DIGITAL IMPAIRMENT COMPENSATION FOR V.90 MODEM

Abstract: An impairment compensation sequence (e.g., 340) for use in a communications system (e.g., 100) susceptible to one or more potential impairments each periodic in an integer number of symbols includes N phases, wherein N is selected such that each potential impairment, if present, is periodic therein, and a sequence of symbols, the sequence organized to place at least one instance of each symbol from a predetermined set of symbols in each phase to allow detection of the potential impairments in each of the N phases. The potential impairments may include robbed-bit signaling and padding. Using estimates prepared based on such an impairment compensation sequence, individual phase intervals may be grouped according to similarity of apparent aggregate effect of the impairments thereon without identification of individual impairments active in the particular phases. Constellation points may then be assigned based on group characteristics corresponding to phase intervals. In an exemplary realization, constellation points are assigned for each of 6 constellation indices based on amplitude estimates characteristic of the groups with which each of 4 corresponding phase intervals are associated.

Description: DIGITAL IMPAIRMENT COMPENSATION FOR V.90 MODEM Technical Field The present invention relates to data communications, and more particularly, to digital impairment learning for pulse-code modulation (PCM) modems used m transmission paths compatible with digital modulation. Background Art Much of the public switched telecommunications network (PSTN) is implemented using digital data transport Nonetheless, sigmficant portions of the PSTN are still based on analog technology For example, the "local loop" portion of PSTN that connects a telephone subscπber to a central office (CO) is typically an analog loop Additionally, analog portions may exist at other pomts along a communications path, e g , as an analog channel m an otherwise digital circuit To achieve high downstream data rates m the PSTN, the current generation of 56 Kbps modems no longer treat the communications path as an analog channel Instead, such modems assume that there is only one (1) analog portion m a downstream transmission path from a digitally connected server modem to a client modem connected to an analog local loop This configuration is reasonable m areas where most Internet Service Providers (ISPs) and business customers are digitally connected to the network and allows data signaling rates of up to 56 Kbps in the downstream transmission path Although a vanety of similar designs are available, modems conforming to the ITU-T Recommendation V 90 are illustrative See generally, ITU-T Recommendation V 90, A Digital Modem and Analogue Modem Pair for Use on the Public Switched Telephone Network (PSTN) at Data Signalling Rates of up to 56000 Bit/S Downstream and up to 33 600 Bit/S Upstream (09/98) (heremafter referred to as "Recommendation V 90") Recommendation V 90 defines a method for signaling between a modem connected to an analog loop (the analog modem) and a modem connected to the digital trunk (the digital modem) Modems in accordance with Recommendation V 90 take advantage of this particular arrangement to increase the data rate from the digital modem to the analog modem Earlier generations of modems, e g , those conforming to ITU-T Recommendation V 34 and/or earlier standards, suffered from quantization noise introduced in the conversion of analog signals to digital, Pulse Code Modulation (PCM) codewords for transmission in digital portions of the PSTN Modern modems, including modems conforming to Recommendation V 90, avoid the effects of PCM quantization by usmg a form of amplitude modulation that allows voltage amplitude levels to be chosen from the quantization levels of the PCM coder/decoder (codec) in the CO Recommendation V 90 matches the quantization levels to potentially eliminate quantization noise entirely m a downstream path from a server modem to a line interface Thus, Recommendation V 90 achieves higher data rates when there are no analog-to-digital conversions between a V 90 digital modem and the PSTN A digital V 90 modem transmits PCM codewords that are converted to discrete analog voltage levels in the local CO and are sent to the analog V.90 modem via the analog local loop. The analog modem's receiver then reconstructs the discrete network PCM codewords from the analog signals received. There is no specific technique provided in Recommendation V.90 by which the analog client modem is to decide whether PCM signaling can be supported by the downstream channel. Rather, the client modem must make an inference about the condition of the channel during the training process. Moreover, part of the modem training procedure, defined in Recommendation V.90, includes estimating the digital impairments present on the telephone channel. Several common sources of digital impairments are Robbed Bit Signaling (RBS), transcoding (e.g., A-law to μ-law conversion), and digital attenuation pad. Using current techniques, 56 Kbps downstream signaling rates can often be achieved. However, actual signaling rates may be limited by distortion introduced in the digital backbone itself. One source of distortion is Robbed Bit Signaling (RBS). RBS is an in-band signaling technique used i some portions of the PSTN to perform control functions such as conveyance of ring and call progress indications in the telephone network. In short, RBS involves modification by the PSTN of data transmitted. Generally, a least significant bit (LSB) of certain PCM codewords may be used (or usurped) by a portion of the digital backbone. Although usurpation of bits is generally acceptable when codewords carry a voice signal, RBS effectively acts as noise or distortion when codewords carry a data signal. Moreover, RBS limits the information carrying capacity of a communications channel that includes a portion employing it. Recognizing these limitations, techniques have been developed for detecting, characterizing and mitigating RBS. For example, U.S. Patent 5,875,229 to Eyuboglu et al., proposes detection and characterization of RBS during a "training" phase prior to other training operations such as initialization of equalizer coefficients. Eyuboglu's characterization technique is based on counting LSB values equal to logic zero and logic one in each of 24 intervals of a received training signal. U.S. Patent 5,859,872 to Townshend also proposes a scheme for handling RBS in which RBS is detected during an initial training phase. In Townshend, a decoder first attempts to equalize a received training signal having a known pattern by mmimizing the difference between its output and a known pattern under the assumption that no robbing of bits occurred. The decoder then measures the average equalized values at each of six phases and determines for each phase which of 4 bit robbing schemes (including no bit robbing) has been employed. Once the bit robbing that occurred in each phase is determined, the equalization process is rerun, since the first equalization was performed without knowledge of the bit robbing. Recommendation V.90 specifies a protocol between digital and analog PCM modems, whereby a Digital Impairment Learning (DIL) sequence is supplied to allow the analog PCM modem to detect digital impairments in a communication path. In this way, the analog PCM modem may define a constellation (or constellations) that tend to avoid or compensate for the discovered impairments. The protocol defined in Recommendation V.90 allows each manufacturer to define a Digital Impairment Learning (DIL) sequence to meet its objectives. DISCLOSURE OF INVENTION Accordingly, a class of impairment compensation sequences and related techniques have been developed. In some realizations, impairment compensation sequences are intended to be requested and supplied as a DIL sequence in accordance with the protocols of Recommendation V.90. However, other similar realizations, e.g., based on proprietary or future protocols, as well as realizations without a formal or generic request/response protocol, are also envisioned and will be appreciated by persons of ordinary skill in the art based on the description herein. In general, techniques in accordance with the present invention employing impairment compensation sequences accept the characteristics of each timing phase as presented and need not make a priori assumptions about RBS or other digital impairments in the communication channel. Accordingly, the impairment compensation sequences and related techniques are robust to digital impairments, including new digital impairments encountered in the field, that are periodic in N phases of the impairment compensation sequence as explained in greater detail herein. For example, in the case of RBS impairments, previous approaches have required knowledge of the RBS forms present in the telephone network. In contrast, 24-phase impairment compensation sequences in accordance with some embodiments of the present invention are indifferent to the particular form of RBS present, including overlaid contributions of various digital portions of a communications path, as long as all RBS is periodic in the 24 phases. Indeed, using complementary techniques described herein, impairment compensation may be achieved without identification of the specific RBS forms affecting a communications path. Accordingly, realizations of communications apparatus and methods employing such impairment compensation sequences and/or related techniques generally do not require updated software or hardware if and when new impairments are encountered in the field or when used in a particular communication networks. For impairments periodic in a predetermined number of phases of a impairment compensation sequence (e.g., RBS-related impairments with periods of 6, 12, or 24 phases of a DIL sequence in accordance with the present invention), new types and/or combinations of impairments may be tolerated and communications facilities employing impairment compensation sequences and/or techniques in accordance with the present invention may conveniently adapt thereto. In some realizations, an impairment compensation sequence in accordance with the present invention is requested by an analog PCM modem using a DIL descriptor and supplied by a digital PCM modem as a DIL sequence all in accordance with phase 3 of a start-up sequence for modems that conform to ITU-T Recommendation V.90. However, the techniques described herein are also applicable to other data communications configurations and equipment. These and other suitable configurations will be better appreciated by persons of ordinary skill in the art based on the specification that follows. BRIEF DESCRIPTION OF DRAWINGS The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings. FIG. 1 is a generalized flow diagram illustrating a digital network including a public switched telephone network (PSTN) that employs robbed-bit signaling (RBS). FIG. 2 illustrates a communications path for server and client modems showing a configuration in which RBS is present in a digital channel. FIG. 3A is a diagram illustrating phases of a symbol sequence that may be transmitted over a communications path in accordance with an embodiment of the present invention. FIG.3B is a diagram illustrating the organization of an illustrative impairment compensation sequence in accordance with the present invention as supplied as a DIL sequence using the DIL descriptor request and DIL segment supply framework of Recommendation V.90. FIG. 4 is a flow diagram illustrating a method for grouping phase intervals according to similarity of aggregate impairments in accordance with an embodiment of the present invention. FIG. 5 is a flow diagram illustrating a method for assigning constellation points in accordance with an embodiment of the present invention. FIG. 6 depicts an exemplary V.90 modem realization in which Digital Impairment Learning (DIL) techniques in accordance with the present invention may be employed. The use of the same reference symbols in different drawings indicates similar or identical items. MODE(S) FOR CARRYING OUT THE INVENTION Although not limited thereto, some aspects of the present invention are described herein in the context of terminology, protocols, standards, facilities, encoding and modulation schemes, communication networks and or interfaces typical of the Public Switched Telephone Network (PSTN). In particular, some aspects of the present invention are illustrated in the context of a digital Pulse Code Modulation (PCM) network, Robbed-Bit Signaling (RBS) typically employed in portions of the current PSTN, and modem realizations conforming to ITU-T Recommendation V.90 and related standards. Nonetheless, this context is merely illustrative. Indeed, based on the description herein, persons of ordinary skill in the art will appreciate realizations in accordance with the present invention suitable for other communication facilities and networks, including point-to-point circuits, private networks and communications channels generally susceptible to digital impairments. Accordingly, and subject to the foregoing, the following nomenclature is presented for clarity in the description of certain illustrative embodiments particularly suitable in the context of a PSTN/V.90-oriented implementation framework: Glossary of Terms A,,k estimate of the ;* amplitude at the &* timing phase A'.j an averaged version of A, ,k c index used to point to a Uchord Φ denotes the empty set 7ι j" 1 subset of Σ consisting of all sets {<7 } that exhibit the same or similar aggregate impairments (e.g , from one or more RBS forms active m correspondmg time mdices) r the set, {y Q ,γ v .