Abstract:
A digital subscriber line (DSL) communication system that utilizes the high frequency band of a standard telephone line does not require the use of a plain old telephone service (POTS) splitter in the resident&#39;s home, which provided isolation between the POTS frequency band (0 to 4 kHz) and the DSL frequency band. Digital signal processing techniques are utilized to adapt to varying subscriber line conditions due to POTS telephone equipment. The digital signal processing techniques eliminate the need for a splitter by reducing susceptibility to distortion due to varying subscriber line characteristics. The digital subscriber line modem utilizes constant envelope modulated signals and frequency division multiplexing, where the constant envelope modulations lessens the intermodulation distortion products due to DSL signals that are transmitted by the modem and which may result in audible noise at the POTS telephone equipment due to non-linearities of the POTS telephone equipment.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATIONS 
     The present application is related to U.S. patent application Ser. No. 08/943,484, filed Oct. 3, 1997, by Henderson, Ko, Zuranski, Haque, Patravali, Rodriguez, Souders, and Tzouris, and entitled, “Splitterless Digital Subscriber Communication System”; U.S. patent application Ser. No. 09/028,141, by Zuranski, Ko, Haque, Patravali, Rodriguez, Souders, and Tzouris, and entitled “Digital Subscriber Line Modem Utilizing Echo Cancellation to Reduce Near-End Cross-Talk Noise”; and U.S. patent application Ser. No. 08/982,421, filed Dec. 2, 1997, by Anderton, Eldumiati, Gronemeyer, Harmer, Henderson, Ko, Peshkin, Rahamim, Stubbe, J. Walley, S. Walley, Wan, and Zuranski, and entitled “Modulation Switching For DSL Signal Transmission”. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a modulation technique for a communication system. In particular, the present invention relates to a splitterless communication system utilizing constant envelope modulation. 
     DESCRIPTION OF THE RELATED ART 
     Explosive growth of the internet and the worldwide web is driving a need for increased communication data rates. In the corporate world, the need for high-speed access or data rates is met by dedicated high-speed links (such as T1/E1 frame relays or OCI ATM systems) from the company to an internet access provider. Users in the company typically utilize a local area network (LAN) to gain access to an internet access router that is coupled to the high-speed link. Unfortunately, home users of the internet do not often have access to a high-speed link and must rely on a standard analog or plain old telephone service (POTS) subscriber line. 
     The need for high-speed access to the home is ever increasing due to the increased popularity of telecommuting and the availability of information, data, programs, entertainment, and other computer applications on the worldwide web and the internet. For example, designers of web technology are constantly developing new ways to provide sensory experiences, including audio and video, to users of the web (web surfers). Higher-speed modems are required so the home user can fully interact with incoming web and communication technologies. 
     Although designers of modems are continuously attempting to increase data rates, analog or POTS line modems are presently only able to 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 (PSTN). The internet access provider is also coupled to the PSTN and transmits and receives information through the PSTN to the subscriber line. 
     Some home users have utilized ISDN equipment and subscriptions to obtain up to 128 Kbps access or data rates by the use of two data channels (B channels) and one control channel (D channel). ISDN equipment and subscriptions can be expensive and require a dedicated subscriber line. Neither ISDN modems nor conventional analog modems are capable of providing 256 Kbps or higher access between the home and the internet. 
     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, the asymmetric digital subscriber line 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). 
     DSL networks and protocols were developed in the early 1990&#39;s to allow telephone companies to provide video-on-demand service over the same wires which were being used to provide POTS. DSL technologies include discrete multitone (DMT), carrierless amplitude and phase modulation (CAP), high-speed DSL (VDSL), and other technologies. Although the video-on-demand market has been less than originally expected, telephone companies have recognized the potential application of DSL technology for internet access and have begun limited offerings. 
     DSL technology allows telephone companies to offer high-speed internet access and also allows telephone companies to remove internet traffic from the telephone switch network. Telephone companies cannot significantly profit from internet traffic within the telephone switch network due to regulatory considerations. In contrast, the telephone company can charge a separate access fee for DSL services. The separate fee is not as restricted by regulatory considerations. 
