Abstract:
A repeater for regenerating signals communicated along a telecommunication line uses a constellation providing a sufficiently high constellation density to ensure that regenerated signals are spectrally compatible. In one exemplary embodiment, such a repeater is configured to implement a method comprising the steps of: receiving first data signals from a first telecommunication line segment; demodulating the first data signals thereby recovering digital data using a constellation that provides a first constellation density; modulating second data signals with the digital data; transmitting the second data signals across a second telecommunication line segment that is bound within a cable; selecting a second constellation density for use in the modulating step such that the second data signals are spectrally compatible with other signals transmitted across a third telecommunication line segment bound within the cable, wherein the second constellation density is higher than the first constellation density.

Description:
RELATED ART 
     Segments of telecommunication lines are usually bundled in cables that extend over large distances from transceivers at a central office to transceivers at remote locations, sometimes referred to as “customer premises.” Signals communicated across telecommunication line segments bundled within the same cable couple from line-to-line causing crosstalk. The crosstalk between signals using the same frequencies may degrade signal performance and may limit the cable&#39;s capacity or data rate. 
     A wide variety of telecommunication technologies may be used to communicate across telecommunication line segments bound by the same cable. In order to allow signals from different technologies to co-exist in the same cable, spectrum management standards (e.g., T1.417-2001 Spectrum Management for Loop Transmission) have been developed. Such standards specify crosstalk limits to ensure that crosstalk will not reduce signal quality below a specified level. A telecommunication service provider must ensure that signals communicated by its equipment satisfy the limits imposed by applicable spectrum management standards. 
     Signals that violate the spectrum management standards by causing an unacceptable amount of crosstalk to affect other signals communicated through the same cable are referred to as “spectrally incompatible” with such other signals. Signals that adhere to the spectrum management standards and, therefore, do not induce an unacceptable amount of crosstalk are referred to as “spectrally compatible” with the other signals communicated through the same cable. 
     DSL services, such as high-data-rate digital subscriber line, second generation (HDSL2) and asymmetric digital subscriber line (ADSL) services, are very popular due to the relatively high data rates and relatively low costs associated with these types of services. HDSL2 transceivers communicate over conventional copper loops and, therefore, are able to utilize a substantial portion of the vast telecommunication copper infrastructure that has been in place for decades. 
     HDSL2 has two deployment technologies, HDSL2 and HDSL4. HDSL2 transmits a 1.544 Mega-bit per second (Mbps) DS1 payload on a single copper loop. HDSL4 uses a similar transmission technology as HDSL2 but uses two copper loops each carrying half of the 1.544 Mbps DS1 payload. In general, HDSL2 signals communicated in accordance with existing standards can be transmitted with acceptable signal quality up to approximately 9,000 feet (ft) before being regenerated by a repeater. ADSL signals, on the other hand, can often be transmitted up to approximately 18,000 ft without regeneration. 
     Moreover, HDSL2 and ADSL signals transmitted in accordance with existing standards are spectrally compatible and, therefore, can be transmitted in the same cable up to a point where the HDSL2 signals are regenerated by a repeater. If an HDSL2 signal is regenerated beyond approximately 9,000 feet, then unacceptable interference occurs with ADSL signals communicated in the same cable, and the repeatered HDSL2 signal is, therefore, spectrally incompatible with such ADSL signals. Thus, deployment of HDSL2 is often limited to the first 9,000 ft of a cable that extends from a central office so that regeneration of HDSL2 signals is unnecessary. Such a limitation ensures spectral compatibility between HDSL2 and ADSL signals but undesirably limits the deployment of HDSL2 services. 
     SUMMARY OF THE DISCLOSURE 
     Generally, embodiments of the present disclosure pertain to systems and methods for regenerating telecommunication signals using sufficiently high constellation densities to ensure spectral compatibility. 
     A method in accordance with one exemplary embodiment of the present disclosure comprises the steps of: receiving first data signals from a first telecommunication line segment; demodulating the first data signals, thereby recovering digital data, using a constellation providing a first constellation density; modulating second data signals with the digital data; transmitting the second data signals across a second telecommunication line segment that is bound within a cable; selecting a second constellation density for use in the modulating step such that the second data signals are spectrally compatible with other signals transmitted across a third telecommunication line segment bound within the cable, wherein the second constellation density is higher than the first constellation density. 