-.,y u _ , of subsets of Σ d(σ,,σj) sum of the amplitude differences-squared m the sets cT ( and o~ (set difference) DIL Digital Impairment Learning sequence mmimum distance parameter He used to calculate the length of the DIL-segments containmg training symbols from Uchord c arbitrary mteger mdex arbitrary mteger mdex arbitrary mteger mdex Lc length of one DIL-segment LSP length of the sign pattern LTP length of the trammg pattern £ arbitrary mteger mdex m arbitrary mteger mdex M the number of elements m T N number of DIL-segments r arbitrary mteger mdex REF, reference symbol m the DIL-segment contammg trammg symbols from Uchord c PCM Pulse Code Modulation maximum power transmitted by digital modem Ri reference symbol at time index i number of elements in (i.e. the size of) the set γ m SE sign pattern a representative element of γ i σk set of all estimated amplitudes at time index k Σ {σ t ;* = 0,l,...,23} Ti training symbol at time index i TP training pattern θ threshold less than which a set difference is defined to be small Ucode A universal code used to refer to a symbol value whether encoded as a μ-law or an A-law PCM code word. Uchord, The c* subset of Ucodes. Ucodes are grouped into eight Uchords. Uchord , contains Ucodes 0 to 15; Uchord 2 contains Ucodes 16 to 31; ...; and Uchord % contains Ucodes 112 to 127. x(k) denotes equalizer input sample at time index k y(k) denotes equalizer output sample at time index k Digital Impairments (including RBS) in the PSTN A conventional network is shown in FIG. 1. Network 100 includes a digital network such as public switched telephone network (PSTN) 100 that includes a digital backbone 110. In the configuration shown, data processing equipment 121, such as an internet server or gateway thereto, communicates with a digital adapter 123, such as a digital V.90 modem, during data communications with data processing equipment 122, e.g., a user's computer. Digital adapter 123 transmits a sequence of symbols over digital backbone 110 to line interface 124. Line interface 124 in turn transmits baseband signals on local loop 126. Local loop 126 typically transmits voice signals or binary data in the form of baseband signals to analog adapter 125. Analog adapter 125, e.g., an analog V.90 modem, receives the baseband signals and may, in turn, equalize and sample the baseband signal, detect the binary information in the demodulated signal and supply results to data processing equipment 122. A reverse path from analog adapter 125 to digital adapter 123 may be constructed using conventional analog modem signaling techniques, such as, for example, V.34 technology, or other suitable techniques. See generally, U.S. Patents 5,875,229 and 5,859,872; see also Recommendation V.90 for a description of digital/analog modem pair technology. Digital backbone 110 typically transmits encoded representations of symbol sequences. In an illustrative realization, symbols are encoded as A-law or / -law pulse code modulation (PCM) codewords, each eight bits in length. Line interface 124 receives the encoded symbols from digital backbone 110 and an A-law or a / -law decoder prepares the symbols for transmission via local loop 126. In a typical realization, line interface 124 produces discrete baseband signals that correspond to inverse quantized symbols that are transmitted to analog adapter 125. Analog adapter 125 receives the baseband signals and obtains the transmitted data through signal processing techniques. Data processing equipment 122, in turn, receives the transmitted data from analog adapter 125. In an illustrative communications network, digital backbone 110 is part of a PSTN that either employs robbed bit signaling (RBS) or exhibits other digital impairments. Such impairments typically introduce disturbances that have a very small effect on human perception of voice signals, but effectively act as noise or distortion when codewords encode data. Accordingly, digital impairments such as RBS limit the information carrying capacity of a communications channel. Focusing on an illustrative digital impairment, RBS is an in-band signaling technique used in some portions of the PSTN to perform control functions such as conveyance of ring and call progress indications in the telephone network. In portions of the PSTN employing the technique, RBS may result in modification of data transmitted. Generally, a least significant bit (LSB) of certain PCM codewords may be used (or usurped) by a portion of the digital backbone. FIG. 2 illustrates a downstream communications path in which RBS is present in a digital channel 216. Server modem 212, e.g., a digital V.90 modem, generates a digital signal that forms an input to a decoder 217, e.g., an A-law or a μ-law decoder. At least some portion of digital channel 216 employs RBS, which "steals" certain low order bits of certain transmitted symbols to instead convey call progress or other indications. Persons of ordinary skill in the art will appreciate that individual portions of the digital channel 216 may each employ RBS affecting the same or different transmitted symbols, such that a conveyed sequence of symbols may be affected by multiple, possibly overlapping, RBS-type digital impairments. Decoder 217 converts the digitally encoded signals received from modem 212 via digital channel 216 and supplies a corresponding modulated analog signal via analog channel 218. Analog channel 218 conveys the modulated analog signal to client modem 219, e.g., an analog V.90 modem. Accordingly, the received signal may be affected by multiple, possibly overlapping, RBS-type digital impairments. FIG. 3A illustrates a symbol sequence 300 such as that transmitted by digital adapter 123 over digital backbone 110. Referring to FIGS. 2 and 3 in combination, some symbols, namely those PCM codewords transmitted in an interval affected by RBS, may be modified by digital channel 216. Typical sources of RBS tends to affect every sixth PCM codeword and, depending on the form employed, may force least significant bits (LSBs) of affected codewords to logic 1, to logic 0, or to logic 1 in some intervals and logic 0 in others. See generally, U.S. Patents 5,875,229 and 5,859,872 for a discussion of RBS forms employed in the digital PSTN. Although a most basic period for RBS is six intervals, it has been observed that RBS, if employed, may exhibit a period of six, twelve or twenty-four. For example, an all-one RBS pattern will exhibit an RBS period of 6 phase intervals and an alternating zero-one pattern will exhibit an RBS period of 12. Other patterns, such as a pattern consisting of repetitions of 0 and an alternating 1/0 (e.g., 01000100...) will exhibit an RBS period of 24. See generally, TIA Draft Standard, TR30.3/ 00-05-Oxx (DRAFT 15 PN3857), North American Telephone Network Transmission Model for Evaluating Analog Client and Digitally Connected Server Modems (May 2000) for a description of RBS patterns and related modem test procedures. Therefore, for illustrative purposes, symbol sequence 300 can be viewed as exhibiting twenty-four timing phases, e.g., phase 1, phase 2, ... phase 24. Of course, symbols transmitted in given phase may be affected by multiple digital impairments, including multiple forms or sources of RBS. For example, in a digital channel including at least two portions, symbols transmitted in intervals corresponding to phase 2 (see FIG.3A) may be affected by overlapping RBS contributions (or other digital impairment contributions) from each of the portions. Alternatively, RBS contributions (or other digital impairment contributions) may affect different phases. Whatever the overlap and phasing characteristics of the impairments, the resulting effects are guaranteed to be periodic in 24 intervals. DIL Sequence Framework The Digital Impairment Learning (DIL) framework specified by Recommendation V.90 may be employed in some realizations to allow an analog V.90 modem to request (and receive from a digital V.90 modem) an impairment compensation sequence in accordance with the present invention. In such realizations, the impairment compensation sequence can be encoded as a DIL sequence. Accordingly, certain aspects of the present invention may be best understood when described in the context of the DIL descriptor request/ sequence supply protocol specified by Recommendation V.90. Nonetheless, based on the description herein, persons of ordinary skill in the art will appreciate realizations in which (1) an alternative descriptor request/ sequence supply protocol is employed or (2) no descriptor request/ sequence supply protocol is employed. In general, impairment compensation sequences in accordance with the present invention may be requested, supplied, supplied without request, encoded, etc. in accordance with any suitable implementation framework. Subject to the forgoing, and without limitation, a realization particularly appropriate for the DIL framework of Recommendation V.90 is now described. To facilitate compensation for digital impairments such as RBS, modem designs in accordance with Recommendation V.90, implement a digital impairment learning protocol that supports a wide variety of implementations techniques (often vendor specific techniques) for ascertaining, compensating and/or avoiding impairments present in a given communications channel. In some embodiments, equalizer and echo cancellation training (or retraining) at an analog V.90 modem may exploit a digital impairment learning (DIL) sequence transmitted by the digital V.90 modem. Typically, the DIL sequence is received and processed by the analog modem to determine the digital impairments present in the connection, for example, the digital PAD value, the RBS pattern, and nonlinear distortion. In an illustrative realization, an impairment compensation sequence in accordance with the present invention is requested and supplied using the DIL framework of Recommendation V.90. Referring illustratively to FIG. 2, during a training phase of operation (e.g., phase 3 of the startup sequence defined by Recommendation V.90), analog client modem 219 requests a particular DIL sequence using a DIL sequence descriptor. The DIL sequence descriptor is sent from analog client modem 219 to digital modem 212. In response, digital modem 212 supplies a corresponding DIL via digital channel 216. The specific requirements of Recommendation V.90 are well known (see generally, ITU-T Recommendation V.90, §§ 8.3, 9.3) and suitable implementations in accordance therewith will be appreciated by persons of ordinary skill in the art. However, to summarize operations in the context of FIG. 2, analog client modem 219 transmits a sequence requesting that digital modem 212 begin equalizer