Abstract:
A coaxial-based communication system connects network-side xDSL services to xDSL CPE devices and avoids interference from a noise spectrum that is degrading to xDSL performance or emission compliance on either the network or customer premises sides on the NID. The system provides an NID and a CPE interface device at the two ends of the coaxial wiring to perform a spectral relocation of the xDSL signal to a lower noise frequency band than is used for the xDSL service. Interfaces to the system are industry standard xDSL, POTS, and CATV services, while the transport medium is the normal coaxial type wiring and CATV splitters found on the customer premise.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/110,544, filed Dec. 2, 1998. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to a digital subscriber loop (DSL) system which utilizes un-shielded twisted pair (UTP) or shielded twisted pair (STP) for network connection to customer premises equipment (CPE) through a network interface device (NID) and standard (i.e., in-home) wiring using coaxial cable for CATV and UTP for Plain Old Telephone Service (POTS). The invention also relates to a DSL system which uses a passive NID-based end station and an active CPE-based end station for spectrally relocating xDSL frequency signals to lower noise locations on customer premises wiring. 
     BACKGROUND OF THE INVENTION 
     Digital subscriber loop signaling (e.g., ADSL, HDSL, VDSL and so on which are hereinafter generally referred to as xDSL) provides a method for high speed data transfer across existing telephone lines. Plain Old Telephone Service (POTS) transmission occurs in a frequency range of approximately 0 Hz to 4 kHz. xDSL utilizes a higher set of frequencies from 20 kHz to 1.1 MHz. Using a different frequency band gives xDSL several advantages over current analog modem technology. For example, fast data transmission downstream from the network to the user is achieved (e.g., on the order of 8 Mb/s), as well as improved data transmission speed upstream from the user to the network (e.g., on the order of 640 kb/s). In addition, xDSL allows simultaneous data transfer in both directions (i.e., upstream and downstream) and does not interfere with telephone transmissions. Thus, both telephone and xDSL transmissions can occur simultaneously. 
     xDSL gains these advantages over current technology in a relatively simple manner. As mentioned earlier, xDSL utilizes a higher frequency band than POTS. This higher frequency band of 20 kHz to 1.1 MHz is divided into two sections, that is, one for upstream data and one for downstream data. Thus, xDSL is able to allow data transfer in both directions at the same time. FIG. 1 depicts the manner in which the frequency spectrum can be divided for POTS and xDSL. The upstream data spectrum  12  ranges from 20 kHz to 160 kHz, for example, and the downstream data spectrum  10  ranges from 240 kHz to 1.1 MHz. In accordance with xDSL, the upstream and downstream spectrums  12  and  10  are further divided into 256 4.3 kHz blocks. These blocks are referred to as “tones”. The downstream spectrum  10  contains more tones and thus has the capability of transmitting data faster. The reason for further dividing the spectrums into tones is so that, if interference noise exists at a certain frequency and the data associated with the tone at that frequency is being destroyed, an xDSL system can refuse to transmit data on that tone. The system will then use a different tone to transmit the data safely. If this should occur, the xDSL system does not transmit data at its maximum rate; however, data integrity is high. When an xDSL system is first powered on, the system checks all of the tones available in the frequency spectrums  10  and  12  to see if data can be transferred on each tone. If the system finds that a sufficient number of good tones are available, the system is said to be “trained”, and data can be transmitted. 
     With this new DSL technology, new problems have also arisen. A problem with xDSL transmissions which is currently foreseeable in virtually all residential and commercial facilities is noise. Essentially all of these facilities have electronic devices (e.g., motor driven devices and variable switches) which generate noise in the form of electromagnetic interference (EMI). Additional examples of these electronic devices include TRIAC devices found in light dimmers and hair-dryers, and brushes in electric motors located in ceiling fans and air-compressors in refrigerators, heat-pumps, and so on. This noise is generated in a frequency range of approximately 10 kHz to 5 MHz, encompassing the entire xDSL spectrum. This poses a problem with existing unshielded twisted pair copper telephone lines because the household EMI noise couples onto these telephone lines and can potentially destroy any xDSL transmission. If the EMI noise is very severe, other network xDSL subscribers may also be degraded from crosstalk in the network feeder cable&#39;s binder group. 
     A need therefore exists for a method that relocates the xDSL sign spectrum away from the most severe EMI noise spectrums located on the customer premises. Shifting the frequency range of the xDSL transmission above the EMI noise range, however, also poses problems. Existing unshielded twisted pair telephone lines have a frequency response similar to a low-pass filter. If the xDSL frequency range were shifted up above the EMI noise range, much of the transmission could be lost due to the low-pass filter effects of the twisted pair telephone lines. The problem also increases as the length of the telephone line increases. As the length of the telephone line increases, the cutoff frequency of the telephone line decreases, which increases the amount of data that could be lost. A need exists for an xDSL system which addresses and solves this problem, as well. 
     In addition, when an xDSL set-top service is supplied for Video On Demand (VOD) within a customer&#39;s premises, the connection is specified to be UTP or STP. The CPE connection from the network interface device (NID) to the television (TV) or set-top box, however, is typically coaxial. A method is needed to reuse the existing customer premises coaxial wiring to deliver the xDSL service from the NID to the xDSL set-top box. 
