Abstract
For wireless multimedia networks, error control beyond what provided by conventional wire-line networks is required to realize reliable communications in very noisy environment. In this thesis, an adaptive hybrid automatic repeat request scheme is designed and analyzed over a Gillbert-Elliott channel model. The new method considers timing constraint and makes use of a powerful concatenated coding scheme. The basic idea is to correct main errors in the first transmission by the outer code. Suppose that an uncorrectable error pattern is detected during the first transmission, additional parity bits are transmitted by using the inner code. The redundancy must be designed to satisfy the constraint of time delays and required cell error rates. Finally, performance evaluations and simulation results are presented, and comparisons of the proposed scheme with other conventional ARQ schemes under the requirements of packet error rates and time delays are also given. In wireless networks, the channel is time-varying and high bit error rate as a result of user's mobility and variation of environment. In order to obtain the wire-line like reliability, we generally adopt two approaches as error controlling techniques: Automatic Repeat reQuest (ARQ) and Forward Error Correction (FEC). In an ARQ system, the receiver informs the transmitter to retransmit message when errors are detected, and this procedure continues until the receiver obtains the message successfully. In a FEC system, the receiver tries to decode the message by the forward error correction code. For wireless channel, ARQ is not efficient because of the frequent retransmission and FEC is not reliable because errors are out of its correcting capability. Recent approach is to combine FEC and ARQ techniques, called hybrid ARQ. Hybrid ARQ has many proposals for satellite channel, but the case is that the packet size is large with respect to propagation delay. However, they may not suit for wireless multimedia networks because of short packet size and timing constraint for supporting QoS. In this thesis, we will investigate a suitable hybrid ARQ scheme as efficient as possible under the requirements of packet error rate and timing constraint. In chapter 2, some conventional ARQ techniques are introduced. Essential techniques in our proposed ARQ scheme are described in chapter 3. In chapter 4, we propose the new adaptive hybrid ARQ and evaluate its performance bound. Finally, simulation results are presented in chapter 5, and some conclusions are made in chapter 6.