Abstract:
A method and apparatus for achieving longer data transmission distances in Symmetrical Digital Subscriber Lines (SDSL) systems, while providing spectral compatibility with Asymmetrical Digital Subscriber Lines (ADSL) systems is disclosed. The method and apparatus enables deployment of SDSL and ADSL in any mix in a binder group of twisted pair copper wires without sacrificing performance of either system. The apparatus transmits signals from opposite ends of the twisted pair subscriber loop using first and second discrete frequency bands. The advantages include longer service reach and deployment without regard to ADSL/SDSL services mix in a binder group.

Description:
CROSS-REFERENCE TO RELATED PROVISIONAL APPLICATION 
     This application claims priority from Applicant&#39;s Provisional Application Serial No. 60/123,668 filed on Mar. 9, 1999 and entitled LONG REACH SDSL SYSTEM SPECTRALLY COMPATIBLE WITH ADSL SYSTEMS. 
    
    
     TECHNICAL FIELD 
     The invention relates to the delivery of data services to customer premises, and in particular to a Symmetrical Digital Subscriber Line (SDSL) system adapted to be spectrally compatible with an Asymmetric Digital Subscriber Line (ADSL) standard system. 
     BACKGROUND OF THE INVENTION 
     SDSL is a high-speed data service that can be provided over a standard telephone line (twisted copper wire pair) referred to as a “subscriber loop”. Unlike Asynchronous Digital Subscriber Line (ADSL) which provides fast downstream data transport to a subscriber&#39;s premises and slow upstream data transport to the service provider, SDSL provides data transport at the same speed in each direction. Many World Wide Web-based applications that benefit from SDSL are now being developed and the demand for the service is growing. 
     Traditional SDSL systems achieve symmetry in the data rates by using the same modulation technique, using the same signal bandwidth and transmitting at the same output power from each end of the communication medium. The use of that type of signals creates Near End CrossTalk (NEXT) between co-located transmitters and receivers and limits the service reach, i.e. the length of the subscriber loop over which the service is supported is limited. 
     Various techniques have been proposed for extending the data transmission reach of SDSL systems. These techniques include the use of coding gain to permit operation at very low SNR, and the use of higher Pulse Amplitude Modulation indexes to reduce the bandwidth of the transmitted signal in order to operate in a frequency band where Near End CrossTalk (NEXT) interference is not severe. Each of these techniques provide only a small incremental benefit. 
     Another proposed technique for extending SDSL reach is the use of signal repeaters. This however creates additional costs and operational problems in the deployment, the powering and the maintenance of such repeaters. The use of signal repeaters also creates a spectral compatibility problem by introducing high signal levels into the cable at a location where the signal levels are normally low. This problem can be solved by installing a signal repeater on all the pairs carrying a digital data signal and sharing the same cable, but is very difficult to manage when the cable is shared by multiple service providers. For those reasons, it is preferable to deploy digital data services without using signal repeaters. 
     Data services are also offered using Asymmetric Digital Subscriber Line (ADSL) service over copper twisted pairs. ADSL systems are adapted to reduce NEXT interference from other ADSL equipment using a Frequency Division Multiplexing (FDM) technique to transmit and receive signals. Data transmission over ADSL systems is regulated under the ANSI T1.413 standard which defines a spectral density for the upstream and downstream frequency bands. 
     When ADSL data service is provided along with SDSL service from the same central location, via the same cable and over adjacent twisted copper pairs, NEXT interference cannot be reduced in either system using signal filters because the interference is within the input frequency band of each receiver. Managing cross-system interference generates high operating costs to service providers, which is ultimately passed on to subscribers. 
     Solutions that have been proposed to mitigate the interference of SDSL and ADSL services deployed over the same cable include deployment rules that limit the number of SDSL and/or ADSL subscriber loops per cable or cable binder, or require binder group segregation between SDSL and ADSL systems. This solution introduces additional operating costs when both services are provided by a single service provider, and does not support a de-regulated environment in which multiple service providers provide data services from the same central location. 