training. Analog client modem 219 then transmits a training signal, TRN. After training its receiver, digital modem 212 receives a sequence, J a , from the analog client modem 219 and subsequently transmits a training signal, TRNi d , for used by analog client modem 219 in its equalizer and/or echo cancellation training. In an exemplary realization in accordance with Recommendation V.90, the sequence, J a , encodes a DIL descriptor. A suitable format for the DIL descriptor appears in ITU-T Recommendation V.90, § 8.3, Table 12, which is incoφorated herein by reference. Digital modem 212 and analog client modem 219 subsequently exchange sequences, J < ι and J d ', and signals, S and S , whereupon digital modem 212 transmits a DIL sequence corresponding to the DIL descriptor. DIL Sequence and Descriptor Formats In some realizations in accordance with the present invention, an impairment compensation sequence is supplied as a DIL sequence and specified using parameters of a DIL descriptor supplied (e.g., by analog modem 219) in accordance with Recommendation V.90. In such realizations, the set of suitable impairment compensation sequences is constrained to a subset of all the possible sequences otherwise in accordance with the description and claims herein. In particular, the suitable subset is constrained by the DIL sequence organization specified by Recommendation V.90 and by the limited expressiveness of the DIL descriptor request format specified in Recommendation V.90. Persons of ordinary skill in the art will appreciate nature of these Recommendation V.90-related constraints based on the following description. According to Recommendation V.90, a DIL sequence consists of TV DIL-segments of length c where: 0 < N < 255 ; 1 < c ≤ 8 ; and L c = (H c + 1) * 6 symbols. In the DIL scheme of Recommendation V.90, N, a set of N training symbols, a set of reference symbols, segment lengths, and a pattern of training and reference symbols (including a sign pattern) are all specified as part of the DIL descriptor. In particular, each DIL segment in accordance with Recommendation V.90 includes a single training symbol (e.g., a Ucode) from the set specified in the DIL descriptor. Both a segment length and a reference symbol value may be individually specified for the set of Ucodes (and DIL segments) corresponding to a particular Uchord. For example, each H c specifies the length of the DIL-segments containing training symbols from the corresponding Uchord c . Similarly, each REF C , specifies the reference symbol for use in DIL- segments containing training symbols from the corresponding Uchord c . A sign pattern (SP) and training pattern (TP ) are used for the entire DIL sequence. The pattern of SP bits determines the pattern of sign for symbols of the DIL sequence. More particularly, values of 0 and 1 indicate negative and positive Ucode signs, respectively. Similarly, the pattern of TP bits determines the pattern of reference and training symbols in the DIL sequence. More particularly, values of 0 and 1 indicate a reference symbol ( REF C ) and training symbol, respectively. Patterns are restarted at the beginning of each DIL-segment. The lengths of these patterns are: 1 < E S/J < 128 ; and Based on the description herein, DIL-descriptor formats and corresponding DIL sequences will be readily understood by persons of ordinary skill in the art familiar with Recommendation V.90. Accordingly, DIL- descriptors may be used to specify DIL sequence realizations of impairment compensation sequences in accordance with the claims that follow and provide a useful framework for description. Nonetheless, DIL sequence realizations are merely exemplary and impairment compensation sequences in accordance with the present invention need not comport with Recommendation V.90 or with a descriptor request/ sequence supply protocol. Impairment Compensation Sequences In an exemplary embodiment in accordance with the present invention, an impairment compensation sequence may be employed to conform a communications device to at least a subset of impairments that may be present in a communications channel. For example, estimates (e.g., amplitude estimates) corresponding to training symbols (e.g., PCM code words) may be computed and adaptive signal processing structures (such as equalizers, echo cancellers, etc.) may be trained using a received impairment compensation sequence transmitted over the affected communications channel. In addition, symbol sets may be selected based on the characteristics of the received impairment compensation sequence. In accordance with the present invention, an exemplary DIL-sequence described herein is designed in order to estimate the received amplitudes associated with all possible ucodes that could be transmitted by the remote modem. Further, the received amplitudes are observed at each of 24 possible time phases. The period of 24 is chosen to coincide with all possible periods of RBS sequences that might be employed on the digital channel. Actual RBS sequences have unknown periods of either 6, 12, or 24, so observing the received amplitudes at 24 distinct time phases subsumes all cases. In particular realizations m accordance with the present invention, impairments that are periodic in N phases of a signal conveyed by the communications channel are addressed using impairment compensation sequences wherem at least one mstance of each symbol is placed m each of the N phases thereof An illustrative peπodic unpairment is RBS, which, m current communication systems, is peπodic m 6, 12 or 24 symbol mtervals of the PSTN Accordmgly, m an RBS-oπented embodiment, an impairment compensation sequence preferably presents each symbol that is, or may be, employed m communication via the affected communications channel in each of 24 phases of the unpairment compensation sequence In this way, estimates may be calculated for each symbol m each phase that may be affected by one or more RBS-type impairments Since actual RBS impairments have peπods that are not known a pπoπ, but which must be 6, 12, or 24 symbol mtervals, estimating correspondmg amplitudes at each of 24 phases subsumes all cases In general, an unpairment compensation sequence in accordance with the present invention mcludes N phases, wherem N is selected such that each potential unpairment, if present, is peπodic therein Accordmg to an exemplary embodiment, N is 24, as m the maximum penod for RBS The impairment sequence is a sequence of symbols organized to place at least one mstance of each symbol from a predetermined set of symbols m each time phase to allow detection of the potential impairments, such as RBS or PAD distortion, in each of the N phases Another quality of the unpairment compensation sequence is that N is chosen to be a least common multiple of respective penods of each of the potential impairments Thus, the impairment compensation sequence establishes communication avoiding impairments that have any mteger multiple, and is not limited to RBS and PAD-type penodic impairments Suitable realizations of such impairment compensation sequences are, in general, shaped by the charactenstics of receiver signal processmg structures and by limitations of (or constraints imposed by) any descriptor request/ sequence supply protocol employed For example, m a receiver that employs a particular equalizer design, e g , a partial response, class five (PRV) equalizer, the organization of a suitable impairment compensation sequence typically allows individual trammg symbols to be isolated m equalizer output sequence Similarly, m a descriptor request/ sequence supply protocol (such as the DIL descnptor request/ sequence supply protocol of Recommendation V 90) wherem a portions (e g , segments) thereof each present a smgle trammg symbol, the organization of a suitable impairment compensation sequence typically presents at least one mstance of a first trammg symbol (typically interspersed with reference symbols), then at least one mstance of a second trammg symbol, then , in each of the N phases of the impairment compensation sequence In general, to improve the quality of estimation of symbols received, the impairment compensation sequence is organized to place at least two mstances of each symbol from a predetermined set of symbols m each phase The two or more mstances can then provide an average of received values correspondmg to the symbol Exemplary DIL Sequences Based on the descnption and claims herem, persons of ordinary skill in the art will appreciate the general charactenstics of impairment compensation sequences m accordance with the present mvention m the context of a particular realization thereof Accordmgly, an impairment compensation sequence is now described which is embodied as an exemplary DIL sequence of symbols selected from the set of Ucodes employed in the PSTN, which is suitable for supply in response to a startup sequence in accordance with Recommendation V.90 and its descriptor request/ sequence supply protocol, and which is particularly adapted for amplitude estimation at a communications device (e.g., an analog V.90 modem) implementation employing a PRV or partial-response, class four (PRTV) equalizer. While the exemplary DIL sequence is particularly suited to such a configuration, persons of ordinary skill in art will understand, based on the more general description herein, that the invention is not limited thereto. Referring to FIG. 3B and Table 1 below, a first portion 351 (104 symbols) of a DIL segment 350 of an exemplary DIL sequence 340 is illustrated. The complete DIL sequence is typically supplied in response to a DIL descriptor (e.g., DIL descriptor 330) that specifies the characteristics of the desired DIL sequence. Column 1 of the table indicates a time index k where a symbol is supplied at each time k= {0,1,2,... 104, ...}, while column 2 identifies each of 24 phases. For each time index, a symbol (e.g., a reference or training symbol) is supplied via a digital transport in which zero or more phases may be affected by zero or more RBS- type impairments. Each such symbol is received as a corresponding amplitude, x(k), which is supplied as an input to an equalizer of the receiver (e.g., a PRV or PRIV equalizer of a analog V.90 modem implementation). In the table, R represents instances a reference symbol and T represents a training symbol. Table 1 — DIL-Sequence A first portion of a DIL segment is illustrated in column 3 of the table In particular, the organization of the first 104 symbols of an exemplary DIL segment is illustrated as a sequence of trammg and reference symbols Correspondmg amplitudes, x(k), are supplied as an mput to an equalizer of the receiver Column 4 illustrates idealized scaled outputs of a receiver employmg a PRV equalizer wherem an output time sequence y(k) is related to the mput time sequence x(k) by the relationship: y(k) = x(k) - 2x(k-2) + x(k-4) Similarly, column 5 illustrates idealized scaled outputs of a receiver employmg a PRIV equalizer wherem an output time sequence y(k) is related to the mput time sequence x(k) by the relationship: y(k) = x(k) - x(k-2) In each case, interesting outputs, i.e , those that are a function of a single training symbol mstance, are listed Other outputs, mdicated as " — " may be ignored Note that m the case of outputs for the PR IV equalizer, only a smgle output based on each trammg symbol are illustrated Dup cative outputs are ignored and the illustrated output is selected for correspondence with PRV outputs although other selections could also be made Inspection of the set of interesting equalizer outputs m Table 1 confirms that a trammg symbol appears once with a positive sign and once with a negative sign m each of the 24 possible time phases of the illustrated portion of a DIL segment. Each of these appearances corresponds to an estimate of the amplitude level of the trammg symbol when observed at the receiver. In an exemplary realization, a complete DIL segment (e.g , DIL segment 350) mcludes two repetitions (351 and 352) of the partial sequence illustrated m table 1 and therefore yields two such estimates, thereby reducmg the error associated with each estimate by 3 dB, assuming that the errors are uncorrelated In an exemplary realization, DIL sequence (e