Method and apparatus for tailoring carrier power requirements according to availability in layered modulation systems

A method and apparatus transmitting a layered modulation signal having a first signal layer having first signal symbols and a second signal layer having second signal symbols is disclosed. The method comprises the steps of determining a first signal layer modulation carrier power CL at least in part according to a first signal layer clear sky margin ML and a first signal layer availability, determining an second signal layer modulation carrier power CU at least in part according to an second signal layer clear sky margin MU and an second signal layer availability, modulating the first signal symbols according to a first carrier at the determined first signal layer modulation carrier power; modulating the second signal symbols according to a second carrier at the determined second signal layer modulation carrier power, and transmitting the two layers independently.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to systems and methods for transmitting data, and in particular to a system and method for tailoring carrier power requirements in a layered modulation system.

2. Description of the Related Art

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 and 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 (8PSK) or sixteen quadrature amplitude modulation (16QAM). Unfortunately, QPSK receivers cannot demodulate conventional 8PSK or 16QAM signals. As a result, legacy customers with QPSK receivers must upgrade their receivers in order to continue to receive any signals transmitted with an 8PSK or 16QAM modulation.

It is advantageous for systems and methods of transmitting signals to accommodate enhanced and increased data throughput without requiring additional spectrum. 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 TWTA peak power significantly lower than that for a conventional 8PSK or 16QAM modulation format for a given throughput.

However, a significant roadblock associated with implementing layered modulation is the requirement for satellite transponder powers levels that are significantly higher than those currently deployed for given Earth coverage area.

Accordingly, there is a need for systems and methods for implementing layered modulation systems at lower transponder power levels. The present invention meets this need and provides further advantages as detailed hereafter.

SUMMARY OF THE INVENTION

To address the requirements described above, the present invention discloses a method and apparatus transmitting a layered modulation signal having a first signal layer having first signal symbols and a second signal layer having second signal symbols. The method comprises the steps of determining a first signal layer modulation carrier power CLat least in part according to a first signal layer clear sky margin MLand a first signal layer availability, determining an second signal layer modulation carrier power CUat least in part according to an second signal layer clear sky margin MUand an second signal layer availability, modulating the first signal symbols according to a first carrier at the determined first signal layer modulation carrier power; modulating the second signal symbols according to a second carrier at the determined second signal layer modulation carrier power to generate the layered modulation signal, and transmitting the layered modulation signal. In one embodiment, the second signal layer clear sky margin is less than the first signal layer clear sky margin when the first signal layer availability and the second signal layer availability are substantially equal. In another embodiment, the second signal layer availability is greater than the first signal layer availability and the second signal layer clear sky margin MUequals

βUαU⁢βU+βL⁢TLαL+βL⁢TL,
wherein αUat least partially represents the rain attenuation of the second modulation carrier, αLat least partially represents the rain attenuation of the first layer modulation carrier, βUat least partially represents the additional noise in the second modulation carrier due to rain, and βLat least partially represents the additional noise in the first modulation carrier due to rain.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Video Distribution System

FIG. 1is a diagram illustrating an overview of a single satellite video distribution system100. The video distribution system100comprises a control center102in communication with an uplink center104via a ground or other link114and with a subscriber receiver station10via a public switched telephone network (PSTN) or other link120. The control center102provides program material (e.g. video programs, audio programs and data) to the uplink center104and coordinates with the subscriber receiver stations110to offer, for example, pay-per-view (PPV) program services, including billing and associated decryption of video programs.

The uplink center104receives program material and program control information from the control center102, and using an uplink antenna106and transmitter105, transmits the program material and program control information to the satellite108. The satellite receives and processes this information, and transmits the video programs and control information to the subscriber receiver station110via downlink118using transmitter107. The subscriber receiving station110receives this information using the outdoor unit (ODU)112, which includes a subscriber antenna and a low noise block converter (LNB).

In one embodiment, the subscriber receiving station antenna is an 18-inch slightly oval-shaped Ku-band antenna. The slight oval shape is due to the 22.5 degree offset feed of the LNB (low noise block converter) which is used to receive signals reflected from the subscriber antenna. The offset feed positions the LNB out of the way so it does not block any surface area of the antenna minimizing attenuation of the incoming microwave signal.

The video distribution system100can comprise a plurality of satellites108in order to provide wider terrestrial coverage, to provide additional channels, or to provide additional bandwidth per channel. In one embodiment of the invention, each satellite comprises16transponders to receive and transmit program material and other control data from the uplink center104and provide it to the subscriber receiving stations110. Using data compression and multiplexing techniques the channel capabilities, two satellites108working together can receive and broadcast over 150 conventional (non-HDTV) audio and video channels via32transponders.

While the invention disclosed herein will be described with reference to a satellite-based video distribution system100, the present invention may also be practiced with terrestrial-based transmission of program information, whether by broadcasting means, cable, or other means. Further, the different functions collectively allocated among the control center102and the uplink center104as described above can be reallocated as desired without departing from the intended scope of the present invention.

Although the foregoing has been described with respect to an embodiment in which the program material delivered to the subscriber122is video (and audio) program material such as a movie, the foregoing method can be used to deliver program material comprising purely audio information or other data as well.