     A method in accordance with another exemplary embodiment of the present disclosure comprises the steps of: receiving first data signals from a first telecommunication line segment; demodulating the first data signals to recover digital data using a constellation that provides a first constellation density; modulating second data signals with the digital data; transmitting the second data signals across a second telecommunication line segment that is bound within a cable; and selecting a higher constellation density for use in the modulating step, as compared to the first constellation density, and spectrally shaping the second data signals based on the selected higher constellation density such that the second data signals are spectrally compatible with other signals transmitted across a third telecommunication line segment bound within the cable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be better understood with reference to the following drawings. The elements of the drawings are not necessarily to scale relative to each other, emphasis instead being placed upon clearly illustrating the principles of the disclosure. Furthermore, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is a block diagram illustrating a conventional communication system. 
         FIG. 2  is a block diagram illustrating a communication system in accordance with an exemplary embodiment of the present disclosure. 
         FIG. 3  is a block diagram illustrating a repeater depicted in  FIG. 2 . 
         FIG. 4  is a graph illustrating exemplary power spectral density (PSD) functions for HDSL2 transceivers communicating across one of the tele-communication lines of  FIG. 2 . 
         FIG. 5  is a block diagram illustrating an exemplary transceiver that may be used to implement one or more transceivers depicted in  FIGS. 2 and 3 . 
         FIG. 6  is a block diagram illustrating an exemplary embodiment of a transmitter depicted in  FIG. 5 . 
         FIG. 7  is a block diagram illustrating an exemplary embodiment of a receiver depicted in  FIG. 5 . 
         FIG. 8  is a flow chart illustrating an exemplary architecture and functionality of control logic depicted in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure pertain to systems and methods for regenerating signals along a telecommunication line using a constellation that provides a sufficiently high constellation density for ensuring that the signals are spectrally compatible with other signals communicated through the same cable. A system in accordance with an exemplary embodiment of the present disclosure comprises a telecommunication line that provides a communication channel between a central office transceiver and a remote transceiver. Segments of the telecommunication line are coupled to a repeater, which regenerates signals communicated along the telecommunication line. In forming the regenerated signals, the repeater uses a constellation providing a sufficiently high constellation density to ensure that the regenerated signals are spectrally compatible with other signals communicated within a close proximity of the regenerated signals (e.g., communicated through the same cable). 
       FIG. 1  depicts a conventional telecommunication system  15 . The system  15  of  FIG. 1  comprises a high-data-rate digital subscriber line, second generation (HDSL2) transceiver  18  and an asymmetric digital subscriber line (ADSL) transceiver  21  residing at a central office  23  of a telecommunication network. The HDSL2 transceiver  18  of  FIG. 1  communicates with a remote HDSL2 transceiver  27  via a telecommunication line  29 . The telecommunication line  29  depicted in  FIG. 1  has two repeaterless segments  31  and  32 , each of which comprises a twisted wire pair, sometimes referred to as a “loop.” Segment  31  couples the HDSL2 transceiver  18  of the central office  23  to a repeater  35 , and segment  32  couples the repeater  35  to the remote HDSL2 transceiver  27 . Note that the remote HDSL2 transceiver  27  may reside at a customer premises or may reside within another repeater in the event that telecommunication line  29  extends beyond the remote HDSL2 transceiver  27 . 
     The ADSL transceiver  21  communicates with a remote ADSL transceiver  39  over a repeaterless telecommunication line segment  41 , which comprises a twisted wire pair. The ADSL transceiver  39  resides at a customer premises. 
     In accordance with current HDSL2 and ADSL specifications, as defined by T1.418, the repeater  35  may be positioned up to approximately 9,000 feet (ft) from the central office  23 , and the remote HDSL2 transceiver  27  may be positioned up to approximately 9,000 feet from the repeater  35  (i.e., approximately 18,000 feet from the central office  23 ). Further, the remote ADSL transceiver  39  may be positioned up to approximately 18,000 ft from the central office  23 . For illustrative purposes, assume that each of the transceivers  27  and  39 , as well as the repeater  35 , are positioned at their maximum respective distances from the central office  23 . 
