Abstract:
The measurement of far-end crosstalk (FEXT) in a Digital Subscriber Line communications is instrumental in the ability of using a multiple input multiple output (MIMO) pre-coder to cancel FEXT. A reliable robust back channel for transmission of error is instrumental to provide error samples for the proper operation of a MIMO pre-coder. Bins can be dedicated to insure bandwidth from the customer premises equipment (CPE) to the central office (CO). By increasing the margin used in the bins, robustness can be added to this back channel between the CPE and CO.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Under 35 U.S.C. 119, this application claims priority to, and the benefit of, U.S. Provisional Patent Application entitled, “Definition of a Signaling Channel for Vectoring of DSL Systems,” having Ser. No. 61/039,714, filed on Mar. 26, 2008, which is incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the Digital Subscriber Line (DSL) Systems and specifically to back channel signaling in the DSL system. 
     2. Related Art 
     High-bandwidth systems, including DSL systems, use baseband modulation, single-carrier modulation as well as multi-carrier modulation schemes. Both DSL and other high-bandwidth systems such as wireless use modulation schemes such as Quadrature Amplitude Modulation (QAM), Carrier-less Amplitude and Phase Modulation (CAP), and Discrete Multi-tone (DMT) for wired media and Orthogonal Frequency Division Multiplexing (OFDM) for wireless communication. One advantage of such schemes is that they are suited for high-bandwidth applications of 2 Mbps or higher upstream (subscriber to provider) and 8 Mbps or higher downstream (provider to subscriber). Quadrature Phase Shift Keying (QPSK) utilizes quadrature carrier phase shift keying to encode 2 bits of information on a carrier frequency by employing waves in the same carrier frequency shifted by increments of 90°, which can be thought of as sine and cosine waves of the same frequency. Since the sine and cosine waves are orthogonal, data can be encoded in the amplitudes of the sine and cosine waves. Therefore, 2 bits can be sent over a single frequency using the quadrature keying. 
     CAP is similar to QAM. For transmission in each direction, CAP systems use two carriers of identical frequency above the 4 kHz voice band, one shifted 90° relative to the other. CAP also uses a constellation to encode bits at the transmitter and to decode bits at the receiver. A constellation encoder maps a bit pattern of a known length to a sinusoid wave of a specified magnitude and phase. Conceptually, a sinusoidal wave can be viewed to be in one-to-one correspondence with a complex number where the phase of the sinusoidal is the argument (angle) of the complex number, and the magnitude of the sinusoidal wave is the amplitude or modulus of the complex number, which in turn can be represented as a point on a real-imaginary plane. Points on the real-imaginary plane can have bit patterns associated with them, and this is referred to as a constellation and is known to one of ordinary skill in the art. 
     DMT modulation, sometimes called OFDM, builds on some of the ideas of QAM but, unlike QAM, it uses more than one constellation encoder where each encoder receives a set of bits that are encoded and outputs sinusoid waves of varying magnitudes and phases. However, different frequencies are used for each constellation encoder. The outputs from these different encoders are summed together and sent over a single channel for each direction of transmission. For example, common Asymmetric Digital Subscriber Line (ADSL) DMT systems divide the spectrum from 0 kHz to 1104 kHz into 256 narrow channels called bins (sometimes referred to as tones, DMT tones, or sub-carriers). These bins are 4.3125 kHz wide. The waveforms in each bin are completely separable from one another, i.e., orthogonal to each other. In order to maintain “orthogonality,” the frequencies of the sinusoids used in each bin should be multiples of a common frequency known as the fundamental frequency and in addition the symbol period τ, must be a multiple of the period of the fundamental frequency or a multiple thereof. In a DMT system, the number of bins is 256 for ADSL/ADSL2, 512 for ADSL2+ and 4096 for Very High Speed Digital Subscriber Line 2 (VDSL2). Each bin can carry a certain number of bits; this number can vary due to factors such as attenuation on the line, noise and crosstalk in the cable, and the transmit signal power spectral density (PSD). The aggregate bit pattern which comprises the bit patterns mapped to constellations in each of the bins during a symbol period is often referred to as a DMT symbol. For the purposes here, time domain references are often referred to in terms of DMT symbol periods also referred to as symbol periods. 
       FIG. 1  illustrates DSL communications layering in communications between central office (CO)  172  and customer premises equipment (CPE)  174 . The communications has a downstream component from CO  172  to CPE  174  and an upstream from CPE  174  to CO  172  component. 
     Within CO  172  is layer  142 , the Transport Protocol Specific-Transmission Convergence (TPS-TC) layer. At this layer support for application specific transports such as ATM or Ethernet are implemented. The function of the TPS-TC is to convert the payload data with appropriate framing into a bit rate for transport to the Physical Media Specific-Transmission Convergence (PMS-TC) layer. The gamma interface delineates the TPS-TC from the layer 2 applications above. 
     The next layer is layer  144 , the PMS-TC layer. This layer manages framing, transmission, and error control over the line. In particular, this layer comprises the forward error correction (FEC) codes such as the Reed-Solomon (RS) Codes and interleaving. The interface between the TPS-TC and the PMS-TC layer is referred to as the alpha interface in the CO unit  172  and the beta interface in the CPE unit  174 . It is very common that the transmit portion of the PMS-TC layer  144  comprises scrambler  102 , RS encoder  104  and interleaver  106  to implement a RS code with interleaving (RS-ILV). DSL standards mandate the use of RS as the FEC. 
     The next layer is layer  146 , the physical media dependent (PMD) layer. In the transmit portion, the PMD layer encodes, modulates and transmits data across physical links on the network. It also defines the network&#39;s physical signaling characteristics. In particular, constellation encoding and trellis codes map data into DMT symbols which are converted into time domain signals through the use of an IFFT where the time domain signal can be transmitted across subscriber line  160 . The trellis code can also supply additional error correction. The interface between the PMD layer and the PMS-TC layer is referred to as the delta interface. 