Uplink Configuration

FIG. 2is a block diagram showing a typical uplink configuration for a single satellite108transponder, showing how video program material is uplinked to the satellite108by the control center102and the uplink center104.FIG. 2shows three video channels (which could be augmented respectively with one or more audio channels for high fidelity music, soundtrack information, or a secondary audio program for transmitting foreign languages), a data channel from a program guide subsystem206and computer data information from a computer data source208.

The video channels are provided by a program source of video material200A–200C (collectively referred to hereinafter as video source(s)200). The data from each video program source200is provided to an encoder202A–202C (collectively referred to hereinafter as encoder(s)202). Each of the encoders accepts a program time stamp (PTS) from the controller216. 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.

In one embodiment of the present invention, each encoder202is a second generation Motion Picture Experts Group (MPEG-2) encoder, but other decoders implementing other coding techniques can be used as well. The data channel can be subjected to a similar compression scheme by an encoder (not shown), but such compression is usually either unnecessary, or performed by computer programs in the computer data source (for example, photographic data is typically compressed into *.TIF files or *.JPG files before transmission). After encoding by the encoders202, the signals are converted into data packets by a packetizer204A–204F (collectively referred to hereinafter as packetizer(s)204) associated with each source200.

The data packets are assembled using a reference from the system clock214(SCR), and from the conditional access manager210, which provides the SCD to the packetizers204for use in generating the data packets. These data packets are then multiplexed into serial data and transmitted.

Broadcast Data Stream Format and Protocol

FIG. 3Ais a diagram of a representative data stream. The first packet segment302comprises information from video channel1(data coming from, for example, the first video program source200A). The next packet segment304comprises computer data information that was obtained, for example from the computer data source208. The next packet segment306comprises information from video channel5(from one of the video program sources200). The next packet segment308comprises program guide information such as the information provided by the program guide subsystem206. As shown inFIG. 3A, null packets310created by the null packet module310may be inserted into the data stream as desired.

The data stream therefore comprises a series of packets from any one of the data sources in an order determined by the controller216. The data stream is encrypted by the encryption module218, modulated by the modulator220(typically using a QPSK modulation scheme), and provided to the transmitter222, which broadcasts the modulated data stream on a frequency bandwidth to the satellite via the antenna106. The receiver500receives these signals, and using the SCID, reassembles the packets to regenerate the program material for each of the channels.

FIG. 3Bis a diagram of a data packet. Each data packet (e.g.302–316) is 130 bytes long, and comprises a number of packet segments. The first packet segment320comprises 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 segment322is 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 segment324comprises 127 bytes of payload data, which in the cases of packets302or306is a portion of the video program provided by the video program source200. The final packet segment326is data required to perform forward error correction.

FIG. 4is a block diagram showing one embodiment of the modulator220. The modulator220optionally comprises a forward error correction (FEC) encoder404which accepts the first signal symbols402and adds redundant information that are used to reduce transmission errors. The coded symbols405are modulated by modulator406according to a first carrier408to produce an upper layer modulated signal410. Second symbols420are likewise provided to an optional second FEC encoder422to produce coded second symbols422. The coded second symbols422are provided to a second modulator414, which modulates the coded second signals according to a second carrier416to produce a lower layer modulated signal418. The resulting signals are then transmitted by one or more transmitters420,422. The upper layer modulated signal410and the lower layer modulated signal418are therefore uncorrelated, and the frequency range used to transmit each layer can substantially or completely overlap the frequency spectrum used to transmit the other. For example, as shown inFIG. 4, the frequency spectrum f1→f3432of the upper layer signal410may overlap the frequency spectrum f2→f4434of the lower layer signal418in frequency band f2-f3436. The upper layer signal410, however, must be a sufficiently greater amplitude signal than the lower layer signal418, in order to maintain the signal constellations shown inFIG. 6andFIG. 7. The modulator220may also employ pulse shaping techniques (illustrated by pulse p(t)430) to account for the limited channel bandwidth. AlthoughFIG. 4illustrates the same pulse shaping p(t)430being applied to both layers, different pulse shaping can be applied to each layer as well.

FIG. 5is a block diagram of an integrated receiver/decoder (IRD)500(also hereinafter alternatively referred to as receiver500). The receiver500comprises a tuner/demodulator504communicatively coupled to an ODU112having one or more LNBs502. The LNB502converts the 12.2- to 12.7 GHz downlink118signal from the satellites108to, e.g., a 950–1450 MHz signal required by the IRD's500tuner/demodulator504. The LNB502may provide either a dual or a single output. The single-output LNB502has only one RF connector, while the dual output LNB502has two RF output connectors and can be used to feed a second tuner504, a second receiver500, or some other form of distribution system.

The tuner/demodulator504isolates a single, digitally modulated 24 MHz transponder, and converts the modulated data to a digital data stream. Further details regarding the demodulation of the received signal follow.

The digital data stream is then supplied to a forward error correction (FEC) decoder506. This allows the IRD500to reassemble the data transmitted by the uplink center104(which applied the forward error correction to the desired signal before transmission to the subscriber receiving station110) verifying that the correct data signal was received, and correcting errors, if any. The error-corrected data may be fed from the FEC decoder module506to the transport module508via an 8-bit parallel interface.

