Abstract:
A system and method for transmitting data over a twisted pair with or without load coils using a transmitter and receiver is disclosed. The system detects load coils by generating and transmitting a test signal having signal power concentrated in two different frequency bands across the twisted pair and comparing the signal power of the received signal to determine whether the twisted pair is loaded or unloaded. If load coils are detected, an adjustment circuit is used to configure the receiver for reception of data over a twisted pair having load coils. Otherwise, the adjustment circuit configures the receiver for reception of data over a twisted pair without load coils.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to high-speed data communications over conventional telephone lines with or without load coils. More particularly, this invention pertains to a system and method built into a high-speed digital communication transceiver pair for detecting the presence and absence of load coils on a telephone line and adapting each transceiver based on the presence or absence of load coils. The adaptation of each transceiver provides for improved data transmission reliability over both loaded and unloaded lines. 
     Telephone companies and other providers of communications services have sought in recent years to develop improved hardware and techniques for using existing conventional copper telephone lines, such as twisted pairs, to transmit and receive digital data. One of the services offered by telephone companies is digital data services (DDS) at up to 64 Kbps over four wire unloaded twisted pairs. 
     One of the disadvantages of transferring data over conventional telephone lines, i.e., over twisted wire pairs, is dealing with a variety of line impairments, including hardware added to the twisted wire pairs that was intended to be used for analog voice services. For example, telephone companies have conventionally attached load coils at periodic intervals along the twisted pairs connecting the central office to the customer premises. The load coils reduce attenuation across the voice frequency band thereby maintaining voice quality over a range of line lengths. 
     Unfortunately, the load coils also cause a substantial roll-off or attenuation of the frequency response of the twisted pair above 3500 Hz. The roll-off limits the frequency bandwidth available for data transfer using the twisted pair. This can cause significant problems if the twisted pair is used for DDS because the frequency bandwidth necessary for DDS is greater than 3500 Hz. 
     As a result, service providers must first determine if load coils are present before they can use prior art transceivers with a given twisted pair. If load coils are present they must be removed in order to deploy DDS. Information regarding the presence or absence of load coils on a given twisted pair, however, is not always readily available. 
     What is needed, then, is a system and method that may be incorporated into a high-speed digital transceiver pair for detecting and compensating for load coils in a twisted pair. 
     SUMMARY OF THE INVENTION 
     The present invention provides a system and method that is incorporated into a high-speed digital communications transceiver pair and is operable to determine whether or not load coils are connected to a twisted pair. In addition, the system and method automatically adjust the transceiver pair to compensate for the presence or absence of load coils on the twisted pair. The present invention determines whether or not load coils are connected to a twisted pair by transmitting a test signal, which has signal power (i.e., frequency components) concentrated in the voice frequency band and signal power (i.e., frequency components) concentrated outside of the voice frequency band, over the twisted pair. The voice frequency band is defined within the present invention as the band of frequencies ranging from approximately 300 Hz to 3300 Hz. The signal power concentrated in each band is not critical so long as it is possible to distinguish the signal power in one band from the signal power in the other band. 
     When load coils are connected to the twisted pair, the load coils reduce the attenuation of frequency components inside the voice frequency band, while at the same time, significantly increasing the attenuation of frequency components outside of the voice frequency band as compared to unloaded loops. 
     By measuring the signal power that passes through the twisted pair it is possible to determine whether or not load coils are connected to the twisted pair. For example, if a signal having frequency components concentrated inside and outside the voice frequency band is transmitted over a twisted pair, and the signal received on the an other side of the twisted pair has frequency components concentrated outside of the voice frequency band that have been significantly attenuated by the transmission through the twisted pair relative to the frequency components concentrated inside the voice frequency band, load coils are connected to the twisted pair. Otherwise, load coils are not present. 
