We consider the problem of synchronous video transmission over noisy channels. Unlike data or image transmission, transmission of video is a delay constrained problem. In a situation where end-to-end delay is a bottleneck, an approach based on Automatic Repeat reQuest (ARQ) alone may not be the best solution, because of the retransmission delays involved. In situations where end-to-end delay requirements permit the use of ARQ, ARQ schemes have been found to work best when the probability of bit error in the transmission channel is low so that there are only occasional retransmissions. When error probabilities are higher, use of ARQ based schemes is not advisable because of the excessive number of retransmissions involved which eat away the available bandwidth and moreover result in large delays. Forward error correction (FEC) techniques, where error check information is appended to data for error correction are useful in such situations but they are wasteful for the low error case since they constantly burn the available bandwidth. We propose that for the case of synchronous video transmission over channels with small propagation delays (so that ARQ is not forbidden), hybrid ARQ/FEC schemes which have the flexibility to switch between the two error recovery modes in a ``soft'' fashion depending upon the error regime in which the channel is, have the potential to outperform the individual schemes.
We propose an Automatic Repeat Request (ARQ) / Forward Error Correction
(FEC) scheme for synchronous transmission of video over a binary symmetric
constant rate channel. The approach consists of jointly allocating source and
channel rates to video blocks from a given admissible set subject
to the buffer or equivalently end-end delay constraints. The channel
codes used are the popular class of powerful FEC codes known as
Rate-Compatible Punctured Convolutional (RCPC) Codes. These codes have
the rate compatibility property in the sense that a weaker channel code
is a prefix of a stronger channel code.
The method used
involves independent coding of the video units and the optimal
partitioning of source rates and channel rates so as to maximize the
expected delivered end-to-end
video quality. The existence of a return channel is assumed through which the
decoder informs the encoder about the success/failure of the transmission. In
the event of a failure, the "incremental" parity information as opposed
to retransmission of the whole unit (modified Type III ARQ) is sent to
the decoder
for correcting errors and a reallocation performed at the encoder.
A block diagram of the system follows in the following figure.
