A comparative study of predictive transmit power control schemes for S-UMTS

Transmit power control (TPC) is indispensable in direct sequence code division multiple access (DS/CDMA) based systems such as the Satellite Universal Mobile Telecommunication System (S-UMTS). Since S-UMTS aims at achieving close integration with the terrestrial component (T-UMTS) in its complementary role, it is going to implement closed-loop TPC at a slow rate of once every frame. In addition, predictive schemes can be used to mitigate the effects of delay. In this regard, recursive-least-squares (RLS) and least-mean-square (LMS) algorithms are normally employed. The RLS algorithm has a higher convergence rate than the LMS algorithm, an attractive attribute when fading abruptly changes. On the other hand, the LMS algorithm has a better tracking property than the RLS algorithm, an attractive attribute when fading changes are persistently slow. However, the mobile satellite system channel exhibits both attributes - abrupt changes and slow drifts. We compare the performance of predictive TPC based on RLS and LMS algorithms for S-UMTS against conventional TPC. We demonstrate that the predictive TPC schemes perform better than the conventional TPC scheme. However, the performance gain achieved depends on the predictive algorithm used, the environment in which the UE (user equipment) is operating, and loop delays. We show that, in general, the LMS based predictive TPC offers better performance than the RLS based predictive TPC scheme in the S-UMTS environment.