     Also, UTP wiring within the customer premises is an uncontrolled element to the network provider. This uncontrolled element has the potential to radiate an xDSL signal much like an antenna radiates a signal, thereby violating FCC emission limits. A method is needed to isolate the existing POTS wiring from the network without having to install a dedicated drop for the xDSL service. 
     SUMMARY OF THE INVENTION 
     In order to keep EMI noise from potentially destroying an xDSL transmission, an xDSL spectrum relocation system is provided in accordance with the present invention which shifts the frequency range of an xDSL transmission above the EMI noise range. Also, to solve the problem of the low-pass filter characteristics of the existing twisted pair telephone lines, the system of the present invention utilizes the existing coaxial cable TV lines within the house. 
     The xDSL spectrum relocation system of the present invention shifts signals from one form of wire to another. With the implementation of xDSL, the incoming telephone line to a house carries both POTS and xDSL transmissions. The xDSL signal is removed from an upstream telephone line and then shifted up in frequency using amplitude modulation (AM) to approximately 25 MHz, which is well above the household EMI noise range. Amplitude modulation essentially uses the amplitude of the data signal to vary the amplitude of a carrier signal. Thus, the data and carrier signals appear to be one signal being transmitted at the frequency of the carrier. This signal is then placed on coaxial cable at a NID with the existing cable TV signals. Since existing cable TV frequencies begin at about 50 MHz, there is no interference between the two signals. 
     Because the existing standard for xDSL does not involve signals outside a selected frequency range (e.g., signals outside a range between 20 kHz and 1.1 MHz), the signal from the coaxial cable is demodulated after it has been transmitted through the house without being affected by EMI noise. Thus, the signal is removed from the coaxial cable, demodulated and placed back on twisted pair telephone line so that the existing xDSL modems can accept the signal. 
     The system of the present invention comprises two devices located at the customer premises which provide spectrum relocation of xDSL signals. Other cable and active xDSL devices do not require modification. The first device is a spectrum relocating NID which can be substituted for an existing POTS ND and the existing CATV NID. This four-port device provides two network side connections and two customer premises connections. The four-port NID device supports a number of features and functions such as: a spectrally band-limited (i.e., for POTS and xDSL) UTP port for connection to the public network; a spectrally band-limited (Le., for broadcast CATV and upstream pay-per-view PPV signaling coaxial port for connection to the public CATV provider network; a spectrally band-limited (i.e., for POTS only) MPT port for connection to existing customer premises POTS wiring; a spectrally band-limited (i.e., for CATV, PPV signaling, and relocated xDSL) coaxial port for connection to existing customer premises coaxial wiring, a function to relocate the downstream xDSL signal (i.e., a network-to-CPE signal) from its normal spectral location on the network side UTP port to an unused spectral location between 5 and 30 MHz on the customer premises side coaxial port; a function to relocate the upstream xDSL signal (Le., a CPE-to-network signal from an unused spectral location between 5 and 30 MHz on the customer premises side coaxial port to its normal xDSL spectral location on the network side UTP port; a function to pass through the POTS signal between the UTP ports on the network and customer premises sides; and a function to pass through the CATV signals (i.e., downstream and PPV upstream signals) between the coaxial ports on the network and customer premises sides. All conventional NID functions such as grounding and surge protection provided by existing types of POTS and CATV NIDs are provided by the new four-port NID. 
     The second spectrum relocating device of the present invention is a CPE interface device co-located with an existing CPE xDSL device such as a personal computer (PC) network interface card (NIC), an xDSL modem, or an xDSL VOD set-top box. The CPE interface device supports a number of features and functions such as: a spectrally band-limited (i.e., for xDSL only) UTP port for connection to the CPE xDSL termination device; a spectrally band-limited (i.e., for CATV, PPV upstream, and relocated xDSL signaling) coaxial port for connection to customer premises existing coaxial wiring; a spectrally band-limited (Le., for broadcast CATV and PPV upstream signaling) coaxial port for connection to a co-located TV or existing set-top device; a function to relocate the downstream (network-to-CPE) xDSL signal from its relocated spectral position between 5 and 30 MHz on the customer premises coaxial port to its normal spectral position on the UTP port connecting to the xDSL CPE device; a function to relocate the upstream xDSL signal (i.e., a CPE-to-network signal from its normal xDSL spectral position on the UTP port connecting to the xDSL CPE device to an unused spectral location between 5 and 30 MHz on the customer premises coaxial port; a function to provide a carrier signal between 5 and 30 MHz over the customer premises coaxial wiring to be used by the NID to obtain power and local oscillator function for frequency relocation; a function to pass through the CATV signals (i.e., downstream and PPV upstream signals) between the coaxial ports for the customer premises coaxial wiring and the co-located TV or set-top device; and a standard UL compliant AC (117 volts and 60 Hz) power connection. 
     By shifting of the xDSL spectrum to a lower noise frequency band on the in-home coaxial wiring, the above-mentioned problem areas are minimized. Also, by keeping the NID device simple and mostly passive, high reliability and low cost can be achieved, thereby meeting the traditional design goals of NIDs owned by the Public Switched Network (PSN). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various aspects, advantages and novel features of the present invention will be more readily comprehended from the following detailed description when read in conjunction with the appended drawings, in which: 
