WO2023164231A1 - Signal peak reduction circuit - Google Patents

Abstract

Devices, systems, and methods for minimizing distortion of power amplifiers by reducing peaks in an input signal to be amplified. The peaks are extracted from the signal and in some embodiments the peaks in the input signal are reduced by using a cancellation signal derived from the extracted peaks.

Description

SIGNAL PEAK REDUCTION CIRCUIT

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority under 35 U.S.C. § 119(e) to U.S. Provisional Application Ser. No. 63/314,467 filed on February 27, 2022, the contents of which are hereby incorporated by reference in its entirety.

BACKGROUND

[0002] The subject matter of this application relates to processing methods for signals transmitted in a communications network.

[0003] Cable Television (CATV) services have historically provided content to large groups of subscribers from a central delivery unit, called a "head end," which distributes channels of content to its subscribers from this central unit through a branch network comprising a multitude of intermediate nodes. Modem Cable Television (CATV) service networks, however, not only provide media content such as television channels and music channels to a customer, but also provide a host of digital communication services such as Internet Service, Video-on-Demand, telephone service such as VoIP, and so forth. These digital communication services, in turn, require not only communication in a downstream direction from the head end, through the intermediate nodes and to a subscriber, but also require communication in an upstream direction from a subscriber and to the content provider through the branch network.

[0004] To this end, such CATV head ends included a separate Cable Modem Termination System (CMTS), used to provide high speed data services, such as video, cable Internet, Voice over Internet Protocol, etc. to cable subscribers. Typically, a CMTS will include both Ethernet interfaces (or other more traditional high-speed data interfaces) as well as RF interfaces so that traffic coming from the Internet can be routed (or bridged) through the Ethernet interface, through the CMTS, and then onto the optical RF interfaces that are connected to the cable company's hybrid fiber coax (HFC) system. Downstream traffic is delivered from the CMTS to a cable modem in a subscriber's home, while upstream traffic is delivered from a cable modem in a subscriber’s home back to the CMTS Many modem CATV systems have combined the functionality of the CMTS with the video delivery system (EdgeQAM) in a single platform called the Converged Cable Access Platform (CCAP). Still other modem CATV architectures (referred to as Distributed Access Architectures or DAA) relocate the physical layer (e.g.., a Remote PHY or R-PHY architecture) and sometimes the MAC layer as well (e.g., a Remote MACPHY or R-MACPHY architecture) of a traditional CCAP by pushing it/them to the network’s fiber nodes. Thus, while the core in the CCAP performs the higher layer processing, the remote device in the node converts the downstream data sent by the core from digital-to- analog to be transmitted on radio frequency, and converts the upstream RF data sent by cable modems from analog-to-digital format to be transmitted optically to the core.

[0005] A critical component of any communications system, such as the CATV communications networks just described, is the power amplifier in the transmitter. The function of the power amplifier is to amplify an input data signal and thereby create a high-powered version of that input data signal for subsequent output into a transmission channel. The greater the amplification capability of the power amplifier, the greater the resulting output power level, and therefore the larger the geographic area covered by the communications system. In addition to the increased coverage area, an increased output power level typically results in increased efficiency (as measured by the ratio of resulting output power to input direct current (DC) power to the power amplifier).

[0006] However, concurrent with these improvements in coverage and power amplifier efficiency, the distortion in the output data signal unfortunately increases with increasing power level. Such distortion reveals itself in many ways, including as a spillover of transmitted power into frequencies outside the intended frequency band of transmission. It also reveals itself as a degradation of the quality of the in-band signal such that larger constellations and higher throughput rates are inhibited at higher transmission powers. Such distortion, a result of the nonlinearities in the power amplifier, directly diminishes the useful output power range of the power amplifier. It is therefore desirable to minimize the distortion and to thereby capitalize on as much of the available output power from the power amplifier as possible.

[0007] What is desired, therefore, are systems and methods that reduce distortion through power amplifiers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:

[0009] FIG. 1 shows an exemplary communications network used to illustrate embodiments of the disclosed systems and methods.

