Abstract:
The present invention provides methods and apparatus for transmitting a data-bearing signal and a non-data-bearing signal. One embodiment comprises communicatively coupling a data-bearing signal during a first time period and communicatively coupling a non-data-bearing signal during a second time period. The data-bearing signal has a first PSD. The non-data-bearing signal has a second PSD substantially the same as the first over a range of frequencies. The non-data-bearing signal has characteristics facilitating echo cancellation. Another embodiment comprises a line interface and a transmitter coupled to the line interface, comprising a data encoder and a periodic signal generator. The encoder is configured to produce a data-bearing signal with a first PSD. The periodic signal generator is configured to produce a non-data-bearing signal with a second PSD. The second PSD is substantially the same as the first PSD over a first range of frequencies. The non-data-bearing signal has characteristics facilitating echo cancellation.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 10/654,718, filed Sep. 3, 2003, now U.S. Pat. No. 7,046,798, which claims priority to provisional application Ser. No. 60/407,915, filed Sep. 3, 2002. Both applications are hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to data communications, and more particularly, to a system and method for transmitting a data-bearing signal and a non-data-bearing signal that has characteristics that facilitate echo cancellation. 
     BACKGROUND 
     Crosstalk is a well-known phenomenon in which an electrical signal transmitted on one wire pair in a cable bundle causes interference on other pairs in the same cable. This interference by a “crosstalk disturber” can result in data errors for communications equipment using the affected pair (“crosstalk victims”), such as analog modems, ISDN adapters, and DSL modems. Various techniques are used to reduce errors resulting from crosstalk. For example, some DSL modems test the wire pair when initializing a connection, and only utilize those parts of the spectrum that have low crosstalk impairment. 
     This technique is of limited value when the disturber uses time domain duplexing (TDD), as explained by  FIG. 1 . Remote device  101  and local device  102  are modems at two ends of a DSL connection. Remote device  101  and local device  102  take turns transmitting, with only one of the transmitters active at any one time. In this example, remote device  101  transmits during period  103  and period  104 , while local device  102  transmits during period  105  and period  106 . Victim device  107  is another communications device using a wire pair colocated with the pair used by local device  102 , and therefore a victim subject to possible crosstalk from local device  102 . Local device  102  is the disturber. 
     In order to minimize the effect of crosstalk, victim device  107  measures impairments at the start of a connection, and adjusts spectrum usage to avoid any frequency ranges that are strongly affected by crosstalk. The effectiveness of this strategy depends on whether or not the disturber (local device  102 ) is actually transmitting at the same time that victim device  107  measures the line impairments. If the victim device  107  measures at time  108 , this is an accurate measurement, because the measurement time  108  coincides with the period  105  when the disturber is transmitting. However, if the crosstalk victim device  107  measures at time  109 , this is inaccurate because the disturber is not transmitting at this time. Since crosstalk victim device  107  has no information about when the disturber (local device  102 ) transmits, the effectiveness of the crosstalk avoidance strategy when used with TDD disturbers is unpredictable. A need therefore exists to address these and other shortcomings in the prior art. 
     SUMMARY 
     The present invention is directed to unique methods and apparatus for communicatively coupling a data-bearing signal and a non-data-bearing signal, where the non-data-bearing signal has characteristics that facilitate echo cancellation. One representative embodiment comprises the steps of: sending a data-bearing signal during a first time period; and sending a non-data-bearing signal during a second time period. The data-bearing signal has a first PSD, and the non-data-bearing signal has a second PSD substantially the same as the first PSD over a first range of frequencies. The non-data-bearing signal has characteristics that facilitate echo cancellation. 
     Another embodiment, among others, comprises a line interface and a transmitter coupled to the line interface. The transmitter further comprises a data encoder and a periodic signal generator. The data encoder is configured to produce a data-bearing signal with a first PSD. The periodic signal generator is configured to produce a non-data-bearing signal with a second PSD. The second PSD is substantially the same as the first PSD over a first range of frequencies. The non-data-bearing signal has characteristics that facilitate echo cancellation. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings illustrate several aspects of the present invention, and together with the description serve to explain the principles of the invention. 
         FIG. 1  illustrates a prior art system, with the crosstalk victim testing the wire pair and a TDD disturber colocated with the wire pair. 
         FIG. 2  is a block diagram illustrating a multipoint communications system in which devices employing the subliminal time domain duplex modulation (STDD) modulation of the present invention are used. 
         FIG. 3  is a block diagram illustrating the control device of  FIG. 2 , including the (STDD) modulation logic of the present invention. 
