Abstract:
A system and method for shaping a transmit signal spectrum allows a transmit precoder and a spectral shaping filter to precode and shape a transmit signal without the need for receiving communication channel response information from a receiver or the need for a training period. The resultant shaped transmit spectrum signal meets regulatory requirements for the amount of energy transmitted per hertz of bandwidth.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This document claims priority to and the benefit of the filing date of co-pending provisional application entitled SPECTRAL SHAPING, assigned Ser. No. 60/039,050, and filed Mar. 5, 1997 and is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to communication systems, and more specifically, to a system and method which allows for the shaping of a transmit signal spectrum in a modem in order to control the amount of power transmitted in a particular frequency bandwidth. 
     BACKGROUND OF THE INVENTION 
     Modems, or remote terminal units, convey information from one location to another over a data communications connection. Digital Subscriber Line (DSL) technology now enables modems to communicate large amounts of data. Modems normally communicate by modulating a baseband signal carrying digital data, precoding the transmit signal to match channel impulse response characteristics, converting the modulated digital data signal to an analog signal, and transmitting the analog signal over a conventional copper wire pair using techniques that are known in the art. These known techniques include mapping the information to be transmitted into a signal space constellation, differentially encoding the information, and modulating the information. The signal space constellation can include both analog and digital information or merely digital information. 
     Communication signals, such as those used in digital subscriber line (DSL) technology, that are transmitted over an analog copper wire pair must meet specific power and frequency requirements as required by government regulations. These requirements dictate that a limited amount of energy per unit of bandwidth be transmitted for a given portion of the frequency spectrum. Typically, higher power is permitted to be transmitted at lower frequencies, while as transmit frequency increases, the power transmitted must be decreased. These government regulatory requirements are designed to limit interference and are imposed on all services that share the analog communications system. Previously, there has been no way to provide the required spectral shaping without unduly limiting the available transmit power. 
     Furthermore, conventional preceding techniques as described in U.S. Pat. No. 5,396,515, U.S. Pat. No. 5,559,835 and U.S. Pat. No. 5,159,610, and specified in the International Telecommunications Union-Telecommunications (ITU-T) V0.34 standard require a receiver to reply to a transmitter with information regarding the channel response of the communications channel during a training period so as to enable the transmitter to set its precoder. This requires a lengthy training period in which a modem transmitter awaits information from a receiver in order to set the transmitter&#39;s precoder in order to maximize transmission efficiency. 
     SUMMARY OF THE INVENTION 
     It is desirable to have a precoder in a modem transmitter that also contains a shaping filter that can shape the transmit signal spectrum to meet regulatory requirements without requiring a training period. The present invention provides a system and method for shaping a transmit signal spectrum. The system includes a filter having a set of filter coefficients defining a particular transmit spectrum. The defined transmit spectrum is one that meets regulatory requirements regarding power transmitted within particular frequency limits. In the preferred embodiment, a finite impulse response (FIR) filter is illustrated, however other filter configurations are possible. The filter is designed to receive a signal space mapped signal from a mapper as is known in the art of modem communications. This signal space mapped signal can be a multidimensional mapped signal, for example a quadrature amplitude modulated (QAM) signal, or it can be a one-dimensional signal, such as that generated by pulse amplitude modulation (PAM). This signal space mapped signal has subtracted from it a substantially flat spectrum signal from a modulo operator. The output of the filter is added to the signal space mapped substantially flat spectrum signal and provides as an output a precoded signal having the desired transmit signal spectrum. 
     The filter signal is fed back into a feedback loop designed to receive the same signal space mapped substantially flat spectrum signal that was input to the filter and, due to feedback, generate the inverse transfer function, resulting in an output that has a frequency response exactly opposite that of the output of the filter. A modulo operator in the feedback loop follows the filter. The modulo operator is designed to operate on the signal from the filter in order to restrict the magnitude of the output of the filter resulting in a substantially flat spectrum signal. The modulo operator operates on the signal from the filter whereby if the voltage of the signal is greater than a preset value, twice that value is subtracted from the signal. A subtractor designed to subtract the output signal of the modulo operator from the signal space mapped signal provides the input to the filter. The input to the filter is essentially a substantially flat spectrum signal and results in a precoded transmit signal output from the filter that is supplied to an adder, which is configured to add the output of the filter to the substantially flat spectrum signal and output a transmit signal that meets the desired transmit spectrum characteristics. 