     In the instant embodiment in which HDSL2 signals are communicated across telecommunication line  29 , pulse amplitude modulation (PAM) is used to form such signals. However, other types of signals may be communicated across telecommunication line  29  in other embodiments, and these signals may be formed using other types of modulation schemes, such as quadrature amplitude modulation (QAM). For example, the signals communicated over telecommunication lines  29  and  41  may be in accordance with HDSL2, HDSL4, G.SHDSL.bis, SDSL, or other known or future-developed standards. In amplitude modulation, such as PAM or QAM, a constellation is used to map digital data words to points or levels, referred to as “symbols,” corresponding to the values of the digital data words. In this regard, a constellation defines different symbols to which digital data words can be mapped, and the total number of symbols within a constellation controls the size of the data words that may be mapped by the constellation. 
     Moreover, “constellation density” is a term that refers to the number of payload bits that are mapped per symbol by a given constellation. For example, if a constellation enables 3.0 payload bits to be mapped per symbol, then the constellation is said to provide a constellation density of 3.0. Thus, a transmitter that modulates data using a constellation providing a constellation density of 3.0, outputs symbols that each contain 3.0 bits of payload information, and a transmitter that modulates data using a constellation providing a constellation density of 4.0, outputs symbols that each contain 4.0 bits of payload information. 
     If error correction is not employed, then a constellation having a density of 3.0 generally has eight different symbols to which data words can be mapped. In such an embodiment, each data word has three bits, all of which define payload information, and there is one symbol for each possible value of a data word. However, if error correction is employed, then a constellation providing a constellation density of 3.0 generally has a number of symbols greater than eight to accommodate the additional error checking bit or bits. For example, in Trellis coded PAM, it is common to define code words having three bits of payload information and one bit for error checking. In such an embodiment, each code word is four bits in length. Therefore, a constellation having a constellation density of 3.0 in such an embodiment has sixteen different symbols (i.e., one symbol for each possible code word value). 
     The constellation density of a constellation used to modulate data transmitted over a telecommunication line affects signal performance. In general, increasing the constellation density enables the same amount of payload information to be communicated at a lower bandwidth. However, for a given length of a telecommunication line segment, increasing the constellation density reduces the signal quality of the signals transmitted over the line segment. Thus, to keep the signal quality of such signals within an acceptable range, the acceptable maximum length of a repeaterless telecommunication line segment is often significantly reduced as the constellation density for the signals communicated over the segment is increased. Moreover, in selecting the constellation density, significant trade-offs exist between bandwidth, signal quality, and reach (i.e., maximum possible repeaterless line length). 
     It is generally well known that a constellation density of a little less than 3.0 provides an optimum solution for PAM considering the factors of bandwidth, signal quality, and reach. Thus, conventional PAM transceivers are typically configured to communicate using constellations providing constellation densities of 3.0. Accordingly, the transceivers  18  and  27  depicted by  FIG. 1 , as well as transceivers (not shown) included in the repeater  35 , are configured to transmit symbols that contain three bits of payload information. 
     In some embodiments, it is possible for the telecommunication line segments  32  and  41  to be positioned in close proximity to one another (e.g., bound within the same cable). When the segments  32  and  41  extending across distance d are bound within the same cable, an unacceptable level of crosstalk, as defined by T1.417, induced by the regenerated signals transmitted across segment  32  interferes with the signals transmitted across segment  41 . In other words, the regenerated HDSL2 signals transmitted across segment  32  are spectrally incompatible with the ADSL signals transmitted across segment  41 . 
     Significant effort has been expended to modify the regenerated signals transmitted across segment  32  in order to make these signals spectrally compatible with the signals transmitted across segment  41 . For example, various techniques for spectrally shaping and adjusting the power levels of the signals transmitted by the repeater  35  have been employed in an effort to make these signals spectrally compatible. Unfortunately, such conventional efforts have been unsuccessful. 
     Thus, to provide HDSL2 services to the HDSL2 transceiver  27 , service providers are generally faced with a decision to either ensure that segment  32  is not bundled in the same cable with an ADSL telecommunication line segment or to allow the signals transmitted across segment  32  to be spectrally incompatible as defined by T1.417. Moreover, many service providers simply choose to limit HDSL2 service to distances of less than approximately 9,000 ft from the central office in order to avoid the incompatibility problems described above. 
