Abstract:
A novel method and apparatus is described for reducing the number of CDMA codes for a constellation of multiple trasponder platforms serving a number of subscribers in the same service area. A coherent processing technique synchronizes the phase of CDMA signals arriving at a subscriber from multiple trasponder platforms to increase the code capacity and thus the number of possible subscribers for most of the multiple transponder platform systems in current use.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to code division multiple access (CDMA) communications systems. More specifically, but without limitation thereto, the present invention relates to a method for reducing the number of CDMA codes required for a number of subscribers serviced by multiple transponder platforms. 
   Traditionally, when multiple satellites become available over a given geographic location, two or more nearby users may not use the same frequency spectrum or code space due to interference. Also, as the number of subscribers within a service area increases, the frequency bandwidth, the number of CDMA codes, or both must be increased to avoid interference from messages intended for other subscribers. The number of subscribers is therefore limited by the frequency bandwidth and the number of CDMA codes. 
   Methods for reducing the number of CDMA codes for a service area effectively increase the bandwidth of the frequency spectrum by providing a greater portion of the information in communications signals to be used for subscriber communication rather than for distinguishing one subscriber from another. 
   Although multiple transponder platforms, e.g. satellites, increase the system availability, their full potential has been unrealized because of the limit on the number of users imposed by the assigned frequency bandwidth and the number of available codes. In conventional asynchronous CDMA single satellite communication systems, unique CDMA codes are assigned to each user to ensure that information directed to one subscriber does not interfere with information directed to another subscriber. Similarly, in multiple satellite communication systems, when two or more satellites are serving in the same geographical location, unique CDMA codes within the same frequency bandwidth are generally used to distinguish each subscriber. Using the same CDMA code for multiple subscribers would result in mutual interference that would prevent the proper decoding of information, because the omnidirectional receiving antennas of the subscribers&#39; terminals lack the capability to discriminate spatially among the satellites. 
   SUMMARY OF THE INVENTION 
   A novel method and apparatus is described for reducing the number of CDMA codes for a constellation of multiple transponder platforms serving a number of subscribers in the same service area. A coherent processing technique synchronizes the phase of CDMA signals arriving at a subscriber from multiple transponder platforms to increase the code capacity and thus the number of possible subscribers for most of the multiple transponder platform systems in current use. For example, the subscribers may use simple terminals with nearly omnidirectional antennas for receiving signals from multiple satellites concurrently. 
   One advantage of the present invention is that a greater number of subscribers may be accommodated within a service area without increasing the frequency spectrum or the number of CDMA codes. 
   Another advantage is that multiple transponder platforms may be used to enhance the signal gain for most of the subscribers in the service area. 
   Still another advantage is that the cost of receiving terminals is substantially reduced due to simpler operation and fewer components. 
   Yet another advantage is that the positions of each transponder platform and subscriber in the constellation need not be known to practice the present invention. 
   Still another advantage is that inexpensive terminals may be used with omnidirectional antennas without sacrificing performance. 
   The features and advantages summarized above in addition to other aspects of the present invention will become more apparent from the description, presented in conjunction with the following drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram illustrating an exemplary multiple transponder platform communications system suitable for use with the present invention. 
       FIG. 2  is an exemplary plot of interference noise power integrated over the entire communications channel vs. distance from the intended subscriber. 
       FIG. 3  is an exemplary plot of interference noise power integrated over the entire communications channel vs. distance from the intended subscriber on a larger distance scale. 
       FIG. 4  is an exemplary plot of interference noise power integrated over the entire communications channel vs. distance from an intended subscriber using the same CDMA codes with different CDMA code lengths. 
       FIG. 5  is an exemplary plot of interference noise power integrated over the entire communications channel vs. distance from an intended subscriber using different CDMA codes and different CDMA code lengths. 
       FIG. 6  is an exemplary family of plot curves of ranging calibration data overhead vs. chip rate. 
