Source: http://www.google.com/patents/US7502429?dq=6,757,682
Timestamp: 2015-05-03 12:01:54
Document Index: 759016935

Matched Legal Cases: ['Application No. 60', 'Application No. 60', 'Application No. 2502924', 'Application No. 2503532', 'Application No. 2515167', 'Application No. 2503530', 'Application No. 2487817', 'Application No. 2484313', 'Application No. 2442400', 'Application No. 2491259', 'Application No. 200410100591', 'Application No. 03757359', 'application No. 03794510', 'Application No. 04256234', 'Application No. 03742400', 'Application No. 03777694', 'Application No. 03742393', 'Application No. 03774848', 'Application No. 03777627', 'Application No. 03742393', 'Application No. 03774848', 'Application No. 02728894', 'Application No. 07075745', 'Application No. 2004', 'Application No. 20026115', 'art 1', 'Application No. 092129629']

Patent US7502429 - Equalization for traveling wave tube amplifier nonlinearity measurements - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inAdvanced Patent SearchPatentsA method and system provide the ability to measure a transmission performance characteristic. A signal is received and equalized. The equalized signal is demodulated and an ideal signal is generated from the demodulated equalized signal. A performance characteristic is the estimated from a difference...http://www.google.com/patents/US7502429?utm_source=gb-gplus-sharePatent US7502429 - Equalization for traveling wave tube amplifier nonlinearity measurementsAdvanced Patent SearchPublication numberUS7502429 B2Publication typeGrantApplication numberUS 10/961,579Publication dateMar 10, 2009Filing dateOct 8, 2004Priority dateOct 10, 2003Fee statusPaidAlso published asUS20050123032Publication number10961579, 961579, US 7502429 B2, US 7502429B2, US-B2-7502429, US7502429 B2, US7502429B2InventorsErnest C. ChenOriginal AssigneeThe Directv Group, Inc.Export CitationBiBTeX, EndNote, RefManPatent Citations (112), Non-Patent Citations (83), Referenced by (2), Classifications (19), Legal Events (2) External Links: USPTO, USPTO Assignment, EspacenetEqualization for traveling wave tube amplifier nonlinearity measurements
US 7502429 B2Abstract
A method and system provide the ability to measure a transmission performance characteristic. A signal is received and equalized. The equalized signal is demodulated and an ideal signal is generated from the demodulated equalized signal. A performance characteristic is the estimated from a difference between the ideal signal and the equalized signal.
1. A method of measuring a transmission performance characteristic for a traveling wave tube amplifier, comprising:
receiving, in a receiver, a signal transmitted from a satellite;
equalizing, in the receiver, the signal using a linear equalizer to restore nonlinearity;
demodulating, in the receiver, the equalized signal;
generating, in the receiver, an ideal signal from the demodulated equalized signal; and
estimating the performance characteristic of the traveling wave tube amplifier from a difference between the ideal signal and the received signal for use in a layered modulation scheme.
2. The method of claim 1, further comprising coherently averaging the received signal to reduce noise.
3. The method of claim 1, wherein the step of estimating the performance characteristic includes fitting a curve of the received signal versus the ideal signal.
4. The method of claim 3, wherein the curve is an AM-AM plot.
5. The method of claim 3, wherein the curve is an AM-PM plot.
6. The method of claim 1, further comprising inverse filtering the received signal to restore nonlinearity.
7. A system for measuring a transmission performance characteristic for a traveling wave tube amplifier, comprising:
(a) an equalizer in a receiver, for equalizing a received signal transmitted from a satellite;
(b) a demodulator in the receiver, for demodulating the equalized signal;
(c) a signal generator in the receiver, for producing an ideal signal from the demodulated equalized signal; and
(d) a processor, in the receiver, for estimating the performance characteristic of the traveling wave tube amplifier from a difference between the ideal signal and the equalized signal for use in a layered modulation scheme.
8. The system of claim 7, wherein the processor is further configured to coherently average the ideal signal to reduce noise and wherein the performance characteristic is based on the difference between the coherently averaged ideal signal and coherently averaged equalized signal.
9. The system of claim 7, wherein the processor is configured to estimate the performance characteristic by fitting a curve of the received signal versus the ideal signal.
10. The system of claim 9, wherein the curve is an AM-AM plot.
11. The system of claim 9, wherein the curve is an AM-PM plot.
12. The system of claim 7, further comprising an inverse filter that is used on the received signal to restore nonlinearity. Description
This is a continuation-in-part application and claims the benefit under 35 U.S.C. �120 of the following co-pending and commonly-assigned utility patent applications, which are incorporated by reference herein:
Utility Application Serial No. 10/962,346, filed on October 8, 2004, the same date herewith, by Ernest C. Chen, entitled �COHERENT AVERAGING FOR MEASURING TRAVELING WAVE TUBE AMPLIFIER NONLINEARITY�, which application claims the benefit of U.S. Provisional Patent Application No. 60/510,368, filed on Oct. 10, 2003, by Ernest C. Chen, entitled �IMPROVING TWTA AM-AM AND AM-PM MEASUREMENT�.
Utility application Ser. No. 10/691,133, filed Oct. 22, 2003, by Ernest C. Chen, Tung-Sheng Lin, Weizheng Wang, and William C. Lindsey, entitled �EQUALIZERS FOR LAYERED MODULATED AND OTHER SIGNALS,� which application claims the benefit of U.S. Provisional Patent Application No. 60/421,241, filed on Oct. 25, 2002, by Ernest C. Chen, Tung-Sheng Lin, Weizheng Wang, and William C. Lindsey, entitled �EQUALIZERS FOR LAYERED MODULATED AND OTHER SIGNALS�.
Utility application Ser. No. 09/844,401, filed Apr. 27, 2001, by Ernest C. Chen, entitled �LAYERED MODULATION FOR DIGITAL SIGNALS,�.
U.S. application Ser. No. 10/165,710, filed on Jun. 7, 2002, by Ernest C. Chen, entitled �SATELLITE TWTA ON-LINE NON-LINEARITY MEASUREMENT,�.
PCT International Patent Application Serial No. PCT/US03/33130, filed on Oct. 17, 2003, by Ernest C. Chen and Shamik Maitra, entitled �ESTIMATING THE OPERATING POINT ON A NONLINEAR TRAVELING WAVE TUBE AMPLIFIER,�.
The present invention relates to systems and methods for transmitting data, and in particular to a system and method for using equalization to restore nonlinearity for full traveling wave tube amplifier curve measurements.
Digital signal communication systems have been used in various fields, including digital TV signal transmission, either terrestrial or satellite. As the various digital signal communication systems and services evolve, there is a burgeoning demand for increased data throughput and added services. However, it is more difficult to implement either improvement in old systems or new services when it is necessary to replace existing legacy hardware, such as transmitters and receivers. New systems and services are advantaged when they can utilize existing legacy hardware. In the realm of wireless communications, this principle is further highlighted by the limited availability of electromagnetic spectrum. Thus, it is not possible (or at least not practical) to merely transmit enhanced or additional data at a new frequency.
