Abstract:
A splitterless digital subscriber line modem adapted to be coupled to a subscriber line including a sending end and a receiving end, the modem being capable of simultaneous access to the subscriber line with other telephone equipment operating in a frequency band below four kilohertz is disclosed herein. The modem includes a data terminal and a control circuit. The data terminal couples the modem to the subscriber line. The control circuit is coupled to the data terminal and receives and transmits signals to and from the data terminal. The control circuit utilizes line coding techniques to measure signal and noise at the receiving end and adjusts amplitude of the signal in response to the signal and the noise whereby power of the signal is optimized.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to communication systems. More particularly, the present system relates to a digital subscriber line modem. 
     BACKGROUND OF THE INVENTION 
     Explosive growth of the internet and the worldwide web drives increasing demands for faster communication data rates. In the corporate world, dedicated high-speed links (perhaps T1/E1 frame relays or OC1 ATM systems) from the company to an internet access provider satisfy current needs for highspeed access or data rates. Corporate users gain access to an internet router using a local area network (LAN). The router then connects to a high-speed link (e.g., T1/E1 lines). Unfortunately, residential users of the internet do not often have a high-speed link and must rely on standard analog or plain old telephone service (POTS) lines. 
     The increasing availability of information, data programs, entertainment, and other computer applications on the worldwide web and the internet strengthens the demand for high-speed access to the home. For example, designers of web technology constantly develop new ways to provide sensory experiences, including audio and video, to users of the web. Higher-speed modems will be required so the residential user can fully interact with future web and communication technologies. 
     Although designers of modems are continuously attempting to increase data rates, analog or POTS line modems can presently only reach data rates of up to 56 kilobits per second (Kbps). These conventional analog modems transmit and receive information on POTS subscriber lines through the public switched telephone network. The internet access provider is also coupled to the switched telephone network and transmits and receives information through it to the subscriber line. 
     Some residential users utilize integrated services digital network (ISDN) equipment and subscriptions to obtain up to 128 Kbps access or data rates the use of 2 B channels. ISDN equipment and subscriptions can, however, be expensive and require a dedicated subscriber line. Thus far, neither ISDN modems nor analog modems are capable of providing 256 Kbps or higher access between the home and the internet. Over one megabit per second (Mbps) data rates with analog modems or ISDN equipment do not seem feasible at this time. 
     A variety of communication technologies are competing to provide high-speed access to the home. For example, asymmetric digital subscriber lines (ADSL), cable modems, satellite broadcast, wireless LANs, and direct fiber connections to the home have all been suggested. Of these technologies, ADSL can utilize the POTS subscriber line (the wire currently being utilized for POTS) between the home user (the residence) and the telephone company (the central office). 
     ADSL networks and protocols were developed in the early 1990&#39;s with the purpose of allowing telephone companies to provide video-on-demand service over the same wires which were being used to provide POTS. ADSL technologies include discrete multitone (DMT), carrier less amplitude and phase modulation (CAP), and other technologies. Although the video-on-demand market has been less than originally expected, telephone companies have recognized the potential application of ADSL technology for internet access and have begun limited offerings. 
     ADSL technology allows telephone companies to offer high-speed internet access. ADSL also permits telephone companies to remove internet traffic from the telephone switch network. Currently, telephone companies cannot significantly profit from internet traffic in the telephone switch network due to regulatory considerations. In contrast, ADSL allows the telephone company to charge a separate access fee for data services. The separate fee is not as restricted by regulatory considerations. 
     With reference to FIG. 1, a conventional asymmetric ADSL (ADSL) system  10  includes a copper twisted pair analog subscriber line  12 , an ADSL modem  14 , an ADSL modem  16 , a band splitter  18 , and a band splitter  20 . Line  12  is a POTS local loop or wire connecting a central office  32  of the telephone company and a user&#39;s residence  22 . 
     ADSL modem  14  is located in user&#39;s residence  22  and provides data to and from subscriber line  12 . The data can be provided from line  12  through modem  14  to various equipment (not shown) coupled to modem  14 . Equipment, such as, computers, network devices, servers, or other devices, can be attached to modem  14 . Modem  14  communicates with a data network (not shown) coupled to modem  16  across line  12 . ADSL modem  16  receives and transmits signals to and from line  12  to the data network, The data network can be coupled to other networks (not shown), including the internet. 
