The focus of the paper is to analyze the downlink performance of a Wideband Code Division Multiple Access (WCDMA) system with Site Selection Diversity Transmission (SSDT) power control during Soft Handover (SHO) mode and to find the optimum SHO margin (M_SH) in terms of maximum system capacity under energy-per-bit to noise spectral density ratio (E_b/N₀) quality requirements. Besides analyzing the dependency of the connection probabilities, percentage of users connected to one, two, or three Base Stations (BSs) simultaneously as a function of the M_SH, it also studies the system capacity and required resources dependency on M_SH for voice service and combined multimedia mobile services. System capacity dependency on M_SH is analyzed by considering two types of user distributions: (1) uniform user distribution; and (2) a concentration of all the users in the location that requires maximum transmitted power (the worst case). The result of this analysis shows an increase in the user capacity by about 16-20% for the optimum M_SH of 5-5.5 dB. Nevertheless, the resources required (number of scrambling codes) by the base station increase faster than the number of active users in terms of M_SH.

Soft handover is a technique that allows wireless user equipment to stay connected to several Base Stations (BSs) in a Wideband Code Division Multiple Access (WCDMA) system (Holma and Toskala, 2004; and Reig, 2006). Code Division Multiple Access (CDMA) technique makes it possible to maintain an old connection, while adding a new one (make before break); however, additional resources from several BSs are required. In downlink, this is achieved by multiple site transmission, which implies that several BSs transmit the same signal to a certain Mobile Station (MS) (Wong and Lim, 1997; and Yang et al., 2001). The capacity in WCDMA system is commonly limited by interference. In order to reduce the interference level in downlink, power control techniques are proposed in WCDMA system. By using a power control algorithm that is based on Signal to Interference Ratio (SIR)-based power control in downlink, the power that is transmitted to the MS is adjusted to achieve the E_b/N₀ requirements (Akhtar et al., 2001). Therefore, more users can be served by the system if SIR-based power control techniques are used. Hence, system interferences are reduced.

In third-generation cellular systems, SSDT power control is employed to mitigate the interference produced by soft handover multiple transmissions. When using SSDT during soft handover mode, only the best server BS is transmitted to the MS, while the rest of the BSs included in the active set turn off the power transmitted to this MS, maintaining only their Dedicated Physical Control Channel (DPCCH) (Furukawa et al., 2000; and Takano and Hamabe, 2001). Two issues must be taken into account in SHO performance evaluation: (1) the active set; and (2) the SHO margin (M_SH). The active set is the group of BSs to which a user terminal is connected. M_SH is the maximum allowed difference (measured in decibels) between the power that is received from the best server BS and the power that is received from a candidate BS that is included in the active set of the user terminal.

Telecommunications Journal, Soft Handover, Uniform Distribution, Capacity, Site Selection Diversity Transmission, Wideband Code Division Multiple Access, WCDMA, Site Selection Diversity Transmission, SSDT, Signal to Interference Ratio, SIR, Dedicated Physical Control Channe, DPCCH.