An equalizer design technique for the PCM modem: a new modem for the digital public switched network

Abstract

Modems designed for the public switched telephone network (PSTN) have conventionally been based on modeling assumptions which view the PSTN connection as an essentially analog medium. However, as the PSTN evolves toward all digital transport and switching, and particularly as major traffic sources, such as Internet service providers, increasingly have direct digital connections to the PSTN, it is appropriate to revisit the model assumptions. Recently, several modem and chipset manufacturers have announced "56K" modems based on an emerging system paradigm in which one user (a residential Internet subscriber) has an analog connection to the PSTN, and the other (an Internet service provider) has a digital one. ITU-T is expected to finalize details of a corresponding recommendation in 1998. With this configuration, modem designs based on signaling with the law alphabet become feasible, and the conventional Shannon limit disappears as the quantization noise is avoided. Thus, the conventional Shannon limit of about 36 kb/s can be beaten, and it is possible to approach the digital transmission rate of 64 kb/s. Modems employing this general approach have become known as-law or pulse-code modulation (PCM) modems. In this paper we present a signaling technique and the sampling theory based on this technique, display the structure and operating principles of a PCM modem equalizer, and show how this equalizer problem can be cast in the language of multiinput-multioutput (MIMO) system theory.

Design of Multiple MMSE Subequalizers for Faster-Than-Nyquist-Rate Transmission

“…As an alternative to the pre-equalization method, the use of a multiple subequalizer structure was proposed in [6]. Instead of using the channel bandwidth of up to 4 kHz, as in the V.92 scheme, this equalizer scheme considers the overall system as a faster-than-Nyquist signaling case [7], [8], corresponding to the case when the transmit signal bandwidth exceeds the available channel bandwidth.…”

“…However, this method may not be applicable to an arbitrary-shaped channel, since the pulse signals should be redesigned according to the channel shape. This problem can be solved by using transmit symbol sequences comprised of the user data and zeros with the use of multiple equalizers in the receiver, where and are integers such that [6], [8], [11]. This scheme makes the overall response of the equalizer and channel have the same effect as the signal generated for an ideal bandlimited channel in [7] and [8].…”

“…We consider a transmission scheme that employs multiple subequalizers and special formats for the transmit sequence [6]. Although this transmission scheme does not comply with the V.92 Recommendation, it has some advantages over the V.92 pre-equalization scheme.…”

A filterbank structure for voice-band PCM channel pre-equalization

“…An alternative approach for PCM channel equalization was reported in [4] where the channel is assumed unknown…”

“…The signal in the down-stream direction can be treated as a base-band Pulse Amplitude Modulated (PAM) signal passing through a band-limited channel. Modems based on this approach are known as PCM modems [4].…”