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In this paper, we investigate stall avoidance mechanisms for the W-CDMA system with high speed downlink packet access (HSDPA). The stall avoidance mechanisms are aimed to reduce transmission delays and keep in-sequence delivery of MAC layer data to the upper layer. By analysis and simulation, we compare three stall avoidance mechanisms, including the timer-based, the window-based, and the indicator-based methods. For comparison, we introduce a new performance metric called the gap processing time. Our results show that for Eb/N0 equal to 7 dB in the AWGN channel, the average gap processing time for the indicator-based method is 8.49 transmission time intervals (TTIs) and those for the window-based method and the timer-based method are 36.38 TTIs and 175.5 TTIs, respectively. In a Rayleigh fading channel, we find that the gap processing time of the window-based and the indicator-based methods are higher than in the AWGN channel, while the timer-based method has the same gap processing time in both the AWGN channel and the Rayleigh fading channel.