The transport module508performs many of the data processing functions performed by the IRD500. The transport module508processes data received from the FEC decoder module506and provides the processed data to the video MPEG decoder514and the audio MPEG decoder517. In one embodiment of the present invention, the transport module, video MPEG decoder and audio MPEG decoder 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 module508. The transport module508also provides a passage for communications between the microcontroller510and the video and audio MPEG decoders514,517. As set forth more fully hereinafter, the transport module also works with the conditional access module (CAM)512to determine whether the subscriber receiving station110is permitted to access certain program material. Data from the transport module can also be supplied to external communication module526.

The CAM512functions in association with other elements to decode an encrypted signal from the transport module508. The CAM512may also be used for tracking and billing these services. In one embodiment of the present invention, the CAM512is a smart card, having contacts cooperatively interacting with contacts in the IRD500to pass information. In order to implement the processing performed in the CAM512, the IRD500, and specifically the transport module508provides a clock signal to the CAM512.

Video data is processed by the MPEG video decoder514. Using the video random access memory (RAM)536, the MPEG video decoder514decodes the compressed video data and sends it to an encoder or video processor516, which converts the digital video information received from the video MPEG module514into an output signal usable by a display or other output device. By way of example, processor516may comprise a National TV Standards Committee (NTSC) or Advanced Television Systems Committee (ATSC) encoder. In one embodiment of the invention both S-Video and ordinary video (NTSC or ATSC) signals are provided. Other outputs may also be utilized, and are advantageous if high definition programming is processed.

Audio data is likewise decoded by the MPEG audio decoder517. The decoded audio data may then be sent to a digital to analog (D/A) converter518. In one embodiment of the present invention, the D/A converter518is a dual D/A converter, one for the right and left channels. If desired, additional channels can be added for use in surround sound processing or secondary audio programs (SAPs). In one embodiment of the invention, the dual D/A converter518itself separates the left and right channel information, as well as any additional channel information. Other audio formats may similarly be supported. For example, other audio formats such as multi-channel DOLBY DIGITAL AC-3 may be supported.

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 microcontroller510receives and processes command signals from the remote control524, an IRD500keyboard interface, and/or another input device. The microcontroller 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)522or, similar memory device. The microcontroller510also controls the other digital devices of the IRD500via address and data lines (denoted “A” and “D” respectively, inFIG. 5).

The modem540connects to the customer's phone line via the PSTN port120. It calls, e.g. the program provider, and transmits the customer's purchase information for billing purposes, and/or other information. The modem540is controlled by the microprocessor510. The modem540can output data to other I/O port types including standard parallel and serial computer I/O ports.

The present invention also comprises a local storage unit such as the video storage device532for storing video and/or audio data obtained from the transport module508. Video storage device532can be a hard disk drive, a read/writable compact disc of DVD, a solid state RAM, or any other storage medium. In one embodiment of the present invention, the video storage device532is a hard disk drive with specialized parallel read/write capability so that data may be read from the video storage device532and written to the device532at the same time. To accomplish this feat, additional buffer memory accessible by the video storage532or its controller may be used. Optionally, a video storage processor530can be used to manage the storage and retrieval of the video data from the video storage device532. The video storage processor530may also comprise memory for buffering data passing into and out of the video storage device532. Alternatively or in combination with the foregoing, a plurality of video storage devices532can be used. Also alternatively or in combination with the foregoing, the microcontroller510can also perform the operations required to store and or retrieve video and other data in the video storage device532.

The video processing module516input 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 modulator534to produce an RF output and/or 8 vestigial side band (VSB) suitable as an input signal to a digital terrestrial television tuner. This allows the receiver500to operate with televisions without a video output.

Each of the satellites108comprises a transponder, which accepts program information from the uplink center104, and relays this information to the subscriber receiving station110. Known multiplexing techniques are used so that multiple channels can be provided to the user. These multiplexing techniques include, by way of example, various statistical or other time domain multiplexing techniques and polarization multiplexing. In one embodiment of the invention, a single transponder operating at a single frequency band carries a plurality of channels identified by respective service channel identification (SCID).

Preferably, the IRD500also receives and stores a program guide in a memory available to the microcontroller510. Typically, the program guide is received in one or more data packets in the data stream from the satellite108. The program guide can be accessed and searched by the execution of suitable operation steps implemented by the microcontroller510and stored in the processor ROM538. The program guide may include data to map viewer channel numbers to satellite transponders and service channel identifications (SCIDs), and also provide TV program listing information to the subscriber122identifying program events.

The functionality implemented in the IRD500depicted inFIG. 5can be implemented by one or more hardware modules, one or more software modules defining instructions performed by a processor, or a combination of both.

The present invention provides for the modulation of signals at different power levels and advantageously for the signals to be non-coherent from each layer. In addition, independent modulation and coding of the signals may be performed. Backwards compatibility with legacy receivers, such as a quadrature phase shift keying (QPSK) receiver is enabled and new services are provided to new receivers. A typical new receiver of the present invention uses two demodulators and one remodulator as will be described in detail hereafter.

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. 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 combined layered signal is demodulated and decoded by first demodulating the upper layer to remove the upper carrier. The stabilized layered signal may then have the upper layer FEC decoded and the output upper layer symbols communicated to the upper layer transport. The upper layer symbols are also employed in a remodulator, to generate an idealized upper layer signal. The idealized upper layer signal is then subtracted from the stable layered signal to reveal the lower layer signal. The lower layer signal is then demodulated and FEC decoded and communicated to the lower layer transport.