     The invention has numerous advantages, a few of which are indicated hereafter, as examples. 
     An advantage of the present invention is that it allows the shaping of a transmit signal using a precoder without the requirement of a receiver sending back to a transmitter a signal containing channel response information. For example, the present invention can be implemented as a standard preset precoder that uses existing coefficients or in combination with a standard precoder using remotely computed coefficients. 
     Another advantage of the present invention is that it optimizes performance in a modem that implements spectral shaping. 
     Another advantage of the present invention is that modem training is not required, thus allowing the transmitter to develop the required transmit signal spectrum including precoding without the need for modem training. This allows the modem to be used in broadcast applications where multiple receivers monitor a common transmitter. 
     Another advantage is that the modem can be used in multipoint applications where multiple transmitters are used to transmit to a common receiver. 
     Other features and advantages of the present invention will become apparent to one of skill in the art upon review of the following drawings and the detailed description of the preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed on clearly illustrating the principles of the present invention. 
     FIG. 1 is a block diagram illustrating a network topography containing a plurality of digital subscriber line (DSL) remote modems and control modem embodying the concepts and features of the present invention; 
     FIG. 2 is a block diagram illustrating a modem of FIG. 1 containing the concepts and features of the present invention; 
     FIG. 3 is a block diagram illustrating the transmitter of FIG. 2; 
     FIG. 4 is a block diagram illustrating the precoder and signal shaping filter of the transmitter of FIG. 3; and 
     FIG. 5 is a graphical representation of the shaped transmit signal spectrum of the transmitter of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention can be implemented in software, hardware, or a combination thereof. In the preferred embodiment, the elements of the present invention are implemented in software that is stored in a memory and that configures and drives a suitable digital signal processor (DSP) situated in a modem. However, the foregoing software can be stored on any computer-readable medium 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 preferred embodiment will be described in the context of a multidimensional signal space signal with the implementation of FIR filters, however, other filter designs and one-dimensional signal space signals are contemplated to be within the spirit and scope of the present invention. 
     With reference now to the figures wherein like reference numerals designate corresponding parts throughout the several views, FIG. 1 shows a view illustrating a multipoint communications topography  11  in which digital subscriber line (DSL) remote modems  18  and control modem  13  employing the concepts and features of the system and method for transmit signal spectral shaping are used. Remote location  16  is connected to central office location  12  and control modem  13  via communications channel  14 . Channel  14  is typically a copper wire pair that runs between a telephone company central office and a remote residential or business location. Remote location  16  contains one or more remote modems  18  connecting one or more user devices  17  to communication channel  14 . Remote location  16  can be a residential, business or any other location. By using modems  18  and  13  employing the concepts and features of the present invention, it is possible to transmit a signal spectrum that is shaped to meet regulatory requirements while taking advantage of precoding the signal in the transmitter of the modem without the need for a training period in which a receiver communicates channel impulse response characteristics back to the transmitter. The present invention benefits from the use of circular constellations with either coded or uncoded modulation, while taking advantage of the improved signal-to-noise ratio made possible with precoding and nonlinear encoding. While the following preferred embodiment is described with reference to modem  18 , the concepts and features of the system and method for transmit signal spectral shaping are equally applicable to control modem  13 . 