The conventional method of increasing spectral capacity is to move to a higher-order modulation, such as from quadrature phase shift keying (QPSK) to eight phase shift keying (8 PSK) or sixteen quadrature amplitude modulation (16 QAM). Unfortunately, QPSK receivers cannot demodulate conventional 8 PSK or 16 QAM signals. As a result, legacy customers with QPSK receivers must upgrade their receivers in order to continue to receive any signals transmitted with an 8 PSK or 16 QAM modulation.
It is advantageous for systems and methods of transmitting signals to accommodate enhanced and increased data throughput without requiring additional frequency. In addition, it is advantageous for enhanced and increased throughput signals for new receivers to be backwards compatible with legacy receivers. There is further an advantage for systems and methods which allow transmission signals to be upgraded from a source separate from the legacy transmitter.
It has been proposed that a layered modulation signal, transmitting non-coherently both upper and lower layer signals, can be employed to meet these needs. Such layered modulation systems allow higher information throughput with backwards compatibility. However, even when backward compatibility is not required (such as with an entirely new system), layered modulation can still be advantageous because it requires a traveling wave tube amplifier (TWTA) peak power significantly lower than that for a conventional 8 PSK or 16 QAM modulation format for a given throughput.
To provide a layered modulation scheme (as described in detail below), a reconstructed upper layer signal is subtracted from a received composite signal to reveal a lower layer signal. As such, the lower-layer signal performance is impacted by how closely the upper-layer signal can be reconstructed relative to the original signal. In other words, the lower layer signal performance is impacted by the fidelity of the reconstructed signal. Thus, layered modulation requires clean cancellation of the upper-layer signal to expose the lower-layer signal for further processing. Clean cancellation requires TWTA non-linearity/distortion to be accurately reproduced in the reconstruction of the upper-layer signal. On-line estimation of the required TWTA characteristics is imperative in minimizing the required TWTA power and the complexity of satellite operation. In addition, the non-linearity measurement may be used to monitor the health of satellite TWTAs and perform other communications diagnostics. However, such an accurate reproduction and knowledge of the TWTA non-linearity presents a significant roadblock.
With a TWTA, there is a region of approximate linearity, in which the output power is nearly proportional to the input power, followed by a curved transition to a point where the output power levels off and reaches a maximum. At this point (i.e., when the TWTA curve is well in the non-linear region), the amplifier is said to have reached saturation. Due to this non-linearity and to avoid intermodulation, the input power is often �backed off� by a particular amount (e.g., 6 dB). The resulting point on the curve after the input power is �backed off� is referred to as the operating point of the TWTA. When subsequently reconstructing the upper layer signal, the amount of distortion/non-linearity used to create the original signal serves to increase the fidelity of the reconstructed signal. Thus, to produce a high fidelity reconstructed upper layer signal, knowledge of the non-linearity (and the operating point) is important. Accordingly, the inclusion of (or taking into account) TWTA non-linearity may improve upper-layer signal cancellation ratio by 10 dB or more (the cancellation ratio between non-linearity-induced noise before and after cancellation is improved). However, linear distortions in a satellite communication channel may block and/or obscure a part of the TWTA nonlinearity visible in the process of TWTA nonlinearity measurement.
Errors in the estimation of the operating point can have a significant impact when reconstructing the upper layer-signal. The impact of amplitude (AM-AM [amplitude modulation to amplitude modulation]) and phase (AM-PM [amplitude modulation to phase modulation]) nonlinearity due to operating point errors may be individually analyzed based on shift analysis. Individual impacts may then be combined for total impact. To evaluate performance impacts, the synthesis of a layer-modulated signal with known TWTA non-linearity and system/representative operating CNR (carrier to noise ratio) may be used. The upper-layer cancellation error may then be calculated for each amount of simulated operating point error in the signal reconstruction process. Thus, the upper layer cancellation ratio may be plotted against the operating point displacement. The cancellation error can then be converted into an amount of lower-layer CNR degradation, which increases the CNR required for signals of both upper and lower layers. Such an increased CNR illustrates the impact of operating point estimation errors.
FIGS. 14A and 14B illustrate the impact of operating point errors in signal reconstruction with an example AM-AM and AM-PM nonlinearity. In FIGS. 14A and 14B, the sensitivity of signal reconstruction error is plotted against the TWTA input operating point error. The effective noise is calculated as a measure of signal reconstruction error.
In FIG. 14A, a set of generic TWTA non-linearity curves are used. The signal reconstruction process is assumed to have full knowledge about the non-linearity curves but is otherwise uncertain about the operating point. The performance plots of FIG. 14A indicate that cancellation errors are below −25 dB for an input operating point error up to about +/−1 dB.
In FIG. 14B, the performance plots are based on the same TWTA non-linearity but with an input backoff of 8 dB. With such an input backoff, the linearity is improved and is less susceptible to TWTA operating point error. As a result, reconstruction and cancellation errors are greatly reduced as indicated in FIG. 14B. The effective noise is below −33 dB with the same input operating error up to about +/−1 dB as in FIG. 14A.
Accordingly, there is a need for systems and methods for implementing layered modulation systems that accurately determine TWTA non-linearity and the operating point.
In the prior art, TWTA non-linearity measurements are performed on the ground before a satellite is launched. The TWTA operating point is then obtained from telemetry tracking and control (TT&C) commands that set the operating point of the TWTA (the procedure assumes that TWTA characteristics have little changed since the satellite was launched). In other words, the operating point set by TT&C commands during pre-launch measurements is used post-launch after receiving the signals from the satellite. However, TWTA characteristics including the non-linearity and the effective and operating point may change over time (including after satellite launch) and temperature. In this regard, the upper-layer signal cancellation in layered modulation may not be as accurately done without non-linearity updates.
Another prior art method is to estimate the non-linearity by trial and error, hoping to converge to a required accuracy within a reasonable effort and cost.
Accordingly, what is needed is a system and method for accurately determining the non-linearity of a TWTA as it changes over time/temperature. Further, what is desired is the capability to make such determinations at any time, from anywhere within the satellite downlink footprint, automatically and accurately. The present invention meets this need and provides further advantages as detailed hereafter.
To address the requirements described above, the present invention discloses a method and apparatus for accurately measuring traveling wave tube amplifier (TWTA) nonlinearity. In this regard, the invention aids in the accurate extraction of a lower-layer signal in a layered modulation scheme. Such an accurate extraction minimizes the amounts of power required for both layers of a signal and also helps to monitor the health of a TWTA. The accurate measurements are possible despite the various linear distortions along the signal path that tend to obscure the visibility of the nonlinearity that is to be measured. Such distortion sources may include a satellite TWTA output multiplex (OMUX) filter, distribution multi-path, and a receiver front-end low pass filter (LPF).
To measure the non-linearity of the TWTA, the measurement procedure begins with capturing a received signal and filtering/equalizing the received signal followed by a demodulation of the equalized signal. The demodulated equalized signal is then used to generate an ideal signal that is noiseless and without TWTA nonlinearity. The ideal signal may be generated by re-encoding and re-modulating the symbols decoded with quasi-error-free (QEF) performance. The ideal signal represents the input to the TWTA and the captured and equalized signal represents the output of the TWTA plus downlink noise and other impairments. The two signals are aligned sample-for-sample by the timing recovery loop of a demodulator. Data pairs are then formed for all corresponding samples. Data sorting sorts the data pairs to the powers/magnitudes of the ideal signal samples.
