Optimal SIR target determination for outer-loop control in the W-CDMA system

In interference limited cellular radio systems, the control of target signal-to-interference (SIR) ratios in the outer loop in order to satisfy a given quality criteria, is a challenge due to the changing fading characteristics of the radio channel. The performance of the systems based on the commonly adopted technique of relatively infrequent frame error rate (FER) measurements is very deteriorated under dynamic environments. The method for the outer loop presented in this paper, exploits the knowledge of SIR fading stochastic models (Suzuki or Loo models for the desired signal, and Gaussian distribution for the interference) to on-line tune the mentioned target value. Thus, the Newton-Raphson method is applied to numerical approximations of the SIR cumulative distribution functions (CDF´s), so that it is possible to obtain the minimum SIR target fade margin that satisfies a given outage probability specification (quality criteria) for that link and for second order statistical moments (shadowing variance, Rice Factor...) which are estimated dynamically. The validity of the SIR CDF´s numerical approximations derived has been tested throughout comparison of their plots with the existent ones. The outage probability obtained from those numerical approximations for given values of the SIR margin and second order statistical moments, is then used as input of the Newton-Raphson process. The outcome of this process is, as desired, the previously mentioned SIR margin value. The method performance has been tested under a realtime UMTS demonstrator developed within the IST ARROWS (advanced radio resource management on wireless services) European Collaborative project. The outcomes from the dynamic simulation show that the outer loop method proposed here is able to maintain the QoS required under dynamic environments.