Patent Document

BACKGROUND 
     Field of the Disclosure 
     The present disclosure relates generally to systems for improving communications in cable modem and other systems. More specifically, the present disclosure relates to system and method for reducing interference in ODFM channels. 
     Related Art 
     Cable modems (CMs) can be found in both homes and businesses, and are used to transmit and receive digital information (e.g., to access the Internet, view television, and/or view on-demand video, etc.). Numerous CMs can communicate with a device known as a Cable Modem Termination System (CMTS), which is installed at a central location and used to transmit information to CMs, as well as receive information from CMs. The signal between these devices traverses a communications network that includes both coaxial cable and fiber optic cable, and is known as a Hybrid Fiber-Coax (HFC) network or cable “plant.” The HFC allows for bi-directional communication between the CMTS and the CMs. The protocol used to communicate between the CMTS and CMs has been standardized by the CableLabs organization and is collectively known as DOCSIS (Data Over Cable Service Interface Specifications). The set of DOCSIS specifications define all levels of communication including the physical layer, media access control layer, and an application interface layer. 
     Typically, many CMs share the bandwidth of a single coaxial cable, which usually has a bandwidth of approximately 1 GHz. The 1 GHz spectrum is divided into multiple channels. Each defined channel is typically shared by many CMs. In the downstream direction, from the CMTS to the CM, the CMTS will use time division multiplexing to send data to all CMs using a unique address to send data to a unique CM. In the upstream direction, from the CMs to the CMTS, many CMs must share the same channel. To accomplish this, the CMTS schedules time slots for each CM known as “MAPs.” A given CM is only allowed to send data during its assigned time slot and assigned frequency mini slots. Synchronization signals from the CMTS to the CM keep the different CMs synchronized. 
     The HFC plant is subject to many different types of impairments that can degrade the quality of the signal. This is especially true in the upstream direction, where noise contributions from many CMs and households combine. These impairments are typically caused by problems such as loose or corroded connections, unterminated lines, faulty equipment, and other noise caused by sources such as motors and lightning. The DOCSIS specification provides a number of different tools to address the most common types of impairments such as: a variety of quadrature amplitude modulation (QAM) constellations; different channel widths; Reed-Solomon Forward Error Correction (R-S FEC); pre-equalization; interleaving; Advanced Time Division Multiple Access (“ATDMA”) (DOCSIS 3.0); and Orthogonal Frequency Division Multiplexing (“OFDM”) (DOCSIS 3.1). By manually varying these parameters, a cable operator can seek to improve signal quality, making tradeoffs between throughput and improved noise immunity. 
     DOCSIS 3.1 is the new standard for Data-Over-Cable-Service. OFDM technology is first implemented in cable data transfer. During the conversion from DOCSIS 3.0 to DOCSIS 3.1, OFDM and ATDMA signals may exist in the same plant for backward compatibility. The need to support a DOCSIS 3.0 modem will last for many years. Both theoretical simulations and field tests show that once the OFDM fast Fourier transform is performed on the combined signals, the ATDMA signal will have significant spectral spread to each side of the signal in the frequency domain due to a rectangular window function being applied to the OFDM fast Fourier transform function. This will cause a spectral region of 8-10 MHz on each side of the ATDMA signal to be unusable by OFDM carriers, which is unacceptable. 
     For example,  FIG. 1  illustrates a prior art version of the current system. RF signal  10  is received by the analog-to-digital converter  12 . The analog-to-digital converter  12  outputs the signal to a plurality ATDMA channel processors  14   a - 14   n . Each of the ATDMA channel processors  14   a - 14   n  are identical in the signal processing methods that are employed. The output of the analog-to-digital converter  12  first goes to mixers  16   a - 16   n  to shift the signal to a common known frequency, which moves the selected ATDMA channels to a baseband. The outputs from the mixers  16   a - 16   n  are then received by filters  18   a - 18   n  to recover the ATDMA signal from either combined signals or adjacent ATDMA signals. The clean ATDMA outputs from filters  18   a - 18   n  are then received by modules  20   a - 20   n  for timing and carrier recovery. The outputs are then received by time domain equalizers  22   a - 22   n  for reconstructing the QAM signal. Finally, the outputs from equalizers  22   a - 22   n  are received by slicers  24   a - 24   n  for eliminating a portion of the signal to obtain the output ATDMA signals  26   a - 26   n.    
     The analog-to-digital converter  12  also outputs the signal to a OFDM channel process  30 . The output of the analog-to-digital converter  12  first goes to a mixer  32  to shift the signal to a common known frequency, which moves the whole OFDM channel to baseband. The baseband channel is up to 95 MHz in bandwidth in DOCSIS 3.1 upstream and up to 190 MHz in bandwidth in DOCSIS 3.1 downstream. The output from the mixer  32  is then received by a filter  34  to obtain a clean OFDM signal from combined signals or OFDM only signals. The output from the filter  34  is then received by a module  36  for fast Fourier transformation. The output is then received by an equalizer  38  for adjusting the amplitude and reconstructing the signal. Finally, the output from the equalizer  38  is received by a slicer  40  for eliminating a portion of the signal to obtain the output OFDM signal  42 . However, the output OFDM signal  42  and the output ATDMA signal  26  may exist in the same plant, and as such, may interfere with each other. Therefore, there exists a need to improve the signal processing in these systems, so that the presence of ATDMA and OFDM signals together do not result in interference and decreased performance. 
