Abstract:
A nodal division multiple access technique for multiple access to a communications channel such as a satellite transponder. The present invention provides multiple access into a communications channel where each accessing site utilizes one signal from a composite amplitude/phase digital signal constellation, such that demodulators receive the composite signal without changes in the system design related to the multiple access operation.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of U.S. application Ser. No. 10/153,250, filed May 22, 2002 now U.S. Pat. No. 7,292,547, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to a system and method for multiple access to a given transponder and more particularly to multiple access in satellite communications. 
     BACKGROUND OF THE INVENTION 
     Communications channels, such as the RF communications channel represented by a geosynchronous communications satellite transponder, are an important, economically valuable resource. A multitude of schemes have been developed for efficient modulation and coding of a single carrier using a communications channel and for multiple access techniques where multiple carriers share a channel. In multiple access, the typical design case involves transmissions from dispersed geographic locations where any one site has a capacity demand less than the total capacity available, but where the aggregate demand from all sites is equal to the total capacity. 
     In the specific case of satellite communications, multiple access to a given transponder has been achieved using frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and various combinations of these techniques. The FDMA, TDMA, CDMA and variations thereof all require specific equipment features at both the transmit and the receive ends of the system. 
     In a satellite system, including a so-called geostationary system, the satellite is always moving with respect to the modulators and demodulators. The geometry and satellite motion causes slowly varying distances between the transmit sites and the satellite and between the satellite and the receive sites. In turn, the varying distance along the line-of-sight causes changes in carrier frequency, and hence phase, due to Doppler. Since the multiple access signals of interest herein share the satellite transponder and the downlink path, the key issues are the differences in paths of the uplink signals. These differences are primarily due to slight differences in the geometry between uplink site and the satellites and slight differences in signal propagation through the atmosphere such as phase scintillation. 
     These variations present design challenges for conventional multiple access techniques such as FDMA, TDMA and CDMA. Any new technique must also accommodate the time-varying geometry that occurs in a communications satellite system. 
     SUMMARY OF THE INVENTION 
     The present invention is a system and method for communications signal processing including the capability to handle time-varying effects such as that which occur in a communications satellite system. The present invention provides multiple access into a communications channel where each accessing site utilizes one signal from a composite amplitude/phase modulated digital signal constellation, such that the demodulators receive the composite signal without changes in the receiver design related to the multiple access operation. 
     The present invention permits nodal division multiple access (NDMA) where standard modulation techniques are used but where innovative processing at the modulator locations permits multiple carriers to share a single communications channel, such as a satellite transponder. With modem signal processing technology, the implementation at the modulator end is practical and economical. No changes are necessary in the demodulators. This is a very important advantage in asymmetrical applications, such as direct broadcast satellite (DBS), where there are many more demodulators than modulators. 
     According to the NDMA technique of the present invention, the user receivers are generally the same as receivers for certain modulation formats without multiple access. NDMA can directly utilize the major body of theory and practice already available in digital communications, particularly in amplitude phase shift keying (APSK). NDMA can be used as a network evolution technique in that all deployed receivers would have the capability to demodulate any appropriate signal, but as the network evolves the transmitted signal becomes a multiple access composite of signals from different geographic points. 
     As a specific application, the invention could be utilized in an advanced system for DBS re-broadcast of local television signals. In existing systems, the local television signal is transported, by terrestrial means, to a small number of major satellite uplink sites. Each of the uplink sites aggregates channels into groups matching the capacity of a single satellite transponder. Each uplink carrier then is a “single access” carrier from one of the major satellite sites to a direct broadcast satellite (DBS) transponder. 
     According to the NDMA system and method described herein, the local channels can be uplinked from less expensive sites nearer to the television signal&#39;s point of origin. For example, the capacity of a transponder could be shared between two local television markets with an uplink in each market. The NDMA invention eliminates the terrestrial transmission costs to a more complex, distant uplink facility. 
     It is an object of the present invention to utilize PDMA in satellite communications. It is another object of the present invention to utilize PDMA as a network evolution technique. It is still another object of the present invention to permit a new signal to be uplinked from a new geographic site distant from an original uplink site. 
     A further object of the present invention is to provide a method for making a transmitted signal become a composite of multiple signals from different geographic sites. 