g., DIL sequence 340) mcludes 128 similarly organized DIL segments, one for each possible (positive) Ucode employed m the PSTN (see Table 1 /Recommendation V.90) In realizations m accordance with Recommendation V 90, each DIL segment presents a particular Ucode as the trammg symbol T Accordmgly, a DIL sequence employmg 128 such DIL segments presents each symbol (i.e., A, { i = —127, — 126,..., — 1,- 0, 0,1,...,127 }) in each of the 24 possible time phases of the DIL sequence Such a DIL sequence therefore provides 128 (Ucodes) x 2 (positive and negative) x 24 (timmg phases) = 6144 estimates, each of which mcludes the average of two observations of each of 256 amplitudes at each of the 24 timmg phases. In the descnption that follows, an estimate of the ;* amplitude at the A* timing phase is denoted by A, ,k for i = -127, - 126,...,- 1,- 0, 0,1,...,127 and k =0,1,..., 23 . (Note that the elements A-o, k an d Ao. k are different m general.) Exemplary DIL Descriptors Although persons of ordinary skill m the art will appreciate a vanety of impairment compensation sequences m accordance with the present mvention, mcludmg sequences supplied without use of descnptor request/ sequence supply protocols or usmg alternate protocols, realizations m accordance with the particular descriptor request/ sequence supply protocols of Recommendation V 90 are illustrative As discussed above, an impairment compensation sequence that is a DIL sequence of reference (R) and trammg (T) symbols m accordance with Recommendation V 90, can be specified accordmg to a logical combination of sign pattern bits (SP) and trammg pattern bits (TP) More specifically, a server modem respondmg m accordance Recommendation V 90 supplies reference and trammg symbols accordmg to the sign pattern and the trammg pattern specified by a DIL descnptor Typically, the particular symbol (reference or trammg) supplied at indexy of a DIL segment is a function of correspondmg SP and TP bit values R if SP m =l and TP n =0, -R if SP m =0 and TP n =0, T if SP m =l and TP n =l ; and -T if SP m =0 and TP n =l where 7 = 0, .. , L c -1 ; m =j mod L SP ; and n =j mod L TP In the context of realizations in accordance with Recommendation V.90, a smgle trammg amplitude is supplied with positive and negative sign m the specified positions of a particular DIL segment. At least one DIL segment is requested for each trammg Ucode Coπesponding reference symbols for a group or Uchord of symbols are specified m accordance with the Recommendation V 90-style DIL descnptor format Accordmgly, m a realization employmg a Recommendation V 90-style DIL descnptor to request a DIL sequence of 128 DIL segments, each as illustrated above with reference to Table 1 and respectively correspondmg to one of the 128 PCM encoded Ucodes employed m the digital PSTN, a suitable DIL descnptor encodes the following X=128, SP = {0xA5, 0xA5, 0xA5, 0xA5, 0xA5, 0xA5, 0x05), TP = {OxCC, OxCC, OxCC, 0x00), Lc=2lO, Hc=34, and REFc=0, c=l,2, 8 The DIL segment length, L c , is specified as 210, which mcludes two 104 symbol iterations of the partial DIL segment illustrated m Table 1 with an extra two bits of paddmg to provide a DIL segment length, L c , that is a multiple of six Sign pattern length, L SP , and trammg pattern length, L T p, are each selected to repeat evenly withm the 208 symbol positions (210 less paddmg) of the DIL segment and provide two (2) positive sign mstances and two (2) negative sign mstances of a particular trammg Ucode m each of the 24 phases of the DIL segment While a complete set of available symbols may be employed m some realizations, a subset may be selected m accordance with power constraints of the system More specifically, a specification such as Recommendation V 90, typically contemplates a limit to the amount of power transmitted across a communication channel The PCM codeword mcludes Uchords that transmit at different power levels Therefore, the set of symbols employed for communication (and mcluded in a DIL sequence), may preferably be chosen to convey as much mformation as possible withm power constraints In another realization, the DIL sequence employs 112, rather than 128, Ucodes Determination of Characteristic Groups Based on a received sequence such as the exemplary DIL-sequences descnbed above, a communications device operating m accordance with some embodiments of the present mvention groups phases accordmg conespondence of respective estimates For example, in some realizations, phases may be grouped by calculatmg a distance measure between phase-specific amplitude estimates of representative sets of symbols An illustrative set difference calculation is described in greater detail below Because the exemplary sequences descnbed above guarantee that individual symbols of a predetermined set appear at least once m each phase, such difference measure calculations are facilitated Usmg distance measure calculations or other conespondence measurement techniques, phases affected by the same or like impairments can be identified and appropriate compensation can be applied to the group It is notable that a receiver employmg techniques such as described herem makes few a priori assumptions about the communications channel For example, the receiver of an analog client modem employmg such techniques (see e g , FIG. 2 m which analog client modem 219 receives an unpairment compensation sequence from digital server modem 212) need not make assumptions about which timmg phases are perturbed by particular forms of RBS, paddmg or other artifacts of a digital communications channel Instead, such techniques exploit the fact that the dιsturbance(s), if any, introduced by the digital network is penodic m a predetermined number of phases (typically 24) This assumption mcludes the common cases m which an RBS pattern exhibits a period of 6 or 12 In the general case, each of the 24 phase mtervals (see e g , FIG. 3A) may, m theory, be affected by a different impairment or superimposed collection of impairments Alternatively, each of the 24 phase intervals may be affected by a single impairment (or superimposed collection of impairments) or no unpairment at all Focusmg illustratively on RBS impairments, presence of one or two distinct forms of RBS is common with less than all of the phase mtervals affected In general, the forms of RBS that may affect a phase interval result in (1) forcmg a least significant bit (LSB) of an octet to one, (2) forcmg the LSB of an octet to zero, and (3) forcmg the LSB of an octet to a "randomly" selected value, either a zero or a one Note that selection of a zero or one is only "random" with respect to the symbols conveyed over the communications channel Selection is, of course, deterministic from the pomt of view of the ln-band signaling that employs it For simplicity, much of the art assumes that a given phase interval is affected by a single form of RBS, rather than a superposition of multiple forms Similarly, it is often assumed that the smgle form of RBS affecting a particular sample is penodic in 6 phase mtervals Stnctly speakmg, neither simplification is necessanly valid Instead, persons of ordinary skill m the art will recognize that individual portions of a communication channel may employ differmg forms of RBS m vanous phase mtervals Accordmgly, symbols conveyed over a communication channel m a given phase interval may be affected by multiple RBS forms or other impairments Furthermore, as descnbed above, the effects of RBS impauments may have penodicity of 12 or 24 phase mtervals Embodiments m accordance with the present mvention may be particularly advantageous m that diagnosis of a particular RBS or other impairment is unnecessary Whatever the particular forms of RBS active in a communications channel, realizations of the present mvention may avoid diagnosis of the particular form(s) of RBS present m a individual phase interval by mstead categorizing the phase mtervals mto charactenstic groups accordmg to the siπulaπty of observed impairments Of course, m addition to RBS, other sources of impairments (such as digital attenuation paddmg) may also affect some or all of the phase mtervals Based on the descnption herem, persons of ordinary skill in the art will appreciate a wide vanety of potential impairments and combinations of impairments that may affect the vanous phase mtervals Nonetheless, for clanty and without loss of generality, the description that follows focuses on RBS as the illustrative impairment Let the amplitude estimate for the l * symbol of a predetermined alphabet at the k ώ timmg phase be denoted by A, k for i = —127, - 126,.. , — 1,- 0,0,1,...,127 and & =0,1,...,23 (Note that, m general, the elements A 0k and A 0 k stand for different estimates ) Further, define the sets <j k = {A t k ; — \21 ≤ l < 127} for k = 0,1,..., 23 That is, o~ k IS e set of all amplitude estimates made at timmg phase k Finally, define ∑ = {(J0> O~\ , — , O~21f ■ Based on the above, an illustrative set difference between o~, and 7 3 > both elements of Σ , can be defined as: where the range of mdex values / for the summation operators covers a representative subset of amplitude estimates (e.g., amplitude estimates for PCM code words conespondmg to Uchord t and Uchord 5 as defined employed in modem communications m accordance with Table 1 of Recommendation V.90). Of course, the particular subset is merely illustrative and persons of ordinary skill m the art will recognize that any representative subset of transmitted symbols (mcludmg all the transmitted symbols) may be employed. Typically, unpairment compensation sequences and constellations employed for data communication do not employ a complete set of symbols (e.g., all 256 signed Ucodes) based on power considerations. Note that the above sum-of-squares set difference calculation is merely illustrative, and any of a vanety of measures for establishing correspondence between phase-specific sets of estimates is suitable For any suitable difference measure, a threshold or similar technique may be employed to discriminate between phases to be attributed to the same or different characteristic groups Based on the above, it is possible to define various subsets of Σ , T = \y § ,y > ,•■•, y M _^ , with the A -1 properties that [ y ι = Σ and y, C\ y = Φ-, ι ≠ J > where φ denotes the empty set. That is, y m mcludes the ι=0 J set of amplitude estimates correspondmg to phase mtervals that can be associated with an m th charactenstic group, where m — 0, 1, ... M — 1 and 1 < M ≤ 24 . Define s m to be the number of elements of y m . In general, a value of M = 24 conespondmg to 24 charactenstic groups each with a smgle conespondmg phase interval ( s m = 1) may anse if each phase interval exhibits dissimilar impairment charactenstics (e g., 24 distmct RBS forms). More typically, a value such as M = 3 may anse may anse if the 24 phase mtervals can be associated with 3 dissimilar sets of impairment charactenstics (e.g , 3 distmct RBS forms, mcludmg no 2 RBS) wherem / £„ = 24 To facilitate groupmg of phase mtervals mto charactenstic groups, it is possible, =0 for each γ m , to choose a representative set of amplitude estimates, cr' m , such that r m e Σ , o~' m e γ and σ", i y if l ≠ m . In general, τ' m is any set of amplitude estimates that is representative of the m ώ characteristic group In some realizations, ( m is a composite (or average) of estimates from the vanous phase mtervals that have been associated with the m Λ charactenstic group. Based on the above definitions, the following pseudocode illustrates a sequence of operations that may be implemented m a communication device (e.g., m software or firmware) for defining a set of charactenstic groups and associating individual phase mtervals with the characteristic groups. Initial step' M = \ y 0 = iσ 0 } σ 'o = σ 0 s 0 = \ General step: for k = \ to 23 { found = false for m = 0 to M - \ { if { d( σ ' m ,σ k ) < θ) { y m = m ^ {σ k } s m = s m + l found = true break } } if ! found { M = M + 1 y M . = W } } where θ is any suitable threshold for discriminating between phase mtervals exhibiting similar and dissimilar impairment charactenstics (e.g., m one realization, d(o- t , <j j ) is small and cr, and C are to be grouped if d(o~ ι , 0~ j ) — θ ). Note that