Exploiting variable-length padding bits for decoder performance improvement with its application to compressed video transmission

In many practical communications systems, the number of bits transmitted by each frame is fixed, whereas the number of information bits to be transmitted can be variable. One example is a typical communication system that consists of a source encoder followed by a channel encoder, where the former generates a variable-length information sequence per source data and the latter converts a fixed-length information sequence into a fixed-length codeword. Therefore, dummy bits are often padded (or stuffed) to the information bits prior to channel encoding at the transmitter so as to fill the gap, and they are usually discarded after channel decoding at the receiver. This paper proposes a novel use of such padding bits (PBs) for improving the performance of channel codes. We first observe that by considering the PBs embedded in the information part of systematic block codes as known bits and expurgating them at the receiver, it is possible to enhance the performance of the channel decoders, provided that the PBs are judiciously rearranged in the information sequence and they are perfectly known at the receiver. Nevertheless, the length of the PBs is random by nature, and transmitting such side information per frame would reduce the throughput. Therefore, we also develop a novel detector that can estimate the PB length reliably without the help of explicit side information. Through computer simulations employing an actual turbo code, the effectiveness of the proposed receiver with the PB length detector is demonstrated in terms of frame error rate (FER) performance. Furthermore, as a practical example of the proposed technique, its application to typical video transmission systems is addressed, where the performance improvement in terms of peak signal-to-noise ratio (PSNR) is observed.