System and method for reducing interference in OFDM channels

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.

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.

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.

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.

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.