[0010] FIG. 2 shows an exemplary block diagram of a downstream signal generation system.

[0011] FIG. 3 shows a typical RF spectrum of downstream signal in cable network.

[0012] FIG. 4 shows a time domain representation of the signal of FIG. 3.

[0013] FIG. 5 shows an exemplary block diagram of a downstream signal generation system having a peak reduction module.

[0014] FIG. 6 shows an embodiment of the peak reduction module of FIG. 5.

[0015] FIG. 7 shows a typical downstream signal in both the time and frequency domains.

[0016] FIG. 8 shows a time-domain “peak” signal extracted from the signal of FIG. 7.

[0017] FIG. 9 shows the spectrum of the peak signal of FIG. 8. [0018] FIGS. 10 and 11 shows the frequency and time domain representations of the peak signal oof FIG. 8 after being fdtered by the band-pass fdter of FIG. 6.

[0019] FIGS. 12-14 show zoomed in portions of the output of the bandpass filter of FIG. 6, the input downstream signal of FIG. 6, and the output of the subtractor module of FIG. 6.

[0020] FIG. 15 shows a zoomed out portion of the subtractor module of FIG. 6.

[0021] FIG. 16 shows the spectrum of the signal of FIG. 15

DETAILED DESCRIPTION

[0022] FIG. 1 generally shows a communication system used to illustrate the devices, systems and methods disclosed in the present application. Specifically, FIG.

1 shows a Hybrid Fiber Coaxial (HFC) broadband network 100 that may employ the various embodiments described in this specification. The HFC network 100 may combine the use of optical fiber and coaxial connections. The network 100 includes a head end 102 that receives analog or digital video signals and digital bit streams representing different services (e.g., video, voice, and Internet) from various digital information sources. For example, the head end 102 may receive content from one or more video on demand (VOD) servers, IPTV broadcast video servers, Internet video sources, or other suitable sources for providing IP content.

[0023] An IP network 108 may include a web server 110 and a data source 112. The web server 110 is a streaming server that uses the IP protocol to deliver video-on- demand, audio-on-demand, and pay-per view streams to the IP network 108. The IP data source 112 may be connected to a regional area or backbone network (not shown) that transmits IP content. For example, the regional area network can be or include the Internet or an IP-based network, a computer network, a web-based network or other suitable wired or wireless network or network system.

[0024] At the head end 102, the various services are encoded, modulated and up- converted onto RF earners, combined onto a single electrical signal and inserted into a broadband optical transmiter. A fiber optic network extends from the cable operator’s master/regional head end 102 to a plurality of fiber optic nodes 104. The head end 102 may contain an optical transmitter or transceiver to provide optical communications through optical fibers 103. Regional head ends and/or neighborhood hub sites may also exist between the head end and one or more nodes. The fiber optic portion of the example HFC network 100 extends from the head end 102 to the regional head end/hub and/or to a plurality of nodes 104. The optical transmiter converts the electrical signal to a downstream optically modulated signal that is sent to the nodes. In turn, the optical nodes convert inbound signals to RF energy and return RF signals to optical signals along a return path.

[0025] Each node 104 serves a service group comprising one or more customer locations. By way of example, a single node 104 may be connected to thousands of cable modems or other subscriber devices 106. In an example, a fiber node may serve between one and two thousand or more customer locations. In an HFC network, the fiber optic node 104 may be connected to a plurality of subscriber devices 106 via coaxial cable cascade 111, though those of ordinary skill in the art will appreciate that the coaxial cascade may comprise a combination of fiber optic cable and coaxial cable. In some implementations, each node 104 may include a broadband optical receiver to convert the dow nstream optically modulated signal received from the head end or a hub to an electrical signal provided to the subscribers’ devices 106 through the coaxial cascade 111. Signals may pass from the node 104 to the subscriber devices 106 via the RF cascade of amplifiers, which may be comprised of multiple amplifiers and active or passive devices including cabling, taps, splitters, and in-line equalizers. It should be understood that the amplifiers in the RF cascade may be bidirectional, and may be cascaded such that an amplifier may not only feed an amplifier further along in the cascade but may also feed a large number of subscribers. The tap is the customer’s drop interface to the coaxial system. Taps are designed in various values to allow amplitude consistency along the distribution system. [0026] The subscriber devices 106 may reside at a customer location, such as a home of a cable subscriber, and are connected to the cable modem termination system (CMTS) 120 or comparable component located in a head end. A client device 106 may be a modem, e.g., cable modem, MTA (media terminal adaptor), set top box, terminal device, television equipped with set top box, Data Over Cable Service Interface Specification (DOCSIS) terminal device, customer premises equipment (CPE), router, or similar electronic client, end, or terminal devices of subscribers. For example, cable modems and IP set top boxes may support data connection to the Internet and other computer networks via the cable network, and the cable network provides bi-directional communication systems in which data can be sent downstream from the head end to a subscriber and upstream from a subscriber to the head end.