     FIG. 1 illustrates exemplary frequency spectra of POTS and xDSL signals; 
     FIG. 2 is a block diagram of an NID and customer premises equipment configured in accordance with an embodiment of the present invention to relocate the frequency spectrum of xDSL signals; 
     FIG. 3 depicts an overlap in the frequency spectrum of customer premises EMI and xDSL signals; 
     FIG. 4 depicts relocation of the xDSL frequency spectrum in accordance with an embodiment of the present invention; 
     FIG. 5 is a block diagram of NID and CPE devices constructed in accordance with an embodiment of the present invention to employ spectral relocation of xDSL signals; 
     FIG. 6 is a block diagram of a network interface device constructed in accordance with an embodiment of the present invention to provide spectral relocation of xDSL signals; 
     FIG. 7 is a block diagram of a bidirectional bandpass filter for xDSL signals with downstream and upstream equalization constructed in accordance with an embodiment of the present invention to compensate the upstream and downstream xDSL signals at a CPE for insertion loss of the system; and 
     FIG. 8 is a block diagram of a CPE interface device constructed in accordance with an embodiment of the present invention to provide spectral relocation of xDSL signals. 
     Throughout the drawing figures, like reference numerals will be understood to refer to like parts and components. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Exemplary customer premises  16  having POTS and xDSL is depicted in FIG. 2. A conventional telephone line  18  and a conventional coaxial cable  20  are connected to the customer premises via an NID  22 . The customer premises  16  can comprise a number of telephone devices which are each indicated at  24 . The customer premises  16  can also comprise a number of cable television (CATV) devices indicated at  26  and an xDSL device  28  which are connected to coaxial cable. 
     The xDSL spectrum relocation system of the present invention shifts signals from one type of signal conductor to another. With the implementation of xDSL, the incoming telephone line  18  to a house carries both POTS and xDSL transmissions. Many of the problems associated with xDSL transmissions arise because of an overlap in frequency ranges between transmission signals and EMI noise. The xDSL signal is removed from the telephone line  18  and then shifted up in frequency using amplitude modulation (AM)  30  to approximately 25 MHz, for example, and therefore above the typical household EMI noise range. FIG. 3 depicts overlaps between xDSL  31  and EMI  33 . FIG. 4 illustrates how the xDSL spectrum relocation system of the present invention avoids these interference problems by relocating the xDSL spectrum  35  above EMI  33 . 
     With continued reference to FIG. 2, an amplitude modulator  30  essentially uses the amplitude of the data signal to vary the amplitude of a carrier signal Thus, the data and carrier signals appear to be one signal being transmitted at the frequency of the carrier. This signal is then placed on coaxial cable  32  with the existing cable TV signals  37  (FIG.  4 ). Since existing cable TV frequencies begin at about 50 MHz, there is no interference between the two signals. Because the existing standard for xDSL does not involve signals which are beyond a selected frequency range (e.g., outside a range between 20 kHz and 1.1 MHz), the signal from the coaxial cable is demodulated, as indicated at  34  in FIG. 2, after it has been transmitted through the house without being affected by EMI noise. Thus, the signal is removed from the coaxial cable, demodulated and placed back on twisted pair telephone line  36  so that the existing xDSL modem  28  can accept the signal. 
     As stated previously, the xDSL spectrum is shifted down in frequency so that the xDSL modem  28  is able to accept and process the information. Once the xDSL data has been relocated to its original frequency spectrum location  31 , the xDSL data is once again susceptible to noise. The overlap between EMI  31  and xDSL  33  data depicted in FIG. 3 may once again be present Since the xDSL data travels only a relatively short distance (e.g., a few feet) on unshielded twisted pair telephone line before reaching the xDSL modem  28 , very little noise will be able to couple onto the telephone line  36 . Therefore, the xDSL data can reach the modem  38  uncorrupted by noise such as household EMI. Thus, by using the xDSL spectrum relocation system, the user has more tones available to transmit and receive data, and can therefore transmit and receive data much faster than conventional data transmission systems. 
     With reference to FIG. 5, the xDSL spectrum relocation system of the present invention comprises two devices located at the customer premises  16  which provide spectrum relocation of xDSL signals. Other cable and active xDSL devices do not require modification. The first device is a spectrum relocating NID  22  which provides for the frequency relocation of an xDSL spectrum in accordance with the present invention, and which can be substituted for an existing POTS NID and an existing CATV NID. This four-port device provides two network side connections and two customer premises connections. The four-port NID device supports a number of features and functions such as: a spectrally band-limited (i.e., for POTS and xDSL) UTP port  38  for connection to the public switched network; a spectrally band-limited (i.e., for broadcast CATV and upstream pay-per-view (PPV) signaling coaxial port  40  for connection to the public CATV provider network; a spectrally band-limited (i.e., for POTS only) UTP port  42  for connection to existing customer premises POTS wiring and POTS devices; and a spectrally band-limited (i.e., for CATV, PPV signaling, and relocated xDSL) coaxial port  44  for connection to existing customer premises coaxial wiring and corresponding devices. The NID  22  relocates the downstream xDSL signal (i.e., a network-to-CPE signal) from its normal spectral location on the network side UTP port  38  to an unused spectral location between, for example, 5 and 30 MHz on the customer premises side coaxial port  44 . The NID  22  also relocates the upstream xDSL signal (i.e., a CPE-to-network signal) from an unused spectral location between, for example, 5 and 30 MHz on the customer premises side coaxial port  44  to its normal xDSL spectral location on the network side UTP port  38 . In addition, the NID  22  passes POTS signals between the UTP ports  38  and  42  on the network and customer premises sides, respectively. The NID  22  also passes through the CATV signals (i.e., downstream and PPV upstream signals) between the coaxial ports  40  and  44  on the network and customer premises sides, respectively. Conventional NID functions such as grounding and surge protection are preferably provided by the NID  22  in the same manner as existing types of POTS and CATV NIDs. 