       FIG. 2  depicts a telecommunication system  100  in accordance with an exemplary embodiment of the present disclosure. As can be seen by comparing  FIGS. 1 and 2 , the system  100  may be similar to the conventional communication system  15  described above. Indeed, components having the same reference number in  FIGS. 1 and 2  are identically configured. However, the configuration of repeater  135  and HDSL2 transceivers  118  and  127  of  FIG. 2  are different than the configuration of repeater  35  and HDSL2 transceivers  18  and  27 , respectively, of  FIG. 1 . In this regard, rather than communicating signals across segment  32  using a constellation that provides a constellation density of 3.0, the repeater  135  and transceiver  127  of  FIG. 2  communicate signals across segment  32  using a constellation that provides a constellation density higher than 3.0. In a preferred embodiment, the repeater  135  and transceiver  127  use a constellation having a density of 4.0, although other constellation densities higher than 3.0 may be used in other embodiments. 
     By increasing the constellation density used to modulate and demodulate the signals transmitted across segment  32 , as compared to conventional system  15 , it is possible for such signals to have a lower bandwidth. Indeed, by using a constellation density of 4.0 or higher, it is possible, to transmit signals across segment  32  such that these signals are spectrally compatible, as defined by T1.417, with the ADSL signals transmitted across segment  41  even when the telecommunication line segments  32  and  41  are in close proximity (e.g., bound within the same cable). Moreover, by using a higher constellation density, as described above, the aforedescribed incompatibly problems plaguing HDSL2 service providers in conventional systems can be avoided. 
     Note that by using spectral shaping and/or power back-off techniques for the signals transmitted across segment  32 , it is possible to reduce the minimum constellation density that is sufficient for ensuring spectral compatibility. In this regard, spectral shaping generally refers to processes that modify the spectral shape of transmitted signals, and power back-off generally refers to processes that reduce the transmission power of transmitted signals. Such spectral shaping and/or power back-off techniques may be used to lower crosstalk effects within selected bandwidths in an effort to make transmitted signals spectrally compatible. 
     As an example, assume that the signals transmitted by repeater  135  across telecommunication line segment  32  cause an unacceptably high level of crosstalk in a particular bandwidth. In such an example, power back-off techniques may be used to lower the transmission power level of the repeater  135  for the particular bandwidth. Lowering the transmission power level has the effect of reducing the crosstalk occurring in the particular bandwidth, and it is possible for the power back-off techniques to reduce the crosstalk in the particular bandwidth to a low enough level such that the transmitted signals become spectrally compatible. Indeed, analyses have shown that, when spectral shaping and power back-off techniques, as well as modulation using a constellation that provides a constellation density of 4.0, are employed to transmit HDSL2 signals across segment  32 , it is possible for such signals to be spectrally compatible with ADSL signals communicated across segment  41 , assuming a symmetric payload of 1.544 megabits per second (Mbps) across segment  32 , a length of 14.5 kft or less for segment  41 , and 26 AWG for segments  32  and  41 . 
       FIG. 3  depicts an exemplary configuration of the repeater  135 . The repeater  135  of  FIG. 3  comprises an HDSL2 transceiver  148  coupled to telecommunication line segment  31  and an HDSL2 transceiver  152  coupled to telecommunication line segment  32 . Further, each of the HDSL2 transceivers  148  and  152  is coupled to a set of buffers  155  and  156 . Signals received from the segment  31  by the HDSL2 transceiver  148  are demodulated by the transceiver  148 . In one exemplary embodiment, the transceiver  148  communicates with the central office HDSL2 transceiver  118  ( FIG. 2 ) using a constellation that provides a constellation density of 3.0. In such an embodiment, each received symbol is demodulated to recover three bits of payload information. Such data is then transmitted to buffer  155 , which buffers the data before transmitting it to HDSL2 transceiver  152 . 
     As described above, the data communicated across segment  32  are preferably modulated using a constellation that provides a constellation density 4.0. In such an embodiment, each symbol communicated across segment  32  by transceiver  152  contains four bits of payload information. 