     With reference to FIG. 1, a conventional asymmetric DSL (ADSL) system  10  includes a copper twisted pair analog telephone 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 across line  12  with a data network (not shown) which is coupled to modem  16 . ADSL modem  16  receives signals from line  12  and transmits signals 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  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 DSL 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 DSL modem  16 . Similarly, band splitter  20  prevents any of the POTS signals from reaching modem  14 . 
     FIG. 2 shows the separate frequency bands for POTS signals and DSL signals. The POTS signals (signals transmitted between telephone  26  and telephone switch network  28 ) utilize a first frequency band  210 , uplink DSL signals (signals transmitted from modem  14  to modem  16 ) utilize a second frequency band  220  that is higher in frequency than the first frequency band  210 , and downlink DSL signals (signals transmitted from modem  16  to modem  14 ) utilize a third frequency band  230  that is higher in frequency than the second frequency band  220 . 
     Referring back to FIG. 1, ADSL modem  16  and DSL 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 50 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 DSL 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 or telephone 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 DSL signals. Telephone  26  can further generate harmonics which can reach the frequency ranges associated with the DSL signals. The nonlinear components can also demodulate DSL 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 (DSL 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  32 , 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 residence  12 , such as, modem  14 , is expensive because the most complex component of system  10  (e.g., the high-speed receiver) is located at residence  22 . 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. 
     Thus, there is a need for a digital subscriber line (DSL) communication system which does not require the use of a splitter in the residence. Further, there is a need for a communication system which allows a DSL modem to be connected directly to the subscriber line similar to the use of a conventional analog modem. Even further there is a need for a DSL modem that provides a modulation technique that results in less intermodulation distortion products for telephones that communicate over the POTS frequency band. The intermodulation distortion products can cause audible noise at the telephones, which is undesirable. 
     SUMMARY OF THE INVENTION 
     The present invention relates generally to a digital subscriber line modem adapted to be coupled directly to a subscriber line. The modem is capable of simultaneous access to the subscriber line with other telephone equipment operating in a frequency band below a predetermined frequency value. The modem includes a modulator that provides a constant envelope modulation for DSL signals that are transmitted over the subscriber line, and that are sent within a frequency band above the predetermined frequency value. 
     The present invention also relates to a telephone communication system that includes a subscriber line between a telephone central office and a remote location, The remote location is capable of transmitting and receiving both POTS signals within a first frequency band and DSL signals within a second frequency band over the subscriber line at the same time without using a splitter at the remote location. The system includes a modulator connected to the subscriber line, where the modulator is configured to provide a constant envelope modulation for the DSL signals that are transmitted over the subscriber line in the second frequency band, so as to lessen an amount of interference with the POTS signals the first frequency band created due to non-linearities that cause demodulation of the DSL signals into the first frequency band. 
     The present invention further relates to a method of providing simultaneous communication over a telephone subscriber line. The simultaneous communication includes at least a first signal in a first frequency band that is either being sent to or received from a telephone, and a second signal in a second frequency band that is higher than the first frequency band, the second signal being sent from a modem. The method includes a step of providing, by the modem, a constant envelope modulation for the second signal in the second frequency band. The constant envelope modulation limits an amount of audible noise heard at the telephone due to a nonlinearity in the telephone that demodulates the second signal into the first frequency band. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be further described with reference to the accompanying drawings, wherein like numerals denote like elements, and: 
     FIG. 1 is a schematic block diagram of a conventional ADSL communication system; 
     FIG. 2 is a frequency plot of a POTS frequency band, an uplink ADSL band, and a downlink DSL band used in a conventional ADSL communication system; 
     FIG. 3 is a block diagram of a splitterless communication system that utilizes a DSL modem according to the present invention; and 
     FIG. 4 is a block diagram of an MSK modulator used in the DSL modem according to the present invention; 
     FIG. 5 is a diagram showing the relationship of an I-channel and a Q-channel in an MSK system; 
     FIG. 6A is a timing diagram showing eight samples of a “+1” PAM symbol using a sinusoidal waveform; 
     FIG. 6B is a timing diagram showing eight samples of a “−1” PAM symbol using a sinusoidal waveform; 
     FIG. 7A shows a first sine/cosine lookup table that can be used according to the system and method of the invention; 
     FIG. 7B shows a second sine/cosine lookup table that can be used according to the system and method of the invention; 
     FIG. 8 is a block diagram of a first embodiment of a DSL modem according to the invention; and 
     FIG. 9 is a block diagram of a second embodiment of a DSL modem according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     U.S. patent application Ser. No. 08/943,484, entitled “Splitterless Digital Subscriber Communication System”, filed Oct. 3, 1997 by Henderson et al., and assigned to Rockwell International Corporation, and incorporated in its entirety herein by reference, discloses a splitterless digital subscriber line communication system that allows for both DSL signals and for standard telephone signals (e.g., sent over the POTS) to coexist without much interference between these two signals. In conventional communication systems that provide for DSL and POTS transmission of voice and/or data, a POTS splitter provides hardware isolation between the POTS frequency band (e.g., 0 to 4 kHz) and the DSL frequency band (e.g., 30 kHz to 1 MHz). 