     With the rapid deployment of consumer ADSL services, the demand for SDSL services is expected to increase. SDSL services are important, for example, in web site provisioning. Satisfying the demand for SDSL services should not be done at the expense of degrading the performance of ADSL services. 
     There is therefore a need for apparatus and methods to provide a long data transmission reach for the delivery of SDSL services while reducing NEXT interference. Reduction of NEXT interference between SDSL and ADSL services deployed over the same cable is also desirable. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to enable the deployment of SDSL and ADSL services from the same copper plant by reducing electromagnetic interference between SDSL and ADSL services deployed in the same binder group. 
     It is a further object of the invention to enable the deployment of SDSL and ADSL services on adjacent copper pairs in the same binder group. 
     In accordance with one aspect of the invention, there is provided a method of transmitting Symmetric Digital Subscriber Line (SDSL) signals over twisted pair copper wires in a cable that interconnects a central location having a plurality of Digital Transmission Units (DTU-C) with a plurality of subscriber premises respectively having at least one Digital Transmission Units (DTU-R), the method comprises a first step of dividing a transmit power spectrum into an upstream frequency band and a downstream frequency band. Signals from the DTU-R are transmitted using the upstream frequency band, and signals from the DTU-C are transmitted using the downstream frequency band. 
     In accordance with another aspect of the invention, there is provided an apparatus for transmitting Symmetric Digital Subscriber Line (SDSL) signals over twisted pair copper wires in a cable that interconnects a central location having a plurality of Digital Transmission Units (DTU-C) with a plurality of subscriber premises respectively having at least one Digital Transmission Units (DTU-R). The apparatus comprises a Digital Transmission Unit (DTU-R) for transmitting signals from the customer&#39;s premises using an upstream frequency band, and a Digital Transmission Unit (DTU-C) for transmitting signals from the central location using a downstream frequency band. The upstream and the downstream frequency bands are discrete frequency bands that do not overlap. 
     In accordance with yet a further aspect of the invention, there is provided a method of providing Asynchronous Digital Subscriber Line (ADSL) and Symmetric Digital Subscriber Line (SDSL) services from a central location, comprising a step of offering the SDSL service using Digital Transmission Units (DTU) at the central location (DTU-C) that generate transmit signals in a first frequency band, and Digital Transmission Units (DTU) at customer premises (DTU-R) that generate transmit signals in a second frequency band that does not overlap the first frequency band. The first and second frequency bands are spectrally compatible with corresponding frequency bands used by the ADSL service. 
     In accordance with yet another aspect of the invention, there is provided a method of providing extended-reach Symmetric Digital Subscriber Line (SDSL) service, comprising using Digital Transmission Units (DTU) at a central location (DTU-C) that generate transmit signals in a first frequency band, and Digital Transmission Units (DTU) at customer&#39; premises (DTU-R) that generate transmit signals in a second frequency band that does not overlap the first frequency band. 
     The invention thereby provides methods that enable widespread deployment of SDSL at a reduced cost while improving the service range by increasing the SDSL service reach. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be explained by way of example only, and with reference to the following drawings, in which: 
     FIG. 1 is a schematic diagram showing elements used in providing data services from a central location; 
     FIG. 2 is a spectral density diagram showing a signal power distribution of a traditional SDSL system; 
     FIG. 3 is a spectral density diagram showing a signal power distribution of an ADSL service in accordance with the ANSI T1.413 standard; 
     FIG. 4 is a spectral density diagram showing a signal power distribution in providing SDSL services in accordance with a preferred embodiment of the invention; 
     FIG. 5, which appears on sheet one of the drawings, is a block diagram of a DTU-C in accordance with a preferred embodiment of the invention; 
     FIG. 6, which also appears on sheet one of the drawings, is a block diagram of a DTU-R in accordance with a preferred embodiment of the invention; 
     FIG. 7 is a table defining signal parameters as they apply to an embodiment of the invention using QAM/CAP signal modulation techniques for downstream transmission signals and PAM signal modulation techniques for the upstream transmission signals; 
     FIG. 8 is a table defining signal parameters as they apply to an embodiment of the invention using QAM/CAP signal modulation techniques for both downstream and upstream transmission signals; and 
     FIG. 9 is a table defining carrier allocations as they apply to two embodiments of the invention using DMT signal modulation techniques for the downstream and upstream transmission signals. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The invention relates to digital transmission systems adapted to provide symmetrical data communications over a voice communication network such as a distribution sector of a Public Switched Telephone Network (PSTN). 