     Signals received from the segment  32  by the HDSL2 transceiver  152  are demodulated by the transceiver  152 . As described above, in a preferred embodiment, each of the HDSL2 transceivers  127  and  152  uses a constellation that provides a constellation density of 4.0. In such an embodiment, each symbol received by HDSL2 transceiver  152  is demodulated to recover four bits of payload information. The demodulated data is then transmitted to buffer  156 , which buffers the data before transmitting it to HDSL2 transceiver  148 . 
     The HDSL2 transceiver  148  modulates the digital data received from buffer  156  to form signals that are then transmitted across segment  31 . In a preferred embodiment, the transceiver  148  modulates the foregoing data using a constellation that provides a constellation density of 3.0. In such an embodiment, each symbol communicated across segment  31  by transceiver  148  contains three bits of payload information. 
     As described above, it is possible for HDSL2 signals transmitted across segment  31  to be spectrally compatible with ADSL signals transmitted across segment  41  even when the HDSL2 signals are modulated via a constellation providing a constellation density of 3.0. Further, a constellation density of 3.0 is close to optimum in considering bandwidth, signal quality and reach. Thus, having the HDSL2 transceivers  148  ( FIG. 3) and 118  ( FIG. 2 ) communicate based on a constellation density of 3.0 generally provides an optimal solution. However, other types of constellation densities are possible in other embodiments. For example, if desired, the HDSL2 transceiver  148 , like the HDSL2 transceiver  152  described above, may be configured to communicate with the central office HDSL2 transceiver  118  ( FIG. 2 ) using a constellation that provides a constellation density of 4.0 or higher. 
     When transceivers  127  and  152  are configured to communicate HDSL2 signals, the following equation may be used to define the power spectral density (PSD) of transceivers  127  and  152 : 
               PSD   ⁡     (   f   )       =     {               10       -   PBO     10       ×       K   transmit     135     ×     1     f   sym       ×         [     sin   ⁡     (       π   ⁢           ⁢   f       f   sym       )       ]     2         (       π   ⁢           ⁢   f       f   sym       )     2       ×     1     1   +       (     f     f     3   ⁢   dB         )       2   ×   order           ×       f   2         f   2     +     f   c   2           ,     f   &lt;     f   int                     0.5683   ×     10     -   4       ×     f     -   1.5         ,       f   int     ≤   f   ≤     1.1   ⁢           ⁢   MHz                       
where K transmit , order, f sym , and f 3db  are specified below in Table 1 and where f int  is the frequency at which the PSD functions of transceivers  127  and  152  are equal.
 
                                     TABLE 1                   K transmit     order   f sym  (kHz)   F3dB                   Repeater Transceiver 152   7.86   6   x/n   f sym /2       Remote Transceiver 127   8.32   6   x/n   0.9 × f sym /2                    
For HDSL2, x equals 1552 kilo-Hertz (kHz), which corresponds to 1544 kHz data channel and 8 kHz control channel. Further, n corresponds to the constellation density used by transceivers  152  and  148 . In the instant embodiment where transceivers  152  and  127  employ a four-bit constellation density, n is equal to 4.0.
 
       FIG. 4  depicts exemplary PSD functions for the transceivers  118 ,  127 ,  148 , and  152 . Curve  136  represents an exemplary PSD function for HDSL2 transceiver  118  in accordance with HDSL2 standards when this transceiver  118  employs a constellation providing a constellation density of 3.0. Curve  137  represents an exemplary PSD function for HDSL2 transceiver  148  in accordance with HDSL2 standards when this transceiver  148  employs a constellation providing a constellation density of 3.0. Curve  138  represents an exemplary PSD function for HDSL2 transceiver  152  when this transceiver  152  employs a constellation providing a constellation density of 4.0, and curve  139  represents an exemplary PSD function for HDSL2 transceiver  127  when this transceiver  118  employs a constellation providing a constellation density of 4.0. Note that curves  138  and  139  may be obtained from the PSD function specified above using the values specified in Table 1 and setting the value of PBO (power back-off) to 0. 
