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Soft handover has two opposing effects on UMTS downlink systems, and there is a trade-off between them. The optimum size of the soft handover zone is analysed in a power-controlled UMTS downlink system. The sensitivity of this optimum size to different system parameters, such as downlink orthogonal factor, path loss exponent, shadowing fading variable, is also analysed.
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Introduction: Soft handover is a technology used in code division multiple access (CDMA) systems to improve the perceived quality of service (QoS) [1]. Much research has been carried out on soft handover, mostly focused on the algorithms of soft handover and its impact in the uplink direction [2, 3], or in the downlink direction without considering power control [4]. Motivated by the asymmetric Internet services of UMTS, in this Letter we analyse the trade-off between the positive and the negative effects of soft handover on a power-controlled downlink system. The optimum size of the soft handover zone is obtained and its sensitivity to different system parameters is analysed.
Analysis: We consider a perfect power-controlled UMTS downlink system with ideal hexagonal topology as shown in Fig. 1. The dotted area is the soft handover zone. All the mobiles in this area are in soft handover status. The coverage of the middle cell comprises three areas, Re , Rse and Ree being the effective radius of different areas, respectively. Assuming a uniform distribution of users and normal- ising the radius of each cell, the density of users r equals 2N=3 3. N is the number of active users per cell. Path loss and shadowing losses are considered and the radio channel is modelled...