     The provision of data services from a central office in a telephone network is well known. As shown in FIG. 1, in the provision of such services a plurality of Digital Transmission Units (DTU-Cs)  30 , 40  are typically located at a central location  10 . The plurality of DTU-Cs communicate with a plurality of distributed Digital Transmission Units (DTU-R)  50 , 60  located at customer premises. Communication takes place between the DTU-Cs and the DTU-Rs over a shared medium such as a cable or a set of cables containing a plurality of twisted copper wire pairs referred to as “local loops”. The effective data transmission rate of a DTU-C to a DTU-R (downstream direction) is the same as the effective data transmission rate of the DTU-R to DTU-C (upstream direction). The service is referred to as a Symmetrical Digital Subscriber Line (SDSL) service. 
     Each DTU-C is connected to a DTU-R by a single twisted copper pair  24 . However because several twisted copper pairs are bundled together in a cable  20 , in what is referred to as binder groups, some signal leakage or “crosstalk” occurs between adjacent twisted copper pairs  24  when data is transmitted. This crosstalk causes interference in adjacent DTU-Cs and DTU-Rs and affects the quality of the received signal. 
     Traditional SDSL systems achieve symmetry in the data rates by using the same modulation technique, the same signal bandwidth, and transmitting at the same output power from each end of the twisted pair  24 . A graphical representation of the power spectral density is shown in FIG.  2 . The crosstalk between adjacent twisted copper pairs  24  causes a signal interference to occur between the output transmitter of a DTU-C  30  to the input receiver of another DTU-C  40 . There is also a similar signal interference path from the output transmitter of a DTU-R  50  and the input receiver of the DTU-R  60 . This interference path induces the highest level of interference in each unit when the two DTU-Rs  50 , 60  are co-located. If the units are not co-located, the DTU-R  50  is partially protected from noise induced by a signal originating from the DTU-R  60  due to attenuation resulting from signal propagation over a second section of cable  20 . 
     However, with respect to interference induced by the transmitter of DTU-R  50  in the receiver of DTU-R  60 , the effective Signal-to-Noise Ratio (SNR) remains constant regardless of the length of the second section of cable  20  and is the same as if DTU-Rs  50  and  60  were co-located at the location of DTU-R  50 . This is because a downstream signal transmitted from DTU-C  40  and the crosstalk signal generated by an upstream signal transmitted by DTU-R  50  are subject to substantially the same attenuation over the length of the second section of cable  20  as the signals propagate towards the DTU-R  60 . 
     Interference from a transmitter co-located with a receiver is called Near-End CrossTalk (NEXT) and limits the distance (reach) over which the DTU-C  40  and the DTU-R  60  can communicate. If the same modulation technique and the same signal bandwidth are used at each end of the twisted pair  20 , NEXT interference is within the input frequency band of the receiver and cannot be segregated out using signal filters. 
     FIG. 3 shows signal power distribution of an Asymmetrical Digital Subscriber Line (ADSL) service in accordance with the ANSI T1.413 standard. 
     FIG. 4 shows a signal power distribution for providing SDSL services in accordance with a preferred embodiment of the invention. In accordance with the invention, Frequency Division Multiplexing (FDM) is applied to SDSL systems. For spectral compatibility with ADSL systems, the low frequency band is reserved for DTU-R to DTU-C (upstream) communication and the high frequency band is reserved for DTU-C to DTU-R (downstream) communication. 