     After processing by the PMD layer, the data is transmitted across subscriber line  160 , where it is received by CPE  174  using its PMD layer  156 . In a receiving capacity PMD layer  156  decodes, demodulates and recovers the data received across physical links on the network. Furthermore, PMS-TC layer  154  in a receiving capacity decodes data encoded by PMS-TC  144  and extracts data from the framing scheme. In particular it can comprise de-interleaver  108 , RS decoder  110 , and descrambler  112  to extract data encoded by PMS-TC  144 . Finally, TPS-TC layer  152  is the transport protocol specific transmission conversion layer, configured for interfacing to specific transport protocols such as ATM or Ethernet. 
     In the upstream direction, CPE  174  transmits to CO  172 . In the transmission, TPS-TC layer  152  functions in a similar fashion for CPE  174  as described for TPS-TC  142  in CO  172 . Similarly PMS-TC  154  functions in a similar fashion as described for PMS-TC  144  and may additionally comprise scrambler  122  analogous to scrambler  102  in the downstream portion of TPC-TC  142 , RS encoder  124  analogous to RS encoder  104  in the downstream portion of TPC-TC  142 , and interleaver  126  analogous to interleaver  126  in the downstream portion of TPC-TC  142 . PMD layer  156  functions in a similar fashion as described for PMD layer  146  in the downstream portion. The encoded and modulated signal is then transmitted from CPE  174  to CO  172  through subscriber line  160 . 
     CO  172  in PMD layer  146 , upon receiving the upstream encoded and modulated signal, performs the analogous decoding, demodulating and receiving of data across DSL to the downstream portion of PMD layer  156 . The upstream portion of PMS-TC  144  functions similarly to the downstream portion of PMS-TC  156  and can comprise de-interleaver  128 , RS decoder  130  and descrambler  132  analogous to de-interleaver  108 , RS decoder  110 , and descrambler  134 , respectively. Finally, TPS-TC  142  functions in the upstream portion function similarly to TPS-TC  152  in the downstream portion. 
     More specifically,  FIG. 2  illustrates a more detailed description of the transmission side of the PMS-TC layer (i.e., the downstream portion of PMS-TC  144  or the upstream portion of PMS-TC  154 ) as disclosed by present xDSL standards. Data from the TPS-TC layer transmitted to the PMS-TC layer can use one of two latency paths and one of two bearer channels. Input  202  represents data on the first bearer channel designated for latency path # 0 . Input  204  represents data on the second and optional bearer channel designated for latency path # 0 . Input  206  represents data on the first bearer channel optionally designated for latency path # 1  when not designated in latency path # 0 . Input  208  represents data on the second bearer channel optionally designated for latency path # 1 . In addition, overhead data can be received by the PMS-TC layer including Embedded Operations Channel (EOC)  210 , Indicator Bits (IB) Channel  212  and Network Timing Reference (NTR)  214 , which can be combined by overhead multiplexer (MUX)  218 . In addition, MUX  216  combines inputs  202  and  204 , and MUX  220  as part of optional latency path # 1  combines input  206  and  208 . MUX  222  combines the output of MUX  216  with a sync byte and overhead data from MUX  218 . Similarly MUX  224  combines the output of MUX  220  with an overhead sync byte and overhead data from MUX  218 . Along each latency path, scrambler  226  and corresponding scrambler  228  scramble the data received from MUX  222  and MUX  224 , respectively. FEC  230  and FEC  232  apply an FEC to the scrambled data from scramblers  226  and  228 , respectively. Typically, the FEC used is a RS code and in the example of  FIG. 1 , FEC  230  and/or  232  can be RS encoder  104 . Interleaver  106  of  FIG. 1  comprises interleaver  234  and interleaver  236  which performs the interleaving of the encoded data received from FEC  230  and FEC  232 , respectively. It should be noted that the FEC and interleaver parameters can generally be configured differently for each latency path. Finally, MUX  238  combines the encoded interleaved data for both latency paths to produce output  240  which is ready for processing by the PMD layer. 
       FIG. 3A  illustrates a more detailed description of the transmission side of the PMD layer. Data is received as serial data by the PMD layer from the PMS-TC layer. This serial data is converted from a serial bit stream to parallel form by serial-to-parallel converter  302  for mapping into M parallel sub-channels, each sub-channel representing a specific sub-carrier. Each parallel sub-channel represents one of the bins or sub-carriers used, so for ADSL, serial-to-parallel converter  302  would produce 256 parallel sub-channels and for VDSL, serial-to-parallel converter  302  would produce 4096 parallel sub-channels. The various parallel bit sequences are passed to symbol mapper  304 . Symbol mapper  304  maps each bit sequence into a constellation point within each sub-carrier (i.e., a DMT sub-symbol). For example, if the bit sequence relative to a specific sub-carrier comprises 3 bits, depending on the value of the three bits it is mapped to one of the eight points indicated by constellation  554  in  FIG. 5 . If the bit sequence comprises 5 bits, the bit sequence is mapped to one of the 32 points indicated by constellation  558 . Furthermore, to increase robustness, the bit sequence can be transformed by a trellis encoder prior to the mapping onto a constellation point. The collection of the DMT sub-symbols mapped by symbol mapper  304  comprises the DMT symbol. Each subsystem is a complex number represented by a constellation point where the corresponding constellation represents points on the complex plane. As such inverse fast Fourier transform (IFFT)  306  becomes a modulator taking the collection of DMT sub-symbols as complex-valued numbers and provides M complex output samples which are converted to 2×M output real samples by taking the complex conjugates of the M samples. The parallel outputs of IFFT  306  are applied to parallel-to-serial converter  308  to provide a serial output signal. Essentially, the serial output signal is a digital time domain signal which undergoes additional time domain processing by time domain module  310  to produce a signal suitable for transmission of a subscriber line. Time domain module  310  can comprise components such as a cyclic prefix block, an up-sampler, and interpolator and a digital-to-analog converter to produce a continuous time domain signal. 