     Now referring to FIG. 2, shown is a schematic view illustrating a modem  18  of FIG. 1 employing the concepts and features of the present invention. Modem  18  contains conventional components as is known in the art of data communications. Central processor  21  controls the operation of the modem&#39;s transmitter  35  and receiver  45  through logical interface  24 . Memory  23  contains precoder and spectral shaping filter logic  50  of the present invention. Illustratively, memory  23  stores precoder and spectral shaping filter logic  50  that is executed by the central processor  21 . The components of the modem connect to communications channel  14  through line interface  22 . By employing the concepts of the present invention, a transmit signal spectrum that meets regulatory requirements can be achieved without the need for a training period, resulting in a shaped signal spectrum having a high degree of stability. 
     With reference to FIG. 3, shown is a preferred embodiment of a transmitter  35  of modem  18  employing the concepts of the present invention. This embodiment is merely an example of a possible implementation. An International Standards Architecture (ISA) bus, a standard computer bus which eliminates the need for interfaces, supplies data, in the form of a data word that can be either 8 or 16 bits for the preferred embodiment, on connection  31  to scrambler  32 . This data word is transformed into an N bit word by counting bits and shifting to arrive at a smaller number of bits, in this example, an N bit data word. By employing a circular constellation, N can be any number. The conversion of the 8 or 16 bit word to an N bit word is known in the art and omitted in FIG. 3 for clarity. 
     The N bit word is then scrambled by scrambler  32 . Scrambler  32  can be either a self synchronized scrambler or a preset free running scrambler as is known in the art. Depending on the application, the preset scrambler may have some advantages, as in the case of using Reed-Solomon coding, as is known in the art. A scrambled N bit word is output on connection  33 . The 2 least significant bits of the scrambled N bit word are separated and input on connection  36  to a differential phase encoder  37  which has logic configured to add the 2 least significant bits to the existing 2 bits in its memory, resulting in 2 differential bits that are used for phase rotation and supplied on connection  39  to rotator  41 . The resulting N-2 bit word on connection  33  is supplied to mapper  34  which maps the N-2 bit word into a signal space, resulting in a mapped N-2 bit word which is supplied on connection  38  to rotator  41 . Optionally, fractional bit rate encoding can be accomplished by inserting a modulus converter  66 , or other fractional bit rate device such as a shell mapper or a device which enables constellation switching in the transmitter between scrambler  32  and mapper  34 . Modulus converter  66  allows the transmission of signal space constellations having a quantity of symbols other than ones of powers of 2 and is depicted in a dashed line to indicate that it is optional. Modulus conversion is a well known technique in the art of communications for allowing the transmission of fractional bit rates, and is described in U.S. Pat. No. 5,103,227. Constellation switching allows the transmission of fractional bit rates by, for example, first transmitting 6 bits in one symbol and 7 bits in the next symbol if it is desired to transmit 6 ½ bits. For 6 ¾ bits one would transmit 7 bits per symbol for three symbol cycles and transmit 6 bits per symbol for the fourth symbol cycle. Shell mapping blocks the data into frames and a shell mapping algorithm, such as that described in the ITU-T V0.34 specification, is used to map the frames of data into a constellation of a certain size. 
     Rotator  41  performs vector multiplication on the 2 differential bits and the mapped N-2 bit word to cause a phase rotation, thus producing the final signal space mapped signal which is embodied in a circular constellation on connection  42 . The circular constellation on connection  42  is then supplied to precoder and spectral shaping filter  50  of the present invention. 
     With reference now to FIG. 4, shown is the precoder and spectral shaping filter  50  of FIG. 3. A filter  84  having a set of complex filter coefficients defining a particular transmit spectrum receives as input a signal space mapped substantially flat spectrum signal on connection  87 . Filter  84  can also use real filter coefficients, however the bandwidth resolution would be unnecessarily restricted. Complex filter coefficients provide wider bandwidth resolution. The defined transmit spectrum is one that meets regulatory requirements regarding power transmitted within particular frequency limits. Filter  84  then processes the signal space mapped substantially flat spectrum signal on connection  87 . The output of filter  84  is added to its input from line  87  by adder  92  to provide as an output a precoded signal having the desired transmit signal spectrum on connection  88 . 
