Effects of soft handover on the UMTS downlink performance

The 3GPP standard proposes two methods to perform soft handover on the downlink. In the first one all base stations (BS) or node-B belonging to the active set (AS) transmit simultaneously to the mobile station (MS). All these signals are added at the MS using maximal ratio combining. The downlink power control adjusts the BS transmit powers such that the E_b.N_o achieved by maximal ratio combining reaches the desired target E_b/N_o. According to the 3GPP standard all BS in the AS transmit with an equal power to the MS. The other possibility to perform soft handover is called site selection diversity. In this case a single BS of the AS is selected and only this BS transmits to the MS while all other BS in the AS switch their power off. The advantage of this strategy is that the other BS in the AS produce no interference while the benefits of soft handover are maintained by fast site selection. If the radio link quality drops abruptly due to fading effects the MS can switch rapidly to another BS. In this paper we analytically compare the two methods with respect to the system capacity. Furthermore, we investigate soft handover mechanisms that allocate not necessarily equal proportions of the total power to the BS in the AS. These mechanism provide a compromise between equal power allocation and SSDT.