Abstruct-We consider the transmission of QCIF resolution (176 x 144 pixels) video signals over wireless channels at transmission rates of 64 kbls and below. The bursty nature of the errors on the wireless channel requires careful control of transmission performance without unduly increasing the overhead for error protection. A dual-rate source coder is presented that adaptively selects a coding rate according to the current channel conditions. An Automatic Repeat reQuest (ARQ) error control technique is employed to retransmit erroneous data-frames. The source coding rate is selected based on the occupancy level of the ARQ transmission buffer. Error detection followed by retransmission results in less overhead than forward error correction for the same quality. Simulation results are provided for the statistics of the frame-error bursts of the proposed system over code division multiple access (CDMA) channels with average bit error rates of to w4.

Visual communications over wireless networks require the efficient and robust coding of video signals for transmission over wireless links having time-varying channel capacity.We compare several schemes for encoding video data into two priority streams, thereby enabling the transmission of video data over wireless links to be switched between two bit rates. An H.261 (p x 64) a1 orithm is modified to implement each candidate scheme. h e algorithms are evaluated for a microcellular wireless environment and a clearchannel bit rate of 65 kb/s. Our results show that by combining layering with automatic-repeat request (wireless-)link control, almost-wireline visual quality can be achieved.

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Video transmission over wireless links is a n emerging application which involves a time-varying channel. In this paper we propose that rate control algorithms should be used a t the video encoders, along with models of the channel behavior, to improve the performance of such systems. Rather than letting information be lost as the channel conditions change, in our scheme channel state information is fedback to the encoder. We propose a method, based on dynamic programming, to compute the rate-distortion performance for a given channel and source realization. We show how the rate-distortion performance changes with end-to-end delay and feedback delay.