     The second spectrum relocating device of the present invention is a CPE interface device  46  which is co-located at the customer premises  16  with an existing CPE xDSL device  28  such as a PC NIC, an xDSL modem, or an xDSL VOD set-top box. The CPE interface device  46  supports a number of features and functions such as: a spectrally band-limited (i.e., for xDSL only) UTP port  48  for connection to the CPE xDSL termination device  28 ; a spectrally band-limited (i.e., for CATV, PPV upstream, and relocated xDSL signaling) coaxial port  50  for connection to customer premises existing coaxial wiring, and a spectrally band-limited (i.e., for broadcast CATV and PPV upstream signaling coaxial port  52  for connection to a co-located TV or existing set-top device  26 . The CPE interface device  46  of the present invention relocates the downstream (network-to-CPE) xDSL signal from its relocated spectral position between 5 and 30 MHz, for example, on the customer premises coaxial port  50  to its normal spectral position on the UTP port  48  connecting to the xDSL CPE device  28 . The CPE interface device  46  relocates the upstream xDSL signal (i.e., a CPE-to-network signal) from its normal xDSL spectral position on the UTP port  48  connecting to the xDSL CPE device to an unused spectral location between 5 and 30 MHz, for example, on the customer premises coaxial port  50 . The CPE interface device  46  also provides a carrier signal between 5 and 30 MHz over the customer premises coaxial wiring to be used by the NID  22  to obtain power and local oscillator function for frequency relocation. The CPE interface device  46  passes through the CATV signals (e.g., downstream and PPV upstream signals) between the coaxial ports  50  and  52  for the customer premises coaxial wiring and the co-located TV or set-top device, respectively. A standard UL compliant AC (117 volts and 60 Hz) power connection  54  is provided. 
     The NID 
     With reference to FIG. 6, a NID  22  provides the functions of frequency up-shifting the downstream xDSL spectrum coming from the network to a frequency above 5 MHz, for example, and down-shifting the upstream xDSL spectrum coming from the CPE. This is preferably accomplished by a mixer  56  located in the NID  22 . The up-shifted xDSL signals arrive at and depart from the RF port  58  of the mixer  56 . The standard xDSL signals arrive at and depart from the IF port  60  of the mixer  56 . The ports  58  and  60  are band-limited by filters  62  and  64 , respectively, to remove any out-of-band components generated by the mixer  56  and to keep all unwanted signals from reaching the mixer  56 . These filters are preferably passive filters that allow signals to pass in both directions. Active implementations of these filters are depicted in FIG. 7, which is discussed in more detail below, and are used for insertion-loss compensation. 
     The carrier signal used at the LO port  66  of the mixer  56  is generated from a half-carrier signal originated at the CPE interface device  46  and transmitted to the NID  22 . This allows frequency and amplitude of the carrier signal to be adjusted by the CPE interface device  46 . As this CPE-generated signal is received at the NID  22 , a band-pass filter  68  extracts the half-carrier frequency component and a rectifier  70  full-wave rectifies the signal. The full-wave rectified signal serves two purposes. First, a carrier band-pass filter  72  removes the carrier frequency generated by the full-wave rectifier function, which is then amplified to the correct level before being applied to the mixer LO port  66 . Second, the full-wave rectified signal is half-wave rectified and passed through a low-pass filter, as indicated at  74 , to generate bias voltages used by the NID  22 . These bias voltages are used by the amplifier  76 , as well as the mixer  56 , depending on the mixer-type. If the LO signal power requirements of the mixer  56  are large, an alternative method can be used in lieu of generating power using half-carrier frequency. 
     Bi-directional filters  78 ,  80 ,  82  and  84  for respective ones of network ports  38  and  40  and the CPE ports  42  and  44  are used to prevent out-of-band signals from reaching the mixer  56 . These filters are preferably passive to allow signals to pass in both upstream and downstream directions. 
     The CPE Interface Device 
     A CPE interface device  46  constructed in accordance with an embodiment of the present invention is depicted in FIG.  8 . The CPE interface device  46  provides a number of functions that are similar to those performed by the NID  22 . A mixer  86  is used to frequency up-shift the upstream xDSL signal from the CPE, and to frequency down-shift the downstream xDSL coming from the network. As with the NID  22 , ports  50  and  52  are provided with band-pass passive filters  96  and  94 , respectively, to allow upstream and downstream signals to pass. The filter  92  and equalization for the port  48  that connects to the xDSL CPE terminating device  28  is shown in FIG.  7 . The upstream and downstream xDSL signals are amplified independently to compensate for the insertion loss of the system. This is accomplished by separating the upstream and downstream signals with power splitters/combiners  98  and  100  and band-pass filters  102  and  104 . Due to the single direction of these filters, they can be active or passive. The amplifiers  106  and  108  are set to compensate for system losses of the CPE interface device  46  and the NID  22 , allowing for the xDSL signal to meet level requirements of the xDSL standard. 