use of a distance measure d(cτ, , <jf) such as the above-descnbed sum-of- squares set difference calculation is merely illustrative and persons of ordmary skill m the art will appreciate a wide vanety of suitable discrimination functions. In some realizations, the threshold value, θ , employed may be predetermined (e.g., empirically), while in others, the threshold value may be computed based on observed sets of estimates. In one such realization, a threshold may be established based on observed distance measures between the sets of amplitude estimates. For example, one such realization: θ = a in{d{σ l , σ J )) where α is selected to baseline the threshold off of a minimum distance between the sets of amplitude "1 / estimates. In one realization = y~ . While the above illustrated pseudocode groups a given phase mterval with the first charactenstic group for which a distance measure is below a threshold, it has been observed that such an implementation may not group a particular phase mterval with those that best match its unpairment charactenstics. Accordingly, an unproved realization (below) illustrates a sequence of operations that may be implemented m a communication device (e.g , m software or firmware) for defining a set of charactenstic groups and associating individual phase mtervals with the characteristic groups that better conesponds to observed impairment charactenstics Initial step M =\ y 0 = W s 0 = l General step: for k = \ to 23 { MinDistance = d O ~ 0 ' , O ~ k ) Minlndex = 0 for m = l to M - \ { distance = d(σ m ' , σ k ) if (distance < MinDistance) { MinDistance = distance Minlndex = m } } if (MinDistance < θ) { Minlndex ~ ? Minlndex ^ \0~ k f S Minlndex S Minlndex + * ■ } else { M = M + l y M . x = cτV-ι = < k S M-> = 1 } } where each set of amplitude estimates, σ k , for a particular phase mterval is added to a charactenstic group to which it best conesponds, y MιnIndex , if the distance measure between that set of amplitude estimates and the characteristic group is less than a threshold Otherwise, a new group is defined. While a variety of suitable implementations will be appreciated by persons of ordinary skill in the art based on the above descriptions and pseudocode, the flow chart of FIG. 4 illustrates one suitable sequence of operations. Initially, amplitudes estimates for one of the 24 phase intervals are treated (402) as representatives of a first characteristic group. Typically, estimates for phase interval 1 are assigned to the first characteristic group, although the particular phase interval assigned is arbitrary. In general, phase interval 1 may be subject to RBS, padding or no periodic impairment at all. Next, amplitude estimates associated with another phase interval (typically, phase interval 2) are evaluated against then cunent characteristic groups. In the illustrative realization of FIG.4, evaluation includes calculating (406) a set difference measure (such as described above) between amplitude estimates for the cunent phase interval (e.g., phase interval 2) and representatives of each characteristic group (e.g., group 1 characterized by amplitude estimates of phase interval 1). Based on comparison with each of the characteristic groups, a best matching characteristic group is identified. If the set difference measure meets a match criterion (e.g., if the set difference measure is below a threshold value), then the cunent phase interval may be added (408) to the matching characteristic group. If not, a new characteristic group is defined (410) using amplitude estimates of the cunent phase interval as representative thereof. Focusing on an exemplary realization, assignment of a phase interval to a characteristic group may be based on use of a distance calculation that focuses on a representative subset of amplitude estimates. For example, in some realizations, estimates of symbols conesponding to Uchords four and five are employed. In one such realization, a set difference calculation is performed during a first pass through loop 404, comparing amplitude estimates σ 2 obtained in the second timing phase with a first group y characterized by amplitude estimates ϋ~[ obtained in the first timing phase, and can be represented as follows: d(σ 2 ,σ ) = . - A',/ Later passes through loop 404 may include comparisons with larger numbers of then-defined characteristic groups. The quality of amplitude estimates used to characterize individual groups can be improved by combining contributions from the various phase intervals that are assigned to the characteristic group. For example, in a characteristic group that includes two or more timing intervals, the set of amplitude estimates conesponding to the same codeword may be averaged or otherwise combined (e.g., using any suitable computational method) for each of the conesponding phase intervals. In an illustrative realization, an averaged value A' t m for the r* amplitude estimate characteristic of the /n* group may be calculated as follows: for i = —127 to 127 (each amplitude estimate) { for m = 0 to M -1 (each group) { ι,m for k = 0 to 23 ( each phase) { if ( τ, G m ) { } A ι .,m „ — A ι ,,m „ I -j m„ } } where σ' m = {A m ;— 127 < l ≤ 127} , 0 < m < M — 1 . Persons of ordmary skill m the art will appreciate that the above pseudocode is merely illustrative and that a particular implementation may employ similar techniques at any of a vanety of pomts m a computation. For example, while some realizations may average amplitude estimates associated with a charactenstic group after assignment of phase mtervals to charactenstic groups, others incrementally refine (see 409, FIG. 4) a representative set σ m ' of amplitude estimates for each charactenstic group usmg amplitude estimates from each assigned phase mterval. Assignment of Constellation Points Based on a groupmg of phase mtervals, such as the exemplary set difference measure based groupmg descnbed above, a communications device operating m accordance with some embodiments of the present mvention assigns constellation pomts based on the charactenstics of grouped phases. For example, m some realizations, constellation pomts are assigned for each of several distinct constellation mdices by selecting, for each such constellation mdex, successive candidate next constellation pomts that, based on symbol estimates for the one or more charactenstic sets conesponding to the constellation mdex, satisfy a distance metnc with respect to a last assigned constellation pomt for the constellation mdex. From such a set of candidates, successive lowest power candidates are assigned as next constellation pomts m respective constellations. As before, realizations of the present mvention may be better understood m the context of an illustrative embodiment m accordance with Recommendation V.90. Also as before, illustrative V.90 embodiments are not meant to be limiting To the contrary, based thereon, persons of ordmary skill m the art will appreciate a vanety of suitable realizations mcludmg those suitable for operation m the context of other standards, future standards and non-standard communication facilities or protocols. Similarly, the precedmg descnption identifies a particularly advantageous organization for an unpairment compensation sequence (sometimes encoded as a Digital Impairment Leammg (DIL) sequence m accordance with Recommendation V 90) and related techniques for groupmg phase mtervals based on conespondence of apparent aggregate impairments While groupmg of phase mtervals based on estimates of received symbols conesponding to a 24 phase DIL sequence with charactenstics such as descnbed above is preferable, other methods of groupmg may be employed as a precursor to the assignment of constellation pomts now descnbed For example, m one realization groupmg is based on a 6 phase view of received sequences such that there is a one-to-one conespondence between phase mtervals and constellation mdices These and other vanations will be appreciated by persons of ordmary skill m the art based the description that follows In view of the foregoing, an illustrative V 90-onented embodiment is now descnbed Recommendation V 90 provides for six sets of constellation pomts, one for each of six timmg mstants (or constellation mdices) from 0 to 5 As descnbed above, one particularly advantageous view of a DIL sequence is as a 24-phase symbol sequence, wherem a vanety of potential impairments are known to be penodic at least m 24 phase mtervals (if not 6 or 12 phase mtervals) In such an illustrative framework, a tf h constellation mdex conesponds to four phase mtervals k, k+6, k+12 and k+18 Accordmgly, m realizations that employ a 24-phase view of symbol sequences, group associations for each of the conespondmg four phase mtervals may be employed in the assignment of successive constellation pomts for a given constellation mdex Of course, in general, actual groupmgs are dependent on the particular impairment charactenstics of a communication channel Accordmgly, based on the particular impairments active m particular timing phases of a particular communications channel, the four phase mtervals conespondmg to a particular constellation mdex may be associated with 1, 2, 3 or 4 groups In a simplified realization, two general cases need to be considered In the first case, each of the phase mtervals conespondmg to a constellation mdex is associated with a same charactenstic group In mathematical terms, each of the phase mtervals is associated with set of amplitude estimates charactenstic of group y m , ι e , σ k e y m , σ k+6 e y m , σ k+n e y m , and σ k+ιs e y m for some m In this case, the set of symbol estimates & ' m = {A m ;—\21 ≤ l < \21} can be used for selection of constellation pomts as follows Let z, be the smallest positive value of i such that A m —A , _ l m ≥ d mm For r ≥ 2 , let ι r be the smallest p rositive value of i such that A' t. m„ —A' ι. r i m > d m mi„n and A J *' -ι r - \ m -A' -t m > d ^ πun Continue this process for r — 3,4,. . as long as the constellation power (e g , as defined m Clause 8 5 2 of Recommendation V 90) does not exceed the maximum average power ( EJ naχ ) or until all the pomts in the set cτ' m (I e , pomts conespondmg to 0 ≤ l ≤ 127 ) have been exhausted The resultmg set {l : ,l 2 ,...,l r } defines the constellation for constellation mdex k In the second case, the conespondmg phase mtervals are not all associated with the same charactenstic group Instead, the phase mtervals may be associated with two, three or four charactenstic groups For example, m the case of σ k γ^ , σ k+6 € y mι , σ k+l2 e y mj , and σ k+ i € m ^ where m ] , m 2 , m 3 , m are not all equal, the sets of symbol estimates o~ , o" m , & ' m , and σ" m are used as follows for selection of constellation pomts for constellation mdex k Let z, be the smallest positive value of I for which symbol estimates A', m , ~ A'-, m , - ^ mm rør everv ^ e {1 * 2,3,4} For r ≥ 2 , let ι r be the smallest positive value of i such that mm{(A' ι m -A m ),(A'_ t m —A'_ ι m )} ≥ d mn for every £ 6 {1,2,3,4} Continue this process for r = 3,4,... as long as the constellation power does not exceed R jjgx or until all the pomts m the set conespondmg to 0 < l ≤ 127 have been exhausted The resulting set {ι l ,ι 2 ,...,ι r } defines the constellation for constellation mdex k Note that the final constellation is typically subject to additional manipulation that is somewhat independent of the techniques described herein. In particular, legal regulations prohibit modems from transmitting at a power level that is greater than a specified maximum. This restriction sometimes leads to creation of a constellation for which addition of an additional point would exceed the maximum power, but such that using the derived set of points fails to use all of the power available. In that instance the distance between the points in the resulting set can be increased in order to improve performance. Therefore, in practice an additional iteration can be performed on the set of constellation points obtained by following the above procedure. This iteration can be applied in addition to the steps described above. Building on the above, some additional realizations are notable. For example, FIG. 5 illustrates a variation in which constellation