Study Of Soft Handover In WCDMA System

Soft handover is one of the key techniques in WCDMA system. It is the emphasis of radio resource management (RRM) and optimization. The soft handover algorithm and correlative parameters setting have an immediate effect on the system capacity and quality of service.Firstly, RRM and handover technology in WCDMA system are summarized. A particular description of the soft handover procedure in WCDMA system is given. The soft handover algorithms in different CDMA based systems, including WCDMA, IS-95A and cdma2000, are compared.Then four soft handover algorithms are investigated based on a simulation system. In the simple algorithm, the average dedicated channel overhead and base station connection probability increase along with the augment of soft handover threshold, and moreover, the average signaling overhead is maximal when the soft handover threshold equals 9dB approximatively. In the conventional algorithm, reporting range determines the soft handover region, hysteresis parameter and trigger time reduce the"ping-pong"effect and fast fading, maximum size of active set restricts the active set number, and all the parameters above affect the system performance. Two enhanced algorithms are investigated on the basis of the conventional algorithm. Parameter W is introduced to improve the handover performances. If the parameter W is set to 0.2, the quality of service is the best. If the parameter W is set to 2, the system overheads are the least. Furthermore, with the help of position information, the system overheads can also be reduced.The inner loop power control strategies for soft handover are studied afterwards. The user equipment (UE) detects the reliability of transmit power control (TPC) commands from different cells in the active set and combines them. Adjustment loop or the reduction of TPC frequency can be used to balance downlink power among active set cells during soft handover. Site selection diversity TPC (SSDT) can be used to reduce the downlink interference when the UE is in soft handover.Lastly, the effect of soft handover on the system capacity is analyzed in theory. In the uplink, soft handover extends cell coverage and increases uplink capacity. In the downlink, soft handover introduces more interference. If the macrodiversity gain can't compensate for the interference loss, downlink capacity will decrease.