         FIG. 4  is another view of the communications system of  FIG. 2 , with the crosstalk victim testing the wire pair and a TDD disturber colocated with the wire pair. 
         FIG. 5A  is a graph of a simple periodic signal used by the control device of  FIG. 3 . 
         FIG. 5B  is a graph of another periodic signal used by another embodiment of the control device of  FIG. 3 . 
         FIG. 6A  is a graph of the PSD of the data-bearing signal transmitted by the control device of  FIG. 3 . 
         FIG. 6B  is a graph of the frequency range in which the crosstalk victim of  FIG. 4  is susceptible to crosstalk. 
         FIG. 6C  is a graph of the PSD of the non-data-bearing signal transmitted by the control device of  FIG. 3 . 
         FIG. 7  is a block diagram of an example embodiment of a transmitter in the control device of  FIG. 3 . 
         FIG. 8  is a block diagram of an example embodiment of a receiver in the control device of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     Having summarized the inventive concepts of the present invention, reference is now made in detail to the description of the invention as illustrated in the drawings. While the invention will be described in connection with these drawings, there is no intent to limit it to the embodiment or embodiments disclosed therein. On the contrary, the intent is to cover all alternatives, modifications, and equivalents included within the spirit and scope of the invention as defined by the appended claims. 
     The present invention can be implemented in software, hardware, or a combination of the two. In the preferred embodiment, the elements of the present invention are implemented in software that is stored in a memory and that configures and is executed by a suitable digital signal processor (DSP) situated in a communication device. However, this software can be stored on any computer-readable medium, for transport or for use by or in connection with any suitable computer-related system or method. In the context of this document, a computer-readable medium is an electronic, magnetic, optical, or other physical device or means that can contain or store a computer program for use by or in connection with a computer-related system or method. 
     The present invention is generally directed to methods and apparatuses for subliminal time domain duplex modulation (STDD). A transmitter in a conventional TDD system transmits a data-bearing signal during specific time periods, and does not transmit at all during the remaining time periods. A transmitter in an STDD system implemented in accordance with the invention transmits a data-bearing signal during specific time periods, and a non-data-bearing signal during the remaining time periods. The non-data-bearing signal has the same power spectrum density (PSD) as the data-bearing signal, and has characteristics that facilitate echo cancellation by the transmitter. 
       FIG. 2  is a block diagram illustrating a multipoint communications system  201  in which devices employing the STDD modulation of the present invention are used. Remote location  202   a  contains one or more remote devices  203 . Each remote device  203  connects one or more user devices  204  to communication channel  205   a  via communication bus  206 . In addition, user devices such as telephones may be connected directly to communication bus  206 . Communication bus  206  is illustratively the copper wiring infrastructure used throughout a remote location to connect remote devices  203  to communication channel  205 . 
     Communication channel  205  is typically the copper wire pair that extends between a telephone company central office and a remote residential, business, or any other location served by local telephone service. Communication channel  205  connects remote location  202  to either central office  207  or headend  208 . Control device  209  is located at central office  207  or headend  208 . 
     As is well-known in the art, central office  207  contains a switch, and acts to connect remote devices  203  to the public switched telephone network (PSTN). Headend  208  is usually located close to a relatively small group of subscribers, for example, a neighborhood. Headend  208  contains a group of modems used in common by the group of subscribers, and is connected to each subscriber pair in the group. Headend  208  is connected to central office  207  by communication channel  205 . Both headend  208  and central office  207  are capable of communicating with multiple remote locations  202  and multiple remote devices  203 . 
     In the preferred embodiment, control device  209  and remote device  203  are illustratively digital subscriber line (DSL) communication devices. However, the concepts of the present invention are applicable to various other types of communication devices. 
     Signal generating sources may be located in the vicinity of remote device  203  or in the vicinity of control device  209 . For example, switching relay devices located at central office  207  or headend  208  can impart random impulse noise or crosstalk to the communication channel, thus impairing the subscriber line connecting control device  209  to remote device  203 . 
     The subscriber line connecting control device  209  to remote device  203  can be degraded more severely by transmissions from the headend  208 . This is true because headend  208  may be located in a cross-connect box between the central office  207  and remote location  202 , so that the power of the communicated signals has been attenuated by the long communication channel. By using control device  209  and remote devices  203  employing STDD of the present invention, it is possible for the receiver located in either remote device  203  or control device  209  to efficiently measure crosstalk generated in its own vicinity and elsewhere in the communication system. 