     At least one analog telephone  26 , located in residence  22 , can be coupled to subscriber line  12  through splitter  20  for communications across line  12  with telephone switch network  28 . Telephone  26  and telephone switch network  28  (e.g., public-switched telephone (PST) network) are conventional systems well-known in the art. Alternatively, other analog equipment, such as, facsimile machines, POTS modems, answering machines, and other telephonic equipment, can be coupled to line  12  through splitter  20 . 
     System  10  requires that band splitter  18  and band splitter  20  be utilized to separate higher frequency ADSL signals and lower frequency POTS signals. For example, when the user makes a call from residence  22  on telephone  26 , lower frequency signals (under 4 kilohertz (kHz)) are provided through band splitter  20  to subscriber line  12  and through band splitter  18  to telephone switch network  28 . Band splitter  18  prevents the lower frequency POTS signals from reaching ADSL modem  16 . Similarly, band splitter  20  prevents any of the POTS signals from reaching modem  14 . 
     ADSL modem  16  and ADSL modem  14  communicate higher frequency ADSL signals across subscriber line  12 . The higher frequency ADSL signals are prevented from reaching telephone  26  and telephone switch network  28  by band splitters  20  and  18 , respectively. Splitters  18  and  20  can be passive analog filters or other devices which separate lower frequency POTS signals (below 4 kHz) from higher frequency ADSL signals (above 25 kHz). 
     The separation of the POTS signals and ADSL signals by splitters  18  and  20  is necessary to preserve POTS voice and data traffic and ADSL data traffic. More particularly, splitters  18  and  20  can eliminate various effects associated with POTS equipment which may affect the transmission of ADSL signals on subscriber line  12 . For example, the impedance of subscriber line  12  can vary greatly as at least one telephone  26  is placed on-hook or off-hook. Additionally, the changes in impedance of subscriber line  12  can change the ADSL channel characteristics associated with subscriber line  12 . These changes in characteristics can be particularly destructive at the higher frequencies associated with ADSL signals (e.g., from 30 kHz to 1 megahertz (MHz) or more). 
     Additionally, splitters  18  and  20  isolate subscriber line wiring within residence  22 . The impedance of such wiring is difficult to predict. Further still, the POTS equipment, such as, telephone  26 , provides a source of noise and nonlinear distortion. Noise can be caused by POTS voice traffic (e.g., shouting, loud laughter, etc.) and by POTS protocol, such as, the ringing signal. The nonlinear distortion is due to the nonlinear devices included in conventional telephones. For example, transistor and diode circuits in telephone  26  can add nonlinear distortion and cause hard clipping of ADSL signals. Telephone  26  can further generate harmonics which can reach the frequency ranges associated with the ADSL signals. The nonlinear components can also demodulate ADSL signals to cause a hiss in the audio range which affects the POTS. 
     Conventional ADSL technology has several significant drawbacks. First, the costs associated with ADSL services can be quite large. Telephone companies incur costs related to central office equipment (ADSL modems and ADSL network equipment) and installation costs associated with the ADSL modems and network equipment. Residential users incur subscriber equipment costs (ADSL modems) and installation costs. 
     Installation costs are particularly expensive for the residential user because trained service personnel must travel to residence  22  to install band splitter  20  (FIG.  1 ). Although band splitter  18  must be installed at the central office, this cost is somewhat less because service personnel can install band splitter  18  within central office  32 . Also, at office  32 , splitter  18  can be included in ADSL modem  16 . However, in residence  22 , splitter  20  must be provided at the end of subscriber line  12 . 
     Additionally, ADSL equipment for the residence, such as, modem  14 , is expensive because the most complex component of modem  14  (e.g., the receiver) is located at residence  22  since high-speed transmissions are generally received within residence  22 , and lower-speed transmissions are received by central office  32 . In most internet applications, larger amounts of data are requested by the residential user rather than by the internet source. Receivers are typically much more complex than transmitters. These high-speed receivers often receive data at rates of over 6 Mbps. 