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 8PSK, using the conventional concatenated FEC code or using a new Turbo code. The lower layer signal may even be an analog signal.

Signals, systems and methods using the present invention may be used to supplement a pre-existing transmission compatible with legacy receiving hardware in a backwards-compatible application or as part of a preplanned layered modulation architecture providing one or more additional layers at a present or at a later date.

Layered Signals

FIGS. 6A–6Cillustrate the basic relationship of signal layers in a layered modulation transmission. In these figures the horizontal axis is for the in-phase, or “I” value of the displayed symbol, and the vertical axis for the quadratue, or “Q” value of the displayed symbol.FIG. 6Aillustrates a first layer signal constellation600of a transmission signal showing the signal points or symbols602. This signal constellation isFIG. 6Billustrates the second layer signal constellation of symbols604over the first layer signal constellation600where the layers are coherent. FIG.2C illustrates a second signal layer606of a second transmission layer over the first layer constellation where the layers may be non-coherent. The second layer606rotates about the first layer constellation602due to the relative modulating frequencies of the two layers in a non-coherent transmission. Both the first and second layers rotate about the origin due to the first layer modulation frequency as described by path608.

FIGS. 7A–7Care diagrams illustrating a signal constellation of a second transmission layer over the first transmission layer after first layer demodulation.FIG. 7Ashows the constellation700before the first carrier recovery loop (CRL) andFIG. 7Bshows the constellation704after CRL. In this case, the signal points of the second layer are actually rings702.FIG. 7Cdepicts a phase distribution of the received signal with respect to nodes602.

Relative modulating frequencies cause the second layer constellation to rotate around the nodes of the first layer constellation. After the second layer CRL this rotation is eliminated. The radius of the second layer constellation is determined by its power level. The thickness of the rings702is determined by the carrier to noise ratio (CNR) of the second layer. As the two layers are non-coherent, the second layer may also be used to transmit analog or digital signals.

FIG. 8is a diagram showing a system for transmitting and receiving layered modulation signals. Separate transmitters107A,107B, as may be located on any suitable platform, such as satellites108A,108B, are used to non-coherently transmit different layers of a signal of the present invention. Uplink signals are typically transmitted to each satellite108A,108B from one or more transmitters105via an antenna106. The layered signals808A,808B (downlink signals) are received at receiver antennas112A,112B, 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. Because the signal layers may be transmitted non-coherently, separate transmission layers may be added at any time using different satellites108A,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 receivers500, 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.

Layered modulation applications include backwards compatible and non-backwards compatible applications. “Backwards compatible” in this sense, describes systems in which legacy receivers500are not rendered obsolete by the additional signal layer(s). Instead, even if the legacy receivers500are 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 and coding scheme employed is such that pre-existing equipment is incapable of receiving and decoding the information on additional signal layer(s).

The pre-existing legacy IRDs500decode and make use of data only from the layer (or layers) they were designed to receive, unaffected by the additional layers. The present invention may be applied to existing direct satellite services which are broadcast to individual users in order to enable additional features and services with new receivers without adversely affecting legacy receivers and without requiring additional signal frequencies.

Demodulator and Decoder

FIG. 9is a block diagram depicting one embodiment of an enhanced IRD802capable of receiving layered modulation signals. The enhanced IRD802includes a feedback path902in which the FEC decoded symbols are fed back to a enhanced modified tuner/demodulator904and transport module908.

FIG. 10Ais a block diagram of one embodiment of the enhanced tuner/modulator904and FEC encoder506.FIG. 10Adepicts reception where layer subtraction is performed on a signal where the upper carrier has been demodulated. The upper layer of the received combined signal1016from the LNB502, which may contain legacy modulation format, is provided to and processed by an upper layer demodulator1004to produce the stable demodulated signal1020. The demodulated signal1020is fed to a communicatively coupled FEC decoder1002which decodes the upper layer to produce the upper layer symbols which are output to an upper layer transport. The upper layer symbols are also used to generate an idealized upper layer signal. The upper layer symbols may be produced from the decoder1002after Viterbi decode (BER<10−3or so) or after Reed-Solomon (RS) decode (BER<10−9or so), in typical decoding operations known to those skilled in the art. The upper layer symbols are provided via feedback path902from the upper layer decoder1002to a remodulator1006and then a module which applies the distortion that would be introduced by the satellite downlink network. This effectively produces an idealized upper layer signal. The idealized upper level signal is subtracted from the demodulated upper layer signal1020.

In order for the subtraction to leave a clean lower layer signal, the upper layer signal must be precisely reproduced. The modulated signal may have been distorted, for example, by traveling wave tube amplifier (TWTA) non-linearity or other non-linear or linear distortions in the transmission channel. The distortion effects are estimated from the received signal after the fact or from TWTA characteristics which may be downloaded into the IRD in AM-AM and/or AM-PM maps1014.

A subtractor1012then subtracts the idealized upper layer signal from the stable demodulated signal1020. This leaves the lower-power second layer signal. The subtractor1012may include a buffer or delay function to retain the stable demodulated signal1020while the idealized upper layer signal is being constructed. The second layer signal is demodulated by the lower level demodulator1010and FEC decoded by decoder1008according to its signal format to produce the lower layer symbols, which are provided to a transport module similar to508but for the lower layer.