     In one embodiment, the present invention determines whether or not load coils are connected to a twisted pair by calculating the ratio of the power of the received signal outside of the voice frequency band and the power of the received signal inside the voice frequency band and comparing this ratio to a predetermined constant. If the ratio of powers is greater than the predetermined constant, then load coils are not connected to the twisted pair. If the ratio is less than the predetermined constant, then load coils are connected to the twisted pair. 
     Once the present invention determines whether or not load coils are connected to the twisted pair, an adjustment circuit is used to adjust the transceiver parameters accordingly. To put it another way, if load coils are detected, the adjustment circuit causes the transceiver to use one set of transceiver circuits and parameters and, if load coils are not detected, the adjustment circuit causes the transceiver to use another set of transceiver circuits and parameters. 
     In one embodiment, the present invention includes a transmitter connected to a receiver using a twisted pair. The transmitter includes a signal generating circuit that is operable to generate and transmit a test signal that contains a first signal at a first frequency and a second signal at a second frequency. The first signal has a frequency that is higher than the highest frequency in the voice frequency band and the second signal has a frequency that falls within the voice frequency band. In other words, the signal power of the test signal is concentrated inside and outside of the voice frequency band. In one embodiment, the first signal has a frequency of 4 kHz and the second signal has a frequency of 1.5 kHz. 
     The receiver includes a load coil detection circuit that is operable to receive and filter the test signal, which is modified by transmission across the twisted pair, to separate out the first and second signals. The load coil detection circuit further includes a signal power measurement circuit for measuring the signal power of the first and second signals and a comparator circuit for comparing the measured power of the first signal with a scaled measured power of the second signal. If load coils are present, then the first signal will be more attenuated than it would be if load coils were not present. In a similar manner, the second signal will be less attenuated if load coils are present. 
     Finally, the receiver includes an adjustment circuit that is used to adjust the receiver based on the presence or absence of load coils on the twisted pair. 
     In an alternative embodiment, the present invention is incorporated into a pair of transceivers. In this alternative embodiment, the present invention simply uses a transmitter located in the first transceiver and a receiver located in the second transceiver to determine if load coils are present on the twisted pair. Once this determination is made, the adjustment circuit in the second transceiver is used to adjust the receiver in the second transceiver accordingly. In addition, this information is sent to the first transceiver and the first transceiver is adjusted accordingly. 
     In another alternative embodiment, the present invention uses the transmitter located in the first transceiver and the receiver located in the second transceiver to determine if load coils are present on the twisted pair. In addition, in this embodiment the present invention also uses the transmitter located in the second transceiver and the receiver located in the first transceiver to determine if load coils are connected to the twisted pair. In this case, the adjustment circuits in both the first and second transceivers are used to adjust the transmitters and receivers in each transceiver accordingly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of one embodiment of the present invention. 
     FIG. 2 is a block diagram of one embodiment of the load coil detection circuit of the present invention. 
     FIG. 3 is a plot of insertion loss versus frequency for a loaded and unloaded twisted pair. 
     FIG. 4 is a block diagram showing receiver parameters that are adjusted based on the presence or absence of load coils. 
     FIG. 5 is a block diagram of a second embodiment of the present invention. 
     FIG. 6 is a block diagram of a third embodiment of the present invention. 
     FIG. 7 is a flow chart showing the pre-training, training, and data transmitting modes of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As an initial note, the present invention is preferably integral to a high-speed data communication transceiver pair such as the system described in detail in Applicant&#39;s co-pending U.S. patent application Ser. No. 09/668,856 filed on Sep. 25, 2000 and entitled “A Method and Apparatus for Duplex Transmission on a Four Wire Communication System.” Accordingly, the disclosure of that application is hereby incorporated by reference in its entirety into this application. 
     A block diagram of one embodiment of the present invention is shown in FIG.  1 . This embodiment includes a transmitter  10  connected to a receiver  12  using a twisted pair  14 , which may or may not be connected to load coils (not shown). Transmitter  10  includes a test signal generating circuit  16  that is operable to generate and transmit a test signal  17  over the twisted pair  14  to the receiver  12 . The receiver  12  includes a load coil detection circuit  18  for detecting the presence or absence of load coils on the twisted pair  14  and an adjustment circuit  20  for adjusting parameters in the receiver  12  to compensate for the presence or absence of load coils. 