     ADSL equipment can be subject to cross-talk noise from other subscriber lines situated adjacent to subscriber line  12 . For example, subscriber lines are often provided in a closely contained bundle. The close containment can cause cross-talk from other subscriber lines to be placed on subscriber line  12 . Modem  14  must compensate for cross-talk noise. 
     U.S. application Ser. No. 08/943,484, entitled, “Splitterless Digital Subscriber Line Communication system,” filed on Oct. 3, 1997, by Henderson, et al. describes a digital subscriber line (DSL) communication system which does not require the use of a splitter in the residence. The splitterless communication system allows a DSL modem to be connected directly to the subscriber line similar to the use of a conventional analog modem. The DSL modem used in the splitterless communication system is less expensive and does not utilize a considerably expensive high-speed receiver which operates at data rates over 2 Mbps. 
     As mentioned above, however, the presence of transistor and diode circuits in telephones can add non-linear distortion and cause hard clipping of ADSL signals. Non-linear components can also demodulate ADSL signals to cause a hiss in the audio range. The demodulation, distortion, and hard clipping which in conventional ADSL systems is shielded to a large degree by band splitter  20  can affect splitterless ADSL systems much more severely, since three is no band splitter at the user&#39;s residence. 
     Thus, there is a need for a power cutback level in splitterless DSL systems that achieves acceptable levels of noise reduction. Further, there is a need for reducing power as much as possible while preserving the signal to noise ratio at an acceptable level. Further still, there is a need to counter the demodulation effects of non-linear telephone devices on the telephone line. 
     SUMMARY OF THE INVENTION 
     One embodiment of the invention relates to a splitterless digital subscriber line modem adapted to be coupled to a subscriber line including a sending end and a receiving end, the modem being capable of simultaneous access to the subscriber line with other telephone equipment operating in a frequency band below four kilohertz. The modem includes a data terminal and a control circuit. The data terminal couples the modem to the subscriber line. The control circuit is coupled to the data terminal and receives and transmits signals to and from the data terminal. The control circuit utilizes line coding techniques to measure signal and noise at the receiving end and adjusts amplitude of the signal at the sending end in response to the signal and the noise whereby power of the signal is optimized. 
     Another embodiment of the invention relates to a communication system for use with a subscriber line. The communication system includes a user splitterless digital subscriber line modem, a splitter, and an office digital subscriber line modem. The user splitterless digital subscriber line modem is located at a office site and is coupled directly to the subscriber line. The modem receives downstream signals from the subscriber line and transmits upstream signals to the subscriber line. The office digital subscriber line modem utilizes line coding techniques to measure signal and noise at the office site and transmits control signals to the user splitterless digital subscriber line modem allowing it to adjust amplitude of the signal in response to the signal and the noise whereby power of the signal is optimized. The splitter is located remote from the user site and has a signal terminal, a lower frequency path terminal, and a higher-frequency path terminal. The signal terminal is coupled to the subscriber line. The lower frequency path terminal is coupled to a switched telephone network. The office digital subscriber line modem is coupled to the higher frequency path terminal. The office digital subscriber transmits the down stream signals to the subscriber line to the splitter and receives the upstream signals from the subscriber line through the splitter. 
     Another embodiment of the invention relates to a method of optimizing total transmitted power over a subscriber line including a sending end and receiving end in a splitterless asynchronous digital subscriber line (ADSL) system. The method includes utilizing line coding techniques to measure signal and noise at the receiving end and adjusting signal amplitude at the sending end based on signal and noise measured at the receiving end. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will become more fully understood from the following detailed description, taken in conjunction with the accompanying drawings, wherein like reference numerals refer to like parts, in which: 
     FIG. 1 is a schematic block diagram of a prior art ADSL communication system; 
     FIG. 2 is a schematic block diagram of a communication system in accordance with an exemplary embodiment of the present invention, the communication system includes a DSL modem in accordance with another exemplary embodiment of the present invention; 
     FIG. 3 is a more detailed schematic block diagram of the DSL modem illustrated in FIG. 2, the DSL modem includes a control circuit in accordance with yet another exemplary embodiment of the present invention; 
     FIG. 4 is a more detailed schematic block diagram of the control circuit illustrated in FIG. 3; and 
     FIG. 5 is a state diagram showing an example of the operation of DSL modem illustrated in FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 2, a DSL communication system  50  includes a copper twisted pair subscriber line  52 , a customer or residential DSL modem  54 , a central office DSL modem  56 , and a band splitter  58 . Subscriber line  52  is a local loop, such as, a twisted pair of American wire gauge (AWG)  24  or  26  copper wires, which connects a central office  60  and a residence  62 . Residence  62  can also be an office, building, or other facility. Similarly, central office  60  can be any facility associated with a provider of telephone services. 