       FIG. 7  is an exemplary family of plot curves of signal-to-noise ratio integrated over the entire communications channel vs. number of phase coherency bits. 
       FIG. 8  is an exemplary plot of Fourier processing overhead as a fraction of resource vs. data rate. 
       FIG. 9  is an exemplary flow chart  90  of a computer program for performing the coherent phase synchronization of the present invention. 
       FIG. 9A  is a flow chart of step  906  in FIG.  9 . 
       FIG. 9B  is a flow chart of step  910  in FIG.  9 . 
       FIG. 9C  is a flow chart of step  914  in FIG.  9 . 
       FIG. 10  is an embodiment of the present invention in a gateway  1000  for performing the functions in FIG.  9 . 
   

   DESCRIPTION OF THE INVENTION 
   The following description is presented to disclose the currently known best mode for making and using the present invention. The scope of the invention is defined by the claims. 
     FIG. 1  is a ram illustrating an exemplary multiple satellite communications system  10  suitable for use With the present invention for coherent synchronization of CDMA communications signals. In this example, single transponder satellites represent transponder platforms and cellular telephones represent subscribers. Alternatively, the transponder platforms may also be carrier signal frequency reflecting surfaces, and the subscribers may also be fixed or mobile terminals. Other suitable devices with sufficient field of view to cover the directions from which subscriber signals arrive and combinations thereof for relaying a signal from a gateway to a subscriber may also be use, whether fixed or mobile, on the ground, in the air, or in space. Similarly, subscribers may be any suitable devices and combinations thereof employed for CDMA communications, whether fixed or mobile, on the ground, airborne, or in space. 
   A first forward link CDMA signal  120  is transmitted by a hub or gateway  104  to satellite  106  and relayed from satellite  106  to intended subscriber  102 . A second forward link CDMA signal  122  is sent by gateway  104  to satellite  108  and relayed from satellite  108  to intended subscriber  102 . The sequence of forward link CDMA signals may be sent at different times or otherwise arranged by well known techniques to avoid mutual interference during the synchronization process. Subscriber  102  logs the time each forward link CDMA signal is received according to a reference clock and inserts the time data in a return link CDMA signal corresponding to each forward link CDMA signal received. 
   Gateway  104  uses the time data in each return link CDMA signal to calculate a corresponding time delay and performs a Fourier analysis of the return link CDMA signals to calculate a corresponding signal carrier frequency shift due to Doppler. Using the time delay and frequency shift calculations, gateway  104  inserts a delay in the transmission of each subsequent CDMA signal from gateway  104  so that the CDMA signals directed to intended subscriber  102  arrive at intended subscriber  102  from satellites  106  and  108  in coherent phase. The in-phase signals add constructively at intended subscriber  102 &#39;s location, increasing the signal-to-noise ratio. 
   On the other hand, signals from gateway  104  directed to intended subscriber  102  arrive out of phase at unintended subscriber  112  located at a distance  110  from intended subscriber  102 . The phase difference is determined by the geometry of the communications system and distance  110  between intended subscriber  102  and unintended subscriber  112 . The out-phase signals interfere with each other and appear as interference noise. 
     FIG. 2  is an exemplary family of plot curves of interference noise power vs. distance from the intended subscriber using coherent phase synchronization for a randomly distributed multiple transponder platform constellation of seven satellites, a CDMA code length of six, and a frequency bandwidth of 100 kHz. Using the same CDMA code for the intended subscriber results in a maximum interference noise power of 0 dB at zero distance shown in curve  202 , while using a different CDMA code for the unintended subscriber reduces integrated interference noise power at zero distance by about 7.5 dB as shown in curve  204 . As the distance increases, the difference in interference noise power between using the same CDMA code vs. using different CDMA codes becomes less significant. The present invention exploits this feature to make CDMA codes reusable given adequate distance between subscribers within the same service area. 