As described above, the presence of filters along the signal path before the data is collected may reduce the accuracy of the estimate. Accordingly, the equalization and/or inverse filtering of the invention (prior to demodulation) undo the filtering effect so that a full TWTA non-linearity may be restored for most accurate measurements.
FIG. 4 is a block diagram showing one embodiment of the modulator for the uplink signal;
FIGS. 6A-6C are diagrams illustrating the basic relationship of signal layers in a layered modulation transmission;
FIGS. 7A-7C are diagrams illustrating a signal constellation of a second transmission layer over the first transmission layer after first layer demodulation;
FIG. 8A is a diagram showing a system for transmitting and receiving layered modulation signals;
FIG. 8B is a diagram showing an exemplary satellite transponder for receiving and transmitting layered modulation signals;
FIG. 8C is a diagram illustrating details of a simulation of an exemplary satellite transponder and signal processing;
FIG. 9 is a block diagram depicting one embodiment of an enhanced IRD capable of receiving layered modulation signals;
FIGS. 11A and 11B depict the relative power levels of example embodiments of the present invention;
FIG. 12 illustrates an exemplary computer system that could be used to implement selected modules or functions the present invention;
FIGS. 13A, 13B, 13C, and 13D are block diagrams of a basic system for measuring performance maps in accordance with one or more embodiments of the invention;
FIG. 13E outlines the flow of a method of the present invention;
FIGS. 14A and 14B illustrate the impact of operating point errors in signal reconstruction.
The invention provides a method of determining/estimating on-line non-linearity measurements of a TWTA. The invention provides for the use of an equalizer/inverse filter that is used to restore part of the TWTA non-linearity that is non-observable because of linear filters along the signal path. Such equalization may be performed prior to demodulation thereby affecting the creation of an ideal signal and the signal used to produce the nonlinearity measurements. Allowing current TWTA non-linearity to be measured for compensation from layer-modulated signals provides for continuous layered modulation operation without service disruptions and with best CNR performance. Additionally, a system design/operational option is available for individual receivers to generate non-linearity curves locally, thereby eliminating the need to transmit the curves from the broadcast center.
The uplink center 104 receives program material and program control information from the control center 102, and using an uplink antenna 106 and transmitter 105, transmits the program material and program control information to the satellite 108 via uplink signal 116. The satellite receives and processes this information, and transmits the video programs and control information to the subscriber receiver station 110 via downlink signal 118 using transmitter 107. The subscriber receiving station 110 receives this information using the outdoor unit (ODU) 112, which includes a subscriber antenna and a low noise block converter (LNB).
The video channels are provided by a program source 200A-200C of video material (collectively referred to hereinafter as program source(s) 200). The data from each program source 200 is provided to an encoder 202A-202C (collectively referred to hereinafter as encoder(s) 202). Each of the encoders accepts a program time stamp (PTS) from the controller 216. The PTS is a wrap-around binary time stamp that is used to assure that the video information is properly synchronized with the audio information after encoding and decoding. A PTS time stamp is sent with each I-frame of the MPEG encoded data.
The data packets are assembled using a reference from the system clock 214 (S CR), and from the conditional access manager 210, which provides the service channel identifier (SCID) to the packetizers 204 for use in generating the data packets. These data packets are then multiplexed into serial data and transmitted.
FIG. 3A is a diagram of a representative data stream. The first packet segment 302 comprises information from video channel 1 (data coming from, for example, the first video program source 200A). The next packet segment 304 comprises computer data information that was obtained, for example from the computer data source 208. The next packet segment 306 comprises information from video channel 5 (from one of the video program sources 200). The next packet segment 308 comprises program guide information such as the information provided by the program guide subsystem 206. As shown in FIG. 3A, null packets 310 created by the null packet module 212 may be inserted into the data stream as desired followed by further data packets 312, 314, 316 from the program sources 200.
The data stream therefore comprises a series of packets (302-316) from any one of the data sources (e.g. program sources 200, program guide subsystem 206, computer data source 208) in an order determined by the controller 216. The data stream is encrypted by the encryption module 218, modulated by the modulator 220 (typically using a QPSK modulation scheme), and provided to the transmitter 105/222, which broadcasts the modulated data stream on a frequency bandwidth to the satellite via the antenna 106. The receiver 500 at the receiver station 110 receives these signals, and using the SCID, reassembles the packets to regenerate the program material for each of the channels.
FIG. 3B is a diagram of a data packet. Each data packet (e.g. 302-316) is 147 bytes long, and comprises a number of packet segments. The first packet segment 320 comprises two bytes of information containing the SCID and flags. The SCID is a unique 12-bit number that uniquely identifies the data packet's data channel. The flags include 4 bits that are used to control other features. The second packet segment 322 is made up of a 4-bit packet type indicator and a 4-bit continuity counter. The packet type identifies the packet as one of the four data types (video, audio, data, or null). When combined with the SCID, the packet type determines how the data packet will be used. The continuity counter increments once for each packet type and SCID. The next packet segment 324 comprises 127 bytes of payload data, which in the cases of packets 302 or 306 is a portion of the video program provided by the video program source 200. The final packet segment 326 is data required to perform forward error correction.
FIG. 4 is a block diagram showing one embodiment of the modulator 220. The modulator 220 optionally comprises a forward error correction (FEC) encoder 404 which accepts the first signal symbols 402 and adds redundant information that are used to reduce transmission errors. The coded symbols 405 are modulated by modulator 406 according to a first carrier 408 to produce an upper layer modulated signal 410. Second symbols 420 are likewise provided to an optional second FEC encoder 422 to produce coded second symbols 424. The coded second symbols 424 are provided to a second modulator 414, which modulates the coded second symbols 424 according to a second carrier 416 to produce a lower layer modulated signal 418. The upper layer modulated signal 410 and the lower layer modulated signal 418 are therefore uncorrelated. Thus, the upper layer signal 410 and the lower layer signal 418 can be transmitted to separate transponders on one or more satellites 108 via separate uplink signals 116. Thus, the lower layer signal 418 can be implemented from a separate satellite 108 that receives a separate uplink signal 116. However, in the downlink signal 118 the upper layer signal 410 must be a sufficiently greater amplitude signal than the lower layer signal 418 to maintain the signal constellations shown in FIG. 6 and FIG. 7.
It should be noted that it may be more efficient to retrofit an existing system by using a transponder on a separate satellite 108 to transmit the lower layer downlink signal over the existing legacy downlink signal rather than replacing the legacy satellite with one that will transmit both downlink signal layers. Emphasis can be given to accommodating the downlink legacy signal in implementing a layered downlink broadcast.
The tuner/demodulator 504 isolates a single, digitally modulated 24 MHz transponder signal, and converts the modulated data to a digital data stream. Further details regarding the demodulation of the received signal follow.
