Multiplex communications – Diagnostic testing – Determination of communication parameters
Reexamination Certificate
2000-02-02
2004-04-13
Vu, Huy D. (Department: 2663)
Multiplex communications
Diagnostic testing
Determination of communication parameters
C370S522000, C370S523000
Reexamination Certificate
active
06721279
ABSTRACT:
TECHNICAL FIELD
The present invention relates, generally, to modem technology and, more particularly, to adaptive constellation techniques for improving the performance of pulse code modulation (PCM) modems using adaptive constellation design.
BACKGROUND ART AND TECHNICAL PROBLEMS
Modem systems operating at about 56 kbps were developed to take advantage of the fact that ISPs are connected to the PSTN through a digital line rather than a twisted pair of copper wires terminating at the central office. That is, referring now to
FIG. 1
, an ISP
120
generally comprises a server
102
coupled to a digital modem
144
which is connected to PSTN
108
through a digital line
134
and is capable of transmitting data at about 64 kbps. Data is transmitted to central office
110
over a digital line
132
, and then over the analog local line
134
to an analog modem
112
associated with a user system
114
(e.g., a PC or the like).
PSTN
108
comprises assorted networks and components used to provide, among other things, standard telephone service. PSTN
108
might include foreign exchange services (FX), local exchange carriers (LECs), inter-exchange carriers (IECs), digital loop carriers (DLCs) and the like. As the parameters of PSTN
108
and a line card coder/decoder (codec) provided within central office
110
are dictated by network specifications (e.g., the use of &mgr;-law or A-law encoding), ISP modem
144
is configured to transmit digital data in such a way as to fully exploit its digital connection to the network. See, e.g., the ITU-T V.90 specification, hereby incorporated by reference.
Digital communication systems may employ a number of initialization, training, and adaptive learning protocols that are designed to equalize the channel distortions, optimize the data transmission speed, reduce transmission errors, and improve the quality of the received signal. For example, the current generation of pulse code modulation (PCM) modems, e.g., modem systems compliant with ITU-T Recommendation V.90, perform an initial training procedure to adaptively adjust the equalizer structure resident at client-side analog modem
112
(APCM).
V.90 modem systems perform an initial two-point training procedure during which one constellation signal point (based on a particular &mgr;-law or A-law level) is transmitted as a sequence having positive and negative signs. The DPCM transmits the two-point training sequence to the APCM, and the APCM analyzes the received signal to determine the channel characteristics and to adjust its equalizers. After performing this two-point training, a digital impairment learning (DIL) procedure is performed.
FIG. 2
illustrates a typical V.90 modem downstream transmission channel
200
over which such a training sequence may be sent. The signal b(n) (
202
) may represent a sequence of digital symbols, e.g., 8-bit codewords, that are to be transmitted by a DPCM transmitter
204
, where “n” represents the time index for the transmitted symbol.
A number of digital impairments
208
, such as robbed bit signaling (RBS) and digital pads, may be present within the digital network channel associated with DPCM
204
. A digital to analog conversion occurs at a PCM codec
212
to facilitate transmission to the end user over an analog loop as described above. Analog impairments
216
, such as nonlinear and linear distortion, may be associated with the analog loop and/or any number of analog processing components. Furthermore, in practical applications, additive noise
218
may be introduced to the analog signal before the analog signal is received by the APCM receiver
220
, which produces a series of estimated symbols b(n) (
222
).
Digital impairments significantly limit the performance of PCM modem receiver
220
. Digital impairment includes, for example, digital pads and robbed-bit signaling (RBS). Since reliable operation of a PCM modem is predicated on PCM receiver
220
knowing which levels digital transmitter
204
is sending out, PCM receiver
220
must detect what type of digital impairment has been encountered on a particular telephone line, or estimate all available PCM levels. In typical V.90 systems, RBS-altered symbols are periodic in nature based on the symbol count; e.g., RBS may occur every six or twelve symbols. Furthermore, the effect of RBS is deterministic but unknown to the APCM, while digital pads cause a constant, deterministic, and level-dependent (nonlinear) effect.
Prior art modem systems may compensate for linear analog impairments, such as amplitude and phase distortions, with well known linear equalization techniques. Such techniques, however, may not adequately compensate for the presence of digital impairments, and may therefore alter the level associated with the predetermined training point. Consequently, the initial training procedure performed by known V.90 modem systems do not provide the most efficient and effective result.
Methods are therefore needed which overcome these and other limitations of the prior art.
SUMMARY OF THE INVENTION
The present invention provides systems for compensating for the presence of digital impairments using adaptive constellation techniques. In accordance with various aspects of the present invention, actual sample magnitudes are estimated from received samples transmitted during a training period using an algorithm based on minimizing the sum of squares of the estimated errors. After which a suitable constellation is derived. In one embodiment, an optimum least-squares algorithm is employed. In an alternate embodiment, a least mean squares (LMS) or normalized LMS algorithm is performed.
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ITU-T V.90 Recommendation.
Olafsson Sverrir
Zhang Xuming
Farjami & Farjami LLP
PC-TEL, Inc.
Ryman Daniel
Vu Huy D.
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