     FIG. 5 shows a block diagram of a DTU-C unit  50  in accordance with a preferred embodiment of the invention. A signal modulator  22  produces a modulated high frequency data signal that is passed through a high-pass filter  32  to remove unwanted signal energy within a local receiver low frequency band, and provide a high frequency transmit signal. A 4-wire to 2-wire conversion circuit  34  applies the high frequency transmit signal to the twisted copper pair  24  and extracts a low frequency received signal from it. The 4-wire to 2-wire conversion circuit  34  therefore operates as a signal coupler to couple the high frequency transmit signal to the twisted pair  24  and as a signal decoupler to decouple the received low frequency signal from the twisted pair  24 . 
     The low frequency received signal contains the low frequency data signal transmitted from the distant DTU-R  70  (FIG. 6) combined with the output signal of the DTU-C  50 , as attenuated by the high-pass filter  32 , the 4-wire to 2-wire conversion circuit  34 , and combined with the NEXT noise from any other DTU-C sharing the same cable with the twisted pair  24 . The received low frequency signal is passed through a low-pass filter  36  that is designed to remove the combined interference resulting from each of the DTU-C output signals. The received signal low frequency signal is then passed to a signal demodulator  38 . 
     The attenuation characteristic of the high-pass filter  32  is based on the level of out-of-band energy present at the output of the signal modulator  22  within the pass-band of the signal demodulator  38 , combined with the signal rejection achieved through the 4-wire to 2-wire conversion circuit  34  and the level of the received upstream signal. The resulting SNR must enable adequate reception of the DTU-R upstream signal. Care must be taken to ensure adequate linearity in the high-pass filter  32  and the 4-wire to 2-wire circuit  34  in order to avoid generating distortion products that could replace the unwanted signal energy that was filtered out. 
     The attenuation characteristic of the low-pass filter  36  is based on the transmit signal rejection achievable through the 4-wire to 2-wire conversion circuit  34 , combined with the level of the received upstream signal at the end of the longest twisted pair  24 , and the dynamic range available at the input of the signal demodulator  38 . 
     FIG. 6, is a block diagram of a DTU-R unit  70  in accordance with a preferred embodiment of the invention. A signal modulator  26  produces a modulated low frequency data signal that is passed through a low-pass filter  42  to remove the unwanted signal energy within the local receiver high frequency band and provide a low frequency transmit signal. 
     A 4-wire to 2-wire conversion circuit  44  applies the low frequency transmit signal to the twisted copper pair  24  and extracts a received high frequency downstream signal from the twisted copper pair. The 4-wire to 2-wire conversion circuit  44  therefore operates as a signal coupler to couple the low frequency transmit signal to the twisted pair  24  and as a signal decoupler to decouple the received high frequency signal from the twisted pair  24 . 
     The high frequency received signal contains the high frequency data signal transmitted from the distant DTU-C  50  (FIG. 5) combined with the transmit signal of the DTU-R  70 , as attenuated by the low pass filter  42 , the 4-wire to 2-wire conversion circuit  44  and combined with the NEXT noise from all other DTU-Rs sharing the same cable  20  (FIG. 1) with the twisted pair  24 . The received downstream signal is passed through a high-pass filter  42  designed to remove the combined interference resulting from all the DTU-R output signals. The received downstream signal is then passed to signal demodulator  48 . 
     The DTU-R low-pass filter  42  attenuation characteristic is based on the level of out-of-band energy present at the output of the signal modulator  26  within the pass-band of the signal demodulator  48 , combined with the signal rejection achieved in the 4-wire to 2-wire conversion circuit  44  and the level of the downstream signal received on the twisted pair  24 . The resulting SNR must be adequate to permit reception of the DTU-C downstream signal. Care should be taken to ensure adequate linearity in the low-pass filter  42  and the 4-wire to 2-wire circuit  44  so as to avoid generating distortion products that could replace the unwanted signal energy that was filtered out. 
