Pulse or digital communications – Equalizers
Reexamination Certificate
1999-09-10
2002-07-02
Corrielus, Jean (Department: 2631)
Pulse or digital communications
Equalizers
C375S222000
Reexamination Certificate
active
06414989
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a pulse code modulation modem system. More particularly, the present invention relates to an upstream transmission scheme for transmitting data from a client modem coupled to a digital telephone network via an analog connection to a central site modem coupled to the digital telephone network via a digital connection.
BACKGROUND OF THE INVENTION
FIG. 1
is a block diagram representation of a prior art V.90 modem system
100
. V.90 modem systems (i.e., systems compliant with the ITU-T Recommendation V.90) utilize pulse code modulation (PCM) techniques in the downstream direction and conventional V.34 voice band techniques in the upstream direction. As used herein, the downstream direction refers to transmissions from a digital PCM modem (DPCM)
102
to an analog PCM modem (APCM)
104
, while the upstream direction refers to transmissions from APCM
104
to DPCM
102
. In the context of a common practical application, APCM
104
may be considered to be a client modem and DPCM
102
may be considered to be a server modem or a central site modem.
APCM
104
is coupled to a digital communication network, e.g., the public switched telephone network (PSTN)
106
via an analog connection
108
, e.g., the local loop associated with a user's telephone line. Analog connection
108
may be connected to a central office
110
, which in turn is coupled to PSTN
106
via a digital connection
112
. DPCM
102
is coupled to PSTN
106
via a digital connection
114
. Modem system
100
takes advantage of the digital nature of PSTN
106
to enable downstream transmission at a theoretical data rate of 56 kbps.
Upstream transmissions in modem system
100
are limited by the quantization noise introduced by the codec resident at central office
110
. The central office codec filters incoming signals before an analog-to-digital conversion. The filters at the codec remove low frequency components below approximately 100 Hz (the −3 dB cutoff frequency) and high frequency components above approximately 3.5 kHz. Without taking advantage of the details of the central office codec, the upstream channel can be modeled as a channel having a signal-to-noise ratio (SNR) of about 37 dB. Thus, even if an upstream signal is adequately equalized by DPCM
102
, the overall performance will be limited by the 37 dB SNR. Consequently, V.34 modems (and upstream V.90 transmissions) are currently limited to a maximum theoretical data rate of 33.6 kbps.
In a V.34 system, a form of Tomlinson-Harashima precoding is employed to pre-equalize the transmit signal in a manner that compensates for the channel characteristics. The transmit modem includes an equalizer structure that is tuned after a training procedure during which the receive modem trains an equivalent equalizer structure in response to a known training sequence. The receive modem communicates the equalizer filter tap settings back to the transmit modem for use in the precoding scheme. However, merely shifting the equalization function to the transmit modem may result in a dramatic increase in transmit power. Accordingly, precoding is applied such that the transmit power penalty is decreased. Tomlinson-Harashima (and other) precoding techniques are generally known to those skilled in the art. For example, such precoding techniques are set forth in detail in Lee & Messerschmitt, DIGITAL COMMUNICATION (Kluwer Academic Publishers, 2d ed., 1994), the content of which is hereby incorporated by reference.
PCM modulation for the upstream channel has been proposed in an effort to increase the upstream data rate in V.90 modem systems. In other words, the PCM scheme utilized for the downstream channel (or a modified version thereof) may be utilized for upstream data communication. Using upstream PCM modulation, theoretical data rates up to 48 kbps can be achieved if suitable equalization and processing are performed by modem system
100
.
The Tomlinson-Harashima and V.34 preceding techniques are not effective in the context of a data communication system that employs a PCM modulation scheme. These prior art precoding techniques are based on the fundamental assumption that the transmit signal points are spaced apart in a linear or uniform manner. In other words, any two “neighboring” signal points in a V.34 modem system are separated by the same distance. In contrast, mu-law and A-law signal point constellations (either of which may be used in a practical PCM modem system) and constellations derived from mu-law or A-law constellations include signal points that are nonlinearly spaced. For example, signal points associated with relatively lower transmit power are spaced closer together than signal points associated with relatively higher transmit power.
Due to the inapplicability of V.34 preceding for PCM signal point sequences, a need exists for an appropriate technique that is compatible with upstream PCM transmissions.
BRIEF SUMMARY OF THE INVENTION
A data communication system in accordance with the present invention provides an improved technique for providing PCM transmissions in the upstream direction from a client device to a server or central site device. The techniques of the present invention may be applied in the context of a modified V.90 modem system that utilizes PCM encoding in the upstream direction. The client modem device utilizes pre-equalization to compensate for characteristics of the upstream channel and a preceding technique to achieve upstream data rates that can exceed the maximum theoretical upstream data rate associated with conventional V.90 modem systems. The precoding technique does not rely on known Tomlinson-Harashima or V.34 precoding schemes.
The above and other aspects of the present invention may be carried out in one form by a modem device for transmitting data to a remote device over an upstream channel. One such modern device includes a data mode mapper configured to map digital data into a plurality of equivalent signal point sequences, where each signal point is selected from a nonlinearly-spaced signal point constellation. The modem device also includes a cost metric analyzer configured to obtain a measurable quantity for each of the equivalent signal point sequences and a selection element configured to select a preferred signal point sequence based on a comparison of the measurable quantity for a number of equivalent signal point sequences.
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“Recommendation V.90”, ITU-T Telecommunication Standardization Sector of ITU, Sep. 1998, pp. 7, 9-10.
Warke, N. et al., “On the Next Generation of High-speed Voiceband Modems”, 1999 IEEE International Conference on Communications, vol. 1, Jun. 6-10, 1999, pp. 256-260.
International Search Report, Dec. 11, 2000.
Jonsson Olafur
Olafsson Sverrir
Conexant Systems Inc.
Corrielus Jean
Farjami & Farjami LLP
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