points are assigned to constellations for each of 6 constellation indices in a manner that advantageously utilizes overall constellation power (i.e., power collectively employed by a statistical distribution of information transmitted in phase intervals conesponding to each of the constellation indices) to maximize information capacity. As before, an initial point is selected for each of 6 constellations. In particular, a minimal Ucode is selected (501) such that, for each conesponding group (e.g., in an exemplary embodiment, the 1, 2, 3 or 4 groups associated with the four phase intervals conesponding to a given constellation index), the difference between amplitude estimates for positive and negative receptions of the Ucode is at least d mm . Generally, the d πύn parameter relates to an estimated signal-to-noise ratio on the telephone line and determines the minimum spacing between constellation points consistent with a given level of performance. Additionally, legal regulations prohibit modems from transmitting at certain power levels. Recommendation V.90 defines this restriction, which is hereinafter refened to as maximum power level, P max . Note that in the Ucode set defined by Table 1 of Recommendation V.90, Ucodes are organized in order of increasing power. Accordingly, in an illustrative V.90 realization, a minimal Ucode is a lowest number Ucode that meets the d mm criterion. For each of the 6 constellations such a minimal Ucode is identified and added (502) as a first constellation point. Persons of ordinary skill in the art will appreciate suitable interpretations for other coding schemes. Successive constellation points are added to the 6 constellations as follows. For each of the K constellation indices, a minimal candidate Ucode is identified based on evaluation of a minimum spacing criterion in the context of amplitude estimates from the characteristic group (or groups) conesponding thereto. In an exemplary realization, the minimum spacing criterion is based on the difference between a group- specific amplitude estimate and the Ucode last added to a cunent constellation exceeding d mn . Then, a minimal one of the 6 minimal candidates is selected for addition to its conesponding constellation. For example, assume that based on an evaluation (503) of the d mm criterion in the context of two characteristic groups γ and y 2 of amplitude estimates associated with phase intervals 4, 10, 16, and 22 conesponding to a 4 th constellation index, Ucode 22 is the minimal Ucode for which estimates from both groups exceed the Ucode last added to constellation 4 by at least d mm . Further assume that the similarly calculated (503) minimal candidates for the 0 th , 1 st , 2 nd , 3 rd and 5* constellation indices are Ucode 23 , Ucode 2 , Ucode 24 , Ucode 23 and Ucode 2 , respectively based on the aggregate impairments observed in each of the phase intervals conesponding thereto (i.e., based comparison of the amplitude estimates for the conesponding characteristic groups with respective last added constellation points). Based on the foregoing, Ucode 22 is selected (505) as the minimal one of the candidate Ucodes and is added (504) to the conesponding constellation (i.e., constellation 4). A next candidate is calculated (503) for the 4 th constellation index and a minimal one of the then current candidates is again selected for addition to its conesponding constellation. For example, assuming that the next candidate for the 4 th constellation index is Ucode 24 , a minimal candidate (Ucode 23 ) is added to constellation 0. The process continues until a selected candidate would, if added, cause overall constellation power to exceed a maximum level, e.g., maJ . While the above description has assumed that all 6 constellation indices are distinct, this need not be the case. Indeed, performance advantages may be achieved if constellation assignment calculations are not duplicated for indistinct indices. For example, based on the description herein, persons of ordinary skill in the art will appreciate that if the set of characteristic groups associated with conesponding phase intervals of two or more constellation indices are the same (i.e., the two or more constellation indices are indistinct), then selections performed for one constellation index may be used in each of the others. Resulting constellations may be transmitted to a remote communications device, e.g., to a remove V.90 server modem, using any suitable mechanism. For example, after an analog client modem operating in accordance with an embodiment of the present invention receives a DIL sequence, groups phases and assigns constellation points as described above, it may transmit an encoding of the resulting constellations back to a digital server modem. In one such realization, at least the assignment of constellation points is performed during the Phase — Final Training period defined by Recommendation V.90. Constellation parameter transfer protocols and mechanisms such as that defined by Recommendation V.90 are well known and realizations in accordance with the present invention may use any transfer mechanism suitable for the communications devices and/or communications channels employed. Exemplary Communication Svstems and Device Embodiments A typical communications system in accordance with the present invention includes a receiver (typically including an equalizer) to receive an impairment compensation sequence such as described above. Typically, such a receiver is provided as part of a communications device such as an analog V.90 modem, although other realizations will be appreciated by persons of ordinary skill in the art based on the description herein. In some realizations, characteristic groups are determined and impairment sensitive selection of constellation points is performed based on the received impairment compensation sequence. In general, suitable impairment compensation sequences are selected in accordance with the particular signal processing structures employed by the receiver. For example, some of the description above emphasized impairment compensation sequences suitable for a receiver employing a partial-response IV equalizer (PRIV) or partial- response V equalizer (PRV). While not limited thereto, suitable realizations may be understood in the context of particular communications system and device configurations (including modems conforming to Recommendation V.90) employing a partial response class V (PRV) equalizer structure such as described in greater detail in co-pending U.S. Patent Application Number 09/249,990, filed on February 13, 1999, entitled "Efficient Reduced State Maximum Likelihood Sequence Estimator, " the entirety of which is incorporated by reference herein. Referring illustratively to FIG. 6, an exemplary V.90 modem implementation employs a reduced state maximum likelihood sequence estimator and a partial response class V equalizer. A brief description of the illustrated modem will provide a context for better understanding some device embodiments in accordance with the present invention. A detailed discussion of the operation of the exemplary modem is omitted since modem operation in conformance with Recommendation V.90 is known the art. More detailed information may be found in the above-incorporated U.S. Patent Application and in Recommendation V.90. In an illustrative embodiment, partial response equalizer 919 is implemented in software that may be stored on computer readable media that can be executed on a suitable processor. Those of skill in the art will appreciate that the teachings herein may be applied to a variety of modem and communication device implementations including those employing a programmable digital signal processor and/or specialized logic. The illustrated modem includes both transmitter 601 and receiver 603 portions. In general, DIL descriptor information is supplied using transmitter portion 601, while a conesponding DIL sequence is received using receiver portion 603. Constellations selected as described herein are communicated using transmitter portion 601. Transmitter data module 605 supplies a digital bit stream to encoder 607. Transmitter data module 605 may generate such digital data locally in the case of startup or training (including DIL descriptor supply) or may fetch or receive data from an external data source or application program (not shown) in data mode. Transmitter data module 605 passes the data to the encoder 607, which in turn converts the input bit stream into a sequence of symbols that are then provided to modulator 609. Modulator 609 performs appropriate sampling and filtering operations and provides its output to shaping and preemphasis filter 611, which in turn provides square-root-of raised cosine shaping as well as preemphasis filtering as specified by Recommendation V.90. Note that during certain phases of modem operation, including supply of a DIL descriptor, spectral shaping is disabled. The resultant modulated, encoded signal is supplied to a communications channel (not shown) by digital-to-analog converter 613. Receiver portion 603 of the illustrated modem receives the output from the transmitter (prior to the D/A) into receiver front end 615. Receiver front end also receives a received signal 616 from the communications channel. Receiver front end 615 performs operations such as analog-to-digital conversion on the received signal, various filtering and echo-canceling operations and provides its output to demodulator 617. Demodulator 617 includes partial response class V equalizer 619. Accordingly, DIL sequences organized as described above for PRV equalizers are suitable for supply to the illustrated modem and characteristic group determinations and impairment sensitive selection of constellation points based on the equalized DIL sequences may be employed. In the illustration of FIG. 6, demodulator 617 also includes a second stage near-end echo canceller 620, which cancels the echo at the output of the equalizer and is intended to track slow variations of the echo signal. The output of the partial response equalizer is provided to decoder 621, and more specifically to reduced state maximum likelihood sequence estimator 623 thereof. In data mode, symbols decoded by maximum likelihood sequence estimator 623 are split into sign and amplitude components and passed respectively to the spectral de-shaper 625 and modulus decoder 627. These modules extract the data payload from the symbols that is later converted to a bit stream by data assembler 629. Receive data module 631 descrambles the bit stream and, in general, passes it to an application program (not shown). For DIL operations described herein, amplitude estimates are, in general, processed at the equalizer output although other points in the signal flow of FIG. 6 may also be suitable. In particular, based on such amplitude estimates, the illustrated modem assigns each phase of the received sequence to a characteristic group as described in greater detail above. Similarly, constellation points are selected for each characteristic group of phases as described in greater detail above. Constellation point selections are transmitted to a remote modem (not shown) using transmitter portion 601 in accordance with the "Phase 4 - Final training" protocol of Recommendation V.90. In general, techniques described herein support improved impairment compensation sequences, methods and signal processing configurations. In some realizations, improved digital impairment learning in accordance with Recommendation V.90 is provided. Based on the description herein, persons of ordinary skill in the art will appreciate a variety of applications of the underlying techniques and the breadth of the invention. Without limitation thereto, a V.90 modem configuration provides a useful example. Some of the exemplary embodiments have been described in a largely software context to provide clarity to the invention. While such software implementations of the invention are certainly contemplated, the teachings disclosed herein may also be used by persons of ordinary skill in the art to implement the present invention in a hardware or mixed hardware/software context without departing from the scope of the invention.