[0027] References are made in the present disclosure to a Cable Modem Termination System (CMTS) in the head end 102. In general, the CMTS is a component located at the head end or hub site of the network that exchanges signals between the head end and client devices within the cable network infrastructure. In an example DOCSIS arrangement, for example, the CMTS and the cable modem may be the endpoints of the DOCSIS protocol, with the hybrid fiber coax (HFC) cable plant transmitting information between these endpoints. It will be appreciated that architecture 100 includes one CMTS for illustrative purposes only, as it is in fact customary that multiple CMTSs and their Cable Modems are managed through the management network.

[0028] The CMTS 120 hosts downstream and upstream ports and contains numerous receivers, each receiver handling communications between hundreds of end user network elements connected to the broadband network. For example, each CMTS 120 may be connected to several modems of many subscribers, e.g., a single CMTS may be connected to hundreds of modems that vary widely in communication characteristics. In many instances several nodes, such as fiber optic nodes 104, may serve a particular area of a town or city. DOCSIS enables IP packets to pass between devices on either side of the link between the CMTS and the cable modem. [0029] It should be understood that the CMTS is a non-limiting example of a component in the cable network that may be used to exchange signals between the head end and subscriber devices 106 within the cable network infrastructure. For example, other non-limiting examples include a Modular CMTS (M-CMTSTM) architecture or a Converged Cable Access Platform (CCAP).

[0030] An EdgeQAM (EQ AM) 122 or EQ AM modulator may be in the head end or hub device for receiving packets of digital content, such as video or data, re- packetizing the digital content into an MPEG transport stream, and digitally modulating the digital transport stream onto a downstream RF carrier using Quadrature Amplitude Modulation (QAM). EdgeQAMs may be used for both digital broadcast, and DOCSIS downstream transmission. In CMTS or M-CMTS implementations, data and video QAMs may be implemented on separately managed and controlled platforms. In CCAP implementations, the CMTS and edge QAM functionality may be combined in one hardware solution, thereby combining data and video delivery.

[0031] Although the architecture shown in FIG. 1 is used to illustrate various embodiments of the disclosed device, systems, and methods, other architectures may do so as well. For example, distributed architectures such as those preciously described may also benefit from the devices and techniques disclosed herein, as may any other communication network or other device or system that transmits and amplifies signals.

[0032] As also noted above, communication systems rely upon one or more amplifiers to distribute signals over long distances, and there is typically a trade-off between the amplification power and signal distortion. Referring first to FIG. 2, for example, a typical downstream generation scheme 200 receives input data 202, and from that data creates and transmits an amplified downstream RF signal 204. Specifically, the module 206 which is an electronic device, converts the input data, typically digital, to a baseband downstream signal. The signal bandwidth of the downstream signal output by the system 200 varies based on the applicable communication standard, but typical baseband signals could encompass spectrum up to 1.1 GHz (e.g., DOCSIS 3.1) or up to 1.7 GHz (e.g., DOCSIS 4.0). This signal content could carried upon any applicable modulation scheme, e.g., a number of single QAM carriers with a width of 6 or 8 MHz each, or alternatively could be carried on a number of OFDM blocks of carriers (192 MHz each block), or a mix of both. Module 206 produces I (in-phase) and Q (quadrature) components (together I,Q samples) of a downstream signal with an appropriate selected clock frequency.