     With the POTS splitter  20  in a conventional system, as seen in FIG. 1, the telephone  26  in residence  22  does not receive the DSL signals that are on subscriber line  12 . The DSL signals are prevented from appearing on the ring and tip lines of the telephone circuit  36  in residence  22 . Also, the POTS splitter  20  provides isolation for an DSL modem  14 , so that the POTS signals that are on subscriber line  12  are prevented from being received by the DSL modem  14 . 
     Splitter  20  is three-port device including port  22 A coupled to telephone loop or line  12 , a port  22 B coupled to telephone wiring  36  inside residence  22 , and a port  22 C coupled to DSL modem  14 . Splitter  20  communicates signals at port  22 A at the full bandwidth of subscriber line  12  (e.g., the full bandwidth capability of line  12 ). Splitter  22  low pass filters any signals communicated through port  22 B to or from wiring  36 . Thus, splitter  20  only allows POTS signals to pass through to devices, such as telephone  26 . Splitter generally filters the signals provided through port  22 B with a low pass filter tuned to a 0 to 4 kHz frequency range. Splitter  20  acts as a high pass filter for any signals communicated through port  22 C. 
     As stated above, however, having a POTS splitter at the house or residence  22  requires a telephone company or the like to install such a device for the house, which is a time consuming and costly effort, due in part to the wiring operations necessary to install splitter  20  and the person-hours required to travel to each house desiring such a splitter  20 . 
     Referring now to FIG. 3, splitterless communication system  50  allows for both POTS transmissions and for DSL transmissions to coexist on the same twisted pair copper wires. A DSL modem  54  has the capability to change its data rate in accordance with POTS-related impairments that affect a DSL band, thereby allowing for successful data transmission over the DSL band. DSL modem  54  includes a digital signal processor that can provide for adjusting of automatic gain control, converging of equalizers, error processing, and/or line characterization. Based on the amount of POTS-impairments on the DSL band of the subscriber line  52 , DSL modem  54  provides the highest data rate available, by constantly adjusting the data rate to reach the maximum data rate potential on the subscriber line. DSL modem  54  operates at a data rate lower than the theoretical data rate of conventional DSL modems, but provides a faster data rate than current modems, even faster than conventional modems using 56FLEX™ or X2™ technology. 
     U.S. patent application Ser. No. 08/943,484, filed Oct. 3, 1997 by Henderson et al. and entitled, “Splitterless Digital Subscriber Communication System”, discloses the use of Quadrature Amplitude Modulation (QAM) for the DSL signal transmission. Preferably, the QAM constellation size is a power-of-two value within the range from 4 to 256 constellation points. Additionally, Reed-Solomon encoding may also be utilized for the DSL transmission. 
     Other techniques are known for data transmission using DSL modems, such as Discrete Multitone (DMT). DMT allows for the splitting of the available DSL bandwidth into a number of subchannels. The subchannels are allocated a number of bits (0-8) per hertz in each 4 kHz subchannel band, depending upon the signal-to-noise ratio experienced in the subchannel. DMT allows for the allocation of data so that the throughput of every subchannel is maximized. This data transmission technique is designed to maximize the transmission capability of the DSL band. 