     An optional second filter  83  having a set of complex filter coefficients identical to that of the first filter, and which resides in a feedback loop, is designed to receive, on connection  97 , the same signal space mapped substantially flat spectrum signal that was input to first filter  84  and, by feedback, generate the inverse transfer function, resulting in a signal that has a frequency response exactly opposite that of the signal at the output of first filter  84  on line  88 . The preferred embodiment uses the output of filter  84  on line  89  to avoid using duplicate filter  83 . If a conventional precoder is used in the receiver of remote modem  18  then filter  83  would use the coefficients provided by the remote receiver. 
     Modulo operator  82  in the feedback loop follows filter  84 , or filter  83  if used. Modulo operator  82  is designed to operate on the signal from filter  84  in order to restrict the magnitude of the output of filter  84  resulting in a substantially flat spectrum signal. Modulo operator  82  operates on the signal on connection  89  whereby if the voltage of the signal on connection  89  is greater than a preset value, twice that value is subtracted from the signal, or if the voltage is more negative than the negative of the preset value then twice that value is added to the more negative signal. Following modulo operator  82  is subtractor  81  designed to subtract the output signal of modulo operator  82  on connection  91  from the signal space mapped signal on connection  42  and provide the signal on connection  87  that is input to filter  84  and optionally to filter  83 . The output of filter  84  and adder  92  is essentially a shaped spectrum signal and results in a precoded transmit signal output from adder  92  that meets the required transmit spectrum characteristics. Modulo operator  82  may optionally be located on line  87  after subtractor  81  to implement Tomlinson Precoding as may best be used when transmitting square signal space constellations. 
     Referring back to FIG. 3, after processing by precoder and spectral shaping filter  50  the precoded and spectrally shaped circular constellation is supplied on connection  88  to scaler  43 . Scaler  43  multiplies the circular constellation by a scalar which is a function of the data rate and supplies the scaled constellation on connection  47  to optional nonlinear encoder  44 . Nonlinear encoder  44  encodes the signal as disclosed in U.S. Pat. No. 5,265,127 and is optional as disclosed herein. The signal is then supplied to transmit (TX) Hilbert filter  46  for carrierless amplitude/phase (CAP) modulation. In an alternate embodiment of the present invention, the scaled circular constellation on connection  53  is modulated using modulator  48 , using a technique such as, for example, quadrature amplitude modulation (QAM), or pulse amplitude modulation (PAM) as is known in the art. 
     Because everything disclosed thus far occurs at the symbol rate of the modem, with the symbol rate equal to the bandwidth of the modem, less expensive components may be employed, thus reducing the cost of implementation. 
     Either the CAP modulated signal or the QAM or PAM modulated signal is supplied on connection  49  to transmit preemphasis filter  58 . As is known in the art, transmit pre emphasis filter  58  processes the signal to shape its passband spectrum. The preemphasized transmit signal is next supplied on connection  59  to digital to analog converter  52  for conversion to an analog signal that can be transmitted conventionally over communication channel  14  as is known in the art. Optionally, transmit pre emphasis filter  58  can reside after the DAC to provide pre emphasis in the analog domain. 
     Referring now to FIG. 5, shown is a graphical representation of an example of the shaped transmit signal spectrum  100  of transmitter  35 . Transmitted power is referenced on the vertical axis  107  and frequency is referenced on the horizontal axis  108 . The curve represents power transmitted at various frequency levels. As can be seen, transmitted power is higher at lower frequency levels, as depicted by section  102  of the curve. As transmit frequency increases transmitted power is reduced as evidenced by section  103  of the curve until a lower transmit power is achieved as shown by section  104  of the curve. As frequency gets higher, power continues to be reduced as shown by section  106  of the curve. 
     It will be obvious to those skilled in the art that many modifications and variations may be made to the preferred embodiments of the present invention, as set forth above, without departing substantially from the principles of the present invention. All such modifications and variations are intended to be included herein within the scope of the present invention, as defined in the claims that follow.