Occupancy regulation for reordering buffer at 3GPP's ARQ

3GPP RLC protocol specification adopted an error control scheme based on selective-repeat ARQ. In the 3GPP's ARQ, distinctive windows are installed at the transmitting and receiving stations so that each station is prohibited to send or receive a data PDU out of its window. As in a general selective-repeat ARQ, a classical problem of reordering is inherent in the 3GPP's ARQ. While a decrease in window size reduces the occupancy at the reordering buffer, such a decrease incurs degradation in delay and throughput performance. First, for the purpose of investigating the effect of window size on throughput performance, we develop an approximation method to calculate the maximum throughput of the 3GPP's ARQ. Secondly, aiming at reducing the occupancy at the reordering buffer as well as suppressing a deterioration of delay and throughput performance, we propose a scheme of regulating the occupancy at the reordering buffer as an additive to the 3GPP's ARQ. In the occupancy regulation scheme, a threshold is created in the receiving station's window and a data PDU out of the threshold (but within the window) is treated according to a go-back-N ARQ. For judging the effectiveness of the occupancy regulation scheme, we investigate peak occupancy, mean delay time and maximum throughput exhibited by the proposed scheme in various environments. From numerical examples, we confirm the validity of our approximation method and conclude that the occupancy regulation scheme is able to improve the delay and throughput performance of the 3GPP's ARQ without inflating the occupancy at the reordering buffer.