Optimal design of hybrid FEC/ARQ schemes for real-time applications in wireless networks

Multimedia data transmission over wireless networks often suffers from delay, jitter and packet loss. The main problem to implement a wireless network is the high and variable bit error rate in the radio link, making it necessary to use an additional error control mechanism. The traditional techniques for error control are ARQ (Automatic Repeat Request) and FEC (Forward Error Correction). The ARQ technique works better than FEC, if the bit error rate in the channel is low, and worse, if the bit error rate is high. Thus, hybrid ARQ/FEC techniques can improve the performance. In this paper, a novel hybrid ARQ/FEC scheme is analyzed in an environment of real-time applications. We focus in a simple FEC scheme which has been standardized by IETF [11]. It consists in adding a low quality copy of the original packet n to the packet n + ø. If packet n is lost in the network, it can be correctly recovered if packet n +ø is correctly received. The effect of link layer (LL) parameters like the number of LL transmission attempts, offset ø, LL packet size, and FEC code rate on the TFRC (TCP Friendly Rate Control)and UDP performance is evaluated. We show that the combination of FEC and ARQ reduces the number of retransmissions and the time spent by a packet in the buffer at the output of the wireless link. We validate our analytical results using ns-2 [9]simulation