     Table 2 below specifies exemplary PBO values that may be used for the PSD functions depicted in  FIG. 4  to ensure that the HDSL2 signals communicated by transceivers  118 ,  127 ,  148  and  152  are spectrally compatible with the ADSL signals communicated by transceivers  21  and  39  when such HDSL2 and ADSL signals are propagated through the same binder or binders. Note that the values of Table 2 are based on transceivers  127  and  152  using a constellation density 4.0. 
                                                             TABLE 2                   Telecommunication   Telecommunication       line segment 31   line segment 32            Loop Reach -       Loop Reach -           26 AWG (kft)   PBO (dB)   26 AWG (kft)   PBO (dB)                    length ≧ 8.0   0   length ≧ 5.5   0       8.0 &gt; length ≧ 7.5   1   5.5 &gt; length ≧ 5.0   1       7.5 &gt; length ≧ 7.0   2   5.0 &gt; length ≧ 4.5   2       7.0 &gt; length ≧ 6.5   3   4.5 &gt; length ≧ 4.0   4       6.5 &gt; length ≧ 5.5   5   4.0 &gt; length ≧ 3.5   6       5.5 &gt; length ≧ 4.5   8   3.5 &gt; length ≧ 3.0   8       4.5 &gt; length ≧ 3.0   10   3.0 &gt; length ≧ 1.5   10       3.0 &gt; length ≧ 2.0   12   1.5 &gt; length ≧ 1.0   11       2.0 &gt; length   15   1.0 &gt; length   15                    
As an example, if the length of segment  32  is between 3,500 feet and 4,000 feet, then it can be ensured that the HDSL2 signals communicated by transceivers  127  and  152  are spectrally compatible with ADSL signals communicated by transceivers  21  and  39  through the same binder, if the PSD functions for transceivers  127  and  152  shown by  FIG. 4  are backed-off by 6 decibels (dB). If the length of segment  31  is between 7,500 feet and 7,000 feet, then it can be ensured that the HDSL2 signals communicated by transceivers  118  and  148  are spectrally compatible with ADSL signals communicated by transceivers  21  and  39  through the same binder, if the PSD functions for transceivers  118  and  148  shown by  FIG. 4  are backed-off by 2 decibels (dB). Thus, by employing the transmit power spectrums depicted by  FIG. 4  and backing off the transmit power according to Table 2, it can be ensured that the HDSL2 signals communicated by transceivers  118 ,  148 ,  152 , and  127  are spectrally compatible with the ADSL signals communicated by transceivers  21  and  39 .
 
     Note that spectral compatibility is ensured if the specified PBO values specified in Table 2 are applied across all of the transmitted frequencies. However, in some embodiments, it is possible to use PBO values other than those specified in Table 2, and it is possible to apply different PBO values to different bandwidths instead of applying the same PBO value across all of the transmitted frequencies. 
       FIG. 5  depicts an exemplary transceiver  170  that may be used to implement any of the HDSL2 transceivers  118 ,  127 ,  148 , or  152  of  FIGS. 2 and 3 . The transceiver  170  of  FIG. 5  comprises a pair of digital data ports  174  and  176 . If the transceiver  170  is used to implement HDSL2 transceiver  152  of  FIG. 3 , then the digital data port  174  is coupled to and receives digital data from the buffer  155 . Further, the digital data port  176  is coupled to and transmits digital data to the buffer  156 . If the transceiver  170  is used to implement HDSL2 transceiver  148  of  FIG. 3 , then the digital data port  174  is coupled to and receives digital data from the buffer  156 . Also, the digital data port  176  is coupled to and transmits digital data to the buffer  155 . 
     As shown by  FIG. 5 , the transceiver  170  comprises a transmitter  181  and a receiver  183 . The transmitter  181  has a mapper  184  for mapping the digital data from the digital data port  174  to symbols using a selected constellation, as will be described in more detail hereinbelow. Thus, the transmitter  181  outputs a data signal that comprises the digital data received from the digital data port  174 . A digital filter  185  receives and filters the data signal output by transmitter  181  to provide a filtered digital signal to a digital-to-analog (D/A) converter  188 . The D/A converter  188  converts the filtered digital signal into an analog signal, which is filtered by an analog filter  191 . This filtered analog signal is then applied, via a hybrid network  194  and a line-coupling transformer  196 , to the telecommunication line segment  31  or  32  that is coupled to the transformer  196 . In this regard, if the transceiver  170  is used to implement HDSL2 transceiver  127  or  152 , then the transformer  196  is coupled to telecommunication line segment  32 . If the transceiver  170  is used to implement HDSL2 transceiver  118  or  148 , then the transformer  196  is coupled to telecommunication line segment  31 . 