FIG. 10Bdepicts another embodiment wherein layer subtraction is performed on the received layered signal. In this case, the upper layer demodulator1004produces the upper carrier signal1022. An upper carrier signal1022is provided to the remodulator1006. The remodulator1006provides the remodulated signal to the non-linear distortion mapper1018which effectively produces an idealized upper layer signal. Unlike the embodiment shown inFIG. 10A, in this embodiment, the idealized upper layer signal includes the upper layer carrier for subtraction from the received combined signal416.

Other equivalent methods of layer subtraction will occur to those skilled in the art and the present invention should not be limited to the examples provided here. Furthermore, those skilled in the art will understand that the present invention is not limited to two layers; additional layers may be included. Idealized upper layers are produced through remodulation from their respective layer symbols and subtracted. Subtraction may be performed on either the received combined signal or a demodulated signal. Finally, it is not necessary for all signal layers to be digital transmissions; the lowest layer may be an analog transmission.

The following analysis describes the exemplary two layer demodulation and decoding. It will be apparent to those skilled in the art that additional layers may be demodulated and decoded in a similar manner. The incoming combined signal is represented as:

sUL⁡(t)=fU⁡(MU⁢⁢exp⁡(j⁢⁢ωU⁢t+θU)⁢⁢∑m=-∞∞⁢SUm⁢p⁡(t-mT))+fL⁡(ML⁢⁢exp⁡(j⁢⁢ωL⁢t+θL)⁢⁢∑m=-∞∞⁢SLm⁢p⁡(t-mT+Δ⁢⁢Tm))+n⁡(t)
where, MUis the magnitude of the upper layer QPSK signal and MLis the magnitude of the lower layer QPSK signal and ML<<MU. The signal frequencies and phase for the upper and lower layer signals are respectively ωU, θUand ωU, θU, respectively. The symbol timing misalignment between the upper and lower layers is ΔTm. The expression p(t−mT) represents the time shifted version of the pulse shaping filter p(t)430employed in signal modulation. QPSK symbols SUmand SLmare elements of

{exp⁡(j⁢⁢n⁢⁢π2),⁢n=0,1,2,3}.
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 output of the demodulator1004to the FEC decoder1002after removing the upper carrier:

After subtracting the upper layer from sUL(t) in the subtractor1012, the following remains:

sL⁡(t)=ML⁢⁢exp⁢{j⁡(ωL-ωU)⁢t+θL-θU}⁢⁢∑m=-∞∞⁢SLm⁢p⁡(t-mT+Δ⁢⁢Tm)
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 the current legacy system capacity that uses a legacy operating mode with a 6/7 FEC code rate. This capacity increase is enabled by transmitting a backward compatible upper layer carrier through a TWTA that is approximately 6.2 dB above the power used in the legacy system. The new lower layer QPSK signals may be transmitted from a separate transmitter, or from a different satellite for example.

Systems using 16QAM modulation could be designed to provide similar transmission capacity, but this modulation format requires reasonably linear transmitting amplifiers. With layered modulation, separate amplifiers can be used for each layer, and if QPSK signals are used for these layers then these amplifiers can be used in a more efficient non-linear mode. Thus layered modulation eliminates the need for less efficient linear travelling wave tube amplifiers (TWTAS) as are needed for 16QAM. Also, no phase error penalty is imposed on higher order modulations such as 8PSK and 16QAM.

Backward Compatible Applications

FIG. 11Adepicts the relative power levels1100of example embodiments of the present invention without taking into account the effects of rain. Accommodation of rain fade effects comes through the inclusion of clear sky margin in the calculation of transmit power levels, and this is treated in a later section.FIG. 11Ais not a scale drawing. This embodiment doubles the pre-existing rate 6/7 capacity by using a TWTA whose power level is 6.2 dB above a pre-existing (legacy) TWTA, and a second TWTA whose power level is 2 dB below that of a pre-existing (legacy) TWTA. This embodiment uses upper and lower QPSK layers which are non-coherent. An FEC code rate of 6/7 is also used for both layers. In this embodiment, the signal of the legacy QPSK signal1102is used to generate the upper layer1104and a new QPSK layer is the lower layer1110. The legacy QPSK signal1102has a threshold CNR (i.e., the carrier to noise ratio required to achieve acceptable performance) of approximately 7 dB. The new lower QPSK layer1110has a threshold CNR of approximately 5 dB. In the present invention, then, the lower QPSK layer transmit power level1110is first set so that the received lower layer power is 5 dB above the reference thermal noise power level1108. Both the thermal noise and the lower layer signal will appear as noise to the upper layer legacy QPSK signal, and this combined noise power must be taken into account when setting the upper layer transmit power level. The combined power of these two noise sources1106is 6.2 dB above the reference thermal noise floor1108. The legacy QPSK signal must then be boosted in power by approximately 6.2 dB above the legacy signal power level1102bringing the new power level to approximately 13.2 dB as the upper layer1104. In this way the combined lower layer signal power and thermal noise power is kept at or below the tolerable noise floor1106of the upper layer. It should be noted that the invention may be extended to multiple layers with mixed modulations, coding and code rates.