     Test signal generating circuit  16  includes a first signal generator  22  for m generating a first signal  24  having a first frequency, f 1 , which results in a signal having a signal power concentrated outside of the voice frequency band. In one embodiment, the first frequency is 4000 Hz. In alternative embodiments, other frequencies may be used as long as those frequencies are higher than the highest frequency in the voice frequency band or, in other words, the first signal has a frequency that results in a signal power concentrated outside of the voice frequency band. For example, the first signal might include a series of pulses, a single pulse, a square wave, or multiple sine waves. 
     Test signal generating circuit  16  also includes a second signal generator  26  for generating a second signal  28  having a second frequency, f 2 , which results in a signal having a signal power concentrated inside of the voice frequency band. In one embodiment, the second frequency is 1500 Hz. Alternative embodiments may include a second frequency chosen from any frequency within the voice frequency band so long as the resulting signal includes a signal power concentrated in the voice frequency band. In a manner similar to that discussed with regard to the first signal, the second signal might also include a series of pulses, a single pulse, a square wave, or multiple sine waves. 
     A signal combiner (or summing circuit)  30  is included in the test signal generating circuit  16  for combining the first and second signals,  24  and  28 , to form the test signal  17 . In one embodiment, the signal combiner  30  is a summer and simply sums the two signals together to create the test signal  17 . 
     In one embodiment, the test signal generating circuit  16  is implemented using a Digital Signal Processor circuit (not shown). A variety of other conventional signal generating devices and circuits may be used to generate these signals as well. In this embodiment, the test signal  17  is generated in a digital form and must be converted into an analog signal. As a result, the transmitter  10  includes a digital-to-analog (D/A) converter circuit  32  for converting the digital version of test signal  17  into an analog version. In addition, the transmitter  10  in this embodiment also includes a gain and filtering circuit  34  for boosting the signal strength of the resulting analog signal prior to transmission across the twisted pair  14 . Those skilled in the art are aware of how to use DSPs, D/A circuits, and gain and filtering circuits to perform signal generation, combining, filtering, as well as other processes, in the transmitter  10 . 
     Referring again to FIG. 1, the test signal  17  is transmitted across twisted pair  14  to form a received test signal  19 . The received test signal  19 , which is essentially the test signal  17  as modified by the transmission characteristics of the twisted pair  14 , is then applied to the input of receiver  12 . 
     Receiver  12  includes a gain and filtering circuit  36  and an analog-to-digital (A/D) circuit  38 . The gain and filtering circuit  36  is operable to boost the signal strength of the received test signal  19  and the A/D circuit  28  is operable to convert the received test signal  19  into a digital signal. Gain and filtering circuit  36  and A/D circuit  38  are required when the processing of the received test signal  19  is to be performed using a DSP circuit, but would not be necessary if the signal processing is performed using analog circuits. 
     As mentioned previously, receiver  12  includes a load coil detection circuit  18  and an adjustment circuit  20 . In a preferred embodiment, the load coil detection circuit  18  and the adjustment circuit  20  are implemented using a Digital Signal Processor (DSP) circuit (not shown). However, these circuits could be implemented using other conventional circuits as well, such as Fast Fourier Transforms (FFTs), Discrete Fourier Transforms (DFTs), wavelet transforms, and others known to those skilled in the art. 
     Referring to FIGS. 1 and 2, load coil detection circuit  18  in receiver  12  includes a signal power measuring circuit  40  connected to a comparator circuit  42 . Signal power measuring circuit  40  includes a pair of filters,  44  and  46 , for filtering the received test signal  19 . Filter  44  is a bandpass filter centered around f 1  allowing the first frequency f 1  to pass while substantially rejecting f 2 , while filter  46  is a bandpass filter centered around f 2  allowing the second frequency f 2  to pass while substantially rejecting f 1 . 