     FIG. 3  is an exemplary family of plot curves of interference noise power integrated over the entire communications channel vs. distance from the intended subscriber on a larger distance scale. Again, using the same CDMA codes for multiple subscribers shown in curve  302  compared to using different CDMA codes shown in curve  304  does not substantially increase the interference noise power relative to the signal gain achieved by coherent phase synchronization. 
     FIG. 4  is an exemplary family of plot curves of interference noise power integrated over the entire communications channel vs. distance from the intended subscriber using the same CDMA codes with CDMA code lengths of 6 shown in curve  402  and 10 shown in curve  404 . On average, longer code lengths reduce the integrated interference noise power for unintended subscribers, even when using the same CDMA codes. 
     FIG. 5  is an exemplary family of plot curves of interference noise power vs. distance from the intended subscriber using different CDMA codes and CDMA code lengths of 6 shown in curve  502  and 10 shown in curve  504 . When different CDMA codes are used, the difference in interference noise power vs. distance for unintended subscribers is more significant. 
   For synchronization of signals from multiple satellites, there is no need to determine the absolute positions of the platforms. The relative timing, frequency, and phase between platforms is sufficient to perform coherent phase synchronization of CDMA signals. 
   Although the range is substantially the same for a forward link and the corresponding return link, the processing delays and signal channel distortions may be different. 
   To determine the range between the gateway or hub and a subscriber, the gateway includes ranging calibration data in each message sent to each subscriber via each transponder platform to establish a subscriber range corresponding to each transponder platform. The subscriber receives the ranging calibration data from each transponder platform and returns a message to the gateway containing the time read from a reference clock. 
     FIG. 6  is an exemplary family of plot curves of ranging calibration data overhead vs. chip rate for a range rate of 60 m/sec and a data rate of 144 kHz for 10 satellites shown in curve  602 , 5 satellites shown in curve  604 , 2 satellites shown in curve  606 , and 1 satellite shown in curve  608 . 
   The gateway receives the subscriber message and performs a coherent phase synchronization calculation for each subscriber via each satellite. The gateway uses the coherent phase synchronization calculations to delay the signals transmitted to each satellites for each subscriber so that the signals for each subscriber from all of the transponder platforms arrive at each subscriber in coherent phase. 
   The coherent phase synchronization calculations include the steps of comparing the transmission time in the range calibration data with the reception time read from the subscriber&#39;s reference clock. The time difference is included in the subscriber&#39;s return signal. Several such time differences are measured and sent back to the gateway. The fraction of the chip time required for range calibration data is given substantially by: 
               T   X         T   C     +     T   x               (   1   )             
 
where T x  is message time allocated for the range calibration data, and T c  is message time allocated for communications data. The accuracy of the ranging calibration is given substantially by: 
               C   W         n   x               (   2   )             
 
where C w  is the chip width, and n x  is the number of time differences measured. For a maximum range rate of 60 m/sec and a data rate of 144 KHz, a typical value for the fraction of chip time required for ranging calibration data is less than 5%.
 
   The parameters used for the phase synchronization calculation are: 
   n b =number of phase bits required to achieve phase coherency 
   δ f =total frequency uncertainty 
   n b  depends on the number of platforms and the desired signal-to-noise ratio (typically 20 dB). T x  is the product of n b  times the chip duration. 
   δ f  represents the combined effects of CDMA carrier oscillator stability and relative motion among the gateway, the transponder platform, and the subscriber. 
   The Fourier period is determined by the required frequency accuracy substantially from the following formula: 
               δ   f     =     1       T   C     +     T   x                 (   3   )             
 
The total number of samples required for the Fourier processing determines the fraction of the chip time required for range calibration data given by (1).
 
     FIG. 7  is an exemplary family of plot curves of signal-to-noise ratio vs. number of phase coherency bits used in the ranging calibration data for 10 satellites shown in curve  702 , 5 satellites shown in curve  704 , 2 satellites shown in curve  706 , and 1 satellite shown in curve  708 . 