In an alternate embodiment of this backwards compatible application, an FEC code rate of 2/3 may be used for both the upper and lower layers1104,1110. In this case, the threshold CNR of the legacy QPSK signal1102(with an FEC code rate of 2/3) is approximately 5.8 dB. The legacy signal1102is boosted by approximately 5.3 dB to approximately 11.1 dB (4.1 dB above the legacy QPSK signal1102with an FEC code rate of 2/3) to form the upper QPSK layer1104. The new lower QPSK layer1110has a threshold CNR of approximately 3.8 dB. The total signal and noise of the lower layer1110is kept at or below approximately 5.3 dB, the tolerable noise floor1106of the upper QPSK layer. In this case, the total capacity is 1.55 times that the legacy signal1102.

In a further embodiment of a backwards compatible application of the present invention the code rates between the upper and lower layers1104,1110may be mixed. For example, the legacy QPSK signal502may be boosted by approximately 5.3 dB to approximately 12.3 dB with the FEC code rate unchanged at 6/7 to create the upper QPSK layer1104. The new lower QPSK layer1110may use an FEC code rate of 2/3 with a threshold CNR of approximately 3.8 dB. In this case, the total capacity is 1.78 times that of the legacy signal1102.

Non-Backward Compatible Applications

As previously discussed the present invention may also be used in “non-backward compatible” applications. In a first example embodiment, two QPSK layers1104,1110are used each at an FEC code rate of 2/3. The upper QPSK layer504has a threshold CNR of approximately 4.1 dB above its noise floor1106and the lower QPSK layer1110also has a threshold CNR of approximately 4.1 dB. The combined power of the thermal noise and the lower QPSK layer1110is approximately 5.5 dB above the reference thermal noise level1108. The CNR for the upper QPSK signal1104is then set at approximately 9.6 dB (4.1+5.5 dB), merely 2.4 dB above the legacy QPSK signal rate 6/7. The capacity is then a factor of approximately 1.56 compared to the legacy rate 6/7.

FIG. 11Bdepicts the relative power levels of an alternate embodiment wherein both the upper and lower layers1104,1110can be below the legacy signal level1102. The two QPSK layers1104,1110use a code rate of 1/2. The lower and upper QPSK layers have a threshold CNR of approximately 2.0 dB. In this case, the upper QPSK layer1104is approximately 2.0 dB above its noise floor1106of approximately 4.1 dB. The upper layer signal level of 6.1 dB is lower than the 7.0 dB for the legacy signal. The capacity of this embodiment is a factor of approximately 1.17 compared to the legacy rate 6/7.

Hardware Environment

FIG. 12illustrates an exemplary computer system1200that could be used to implement selected modules or functions the present invention. The computer1202comprises a processor1204and a memory, such as random access memory (RAM)1206. The computer1202is operatively coupled to a display1222, which presents images such as windows to the user on a graphical user interface1218B. The computer1202may be coupled to other devices, such as a keyboard1214, a mouse device1216, 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 computer1202.

Generally, the computer1202operates under control of an operating system1208stored in the memory1206, and interfaces with the user to accept inputs and commands and to present results through a graphical user interface (GUI) module1218A. Although the GUI module1218A is depicted as a separate module, the instructions performing the GUI functions can be resident or distributed in the operating system1208, the computer program1210, or implemented with special purpose memory and processors. The computer1202also implements a compiler1212which allows an application program1210written in a programming language such as COBOL, C++, FORTRAN, or other language to be translated into processor1204readable code. After completion, the application1210accesses and manipulates data stored in the memory1206of the computer1202using the relationships and logic that was generated using the compiler1212. The computer1202also 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 system1208, the computer program1210, and the compiler1212are tangibly embodied in a computer-readable medium, e.g., data storage device1220, which could include one or more fixed or removable data storage devices, such as a zip drive, floppy disc drive1224, hard drive, CD-ROM drive, tape drive, etc. Further, the operating system1208and the computer program1210are comprised of instructions which, when read and executed by the computer1202, causes the computer1202to perform the steps necessary to implement and/or use the present invention. Computer program1210and/or operating instructions may also be tangibly embodied in memory1206and/or data communications devices1230, 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.

Those skilled in the art will recognize many modifications may be made to this configuration without departing from the scope of the present invention. For example, 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 present invention.

Using the techniques described herein, as will be shown later, the clear sky margin required for the upper signal layer402is considerably less than the clear sky margin that would be required if the signal were sent by itself. It is also considerably less than that required for the lower signal layer420. In a rain fade condition, the upper and lower layers fade together. Thus, the primary source of noise for the upper signal layer402fades as fast as the upper layer signal itself, allowing for a significantly reduced upper layer clear sky margin. The present invention takes full advantage of this effect. Conversely, the clear sky margin required for the lower layer must be set high enough to account for the fade of the lower layer carrier relative to its primary source of noise, thermal noise, which increases in rain. Hence, the required clear sky margin for the upper signal layer402can be reduced when compared to that which is required for the lower signal layer420. Alternatively or in combination, the technique described below can be used to design a layered modulation system that provides higher availability levels for the upper layer than for the lower layer.