     Filter  44  filters the received test signal  19  to obtain a first filtered signal  48  and filter  46  filters the received test signal  19  to obtain a second filtered signal  50 . Preferably, the filters and other circuits contained in the receiver  12  are implemented using conventional digital techniques in a manner known to those of skill in the art. 
     First filtered signal  48  is essentially a version of the first signal  24  as modified by the transmission characteristics of the twisted pair  14 . In other words, filtered signal  48  has the same frequency as the first signal  24 , but has an amplitude that has been attenuated by the twisted pair  14 . When the twisted pair  14  includes load coils the attenuation of the amplitude of the first signal  24  is greater as compared to the attenuation that occurs when the twisted pair  14  does not contain load coils. 
     In a similar manner, second filtered signal  50  is a modified version of second signal  28 . In this case, however, the attenuation of the amplitude of the second signal  28  is less when load coils are present as compared to the attenuation that occurs when load coils are absent from the twisted pair  14 . 
     In one embodiment, filter  44  is a band pass filter having a center frequency of f 1  and filter  46  is a band pass filter having a center frequency of f 2 . In alternative embodiments, other filters may be used as long as the filters are designed to provide the above described filter characteristics such that there is no significant frequency overlap in the bandpass areas of the filters. 
     Signal power measuring circuit  40  also includes a first squaring circuit  52  for squaring the first filtered signal  48  (the resulting signal is referred to as the first squared signal  54 ) and a second squaring circuit  56  for squaring the second filtered signal  50  (similarly, the resulting signal is referred to as the second squared signal  58 ). 
     First squaring circuit  54  is connected in turn to a first integrate and dump circuit  60  and second squaring circuit  56  is connected to a second integrate and dump circuit  62 . Both integrate and dump circuits are designed to integrate a predetermined number of samples, N, of any signal on their inputs. In one embodiment, N is thirty (30), however, more or less samples may be used as well. 
     One benefit of integrating, or alternatively accumulating, a predetermined number of samples is to reduce the effect of noise. In any event, the outputs of the integrate and dump circuits are considered to be equivalent to the signal power of each filtered signal. 
     Accordingly, first integrate and dump circuit  60  integrates first squared signal  54  to obtain a first integrated signal  64  (also referred to as a first signal power  64 ), which is coupled to the positive input of a summer  66 , which forms part of the comparator circuit  42 . Similarly, second integrate and dump circuit  62  integrates second squared signal  58  to obtain a second integrated signal  68  (also referred to as a second signal power  68 ), which is coupled to the negative input of the summer  66  through a scaling circuit  70 , which is also a part of comparator circuit  42 . 
     Scaling circuit  70  multiplies the second integrated signal  68  by a constant number (or scaling factor), A, in order to generate a scaled signal  72 . The scaling factor is selected such that the output of the summer  66  is positive for unloaded twisted pairs and negative for loaded twisted pairs. In one embodiment, A is {fraction (1/64)}. In alternative embodiments, the scaling factor may vary. 
     To illustrate, consider FIG. 3, which shows a plot of insertion loss (caused by the twisted pair  14 ) with respect to frequency. Line  92  is a plot of the losses caused by twisted pair  14  as the frequency transmitted over the twisted pair  14  varies from 0 to 7000 Hz. Line  94  is a similar plot of the losses when load coils are connected to the twisted pair  14 . 
     Referring to line  92 , the attenuation at a frequency of 1500 Hz caused by the twisted pair  14  is approximately 20 dB, while at a frequency of 4000 Hz the attenuation is approximately 34 dB. Multiplying the second integrated signal  68  by scaling factor A reduces the second integrated signal  68  enabling a direct comparison of the two signals. As the result, scaled signal  72  is less than the first integrated signal  64  and when the first integrated signal  64  is summed with the scaled signal  72 , the resulting difference signal  74  obtained at the output of summer  66  is a positive value. 