       FIG. 3  is a block diagram illustrating the control device  209  of  FIG. 2 , including the STDD modulation logic of the present invention. Typically, control device  209  will transmit signals to remote devices  203  over communications channel  205 . Similarly, remote devices  203  will transmit signals to control device  209 . Control device  209  contains STDD modulation logic  301 , which enables the device to transmit a non-data-bearing signal when not transmitting data, and to cancel the echo from the non-data-bearing signal. 
     Control device  209  contains conventional components as is known in the art of data communications. For example, digital signal processor (DSP)  302  controls the operation of transmitter  303  and receiver  304 , and couples to line interface  305  to gain access to communications channel  205 . Also included in transmitter  303  and receiver  304  is memory  306  which includes STDD modulation logic. In one embodiment, the STDD modulation logic of the present invention is executed within DSP  302  and is therefore shown as residing in both DSP  302  and memory  306 . 
     For simplicity, the system and method for STDD modulation is described as residing in the transmitter and receiver of control device  209 . However, the systems and methods of the present invention can be applied to any communication transmitter and receiver, including the transmitter and receiver located in remote device  203 . 
       FIG. 4  is another view of the communications system  201  of  FIG. 2 , with the crosstalk victim testing the wire pair and a TDD disturber colocated with the wire pair. Remote device  203  and control device  209  are modems at two ends of a DSL connection, as shown in  FIG. 2 . Victim device  401  is another communications device using a wire pair colocated with the pair used by control device  209 . Victim device  401  is therefore a victim subject to possible crosstalk from control device  209 . Control device  209  is the disturber. 
     Remote device  203  and control device  209  take turns transmitting data signal  402 , with only one of the transmitters sending data signal  402  at any one time. (While each data transmission is represented by the same data signal  402 , the data contained in the signal will of course vary from transmission to transmission). When the transmitter in control device  209  is not sending data signal  402 , it sends a non-data-bearing signal  403  which has the same power spectral density (PSD) as data signal  402 . In some TDD systems, data is transmitted for a fixed time period, while in others the period of data transmission can vary, with a guard time in between to let the remote end know the transmission is over. In yet another variation, a variable period with guard time is used but a maximum period is enforced. 
     Victim device  401  measures impairments at the start of a connection, and adjusts spectrum usage to avoid any frequency ranges that are strongly affected by crosstalk. In this example, crosstalk victim device  401  measures at time  404  and time  405 . Because the non-data-bearing signal  403  has the same PSD as data signal  402 , the PSD measured at time  404  has the same value as the PSD measured at time  405 . The resulting adjustments which crosstalk victim device  401  makes to its spectrum usage will therefore be effective no matter what point in time the measurements are taken. A deficiency in the crosstalk victim device  401  can therefore be addressed in the disturber (control device  209 ) by applying the method of the present invention. Further details of the PSD characteristics of the non-data-bearing signal  403  will be discussed later. 
     Non-data-bearing signal  403  also has at least one characteristic which facilitates echo cancellation of the non-data-bearing signal  403  by the receiver. One such characteristic is periodicity, because the echo of a periodic signal is itself periodic. Turning now to  FIGS. 5A and 5B , the nature of a periodic signal which can be used to construct non-data-bearing signal  403  will be discussed.  FIG. 5A  is a graph of a simple periodic signal  501   a  used by the control device of  FIG. 3 . Signal  501   a  has period t.  FIG. 5B  is a graph of another periodic signal,  501   b , used by another embodiment of the control device of  FIG. 3 . Signal  501   b  has period  2   t , and a first portion  502  and second portion  503 . At time t, the phase of first portion  502  is reversed by 180° to produce second portion  503 . 
     The embodiment using signal  501   b  to construct non-data-bearing signal  403  is advantageous in avoiding disruption of the victim&#39;s phase and timing recovery processes. To understand why this is so, suppose the disturber uses single phase periodic signal  501   a  as its non-data-bearing signal  403 , and that crosstalk victim device  401  happens to use a data signal which is in phase with periodic signal  501   a . In this case, the data signal received by the victim device  401  from the associated remote end always adds to periodic signal  501   a  (received as crosstalk). This addition may result in errors in victim&#39;s data signal, and could disrupt the crosstalk victim&#39;s timing recovery processes. 
     But suppose instead that the disturber uses reversing phase periodic signal  501   b  as its non-data-bearing signal  403 . In that case, even if the victim&#39;s data signal is perfectly in phase with the first half of periodic signal  501   b , then by definition it would be perfectly out of phase with the second half, so the additive effects on the victim data signal would cancel each other out. 