Clear Sky Margin

The distribution of power to the upper and lower layer carriers discussed earlier (FIGS. 11A and 11B) did not consider the affects of rain attenuation on the upper and lower layer signals. These affects can be large, acting to both decrease the desired signal level and to increase the noise level. In the case of layered modulation these effects must be carefully considered for each layer. Additional power is added to each layer to accommodate these rain effects, and this added power is called clear sky margin (CSM). In the technique described below, the design of the upper signal layer410takes advantage of the fact that both the lower signal layer418and the upper signal layer410are attenuated by an equal amount in a rain fade condition. Also, since the upper signal layer410must be locked and reconstructed before the lower signal layer418can be accurately demodulated, the lower signal layer418can be no more “available” in a statistical sense than the upper signal layer410. In a critical condition where the upper and lower signal layers have exactly the same availability, both signals will drop to their respective operating thresholds simultaneously when the rain attenuation reaches a sufficient value.

Upper and Lower Signal Layers with Equal Availability

Assuming that the clear sky thermal noise level is represented by N, and that the carrier-to-noise threshold level is given by TLand TU, for the lower and upper signal layers418,410, respectively, the threshold levels TLand TUcan be defined in a number of ways. For purposes of illustration, the analysis that follows assumes that the threshold levels TLand TUare quasi-error-free thresholds. This is the operating point where the number of bit errors detected at the output of the forward error correction decoder506have dropped to about one error per hour or one error per day.

Assuming for the moment that there is a given link availability requirement, from that value, suitable values for rain attenuation and rain noise can be determined. Defining a parameter α to represent the amount of rain attenuation present (α<=1), and β to represent the increase in noise due to atmospheric rain (β>=1), both of which are a function of the desired signal availability, the lower signal layer418carrier power CLrequired to provide the necessary link availability can be determined from the expression:

The values α and β are both a function of the desired availability, and are typically defined through the use of rain attenuation models that would be readily known to someone skilled in the art.

A clear sky margin (defined as a ratio between the clear sky carrier to noise-plus-interference ratio and the threshold carrier to noise-plus-interference ratio) can be computed for each layer. For the lower signal layer418, the clear sky margin MLbecomes:

ML=CLN⁢⁢TL=βαEquation⁢⁢(3)
The upper signal layer410carrier power CUnecessary to provide the required link availability is computed by noting that when the upper signal layer carrier is at a threshold condition, the carrier is attenuated by the factor α. The noise term however, contains both the link thermal noise power (increased by the rain) and the lower level carrier power (attenuated by rain). Consequently, the upper layer carrier power CLnecessary to provide the required link availability is defined by the Equation (4) below:

Using this expression, the required upper level carrier power CUis described in Equation (5) below.

And the clear sky margin for the upper signal layer410becomes

Noting that

The upper signal layer clear sky margin can be written in terms of the lower signal layer threshold as shown in Equation (8) below.

In a typical application, the values of α might change from −1 to −5 dB and the values for β might range from 2 to 4 dB, depending on the desired availability. Since the lower level clear sky margin is (β-α), when expressed in dB, then it can be seen that typical lower signal layer clear sky margins will range from 3 to 9 dB, depending on the desired availability.

It would ordinarily be expected that the lower signal layer clear sky margin would also be required for the upper signal layer, which would require very high transmitter powers. However, this is not necessary because the upper and lower signal layers fade together in rain, as shown in the derivation for the upper layer clear sky margin in Equation (4) above. Hence, the upper layer clear sky margin depends on the carrier to noise threshold and to a lesser extent on the ratio of α to β. The required upper layer clear sky margin is typically 1 dB or less, and approaches 0 dB with increasing lower signal layer420carrier-to-noise threshold.

FIG. 13is a diagram showing both upper and lower signal layer clear sky margins as a function of lower layer threshold and desired availability. Plot1302shows the lower layer clear sky margin as a function of the lower signal layer carrier-to-noise ratio threshold for a lower signal layer availability of 99.95%. Plots1304–1308show the same for lower signal layer availabilities of 99.90%, 99.85%, and 99.80%, respectively. Plots1310–1316show the upper layer clear sky margin for upper signal layer availabilities of 99.95%, 99.90%, 99.85%, and 99.80% respectively. Note in this figure that the upper layer clear sky margins are much less than the lower layer clear sky margins. The smaller clear sky margins for the upper layer are critical to the performance of layered modulation because it lowers the required satellite transmit power of the upper layer carrier.

The ratio of the upper signal layer carrier to thermal noise in clear sky can be computed as a function of α, β, and the upper and lower carrier-to-noise ratios.

Beginning with the relation

we can obtain

If the lower signal layer418were not present (e.g. a legacy signal), the required clear sky carrier to noise ratio would not include the term (1+TL). This added term accounts for the presence of the lower signal layer418as interfering noise to the upper signal layer410. Noting that N refers to the thermal noise only, the total noise plus lower layer interference power seen by the upper signal layer demodulator is dominated by the lower layer signal layer carrier power.

Equation (10) provides a minimum value for CUrelative to the thermal noise for both the upper and lower signal layers to exhibit the same availability. By increasing CUabove this level, the availability of the upper signal layer410can be increased over that of the lower signal layer418.