     Other objects and advantages of the present invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this invention, reference should now be had to the embodiments illustrated in greater detail in the accompanying drawings and described below by way of examples of the invention. In the drawings: 
         FIG. 1  is a diagram of a static geometry system having a terrestrial repeater; 
         FIG. 2  is a diagram of a static geometry local modulating subsystem; 
         FIG. 3  is a system diagram of a dynamic geometry system; 
         FIG. 4  is a diagram of a dynamic geometry local modulating subsystem; 
         FIG. 5  is a signal constellation of a composite input signal according to the present invention; 
         FIG. 6  is a signal constellation of the composite input signal after removal of the local signal according to the present invention; 
         FIG. 7  is a signal constellation of an estimate of distant signal coordinates according to the present invention; 
         FIG. 8  is a signal constellation of local coordinates according to the present invention; 
         FIG. 9  is a signal constellation of the output signal according to the present invention; and 
         FIG. 10  is a system diagram of an alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Some background material and terms are defined herein for the detailed description of the preferred embodiments of the present invention. A “local transmit site” is one of several sites which transmit signals in multiple access with other distant sites into a common channel. Most of the required signal processing is described herein with regard to a single “local” site. Each local site transmits a modulated “carrier” or “signal” which, after the common channel, is a part of the “composite” signal. The term “carrier” denotes an unmodulated transmission. 
     In-phase (I) and quadrature (Q) carrier and signal components are utilized throughout the present invention. Each local signal is described by its location in the I-Q coordinate space of the underlying carrier. 
     The following description applies to a 4PSK satellite communications application. With a 4PSK (QPSK) modulation design, the present invention permits multiple access from two locations, with each transmitting a 2PSK signal. Other applications can be generalized from this particular case. For example, with 16QAM, the invention permits NDMA from two sites, each with 4PSK modulation or from as many as four sites each with 2PSK or 4PSK modulation. It should also be noted that the fundamentals of the invention apply to any system and modulation where the modulations can be combined within the channel and where the distant carriers can be appropriately controlled. For example, the present invention applies to amplitude and phase modulations if the linearity of the communications channel permits quasi-linear superposition of the modulations from the different transmit locations. 
     A variety of forward error control (FEC) techniques may be used with the signals described herein. For the NDMA system described herein, the FEC design is largely independent and, in fact, an important feature of the invention in that the FEC scheme may be different for each accessing signal. For example, an NDMA system may start with a 4PSK signal with one FEC scheme and then later add a 2PSK signal with a different, more advanced FEC. 
       FIG. 1  is a block diagram of a static geometry terrestrial repeater system  10  using 4PSK for multiple access to a terrestrial repeater. The present invention is applicable to static geometry applications, such as a terrestrial repeater. Another example of a static geometry application is a satellite whose motion is not significant and the Doppler effect is negligible. There are two originating sites, site A and site B. Each of the two originating sites transmits 2PSK to a transponder  12 . Transponder  12  transmits a composite signal, A+B, to Site A, Site B, and a plurality of other, much simpler, receiving sites  14 . The composite signal A+B is also used as a frequency and phase reference signal according to the present invention. 
       FIG. 2  is the local modulating subsystem  20  for a static example, which is located at either Site A or Site B. The incoming composite signal A+B is received at a M-ary receiver  22  where it is used as a timing, frequency and phase reference. The composite signal A+B is recovered from the transponder and converted into In-phase and Quadrature components. Ambiguity resolution is performed to determine a reference for the phase of the signal. The converted recovered carrier and the phase ambiguity indicator are sent to the modulator, where the information bit stream is also fed. The outgoing modulated signal is output from the modulator. 
     The static geometry application shown in  FIGS. 1 and 2  is not particularly challenging for the present invention of NDMA, because the geometry is fixed and the differential propagation impairments for the paths A and B are small. 
       FIG. 3  is a block diagram of a system  30  with a time-varying, dynamic geometry involving an orbiting communications satellite  32 . The relative motion of the satellite  32 , and transponder  34 , is shown by a dashed line  33  with arrows indicating the direction of motion. Transmit site A  36  transmits signal A to the transponder  34 . Transmit site B  38  transmits signal B to the transponder. A composite signal A+B is transmitted from the transponder back to each of sites A  36  and B  38 . The composite signal A+B is also transmitted to a plurality of receive sites  40 . The signals are transmitted and received through a variety of atmospheric effects  42 . 
     The application shown in  FIG. 3  introduces two new effects to provide sources of timing, frequency and phase instability. In contrast to the static geometry application shown in  FIG. 1 , the satellite application in  FIG. 3  introduces variations in the signal. Because the satellite  32  is not perfectly “geostationary”, a small relative motion with respect to the transmit sites  36 ,  38  gives timing, frequency and phase changes. Also, since the different uplink signals do not follow the same paths through the atmosphere, it is assumed that small differential delays, or phase changes, will occur. 
       FIG. 4  is a block diagram of the local modulating subsystem  50  for the dynamic geometry system of  FIG. 3 .  FIG. 4  shows the signal processing used at each of the transmit sites. With reference to  FIG. 4  it should be noted that 4PSK modulation is shown, however, with refinements the architecture shown may be generalized and applied to amplitude/phase shift keying such as 16QAM. In the description with respect to  FIGS. 4 through 9 , the concept of a “local I/Q coordinate space” is used to collect incoming signal measurements and synthesize outgoing signals. It should be noted that the I/Q space is only a localized signal processing implementation and it does not directly relate to the composite signal constellation. The local I/Q coordinates are the in-phase and out-of-phase components of a given signal with respect to the local VCO reference. 
