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In this paper, we exploit the rich set of flexible features offered at the medium access control (MAC) layer of WiMax for construction and transmission of {\em MAC protocol data units} (MPDU) for supporting multiple VoIP streams. We study the quality of VoIP calls, usually given by R-score, with respect to delay and loss of packets. We observe that loss is more sensitive than delay, hence we compromise the delay performance within acceptable limits in order to achieve a lower packet loss rate. Through a combination of techniques like forward error correction, automatic repeat request, MPDU aggregation, and minislot allocation, we strike a balance between the desired delay and loss. Simulation experiments are conducted to test the performance of the proposed mechanisms. We assume a three-state Markovian channel model and study the performance with and without retransmissions. We show that the feedback-based technique coupled with retransmissions, aggregation, and variable length MPDUs are effective and increases the R-score and mean opinion score by about 40%.
VoIP, R-score, WiMax, FEC, ARQ, aggregation, fragmentation.

S. Sengupta, S. Ganguly and M. Chatterjee, "Improving Quality of VoIP Streams over WiMax," in IEEE Transactions on Computers, vol. 57, no. , pp. 145-156, 2007.
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