FIG. 14is an illustration showing exemplary lower and upper signal layer clear sky margins as power levels (dB) relative to thermal noise in clear sky conditions. In this example, the lower signal layer carrier-to-noise-plus-interference threshold was set at 6.0 dB, and the upper signal layer carrier-to-noise-plus-interference threshold was set to 5.0 dB. The values for α and β are about −2.0 and +3.0 dB, respectively. Note that the lower signal layer threshold point plus clear sky margin give a clear sky lower layer carrier power of 11.0 dB relative to thermal noise N. The combination of thermal noise and lower signal layer carrier power is 11.4 dB, which is the noise plus interference level seen by the upper signal layer carrier.

Summing (in dB) the upper layer required threshold to the noise-plus-interference value of 5 dB to 11.4 dB gives the upper layer threshold point of 16.4 dB relative to thermal noise N. The required clear sky margin above this point is only 0.6 dB, yet in a rain fade condition, bot the upper and lower signal layers will exhibit the same availability.

Upper and Lower Signal Layer Margins with Improved Upper Layer Availability

The upper and lower signal layers410,418can be designed with different availability objectives a well. As previously noted, the lower signal layer418availability cannot be better than the upper signal layer410availability, since successful demodulation of the lower signal layer418depends on successful demodulation of the upper signal layer410. However, the upper signal layer410can be designed with better availability than the lower signal layer418by increasing the upper signal layer margin. As demonstrated below, significant improvements can be made in the upper signal layer410availability with only small increases in the upper signal layer410margin. This is a significant advantage of the non-coherent layered modulation techniques described herein.

Modifying Equation (1) to differentiate between the parameters α and β for the upper and lower signal layers yields Equation (11) below.

This yields Equations (12) and (13).

For improved availability in the upper layer,
αU<αLEquation (14)
and
βU>βL.  Equation (15)

Noting that when the upper signal layer410is at threshold, the new upper signal layer values for α and β will apply,

Referring to Equation (5), the new upper signal carrier power becomes

Using Equation (6), the following relationship is derived:

we obtain,

Note that Equation (20) reduces to Equation (8) if the availabilities of the upper and lower signal layers are equal (e.g. αL=αUand βL=βU).

FIG. 15is a plot of Equation (20) as a function of the unavailability of the upper signal layer410. In this example, the lower level unavailability is 0.02% (since unavailability is (1-availability), this translates to an availability of 99.8%) and the lower signal layer threshold is 6 dB.

As can be seen in the lower curve ofFIG. 15, which plots MU, the upper layer clear sky margin defined by (18) or (20), the upper signal layer performance can be improved (e.g. lower unavailability) by increasing the upper signal layer clear sky margin by only 10ths of a dB, as shown in curve1504. As upper curve1502shows, in a conventionally modulated, single-layer satellite link, the clear sky margin would have to be improved by 3 dB to achieve the same performance improvement.

Thus, if one of the signal layers requires higher availability than the other, then that layer must be designated as the upper signal layer. Similarly, if backward compatibility is required, then the signal layer that provides such backward compatibility must be designated as the upper signal layer. Normally, there is no conflict between these requirements, as the backwards-compatible layer is normally also desired to be the higher availability layer. If, however, the non-backwards-compatible layer requires higher availability than the backwards-compatible layer, a conflicting requirement exists. This can be resolved by designing the system such that the availability of the signal layers is equal and at the higher availability value.

FIG. 16is a diagram illustrating exemplary method steps that can be used to practice one embodiment of the invention. A first signal layer modulation carrier power CLis determined at least in part according to a first signal layer clear sky margin MLand a first signal layer availability, as shown in block1602. In one embodiment, this is accomplished by determining the first level carrier power CLaccording to

CL=β⁢⁢NTLα,
wherein β/α is the first layer clear sky margin ML, β comprises a value representing an increase in noise of the layered modulation signal due to atmospheric rain, α comprises a value representing rain attenuation of the layered modulation signal, N comprises a value representing clear-sky thermal noise, and TLcomprises a first signal layer carrier-to-noise threshold level. In block1604, a second signal layer modulation carrier power CUis determined at least in part according to a second signal layer clear sky margin MUand a second signal layer availability. In one embodiment, this is accomplished by determining an second level carrier power CUaccording to

CU=(β⁢⁢N+α⁢⁢CL)⁢TUα,
and wherein the second layer clear sky margin

MU=(TL+1)(TL+αβ)
and TUcomprises a second signal layer carrier-to-noise threshold level. Next, the first signal symbols are modulated according to a first carrier at the determined first signal layer modulation carrier power, as shown in block1606. Then the second signal symbols are modulated according to a second carrier at the determined second signal layer modulation carrier power, as shown in block1608. The modulated first and second signals are then transmitted independently to the satellite, as shown in block1610.

In one embodiment wherein the first signal layer availability and the second signal availability are substantially equal (e.g. αL≈αUand βL≈βU), the second signal layer clear sky margin MUis less than the first signal layer clear sky margin ML. In another embodiment, the second signal layer availability is greater than the first signal layer availability (αU<αLand βU>βL, for example), and the second signal layer clear sky margin MUequals

βUαU⁢βU+βL⁢TLαL+βL⁢TL,
wherein αUat least partially represents the rain attenuation of the second modulation carrier, αLat least partially represents the rain attenuation of the first layer modulation carrier, βUat least partially represents the additional noise in the second modulation carrier due to rain, and βLat least partially represents the additional noise in the first modulation carrier due to rain.

CONCLUSION

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.

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 composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.