     Referring to  FIG. 5 , a signal constellation  100  for the composite signal A+B is shown. The composite signal A+B is input to the local modulating subsystem  50  of  FIG. 4 . The local modulation is removed  52  from the composite signal A+B. This is accomplished by coarse and fine synchronization processes wherein a replica of the outgoing signal  54  is subtracted  56  from the composite signal A+B. The synchronization processes drive toward minimization of the difference of the signals and hence provide a “clean” replica of the distant signals. The coarse part of the synchronization process is open loop and uses an archive  58  of the outgoing signal  54  and parameters of the satellite orbit and satellite to local uplink geometry. 
     Using the orbit parameters and geometric data, straightforward calculations provide an estimate  60  of the satellite-to-ground distance, and hence signal delay and the time-rate-of-change of the distance and hence the timing and frequency shift. Since the roundtrip delay is less than 300 msec, the signal archive storage requirements are modest. An example of a fine tracking loop is an early/late delay-locked loop that removes  52  local modulation from the composite signal A+B. The loop is of the type used in spread-spectrum communications systems, an example of which is described in Digital Communications and Spread Spectrum Systems, R. E. Ziemer and R. L. Peterson, Macmillan, 1985, specifically at Chapter 9, pages 419 through 483, which is incorporated herein by reference.  FIG. 6  is a signal constellation  102  of the signal after the local signal has been removed. 
     Referring back to  FIG. 4 , the coarse blocks  58 ,  60  and the fine block  52  will maintain lock for reliable operation except, perhaps, immediately following a satellite orbital maneuver. Therefore, in terms of customer satisfaction, it is suggested that orbital maneuvers be carried out during low customer interest, such as the early morning hours, to minimize the impact on perceived system availability. 
     The carrier A+B is recovered  62  from the input modulation. The carrier is a composite of the carriers from the plurality of distant sites. Since all of the uplink sites have this circuitry, all will continuously drive towards a common frequency. Bit decisions are not made at this signal processing stage. Signal samples are output in the local I/Q coordinate space.  FIG. 7  is a signal constellation  104  of the distant signal coordinates that are estimated. 
     Referring back to  FIG. 4 , signal samples are examined and at each bit time, the optimum location for local signals is recomputed  64  in local I/Q coordinates for the local M-ary signals. An algorithm is used to maximize the distance between the local signals and the signals received from distant sites. Since all uplink sites have the same circuitry, they all will continuously drive toward an optimum signal constellation. For example,  FIG. 8  shows boundaries  106  for the signal constellation. If the received distant signal shown in constellation  104  of  FIG. 7  is slowing rotating in the local I/Q space, the algorithm will cause the outgoing modulation to rotate appropriately to maintain roughly the distance intended in the M-ary modulation design shown by the boundaries  106  in  FIG. 8 . 
     Referring again to  FIG. 4 , orbital parameters and geometric data are used to pre-distort or compensate the outgoing signal  66  such that the effects of the changing geometry are removed  68 . The local signal constellation  108  is shown in  FIG. 9 . Referring back to  FIG. 4 , the compensated signal  68  and the I/Q coordinates of the optimum location  64  and the local bit stream for transmission  72  are used to create  70  the outgoing modulated signal. The I/Q references from the local voltage control oscillator as modified at block  68  are used to create  70  the outgoing phase modulated signal. The implementation described with reference to  FIG. 4  occurs at each uplink site. For example, with 16QAM, the implementation of  FIG. 4  would be deployed at up to four sites, with each site transmitting a 2PSK signal. 
     In an alternative embodiment, shown in  FIG. 10 , there is no need for an outgoing signal archive. In the embodiment shown in  FIG. 10  an initiating sequence  80  is implemented. Site A is the “master” site  82  and transmits first. Site B, called the “slave” site  84 , receives the signal from site A by way of the satellite. Site B demodulates  86  the signal from site A  82  to obtain symbol timing and signal carrier frequency information about the signal from site A. Slaving to this information, site B then transmits  88  its own signal with synchronized symbol timing and a carrier frequency equal to the received frequency. The two signals make a new composite signal. For example, site A transmits a 4 PSK signal and site B transmits a 4 PSK signal, their combination produces a 16 QAM signal as a composite signal. 
     Upon receiving the composite signal, Site B continuously tracks  90  symbol timing and carrier frequency errors between the two signals with a phase locked loop  92  to line up symbol timing and carrier phase with respect to the signal transmitted by Site A, by optimizing the placement of individual signal nodes within the composite constellation. Synchronization of symbol timing and carrier frequency/phase is maintained with the phase locked loop. In this regard, there is no need to store signals for cancellation. It is possible that additional slave sites be sequentially added to transmit higher-order modulation signals. 
     The invention covers all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the appended claims.