     An analog signal on the telecommunication line segment  31  or  32  is coupled through transformer  196  and hybrid network  194  and is applied to an analog filter  202 , which filters the received analog signal and provides a filtered analog signal to an analog-to-digital (A/D) converter  204 . The A/D converter  204  converts the filtered analog signal into a digital signal, which is filtered by a digital filter  207 . A differential summer  209  combines this filtered digital signal with an echo cancellation signal from an echo canceller  212  in order to cancel, from the filtered digital signal, echoes of signals transmitted by the transceiver  170  over the telecommunication line segment  31  or  32  that is coupled to the transformer  196 . The combined signal from the differential summer  209  is then coupled through a channel equalizer  208  to remove intersymbol interference (ISI) and then received by the receiver  183 . 
     The receiver  183  has a decoder  213 , such as a Viterbi decoder, for example, although other types of decoders may be used in other examples. The decoder  213  has an inverse mapper  214 , which maps the symbols received from the equalizer  208  to digital data using a selected constellation, as will be described in more detail hereinbelow. Such digital data is transmitted to the digital data port  176 , which outputs this digital data from the transceiver  170 . 
       FIGS. 6 and 7  depict exemplary embodiments of the transmitter  181  and receiver  183 , respectively. For illustrative purposes, transmitter  181  will be described hereafter as performing Trellis coded PAM to provide code words of four payload bits and one error checking bit. Such code words are mapped into symbols using a constellation that provides a constellation density of 4.0. However, in other examples, other configurations of the transmitter  181  and receiver  183  are possible. For example, it is unnecessary for error correction encoding to be performed, and data words of other bit lengths may be used. Further, other types of coding may be used to encode and decode code words, and constellations having different constellation densities may be used in other embodiments. 
     Digital data from the data port  174  ( FIG. 5 ) is respectively framed and scrambled by framer  242  and scrambler  244 . A serial-to-parallel converter  247  converts the serial stream of data from scrambler  244  to four-bit data words. A Trellis encoder  249  encodes each four-bit data word with an additional error checking bit to provide a five-bit data word. The mapper  184  maps each five-bit data word into a symbol using a constellation that provides a constellation density of 4.0. In such an embodiment, the constellation has thirty-two different symbols (i.e., one symbol for each possible value of the five-bit encoded data words). Note that encoding techniques other than Trellis encoding may be employed by encoder  249  in other embodiments. 
     In  FIG. 7 , the signals received by receiver  183  are decoded into five bit code words by decoder  213 . Each code word comprises four payload bits and one error correction bit. The decoder  213  outputs the four payload bits of each code word to a parallel-to-serial converter  255  that converts the data words into serial data. A descrambler  257  and deframer  259  then respectively descramble and deframe the data from the parallel-to-serial converter  255 . 
     As shown by  FIG. 5 , the transceiver  170  comprises control logic  220  that is in communication with the transmitter  181  and the receiver  183 . Note that the control logic  220  may be implemented in hardware, software, or a combination thereof. The control logic  220  is configured to dynamically select a constellation based on its constellation density and to then instruct the mapper  184  and mapper  214  to use the selected constellation. 
     In this regard, if the transceiver  170  is used to implement the HDSL2 transceiver  127  ( FIG. 2 ) or  152  ( FIG. 3 ), then the control logic  220  preferably instructs the transmitter  181  and receiver  183  to use a constellation providing a constellation density of 4.0. In such an embodiment, the mapper  184  maps each five-bit data word (four payload bits and one error checking bit) received from encoder  249  ( FIG. 6 ) to a single respective symbol, as described above. As a result, each symbol communicated across telecommunication line segment  32  contains four bits of payload information. 