       FIG. 3B  illustrates a more detailed description of the reception side of the PMD layer. An analog signal is received over the subscriber line where time domain module  332  provides a variety of time domain signal processing and can comprise components such as an analog gain module, an analog-to-digital converter and a digital gain adjustment module. The resultant digital time domain signal is converted to parallel sub-channels using serial-to-parallel converter  334 . The parallel sub-channels are converted to frequency domain using FFT  336 . Frequency equalizer  338  which has been previously trained during the initialization can apply selected gain to each frequency. The resultant frequency data corresponds to constellation points in a constellation associated with the sub-carrier corresponding to the sub-channel which is then unmapped by de-mapper  340  which can comprise a trellis code decoder back into digital data. The de-mapped digital data is then reassembled into a serial stream of bits by parallel-to-serial-converter  342  where the data is then handled by the PMS-TC layer. 
     While  FIG. 3A  shows an overview of the coding and modulation process taking place in the PMD layer,  FIG. 4  shows the inclusion of a sync symbol. A sync symbol, which is not to be confused with the sync byte introduced in the PMS-TC layer which is used for overhead framing, is used to define the boundary of a DMT superframe. Typically, a sync symbol is followed by a predetermined number of data symbols. Together they define the DMT superframe. Typically, the superframe comprises a sync symbol and 256 data symbols. Characteristically, a sync symbol comprises 2-bits per available bin and is set to a well defined sequence of all ones or all zeros. A quadrant scrambler may then rotate the sync symbol as mapped onto the appropriate constellation. On the contrary, when user data is mapped on a DMT symbol, each bin has a certain number of bits which can be carried, the numbers vary due to factors such as line attenuation, noise and crosstalk in the cable, and the transmit signal PSD. In order for a bin to be useable, the signal-to-noise ratio (SNR) must be large enough to load 2 bits; alternatively, one bit of information may be loaded across multiple bins with lower SNR (referred as one bit constellations). 
     More specifically, in  FIG. 4  between mapper  304  and IFFT  306  is MUX  404  which includes a sync symbol produced by sync symbol module  402  after a predetermined number of user data symbols have been processed. The sync symbol module  402  generates the sync symbol based on a well defined sequence of 2-bit symbols of all ones or all zeros and with the application of a quadrature scrambler. 
     During an initialization phase, SNR measurements are taken for each bin, which are used in that initialization phase to determine the number of bits that can be transmitted reliably over each bin. The process of determination is referred to as bit loading. To determine a bit loading profile, factors such as the transmit PSD, the ratio of useful receive signal power to total noise power, which includes background noise and other external noise sources such as crosstalk.  FIG. 5  shows an example of a bit loading profile. For illustration purposes only 23 bins are shown in this example; bins  502 ,  504 ,  516 , and  528  carry no data. Other bins carry varying amounts of data. Each combination of bits across all bins comprise a DMT symbol. Additionally, associated with each bin is a constellation for mapping bits onto the subcarrier. For example, bin  520  is only capable of carrying 2-bits, so mapper  304  maps 2-bits of data to a constellation point on constellation  552 . Likewise, bin  522  is capable of carrying 5-bits, so mapper  304  maps a given 5-bits of data to a constellation point on constellation  558 . In the same fashion bin  524  which can carry 3-bits is associated with constellation  554 , and bin  526  which can carry 4-bits is associated with constellation  556 . Each DSL standard defines the association of the number of bits with a particular constellation. 
     As mentioned previously crosstalk is a ubiquitous source of noise in a DSL system.  FIG. 6  illustrates the various types of crosstalk typically experienced in a DSL system. For simplicity, CO  610  comprises two transceivers communicating over two subscriber lines to two CPEs. Transceiver  602  is in communications with CPE  604  and transceiver  606  is in communications with CPE  608 . For the sake of example, the crosstalk from CO  606  and CPE  608  to either CO  602  or CPE  604  is described. However, it should be understood that interference may also be between the transmitter and receiver on the same subscriber line in both the upstream and downstream paths, which is the near-end echo of the transmit signal. The term “far-end” refers to when the source of interference is away from the receiving side and the term “near-end” refers to when the source of interference is close to the receiving side. For example, interference shown by arrow  612  illustrates noise generated by transceiver  606  coupled into the downstream communications and received by CPE  604 . The term “victim” is applied to the line or the circuit being examined for crosstalk, and the term “disturber” is applied to the source of the crosstalk. Since the noise is generated away from the receiving side, this is referred to as downstream far-end crosstalk (FEXT). Likewise, interference shown by arrow  614  illustrates upstream near-end crosstalk (NEXT). Interference shown by arrow  616  illustrates upstream FEXT, and interference shown by arrow  618  illustrates downstream NEXT. In particular, downstream FEXT is a ubiquitous source of noise in VDSL. Accordingly, various needs exist in the industry to address the aforementioned deficiencies and inadequacies, such as mitigating downstream FEXT. 
     SUMMARY OF INVENTION 
     A system and method for providing dedicated back channel communications for error samples in a DSL system comprises initially reserving a set of reserved bins for back channel communications. Once the bins are reserved for back channel communications, error samples can be encoded and mapped onto this set of reserved tones. During a given DMT superframe, the encoded and mapped error samples are transmitted along side of user data on non-reserved tones from the CPE to the CO. An orthogonal pilot sequence transmits samples transmitted during the sync frame associated with a downstream DMT superframe. Error samples are measured from the transmitted orthogonal pilot sequences and transmitted back to the CO in the next upstream DMT superframe. 
