擬結果我們發現所提出的最大頻譜使用效率之中繼選擇方法在轉換比上優於傳統的最大-最小法則, 這也意味著我們所提出的方法能達到最大的頻譜使用效率。

In the conventional decode-and-forward (DaF) cooperative networks, the relay selection method mainly follows the rule of max-min policy, which means the relay associated with the max-min channel gain can be selected to forward the data packets. However, the max-min policy may cause some extra amount of interference to the primary system (PS) such that the so-called transfer ratio (TR) can be lower. Note that the transfer ratio is defined as the ratio of the secondary system (SS)’s capacity gain to the PS’s capacity loss due to the activities of the SS. In this thesis, we proposed a novel relay selection for the underlay cognitive radio (CR) networks called maximum spectrum efficiency relay selection. The key idea here is to select a relay that maximizes the sum capacity of the PS and SS. This method assures that the PS’s capacity loss will not be wasted. At last, through simulation results, we show that the proposed maximum spectrum efficiency relay selection can own a higher TR compared with the conventional max-min rule. Therefore, our relay selection strategy achieves maximum spectrum efficiency utilization.

[1] V. Chakravarthy, X. Li, Z. Wu, M. A. Temple, F. Garber, R. Kannan, and A. Vasilakos, “Novel overlay/underlay cognitive radio waveforms using sd-smse framework to enhance spectrum efficiency-part i: Theoretical framework and analysis in awgn channel,” IEEE Trans. on Communications, vol. 57, pp. 3794–3804, December 2009.
[2] Q. Zhao and B. M. Sadler, “A survey of dynamic spectrum access,” IEEE Signal Process. Mag., vol. 24, pp. 79–89, May 2007.
[3] F. C. C. (FCC), “Spectrum policy task force,” ET Docket no. 02-135, November 15 2002.
[4] S. Haykin, “Cognitive radio: Brain-empowered wireless communications,” IEEE Journal on Selected Areas in Communications, vol. 23, pp. 201–220, Feburary 2005.
[5] A. Ghasemi and E. S. Sousa, “Fundamental limits of spectrum-sharing in fading environments,” IEEE Transaction on Wireless Communications, vol. 6, pp. 649– 658, Feburary 2007.
[6] R. Zhang, X. Kang, and Y. Liang, “Protecting primary users in cognitive radio networks: Peak or averageinterference power constraint?,” in IEEE InternationalConference on Communications, (Dresden, Germany), pp. 1–5, June 2009.
[7] H. A. Suraweera, J. Gao, P. J. Smith, M. Shafi, and M. Faulkner, “Channel capacity limits of cognitive radio in asymmetric fading environments,” in IEEE International
Conference on Communications, (Beijing, China), pp. 4048–4053, May 2008.
[8] X. Kang, R. Zhang, Y. Liang, and H. K. Garg, “Optimal power allocation for cognitive radio under primary user’s outage loss constraint,” in IEEE International Conference on Communications, (Dresden, Germany), pp. 1–5, June 2009.
[9] A. Ribeiro, X. Cai, and G. B. Giannakis, “Symbol error probabilities for general cooperative links,” IEEE Transaction on Wireless Communications, vol. 4, pp. 1264–1273, May 2005.
[10] G. Zhao, J. Ma, G. Y. Li, Y. K. T. Wu, A. Soong, and C. Yang, “Spatial spectrum holes for cognitive radio with relay-assisted directional transmission,” IEEE Transaction on Wireless Communications, vol. 8, pp. 5270–5279, October 2009.
[11] A. Bletsas, H. Shin, and M. Z. Win, “Cooperative communications with outageoptimal opportunistic relaying,” IEEE Transaction on Wireless Communications, vol. 6, pp. 3450–3460, September 2007.
[12] S. S. Ikki and M. H. Ahmed, “On the performance of amplify-and-forward cooperative diversity with the nth best-relay selection scheme,” in IEEE International Conference on Communications, (Dresden, Germany), pp. 1–6, June 2009.
[13] A. Bletsas, H. Shin, and M. Z. Win, “Cooperative diversity in wireless networks: efficient protocols and outage behavior,” IEEE Transaction on INFORMATION THEORY, vol. 50, pp. 3062–3080, December 2004.
[14] Y. Jing and H. Jafarkhani, “Single and multiple relay selection schemes and their achievable diversity orders,” IEEE Transaction on Wireless Communications, vol. 8, pp. 1414–1423, March 2009.
[15] A. Sendonaris, E. Erkip, and B. Aazhang, “User cooperation diversity-part i: System description,” IEEE Trans. on Communications, vol. 51, pp. 1927–1938, November 2003.
[16] A. S. Ibrahim, A. K. Sadek, W. Su, and K. J. R. Liu, “Cooperative communications with relay-selection: When to cooperate and whom to cooperate with?,” IEEE Transaction on Wireless Communications, vol. 7, pp. 2814–2827, July 2008.
[17] I. Krikidis, J. S. Thompson, S. McLaughlin, and N. Goertz, “Max-min relay selection for legacy amplify-and-forward systems with interference,” IEEE Transaction on Wireless Communications, vol. 8, pp. 3016–3027, June 2009.
[18] M. Yuksel and E. Erkip, “Multiple-antenna cooperative wireless systems: A diversity multiplexing tradeoff perspective,” IEEE Trans. on Information Theory, vol. 53, pp. 3371–3391, October 2007.
[19] G. L. St¨uber, Principles of mobile communication. Norwell, MA: Kluwer Academic Publishers, 2nd, ed., 2001.
[20] N. C. Beaulieu and Q. Xie, “An optimal lognormal approximation to lognormal sum distributions,” IEEE Trans. on Vehicular Technology, vol. 53, pp. 479–489, 2004.
[21] L. Zhao and J. Ding, “Least squares approximations to lognormal sum distributions,” IEEE Trans. on Vehicular Technology, vol. 56, pp. 991–997, 2007.
[22] G. Ganesan and Y. G. Li, “Agility improvement through cooperation diversity in cognitive radio,” in IEEE Global Telecommunications Conference, (St. Louis, Missouri, USA), pp. 2505–2509, November 2005.
[23] J. F. Chamberland and V. V. Veeravalli, “The impact of fading on decentralized detection in power constrained wireless sensor networks,” in IEEE International Conference on Acoustics, Speech, and Signal Processing, (Montreal, Quebec, Canada), pp. 837–840, May 2004.
[24] C. Sun, W. Zhang, and K. B. Letaief, “Cluster-based cooperative spectrum sensing in cognitive radio systems,” in IEEE International Conference on Communication, (Glasgow, Scotland), pp. 2511–2515, June 2007.
[25] A. K. Sadek, K. J. R. Liu, and A. Ephremides, “Cognitive multiple access via cooperation: Protocol design and performance analysis,” IEEE on Information Theory, vol. 53, pp. 3677–3696, October 2007.
[26] X. Gong, W. Yuan, W. C. W. Liu, and S. Wang, “A cooperative relay scheme for secondary communication in cognitive radio networks,” in IEEE Global Telecommunications
Conference, (New Orleans, Louisiana), pp. 1–6, November 2008.
[27] V. Asghari and S. Aissa, “End-to-end performance of cooperative relaying in spectrum-sharing systems with quality of service requirements,” IEEE Trans. on Vehicular Technology, vol. 60, pp. 2656–2668, July 2011.
[28] A. Bletsas, A. Khisti, D. P. Reed, and A. Lippman, “A simple cooperative diversity method based on network path selection,” IEEE Journal on Selected Areas in Communications, vol. 24, pp. 659–672, May 2006.