     DSL modem  56  is coupled to a data network  64 . Splitter  58  has a signal input  66  coupled to subscriber line  52 , a higher-frequency output  68  coupled to DSL modem  56 , and a lower-frequency output  70  coupled to a telephone switch  72 . Telephone switch  72  is coupled to a POTS network  74 . DSL modem  56 , splitter  58 , and telephone switch  72  are preferably located in central office  60 . Alternatively, splitter  58  could be included as part of DSL modem  56  (e.g., DSL modem  56  is provided as an in-line device between subscriber line  52  and switch  72 ). 
     In residence  62 , one or more telephones  80 , analog facsimile machine  81 , and analog modem  82  can be coupled directly to subscriber line  52  as is well known in the art. Telephone  80  can be any conventional communication devices, including answering machines, which can be coupled to subscriber line  52  for providing various POTS functions. 
     DSL modem  54  is coupled directly to subscriber line  52  at a data terminal, input/output or subscriber line access  55 . DSL modem  54  is also coupled to a computer  84 . Alternatively, DSL modem  54  could be coupled to other devices (not shown), such as, a network, server, or other communication or computing device. 
     Unlike conventional ADSL or DSL communication systems, such as, system  10  described with reference to FIG. 1, DSL modem  54  does not utilize a splitter between modem  54  and subscriber line  52  and between telephones  80  and subscriber line  52 . DSL modem  54  advantageously utilizes digital signal processing techniques to adapt to varying subscriber line characteristics due to analog equipment, such as, telephones  80 , machine  81 , and modem  82 , thereby eliminating the need for a splitter in residence  62 . DSL modem  54  can operate concurrently with any of telephones  80 , machine  81 , and analog modem  82 . 
     DSL modem  54  preferably includes subscriber line access  55  which is part of a standard connector, such as, an RJ 11  walljack, and is coupled to subscriber line  52  similarly to conventional telephones  80  and analog modems  82 . Access  55  is preferably a two-wire terminal. 
     Modem  54  can be provided as an internal device in computer  84 , such as, on a PCI card, or as an external device. Preferably, modem  54  is an internal device so that high speed communications between modem  54  and computer  84  are not slowed by serial ports associated with computer  84 . As an external device, modem  54  could be coupled through a printer port or a universal serial bus (USB) to computer  84 . 
     Modem  54  preferably adjusts the amplitude of the signal transmitted at access  55  in response to signal-to-noise ratios at access  65  associated with modem  56 . The amplitude can be advantageously adjusted by modem  54  on a tone-by-tone basis to optimize the reception of the signal at access  65  while minimizing the potential for nonlinear interference from telephone  80 , fax machine  81 , and/or analog modem  82 . The same adjustment can take place in the downstream direction, that is, modem  56  can preferably adjust the amplitude of the signal transmitted at access  65  in response to signal-to-noise ratios at access  55  associated with modem  54 . Preferably, cutbacks are possible up to 30 dB in the upstream and downstream directions. Some systems proposed to the International Telecommunications Union (ITU) attempt to solve the problem of high noise level by adjusting total transmitted power (i.e. power cutback) by only 6 to 9 dB in the upstream direction, and up to 12 dB in the downstream direction. However, these proposed systems do not achieve the necessary level of noise reduction. 
     The power cutback approach requires changes in the start of negotiation. Specifically, both the receive end and sending end of the communication line must indicate to each other how much power cutback is possible and how many data bits would be able to be sent per symbol. In one embodiment where DMT line coding techniques are used, the bits and gains determination algorithm is altered to reflect the negotiation between the receive end and sending end. 