     With continued reference to FIG. 8, the carrier frequency is generated from a voltage controlled oscillator (VCO)  110 , under the control of a microcontroller  112 , which preferably has pulse-width generated digital-to-analog (D/A) conversion capability. The output of the VCO  110  is amplified, as indicated at  114 , before being applied to the mixer  86 . The amplitude of the signal is also controlled by the microcontroller  112 . The half-carrier frequency is generated by dividing the carrier by two, amplifying, and band-pass filtering, as indicated at  116 ,  118  and  120 , before being applied to the coaxial port  50 . The amplitude of the half-carrier signal is also under the control of the microcontroller  112 . Alternatively, the frequency doubler circuit of the NID  22  can be used to generate the half-carrier frequency. A band-pass filter  122  and signal quality monitor  124  are used to provide feedback from the up-shifted frequency components to the microcontroller  112  for frequency adjustments and half-carrier or full-carrier amplitude adjustments. Also, from the quality monitor  124 , the upstream and downstream compensation circuit can null insertion loss of the end-to-end system A dedicated power-supply  126  is preferably provided in the CPE interface device  46  to generate all required voltages. 
     The components in the NID  22  and in the CPE interface device  46  preferably have 75 ohm impedance characteristics, including the coaxial cabling. All coaxial wiring connections in the NID  22  and the customer premises  16  are preferably 75 ohm RG-6 or RG-59 coaxial cable with ‘F’ connectors. 
     The Mixer 
     The mixers  56  and  86  are selected to preferably meet a number of criteria For example, each mixer has 75 ohm impedance characteristics because the entire system is preferably a 75 ohm system The mixers  56  and  86  do not require an external power supply. Thus, the NID  22  can remain as simple and reliable as possible. In addition, having an external power supply in the NID  22  is generally not feasible because of the location of the NID  22  at the customer&#39;s premises. 
     Each mixer  56  and  86  preferably allows for a 25 MHz signal to be placed on the corresponding LO port  66  and  91 , respectively, for relocation of the xDSL signal The mixer allows for a 20 kHz to 1.1 MHz signal to be placed on the corresponding IF port  60  and  80  to include the entire xDSL spectrum Because data transmission is bi-directional both the mixer  56  in the NID  22  and the mixer  86  in the CPE interface device  46  can support all possible xDSL frequencies. The Mini Circuits ZAD-35HB mixer was selected for mixers  56  and  86  in the model, although other mixers can be used. In addition to meeting the above criteria, this mixer also can be purchased in a package to be used on a printed circuit board for the printed circuit board design of the NID  22  and the CPE interface device  46 . 
     With the mixer  56  and  86  being a passive device, and thus not needing an external power supply, the mixer generally requires the 25 MHz LO signal to be at a power level of 17 dBm+/−3 dBm to drive the internal circuitry. To create the necessary 25 MHz, 17 dBm LO signal for both the mixer  56  at the NID  22  and the mixer  86  at the CPE interface device  46 , the LO signal is generated by oscillator circuitry at the customer premises  16  where an external power supply is available. To get the LO signal for the mixer  56  at the NID  22 , the LO signal generated at the CPE interface device  46  is preferably transmitted over the coaxial cable linking the customer premises  16  to the NID  22 . This signal is transmitted in parallel with the CATV and xDSL signals over this link. To avoid the LO signal, which is at 25 MHz, from interfering with the modulated xDSL signals that are close to the LO signal in frequency, the 25 MHz LO signal being transmitted to the CPE  16  from the NID  22  is divided in frequency to 12.5 MHz at the CPE  16  and then transmitted. At the NID  22 , circuitry  70  is provided to multiply the 12.5 MHz signal by two to re-create the 25 MHz carrier for the LO port  66  of the mixer  56  at the NID  22 . Accordingly, this configuration allows the NID  22  to remain simple, reliable, and without an external power supply. 
     There is another advantage to the transmission of the LO signal from the CPE  16  to the NID  22 . Since the LO signal for the mixer  56  in the NID  22  is being generated at the CPE  16 , the CPE is able to control the operation of the NID  22  by varying the frequency and/or the amplitude of the LO signal going to the NID. This allows the CPE  16  to compensate for external conditions that could effect the performance of the system. 
     AM Verification 
     An IF signal between 20 kHz and 1.1 MHz was modulated with a 25 MHz carrier (i.e., an LO signal) to test the amplitude modulation technique described above and to verify the modulated output on the RF ports  58  and  90  of the mixers  56  and  86 . To generate the 20 kHz to 1.1 MHz IF signal, a Wavetek Model  134  signal generator, which has a 0 dBm output power level, was used. As stated previously, generation of a 25 MHz, 17 dBm+/−3 dBm signal for the LO port of both mixers  56  and  86  is desired. Most RF signal generators are not capable of producing a 17 dBm signal. The Hewlett Packard 8654B RF signal generator has a maximum output signal strength of 0 dBm. To achieve the necessary 17 dBm on the LO port of the mixer, a −6 dBm loss splttter and a 23 dBm gain amplifier were used in series with the output of the RF signal generator. As the table below shows, the sum of the 0 dBm output of the RF signal generator, the −6 dBm loss of the splitter, and the 23 dBm gain of the amplifier add to create the necessary 17 dBm input to the LO port of the mixer. 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 RF Signal Generator Output 
                 0 
                 dBm 
               
