Adaptive modulation switching level control in high speed downlink packet access transmission

In Release 5 of the UMTS systems, adaptive modulation and coding (AMC) plays a key role in boosting the peak rate of packet transmission up to 10 Mbps. One of the key challenges for the AMC scheme is the selection of the AMC switching levels. This is a challenging issue due to dissimilar sensitivity of the modulation schemes to Doppler velocity or multipath channel impairment. Furthermore, given the wide range of receiver and transmitter technologies, a scheme is required that adaptively estimates the optimal switching point which will maximize the system throughput. We present an adaptive mechanism utilizing the fast hybrid ARQ feedback signal that is available in R5 of the UMTS system. Conceptually, the proposed threshold adjustment algorithm (TAA) is somewhat similar to the outer loop power control scheme that is already adopted in R99. The switching level is adjusted in such a way that the link performance, in terms of block error rate, maintains a target value. Performance of the proposed scheme is also demonstrated using a link level simulation.