     However, if the transceiver  170  is used to implement the HDSL2 transceiver  118  or  148  ( FIG. 3 ), then the control logic  220  preferably instructs the transmitter  181  and receiver  183  to use a constellation providing a constellation density of 3.0. In such an embodiment, the mapper  184  encodes each four-bit data word (three payload bits and one error checking bit) received from encoder  249  ( FIG. 6 ) to a single respective symbol. Note that central office HDSL2 transceiver  118  is similarly configured to use the same type of constellation that is used by transceiver  148 . Therefore, each symbol communicated across telecommunication line segment  31  contains three bits of payload information. 
     Various techniques may be used to enable the control logic  220  to select the appropriate constellation. In one exemplary embodiment, the control logic  220  selects the appropriate constellation based on a duration of at least one pulse received by the transceiver  170  during training. In this regard, according to current standards, transceivers implemented at a central office (C.O.) transmit 0.2 second pulses, and non-C.O. transceivers transmit 0.3 second pulses. The control logic  220  preferably uses this pulse duration difference to select a constellation providing the appropriate constellation density. 
     In particular, in block  231  of  FIG. 8 , the control logic  220  determines whether the duration of pulses received by the transceiver  170  during training is 0.2 seconds or 0.3 seconds. If the transceiver  170  is receiving 0.2 second pulses during training, then the control logic  220  determines that the transceiver  170  is in communication with another transceiver that is located at a central office. In such an embodiment, the control logic  220  instructs the transmitter  181  and receiver  183  to use a constellation providing a constellation density of 3.0, as shown by blocks  235  and  236  of  FIG. 8 . However, if the receiver  183  is receiving 0.3 second pulses during training, then the control logic  220  determines that the transceiver  170  is in communication with a non-C.O. transceiver. In such an embodiment, the control logic  220  instructs the transmitter  181  and receiver  183  to use a constellation providing a constellation density of 4.0, as shown by blocks  238  and  236 . 
     Thus, in operation, the HDSL2 transceivers  127  ( FIG. 2) and 152  ( FIG. 3 ) enter into a training mode prior to entering into a data mode. In the training mode, transceiver  152  transmits 0.3 second pulses across telecommunication line segment  32  similar to existing standards and protocols. Based on the duration of the 0.3 second pulses, transceiver  127  selects a constellation providing a constellation density of 4.0. After completing the training mode and entering into the data mode, the HDSL2 transceivers  127  and  152  communicate with each other using the selected constellation. Signals received by the transceiver  152  from the telecommunication line segment  32  are demodulated to recover digital data, which is buffered by buffer  156 . Digital data buffered by buffer  155  is modulated by the transceiver  152  before being transmitted across telecommunication line segment  32 . 
     In another embodiment, it is possible for transceiver  127  to be coupled to a central office transceiver (not shown) over a repeaterless telecommunication line segment. In such an embodiment, the transceiver  127  receives 0.2 second pulses instead of 0.3 second pulses in accordance with existing standards and protocols, and the transceiver  127  would, therefore, select a constellation providing a constellation density of 3.0. 
     The other transceivers  118 ,  148 , and  152  may be similarly configured to select the appropriate constellation density based on training signals such that the transceivers  118  and  148  select and use a constellation providing a constellation density of 3.0 and such that the transceiver  152  selects and uses a constellation providing a constellation density of 4.0. However, it is possible for any of the transceivers  118 ,  127 ,  148 , and  152  to be hardcoded to select and use the appropriate constellation. For example, in one exemplary embodiment, the remote transceiver  127  is configured to adaptively select, based on training signals, a constellation density providing a constellation density of 4.0. However, the transceivers  118  and  148  are hardcoded to use a constellation providing a constellation density of 3.0, and the transceiver  152  is configured to use a constellation providing a constellation density of 4.0. 
     It should be noted that repeater  135  has generally been described as operating in an HDSL2 environment. However, the techniques described herein for making HDSL2 signals spectrally compatible with ADSL signals may be employed in other types of environments to make other types of signals (i.e., non-HDSL2 signals) spectrally compatible with other signals communicated through the same cable. In addition, the transceivers  118 ,  148 ,  152 , and  127  have been described herein as using PAM. However, in other embodiments, other types of modulation may be employed such as quadrature amplitude modulation (QAM), for example.