     The selection of the reserved bins is related to the bits per bin allocation or bit loading process. For robustness, the allocation of bits per bin for the reserved bins uses a higher SNR margin over bins that are used to carry conventional user data. An alternative approach to selection of the number of bits per bin for the reserved bins is to take the existing bit loading profile and assign fewer bits than the number of bits specified in the bit loading profile. An optimal method of selection of the set of bins to reserve is to select the bins with the highest SNR. 
     In addition to providing the error samples, the back channel can be used to acknowledge commands received from the CPE and can also include error detection information such as a cyclic redundancy code (CRC). The commands can specify the bin group for which error measurements are desired as well as the resolution of these error measurements. Furthermore, the system and methods described above can be embodied in a DSL modem comprising a processor, a line driver, and memory comprising instructions to perform the methods described above. 
     A CO can also be equipped as a system with a method for measuring downstream errors. This method comprises transmitting orthogonal pilot sequences to one or more CPEs during synchronized sync symbols, where the pilot sequences transmitted by any two CPEs are orthogonal. Following the transmission of the DMT superframe containing each sync symbol, error samples are returned in the following upstream DMT superframe in the form of a message block which might include an acknowledgement to the error sampling command issued by the CO. The message block is received over a dedicated back channel as described above. 
     Other systems, methods, features, and advantages of the present disclosure will be or become apparent to one with ordinary skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  illustrates DSL communications layering in communications between a CO and a CPE; 
         FIG. 2  illustrates a more detailed description of the transmission side of the PMS-TC layer as disclosed by present xDSL standards; 
         FIG. 3A  illustrates a more detailed description of the transmission side of the PMD layer; 
         FIG. 3B  illustrates a more detailed description of the reception side of the PMD layer; 
         FIG. 4  illustrates a more detailed description of the transmission side of the PMD layer with the inclusion of a sync symbol; 
         FIG. 5  shows an example of a bit loading profile; 
         FIG. 6  illustrates the various types of crosstalk typically experienced in a DSL system; 
         FIG. 7  illustrates a DSL system in accordance with one embodiment; 
         FIG. 8  illustrates an exemplary embodiment of a vectorized PMD layer; 
         FIG. 9  illustrates a timing diagram showing vectoring enabled CPEs synchronized and aligned; 
         FIG. 10  is a block diagram of an embodiment of one of the vectoring enabled CPEs; 
         FIG. 11  is a diagram indicative of a representative normalized error sample calculation for a given bin; 
         FIG. 12A  illustrates a DMT superframe; 
         FIG. 12B  illustrates a back channel message block split across a DMT superframe boundary; 
         FIG. 13  illustrates a data frame structure which supports the back channel message block; 
         FIG. 14A  shows bins dedicated to the back channel; 
         FIG. 14B  illustrates an example of bins dedicated to the back channel where additional margin is used; 
         FIG. 15  illustrates an implementation of the PMD layer in a CPE in accordance with an embodiment of the invention; and 
         FIG. 16  illustrates an implementation of the PMD layer in a CO transceiver in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     A detailed description of embodiments of the present invention is presented below. While the disclosure will be described in connection with these drawings, there is no intent to limit it to the embodiment or embodiments disclosed herein. On the contrary, the intent is to cover all alternatives, modifications and equivalents included within the spirit and scope of the disclosure as defined by the appended claims. 
       FIG. 7  illustrates a DSL system in accordance with one embodiment. CO  730  comprises a plurality of transceivers represented by transceivers  740   a ,  740   b , and  740   c . Each line card can comprise one or more transceivers. The transceivers are connected to CPEs  710   a ,  710   b , and  710   c , respectively, through separate subscriber lines. In the diagram, each subscriber line is broken down into its upstream and downstream paths. The downstream paths for transceivers  740   a ,  740   b , and  740   c  are indicated by arrows  702   a ,  702   b , and  702   c , respectively. Similarly, the upstream paths for transceivers  740   a ,  740   b , and  740   c  are indicated by arrows  704   a ,  704   b , and  704   c , respectively. In this figure, only three of the M vectoring enabled CPEs are shown as CPEs  710   a ,  710   b  and  710   c.    
     As signals are transmitted downstream from the CO  730  onto the DSL loops, a certain amount of energy effectively leaks from one downstream CO transmitter into an adjacent CPE receiver, thereby creating the undesired FEXT signal into adjacent receivers. To combat FEXT, the transceivers coupled to vectoring enabled CPEs have transmitters that can share information and coordinate transmission in the form of multiple input multiple output (MIMO) pre-coding signals indicated by vectorized PMD layer  770 . 
       FIG. 8  illustrates an exemplary embodiment of vectorized PMD layer  770 . For each transceiver, the PMD layer resembles that shown in  FIG. 3  with MIMO pre-coder  802  inserted between the mapper in each transceiver and the IFFT. The detailed PMD layers for transceivers  740   a ,  740   b  and  740   c  are indicated as PMD layers  810   a ,  810   b  and  810   c , respectively. 