The transport module 508 performs many of the data processing functions performed by the IRD 500. The transport module 508 processes data received from the FEC decoder module 506 and provides the processed data to the video MPEG decoder 514 and the audio MPEG decoder 517. As needed the transport module employs system RAM 528 to process the data. In one embodiment of the present invention, the transport module 508, video MPEG decoder 514 and audio MPEG decoder 517 are all implemented on integrated circuits. This design promotes both space and power efficiency, and increases the security of the functions performed within the transport module 508. The transport module 508 also provides a passage for communications between the microcontroller 510 and the video and audio MPEG decoders 514, 517. As set forth more fully hereinafter, the transport module also works with the conditional access module (CAM) 512 to determine whether the subscriber receiving station 110 is permitted to access certain program material. Data from the transport module 508 can also be supplied to external communication module 526.
A description of the processes performed in the encoding and decoding of video streams, particularly with respect to MPEG and JPEG encoding/decoding, can be found in Chapter 8 of �Digital Television Fundamentals,� by Michael Robin and Michel Poulin, McGraw-Hill, 1998, which is hereby incorporated by reference herein.
The microcontroller 510 receives and processes command signals from the remote control 524, an RD 500 keyboard interface, and/or another input device. The microcontroller 510 receives commands for performing its operations from a processor programming memory, which permanently stores such instructions for performing such commands. The processor programming memory may comprise a read only memory (ROM) 538, an electrically erasable programmable read only memory (EEPROM) 522 or, similar memory device. The microcontroller 510 also controls the other digital devices of the IRD 500 via address and data lines (denoted �A� and �D� respectively, in FIG. 5).
The present invention also comprises a local storage unit such as the video storage device 532 for storing video and/or audio data obtained from the transport module 508. Video storage device 532 can be a hard disk drive, a read/writable compact disc of DVD, a solid state RAM, or any other suitable storage medium. In one embodiment of the present invention, the video storage device 532 is a hard disk drive with specialized parallel read/write capability so that data may be read from the video storage device 532 and written to the device 532 at the same time. To accomplish this feat, additional buffer memory accessible by the video storage 532 or its controller may be used. Optionally, a video storage processor 530 can be used to manage the storage and retrieval of the video data from the video storage device 532. The video storage processor 530 may also comprise memory for buffering data passing into and out of the video storage device 532. Alternatively or in combination with the foregoing, a plurality of video storage devices 532 can be used. Also alternatively or in combination with the foregoing, the microcontroller 510 can also perform the operations required to store and or retrieve video and other data in the video storage device 532.
The video processing module 516 input can be directly supplied as a video output to a viewing device such as a video or computer monitor. In addition, the video and/or audio outputs can be supplied to an RF modulator 534 to produce an RF output and/or 8 vestigal side band (VSB) suitable as an input signal to a conventional television tuner. This allows the receiver 500 to operate with televisions without a video output.
In a typical backwards-compatible embodiment of the present invention, the legacy QPSK signal is boosted in power to a higher transmission (and reception) level. This creates a power �room� in which a new lower layer signal may operate. The legacy receiver will not be able to distinguish the new lower layer signal from additive white Gaussian noise, and thus operates in the usual manner. The optimum selection of the layer power levels is based on accommodating the legacy equipment, as well as the desired new throughput and services.
The new lower layer signal is provided with a sufficient carrier to thermal noise ratio to function properly. The new lower layer signal and the boosted legacy signal are non-coherent with respect to each other. Therefore, the new lower layer signal can be implemented from a different TWTA and even from a different satellite. The new lower layer signal format is also independent of the legacy format, e.g., it may be QPSK or 8 PSK, using the conventional concatenated FEC code or using a new Turbo code. The lower layer signal may even be an analog signal.
2.4 Layered Signals
FIGS. 6A-6C illustrate the basic relationship of signal layers in a received layered modulation transmission. FIG. 6A illustrates an upper layer signal constellation 600 of a transmission signal showing the signal points or symbols 602. FIG. 6B illustrates the lower layer signal constellation of symbols 604 over the upper layer signal constellation 600 where the layers are coherent (or synchronized). FIG. 6C illustrates a lower layer signal 606 of a second transmission layer over the upper layer constellation where the layers are non-coherent. The lower layer 606 rotates about the upper layer constellation 602 due to the relative modulating frequencies of the two layers in a non-coherent transmission. Both the upper and lower layers rotate about the origin due to the first layer modulation frequency as described by path 608.
FIGS. 7A-7C are diagrams illustrating a non-coherent relationship between a lower transmission layer over the upper transmission layer after upper layer demodulation. FIG. 7A shows the constellation 700 before the first carrier recovery loop (CRL) of the upper layer and the constellation rings 702 rotate around the large radius circle indicated by the dashed line. FIG. 7B shows the constellation 704 after CRL of the upper layer where the rotation of the constellation rings 702 is stopped. The constellation rings 702 are the signal points of the lower layer around the nodes 602 of the upper layer. FIG. 7C depicts a phase distribution of the received signal with respect to nodes 602.
Relative modulating frequencies of the non-coherent upper and lower layer signals cause the lower layer constellation to rotate around the nodes 602 of the upper layer constellation to form rings 702. After the lower layer CRL this rotation is eliminated and the nodes of the lower layer are revealed (as shown in FIG. 6B). The radius of the lower layer constellation rings 702 is indicative of the lower layer power level. The thickness of the rings 702 is indicative of the carrier to noise ratio (CNR) of the lower layer. As the two layers are non-coherent, the lower layer may be used to transmit distinct digital or analog signals.
FIG. 8A is a diagram showing a system for transmitting and receiving layered modulation signals. Separate transmitters 107A, 107B (that include TWTAs to amplify the signals), as may be located on any suitable platform, such as satellites 108A, 108B, are used to non-coherently transmit different layers of a signal of the present invention. Uplink signals 116 are typically transmitted to each satellite 108A, 108B from one or more uplink centers 104 with one or more transmitters 105 via an antenna 106.
FIG. 8B is a diagram illustrating an exemplary satellite transponder 107 for receiving and transmitting layered modulation signals on a satellite 108. The uplink signal 116 is received by the satellite 108 and passed through an input multiplexer (IMUX) 814. Following this the signal is amplified with a traveling wave tube amplifier (TWTA) 816 and then through an output muliplexer (OMUX) 818 before the downlink signal 118 is transmitted to the receivers 802, 500.
FIG. 8C is a diagram illustrating details of a simulation of an exemplary satellite transponder and signal processing in accordance with one or more embodiments of the invention. Specifically, FIG. 8C illustrates how a layer-modulated signal can be simulated to generate results as described herein. In FIG. 8C, the blocks with potential impact on nonlinearity measurement accuracy of the upper layer TWTA are highlighted. As illustrated and described in more detail below, no other interferences, such as adjacent or co-channel interferences 820 are included.
The FEC encoders 404 and 422 accept the first and second signal symbols 402 and 420 respectively and add redundant information that are used to reduce transmission errors. The encoded signals are modulated, up-converted in frequency, up-linked, and received in the satellite 108 with relatively small impairment.
