換手，指的是當手機用戶從目前所在細胞移動到到另一個細胞，在此過程中必須在新的基地台尋找到新的通道以保障通話的暢通，另外必須切斷和舊的基地台的連線使其他用戶可以繼續使用此通道。隨著不斷增加的需求高流動性用戶在異構網絡，換手面臨隨著越來越多的挑戰，因為用戶的移動性會影響通話質量水平。
本文分析了兩種類型的換手機制，軟切換(SHO)和硬切換（HHO）應用於寬帶碼分多址多址移動通信系統（WCDMA）中。我們對這兩種換手機制的好處和不足進行了討論以及分析。然而，在實際系統，軟切換提供了可靠的通信距離和通話品質，但其優點來自額外的費用網絡資源和執行成本。兩造相比之下，硬切換提供了一個高效的頻譜利用率，但是，在執行換手時的通話品質以及斷線機率卻不如軟切換。因此，我們進一步提出混合換手機制，結合兩種切換機制。此外，我們利用對稱設計（SD）和準半對稱設計（QSSD）在不同的基地台安排不同的頻段，分別給硬切換和軟切換不同的比例以達最佳效能。為了選擇最佳設計參數，我們將移動用戶的速度和換手機制的建置成本納入考量。所以我們將這個問題轉變為另一個優化問題，採用遺傳演算法解決這個問題。最後的模擬結果，顯示我們的混和換手機制，在不同的地理環境以及不同的使用者移動模型中，展現強大的適應能力。

Handoff is a momentous process when a mobile
station (MS) moves from one cell to another, the channel in the old
base station (BS) is released, and a channel is required in the new
BS. This process is called handoff. With the increasing demand of
high mobility users in heterogeneous network, handoff is confronted
with more and more challenges since user's mobility will affect the
quality of link and interference levels in cellular systems.
This paper analyzes the performance of two type handoffs soft
handoff (SHO) and hard handoff (HHO) of wideband code-division
multiple-access cellular system (WCDMA). The benefits and shortages
of these two schemes are discussed. However, in practical systems,
soft handoff provides robust communication away from call
disconnection although it come from the expense of additional
network resources and implemented cost. By way of contrast, hard
handoff provides a efficient spectrum utilization, however the
blocking probability is worse than soft handoff. Thus, there is a
tradeoff between hard and soft handoff. We compare the performance
of hard and soft handoff schemes. Based on this, we further propose
a hybrid handoff scheme which combining two handoff schemes.
Moreover, we apply symmetric designs (SD) and quasi-semisymmetric
designs (QSSD) to arrange radio resource or frequency bands in
difference base stations (BSs). In order to choose the optimum
design parameters, we take mobile users' speed and the cost between
hard and soft handoffs into account. Moreover, we transform this
problem to an optimization problem and using genetic algorithm to
solve it.

[1] A. Leu, B. Mark, and S. Tang, \Analysis of handoff interference and outage along arbi-
trary trajectories in cellular networks," IEEE transactions on wireless communications,
vol. 7, no. 9, pp. 3597{3607, 2008.
[2] Y. Zhang, \Hando□ performance in wireless mobile networks with unreliable fading
channel," IEEE Transactions on Mobile Computing, vol. 9, no. 2, pp. 188{20, 2009.
[3] A. Viterbi, A. Viterbi, K. Gilhousen, and E. Zehavi, \Soft hando□ extends cdma cell
coverage and increases reverse link capacity," IEEE Journal on Selected Areas in Com-
munications, vol. 12, no. 8, pp. 1281{1288, 1994.
[4] A. Sgora and D. Vergados, \Handoff prioritization and decision schemes in wireless
cellular network : a survey," IEEE Communications Surveys & Tutorials, vol. 11, no. 4,
pp. 57{77, 2009.
[5] S. Kwon, H. Jeon, and H. Lee, \A channel assignment scheme for integrated services
in ds-cdmacellular systems," in 1997 IEEE 6th International Conference on Universal
Personal Communications Record, 1997. Conference Record., vol. 2, 1997.
[6] X. Ma, Y. Liu, and K. Trivedi, \Design and performance analysis of a new soft hando□
scheme for cdma cellular systems," IEEE Transactions on Vehicular Technology, vol. 55,
no. 5, p. 1603, 2006.
[7] D. Wong and T. J. Lim, \Soft handoffs in cdma mobile systems," IEEE Personal Com-
munications, vol. 4, pp. 6{17, December 1997.
[8] W. Mao, Z. He, and et al., \Study on the soft handoff simulation based on received signal
strength in cdma," in Arti‾cial Intelligence and Computational Intelligence, 2009, pp.
145{149.
[9] J. Rojas-Lima, \Comparative study of fade margins with hard and soft handoff in
cdma based microcellular systems," in Proceedings of the 6th international conference
on Symposium on Wireless Communication Systems. IEEE Press, 2009, pp. 555{559.
[10] R. Thomas, H. Gilbert, and G. Mazziotto, \In°uence of the moving of the mobile
stations on the performance of a radio mobile cellular network," in Proc. 3rd Nordic
Seminar on Digital Land Mobile Radio Communications, 1988.
[11] O. L□azaro and D. Girma, \A hop‾eld neural-network-based dynamic channel allocation
with hando□ channel reservation control," IEEE Transactions on Vehicular Technology,
vol. 49, no. 5, pp. 1578{1587, 2000.
[12] D. Gross, Fundamentals of queueing theory. Wiley-India, 2008.
[13] R. Sindal and S. Tokekar, \Modeling and analysis of voice/data call admission control
scheme in cdma cellular network for variation in soft hando□ threshold parameters," in
16th IEEE International Conference on Networks, 2008. ICON 2008, 2008, pp. 1{6.
[14] X. Wang, S. Xie, and X. Hu, \Recursive analysis for soft hando□ schemes in cdma
cellular systems," IEEE Transactions on Wireless Communications, vol. 8, no. 3, pp.
1499{1507, 2009.
[15] H. Zeng and I. Chlamtac, \Adaptive guard channel allocation and blocking probability
estimation in pcs networks," Computer Networks, vol. 43, no. 2, pp. 163{176, 2003.
[16] G. Bolch, Queueing networks and Markov chains: modeling and performance evaluation
with computer science applications. Wiley-Blackwell, 2006.
[17] S. C»amtepe and B. Yener, \Combinatorial design of key distribution mechanisms for
wireless sensor networks," IEEE/ACM Transactions on Networking (TON), vol. 15,
no. 2, p. 358, 2007.
[18] Y. Ionin and M. Shrikhande, Combinatorics of symmetric designs. Cambridge Univ
Pr, 2006.
[19] M. Hall, Combinatorial Theory, 2nd ed. Wiley, 1998.
[20] I. Anderson, Combinatorial Designs: Construction Methods. Ellis Horwood Chichester,
West Sussex England, 1990.
[21] W. Wallis, Combinatorial designs. CRC, 1988.
[22] T. Huang, L. Huang, and M. I. Lin, \On a class of strongly regular designs and quasi-
semisymmetric designs," Recent Developments of Algebra and Related Areas, no. 3, pp.
129{153, 2009.