[0033] The I,Q samples from module 206 are upsampled and upconverted by module 208, which is a device that upsamples/upconverts to a higher-frequency clock of the DAC 210, which provides signal spectrum from 100 MHz (or higher) to 1.2 or to 1.8 GHz. Blocks 206 and 208 are digital and realized in a Field Programmable Gate Array (FPGA) or Application Specific Integrated Circuit (ASIC). Digital to analog converter (DAC) 210 converts signal from digital to analog domain. That signal is amplified by low-noise amplifier 212 and output power amplifier 214. The output RF downstream signal 204 is output onto the communications network. .

[0034] A ty pical RF spectrum of a downstream signal 300 in cable network is presented in FIG. 3. In this example, the downstream signaO 3001 extends from 258 MHz to 1800 MHz (8 OFDM blocks, 192 MHz each), generated with clock 6553.6 MHz. This spectrum has a specific tilt profile, created in the downstream signal to compensate for losses over lengths of transmission media, such as fiber or coaxial cable, where losses not only occur as a function of distance, but those losses are greater at higher frequencies than lower frequencies. Thus, the purpose of the up-tilt shown in FIG. 3 is to achieve unity gain (a level amplification factor of one across all frequencies) at the end of the span of fiber or cable to the next active element or endpoint. This kind of spectrum shape is exemplary, as the disclosed devices, systems and methods are applicable to any kind of spectrum profile, bandwidth and used modulation.

[0035] The required downstream output power in a cable network can vary from 68 to 73 dBmV, depending on user requirements for the network. Required signal to noise ratio (SNR) in cable networks is very high (up to 50 dB) due to the use of high- order modulation (up to 4096QAM). This leads to a requirement of very high linearity of the power amplifier 214 in FIG. 2, which tends to cause distortion, as mentioned above. The difficulty in achieving high power amplifier output power is associated with specific property of wide band signal i.e., a high peak to average ratio, which increases with signal bandwidth and modulation order. This is because, with a high peak-to-average ratio, the amplified signal will tend to clip at the peaks, meaning that the amplifier lacks the power to fully amplify the entire dynamic range of the signal.

[0036] FIG. 4 shows a time domain representation 302 of the broadband signal 300 of FIG. 3. This signal has very visible spikes (positive and negative), which significantly exceed average power. These spikes cause the power amplifier 214 to saturation, causing non-linear distortions and a concomitant reduction in SNR reduction, which makes using of higher-order modulation less feasible and therefore reduces network capacity.

[0037] Disclosed are improved devices, systems and methods that reduce peak-to- average power of a signal amplified by a power amplifier. Such systems reduce distortions and therefore allow higher modulation orders and increase network capacity. In one preferred embodiment, the disclosed systems and methods use a novel out-of-band cancellation signal, which does not affect the SNR in the main part of spectrum.

[0038] FIG. 5 shows an improved system for generating an amplified downstream signal having a novel peak reduction circuit. Specifically, like the system of FIG. 2, a downstream generation system 400 receives input data 401, and from that data creates and transmits an amplified downstream RF signal 403. Specifically, the module 402 which is an electronic device, converts the input data, typically digital, to a baseband downstream signal. The signal bandwidth of the downstream signal output by the system 400 varies based on the applicable communication standard, but typical baseband signals could encompass spectrum up to 1.1 GHz (e.g., DOCSIS 3.1) or up to 1.7 GHz (e.g., DOCSIS 4.0). This signal content could carried upon any applicable modulation scheme, e g., a number of single QAM carriers with a width of 6 or 8 MHz each, or alternatively could be carried on a number of OFDM blocks of carriers (192 MHz each block), or a mix of both. Module 406 produces I (in-phase) and Q (quadrature) components (together T.Q samples) of a downstream signal with an appropriate selected clock frequency.

