An Improved Link Adaptation Scheme for High Speed Downlink Packet Access

Adaptive modulation and coding (AMC) is one of the important features of high-speed downlink packet access (HSDPA). The link adaptation is done at the Node-B on the basis of channel quality indicator (CQI) reports sent by the user equipment (UE). However the performance of AMC scheme is degraded due to the large delay between computation of CQI report at UE and the corresponding transport format and resource indicator (TFRI) selection at the Node-B. An improved link adaptation approach, named CQI averaging herein, is proposed in this paper. In this approach, a number of consecutively received CQI reports from UE are mapped to equivalent signal to interference ratio (SIR) values and passed through a moving average filter. The resulting average is used to derive the CQI index (named Average CQI herein) to be followed at Node-B transmitter. This approach provides a significant gain, especially for medium vehicular speeds. The gain is more pronounced for channels that provide less multipath diversity. Simulation results are used to relate desirable filter length of the moving average filter with the characteristics of the radio channel. Subsequently, an algorithm for adaptation of filter length based on the statistics of CQI reports is proposed.