               
                   
                 Splitter Loss 
                 −6 
                 dBm 
               
               
                   
                 Amplifier Gain 
                 23 
                 dBm 
               
               
                   
                 Total Signal Power to LO port 
                 17 
                 dBm 
               
               
                   
                   
               
             
          
         
       
     
     A ZHL-6A-BNC amplifier and a ZFSC-3-4 splitter by Mini Circuits were used, although components from other manufacturers can be used. Both of these items have 50 ohm impedance characteristics and a desirable frequency range. The amplifier uses an external 24 VDC power supply. Since the 17 dBm LO signal is used at both the NID  22  and at the CPE  16 , and generating this signal at the CPE  16 , as opposed to the NID  22 , is preferred, the LO signal for both mixers  56  and  86  is generated at the CPE interface device  46  where an external power supply  126  exists. The NID  22  is therefore implemented as simply as possible so as to be reliable and reduce the likelihood of the local telephone carrier having to make frequent repairs and replacements. 
     The output of the RF port on the mixer was examined using a spectrum analyzer. Three characteristics were investigated. First, the signal strength of the modulated bands (25 MHz+/− IF frequency) was considered. By examining the signal level, the loss due to the modulation was calculated. Amplitude modulation centers two replicas of the IF signal around the carrier frequency. For example, if the IF frequency is 500 kHz, and this signal is modulated with a 25 MHz carrier, the result is two spectral “bands” at 24.5 MHz and at 25.5 MHz. Ideally, the signal strength of each band is the same as the signal strength of the IF signal. However, due to losses in the mixer, the signal strength of the bands is less than the IF signal strength. These bands are preferably as high in signal strength as possible to ensure that, after demodulation, the original IF signal can be re-created. Also, the signal strength of the modulated “bands” are preferably independent of the IF frequency over the xDSL spectrum. 
     After measuring the signal strength of these modulated bands, it was concluded that over an exemplary xDSL spectrum of 20 kHz to 1.1 MHz, the signal loss of the modulated bands due to the modulation process by the mixer was approximately −2 dBm from the IF port to the RF port. These results meet the criteria for signal strength and frequency independence. The −2 dBm loss is a very small loss, and is not likely to cause problems in re-creating the signal after demodulation. Also, since the signal strength is independent of the IF frequency, the entire xDSL spectrum can be modulated as described above. 
     The signal strength of the 25 MHz component was also considered. Ideally, no carrier component is on the RF port; however, there is usually some component of the carrier that is on the RF port of a mixer. This carrier component is preferably minimized as much as possible to prevent interference with the desired modulated “bands”. The following table shows measured signal strengths of an unwanted 25 MHz carrier component: 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 IF frequency (kHz) Signal strength of unwanted 25 MHz 
               
               
                 carrier component (dBm) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                  20 
                 −30 
               
               
                   
                  50 
                 −32 
               
               
                   
                  75 
                 −35 
               
               
                   
                 100 
                 −36 
               
               
                   
                 125 
                 −39 
               
               
                   
                 160 
                 −42 
               
               
                   
                 240 
                 −48 
               
               
                   
                 400 
                 −26 
               
               
                   
                 600 
                 −24 
               
               
                   
                 800 
                 −23 
               
               
                   
                 1000  
                 −22 
               
               
                   
                 1100  
                 −21 
               
               
                   
                   
               
             
          
         
       