     The purpose of the MIMO pre-coder is to compensate at the transmitter for the undesired FEXT addition to the subscriber lines. Pre-coding (or pre-cancellation) is performed by means of a matrix operation (shown as channel matrix  804 ) that takes as input the transmit data samples (i.e., mapper outputs on the CO side) and outputs pre-compensated data sample for input to the IFFT on the CO side. The pre-compensation is such that the FEXT at each of the far-end receivers in the vectored group is cancelled. In order for pre-coding to work, the data symbols of all users should be synchronized and aligned at the transmitter output, so that the pre-coding matrix presents a complete independence between all subcarriers of the vectored DMT system. With proper synchronization and alignment of DMT symbols, the pre-coder operation can be seen as a matrix multiplication per each subcarrier across all the users in the vectored group. In all generality, the per subcarrier pre-coder coefficient converges to the inverse of the FEXT coupling channel matrix that exists among the vectored users. The derivation of the pre-coder coefficients can be performed after a FEXT coupling channel analysis phase, during which known signal sequences are being transmitted by each transmitter with a well determined pattern. Further detail in deriving optimal pre-coding matrices can be found in U.S. patent application Ser. No. 11/845,040 filed on Aug. 25, 2007, which is hereby incorporated by reference. 
     One key feature that enables vectoring is alignment and synchronization of the transmitted DMT symbols.  FIG. 9  illustrates a timing diagram showing transceivers connected to vectoring enabled CPEs synchronized and aligned. As can be seen, vectoring enabled CPEs represented by timing diagrams associated with CPE  710   a ,  710   b , and  710   c  receive synchronized sync symbols as indicated by the symbol periods referenced as  902 . Consequently, the received DMT symbols as well as the DMT super frames are synchronized in vectoring enabled CPEs. This alignment is controlled by the CO, is required for synchronous operation and ensures orthogonality among the M vectoring enabled users. 
     Returning to  FIG. 7 , another key element among the features needed for vector enabled CPEs is the ability to measure and report error samples in support of vectoring. Each vectoring enabled CPE comprises error measurement and transmission module  720 . Error measurement and transmission module  720  is responsible for measuring the error samples in the sync symbols seen at the CPE and propagating the error measurement back to CO  730  and the vector processing entity. In this way the FEXT coupling channel can be identified, and the proper pre-coder coefficients can be derived. While this can be performed during initialization, due to the slow varying nature of a DSL channel, it is desirable to continue updating the FEXT pre-coders during data mode in order to track the channel variation. Hence, it is desirable for the vectoring enabled CPEs to comprise an error measurement and transmission module to periodically feed the CO with their error samples. 
     To facilitate the measurement of error samples, CO  730  can transmit commands to each CPE. The commands can instruct the CPE on which bin or group of bins to make error measurements, how many measurements to make, and the resolution of the corresponding error samples to be reported. In order to minimize the overhead, at initialization the set of all eligible bins (known as the MEDLEY set) can be divided into specific bin groups and assigned a unique numeric identifier. These bin groups do not have to be mutually exclusive. These bin groups can also contain only one bin. The CO can request measurements for a specific bin group by using the unique identifier in a command to the CPE. Furthermore, the bin groups could comprise M TG  bins and be in the natural numeric order, that is bin group  1  would comprise bins  1  . . . M TG  and bin group  2  would comprise bins M TG +1 . . . 2M TG . Although the preceding example shows contiguous blocks of bins. There may be bins either not in the MEDLEY set or not capable of transmission, which can be excluded from each bin group, so the allocation to each bin group can be adjusted to exclude these bins while maintaining M TG  bins in the bin group. So as a further example, bin group I could comprise bins  1  . . . B BAD −1, B BAD +1 . . . M TG +1 where bin B BAD  is incapable of carrying any traffic. 
     It should be noted that the number of error samples to request and corresponding resolution will depend on the phase of adaptation, i.e., learning, tracking, and joining phases. For example, in a learning phase, as part of the training protocol, specific signals are defined to encode the error samples onto special operations channel (SOC) bins for robust transfer until the channel training is complete and any data transmission is configured reliably. However, during the tracking phase and joining phase (i.e., when new lines join), the error samples are transported through the back channel. Therefore, in order to facilitate the transport of error samples in data mode, a back channel is used as indicated by paths  706   a ,  706   b , and  706   c , and described below. 
       FIG. 10  is a block diagram of an embodiment of one of the vectoring enabled CPEs depicted in  FIG. 7 . In accordance with certain embodiments, the steps for normalized error measurements and back channel signaling described in this disclosure may be incorporated in software within a CPE such as a DSL modem. One of ordinary skill in the art will appreciate that DSL modems comprise other components, which have been omitted for purposes of brevity. Generally, DSL modem  710   a - c  may include processor  1010 , memory component  1040  (which may include volatile and/or nonvolatile memory components), and data storage component  1020  that are communicatively coupled via a local interface  1030  such as a data bus. In addition, the DSL modem  710   a - c  comprises an input/output interface  1070  which can be coupled to an end user device such as a PC, router, wireless access point, etc. and can be an Ethernet interface. DSL modem  710   a - c  further comprises line interface  1080  which can be coupled to the DSL loop to communicate with a CO. Line interface  1080  can comprise elements such as a line driver, analog front end and DSL transceiver. 
     The local interface  1030  may have additional elements, which are omitted for simplicity, such as controllers, buffers (caches), drivers, repeaters, and receivers to enable communications. Further, the local interface may include address, control, and/or data connections to enable appropriate communications among the aforementioned components. Processor  1010  may be a device for executing software, particularly software stored in memory component  1040 . Processor  1010  can be any custom made or commercially available processor, a central processing unit (CPU), an auxiliary processor among several processors associated with DSL modem  410   a - b , a semiconductor based microprocessor (in the form of a microchip or chip set), a macroprocessor, or generally any device for executing software instructions. 
     Memory component  1040  can include any one or combination of volatile memory elements (e.g., random access memory (RAM, such as DRAM, SRAM, SDRAM, etc.)) and/or nonvolatile memory elements (e.g., ROM, hard drive, tape, CDROM, etc.). Moreover, memory component  1040  may incorporate electronic, magnetic, optical, and/or other types of storage media. One should note that some embodiments of memory component  1040  can have a distributed architecture (where various components are situated remotely from one another), but can be accessed by processor  1010 . 