     However, in a splitterless communication system that allows for both POTS transmission and DSL transmission over a subscriber line, the particular modulation scheme utilized for the DSL transmission may have an adverse impact on transmissions over the POTS band. In particular, since there is no hardware device (splitter) providing isolation between POTS signals and DSL signals, audible distortion due to intermodulation of DSL signals may appear at a telephone earpiece of a telephone at the house. Thus, conventional DSL modulation techniques, such as QAM and DMT, may be undesirable with regards to audible noise that is perceptible at the telephone earpiece. 
     For example, an DSL signal can have a first frequency component at 80 kHz and a second frequency component at 82 kHz. The DSL signal, when applied to a non-linearity in the communication system, produces a difference component at a frequency of 82 kHz−80 kHz=2 kHz, which is in the middle of the POTS frequency band. Of course, a typical DSL signal has a continuum of frequencies, which would result in difference components at a continuum of frequencies when the DSL signal interacts with a non-linearity in the system. 
     A non-linearity may appear in communication system  50  due to non-linearities in interface circuitry (not shown) of a telephone such as telephone  86 . These nonlinearities may be due, for example, to transistors and diode circuits in the telephone. When an DSL signal appears at the interface circuitry, the non-linearities will cause intermodulation distortion products, such as the 2 kHz difference component described in the above example, to be picked up by a receiver microphone in the telephone. This “audible noise” is undesirable to a user of the telephone, and results in a hiss or background noise that may interfere with voice signals on the POTS frequency band of the subscriber line. Additionally, the audible noise may adversely interfere the transmission and reception of data signals within the POTS band. In particular, the non-linearities in the telephone act to demodulate the DSL signals so that they appear in the POTS frequency band, producing an undesirable result. 
     In the system and method according to the invention, the intermodulation product distortion is lessened to a great extent in the POTS frequency band by utilizing a modulation scheme that provides a lesser amount of intermodulation product distortion than conventional modulation schemes used for DSL transmission. As stated above, the intermodulation product distortion is not a major problem for a conventional system having a hardware (POTS) splitter. However, for the system according to the invention that does not utilize a POTS splitter at a source/destination site (i.e., a house), the problem of intermodulation product distortion has been recognized by the inventors, and is dealt with in a manner that provides for substantially noise-free simultaneous data and/or voice communications over both the POTS band and the DSL band. By using a constant envelope modulation, intermodulation products appearing at or near the baseband frequency range are lessened to a great extent than by not using constant envelope modulation. Since the baseband frequency range is a part of the POTS band, this is a useful feature for simultaneous use of POTS transmission and DSL transmission over the same lines. 
     In the system and method according to the invention, a constant envelope modulation technique for upstream signal transmission over the DSL band is utilized. Constant envelope modulation corresponds to a non-amplitude modulation technique, such as frequency modulation or phase modulation. Phase modulation, such as phase shift keying (PSK), does modulate the envelope somewhat (at each phase transition in the modulated signal), but provides a sufficient enough “constant” envelope to be useful for a splitterless system for DSL and POTS traffic. Other types of phase modulation, such as continuous phase modulation, may be utilized to provide a substantially constant envelope with increased data rate transmission capability. System and application parameters can affect the meaning of the term constant envelope as used in the present application. For example, the constant envelope is preferably consistent enough so that intermodulation product distortion in the voice band does not annoy the user of the telephone. 
     In the system and method according to the invention, given that there are non-linearities that exist in the communications system and that cannot be entirely eliminated, a modulation technique for signals in the DSL band is used so only a small amount of interference affects standard voice and/or data transmissions over the POTS band. In the example given above with respect to an DSL signal having two frequency components at 80 kHz and 82 kHz, if amplitude modulation was used for the DSL signal, then a strong difference component would be generated by the non-linearities of the interface circuitry of a telephone on the POTS side. However, if constant envelope modulation was used instead, a lesser-sized difference component would be generated by the non-linearities of the interface circuitry of the telephone on the POTS side. 