In the simulation, the upper and lower signals are processed by signal mappers 824A and 824B respectively. The signal mappers 824A and 824B converts the FEC output bits into constellation symbols. To ensure that the fundamental shapes of the pulses of the signal do not interfere with one another at an optimal sampling point later in the receiver, pulse shaping filters 826A and 826B may be used to establish a desirable pulse shape. In this regard, the pulse shaping filters 826A and 826B may comprise a root-raised cosine filter (RRCF) in which the frequency filter provides for expressing the frequency response as the square root of the magnitude spectrum of a full raised cosine filter (RCF).
Continuing the simulation, the pulse shaped signals are independently offset by the baud and carrier offset modules 828A and 828B. As illustrated in FIG. 8B, the signal is then passed through satellite input multiplexer (IMUX) filters 814A and 814B. Following this the signal is amplified with traveling wave tube amplifiers (TWTA) 816A and 816B and then through output muliplexers (OMUX) 818A and 818B. The upper layer signal and lower layer signal are combined with adjacent and co-channel interferences 820, as may be required, before downlink transmission. In the simulation illustrated in FIG. 8C, additive white Gaussian noise (AWGN) 830 may be added to test the robustness of the invention.
As part of the simulation, phase noise 832 may be added to the signal. Further, overlapping multi-path signals 834 may also be added. The multi-path signals 834 account for signal reflection in the distribution cable and could be significant depending on the cable length and installation. The combined signals are then processed by tuner 836. A receiver front-end filter 838 (e.g., a 4th order Butterworth Low Pass Filter (LPF) as an anti-aliasing filter) may be included to model receiver implementation that outputs a simulated received signal to measure TWTA nonlinearity. In this regard, the receiver front-end filter may have a major effect on the TWTA nonlinearity measurement since bandwidth is usually limited for a given A/D (analog/digital) speed. Thus, after passing through the filter 838, the signal is converted from its analog form into digital form by A/D converter 840. The signal may then pass through a matched filter (RRCF) 842. However, the matched filter 842 may be irrelevant with respect to its effect of linear distortions on TWTA nonlinearity measurements since non-linearity can be measured from signals before matched filtering.
As an alternative to the above, a down-linked signal (not shown) may be captured by a receiver for TWTA nonlinearity measurement. As illustrated and described herein, the baud and carrier offset module 828A, satellite TWTA 816A, Satellite OMUX Filter 818A, Downlink AWGN 830, Phase Noise 832, multi-path in cable 834, and Receiver Front-end Filter 838 have a more significant effect than other components of the satellite and receiver.
Referring again to FIG. 8A, the layered signals 808A, 808B (e.g. multiple downlink signals 118) are received at receiver antennas 812A, 812B, such as satellite dishes, each with a low noise block (LNB) 810A, 810B where they are then coupled to integrated receiver/decoders (IRDs) 500, 802. For example, first satellite 108A and transmitter 107A can transmit an upper layer legacy signal 808A and second satellite 108B and transmitter 107B can transmit a lower layer signal 808B. Although both signals 808A, 808B arrive at each antenna 812A, 812B and LNB 810A, 8101B, only the layer modulation IRD 802 is capable of decoding both signals 808A, 808B. The legacy receiver 500 is only capable of decoding the upper layer legacy signal 808A; the lower layer signal 808B appears only as noise to the legacy receiver 500.
Because the signal layers may be transmitted non-coherently, separate transmission layers may be added at any time using different satellites 108A, 108B or other suitable platforms, such as ground based or high altitude platforms. Thus, any composite signal, including new additional signal layers will be backwards compatible with legacy receivers 500, which will disregard the new signal layers. To ensure that the signals do not interfere, the combined signal and noise level for the lower layer must be at or below the allowed noise floor for the upper layer at the particular receiver antenna 812A, 812B.
Layered modulation applications include backwards compatible and non-backwards compatible applications. �Backwards compatible� in this sense, describes systems in which legacy receivers 500 are not rendered obsolete by the additional signal layer(s). Instead, even if the legacy receivers 500 are incapable of decoding the additional signal layer(s), they are capable of receiving the layered modulated signal and decoding the original signal layer. In these applications, the pre-existing system architecture is accommodated by the architecture of the additional signal layers. �Non-backwards compatible� describes a system architecture which makes use of layered modulation, but the modulation scheme employed is such that pre-existing equipment is incapable of receiving and decoding the information on additional signal layer(s).
2.5 Demodulator and Decoder
FIG. 9 is a block diagram depicting one embodiment of an enhanced IRD 802 capable of receiving layered modulation signals. The enhanced IRD 802 includes a feedback path 902 in which the FEC decoded symbols are fed back to an enhanced modified tuner/demodulator 904 and transport module 908 for decoding both signal layers as detailed hereafter.
FIG. 10A is a block diagram of one embodiment of the enhanced tuner/modulator 904 and FEC encoder 506. FIG. 10A depicts reception where layer subtraction is performed on a signal where the upper layer carrier has already been demodulated. The upper layer of the received combined signal 1016 from the LNB 502, which may contain legacy modulation format, is provided to and processed by an upper layer demodulator 1004 to produce the stable demodulated signal 1020. The demodulated signal 1020 is communicatively coupled to a FEC decoder 1002 which decodes the upper layer to produce the upper layer symbols which are output to an upper layer transport module 908. The upper layer symbols are also used to generate an idealized upper layer signal. The upper layer symbols may be produced from the decoder 402 after Viterbi decoding (BER<10−3 or so) or after Reed-Solomon (RS) decoding (BER<10−9 or so), in typical decoding operations known to those skilled in the art. The upper layer symbols are provided via feedback path 902 from the upper layer decoder 1002 to a remodulator 1006 which effectively produces an idealized upper layer signal. The idealized upper level signal is subtracted from the demodulated upper layer signal 1020.
In order for the subtraction to yield a clean lower layer signal, the upper layer signal must be precisely reproduced. The modulated signal has been distorted, for example, by traveling wave tube amplifier (TWTA) non-linearity or other non-linear or linear distortions in the transmission channel. Linear distortions may be caused by a satellite OMUX filter, cable multi-path, receiver LPF, etc. For example, overlapping multi-path signals may cause a delay longer than a coherent signal interval that may be compensated by processing a longer data segment. Further, overlapping multi-path signals may cause a delay shorter than the coherent signal interval thereby obscuring the measurement signal. In another example, filters may remove part of an IM-3 (third-order intermodulation) spectrum resulting in a measurable nonlinearity that is less than the actual nonlinearity.
The distortion effects may be estimated from the received signal after the fact by this invention method with which TWTA characteristics may be downloaded into the IRD in AM-AM and/or AM-PM maps 1018, used to eliminate the distortion (see detailed description below). The time domain response p(t) 1014 is incorporated as a signal characteristic. Alternatively, in accordance with one or more embodiments of the invention, the individual IRDs 500 and 802 may generate the nonlinearity curves locally, thereby eliminating the need to transmit the curves from the broadcast center.
A subtractor 1012 then subtracts the idealized upper layer signal from the stable demodulated signal 1020. This leaves the lower-power second layer signal. The subtractor 1012 may include a buffer or delay function to retain the stable demodulated signal 1020 while the idealized upper layer signal is being constructed. The second layer signal is demodulated by the lower level demodulator 1010 and FEC decoded by decoder 1008 according to its signal format to produce the lower layer symbols, which are provided to the transport module 908.
