On the Effectiveness of Retransmission in Preserving Frame Error Rate over IEEE 802.11e/a WLANs

To preserve the frame error rate (FER) of multimedia applications in the polling-based HCCA of 802.11e/a WLAN is important especially under low signal-to-noise ratio (SNR) conditions. MAC frame retransmission is an effective technique to reduce the FER, especially when there has no better modulation and coding scheme (MCS) can be selected. The Surplus Bandwidth Allowance (SBA) field in the Traffic Specification (TSPEC) element in 802.11e standard specifies the excess allocation of time over and above the stated application rates belonging to this TSPEC, by taking into account the retransmissions. This paper first identifies the relationship between the surplus bandwidth allowance and the retransmissions, which is represented as the number of frame retransmission allowance within a polled TXOP (Transmission Opportunity). Then, it investigates the effectiveness of the retransmission technique in preserving the FER of traffic streams over 802.11e/a WLAN under low SNR, ranging from 5-14 dB. Simulation results show that to preserve the FER below 3% for MPEG-2 video traffic streams, the required SNR is reduced from 14 dB to 11 dB with one retry allowance, and can be further reduced to 9 dB if two retry allowances is adopted. This paper adopts IEEE 802.11 multipath channel model in simulation.