On Noisy Channel Quantizer Design for Unequal Error Protection

Abstract

In the last few years growing efforts have been devoted to greatly enhance the robustness of low bit rate speech coders to errors introduced by the transmission channel. This increasing interest is in great part due to the need for efficient coders geared towards mobile and personal communication systems. Due to the narrow spectral bandwidth assigned to these applications, the number of redundancy bits available for forward error detection and correction will necessarily be small. This will force the coder designer to use different levels of error protection. For example, the TIA standard IS-54 for cellular communications [] incorporates three levels of protection, where only 12 out of the 159 bits in the frame are in the highest protection class while 82 are in the third class, where bits are left unprotected. The selection of which bits should be placed in which class is usually done by subjectively evaluating the impact on the received speech quality of an error in a given bit. In protection schemes like this, it is very likely that a given parameter being encoded with a B-bit quantizer will have n ₁ of these bits highly protected, an average level of protection will be given to the next n ₂ bits and the remaining (B — n ₁ —n ₂) will be left unprotected. Thus, different bits of a certain binary word (index), representing a quantization level, may be subject to different error rates, due to the diverse types of channel coding being used in the protection of each bit or group of bits.

This work was performed while the author was on leave as a Consultant with the Speech Proc. Research Dept., AT&T Bell Laboratories, Murray Hill, NJ, U.S.A.