     With reference to FIG. 3, modem  54  includes a band-pass filter  57  coupled between access  55  which is coupled to subscriber line  52 . Modem  54  also includes a band-pass filter  57 ′ coupled between access  55  which is coupled to subscriber line  52 . Band-pass filters  57  and  57 ′ preferably have a threshold frequency above 4 kHz and beneath the lowest band carrier edge for the DSL signals to prevent POTS signal from entering modem  54 . Filters  57  and  57 ′ can be passive filters with a threshold frequency of 10 kHz. As a person of ordinary skill in the art would understand, although multiple blocks are shown in FIG. 3 for band-pass filters  57  and  57 ′, these functional blocks can be implemented with single components. 
     In one exemplary embodiment, modem  54  includes a receive control circuit  92  and a transmit control circuit  93  which operate with the discrete multitone (DMT) line coding technique. Other line coding techniques, such as carrier less amplitude and phase (CAP) techniques, may be used. 
     In the embodiment using DMT line coding, receive control circuit  92  includes an error processor  96 , a fast fourier transform (FFT) circuit  98 , a detector  100 , a frequency domain equalizer  104 , and an error message processor  106 . Additionally, an automatic gain control circuit (AGC)  102  is disposed between band-pass filter  57  and FFT circuit  98 . Transmit control circuit  93  includes an inverse fast fourier transform (FFT) circuit  98 ′, a tone amplitude module or circuit  94 , and an error message creator  108 . Additionally, a transmit gain scaler circuit  102 ′ is disposed between band-pass filter  57 ′ and inverse FFT circuit  98 ′. 
     Circuits  102  and  102 ′ can be analog circuits. Alternatively, circuits  102  and  102 ′ can be digital circuits located in receive control circuit  92  or transmit control circuit  93 . Circuits  102  and  102 ′ can also be hybrid analog and digital circuits. Additionally, as a person of ordinary skill in the art would understand, although multiple blocks are shown in FIG. 3 for AGC circuits  102  and  102 ′, these functional blocks can be implemented with single components. 
     In the receive control circuit  92 , FFT circuit  98  is disposed between circuit  102  and equalizer  104  and is implemented by a digital signal processor (DSP) (not shown) running a software program. FFT circuit  98  or AGC circuit  102  converts the signal received from line  52  from an analog to a digital representation. FFT circuit  98  converts the digital signal from the time domain to the frequency domain and sends the converted digital, frequency domain signal to equalizer  104 . 
     Frequency domain equalizer  104  is disposed between FFT circuit  98  and detector  100  and error message processor  106 . An output line from error processor  96  leads to computer  84  via line  59  and can also lead other components within modem  54 . Equalizer  104  is an adaptive compensation circuit for counteracting distortions on line  52 . Equalizer  104  can be converged (e.g., tuned) so the constellation associated with the QAM signals are appropriately situated for decoding. Alternatively, equalizer  104  can be any device, digital or analog, for reducing frequency or phase distortion, or both, on subscriber line  52  by the introduction of filtering to compensate for the difference in attenuation or the delay, or both, at various frequencies in the transmission and reception spectrums. 
     Detector  100  is disposed between frequency domain equalizer  104  and error processor  96 . Detector  100  generates an output which includes error signals. In another possible embodiment, detector  100  is a slicer. 
     Error processor  96  monitors signals from detector  100  to determine and provide a signal and noise signal on line  97 . The signal and noise signal on line  97  is received by error message creator  108  in transmit control circuit  93  where an error message is created to be sent along line  52 . 
     Error message processor  106  processes error messages sent along line  52  from modem  56  (FIG.  2 ). Error processor  106  sends a signal to tone amplitude circuit  94  in transmit control circuit  93  indicating the need to adjust the amplitude of individual tones (or frequencies) and/or the amplitude of the entire signal on line  52 . 
     Tone amplitude circuit  94  adjusts the amplitude of individual tones (or frequencies) and/or the amplitude of the entire signal on line  52 . The adjustments in amplitude occur in response to signals from error message processor  106 . As such, tone amplitude circuit  94  optimizes signal power on line  52 . Advantageously, the amplitude of the signal received at access  55  on line  52  can be adjusted on a tone-by-tone basis. Preferably, the adjustments in amplitude result in power cutbacks of up to 30 dB in both the upstream and downstream directions. 