     Such a use of constant envelope modulation has been provided for wireless communications, such as voice and/or data transmissions using satellites, where more than one signal passes through a transponder that may operate in a non-linear region under certain situations (e.g., fully-loaded transponder) operating at or near saturation. In such wireless systems, the use of constant envelope modulation provides for lesser suppression of the weaker signals as compared to stronger signals that are input to the transponder. In the system and method according to the invention, however, the use of constant envelope modulation provides for the lessening of intermodulation distortion products at a telephone on a POTS side of a wired system that does not have a splitter, where the telephone has non-linearities that cause intermodulation distortion to occur. 
     Another useful type of constant envelope modulation technique that can be utilized in the a DSL modem according to the invention is Minimum Shift Keying (MSK) modulation. MSK modulation is a continuous-phase frequency shift keying (FSK) modulation with a minimum modulation index (0.5) that will produce orthogonal signaling. The details of MSK are presented in “Digital and Analog Communication Systems”, by Leon W. Couch II, and are well known in the art. 
     A block diagram of one possible MSK modulation circuit for upstream DSL traffic that can be utilized in the system according to the invention is shown in FIG.  4 . In FIG. 4, an MSK modulator  500  receives data from an I channel  505  and from a Q channel  510 , with both channels receiving data in binary form (i.e., each bit is either “1” or “0”). I channel data is provided to a Binary-to-Pulse Amplitude Modulator (PAM) circuit  520 , and Q channel data is provided to a Binary-to-PAM circuit  530 . Circuits  520 ,  530  each convert the binary data to corresponding PAM data, where a binary “1” is output as a “1”, and where a binary “0” is output as a “−1”. 
     MSK modulator  500  also includes a Full-Wave Rectifier circuit  540 , which converts an input sinusoid of the form sin(ωT/2) into a full-wave-rectified signal. The full-wave-rectifier circuit  540  essentially performs an Absolute Value function for any signal input to it. MSK modulator  500  encodes the I and Q channels into half-wave sinusoids, at a rate of T/2, where T=symbol rate. 
     MSK modulator  500  further includes a first multiplier  550  and a second multiplier  560 . The first multiplier  550  multiplies the PAM data of the binary-to-PAM circuit  520  with the output of the full-wave-rectifier circuit  540 , and the first multiplier  550  outputs a first multiplied signal. The second multiplier  560  multiplies the PAM data of binary-to-PAM circuit  530  with the output of full-wave-rectifier circuit  540 , and the second multiplier  560  outputs a second multiplied signal. The first and second multiplied signals are signals that have a carrier frequency corresponding to the output of the full-wave-rectifier circuit  540 . 
     The second multiplied signal is delayed by a delay circuit  570 , where the delay corresponds to one-half the symbol rate (in digital terms, Z −T/2 ). Delay circuit  570  delays the signal in the Q channel so that it will be at a peak when the signal in the I channel is at a zero value. An adder  580  adds the output of the delay circuit  570  to the output of the first multiplier  550 , where that output corresponds to a constant envelope MSK signal  590 . 
     The constant envelope feature of MSK is shown in FIG. 5, where each zero value of either the I or Q channel is matched in time by a peak value in the other channel. Values in between the peak and minimum values in each channel are matched in time with each other (due to a delay element provided in the Q channel) to maintain a constant envelope modulated signal (the sum of the I and Q channels) at all times. 
     The sin(ωT/2) signal can be generated in a number of ways. One way is to use a sample counter as an index to a sine/cosine table. For example, in the preferred embodiment, the combined symbol rate upstream is 34000 Hz, and the sample rate is 272,000 Hz. Thus, there are 8 samples per symbol. The sample counter counts from 0 to 7 for each input symbol, and is used as an index to a sine/cosine lookup table. FIG. 6A shows eight sample points for one pulse amplitude modulated (PAM) symbol corresponding to a “1” value, with each sample point corresponding to a particular sinusoidal value. If the PAM symbol corresponds to a “−1” value, then the eight sample points would have corresponding sinusoidal values as shown in FIG.  6 B. 