FIG. 10B depicts another embodiment wherein layer subtraction is performed on the received layered signal (prior to upper layer demodulation). In this case, the upper layer demodulator 1004 produces the upper carrier signal 1022 (as well as the stable demodulated signal output 1020). An upper carrier signal 1022 is provided to the recoder/remodulator 1006. The recoder/remodulator 1006 provides the recoded/remodulated signal to the non-linear distortion mapper 1018 which effectively produces an idealized upper layer signal. Unlike the embodiment shown in FIG. 10A, in this embodiment, the idealized upper layer signal includes the upper layer carrier for subtraction from the received combined signal 808A, 808B.
s UL ( t ) = f U ( M U exp ( jω U t + θ U ) ∑ m = - ∞ ∞ S Um p ( t - mT ) + f L ( M L exp ( jω L t + θ L ) ∑ m = - ∞ ∞ S Lm p ( t - mT + Δ T m ) + n ( t ) where, MU is the magnitude of the upper layer QPSK signal and ML is the magnitude of the lower layer QPSK signal and in general ML<<MU. The signal frequencies and phase for the upper and lower layer signals are respectively ωU, φU and ωL,φL. The symbol timing misalignment between the upper and lower layers is ΔTm. p(t−mT) represents the time shifted version of the pulse shaping filter p(t) 414 employed in signal modulation. QPSK symbols SUm and SLm are elements of
{ exp ( j n π 2 ) , n = 0 , 1 , 2 , 3 } . For exemplary QPSK signals. fU(�) and fL(�) denote the distortion function of the TWTAs for the respective signals.
Ignoring fU(�) and fL(�) and noise n(t), the following represents the combined signal after removing the upper carrier:
s UL ′ ( t ) = M U ∑ m = - ∞ ∞ S Um p ( t - mT ) + M L exp { j ( ω L - ω U ) t + θ L - θ U } ∑ m = - ∞ ∞ S Lm p ( t - mT + Δ T m ) Because of the magnitude difference between MU and ML, the upper layer demodulator 1004 and decoder 1002 disregard the ML component of the s′UL (t).
s L ( t ) = M L exp { j ( ω L - ω U ) t + θ L - θ U } ∑ m = - ∞ ∞ S Lm p ( t - mT + Δ T m ) Any distortion effects, such as TWTA nonlinearity effects are estimated for signal subtraction. In a typical embodiment of the present invention, the upper and lower layer frequencies are substantially equal. Significant improvements in system efficiency can be obtained by using a frequency offset between layers.
Using the present invention, two-layered backward compatible modulation with QPSK doubles a current 6/7 rate capacity by adding a TWTA approximately 6.2 dB above an existing TWTA power. New QPSK signals may be transmitted from a separate transmitter, from a different satellite for example. In addition, there is no need for linear travelling wave tube amplifiers (TWTAs) as with 16 QAM. Also, no phase error penalty is imposed on higher order modulations such as 8 PSK and 16 QAM.
3.0 Power Levels of Modulation Layers
In a layered modulation system, the relationship between the individual modulation layers can be structured to facilitate backward compatible applications. Alternately, a new layer structure can be designed to optimize the combined efficiency and/or performance of the layered modulation system.
3.1 Backward Compatible Applications
The present invention may be used in Backward Compatible Applications. In such applications, a lower layer signal may take advantage of advanced forward error correction (FEC) coding techniques to lower the overall transmission power required by the system.
FIG. 11A depicts the relative power levels 1100 of example embodiments of the present invention. FIG. 11A is not a scale drawing. This embodiment doubles the pre-existing rate 6/7 capacity by using a TWTA 6.2 dB above a pre-existing TWTA equivalent isotropic radiated power (EIRP) and second TWTA 2 dB below the pre-existing TWTA power. This embodiment uses upper and lower QPSK layers which are non-coherent. A code rate of 6/7 is also used for both layers. In this embodiment, the signal of the legacy QPSK signal 1102 is used to generate the upper layer 1104 and a new QPSK layer is the lower layer 1110. The CNR of the legacy QPSK signal 1102 is approximately 7 dB. In the present invention, the legacy QPSK signal 1102 is boosted in power by approximately 6.2 dB bringing the new power level to approximately 13.2 dB above thermal noise as the upper layer 1104. The noise floor 1106 of the upper layer is approximately 6.2 dB. The new lower QPSK layer 1110 has a CNR of approximately 5 dB with an advanced FEC such as a turbo code. The total signal and noise of the lower layer is kept at or below the tolerable noise floor 1106 of the upper layer. The power boosted upper layer 1104 of the present invention is also very robust, making it resistant to rain fade. It should be noted that the invention may be extended to multiple layers with mixed modulations, coding and code rates.
In a further embodiment of a backwards compatible application of the present invention the code rates between the upper and lower layers 1104, 1110 may be mixed. For example, the legacy QPSK signal 502 may be boosted by approximately 5.3 dB to approximately 12.3 dB with the code rate unchanged at 6/7 to create the upper QPSK layer 1104. The new lower QPSK layer 1110 may use a code rate of 2/3 with a CNR of approximately 3.8 dB. In this case, the total capacity relative to the legacy signal 1102 is approximately 1.78. In addition, the legacy IRDs will suffer no significant rate decrease.
3.2 Non-Backward Compatible Applications
As previously discussed the present invention may also be used in �non-backward compatible� applications. In such applications, both upper and lower layer signals may take advantage of advanced forward error correction (FEC) coding techniques to lower the overall transmission power required by the system. In a first example embodiment, two QPSK layers 1104, 1110 are used each at a code rate of 2/3. The upper QPSK layer 504 has a CNR of approximately 4.1 dB above its noise floor 1106 and the lower QPSK layer 1110 also has a CNR of approximately 4.1 dB. The total code and noise level of the lower QPSK layer 1110 is approximately 5.5 dB. The total CNR for the upper QPSK signal 1104 is approximately 9.4 dB, merely 2.4 dB above the legacy QPSK signal rate 6/7. The capacity is approximately 1.74 compared to the legacy rate 6/7.
FIG. 11B depicts the relative power levels of an alternate embodiment wherein both the upper and lower layers 1104, 1110 are below the legacy signal level 1102. The two QPSK layers 1104, 1110 use a code rate of 1/2. In this example, the upper QPSK layer 1104 is approximately 2.0 dB above its noise floor 1106 of approximately 4.1 dB. The lower QPSK layer has a CNR of approximately 2.0 dB and a total code and noise level at or below 4.1 dB. The capacity of this embodiment is approximately 1.31 compared to the legacy rate 6/7.
FIG. 12 illustrates an exemplary computer system 1200 that could be used to implement selected modules and/or functions of the present invention. The computer 1202 comprises a processor 1204 and a memory 1206, such as random access memory (RAM). The computer 1202 is operatively coupled to a display 1222, which presents images such as windows to the user on a graphical user interface 1218B. The computer 1202 may be coupled to other devices, such as a keyboard 1214, a mouse device 1216, a printer, etc. Of course, those skilled in the art will recognize that any combination of the above components, or any number of different components, peripherals, and other devices, may be used with the computer 1202.
