在寬頻-直序展頻分碼多重存取系統中,提高OVSF碼樹的使用率將可使通訊系統服務更多的使用者，以及提供各種不同的多媒體服務。然而，重新分配已在使用中的碼將會耗費許多成本。因此動態調整碼樹時必須同時注意重新分配的次數。本文中，我們提出一個狀態分類配碼的方法和依使用者需求的比例來分配碼的策略。我們依據OVSF碼樹的狀態將之分成四種類型。在固定的時間點，系統會檢查目前碼樹的狀態以決定是否要重新分配這些使用中的碼。也就是說，我們不像一般動態分配的方式，當碼樹仍有足夠的容量但卻無法提供空的碼給新的連線時才重新調整碼樹，而是在適當的時機下做重新分配的動作。除此之外，在重新分配碼的同時，我們會依據在不同速率下，使用者需求的比例來保留空的，可使用的碼。因此，處在此種必須經常重新排列碼的緊急狀況機會已減少。在文中我們也根據馬可夫鏈的性質來証明上述的結果。利用所提出的狀態分類配策略和依比例保留碼的原則，將可減少連線無法被服務的機率及重新分配的次數。

In Wideband DS-CDMA system, increasing the utilization of Orthogonal Variable Spreading Factor (OVSF) code tree can serve more users. As Dynamic Code Assignment (DCA) does, the allocated codes will be reassigned when a new call can’t be served even if the capacity is enough. However, reassigning occupied codes is expensive. In this thesis, we have proposed a state classification code assignment principle and a proportional reservation policy. In the proposed algorithm, the OVSF code tree is classified into four states. Then, the system will decide whether to reassign codes or not by checking the current state of the tree. In other words, the occupied codes will be reallocated in an appropriate occasion. Besides, we reserve vacant and available codes corresponding to the probability of requests for each supported rate when reassignment occurs. Therefore, the reassignment is reduced. It is also proved on the basis of the property of Markov Chain. Taking advantage of the proposed states classification code assignment and the policy of proportional code reservation, the call blocking rate and the number of reassignment can be reduced.

Abstract.................................................Ⅰ
Contents.................................................III
Tables...................................................IV
Figures..................................................V
1.Introduction...........................................1
2.Background Materials...................................3
2.1.Direct Sequence Spread Spectrum and Walsh Mapping..3
2.2.OVSF Code Tree.....................................4
2.3.Code Assignment and Code Reassignment..............5
3.Models.................................................6
3.1.Problem Definition.................................6
3.2.Mathematical Model.................................7
4.The Proposed Approach..................................16
4.1.Prepared Table.....................................17
4.2.States Classification Code Assignment (SCCA).......18
4.3.Code Assignment Scheme.............................19
4.4.Code Reassignment Scheme...........................19
4.5.Proportional Reservation Policy....................21
4.6.The Prepared Table Update..........................21
5.Simulation...... .......................................25
5.1.Model..............................................25
5.2.Results............................................27
6.Conclusions..... .......................................29
References........ .......................................30

[1]S. Dixit, Y. Guo and Z. Antoniou, “Resource Management and Quality of Service in Third-Generation Wireless Networks,” IEEE Communications Magazine, February 2001, pp.125-133.
[2]E. Dahlman, B. Gudmundson, M. Nilsson and J. Sköld,“UMTS/IMT-2000 Based on Wideband CDMA,” IEEECommunications Magazine, Sep 1998, pp. 70-80.
[3]Ö. Gürbüz and H. Owen, “Dynamic Resource Scheduling Schemes for W-CDMA Systems,” IEEE Communications Magazine, October 2000, pp. 80-84.
[4]K. S. Gilhousen, “System and method for Orthogonal Spread Spectrum Sequence Generation in variable data rate systems,” U.S. Patent Serial Number: 5751761, May 1998.
[5]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,” ELECTRONICS LETTERS, Vol.33, No.1, 2nd Jan 1997, pp.27-28.
[6]T. Minn and K. Y. Siu, “Dynamic Assignment of Orthogonal Variable – Spreading – Factor Codes in W-CDMA,” IEEE Communications Magazine, Vol.18, NO 8, Aug 2000, pp.1429-1440.
[7]Y. C. Tseng, C. M. Chao and S. L. Wu, “Code Placement and Replacement Strategies for Wideband CDMA OVSF Code Tree Management,” Global Telecommunications Conference, GLOBECOM '01, IEEE, Vol.1, 2001, pp. 562 –566.
[8]W. T. Chen, Y. P. Wu and H.C. Hsiao, “A Novel Code Assignment Scheme for W-CDMA Systems,” Vehicular Technology Conference, IEEE VTS 54th, Vol.2, Fall 2001, pp. 1182 –1186.
[9]M. Dell’Amico, M. L. Merani and F. Maffioli, “Efficient Algorithms for the Assignment of OVSF Codes in Wideband CDMA,” Communications, 2002. ICC 2002. IEEE International Conference, Vol. 5, 2002, pp. 3055 –3060.
[10]F. Shueh, Z. E. P. Liu, and W. S. E. Chen, “A Fair, Efficient, and Exchangeable Channelization Code Assignment Scheme for IMT-2000,” IEEE ICPWC’2000, pp.429-432.
[11]R. G. Cheng and P. Lin, “OVSF Code Channel Assignment for IMT-2000,” Vehicular Technology Conference Proceedings, IEEE, Vol.3, 2000, pp.2188 –2192.
[12]F. Shueh and W. S. E. Chen, “Code Assignment for IMT–2000 on Forward Radio Link,” Vehicular Technology Conference, IEEE, 2001, pp.906-910.
[13]R. Assarut, K. Kawanishi, U. Yamamoto, Y. Onozato and M. Matsushita, “Region Division Assignment of Orthogonal Variable-Spreading-Factor Codes in W-CDMA,” IEEE Communications Magazine, pp. 1884-1888.
[14]R. Assarut, K. Kawanishi, R. Deshpande, U. Yamamoto and Y. Onozato, “Performance Evaluation of Orthogonal Variable-Spreading-Factor Code Assignment Schemes in W-CDMA,” IEEE Communications Magazine, 2002, pp.3050-3054.
[15]E. H. Dinan and B. Jabbari, “Spreading Codes for Direct Sequence CDMA and Wideband CDMA Cellular Networks,” IEEE Communications Magazine, Vol.36 Issue 9, Sep 1998, pp. 48 –54.
[16]F. Adachi, M. Sawahashi and H. Suda, “Wideband DS-CDMA for Next Generation Mobile Communications Systems,” IEEE Communications Magazine, September 1998, pp.56-69.
[17]E. A. Feinberg, “Constrained semi-Markov decision processes with average rewards,” ZOR-Mathematical Methods Operations Res., Vol.39 1994.