     In a first configuration of a sine/cosine lookup table, the sine/cosine lookup table has 2048 pairs of entries, where each entry has a corresponding sine value associated with it. The carrier delta Δ is used to access the appropriate entry in the lookup table. In a second configuration, a sample counter is used as an index to a lookup table that only contains a number of entries corresponding to twice the number of samples per symbol. Thus, in the example described above, a lookup table having only sixteen pairs of entries would be used in the second configuration. 
     In the first configuration, each entry in the first column of the lookup table corresponds to a particular sample position of one positive cycle of a sinusoid, with entry  0  corresponding to a 0 degree position in the positive cycle of a sinusoid, and with entry  1023  corresponding to an 180 degree or π position in that one cycle. Entries  1024  to  2047  correspond to sample positions of one negative cycle of the sinusoid, and correspond to positions between 180 degrees and 360 degrees of the sinusoid. The carrier delta Δ is used as an index to a lookup table. 
     The carrier delta Δ is computed according to the following equation: 
     
       
         Δ=2048*( fc/fs )=2048*( fc/ 272,000),  
       
     
     wherein fc is the symbol rate for each channel, and fs is the sample rate. 
     In the system according to the invention, the symbol rate is 17000 Hz for each channel, and so the total bit rate output by the MSK modulator is 34000 bits/second, since there are two bits per symbol (i.e., and I bit and a Q bit for each symbol that is MSK modulated). Thus: 
     
       
         Δ=2048*(17000/272000)=128.  
       
     
     From this calculation, a half-wave sine table having the values as shown in FIG. 7A is obtained, with the first column corresponding to an address location for each value in the lookup table, and with the second column corresponding to the corresponding sine value (with the integer value 32767 corresponding to a sine value of just below “1”). Using the carrier A that increments by 128 for each sample of a symbol, the sine value corresponding to the zeroth location in the lookup table is retrieved for the zeroth sample of a “+1” PAM symbol, the 128th location in the lookup table is retrieved for the first sample of the “+1” PAM symbol, . . . , the 896th location in the lookup table is retrieved for the seventh sample of the “+1” PAM symbol. Either the zeroth location or the 1024th location in the lookup table is retrieved for the eighth sample, which corresponds to the zeroth sample of a next symbol. The zeroth location is retrieved if the next symbol is a “+1” PAM symbol, and the 1024th location is retrieved if the next symbol is a “−1” PAM symbol. 
     Thus, the zeroth sample for a “+1” PAM symbol in the I channel would map to the sine value of 0, the first sample for the “+1” PAM symbol in the I channel would map to the sine value of 12539, . . . , the fourth sample for the “+1” PAM symbol in the I channel would map to the sine value of 32767 (which equals 2 16 −1, which corresponds to the largest number in a 16-bit integer range, which is a typical maximum integer value for a 16-bit computer), . . . , and the seventh sample for the “+1” PAM symbol in the I channel would map to the sine value of 12539. The next sample in the I channel would correspond to the zeroth sample for the next symbol in the I channel. Note that, due to the T/2 delay element in the path of the Q-channel, data corresponding to the fourth sample of the symbol in the I channel, which is at a peak output value, is added to a T/2-delayed value from the Q-channel, which would correspond to a zeroth sample for a symbol in the Q channel. This zeroth sample for the symbol in the Q-channel is obtained from a similar table to that shown in FIG. 7A, where the corresponding sine value would be “0”. Thus, the adder  590  would add a peak value from the I channel to a minimum value from the Q channel, which maintains the constant envelope feature of this modulation scheme. 
     Entries  1024  to  2047  of the sine table correspond to the corresponding sine values for a “−1” PAM output, and entries  0  to  1023  of the sine table correspond to the corresponding sine values for a “+1” PAM output. Thus, for a “+1” PAM value, the 0th entry in the table is accessed for the first sample of that symbol, and every 128th entry is successively accessed for the next seven samples of that same symbol. For a “−1” PAM value, the 1024th entry in the table is accessed for the first sample of that symbol, and every 128th entry is successively accessed for the next seven samples of that same symbol. 