     The software in memory component  1040  may include one or more separate programs, each of which includes an ordered listing of executable instructions for implementing logical functions. In the example shown in  FIG. 10 , the software in the memory component  1040  may include an operating system  1050 . Furthermore, the software residing in memory  1040  may include application specific software  1060 , which may further include module  720  for normalized error measurements and for back channel signaling. It should be noted, however, that these modules can be implemented in software, hardware or a combination of software and hardware. The operating system  1050  may be configured to control the execution of other computer programs and provides scheduling, input-output control, file and data management, memory management, and communication control and related services. 
     A system component and/or module embodied as software may also be constructed as a source program, executable program (object code), script, or any other entity comprising a set of instructions to be performed. When constructed as a source program, the program is translated via a compiler, assembler, interpreter, or the like, which may or may not be included within the memory component  1040 , so as to operate properly in connection with the operating system  1050 . When the DSL modem  710   a - c  is in operation, the processor  1010  may be configured to execute software stored within the memory component  1040 , communicate data to and from the memory component  1040 , and generally control operations of the DSL modem  710   a - c  pursuant to the software. Software in memory may be read by the processor  1010 , buffered within the processor  1010 , and then executed. 
     To implement the vectoring described above, the CPE supports the reception and processing of the orthogonal pilot sequences and reports the normalized error sample relative to the specific sync symbol back to the CO where the CO can process the information to construct the channel matrix representing the FEXT in the cable. 
     During the tracking phase, to monitor and measure the error, orthogonal pilot sequences are transmitted through the sync symbol to vectoring enabled CPEs. An orthogonal pilot sequence comprises a sequence of a predetermined length such that any two sequences are orthogonal (i.e., their scalar products are zero). To each vector enabled CPE, the CO transmits one of these pilot sequences. The use of orthogonal pilot sequences facilitates the processing for determination of the crosstalk couplings among the subscriber lines connected to vectoring enabled CPEs. Specifically, the crosstalk couplings from a first disturber to a victim are typically computed by correlating the measured error reported by the victim CPE with the orthogonal transmit sequence transmitted by the first disturber CO over the entire duration of the orthogonal sequence. The crosstalk from other disturbers into the victim does not influence the computation of the crosstalk coupling for the crosstalk from the first disturber into the victim if the transmitted signals on the other disturbers are orthogonal to the transmitted signal of the first disturber. 
       FIG. 11  is a diagram indicative of a representative normalized error sample calculation for a given bin. For each bin in an orthogonal pilot sequence, a value of one or zero is encoded to (1, 1) or (−1, −1) in the constellation, shown as and  1108 , respectively. However, because of quadrant scrambling the resultant encoded constellation point could also be (1, −1) or (−1, 1), shown as  1106  and  1110 , respectively. Since the orthogonal pilot sequences are unique for each line and are synchronized to the sequence transmitted from the CO-side, the CPE can regenerate locally an ideal reference for making correct symbol decisions. 
     When sample  1102  is received, it is aligned to one of these constellation points, in this particular example constellation point  1104 . The constellation points and received sample  1102  can be viewed as complex numbers on the complex plane. Because the pilot sequence may be known by the CPE, the particular constellation point may be predicted by the CPE. However, the CPE may not know the pilot sequence or may not be able to predict the constellation point due to the quadrature scrambling. In this case, real axis  1120  and imaginary axis  1122  act as decision boundaries in order to align received sample  1102  to one of the constellation points. Vector  1112  on the complex plane is the difference between the constellation point and the received sample. Vector  1112  can also be viewed as a complex number with real part  1114  and imaginary part  1116 . This is often also referred to as an in-phase component and a quadrature component, respectively. If b bits is the desired resolution for a real number in the error reporting then 2b bits are needed to convey the normalized error sample (b bits for the real part and b bits for the imaginary part). For every bin of a vectored user on which error reporting is required, 2b bits, which are computed on every sync symbol at the rate of the DMT superframe, can therefore be sent back to the CO through the back channel. 
     To be most productive the transport of the error samples from each CPE to the CO should be timely and robust. One possible method for passing error samples is to use the dedicated EOC channel  210  as shown in  FIG. 2 . However, because the EOC is responsible for other functions, only a certain bandwidth would be available for the transport of error samples. A disadvantage of this approach is that this may require an adjustment to the rate of transmission of the error samples, which may affect the update rate of the pre-coding system. Another disadvantage is that the transmission of the error samples may not be associated with the superframe or sync symbol from which the error measurement was made, so additional overhead would be needed to associate the error sample properly with the sync symbol on which the error measurement was made. Associating the error sample with the appropriate sync symbols is essential to the accurate estimation of the pre-coder coefficients. Hence, the creation of a dedicated back channel can ensure adequate bandwidth for the transport of the error samples; as a result it would relieve the complexity in associating each error sample with a sync symbol. 
       FIG. 12A  illustrates a DMT superframe. The superframe comprises sync frame  1202  which has a duration of 0.25 milliseconds followed by 256 data frames exemplified in the figure by data frames  1204 ,  1206 ,  1212 ,  1214 ,  1222  and  1224 . Each data frame has a duration of 0.25 milliseconds so the entire superframe has a duration of 64.25 milliseconds. Sync frame  1202  comprises the sync symbol as described above. Also shown in the figure is sync frame  1230  of the subsequent DMT superframe. 
     During the transmission from CPE to CO (and from CO to CPE for command messages) of the data frames within the DMT superframe is back channel message block  1220  which takes 64 milliseconds to complete the transmission. Back channel message block  1220  can be embedded into some or all of the 256 upstream data frames. Back channel message block  1220  can comprise an acknowledgement to any commands from the CO relating to error measurements such as described above. It can also comprise the measured error samples. Finally, depending on the way the back channel is implemented it can optionally comprise some error detection such as a CRC. For protection against impulse noise, a simple FEC with interleaving can also be applied. 