[0039] The I,Q samples from module 402 are output to a peak reduction circuit 404 as described later, which in turn outputs a filtered signal to be upsampled and upconverted by module 406, which is a device that upsamples/upconverts to a higher- frequency clock of the DAC 410, which provides signal spectrum from 100 MHz (or higher) to 1.2 or to 1.8 GHz. Blocks 402, 404, and 406 may be digital and realized in a Field Programmable Gate Array (FPGA) or Application Specific Integrated Circuit (ASIC). Digital to analog converter (DAC) 408 converts signal from digital to analog domain. That signal is amplified by low-noise amplifier 410 and output power amplifier 412. The output RF downstream signal 4034 is output onto the communications network.

[0040] As just noted, the downstream generation system 400 includes a novel peak reduction circuit 404 that operates on I,Q samples of the downstream signal received from the module 402. A block diagram of the peak reduction circuit 404 block is presented in FIG. 6. Specifically, the peak reduction circuit 420 receives an input downstream signal 422 that, as just mentioned, may be received from the module 402 which generates I, Q samples. That input signal is provides to an I,Q peak extraction circuit 424 that filters the input signal to provide a peak signal having extracted peaks. The peak signal is then provided to a band-pass filter 426 that produces a peak cancellation signal from the peak signal. Delay module 428 operates on the original downstream signal with the original I,Q samples. Thus, a subtractor 430, which is a filter, may receive that input signal and subtract from it the output of the band pass filter 426 (the peak cancellation signal). The output of the subtractor is then provided to the upsampling module 406 of FIG. 5 for further processing. Delay module 428 in FIG. 6 preferably compensates for possible processing delays in circuit 424 and bandpass filter 426.

[0041] FIG. 6 shows a typical signal ( in the time and frequency domain) that may be input to the peak reduction circuit 404. Specifically, the input signal of to the peak reduction circuit comprises I and Q downstream samples from downstream generator and has a spectrum similar to that shown in FIGS. 3 and 4, but the center axis of its amplitude is at zero, in the time domain. As before, this signal has spikes that would ordinarily tend to cause distortion when amplified by a power amplifier.

[0042] Peak extraction circuit 424 extraction peaks from I and Q components according to the algorithm: if I > L then PI = I - L; if I < -L then PI = I + L; else PI = 0; if Q > L then PQ = Q - L; if Q < -L then PQ = Q + L; else PQ = 0 where L is a threshold. In this example, L is 28672, but other embodiments may use different thresholds as appropriate to extract sufficient peaks to avoid power saturation by a power amplifier.

[0043] The peaks of I,Q (Pi, PQ ) extracted by circuit 424 are shown in FIG. 8 and the spectrum of these peaks is shown in FIG. 9. As already noted, the signals Pi, PQ are passed to the bandpass filter 426. The bandwidth of the bandpass filter is preferably a fraction of downstream signal bandwidth i.e., less than that of the downstream signal bandwidth and in one preferred embodiment the center frequency of the bandpass filter 426 is chosen to be just above the high edge of the downstream spectrum high edge so that the bandpass filter output spectrum does not overlap the spectrum of the downstream signal. The spectrum of the output of bandpass filter 426 is shown in FIG. 10 and its time domain representation is shown in FIG. 11. As can be noted by comparing FIG. 11 to FIG. 8, the bandpass filter changed the shape of the peak signal (Pi, PQ ).

[0044] Because it is difficult to see small details in FIG.l 1 and preceding figures, FIGS 12-14 are zoomed versions of the signals around one of the spikes or peaks. Specifically, FIG. 12 is a zoomed version of the cancellation signal from FIG. 11, FIG. 13 is a zoomed in version of the original signal input to the peak extraction circuit 424 and FIG. 14 is a zoomed in version of the output of the subtractor 430 that subtracts the signal of FIG. 12 from the signal of FIG. 13. Each of these figures shows the I-component 440 and the Q component 442 of the respective signals.

