Information-Theoretic Reflections on PCM Voiceband Modems
This paper deals with the downstream transmission in “56 kbit/s” PCM modems. The channel consists of a digital 64 kbit/s PCM voice channel extending from a digital server modem to a local telephone office (LTO) and an analog subscriber line from the LTO to a client modem. We compute the capacity of the ideal PCM downstream channel, which is represented as a perfectly equalized AWGN channel over which A-law or μ-law symbol levels are transmitted at 8 kbaud. The Blahut—Arimoto algorithm is employed to determine the capacity-achieving symbol probabilities, either with or without a constraint on average symbol power. Coded-modulation methods for downstream transmission are still being studied. In the absence of agreement on a suitable coded-modulation method, first-generation PCM modems employ uncoded modulation and thereby do not achieve reliable 56 kbit/s transmission. We examine the performance gap between capacity-achieving modulation and uncoded modulation, and conclude that true 56 kbit/s transmission may become possible by applying suitable shaping and coding techniques. We also offer an observation that may stimulate further thought on the virtue of the quantization-noise avoidance approach adopted for PCM modems.
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- Baseline text for V.pcm, ITU Study Group 16, WP 1, Temporary Document 17E, 1998.Google Scholar