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

Abstract

Transmit power control is indispensable in Direct Sequence Code Division Multiple Access (DS/CDMA) based system such as Satllite Universal Mobile Telecommunicaton System (S-UMTS). Since S-UMTS aims at achleving close integration with the terrestrinl component (T-UMTS) in its complementary role, it is going to implement closed-loop transmit power control (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) algorithans are normally employed. The RLS slgorithm has higher convergence rate than the LMS algorithm: an attractive attribute when the fading process abruptly changes. The LMS algorithm, on the other hand, has better tracking property than the RLS algorithm: an attractive attribute when the changes in the fading process are persistently slow. Howenr, the mobile satellite system channel exhibits both attributes: abrupt changes and slow drifts. In this paper, therefore, we compare the performeace of prodictive TPC based on the RLS and the LMS algorithms for S-UMTS with the conventional TPC as a reference. We demoustrate that the predictive TPC schemes perform better than the conventional TPC scheme. However, the performance gain achieved depends on the predictive algorithrn used, the environment in which the UE 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 S-UMTS environment.