Generally, the computer 1202 operates under control of an operating system 1208 stored in the memory 1206, and interfaces with the user to accept inputs and commands and to present results through a graphical user interface (GUI) module 1218A. Although the GUI module 1218A is depicted as a separate module, the instructions performing the GUI functions can be resident or distributed in the operating system 1208, the computer program 1210, or implemented with special purpose memory and processors. The computer 1202 also implements a compiler 1212 which allows an application program 1210 written in a programming language such as COBOL, C++, FORTRAN, or other language to be translated into processor 1204 readable code. After completion, the application 1210 accesses and manipulates data stored in the memory 1206 of the computer 1202 using the relationships and logic that was generated using the compiler 1212. The computer 1202 also optionally comprises an external communication device such as a modem, satellite link, Ethernet card, or other device for communicating with other computers.
In one embodiment, instructions implementing the operating system 1208, the computer program 1210, and the compiler 1212 are tangibly embodied in a computer-readable medium, e.g., data storage device 1220, which could include one or more fixed or removable data storage devices, such as a zip drive, floppy disc drive 1224, hard drive, CD-ROM drive, tape drive, etc. Further, the operating system 1208 and the computer program 1210 are comprised of instructions which, when read and executed by the computer 1202, causes the computer 1202 to perform the steps necessary to implement and/or use the present invention. Computer program 1210 and/or operating instructions may also be tangibly embodied in memory 1206 and/or data communications devices 1230, thereby making a computer program product or article of manufacture according to the invention. As such, the terms �article of manufacture,� �program storage device� and �computer program product� as used herein are intended to encompass a computer program accessible from any computer readable device or media.
5. Estimating Non-Linearity
Referring again to FIGS. 10A and 10B, non-linear distortion maps 1018 that depict the non-linearity of the TWTA may be used by a non-linear distortion map module during the layered modulation signal reconstruction process. As described above, in an exemplary receiver 802, TWTA AM-AM and AM-PM map are applied using an estimated operating point to a re-encoded and re-modulated signal to more accurately reconstruct the upper layer signal.
While FIGS. 10A and 10B illustrate the use of the non-linear distortion maps, knowledge of the non-linear distortion maps (and operating point) must be determined. It should be noted that the measurement of non-linearity may be conducted in a variety of manners as part of the layered modulation scheme. The TWTA non-linearity may be measured at the local IRDs 500 or 802, in which case the operating point may be automatically calculated from the nonlinearity measurements. The TWTA non-linearity may also be made at a broadcast/uplink center 104 with the operating point similarly obtained, in which case information on TWTA non-linearity and operating point can be downloaded to individual IRDs 500/802, such as through the downlink signal 118, to support the layered modulation signal receiving process.
6. Measuring Non-Linearity
As described above, the measurement of non-linearity may be conducted in a variety of manners as part of the layered modulation scheme. A first mechanism for TWTA non-linearity measurement is fully described in U.S. patent application Ser. No. 10/165,710, entitled �SATELLITE TWTA ON-LINE NON-LINEARITY MEASUREMENT�, filed on Jun. 7, 2002 by Ernest C. Chen.
A second measurement mechanism may provide an improvement over the first mechanism. Such a second measurement mechanism is more fully described in U.S. patent application Ser. No. 10/962,346, entitled �COHERENT AVERAGING FOR MEASURING TRAVELING WAVE TUBE AMPLIFIER NONLINEARITY�, filed on Oct.8, 2004 by Ernest C. Chen. In the second measurement mechanism, non-linearity may be measured at the broadcast center or in each local IRD 500/802 (e.g., using a coherent averaging technique that maximizes processing gains).
The IRD 500/802 which receives the downlink signal 118 (e.g., from the LNB 502) may also include a signal processor which extracts the symbol stream and carrier frequency from the incoming signal and generates an ideal signal, i.e. a signal without the effects of the TWTA and noise. The ideal signal is then used in a comparison processor to produce TWTA characteristic maps (which provide the measurements for TWTA non-linearity). As described herein, the signal processor and comparison processor may be incorporated in IRD 802 within the tuner/demodulator 904, FEC 506. The details concerning the generation of the characteristic maps will be described below in the discussion of FIGS. 13A-13E.
Typically, the TWTA characteristic maps comprise measurements of the output amplitude modulation versus the input amplitude modulation (the AM-AM map) and the output phase modulation versus the input amplitude modulation (the AM-PM map). The received signal represents the TWTA amplifier output (plus noise) and the generated ideal signal represents the amplifier input. In addition to diagnosing and monitoring the amplifier, these characteristic maps may then be used to facilitate and/or improve reception of lower layer signals of a system using a layered modulation transmission scheme.
FIGS. 13A, 13B, 13C, and 13D are block diagrams of the basic system 1300 for measuring the characteristic maps. All of the described functions may be carried out within a receiver 802 used in a direct broadcast satellite system having a basic architecture as described above. The appropriate signal section is captured and demodulated by demodulator 1302 which aligns symbol timing and removes any residual carrier frequency and phase in the signal. The demodulated signal is used in a signal generator 1304 to generate an ideal signal, i.e. one representing the pre-transmitted signal. In the case of a digital signal, the signal will be further decoded to obtain the signal symbols which will be used to generate the ideal signal. The difference between the ideal signal and the received signal is used by processors 1306, 1310, 1308, 1312 to estimate a transmission non-linearity characteristic. Only a small section of the received signal, typically thousands of symbols, may be needed to obtain an estimate depending on the CNR of the received signal.
FIG. 13A depicts an embodiment where the non-linearity characteristic is estimated from a difference between the generated ideal signal (noise-free and without TWTA non-linearity) and the received signal after demodulation (and FEC decoding and remodulation as necessary). Because the ideal signal can be easily generated from only the symbols and symbol timing, obtaining the estimate from the received signal after demodulation simplifies the processing.
FIG. 13B depicts an embodiment where the performance characteristic is estimated from a difference between the ideal signal and the received signal before demodulation. In this case, the ideal signal must also be generated with the carrier frequency of the received signal. This may be done by adding the demodulated symbol timing and carrier frequency and phase to the ideal signal.
FIG. 13C is similar to FIG. 13A in that it depicts an embodiment where the non-linearity characteristic is estimated from a difference between the generated ideal signal (noise-free and without TWTA non-linearity) and the received signal after demodulation. As in FIGS. 13A and 13B, the measurement procedure starts with capturing a received signal and generating (1304) an ideal signal 1320 that is noiseless and without TWTA nonlinearity. The ideal signal 1320 is generated by re-encoding and re-modulating the symbols decoded with quasi-error-free (QEF) performance (as described above). The ideal signal 1320 represents the input to TWTA 816 and the captured signal represents the output of TWTA 816 (with noise).
In normal conditions, non-linearity widens the signal bandwidth through IM-3 (third-order inter-modulation), etc. Further, various noise and interference may impair the non-linearity measurements. For example, thermal noise (additive downlink), phase noise (LNB, tuner) and other interferences (CCI, ACI, lower-layer signal) may impair the measurements. Such impairments typically will not bias the AM-AM and AM-PM estimates. On the other hand, filtering along signal path generally reduces the observable non-linearity.