     The DTU-R high-pass filter  46  attenuation characteristic is based on the transmit signal rejection achieved in the 4-wire to 2-wire conversion circuit  44  combined with the level of the downstream signal received at the end of the longest twisted pair  24  and the dynamic range available at the input of the signal demodulator  48 . 
     The 4-wire to 2-wire conversion circuits  34  and  44  can be any one of several circuits well known in the art, including passive and active hybrid circuits. An active echo-canceller can also be used to increase the effective trans-hybrid loss. 
     FIG. 4 shows the frequency spectrum occupancy of the DTU-C to DTU-R (downstream) and of the DTU-R to DTU-C (upstream) signals. This selection of signal power spectral densities provides spectral compatibility with ADSL systems, and complies with the ANSI T1.413 standard. For compatibility with that standard, the power spectral density of the transmitted signal is set to −40 dBm/Hz in the DTU-C to DTU-R (downstream) direction and to −38 dBm/Hz in the DTU-R to DTU-C (upstream) direction, but other power spectral densities may also be used, if desired. For further spectral compatibility with ADSL systems complying with the ANSI T1.413 standard, the maximum frequency used in the DTU-R to DTU-C (upstream) direction is set to approximately 140 kHz. Of course, a different maximum frequency could be used, if spectral compatibility with ANSI T1.413 is not required. 
     The signal in the DTU-C to DTU-R (downstream) direction occupies a wider bandwidth than the signal in the DTU-R to DTU-C (upstream) direction because of higher cable attenuation at higher frequencies, which requires a lower modulation density. This constraint requires a wider downstream bandwidth in order to convey data at a rate equal to that used in the upstream direction. 
     In accordance with the invention, because the signal used in the DTU-R to DTU-C (upstream) direction is in the low frequency sector of the frequency spectrum, the upstream signal can be modulated by signal modulator  26  using any variant of Pulse Amplitude Modulation (PAM), including 2B1Q coding. Alternatively, the upstream signal can be modulated using a pass-band modulation scheme such as Quadrature Amplitude Modulation (QAM) or Carrier-less Amplitude and Phase (CAP) modulation by adjusting the signal center frequency as close as possible to DC while avoiding signal spill-over around 0 Hertz. A Discrete Multi-Tone (DMT) modulation can also be used to modulate the upstream signal by selecting carrier frequencies close to DC. 
     In accordance with the invention, as the signal used in the DTU-C to DTU-R (downstream) direction is in the high frequency part of the frequency spectrum, the downstream signal can be modulated using any variant of Quadrature Amplitude Modulation (QAM) or Carrier-less Amplitude and Phase (CAP) modulation, by adjusting the signal centre frequency to avoid an overlap with the DTU-R to DTU-C (upstream) signal. A Discrete Multi-Tone (DMT) modulation can also be used to modulate the downstream signal by selecting carrier frequencies to avoid the overlap with the upstream signal. 
     FIGS. 7-9 are tables that provide examples of frequency-divided SDSL signals in accordance with the invention for achieving symmetrical data rates over telephone grade twisted copper pairs, while being spectrally compatible with ANSI T1.413 ADSL systems. The frequency division properties of the SDSL systems in accordance with the invention as well as the spectral compatibility with ADSL ensure maximum reach of both systems by reducing NEXT interference within the SDSL systems and across SDSL and ADSL systems. SDSL and ADSL services can therefore be offered on twisted copper pairs in the same binder group without noticeable degradation of the performance of either one. 
     In the examples described below, it is assumed that the SDSL service is not combined with a Plain Old Telephone Service (POTS) on the same twisted copper pair. Therefore, it is not necessary to preserve a large frequency separation between the voice frequency band and the data signal frequency band. 
     FIG. 7 shows signal options for using a QAM/CAP modulated SDSL downstream signal and a PAM modulated SDSL upstream signal. A different downstream center frequency could be used, while still preserving the frequency division nature of the signals. However, using a different downstream center frequency has the effect of widening or narrowing a gap between the upstream and the downstream signals, which may impact the complexity of the band split filters used. As the PAM modulated signal is a baseband signal, the low 3-dB frequency and center frequency do not apply. The use of a symbol transfer rate of 260 kbaud is assumed in the given examples. 
