On the uplink SIR of a mobile in soft handoff with applications to BLER estimation

We focus on the uplink and derive the joint probability density function of the signal to interference (+noise) ratios (SIRs) at the base stations (BSs) for the general case of n BSs supporting a mobile in soft handoff, each using maximal ratio combining with m-fold diversity. We then calculate the correlations between the SIRs for some example cases. The insights gained here lead to a fresh approach to the estimation of the block error rate (BLER) at the output of the frame selector (FS), which is the usual measure of quality of service in UMTS. For low specified BLER (e.g., 0.1%), the method of counting cyclic redundancy code errors requires an impractically large number of data blocks for an accurate estimate of BLER. We propose to estimate the BLER at the FS output based on measurements performed by the individual BSs. The initial setting of the target SIR in the outer loop power control can then be selected in the same way in order to expedite BLER convergence to the desired setting.