Pulse or digital communications – Pulse code modulation
Reexamination Certificate
1999-10-29
2003-12-09
Chin, Stephen (Department: 2634)
Pulse or digital communications
Pulse code modulation
C375S222000
Reexamination Certificate
active
06661847
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of modems, and, more particularly, to the construction and optimization of signal constellations for data transmission based on available signal alphabets that are either known a priori, determined through identification of model parameters, or estimated during modem start-up by means of learning techniques.
BACKGROUND OF THE INVENTION
Although the preferred embodiment will be described with respect to a pulse code modulation (PCM) modem, the present invention is in no way limited to PCM type modems and may be used with non-PCM modems. The present invention relates to the construction and optimization of signal constellations for the downlink of a PCM modem communication, system. The communications channel of interest for PCM modems is shown in
FIG. 1. A
digital modem
10
is connected to the digital transport
20
, which is connected by means of a D/A converter
30
in a PCM codec
40
to the subscriber line
50
. An analog modem
60
is connected to the opposite end of the subscriber line
50
.
PCM modems such as modems according to ITU-T Recommendation V.90 employ pulse-amplitude modulation (PAM) for downstream signaling, where the signal constellations are sets of PCM codes according to the ITU-T Recommendation G.711 transmitted by the digital modem and corresponding voltage levels, at a given point of reference. Unless specified otherwise, the point of reference will be the output of the D/A converter
30
in the PCM codec
40
, which is typically located at the Central Office.
For downstream data transmission according to Recommendation V.90, data bits are mapped to PCM codes according to Recommendation G.711 by the digital modem. The PCM codes are sent through the digital network and converted to analog voltage levels by the D/A converter in the PCM codec
40
of the Central Office. In mu-law networks, robbed-bit signaling (RBS) is often used for in-band call control, which results in the least-significant bit of a PCM code being unavailable for PCM modem data transmission. Moreover, digital attenuation pads with numerous attenuation levels and implementation characteristics are employed in the digital network. Both RBS and digital attenuation pads act as impairments from the perspective of PCM modem data transmission.
Due to the frame structure present on digital T
1
links, a frame size of 6 modulation intervals T (6 samples at 8000 samples/second) was selected for downstream data transmission in V.90. Generally, the combination of impairments, such as RBS, digital pads, and PCM codec infidelity, may be different for each interval of a 6T frame. Since certain clusters of PCM codes are mapped by the digital impairments to the same PCM code, only a subset of non-overlapping PCM codes (with one representative from each cluster) can be used for data signaling in any of the 6 intervals, and the 6 subsets are generally different. Recomnnendation G.711 specifies the voltage levels at the D/A converter output corresponding to the 256 PCM codes. However, the subset of these PCM codes that is actually used is not known in advance. Moreover, the true voltage levels corresponding to these remaining PCM codes may differ significantly from the ideal levels specified by G.711 due to PCM codec infidelity and other impairments.
The presence of a priori unknown impairments in both the digital network and the PCM codec may require precise identification of the voltage levels corresponding to transmitted PCM codes, independently for each interval of a frame (cf. related U.S. application Ser. No. 09/264,272). The Recommendation V.90 uses a frame size of 6 modulation symbols, at a rate of 8000 symbols/second. In V.90 start-up, for example, this identification can be accomplished by the analog modem by using the DIL (Digital Impairment Learning) sequence of Phase 3 of the startup procedure. After an initial training of the analog modem's equalizer (cf. related U.S. application Ser. No. 09/264,085), the voltage levels (i.e. signal levels) corresponding to transmitted PCM codes are learned separately for each interval in a frame of 6 modulation intervals.
The V.90 Recommendation allows up to 6 different signal constellations, which may be selected and assigned to intervals of a data frame by the analog modem. These constellations may be selected with a suitable spacing between adjacent signal levels to allow for reliable data transmission in the presence of noise and other distortions while maximizing the data rate subject to a constraint on average power. In practice, reliability is specified in terms of a desired probability of symbol error.
A significant constraint may be the power limit as well as the point of reference for measuring power imposed by country-specific regulations. The power limit and point of reference are sent by the digital modem to the analog modem during V.90 start-up. If the point of reference is at the output of the PCM codec and the presence of digital pads is detected, the analog modem can compensate for the attenuation and achieve higher data rates.
The problem of constellation generation in a V.90 modem is made significantly more difficult by the wide range of PCM-modem specific impairments under which near-optimal constellations must be selected. Another complication is that the impairments encountered for a particular connection may be unpredictable and may change from call to call. Furthermore, a V.90 modem may have only a limited amount of time available during start-up for selecting optimal signal constellations.
The problem of constellation generation for PCM modems was described in a TR-30.1 contribution by R. Fischer and G. Ungerboeck. However, no practical solution for solving the optimization problem with finite computational resources was proposed. Also, in U.S. Pat. No. 5,831,561 (hereinafter “the '561 patent”), the use of learned levels as the source of a signaling alphabet is described. However, little detail is provided as to how to build a constellation other than by selecting a larger number of available points and then reducing the number of points to the desired number of points by optimizing the minimum distance between points. Thus, the '561 patent assumes a first data rate and then finds an achievable data rate based on the assumed data rate. The assumption of a first data rate is not necessary in the present invention.
SUMMARY OF THE INVENTION
Certain objects, advantages, and features of the invention will be set forth in the description that follows and will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention.
It is an object of the present invention to provide an improved method for the selection and optimization of a set of signal constellations for data transmission.
It is another object of the present invention that it can be used in a V.90 client or other PCM modem to select a set of signal constellations for downstream data transmission.
It is yet another object of the present invention to allow for the selection of near-optimal signal constellations as subsets of available signal alphabets. Specifically, for PCM modem channels, the available signal alphabets are dependent on the digital impairments encountered in a particular modem connection.
Another object of the present invention is to reduce or eliminate dependencies on a precise model of the impairments in the digital network and the PCM codec in constellation generation.
A further object of the present invention is to automatically take into account the effect of impairments for a wide range of PCM-modem specific impairments.
Another object of the present invention is to provide flexibility in selecting the signal constellations for different power limits and target error probabilities.
These and other objects of the present invention may be provided by methods, systems and computer program products for constellation generation which determine the range of available and usable ucodes and an initial minimum
Davis Gordon Taylor
Dholakia Ajay
Neeser Fredy D.
Chin Stephen
Munoz-Bustamante Carlos
Myers Bigel Sibley & Sajovec P.A.
Odom Curtis
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