Interaction between flow control and packet scheduling algorithms in HSDPA

One of the key elements of the High Speed Downlink Packet Access (HSDPA) is the fast scheduler located at the base station. For the scheduling algorithm to operate efficiently, sufficient amount of traffic need to be buffered at the base station. However, the users' buffers at the base station are managed by a flow control protocol in order to minimize data loss during handover. This thesis studies the impact of this flow control protocol on the scheduling algorithm and on the overall system performance. Two selected flow control algorithms are evaluated and compared to the hypothetical case when flow control is eliminated from the system. The results indicate that eliminating the flow control greatly improves the scheduler performance and increase the overall system throughput at low traffic load. However, at high traffic load, eliminating the flow control increases handover data loss to a point that negates the positive improvement of the scheduler performance and reduces the effective system throughput. A new flow control algorithm is proposed to predict potential handover conditions using Channel Quality Indicators (CQI) in HSDPA and tune the buffer level accordingly. The improvement in the scheduler performance closely matches the case when flow control is eliminated. However, it reduces the handover data loss by at least 44% compared to that case which results in an enhancement of the effective TCP throughput as seen by the end user.