Claims: WHAT IS CLAIMED: 1. An impairment compensation sequence for use m a communications system, the communications system susceptible to one or more potential impairments each periodic m an mteger number of symbols transmitted across a communications channel, the impairment compensation sequence compnsmg: N phases, wherein N is selected such that each potential impairment, if present, is penodic therein; and a sequence of symbols, the sequence organized to place at least one mstance of each symbol from a predetermined set of symbols m each phase to allow detection of the potential impairments m each of the N phases. 2. The unpairment compensation sequence of claim 1 wherem the potential impairments mclude at least one of robbed-bit signaling, paddmg and a combination of robbed-bit signaling and paddmg. 3. The impairment compensation sequence of claim 1 wherein the sequence mcludes a number of segments, the number conespondmg to a number of elements m the predetermined set of symbols. 4. The unpairment compensation sequence of claim 1 wherem the predetermmed set of symbols are chosen from at least a subset of a universal pulse code modulation (PCM) codeword set. 5. The impairment compensation sequence of claim 4 wherem the subset is selected m accordance with power constraints. 6 The impairment compensation sequence of claim 1 wherem the N phases mclude 24 time phases. 7. The impairment compensation sequence of claim 1 wherem N is a least common multiple of respective periods of each of the potential impairments. 8. The impairment compensation sequence of claim 1 wherem the sequence is organized to place at least two mstances of the symbol from the predetermmed set of symbols m each phase, an average of received values conespondmg to the at least two improving an estimation of the symbol. 9. The impairment compensation sequence of claim 1 wherem the sequence mcludes a plurality of segments each conespondmg to respective symbols of the predetermined set thereof, each segment havmg a portion therein repeated at least once, each segment providmg at least two mstances of one of a positive reference symbol, a negative reference symbol, a positive trammg symbol, and a negative trammg symbol. 10. A method of establishing communication across a channel, the communication susceptible to one or more potential impairments each penodic m an mteger number of symbols, the method compnsmg receiving a sequence of symbols, the sequence organized to place at least one mstance of each symbol from a predetermmed set of symbols in each of N phases, wherein N is selected such that each potential impairment, if present, is penodic therem, and detecting potential impairments, if present, conespondmg to each of the N phases 11 The method of claim 10, wherem the channel mcludes a digital portion of a Public Switched Telecommunications Network (PSTN) and wherem the potential impairments mclude at least one of robbed- bit signalmg (RBS), paddmg, and a combmation of robbed-bit signalmg and paddmg m the digital portion of the PSTN 12 The method recited m claim 10 wherem the sequence of symbols is a digital impairment learning (DIL) sequence 13 The method recited m claim 10 wherem N is a least common multiple of respective peπods of each of the potential impairments 14 The method recited m claim 10 wherem the sequence of symbols is organized to place at least two mstances of the symbol from the predetermmed set of symbols m each phase, an average of received values conespondmg to the at least two mstances improving an estimation of the symbol 15 The method recited m claim 10 wherem the sequence of symbols mcludes a number of segments, the number conespondmg to a number of elements m the predetermmed set of symbols 16 The method recited m claim 10 wherem the predetermmed set of symbols are chosen from at least a subset of a universal pulse code modulation (PCM) codeword set 17 The method recited m claim 16 wherem the subset is selected m accordance with power constraints 18 A computer program product compnsmg instructions executable on at least one processor to at least partially implement a communications device, and said instructions mcludmg a communication subset thereof executable to adapt the communications device for communication across a channel susceptible to one or more potential impairments each penodic in an mteger number of symbols transmitted across the channel, the communications subset receivmg an impairment compensation sequence, that includes N phases, wherein N is selected such that each potential impairment, if present, is penodic therem, and a sequence of amplitudes transmitted from terminal equipment, the sequence organized to place at least one mstance of each symbol from a predetermmed set of symbols m each phase to allow detection of the potential unpairments m each of the N phases 19 A computer program product as m claim 18 wherem the instructions are encoded by or transmitted in at least one computer readable medium selected from the set of a disk, tape or other magnetic, optical, or electronic storage medium and a network, wirelme, wueless or other commumcations medium 20 A method of operatmg a commumcations device to compensate for one or more potential unpairments m a communication channel, each periodic m N timmg phases thereof, the method compnsmg receivmg a sequence via the communication channel and estimating symbols thereof, the sequence organized such that conespondmg mstances of one or more symbols of a predetermmed set appear m each of the N phases, groupmg the N phases mto one or more groups thereof based on conespondence of respective estimates of the conespondmg symbol mstances, and for each group, compensating for an aggregate effect of the impairments on the grouped phases without identification of individual impairments active m the particular phases 21 A method, as recited m claim 20, wherem the received sequence mcludes an unpairment compensation sequence transmitted by a remote communications device 22 A method, as recited m claim 20, wherem the received sequence mcludes a Digital Impairment Learning (DIL) sequence m accordance with ITU-T Recommendation V 90 23 A method, as recited m claim 20, wherem the communications channel mcludes at least one portion susceptible to robbed bit signaling (RBS) 24 A method, as recited m claim 20, wherem the communications channel traverses a portion of a public switched telecommunications network (PSTN) susceptible to the one or more potential unpairments each penodic m the N phases 25 A method, as recited m claim 20, wherem N = 24 26 A method, as recited m claim 20, wherem the groupmg based on conespondence mcludes companng amplitude estimates for each of the phases with conespondmg estimates characteristic of the groups 27 A method, as recited m claim 20, wherem the groupmg based on conespondence mcludes computmg distance measures based on the respective estimates, and selecting for inclusion m a particular group, those phases for which the computed distance measures are less than a threshold 28 A method, as recited m claim 27, wherem the distance measures are based on companson of respective estimates for plural symbols of the predetermmed set thereof 29 A method, as recited m claim 28, wherem the plural symbols conespond to one or more Uchords 30 A method, as recited m claim 20, wherem the compensating mcludes trammg an adaptive structure for use m the grouped phases 31 A method, as recited m claim 20, wherem the compensating mcludes defining a constellation for use m the grouped phases 32 A method, as recited m claim 20, wherem at least some of the potential unpairments are not penodic in 6 timing phases 33 In a communications system susceptible to one or more potential impairments, a method of categonzmg N phases of a received sequence of amplitude estimates accordmg to impairments present therem, the method compnsmg organizing the received amplitude estimates mto the N phases, wherem N is selected such that each potential impairment, if present, is periodic therem, and for each of the N phases, comparmg one or more sets of amplitude estimates thereof to respective one or more sets of the amplitude estimates of other phases of the N phases to categorize the N phases mto one or more groups accordmg to similanty of the impairments, if any, present therem 34 The method recited m claim 33 wherem the comparmg mcludes calculatmg a difference measure between the set of amplitude estimates and a set of conespondmg amplitude estimates m another phase, and comparmg the difference measure with a threshold 35. The method recited in claim 34 wherein for a particular phase, the calculating and comparing includes: determining a set of difference measures, each difference measure being a measure between the particular phase and another phase, the another phase representing an identified characteristic group of phases grouped according to similarity of impairments, if any; finding a minimal difference measure from the set of difference measures; if the minimal difference measure is less than the threshold, including the particular phase in the identified characteristic group; and otherwise, defining the particular phase as a member of a new characteristic group. 36. The method recited in claim 34, wherein the difference measure is a determination between the set of amplitude estimates and the conesponding set of amplitude estimates, a result being squared and summed together. 37. The method recited in claim 34, wherein the set of amplitude estimates and the conesponding set of amplitude estimates each have two or more estimates of a same codeword within the respective phase, the two or more estimates of the codeword being averaged to increase accuracy. 38. The method recited in claim 35, further comprising: identifying at least one phase of the N phases as representing a characteristic group. 39. The method recited in claim 33, wherein the potential impairments include at least one form of robbed-bit signaling (RBS). 40. The method recited in claim 33, wherein the potential impairments include digital attenuation padding. 41. The method recited in claim 33, wherein N is 24. 42. The method recited in claim 33, wherein the impairments to which the communications channel is subject include robbed-bit signaling and padding impairments. 43. The method recited in claim 34, wherein the threshold is predetermined. 44. The method recited in claim 34, wherein the threshold is a function of distance measures between respective subsets of the amplitude estimates. 