     In the transmit control circuit  93 , inverse FFT circuit  98 ′ is disposed between circuit  102 ′ and tone amplitude circuit  94  and is implemented by a digital signal processor (DSP) running a software program. A mapper circuit  109  is disposed between line  59  and circuit  104 . Inverse FFT circuit  98 ′ converts the signal received from tone amplitude circuit  94  from the frequency domain to the time domain and from a digital to an analog representation. The inverse FFT circuit  98 ′ sends the converted analog, time domain signal to circuit  102 ′. 
     With reference to FIG. 4, error processor  96  may include a Reed-Solomon decoder  120 , a mean squared error calculator  122 , and a Trellis decoder  124 . Reed-Solomon decoder  120  analyzes frames of data provided from decoder  124  and determines if a frame error occurs and if errors are occurring in the frame. Reed-Solomon decoder  120  can correct errors is well known in the art. Calculator  122  provides error message creator  108  an indication of the signal-to-noise ratio through line  97 . Alternatively, calculator  122  can be replaced by other systems which can provide an indication of signal-to-noise ratios. In another alternative, creator  108  can respond to other error signals, such as signals from decoder  120  or decoder  124 . 
     With reference to FIG. 5, operation of modem  54  is described with reference to FIGS. 2-5. FIG. 5 is a state diagram showing signal power cutback operations for modem  54 . Modem  54  preferably operates at reduced transmitted power levels to counter the demodulation effects of non-linear components on the telephone line (e.g., line  52 ). Such reduced power levels are the result of lower amplitudes of the total signal and lower amplitudes of individual tones (or frequencies). According to this technique, a line probing sequence is performed to measure both the signal and noise at the receiving end, followed by a hand-shaking sequence in which that information is conveyed back to the transmitting end. 
     In FIG. 5, modem  54  (FIG. 2) operates in a data mode  502  when data is communicated across subscriber line  52 . As data is communicated, signal-to-noise ratio (S/N) signals from error processor  96  are consistently checked. If the S/N signals are within an acceptable level, modem  54  is maintained in data mode state  502 . However, if the S/N signals are above a certain threshold, modem  54  enters an increase amplitude state  504  in response to an error message from modem  56 . In state  504 , modem  54  increases the amplitude of individual tones (or frequencies) and/or the amplitude of the entire signal. The increase in amplitude results in an increased signal power. State  504  is maintained until the S/N signals are within the determined threshold. Once the S/N signals are within the determined threshold as determined by modem  56 , modem  54  returns to data mode  502 . 
     If the S/N signals are below a certain threshold, modem  54  enters a decrease amplitude state  506  in response to an error message from modem  56 . In state  506 , modem  54  decreases the amplitude of individual tones (or frequencies) and/or the amplitude of the entire signal. The decrease in amplitude results in a decreased signal power. State  506  is maintained until the S/N signals are within the determined threshold. Once the S/N signals are within the determined threshold by modem  56 , modem  54  returns to data mode  502 . 
     The advantageous architecture of modem  54  can be utilized in modem  56 . The operation of modem  56  can also be similar to modem  54  wherein modem  56  responds to error messages from modem  54  to increase or decrease the amplitude of the downstream signal on line  52 . Modem  56  can also create error messages similar to modem  54 . Alternatively, modem  56  can include different types of circuitry for generating and responding to error messages. 
     Thus, modems  54  and  56  cooperate to optimize the amplitude of downstream and upstream signals on line  52 . The modem on the sending end of line  52  adjusts the amplitude of the signal in response to an error message or control signal from the modem on the receiving end. The error message can be generated in response to signal-to-noise ratios on the receiving end. Either modem  54  or  56  can be on the receiving end or the transmitting end. 
     While the embodiments illustrated in the FIGs. and described above are presently preferred, it should be understood that these embodiments are offered by way of example only. Other embodiments may include, for example, control circuits capable of carrier less amplitude and phase (CAP) line coding techniques. The invention is not limited to a particular embodiment, but extends to various modifications that nevertheless fall within the scope of the appended claims.