     SUMMARY 
     The present disclosure relates to a system for signal processing in a cable modem termination system (CMTS). The present disclosure also applies to CMs when the CMs need to receive a combined QAM and OFDM signal in a downstream signal. The system includes a CMTS receiver in communication with a plurality of cable modems. The system also includes a plurality of ATDMA channel processors including a filter for recovering an ATDMA signal. The system further includes an OFDM channel processor including a filter for processing a combined OFDM and ATDMA signal. Finally, the system also includes a summation module for subtracting the ATDMA signal from the combined ATDMA and OFDM signal to obtain a clean OFDM signal. 
     In another embodiment, a method for signal processing in a cable modem termination system (CMTS) is provided. The method includes the steps of providing a CMTS receiver in communication with a plurality of cable modems; providing a plurality of ATDMA channel processors including a filter; recovering a ATDMA signal; providing an OFDM channel processor including a filter for processing a combined OFDM and ATDMA signal; and subtracting the ATDMA signal from the combined ATDMA and OFDM signal using a summation module to obtain a clean OFDM signal. 
     In another embodiment, a non-transitory, computer-readable medium having computer readable instructions stored thereon is provided. The instructions, when executed by a cable modem termination system (CMTS) receiver in communication with a plurality of cable modems, cause the receiver to perform the steps comprising: providing a plurality of ATDMA channel processors including a filter; recovering an ATDMA signal; providing an OFDM channel processor including a filter for processing a combined OFDM and ATDMA signal; and subtracting the ATDMA signal from the combined ATDMA and OFDM signal using a summation module to obtain a clean OFDM signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing features of the disclosure will be apparent from the following Detailed Description, taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a diagram of a prior art CMTS receiver system; 
         FIG. 2  is a diagram illustrating a CMTS receiver system in accordance with the present disclosure which supports both ATDMA and OFDM signals; 
         FIG. 3  is a diagram showing the combined TDMA and OFDM signal; and 
         FIG. 4  is a diagram showing a clean OFDM signal after filtering by the system of the present disclosure; and 
         FIG. 5  is a diagram showing a clean TDMA signal after filtering by the system of the present disclosure; 
         FIG. 6  is a diagram illustrating a CMTS receiver system in accordance with a second embodiment the present disclosure which supports both ATDMA and OFDM signals. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates to a system and method for signal processing in communications systems, as discussed in detail below in connection with  FIGS. 2-6 . 
     Referring to  FIG. 2 , a block diagram of the system of the present disclosure will be explained in greater detail. RF signal  110  is first received by an analog-to-digital converter  112  which converts the analog signal into a digital signal. After the analog-to-digital converter  112  converts the analog signal to the digital signal, the ATDMA and combined ATDMA/OFDM signals are processed in different channels. For example, ATDMA signals are processed by a plurality of ATDMA channel processors  114 - 114   n . Each of the ATDMA channel processors  114   a - 114   n  are identical in the signal processing methods that are employed. The output of the analog-to-digital converter  112  first goes to mixers  116   a - 116   n  to shift the signal to a common known frequency, which moves the selected ATDMA channels to a baseband. The outputs from the mixers  116   a - 116   n  are then received by filters  118   a - 118   n  to recover the ATDMA signals from either combined signals or adjacent ATDMA signals. The clean ATDMA outputs from filters  118   a - 118   n  are then received by modules  120   a - 120   n  for timing and carrier recovery. The outputs are then received by time domain equalizers  122   a - 122   n  for reconstructing the QAM signals. Finally, the outputs from the equalizers  122   a - 122   n  are received by slicers  124   a - 124   n  for eliminating a portion of the signals to obtain the output ATDMA signals  126   a - 126   n . This process allows DOCSIS 3.0 systems to receive and process ATDMA signals  126   a - 126   n.    
     The analog-to-digital converter  112  also outputs a combined ATDMA and OFDM signal to an OFDM channel processor  130 . The output of the analog-to-digital converter  112  first goes to a mixer  132  to shift the signal to a common known frequency, which moves the whole combined signal channel to a baseband. As noted above, the baseband channel is up to 95 MHz in bandwidth in DOCSIS 3.1 upstream and up to 190 MHz in bandwidth in DOCSIS 3.1 downstream. The output from the mixer  132  is then received by a filter  134  to obtain a clean combined OFDM and ATDMA signal. The filter  134  can remove the band noise in the combined signal. The result is a combined signal including TDMA signals  162  and OFDM signals  160  as shown in  FIG. 3 . Alternatively, the filter  134  could recover a clean OFDM signal from combined signals or OFDM only signals. In statistic combined signal cases, additional filter parameters could be needed in filter  134  to remove all ATDMA signals. In dynamic combined cases, dynamic-adjustable filters could be utilized. 