To account for the filtering effects, an inverse filter/equalizer 1322 may be used to improve the measurement accuracy by restoring the un-filtered signal (i.e., it recovers invisible non-linearity). Accordingly, the embodiment illustrated in FIG. 13C provides for using an inverse filter and/or equalizer 1322 to undo the filtering effect so that TWTA nonlinearity may be restored for most accurate measurements. A linear equalizer 1322 may be used to estimate and remove linear channel distortions and multi-path effects over the entire signal path. Thus, the linear equalizer/inverse filters 1322 effectively performs inverse filtering to �cancel� the combined filter effects. For example, the inverse filters 1322 may estimate and remove linear channel distortion caused by satellite OMUX filters (which may have a significant effect), IRD front-end low pass filters (LPF) (which may also have a major effect since bandwidth is usually limited), and/or other filters between the TWTA and the received/captured data. However, certain filters may not effect the signal. For example, an IMUX filter may have little effect since it happens prior to the bandwidth-widening effect of TWTA nonlinearity.
FIG. 13D is similar to FIG. 13C but differs in one respect, when the inverse filter/equalizer 1322 is applied to the signal. In FIG. 13C, the inverse filter and/or equalizer 1322 is applied after demodulating 1302 the signal. However, in FIG. 13D, the inverse filter/equalizer 1322 is applied to the signal out of the LNB prior to demodulation 1302. In other words, the signal is equalized before both processing the received signal and generating the ideal signal. The use of an equalizer 1322 in both FIG. 13C and FIG. 13D improves the general carrier to noise ratio in higher-order modulation such as in the layered modulation described herein.
In all of the embodiments (i.e., illustrated in FIGS. 13A, 13B, 13C, and/or 13D) the ideal signal and the received signal are next used in processors 1306, 1308 to pair and sort data points of the two signals in a two-dimensional scattergram (a scattergram for purposes herein is the collection of paired points with input and output values represented along X and Y axes, respectively). These processors 1306, 1308 characterize a relationship between an input signal and an output signal of the amplifier plus noise. In this case, the input signal is represented by the generated ideal signal 1320 (re-modulated or otherwise) and the output signal is represented by the received signal. The X-axis of an AM-AM scattergram plots the magnitudes of the ideal signal samples before TWTA nonlinear distortions, and the Y-axis consists of the magnitudes of the received signal samples including the TWTA non-linearity (and noise). An AM-PM scattergram is similarly formed. The X-axis is the same as that for the AM-AM scattergram, and the Y-axis consists of all phase differences between the corresponding samples with and without TWTA non-linearity.
Accordingly, the two signals are aligned sample-for-sample by the timing recovery loop of the demodulator 1302, and data pairs are formed for all corresponding samples. The data sorting (1306 and 1308) sorts the data pairs to the powers of the ideal signal samples. For example, the re-generated ideal (linear) signal samples are sorted in power/magnitude. The received (nonlinear) signal samples remain tagged to the re-generated signal samples after re-ordering.
At this point in the process, the embodiments of FIGS. 13C and 13D may diverge from the embodiments of FIGS. 13A and 13B. The averaging process illustrated in FIGS. 13C and 13D coherently add a given number (representing the averaging ratio) of adjacent sorted TWTA input signals for averaging (i.e., using coherent averaging processors 1324 and 1326) to reduce noise effect. The averaging ratio to be used depends on signal CNR and on the quality of the nonlinearity measurement that is required for a given application. Accordingly, measurement impairments may be compensated by increasing the averaging ratio. For uncorrelated or little correlated noise and interference samples, the averaging process can increase the quality of an unbiased estimate by 3 dB for every doubling of the averaging ratio. However, as described above, the presence of filters along the signal path before the data is collected can reduce the accuracy of the estimate. Including the satellite OMUX filter and IRD front-end low pass filter (LPF), in-path filters can hide a part of the nonlinearity from the captured data.
The use of coherent averaging improves TWTA AM-AM and AM-PM estimates without creating a measurement floor across all input signal powers and works with both classic or layer-modulated signals. Further, the coherent averaging permits TWTA non-linearity to be measured by individual IRDs thereby avoiding the need to download TWTA tables to the IRD if desired.
As part of the coherent averaging, the average powers (operating point) of the input and output signals as well as the actual AM-AM and AM-PM curves may first be rescaled to 0 dB and 0 degree respectively for convenience of subsequent processing. The sorted linear and nonlinear signals are then coherently averaged with the averaging ratio depending on the input signal CNR and quality of non-linearity measurement required as described above.
If necessary, forward error correction (FEC) may be applied to the demodulated signal followed by a re-encoding and remodulation process to ensure that all recovered symbols are error-free.
After the sorting (i.e., in FIGS. 13A and 13B) and coherent averaging (i.e., in FIGS. 13C and 13D), the data points of the ideal signal and the corresponding data points of the received signal are processed by a processor 1310, 1312 to form a line through curve fitting, such as with a polynomial. The result is an estimate of the desired performance characteristic of the TWTA 1314, 1316.
FIG. 13E outlines the flow of a method of the present invention. A signal is received at block 1328. The signal is filtered and/or equalized at step 1330. Thus, at step 1330, an inverse filter and/or linear equalizer 1322 may be used to process the received signal to restore visible nonlinearity. The signal is demodulated at block 1332. Then an ideal signal is generated from the filtered/equalized demodulated (and decoded, re-encoded and remodulated as necessary) signal at block 1334. At step 1336, the received signal (either before or after demodulation) and/or the ideal signal (either with or without re-modulation correspondingly) is coherently averaged to reduce noise. Finally, a performance characteristic (i.e., a TWTA non-linearity curve) is estimated from a difference between the ideal signal (coherently averaged or prior to averaging) and the coherently averaged received signal at block 1338.
As described above, the coherent averaging 1336 and estimating 1338 steps may be performed by aligning the received signal and the ideal signal sample-for sample, forming data pairs for corresponding samples, sorting the data pairs to powers of the ideal signal samples, and coherently averaging the received signals to reduce noise. Such coherent averaging provides the capability to average complex numbers instead of merely examining the magnitude of the signals. In this regard, the coherent averaging process coherently adds a number of adjacent receiving signal samples, with the number of samples representing the averaging ratio.
This concludes the description of the preferred embodiments of the present invention. The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. For example, it is noted that the uplink configurations depicted and described in the foregoing disclosure can be implemented by one or more hardware modules, one or more software modules defining instructions performed by a processor, or a combination of both.
As described above, the received signal (i.e., the signal out of the LNB) may be filtered and/or equalized to restore visible nonlinearity to enable full measurements. The result of such an equalization technique is a significantly improved TWTA nonlinearity measurement for layered modulation (LM) and other applications. Further, the invention enables the TWTA to be accurately measured online (simultaneous with satellite communication operation). Such capabilities avoids service disruption and allows the TWTA nonlinearity measurement to be performed with the highest performance from any receiver and at any time.
It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto. The above specification, examples and data provide a complete description of the manufacture and use of the apparatus and method of the invention. Since many embodiments of the invention can be made without departing from the scope of the invention, the invention resides in the claims hereinafter appended.
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