     Referring to line  94 , the amount of attenuation at 1500 Hz caused by the twisted pair  14  with load coils (or more generally the loaded twisted pair) is approximately 7 dB. The amount of attenuation at 4000 Hz is approximately 71 dB. Thus, the difference in attenuation is much more pronounced than in the case where the twisted pair  14  was unloaded, i.e., with no load coils. 
     In this case, the scaled signal  72  will be larger than the first integrated signal  64 , which will be very small relative to the scaled signal  72 . As a result, the difference signal  74  will be less than zero, i.e., a negative value. Thus, the present invention determines whether load coils are present by determining whether the difference signal  90  is a positive value or a negative value. 
     Note that the polarity of the summer inputs could be reversed and the difference signal  74  could swing positive when load coils are present on the twisted pair  14  and the present invention contemplates this variation as well. 
     The above described comparator circuit may be expressed mathematically as P 1 -AP 2 , where P 1  is the first signal power  64 , A is the scaling factor, and P 2  is the second signal power  68 . When P 1 -AP 2 &gt;0 load coils are not connected to the twisted pair. In a similar manner, when P 1 -AP 2 &lt;0 load coils are connected to the twisted pair. This mathematical expression may also be rearranged to form the equivalent mathematical expression P 1 /P 2 &gt;A and the present invention contemplates implementing the comparator circuit using this type of expression as well. 
     Regardless of which alternative is used, once the present invention determines that load coils are or are not present, the receiver parameters are modified accordingly using the adjustment circuit  20  in the receiver  12 . If load coils are present, then, in a preferred embodiment, the following circuits or parameters are selected in the receiver  12 : a loaded hybrid circuit, loaded coefficients for LEQ initial coefficients, loaded LEQ gains, loaded DFE gains, loaded timing loop gains, and loaded timing loop training lengths. In a similar manner, if the twisted pair  14  is unloaded, then an unloaded version of each of these parameters is selected. These circuits and parameters are shown generally in FIG.  4 . Additional information regarding how these circuits and parameters operate together may be found in the patent application entitled “A Method and Apparatus for Duplex Transmission on a Four Wire Communication System” and referred to earlier in this application. 
     In brief, the LEQ initial coefficients are loaded into the LEQ filter at the start of a data training stage. The LEQ and DFE gains control the speed of the adaptation of the LEQ and DFE, respectively. Timing loop gains determine the acquisition and tracking of the timing loop and timing loop training lengths determine the amount of time spent in the acquisition and training of timing. 
     The details regarding the selection of the appropriate circuits and parameters to be used for loaded and unloaded twisted pairs is known in the art and will not be discussed in detail here since this information is not deemed critical to an understanding of the present invention. 
     The foregoing description of the transmitter  10  and the receiver  12  of the present invention may be implemented in several different embodiments. For example, FIG. 1 shows the implementation of the present invention using a single transmitter  10  and a single receiver  12 . FIG. 5, on the other hand, shows the present invention implemented using a pair of transceivers,  76  and  78 . In this embodiment, the present invention contemplates using the transmitter  10  of FIG. 1 located in the first transceiver  76  and the receiver  12  of FIGS. 1 and 2 located in the second transceiver  78 . Furthermore, in this embodiment, the receiver  12  generates and transmits a signal containing information regarding the presence or absence of load coils on the twisted pair to the first transceiver  76  and an adjustment circuit (not shown) in the first transceiver  76  adjusts the first transceiver  76  accordingly. 
     Finally, FIG. 6 also shows the present invention implemented using a pair of transceivers, each transceiver having a transmitter  10  and a receiver  12  as described previously with respect to FIGS. 1 and 2. In this embodiment, the present invention contemplates sending a test signal  17  from the transmitter  10  in the first transceiver  76  to the receiver  12  in the second transceiver  78  and sending a test signal  17  from the transmitter  10  in the second transceiver  78  to the receiver  12  in the first transceiver  76 . Note in this embodiment there are also two additional components, hybrid circuits,  80  and  82 . 