     The various modulation densities presented in FIG. 7 permit a reduction in data rate in order to increase the distance at which SDSL service may be provided. A rate adaptive system can automatically select a best data rate based on signal attenuation on a twisted copper pair over which the SDSL service is offered. Other techniques known in the art may also be employed to select a most appropriate data rate. 
     FIG. 8 shows signal options for using a QAM/CAP modulated SDSL downstream signal and a QAM/CAP modulated SDSL upstream signal. An excess bandwidth factor of 15% has been used for the band limiting filter of the upstream signal. A different downstream centre frequency could be used, while still preserving frequency division between the upstream and downstream signals. However, this has the effect of widening or narrowing the gap between the upstream and the downstream signals, which may impact the complexity of the band split filters used. All downstream signals use a symbol transfer rate of 260 kbaud and all upstream signals use a symbol transfer rate of 130 kbaud in the examples presented. 
     The various modulation densities presented in FIG. 8 permit a reduction in data rate in order to increase the distance at which SDSL service may be provided. A rate adaptive system can automatically select a most appropriate data rate based on the signal attenuation on a twisted copper pair over which the SDSL service is provided. Other techniques known in the art can be employed to select a most appropriate data rate. 
     The use of PAM or QAM/CAP modulation ensures low end-to-end latency as is desirable in many applications. In the cases where latency is not a concern, Discrete Multi-Tone (DMT) modulation may be used. Many integrated modulator/demodulator circuits compliant with the ANSI T1.413 standard are commercially available. Therefore, signal parameters defined in that standard may be used to provide SDSL service, in order to enable use of existing components and technology. 
     FIG. 9 shows a carrier allocation for two DMT signalling implementations in accordance with the invention. The upstream and downstream signals using DMT modulation preferably use carriers spaced by 4.3125 kHz. Each carrier transports 4 kHz of data plus an overhead associated with a synchronisation symbol and a cyclic prefix characteristic of DMT transmissions. A synchronisation symbol is transmitted after each 68 data symbols. The upstream signal uses a 4-sample cyclic prefix based on a 276 ksample conversion rate and the downstream signal uses a 16-sample cyclic prefix based on a 1104 sample conversion rate. Depending on the number of carriers required, the upstream signal modulator  26  may perform a 64-point inverse discrete Fournier Transform (IDFT) and the downstream signal modulator  22  may perform a 256-point IDFT. Other parameter selections may be used to support different implementations. Due to the 4 kHz signal processing rate, the minimum latency is 0.25 ms at each end, plus a signal processing delay. Actual latencies are commonly between 0.75 ms and 1.0 ms end-to-end, aside from delays introduced by error correction. 
     If the latency is unacceptable, the DMT signal processing rate can be increased, for example, to 8 kHz. If the signal processing rate is 8 kHz with one synchronisation symbol after each 72-data symbols and a cyclic prefix, the carrier spacing is 9.125 kHz. At that signal processing rate, the upstream signal uses a 4-sample cyclic prefix based on a 292-ksample conversion rate and the downstream signal uses a 16-sample cyclic prefix based on an 1168-ksample conversion rate. Depending on the number of carriers required, the upstream signal modulator  26  can perform a 32-point IDFT and the downstream signal modulator  22  can perform a 128-point IDFT. As will be understood by those skilled in the art, other parameters may be used for different implementations. At the 8 kHz signal processing rate, the minimum latency is 0.125 ms at each end plus the signal processing delay. Actual latencies are typically between 0.375 ms and 0.5 ms end-to-end, aside from delays introduced by error correction. 
     One carrier in the downstream direction is preferably reserved for transmission of a timing recovery pilot tone. If a pilot tone is desired in the upstream direction, another carrier may be added for that purpose. Different data rates can be achieved by changing the bit/Hertz allocation for respective carriers. 
     The embodiments of the invention described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.