     
     As indicated by the table data, the strongest component of the unwanted 25 MHz carrier is at 1.1 MHz, which is the highest of the xDSL frequencies. This component had a signal strength of −21 dBm. The −21 dBm carrier interference is not likely to be strong enough to cause problems. Also, a “notch” filter tuned to the 25 MHz carrier can be used to block this carrier from causing interference with the xDSL modulated bands. 
     The signal strength of harmonics was considered. When the IF signal is modulated with the carrier or LO signal, there are unwanted harmonics of the modulated “bands”. For example, if the carrier is 25 MHz and the IF signal is 500 kHz, then there are harmonics at 25 MHz+/−1000 kHz, 25 MHz+/−1500 kHz, 25 MHz+/−2000 kHz, and at other integer multiples of the fundamental 500 kHz IF signal. These harmonics are unwanted because they can interfere with other xDSL frequencies or “tones”, and thus prevent signal re-creation. The following table shows measured signal strengths of unwanted harmonics: 
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                 IF frequency (kHz) 
                 2nd (dBm) 
                 3rd (dBm) 
                 4th (dBm) 
                 5th (dBm) 
               
               
                   
               
             
             
               
                  20 
                 −54 
                 −52 
                 N/A 
                 −57 
               
               
                  50 
                 −62 
                 −50 
                 N/A 
                 −55 
               
               
                  75 
                 −57 
                 −48 
                 N/A 
                 −54 
               
               
                 100 
                 −52 
                 −46 
                 N/A 
                 −53 
               
               
                 125 
                 −50 
                 −46 
                 N/A 
                 −52 
               
               
                 160 
                 −48 
                 −46 
                 N/A 
                 −52 
               
               
                 240 
                 −44 
                 −42 
                 −56 
                 −49 
               
               
                 400 
                 −34 
                 −38 
                 −49 
                 −47 
               
               
                 600 
                 −32 
                 −37 
                 −47 
                 −46 
               
               
                 800 
                 −31 
                 −36 
                 −45 
                 −46 
               
               
                 1000  
                 −30 
                 −34 
                 −44 
                 −46 
               
               
                 1100  
                 −30 
                 −34 
                 −44 
                 −46 
               
               
                   
               
             
          
         
       
     
     The modulated xDSL spectrum was investigated by centering it around 25 MHz on the RF port of the mixer and sending it through a demodulation stage (e.g., sending the modulated signal through the RF port of a second mixer that uses the same carrier as the first mixer on the LO port thereof. The IF output of the second mixer is preferably the same signal as the input of the IF port on the first mixer. However, due to losses and harmonic distortion that occur during the mixing process, the IF signal on the second mixer is not exactly the same as the IF input on the first mixer. The output of the IF port on the second mixer was examined using a spectrum analyzer. Two aspects were considered. First, signal loss from the IF port of first mixer to the IF port of second mixer was considered. This loss is the end-to-end signal loss of the xDSL signal. This loss indicates whether the xDSL signal can be re-created after going through the xDSL spectrum relocation system of the present invention. Also, the IF frequency was varied over the xDSL spectrum to ensure that the loss of the system is independent of IF frequency for all of the xDSL spectrum. The signal loss from the IF port of the first mixer to the IF port of the second mixer was determined to be a −4 dBm loss over the entire xDSL spectrum. This loss is not a significant loss and is not likely to prevent signal re-creation. 
     Also considered was the signal strength of harmonics since the harmonic content of the IF signal can potentially interfere with other xDSL frequencies or tones. Because the signal has now been demodulated, the IF signal of the second mixer and it&#39;s interfering harmonics are centered around 0 Hz. The following table shows the signal strengths of the unwanted harmonics that were measured during the experiment: 
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                 IF frequency (kHz) 
                 2nd (dBm) 
                 3rd (dBm) 
                 4th (dBm) 
                 5th (dBm) 
               
               
                   
               
             
             
               
                  20 
                 −52 
                 −48 
                 N/A 
                 −56 
               
               
                  50 
                 −56 
                 −45 
                 N/A 
                 −55 
               
               
                  75 
                 −60 
                 −46 
                 N/A 
                 −54 
               
               
                 100 
                 −56 
                 −44 
                 N/A 
                 −52 
               
               
                 125 
                 −52 
                 −44 
                 N/A 
                 −52 
               
               
                 160 
                 −49 
                 −43 
                 N/A 
                 −51 
               
               
                 240 
                 −44 
                 −41 
                 −56 
                 −49 
               
               
                 400 
                 −34 
                 −38 
                 −49 
                 −47 
               
               
                 600 
                 −32 
                 −36 
                 −47 
                 −46 
               
               
                 800 
                 −32 
                 −33 
                 −45 
                 −45 
               
               
                 1000  
                 −31 
                 −34 
                 −44 
                 −46 
               
               
                 1100  
                 −30 
                 −34 
                 −44 
                 −46 
               
               
                   
               
             
          
         
       
     