     In yet another embodiment (not shown), two phases  504  and  505  are used, and the phase pseudo-randomly switches at each transmission interval. Like the previous embodiment, the use of phase reversal ensures that on the average the effects on the victim will cancel out. In addition, because the phase is pseudo-random, the non-data-bearing signal  403  is even less likely to be mistaken by the victim for data. 
     As discussed above with reference to  FIG. 4 , non-data-bearing signal  403  is constructed to have a PSD similar to data signal  402 .  FIGS. 6A-C  illustrate how this is accomplished.  FIG. 6A  is a graph of the power spectral density (PSD)  601  of the data signal  402  transmitted by control device  209  of  FIG. 3 . In this example, data signal  402  is transmitted by control device  209  using a single carrier modulation scheme, and the resulting PSD  601  is relatively wide. 
       FIG. 6B  does not show a PSD, but rather the frequency range  602  in which the victim device  401  of  FIG. 4  is susceptible to crosstalk. In this example, the victim device  401  uses DMT modulation, in which the spectrum in use is divided into frequency ranges or bins  603 . 
       FIG. 6C  is a graph of the PSD  604  of the non-data-bearing signal  403  transmitted by the control device  209  of  FIG. 3 . This PSD contains frequencies which are used by both victim device  401  in  FIG. 6B  and by the data signal  402  of control device  209  in  FIG. 6A . The PSD  604  is therefore not usually exactly equivalent to the PSD  601  of data signal  402 , because PSD  604  contains only these overlapping portions. The PSD of non-data-bearing signal  403  is different from that of data signal  402  in another way. The PSD  604  of non-data-bearing signal  403  contains sharp spikes  605  corresponding to the bins  603  in  FIG. 6C , where PSD  601  of data signal  402  is smoother. 
     The sharp spikes  605  are present because the non-data-bearing signal  403  is a composite signal made of multiple pure sine waves (tones) at frequencies corresponding to the overlap with the bins in  FIG. 6B . A composite signal of multiple tones allows the signal to meet the periodicity requirement described above while being trivial to implement. It can be implemented by reading samples out of memory and feeding the samples directly to the digital-to-analog converter. In another embodiment, non-data-bearing signal  403  has the additional characteristic that each bin has constant power over time. This results in measurements that are more accurate when the crosstalk victim device  401  takes measurements over a short period of time. 
       FIG. 7  is a block diagram of an example embodiment of a transmitter  701  in the control device  209  (acting as a disturber). Transmitter  701  operates as a conventional transmitter with an additional periodic signal generator  702 . A conventional control sequencer  703  operates switches  704  and  705  to determine what signal is transmitted. 
     During a data period, switch  704  selects encoder  706  for normal transmission. Transmit data is encoded by encoder  706 , which may use any modulation type such as DMT, quadrature amplitude modulation (QAM), carrierless amplitude/phase (CAP) modulation, or any other modulation for encoding data. The output of the encoder  706  is selected by switch  704  for input to the TX Hilbert filter  707 . The Hilbert filter  707  conditions the signal to meet specific transmit spectrum requirements associated with the line. 
     During initialization, the control sequencer  703  may select train encoder  708 . The train encoder  708  generates a special initialization sequence suitable for training adaptive equipment, such as the adaptive equalizer in the receiver of remote device  203  The train encoder  708  may operate in several phases depending on which algorithms are adapting. Phases may include silence, gain control, timing acquisition, carrier phase acquisition, equalization, and other phases. Control sequencer  703  may also select a nominal zero value from block  709  when no signals are to be transmitted. Zero is useful when training a remote echo canceller or the periodic echo canceller of this invention. 
     Periodic signal generator  702  generates non-data-bearing signal  403 . As described above with reference to  FIG. 6 , non-data-bearing signal  403  is a composite signal made of multiple pure sine waves at frequencies shown in  FIG. 6B . In one embodiment, non-data-bearing signal  403  is efficiently generated by reading a series of samples from a store (not shown), and providing the samples to digital-to-analog converter (DAC)  710  at the same symbol rate used by encoder  706 . 
     The control sequencer  703  selects via switch  705  either the non-data-bearing signal  403  or the modulated data signal  402  output from the Hilbert filter  707  for input to the DAC  710 . The Hilbert filter  707  is selected when actively transmitting and the non-data-bearing signal  403  is selected while receiving. 
     In some applications the Hilbert filter  707  may be adaptive, and in this case the non-data-bearing signal  403  is adjusted to match the filter characteristics. One method is to pass the non-data-bearing signal  403  through Hilbert filter  707 , using switch  705 A to optionally select the non-data-bearing signal  403  for input to Hilbert filter  707 . Another method is to use switch  705  to select either the Hilbert filter  707  output, or the periodic signal generator  702  output. 