     Hybrid circuits  80  and  82  attempt to generate a replica of the transmit echo or reflection caused by the mismatch between the transmit impedance and the loop. The replica is subtracted from the received signal, thereby reducing the transmit echo or reflection level in the receive signal. The function performed by the hybrid circuits  80  and  82  is similar to the function performed by analog echo canceller circuits, which are known in the art. 
     Hybrid circuit  80  includes an unloaded hybrid circuit  84  and a loaded hybrid circuit  86 . In a similar manner, hybrid circuit  82  includes an unloaded hybrid circuit  88  and a loaded hybrid circuit  90 . If the present invention determines that load coils are present on the twisted pair  14 , then the adjustment circuit  20  (not shown in FIG. 6, but see FIG. 1) in the first transceiver  76  causes the loaded hybrid circuit  86  to be selected. If, on the other hand, load coils are not present, the adjustment circuit  20  in the first transceiver  76  causes the unloaded hybrid circuit  84  to be selected. A similar type of selection occurs with respect to unloaded hybrid circuit  88  and loaded hybrid circuit  90  in transceiver  78 . 
     Transceivers,  76  and  78 , also include data inputs,  92  and  94 , and data outputs,  96  and  98 , for inputting and outputting data from the transceivers. Similarly, in FIG. 5, transceiver  76  includes a data input  100  and transceiver  78  includes a data output  102  for inputting and outputting data, respectively. Finally, with respect to FIG. 1, transmitter  10  and receiver  12  include data input  104  and data output  106 , respectively. Inputting and outputting data using transmitters, receivers, and transceivers is well known to those skilled in the art and will not be discussed in further detail. 
     The present invention contemplates using an arrangement of the type shown in FIG. 6 between a telephone company&#39;s central office and the telephone company&#39;s customer&#39;s premises or between two of the telephone company&#39;s central offices. Other connection arrangements are also possible as well. 
     It is also contemplated that the present invention will be used in conjunction with pre-training, training, and data transmitting modes. These modes are described in detail in the patent application entitled “A Method and Apparatus for Duplex Transmission on a Four Wire Communication System” and referenced above. 
     Referring to FIG. 7, a flow chart showing the pre-training, training, and data transmitting modes contemplated by the present invention is shown. In the pre-training mode, the transmitter  10  in transceiver  76  (see FIG. 6) generates and transmits a test signal  17  across the twisted pair  14 . 
     Receiver  12  in the second transceiver  78  receives the transmitted test signal (also referred to as received test signal  19 ) and determines if load coils are present on the twisted pair  14 . If load coils are present, then the adjustment circuit  20  in the second transceiver  78  selects the loaded hybrid circuit  90  and sets receiver parameters in receiver  10  of the second transceiver  78  to the receiver parameters used to transmit data over a loaded twisted pair (also referred to simply as the loaded receiver parameters). If load coils are not present, then the adjustment circuit  20  in the second transceiver  78  selects the unloaded hybrid circuit  88  and the unloaded receiver parameters. 
     Next, the present invention transitions from the pre-training mode to the training mode, which is described in detail in the earlier referenced patent application entitled “A Method and Apparatus for Duplex Transmission on a Four Wire Communication System.” Once the training mode is completed then the present invention transitions from the training mode to the data transmitting mode, which is also described in detail in the earlier referenced patent application entitled “A Method and Apparatus for Duplex Transmission on a Four Wire Communication System.” 
     The system and method of the present invention will automatically begin operating and sensing the presence or absence of a load coil when a transceiver such as that described above is first powered up. Furthermore, it will be apparent to those of skill in the art that the system and method of this invention may be implemented in software and will preferably be integrated into existing digital signal processing (DSP) devices in the transceiver. 
     Thus, although there have been described particular embodiments of the present invention of a new and useful “System And Method For Transmitting Data Over Loaded and Unloaded Twisted Pairs,” it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.