     Bi-directional data transmission was simulated by connecting the IF port of the second mixer to a second Wavetek Model 134 signal generator such that one of the Wavetek signal generators simulates “upstream” data and the other simulates “downstream” data. Three characteristics were of interest. First, simultaneous bi-directional data transmission was verified, which is useful information since xDSL technology employs simultaneous, bidirectional data transmission. With one signal generator transmitting in the “upstream” spectrum (20 kHz to 160 kHz) and the other signal generator transmitting in the “downstream” spectrum (240 kHz to 1.1 MHz), simultaneous bi-directional data transmission is verified. Using a spectrum analyzer, both the “up-stream” and “downstream” spectra can be seen simultaneously on the IF ports of the two mixers. 
     Second, the symmetry of the signal loss of IF data was verified. Ideally, the signal loss of the “upstream” data is the same as that of the “downstream” data going the opposite way through the system. This is because the system is itself a symmetric system, with components being in a mirrored configuration around the RF port of the first mixer with respect to RF port of the second mixer connection. Since the components and coaxial cabling are essentially matched, and the frequencies in the xDSL spectrum have essentially the same end-to-end loss, the signal loss is independent of direction through the system. Also, the signal loss of the “upstream” and “downstream” data signals each match the −4 dBm loss described above. The signal loss matched that of single direction data transmission and was also symmetric. This investigation was conducted over the entire exemplary xDSL spectrum to ensure frequency independence. 
     Third, the 25 MHz carrier component in the modulated signal was considered with both mixers performing modulation and de-modulation. The first mixer is modulating the “downstream” signal, while simultaneously de-modulating the “upstream” signal The second mixer is modulating the “upstream” signal, while simultaneously de-modulating the “downstream” signs The fact that both mixers are modulating apparently makes no significant difference in the signal strength of the 25 MHz carrier component of the modulated signal. However, as was mentioned before, a “notch” filter tuned at 25 MHz placed in the modulated signal path can remove interference from this unwanted component. 
     With regard to frequency doubler circuit and the carrier filtering circuit, the purpose of the frequency doubler  70  is to generate the 25 MHz LO carrier for the NID  22  given a 12.5 MHz input. As stated previously, this is preferably implemented by generating a 12.5 MHz carrier signal at the CPE interface device  46 , and then transmuting this carrier with the modulated data signal to the NID  22  over the coaxial link At the NID  22 , the 12.5 MHz signal is then converted to a 25 MHz signal by the frequency doubler circuit. The carrier filtering circuit then filters out harmonics generated by the frequency doubler to ensure that the input to the LO port of the mixer on the NID is a 25 MHz sine wave. This frequency doubling and filtering process is preferably also used at the CPE  16  to generate the 25 MHz carrier for the mixer  86 . By using the same process at both the NID  22  and the CPE  16 , there is greater likelihood that the LO signal at the NID  22  matches the LO signal at the CPE  16 . The likelihood of constructive or destructive interference of the modulated signal due to the two mixers using different LO frequencies is therefore decreased. 
     The basic concept of relocating the xDSL spectrum away from in-home noise can be implemented other ways. For example, the NID device can be located in the CATV pedestal. In that instance, the drop cable from the feeder cable is not in the xDSL path. Typically, the drop cable is of lower performance characteristics than the feeder cable. With this approach, power for the NID is obtained from the 60 Volts (60 Hz) power feed on the CATV system. The CATV provider, however, would have to agree to allowing the xDSL to be transmitted over this coaxial drop. Secondly, the CPE device can be integrated with the xDSL modem or set-top box. This reduces the cost and the complexity of the CPE device both in component count, microprocessor functions, and filtering requirements. Also, instead of using a bidirectional mixer  56  in the NID, the upstream and downstream xDSL spectrums can be separated by filters and modulated and demodulated separately. Also, instead of using customer premise coaxial wiring, only the frequency shifted xDSL signal can be reinserted on the customer premise POTS wiring. However in this implementation, FCC emission limits could be exceeded. Also, another implementation uses wireless communication between the NID and CPE. However, a sufficiently wide consumer frequency band is required to accommodate single-side-band (SSB) modulation of the xDSL signal. 
     In the illustrated embodiment, a single VCO in the CPE  16  is used. Alternatively, two VCOs can be provided for the NID  22  and the CPE  16 , respectively, which can be controlled by the microcontroller  112 . Using separate VCOs allows for independent phase adjustments at the LO ports  66  and  91  of the mixers  56  and  86 , respectively, to improve insertion loss. The VCO for the NID  22  can be located at the CPE  16  or at the NID  22 . If the VCO is located at the NID, control can be provided using a DC bias over the coaxial conductor, thereby providing frequency control and NID power. In accordance with another embodiment of the present invention, single-sideband modulation can be used in lieu of double-sideband modulation to maximize frequency spectrum usage. Insertion losses, however, are likely to increase and require more compensation. 
     Although the present invention has been described with reference to a preferred embodiment thereof, it will be understood that the invention is not limed to the details thereof. Various modifications and substitutions have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. All such substitutions are intended to be embraced within the scope of the invention as defined in the appended claims.