正交分頻多工（orthogonal frequency division multiplexing，OFDM）因具有
節省頻寬、可對抗頻率選擇性衰退通道（frequency selective fading channel）和高
速率傳輸的優點，在如今的無線通訊中扮演吃重的角色。在無線通道中常會遇到
衰落通道（fading channel）造成效能衰退，適應性調變（adaptive modulation）與
子載波分配（subcarrier allocation）能夠在符合服務品質（quality of service，QOS）
要求之下彌補效能衰退。在多重使用者正交分頻多工架構下，使用Lagrange
method 或heuristics method 的子載波分配技術可利用最少的能量達到傳輸位元
最大化的效果，但這兩種方法複雜度極高，實際上難以實現。Zhang 和Letaief
所提出的子載波偵測的方法可有效減少複雜度，但是子載波在實際系統中動輒上
百甚至上千，此法仍有可改進之處。
本篇論文提出在多重使用者正交分頻多工架構下，針對尚未符合自身最低速
率要求的使用者（需求者）做偵測，計算出需要子載波數目， 再直接由符合自
身最低速率要求的使用者（提供者）提供子載波，如此可用較少的偵測次數達到
相同的效能，進一步使系統簡化，在相同條件下，系統子載波數目越多，效果越
顯著。

Orthogonal frequency division multiplexing（OFDM）systems play an important
role in modern wireless communications due to following advantages: bandwidth
saving、combat with frequency selective fading channel and high throughput. The
performance of wireless communications is often degraded by fading channel .
adaptive modulation and subcarrier allocation are proposed to overcome the degration
to meet the quality of servie（QoS）. Lagrange method and heuristics method, two of
the subcarrier allocation technology under multi-user OFDM, can achieve the goal
that maximizing bit rate with minimizing transmitted power. However, significantly
high complexity of either Lagrange method or heuristics method makes the
implementation difficult. Zhang and Letaief proposed a method of making subcarriers
detected one by one to reduce the complexity. However, in piratical, an OFDM system
accommodates hundred of , or even thousand of subcarriers, so the method can be
improved.
In this thesis, we propose a subcarrier allocation method. The users that are not
satisfied with the QOS requirement are named demander, and the users satisfied with
the QOS requirement are named supplier. In the proposed subcarrier allocation
method, we evaluate the number of subcarriers that demanders need and remove the
subcarriers from supplier to directly compensate demander. Then the system has
lower complexity due to less iterations.

誌謝................................................................................................................................ i
摘要............................................................................................................................... ii
Abstract ........................................................................................................................ iii
目錄............................................................................................................................... iv
圖索引........................................................................................................................... vi
表索引........................................................................................................................ viii
第一章 簡介............................................................................................................ 1
第二章 系統架構........................................................................................................ 3
2.1 正交分頻多工................................................................................................. 3
2.1.1 正交分頻多工訊號模型.................................................................... 4
2. 2 衰退通道........................................................................................................ 9
2.2.1. 小尺度衰退..................................................................................... 10
2. 3 適應性調變.................................................................................................. 14
2. 3. 1 單一調變模式................................................................................ 14
2. 3. 2 多重調變模式................................................................................ 16
2. 4 正交分頻多工系統下的子載波分配.......................................................... 17
第三章簡化子載波分配演算法.............................................................................. 22
3. 1 Y. J. Zhang 和K. B. Letaief 提出之子載波分配演算法 .......................... 22
3. 2 簡化之子載波分配演算法.......................................................................... 26
第四章模擬結果...................................................................................................... 31
4. 1 在不同衰退通道下的比較.......................................................................... 31
4. 2 在不同子載波數目下的比較...................................................................... 35
4. 3 在不同位元速率限制下的比較.................................................................. 37
參考文獻...................................................................................................................... 41

[1] A.Svensson,“An Introduction to adaptive QAM modulation schemes for
known and predicted channels,” Proceedings of the IEEE, vol. 95, no. 12, pp.
2322-2336, Dec. 2007.
[2] S. Ye , R. S. Blum and L. J. Cimini, “Adaptive OFDM systems with imperfect
channel state information, ” IEEE Trans. Wireless Commun., vol. 5, pp. 3255-3265,
Nov. 2006.
[3] A. J. Goldsmith and S. G. Chua,“Adaptive coded modulation for fading
channels,” IEEE Trans. Commun., vol. 46, no. 5, pp. 595-602, May. 1998.
[4] C. Y. Wong, C. Y. Tsui, R. S. Cheng, and K. B. Letaief, “A real-time
subcarrier allocation scheme for multiple access downlink OFDM transmission,”
in Proc. VTC’99, vol. 2, 1999, pp. 1124–1128.
[5] C. Y. Wong, R. S. Cheng, K. B. Letaief, and R. D. Murch, “Multiuser
OFDM with adaptive subcarrier, bit, and power allocation,” IEEE J. Select.
Areas Commun., vol. 17, pp. 1747–1758, Oct. 1999.
[6] H. Yin and L. Hui, “An efficient multiuser loading algorithm for OFDM based
broadband wireless systems,” in Proc. GLOBECOM, vol. 1, 2000, pp. 103–107.
[7] Y. J. Zhang and K. Ben Letaief, “Multiuser adaptive subcarrier-and-bit
allocation with adaptive cell selection for OFDM systems,”
IEEE Trans. Wireless Commun., vol. 3, no. 5, pp. 1566–1575, Sept. 2004.