     Alternatively, it may not be desirable to begin a back channel message block at the beginning of a DMT upstream superframe. For example, to reduce the amount of buffering needed, it may be important to transmit the back channel message block before reception of the next downstream sync symbol is received. Hence, it is not desirable to wait for the start of the next upstream DMT superframe. In this case, the back channel message block can be split across the upstream DMT superframe boundary. 
       FIG. 12B  illustrates a back channel message block split across a upstream DMT superframe boundary. While back channel message block  1280  still comprises up to 256 data frames it does not necessarily start right after a sync symbol. In this particular example, the back channel message block begins with DMT frame  1252  which is the n-th data frame in the DMT superframe. Back channel message block  1280  ends with DMT frame  1266  which is the data frame n−1 in the subsequent data frame. It should be noted that no data can be transmitted during sync frame  1260 . A subsequent back channel message block would then begin with DMT frame  1272 . For convenience, the first data frame of a given back channel message block uses shall be referred to as the back channel start frame. 
       FIG. 13  illustrates a data frame structure which supports the back channel message block. Each data frame such as data frame  1302  is subdivided into back channel part  1304  and data part  1306 . Back channel part  1304  carries L BC  bits and data part  1306  carries L 0  bits. Data frame  1302  is shown starting with the mapping of the L BC  bits first followed by the data part  1306  carrying L 0  bits, but other mapping orders are possible. As a result the message block can hold  256  L BC  bits over a superframe period. 
     The carrying capacity of the back channel depends on the number of bits per data frame allocated, for example to maintain a minimum bit rate of 256 kb/s. This translates to 16,384 bits for each DMT superframe or 64 bits per data frame. If 8 bits are used to represent real numbers, 16 bits will be needed to represent each normalized error sample due to the need to represent the real and imaginary components of the normalized error sample. At a minimum bit rate of 256 kb/s, the back channel message block has the capability of carrying up to 1024 normalized error samples. 
     One method for allocating the L BC  bits per data frame is to assign dedicated bins for the back channel. Returning to the bit loading profile example used in  FIG. 5 , suppose that 12 bits are to be dedicated for the back channel. Using the example of  FIG. 1 ,  FIG. 14A  shows bins  506 ,  508 ,  510  and  524  dedicated to the back channel, each channel contributing 2, 2, 5 and 3 bits, respectively, to the 12 bits. By using this approach, the back channel can be implemented in the PMD layer. By using the PMD layer, the back channel may bypass the FEC supplied by PMS-TC layer. While some degree of error correction is provided by the trellis code, it is recommended that error detection (or error correction) be added to the message block. For example, as suggested above, a CRC could be added to back channel message block  1220 . 
     In order to avoid the overhead needed to identify which sync symbol a given error sample is associated with, the back channel message block in which the back channel information is transmitted should be associated with a corresponding downstream DMT superframe and in particular downstream sync symbol associated with the downstream DMT superframe. This should account for any time delay due to the calculation of the error sample and any round trip delays. For example, if a downstream sync symbol is transmitted at time t 0 , the returning error samples would be expected in the back channel start frame received at time t 0 +t e  where t e  is a predetermined or derived time period offset. Other ways of ensuring correspondence of a particular downstream sync symbol with the back channel start frame can also be used. For example, if it is known that the error samples for a given downstream sync symbol are contained in the back channel message block beginning with data frame n in the upstream DMT superframe being received when the given downstream sync symbol is being transmitted, then a correspondence can be made between the back channel message block beginning with data frame n and the given downstream sync symbol. With the correspondence, the identity of a given error sample can be determined without unduly adding overhead to the communications. 
     To further enhance robustness, the bins selected as dedicated to the back channel should have large margin. Typically, based on the SNR the numbers of bits a given bin can carry is determined to meet a bit error ratio threshold. In particular DSL standards require that the bit error ratio for any given bin should not exceed 10 −7 . Statistically, the bit error ratio is related to the number of bits allocated and the SNR. One approach to making the back channel more robust is to increase the margin on the selected bins. For example, if the margin on the selected bins is increased by a predetermined amount, the number of bits allocated is reduced, but so is the bit error ratio. In a numeric example, suppose bin  510  has an SNR of 32 dB which allows it to support 5 bits with a 6 dB margin (thus the SNR of 32 dB is treated as 26 dB and the value of 26 dB is used to determine the maximum bit-loading that still provides a bit-error-ratio of at most 10 −7 ). However, instead, during the bit loading process when determining the number of bits for bin  510  a margin of 12 dB is incorporated into the SNR, i.e., the bit loading process treats the SNR as 20 dB. In such a case, the bit loading process is likely to allocate only 3 bits for bin  510 . 
     Another method is to take the existing bit loading profile and map the bits per bin values found in the profile to a “robust” bits per bin value. This mapping could be done by a formula or by a table. For example, if bin  534  normally could support 6 bits according to the existing bit loading profile, only 4 bits will be used. If bin  524  normally could support 3 bits only two bits will be used. 
     Regardless of the selection method, additional robustness can be added by using fewer bits per bin for the dedicated back channel bins than the bits per bin that can be supported by those bins.  FIG. 14B  illustrates an example where bins  534 ,  536 , and  538  are dedicated to the back channel, each providing 4 bits each even though they are capable of supporting 6 bits. 