     The analog signals output from the DAC  710  are processed by conventional analog circuitry  711  including a final line driver or operational amplifier. Finally, the analog signals are transformer coupled to the communications line. 
       FIG. 8  is a block diagram of an example embodiment of a receiver  801  in the control device  209  of  FIG. 3 . Receiver  801  operates as a conventional receiver with an additional periodic echo canceller  802 . Any transmitted signal will result in echo at the local receiver, where the echo is produced by impedance mismatch at the hybrid  2 - 4  wire interface. Conventional echo cancellation involves cancelling echo from data signal  402 . This type of echo cancellation requires adaptive filtering, is computation-intensive, and generally cannot remove all distortion. 
     However, echo generated by a non-data-bearing signal  403  can be efficiently cancelled when non-data-bearing signal  403  is a periodic signal. Echo generated by a periodic signal is itself periodic. Therefore, receiver  801  can build an accurate replica of the periodic echo by simply storing the received echo in memory. Then, the echo can be removed in a manner that is computationally efficient and accurate, by recalling the replica from memory and subtracting it from the received signal. 
     The non-data-bearing signal  403  is generated continuously by transmitter  701  while receiver  801  simultaneously receives from the remote modem. The received data signal  402  from the remote modem is extracted by conventional hybrid circuitry  803 . The non-data-bearing signal  403  is echoed from the transmitted signal back into receiver  801 . 
     The analog signal from hybrid circuitry  803  is converted to digital samples for subsequent digital signal processing by the analog-to-digital converter (ADC)  804 . Multiplier  805  and subtraction device  806  process the digital samples from the ADC  804 . Subtraction device  806  subtracts a replica of the periodic signal echo, stored in echo store  807 , for input to the received signal level (RSL) measuring block  808 . The RSL represents the power of the signal from the remote modem with the periodic signal removed. The RSL is used by the Automatic Gain Control (AGC) block  809  to compute the gain G used by multipliers  805  and  810  to scale the received signal from the ADC and the echo signal from the echo store  807  to the appropriate level for input to the adaptive equalizer  811 . This design includes adaptation after automatic gain control to improve convergence at low receive signal levels. The AGC will change with line conditions. 
     Switch  812  selects either the scaled echo signal from multiplier  810  or zero for input to subtraction device  813 . Subtraction device  813  subtracts the scaled echo from the scaled received signal output from multiplier  805  to produce an echo-cancelled signal with no echo. The echo-cancelled signal is scaled by a factor of 2n by multiplier  814  for input to the adaptive equalizer  811 . The adaptive equalizer  811  compensates the echo-cancelled signal for channel distortion. The equalized signal is then decoded by the data decoder  815  to recover the received data (RXD). 
     The echo-cancelled signal from subtraction device  813  is scaled by a factor of 2ES by multiplier  816  for input to adder  817 . Adder  817  adds the echo-cancelled signal to the echo signal from echo store  807  to update the echo samples within the echo store  807 . The echo store  807  is a double precision memory array, which stores one full period of the echo. 
     For rapid initialization, the received echo is simply stored in echo store  807  by opening switch  812  whenever there is no received signal from the remote modem. Then, when there is a received signal, the echo is recalled from echo store  807  and subtracted from the signal by closing switch  812 . The result is a high quality echo cancellation leaving a clean signal for the receiver to decode. The shift exponent ES can be set as high as 2-4 to acquire in less than 40 ms during training when no received signal (only echo) is present. A value as low as 2-20 can be used for tracking in final data mode with received signal present. The received signal is typically more than 50 dB above the residual echo, requiring very slow adaptation. At the end of each message, the received signal is turned off and additional tracking gain can be applied. 
     During initialization, the control device  209  selects the non-data-bearing signal  403  for transmission via switch  705 , while the remote device  203  selects zero for transmission via switch  704  in his transmitter  701 . The control device  209  opens switch  812  to halt echo cancellation and increases the scalar 2ES to rapidly store the incoming echo. Many periods of the echo can be accumulated within the echo store  807  by the action of adder  817 . This effectively averages many samples of the echo to suppress noise or other external sources of distortion. 
     During data reception, the control device  209  selects the non-data-bearing signal  403  for transmission via switch  705 , while the remote device  203  selects the encoder  706  for transmission via remote switch  704 . The local modem control device  209  closes switch  812  to activate echo cancellation and decreases the scalar 2ES to slowly track any changes to the incoming echo. 
     The foregoing description has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments discussed, however, were chosen and described to illustrate the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variation are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly and legally entitled.