[0045] The spike in the initial downstream I component at time stamp 131826 (Fig. 13) and amplitude about 37000 caused a signal with a shape like a sine function (sinX/X) on the bandpass filter output (FIG. 14). The maximum peak of that signal is aligned with the spike of the initial downstream signal because that sine function was obtained from it). Preferably, scaling of the system 400 is accomplished such that the t amplitude of the bandpass filter output is fraction of the amplitude of the initial downstream spike. Subtraction of the signal of FIG.12 from the signal of FIG. 13 produces the signal of FIG. 14. The peak of FIG. 14 can be seen to be significantly lower than that of FIG. 13 i.e., there was a resulting peak reduction. When the signal of FIG. 12 is subtracted from the signal of FIG. 13, other pulses around the peak are also subtracted, and therefore changing samples of the downstream signal around the peak. But because these other samples are much lower than peak, this subtraction does not create new peaks.

[0046] FIG. 15 shows a full signal after subtractor 430. A comparison of this figure with respect to that on the top panel of FIG. 7 shows that a significant peak reduction has occurred. FIG. 16 shows the signal after the subtractor in the frequency domain, and this figure shows that the peak reduction signal 446 is located to the right of the main spectrum and therefore does not distort downstream carriers. Therefore, the presence of the cancellation signal in the downstream spectrum is harmless, because in cable networks this top part of the spectrum is not used.

[0047] Those of ordinary skill in the art will understand that the threshold L and the scaling factor i.e., the relation between the bandpass filter output signal amplitude and the initial signal amplitude could be optimized to get an optimal peak reduction result.

[0048] It will be appreciated that the invention is not restneted to the particular embodiment that has been described, and that variations may be made therein without departing from the scope of the invention as defined in the appended claims, as interpreted in accordance with principles of prevailing law, including the doctrine of equivalents or any other principle that enlarges the enforceable scope of a claim beyond its literal scope. Unless the context indicates otherwise, a reference in a claim to the number of instances of an element, be it a reference to one instance or more than one instance, requires at least the stated number of instances of the element but is not intended to exclude from the scope of the claim a structure or method having more instances of that element than stated. The word "comprise" or a derivative thereof, when used in a claim, is used in a nonexclusive sense that is not intended to exclude the presence of other elements or steps in a claimed structure or method.

Claims

1. A device comprising: an input for receiving an input signal, the input signal having a peak-to- average power ratio; a peak reduction circuit that reduces the peak-to-average power ratio, the peak reduction circuit providing an output signal by extracting peaks from the data signal and using the extracted peaks to reducing the peak-to-average power ratio of the input signal to produce a modified input signal; and an output from a power amplifier that amplifies the modified input signal.

2. The device of claim 1 where the peak reduction circuit extracts I-component peaks and Q-component peaks.

3. The device of claim 1 where the peaks are reduced using a bandpass filter.

4. The device of claim 3 where the bandpass filter has an associated bandwidth that is a fraction of a bandwidth associated with a signal-carrying portion of the input signal.

5. The device of claim 1 where the peak reduction circuit produces a cancellation signal that is subtracted from the input signal.

6. The device of claim 5 where the input signal is associated with a signalcarrying spectrum and the cancellation signal has a cancellation spectrum outside the signal carrying spectrum.

7. The device of claim 1 where the extracted peaks are those exceeding a threshold magnitude.

8. A method comprising: receiving an input signal having an information-carrying spectrum and at least one peak within the information-carrying spectrum; extracting the at least one peak and reducing its magnitude to produce a cancellation signal; subtracting the cancellation signal from the input signal to produce an output signal; amplifying the output signal.

9. The method of claim 8 where the output signal is amplified by a power amplifier, and the cancellation signal reduces distortion of the power amplifier.

10. The method of claim 8 where the extracted at least one peak has an I- component and a Q-component.

11. The method of claim 8 including reducing the reducing the magnitude of the extracted at least one peak.

12. The method of claim 11 where a bandpass filter performs the step of reducing the magnitude of the extracted at least one peak.

13. The method of claim 12 where the bandpass filter has an associated bandwidth that is a fraction of the signal carrying spectrum.

14. The method of claim 8 where the cancellation signal has a spectrum outside the information-carry ing spectrum.

15. The method of claim 8 where the extracted peaks are those exceeding a threshold magnitude.