     Optimally, the selection of bins for the back channel should be made to have the smallest impact over the amount of data that can still be carried. For example, in some bit loading algorithms, it takes 10 dB of SNR for a bin to be viable for carrying one bit of information, but each additional 3 dB of SNR allows the bin to carry roughly an additional bit. For example, 13 dB of SNR allows for 2-bits, 16 dB of SNR allows for 3-bits, 19 dB of SNR allows for 4-bits, etc. Clearly, not all bin reservation schemes are equivalent. As a demonstration, suppose 6 bits are to be transported on dedicated bins with an additional 6 dB of margin. Based on the exemplary bit loading formula just described, 3 bins having 19 dB of SNR could support the 6 bits needed with the additional margin of 6 dB per bin, i.e., each bin would carry 2 bits of dedicated back channel data. However, it would take 12 bits of bandwidth away from regular data. On the other hand, selecting 2 bins having 22 dB of SNR would also support the 6 bits needed, where each bin would carry 3 bits of dedicated back channel data, but cost only 10 bits of regular data bandwidth. Therefore, an ideal reservation of bins would reserve the desired number of bins for the dedicated back channel while minimizing the loss of bandwidth available for regular data. 
     One algorithm for selecting an optimal set of reserved bins is to first select bins with the highest SNR and dedicate these bins to the back channel; then proceeding with the bins with the next highest SNR and dedicating those bins to the back channel until the number of bits required for the back channel has been reached or unless a predetermined threshold has been reached. If the predetermined threshold is reached, the number of bits lost to the back channel from the regular data channel is too high and further allocation to the back channel would result in an unacceptable loss of capacity. 
     A protocol or negotiation can be put into place to accommodate the possibility that the CPE cannot accommodate the bit rate requested by the CO for the back channel. For example, the CO may request that a certain bin group be monitored for its error at a given resolution of 2b bits per complex error sample, which may be translated to k bits per DMT frame needed to be dedicated for the back channel. However, based on the communication capacity of the CPE computed from the bit loading profile, it is determined that by supporting k bits per DMT frame would exceed the allowed drop in true upstream data carrying capability. Then the CPE can respond that it can only carry k′ bits per DMT frame or simply respond stating it can only support a corresponding resolution of 2b′ bits per complex error sample. A lower resolution error sample can still be of use to the CO for determining pre-coder coefficients, but may require more error samples for convergence to the desired set of pre-coder coefficients, leading to longer convergence time. 
       FIG. 15  illustrates an implementation of the PMD layer in a CPE in accordance with an embodiment of the invention. The receiver portion of the PMD layer supports making error measurements and has similar components as that described in the receiver portion shown in  FIG. 3B , but further comprises demultiplexer  1502  which extracts the sync symbol and supplies it to error sampling module  1504 , but passes the other 256 DMT frames in a DMT superframe to de-mapper  340 . The transmitter portion of the PMD layer supports the transmission over a back channel using dedicated bins and has similar components as that described in  FIG. 4 , but further comprises back channel module  1506 . Back channel module  1506  directs serial-to-parallel converter  302  to not map any data to the bins used in the back channel. Back channel module  1506  supplies data to be transmitted over the back channel to the mapper by placing portions of the data on the channels associated with the dedicated bins. Finally, optionally, back channel module  1506  indicates to mapper  304  the number of bits for the dedicated bins. It may do this either by separately supplying the information to mapper  304  or by altering the bit loading profile supplied to mapper  304 . Similarly, back channel module  1506  can prevent serial-to-parallel converter  302  from assigning data to the dedicated bins by either directly supplying the bins to serial-to-parallel converter  302  or by altering the bit loading profile supplied to serial-to-parallel converter  302 . 
     Error measurement and transmission module  720  comprise both error sampling module  1504  and back channel module  1506 . When a sync symbol is received by the CPE, error sampling module  1504  measures the error and may optionally store it for transmission back to the CO. Back channel module  1506  takes the error measurement and transmits it back to the CO as described above. One advantage of using dedicated bins to support the back channel is that the entire error measurement and transmission procedure is performed by the CPE PMD layer without the need for the error transmission to be performed at a higher level. As a result, the higher communications layers need not be aware of the error sampling or transmission activity. 
       FIG. 16  illustrates an implementation of the PMD layer in a CO transceiver in accordance with an embodiment of the invention. The receiver portion of the PMD layer supports receiving back channel information and has similar components as that described in the receiver portion shown in  FIG. 3B , but further comprises back channel receiver  1602 . Back channel receiver  1602  optionally indicates to de-mapper  340 , the number of bits per bin used in the dedicated bins for the back channel. This may be simply relaying that information to de-mapper  340  or altering the bit loading profile seen by de-mapper  340 . Additionally, back channel receiver  1602  optionally informs parallel-to-serial converter  342  which bins are dedicated to the back channel and should not be used in reconstructing the serial data bit stream. Again, the information may simply be supplied to parallel-to-serial converter  342  or an altered bit loading profile with the dedicated bins assigned zero bits could be supplied to parallel-to-serial converter  342 . In addition, the back channel receiver  1602  receives the data modulated on the dedicated bins from de-mapper  340  as demodulated and decoded data. Back channel receiver  1602  then assembles the back channel message block as the DMT superframe is received. 
     Once received, the back channel message block is transmitted to vector processing entity  1604 , which uses the back channel message to calculate or update the channel matrix used in module  804 . Channel matrix  804  is fed into pre-coder  802  as described above for  FIG. 8 . Alternatively, vector processing entity  1604  can update pre-coder  802  directly using the back channel message. In this implementation, the receiving of the back channel data including the error measurements, the calculation of the channel matrix and the adjustment of pre-coder  802  could be implemented without the need for data to leave the PMD layer. This prevents the need of the higher communications layer to be aware of the FEXT reduction taking place. 
     It should be emphasized that the above-described embodiments are merely examples of possible implementations. Many variations and modifications may be made to the above-described embodiments without departing from the principles of the present disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.