臺灣博碩士論文加值系統

在第三代行動通訊系統中，使用了正交變數展頻因子碼(Orthogonal Variable Spreading Factor code； OVSF)來區分不同用戶的資料通道，OVSF碼可以保證不同用戶通道的正交性，讓每一個使用者可以使用相同的頻率傳輸資料而不會互相干擾。OVSF碼可以提供可變傳輸速率的服務滿足多媒體應用時之不同傳輸速度的需求，但由於OVSF碼的正交特性，所以在碼的配置上會有些限制。在OVSF Codes中有兩個研究方向：碼的配置與碼的重配置。其目的是增加系統的使用率，也減少搬移碼時對於系統所造成的負擔。
在之前的研究中，並沒有考慮到使用者在無線通訊的環境中所遭遇到的一些現象，像是路徑損失(path loss)和遮蔽效應(shadowing)。在這樣的環境下載波雜訊比( carrier to interference ratio； CIR) 會隨著使用者的移動而變化，為了維持鏈結品質(link quality)，我們會升高展頻因子(spreading factor)以抵抗雜訊來維持鏈結品質，但是資料速率會因此而被降低，相反的，若是CIR很高，我們可以降低展頻因子來提高資料速率。
在這篇論文中，我們考慮這些效應對系統造成的影響並提出新的配置與重配置方法，分別是Sparse-first、Sparse-first and rightmost、Modified sparse-first and rightmost，並與之前研究中所提出的三種方法Random、Leftmost、Crowded-first做比較。
在我們的模擬中，我們限制單位時間能夠做重新分配的次數，因為這是比較符合實際上的狀況。從我們的模擬結果中我們可以發現不論在通訊品質(quality)、輸出流量(throughput)、碼重分配數目(number of reassigned codes)都比Random、Leftmost、Crowded-first要好。

For the downlink of third-generation (3G) mobile communication system, the orthogonal variable spreading factor (OVSF) codes are used as channelisation codes to support applications with different bandwidth requirements. Two important issues in the utilization of OVSF codes are code assignment problem and code reassignment problem. Previous studies about these two problems do not consider the phenomenon in actual wireless communication environment, which are path loss variation with distance, slow log-normal shadowing and fast multipath fading. In this thesis, we take path loss and shadowing into account and present a rate transition model. We also propose three new assignment strategies, which are sparse-first, sparse-first and rightmost and modified sparse-first and rightmost. These strategies are compared with those proposed in the previous works under the model which is more realistic.

[1] 3GPP TS 25.213, V6.0.0, Spreading and modulation (FDD), December 2003
[2] T. Minn and K.-Y. Siu, “Dynamic Assignment of Orthogonal Variable-Spreading-Factor Codes in W-CDMA,” IEEE J. Selected Areas in Comm., vol. 18, no. 8, pp. 1429-1440, Aug. 2000.
[3] R. Assarut, K. Kawanishi, U. Yamamoto, Y. Onozato, and M. Masahiko, “Region Division Assignment of Orthogonal Variable-Spreading-Factor Codes in W-CDMA”, in Proc. IEEE Vehicular Technology Conf. Fall, pp. 1884-1888, 2001.
[4] Y.-C. Tseng and C.-M. Chao, “Code Placement and Replacement Strategies for Wideband CDMA OVSF Code Tree Management”, IEEE Trans. on Mobile Computing, Vol. 1, No. 4, Oct.-Dec. 2002, pp. 293-302.
[5] A. N. Rouskas and D. N. Skoutas, “OVSF Code Assignment and Reassignment as the Forward Link of W-CDMA 3G systems,” Proceeding of IEEE PIMRC 2002, vol. 5, pp. 2404-2408, 2002
[6] R. G. Cheng and P. Lin, “OVSF Code Channel Assignment for IMT-2000,” Proceedings of IEEE VTC 2000, vol. 3, pp. 2188-2192, 2000
[7] F. Shueh and W. S. E. Chen, “Code Assignment for IMT-2000 on Forward Radio Link,” Proceedings of IEEE VTC 2001, vol. 2, pp. 906-910, 2001
[8] R. L. Peterson , R. E. Ziemer , David E. Borth, Introduction to Spread Spectrum Communications, New Jersey ,Prentice Hall PTR, 1995
[9] E. H. Dinan and B. Jabbari, “Spreading codes for direct sequence CDMA and Wideband CDMA cellular networks,” IEEE Communication magazine, vol. 9, pp. 48–54, Sep. 1998.
[10] F. Adachi, M. Sawahashi, and K. Okawa, “Tree-structured generation of orthogonal spreading codes with different lengths for forward link of DS-CDMA mobile radio,” Electron. Lett., vol. 33, pp. 27–28, Jan. 1997.
[11] G. L. Stuber, Principles of Mobile Communication, 2nd Edition, Boston, Kluwer Academic Publishers, 2001.
[12] T. S. Rappaport, Wireless Communications: Principles and Practice, 2nd Edition, New Jersey, Prentice Hall PTR, 2001