45 The method recited m claim 33, further compnsmg choosmg the set of amplitude estimates for companson accordmg to power constraints 46 The method recited in claim 35, further compnsmg for each identified charactenstic group, averagmg amplitude estimates, wherem groups of the amplitude estimates identify codewords, the averagmg mcludmg identifying a plurality of conespondmg codewords from each phase m the charactenstic group, and averagmg the conespondmg estimates for each codeword 47 The method recited in claim 46, wherem the conespondmg estimates of codewords mclude positive and negative estimates of codewords withm a phase m the charactenstic group 48 The method recited m claim 46, wherem the codewords are PCM codewords 49 The method recited m claim 34, wherem for a particular phase, the comparmg mcludes comparmg a measure between a set of amplitude estimates and another set of amplitude estimates of an identified charactenstic group with a threshold, if the difference measure is less than the threshold, mcludmg the particular phase m the identified charactenstic group, otherwise repeating the comparmg with a next set of amplitude estimates of a next characteristic group, and if no further charactenstic groups are defined, defining the particular phase as a member of a new charactenstic group 50 A computer program product compnsmg mstructions executable on at least one processor to at least partially implement a communications device, and said instructions mcludmg a communication subset thereof executable to adapt the communications device for communication across a channel susceptible to one or more potential unpairments each peπodic m an mteger number of symbols transmitted across the channel, the mstructions executable to cause the communications device to receive a sequence of amplitude estimates, organize the received amplitude estimates mto the N phases, wherein N is selected such that each potential impairment, if present, is periodic therem, and for each of the N phases, compare a set of amplitude estimates thereof to a respective one or more sets of the amplitude estimates of other phases of the N phases to categorize the N phases mto one or more groups accordmg to similarity of impairments present therem 51 A computer program product as m claim 18 wherem the mstructions are encoded by or transmitted m at least one computer readable medium selected from the set of a disk, tape or other magnetic, optical, or electronic storage medium and a network, wirelme, wireless or other communications medium 52 An apparatus compnsmg a receive path coupled to receive sequences susceptible to one or more impairments penodic m N phase mtervals, and means for groupmg the phase mtervals accordmg to apparent aggregate effect of the unpairments thereon without identification of individual impairments active m the particular phases 53 A method of selecting pomts for inclusion m a multipoint constellation, the method compnsmg groupmg phase mtervals based on similaπty of aggregate impairment exhibited therem and calculating a charactenstic set of symbol estimates for each such group, and assignmg constellation pomts for a constellation mdex based on one or more charactenstic sets conespondmg thereto, wherem the one or more charactenstic sets mclude contnbutions of symbol estimates from phase mtervals associated with one or more other constellation mdices 54 A method, as recited m claim 53, further compnsmg performing the assignmg collectively for each of J distmct constellation indices, wherein the assignmg mcludes selecting, for each such constellation mdex, successive candidate next constellation pomts that, based on symbol estimates for the one or more charactenstic sets conespondmg to the constellation mdex, satisfy a distance metric with respect to a last assigned constellation pomt for the constellation index, and assignmg successive lowest power ones of the candidate next constellation pomts to respective constellations 55 A method, as recited m claim 53, wherein each phase interval conespondmg to the constellation index is a member of a same group, and wherem the assignmg mcludes selecting a next constellation pomt such that a distance metnc between a group-characteπstic estimate thereof and a last selected constellation point for the constellation mdex satisfies a mmimum distance cntenon 56 A method, as recited m claim 53, wherem at least two phase mtervals conesponding to the constellation index are members of different groups, and wherein the assigning includes selecting a next constellation point such that a distance metric between group-characteristic estimates thereof and a last selected constellation point for the constellation index satisfies a minimum distance criterion for the at least two phase intervals. 57. A method, as recited in claim 53, wherein the constellation index is a k* one of six constellation indices; and wherein a k" 1 one of the phase intervals, a (k+6) Λ one of the phase intervals, a (k+12)* one of the phase intervals and a (k+lδ)* one of the phase intervals each conespond to the constellation index. 58. A method, as recited in claim 53, wherein multiple phase intervals conespond to each constellation index. 59. A method, as recited in claim 53, wherein a single phase interval conesponds to each constellation index. 60. A method, as recited in claim 53, wherein the phase intervals number twenty-four; and wherein the constellation index is one of six constellation indices. 61. A method, as recited in claim 53, further comprising: performing the grouping based on a received impairment compensation sequence that places at least one instance of each symbol from a predetermined set of symbols in each of the phase intervals. 62. A method, as recited in claim 53, further comprising: communicating the constellation points to a remote communications device. 63. A method, as recited in claim 53, wherein the symbol estimates include amplitude estimates of Ucodes. 64. A method, as recited in claim 53, wherein the distance metric includes a set difference measure based on respective amplitude estimates conesponding to representation Uchords. 65. In a communications system for communicating via a channel susceptible to one or more potential impairments each periodic in an integer N samples of a received signal, a method for mapping constellation points to one or more constellations, the method comprising: receiving a sequence of symbol estimates organized into N phases, a respective one or more of the phases conesponding to each of J constellation indices; grouping the N phases into a set of characteristic groups according to aggregate effects of the periodic impairments, if any, present in the N phases and without a priori identification of individual forms of the periodic impairments present therein; and for each of the J constellation indices, selecting constellation points based on the characteristic groups associated with the one or more respective phases. 66. A method, as recited in claim 65, wherein the selecting includes, for each of the constellation indices, selecting successive candidate next constellation points that, based on the characteristic groups associated with the one or more respective phases, satisfy a distance metric with respect to a last assigned constellation point for the constellation index; and assigning successive lowest power ones of the candidate next constellation points to respective constellations. 67. A method, as recited in claim 65, wherein the constellation point selecting for a particular constellation includes: calculating, for each associated characteristic group, a distance metric between a constellation point last added to the particular constellation and an estimate of a next lowest power constellation point; and selecting for addition to the particular constellation, one of the next lowest power constellation points for which the calculated distance metric exceeds a minimum distance metric for each associated characteristic group. 68. A method, as recited in claim 65, wherein the constellation point selecting for a particular constellation includes: for each associated one of the characteristic groups, finding a next lowest power constellation point for which a distance metric between an estimate of the next lowest power constellation point and a constellation point last added to the particular constellation exceeds a minimum distance metric; and adding to the particular constellation, a highest power one of the next lowest power constellation points. 69. A method, as recited in claim 65, wherein eeaacchh pphhaassee c conesponding a/* constellation index is associated with a single characteristic group; and wherein the constellation point selecting for a particular constellation conesponding to the/'' constellation index includes selecting for addition to the particular constellation, a next lowest power constellation point for which a distance metric between a constellation point last added to the particular constellation and an estimate of the next lowest power constellation point exceeds a minimum distance metric. 70 A method, as recited m claim 65, wherem N is 24 71 A method, as recited m claim 65, wherem J is 6 72 A communication device for communicating, usmg one or more constellations, via a channel susceptible to one or more potential unpairments each peπodic m an mteger N samples of a received signal, the communications device compnsmg a receive path for receivmg from the channel a sequence of symbols organized mto N phases mtervals, wherem respective ones of the phase mtervals conespond to constellation mdices, and an unpairment compensator coupled mto the receive path dunng a trammg mode to receive the sequence and group the N phases thereof mto a set of charactenstic groups accordmg to aggregate effects of the penodic impairments, if any, the impairment compensator selecting, for each of J constellation mdices, constellation pomts based on the charactenstic groups associated with the respective phase mtervals conespondmg thereto 73 The communication device of claim 72 wherem the charactenstic groups associated with respective phase mtervals conespondmg to a particular constellation mdex mclude contnbutions of symbol receptions in grouped phase mtervals conespondmg to other constellation mdices, such that selection of constellation pomts for the particular constellation mdex is improved by symbol receptions from substantially all phase mtervals exhibiting similar aggregate effects of the peπodic impairments 74 The communication device of claim 72, further compnsmg a transmit path, wherem the impairment compensator is coupled to the transmit path to supply an encodmg of the selected constellation pomts to a remote communications device 75 The communication device of claim 72, for a particular received sequence, respective phase mtervals conespondmg to two or more indistinct ones of the constellation mdices are identically grouped, and distmct ones of the constellation mdices number J 76 The communication device of claim 72, wherem N is 24 and J is 6 77. An apparatus comprising: means for organizing a received sequence of symbol estimates into N phases; means for grouping the N phases into a set of characteristic groups according to conespondence of aggregate effects of periodic impairments, if any, present in the N phases; and means for selecting constellation points using symbol estimates characteristic of the grouped phases. 78. The apparatus of claim 77 wherein the means for selecting constellation points using symbol estimates characteristic of the grouped phases includes: means for selecting candidate next constellation points for each of J distinct constellation indices; and means for assigning successive lowest power ones of the candidate next constellation points to respective constellations. 79. A computer program product comprising: instructions executable on at least one processor to at least partially implement a communications device; and said instructions including an impairment compensation subset thereof executable to group N phases of a symbol sequence received by the communications device into a set of characteristic groups according to conespondence of aggregate effects of periodic impairments, if any, present in the N phases, the impairment compensation subset of instructions selecting constellation points using symbol estimates characteristic of the grouped phases. 80. A computer program product as in claim 79 wherein the instructions are encoded by or transmitted in at least one computer readable medium selected from the set of a disk, tape or other magnetic, optical, or electronic storage medium and a network, wireline, wireless or other communications medium.

Inventors: LIANG HAIXIANG, LIANG, HAIXIANG
Applicants: ALTOCOM INC, ALTOCOM, INC.
Application year: 2000
Main classification: H04L25
IPC codes: H04L25