Gap-processing time analysis of stall avoidance mechanisms for high speed downlink packet access with parallel HARQ schemes

The parallel multi-channel stop-and-wait (SAW) hybrid automatic repeat request (HARQ) mechanism is one of key technologies for high speed downlink packet access in the wideband code division multiple access system. However, this parallel HARQ mechanism may encounter a serious stall problem, resulted from the error of the negative acknowledgement (NACK) changing to the acknowledgement (ACK) in the control channel. In the stall situation, the receiver waits for a packet that will be no longer sent by the transmitter and stops delivering the medium access control (MAC) layer packets to the upper layer. The stall issue seriously degrades the quality of service for the high speed mobile terminal owing to the high probability of NACK-to-ACK errors. In this paper, we present an analytical approach to compare three stall avoidance schemes: The timer-based, the window-based, and the indicator-based schemes. To this end, we first define a new performance metric - the gap-processing time - the duration for a nonrecoverable gap appearing in the MAC layer re-ordering buffer until it is recognized. Second, we derive the closed-form expressions for the average gap-processing time of these three stall avoidance schemes. It will be shown that our analytical results match the simulations well. Further, by analysis we demonstrate that the indicator-based stall avoidance scheme outperforms the timer-based and the window-based schemes. The developed analytical approaches provide important insight into determining a proper number of processes in the parallel SAW HARQ mechanism when applying the indicator-based stall avoidance scheme. Moreover, our analysis can facilitate the physical/MAC/radio link control cross-layer design because the gap-processing time is closely related to packet error rate in the physical layer, reordering buffer size in the MAC and RLC layer.