     In the second lookup table configuration, instead of having 2048 entries in the sine/cosine lookup table and then jumping by 128 addresses in the table for each sample, a smaller sine/cosine lookup table having only sixteen entries is used, as shown in FIG.  7 B. In FIG. 7B, the sample counter is used to access the appropriate address location in the sine/cosine table, with the zeroth sample of a “+1” PAM symbol corresponding to a sample count=0, which is used to retrieve the value 0 in the address=0 location of the sine/cosine table. The first sample of the “+1” PAM symbol corresponds to a sample count=1, which is used to retrieve the value “12539” in the address=1 location of the sine/cosine table. After the sample count gets to 7, which corresponds to the eighth or last sample of the “+1” PAM symbol, the sample count resets to zero for a next symbol to be sampled. For a “−1” PAM symbol, the eighth through sixteen entries of the table are respectively accessed for the eight samples of that symbol. 
     FIG. 8 shows a block diagram of a DSL modem  54 A according to a first embodiment of the invention, and many components of the DSL modem  54 A are not shown in order to simplify the explanation. Referring now to FIG. 3, FIG.  4  and FIG. 8, a modulator  500  performs constant envelope modulation on data received from a computer  84 . The constant-envelope-modulated data passes through band-pass filter  810 , which has a response in accordance with the upstream DSL band. Band-pass filter  810  keeps signals in all other bands from passing therethrough. Bandpass-pass filter  810  outputs a filtered signal on subscriber line  52 . 
     Downstream signals are received on subscriber line  52 , and pass through band-pass filter  820 , which has a response in accordance with the downstream DSL band. Note that band-pass filter  810  blocks these downstream DSL signals from being sent to the modulator  500 . Band-pass filter  820  outputs a filtered signal to demodulator  830 . Demodulator  830  performs a demodulation on the downstream signals, and is preferably implemented as a quadrature amplitude demodulator (when the downstream signals are QAM signals). 
     FIG. 9 shows a DSL modem  54 B according to a second embodiment of the invention, in which an echo canceler  910  and other components are used. Such a use of an echo canceler is described in U.S. patent application Ser. No. 09/028,141, entitled “Digital Subscriber Line Modem Utilizing Echo Cancellation To Reduce Near-End Cross-Talk Noise”, by Zuranski et al., which is incorporated in its entirety herein by reference. FIG. 9 substantially corresponds to FIG. 6 of the above-mentioned related patent application, but with the transmitter block replaced by the modulator  500  of FIG.  4 . 
     It is understood that, while the detailed drawings and specific examples given describe preferred exemplary embodiments of the present invention, they are for the purpose of illustration only. The apparatus and method of the invention is not limited to the precise details and conditions disclosed. For example, although an DSL transmission scheme is shown with a POTS transmission scheme, other types of schemes may be utilized according to the teachings of the present invention for a splitterless telephone communication system. One such system would provide for simultaneous communication on telephone wires using both ADSL traffic and POTS traffic, where the upstream ADSL traffic uses constant envelope modulation to lessen the amount of interference to the POTS traffic. 
     Also, while the present invention is directed to a constant envelope scheme for upstream DSL signals sent from a house or remote location to a central office, the downlink DSL signals sent from a server, for example, and arriving at the house also may have constant envelope modulation so as to further reduce the amount of audible noise heard by the telephones. However, the downlink signals have been attenuated by the telephone lines from the server (or other sending site) to the receiving site, and thus do not cause nearly as serious a problem as the closely-located uplink signals that are output by a DSL modem located nearby the telephones. 
     Still further, while constant envelope modulation is described herein as a technique to lessen interference between signals in the ADSL band and signals in the POTS band, other techniques, such as shaped envelope modulation may be utilized in order to lessen the noise heard by the demodulation of ADSL signals by the telephones. For example, by coding the upstream ADSL signal so that the spectrum of the amplitude portion of the modulation is severely attenuated in the audio band (which includes the POTS band), a QAM or other modulation technique may be used which will still keep demodulated components from being heard at the telephones. A phase modulated portion of the signal is not affected, since intermodulation products due to pure phase modulation do not translate to baseband.