     As noted above, for each ATDMA channel processor  114 , the filter  118  recovers the interfering signals. These signals are then received by a plurality of corresponding mixers  150   a - 150   n , which are used to reconstruct the original interfering signals for all the ATDMA channels. An alignment module  152  aligns the combined OFDM and ATDMA signal with the interfering signals for all the ATDMA channels. Each of the plurality of the ATDMA channel processors  114   a - 114   n  know when and in which frequencies the ATDMA signals appear, and can provide in real-time the correct known ATDMA signals to the OFDM channel processor  130  and more specifically, the summation module  154 . Therefore, the summation module  154  can use these known ATDMA signals to obtain clean OFDM signals by subtracting the known ATDMA signals from the combined signal found in  FIG. 3 . The result is a clean OFDM signal shown in  FIG. 4 . Additionally, a clean TDMA signal can be extracted as shown in  FIG. 5 . It should be noted that the system of the present disclosure can be used to cancel any interfering signal, not just ATDMA signals as previously described. 
     The output from the summation module  154  is received by a module  136  for fast Fourier transformation of the signal. The output is then received by an equalizer  138  for adjusting the amplitude and reconstructing the signal. Finally, the output from the equalizer  138  is received by a slicer  140  for eliminating a portion of the signal to obtain the output OFDM signal  142 . The OFDM signal  142  is clean and does not contain any interference with ATDMA signals due to the summation module  154 . 
     Reference will now be made to  FIG. 6  showing an alternative embodiment of the disclosure of the present application. RF signal  210  is first received by an analog-to-digital converter  212  which converts the analog signal into a digital signal. After the analog-to-digital converter  212  converts the analog signal to the digital signal, the ATDMA and combined ATDMA/OFDM signals are processed in different channels. For example, ATDMA signals are processed by a plurality of ATDMA channel processors  214 - 214   n . Each of the ATDMA channel processors  214   a - 214   n  are identical in the signal processing methods that are employed. The output of the analog-to-digital converter  212  first goes to mixers  216   a - 216   n  to shift the signal to a common known frequency, which moves the selected ATDMA channels to a baseband. The outputs from the mixers  216   a - 216   n  are then received by filters  218   a - 218   n  to recover the ATDMA signals from either combined signals or adjacent ATDMA signals. The clean ATDMA outputs from filters  218   a - 218   n  are then received by modules  220   a - 220   n  for timing and carrier recovery. The outputs are then received by time domain equalizers  222   a - 222   n  for reconstructing the QAM signals. Finally, the outputs from the equalizers  222   a - 222   n  are received by slicers  224   a - 224   n  for eliminating a portion of the signals to obtain the output ATDMA signals  226   a - 226   n . This process allows DOCSIS 3.0 systems to receive and process ATDMA signals  226   a - 226   n . As mentioned above, for each ATDMA channel processor  214 , the filter  218  recovers the interfering signals. These signals are then received by a plurality of corresponding mixers  250   a - 250   n , which are used to reconstruct the original interfering signals for all the ATDMA channels. 
     The analog-to-digital converter  212  also outputs a combined ATDMA and OFDM signal to an OFDM channel processor  230 . The output of the analog-to-digital converter  212  first goes to an alignment module  252  for aligning the combined OFDM and ATDMA signal with the interfering signals for all the ATDMA channels, which are being sent from the mixers  250   a - 250   n . Each of the plurality of the ATDMA channel processors  214   a - 214   n  know when and in which frequencies the ATDMA signals appear, and can provide in real-time the correct known ATDMA signals to the OFDM channel processor  230  and more specifically, the summation module  254 . Therefore, the summation module  254  can use these known ATDMA signals to obtain clean OFDM signals by subtracting the known ATDMA signals from the combined signal found in  FIG. 3 . The result is a clean OFDM signal shown in  FIG. 4 . Additionally, a clean TDMA signal can be extracted as shown in  FIG. 5 . It should be noted that the system of the present disclosure can be used to cancel any interfering signal, not just ATDMA signals as previously described. 
     The clean OFDM signal as shown in  FIG. 3  is then received by mixer  232  to shift the signal to a common known frequency, which moves the whole combined signal channel to a baseband. As noted above, the baseband channel is up to 95 MHz in bandwidth in DOCSIS 3.1 upstream and up to 190 MHz in bandwidth in DOCSIS 3.1 downstream. The output from the mixer  232  is then received by a filter  234  to obtain a clean OFDM signal. The filter  234  can remove the band noise in the signal. The output from the filter  234  is received by a module  236  for fast Fourier transformation of the signal. The output is then received by an equalizer  238  for adjusting the amplitude and reconstructing the signal. Finally, the output from the equalizer  238  is received by a slicer  240  for eliminating a portion of the signal to obtain the output OFDM signal  242 . The OFDM signal  242  is clean and does not contain any interference with ATDMA signals due to the summation module  254 . 
     Having thus described the system and method in detail, it is to be understood that the foregoing description is not intended to limit the spirit or scope thereof. It will be understood that the embodiments of the present disclosure described herein are merely exemplary and that a person skilled in the art may make any variations and modification without departing from the spirit and scope of the disclosure. All such variations and modifications, including those discussed above, are intended to be included within the scope of the disclosure.

Technology Category: 5