     FIG. 8  is an exemplary plot of Fourier processing overhead as a fraction of resource vs. data rate for 10 satellites shown in curve  802 , 5 satellites shown in curve  804 , 2 satellites shown in curve  806 , and 1 satellite shown in curve  808 . In this example, the range acceleration is 2 m/sec 2 . As the number of transponder platforms increases, the Fourier processing overhead increases, but the difference becomes less significant with higher data rates. At a data rate of 144 KHz, for example, the overhead is only about 0.1% for a constellation of five satellites having a relative range rate of 2 m/sec 2 . This overhead is insignificant compared to the typical resource fraction of chip time of 10% required for ranging calibration data at the same data rate at a chip rate of 4 mHz. At a range rate of 30 m/sec, the required resource is about 3%, and at 60 m/sec, about 6% assuming sequential processing. Alternatively, the ranging calculations may be done in parallel in the return link to reduce the Fourier processing overhead. 
     FIG. 9  is a exemplary flow chart  900  of a computer program for performing the coherent phase synchronization of the present invention. At step  902  a transponder platform index is initialized that increments up to the total number of transponder platforms in the constellation. 
   At step  904  processing is initialized for the next transponder platform. In this step statistical data is cleared comprising the average signal propagation delay, frequency, and phase between the gateway and each subscriber within the transponder platform&#39;s coverage area. A subscriber index is initialized that increments up to the total number of subscribers. 
   At step  906  the processing for each subscriber within the transponder platform&#39;s coverage area is performed.  FIG. 9A  shows step  906  in further detail. At step  952  the gateway transmits a ranging signal to the subscriber. The ranging signal may contain an identification code unique to the subscriber to avoid interference from other subscribers receiving the ranging signal. When the subscriber receives the ranging signal at step  954 , it computes the signal propagation delay and phase information relative to a local reference clock. At step  956  the subscriber transmits the signal timing and phase information to the gateway. When the gateway receives the information at step  958 , it computes the signal timing, frequency, and phase for the subscriber relative to the transponder platform using, for example, statistical averaging or Fourier analysis. Other methods may be used according to well known techniques. 
   Referring back to  FIG. 9 , the subscriber index is incremented at step  908 . If not all subscribers have been processed, processing continues from step  906  above. Otherwise the processing common to all the subscribers within the transponder platform&#39;s coverage area is performed at step  910 . 
     FIG. 9B  shows step  910  in further detail. At step  962  the gateway computes the relative motion statistics of the transponder platform relative to the subscribers for updating signal timing estimates. 
   Referring back to  FIG. 9 , the transponder platform index is incremented at step  912 . If not all transponder platforms have been processed, processing continues at step  904  above. Otherwise processing continues at step  914 . 
     FIG. 9C  shows step  914  in further detail. At step  972  the gateway updates the average signal timing, frequency, and phase data for all subscribers and all transponder platforms. At step  974  the gateway transmits reference clock corrections to each subscriber to synchronize the signal phase of each subscriber with respect to the gateway. At step  976  the gateway continues transmitting communications messages to each subscriber via each transponder platform using the delays calculated to achieve coherent signal reception by each subscriber. 
   By the process described above the gateway delays the transmission of a CDMA signal for an intended subscriber to each transponder platform or satellite in the constellation by the correct amount to achieve phase coherency of the signals at the subscriber&#39;s location. In addition, the gateway may also adjust the frequency of the CDMA signal for the intended subscriber to each transponder platform to compensate for doppler shift so that the signals arriving at the intended subscriber from each transponder platform have the same frequency. 
   The synchronization process described above is preferably done in the background to minimize interruption of communications messages. When computations are completed for all transponder platforms in the constellation, processing is terminated until another iteration is performed to accommodate changes in position and motion of the transponder platforms and the subscribers. 
     FIG. 10  shows an embodiment of the present invention in a gateway  1000  for performing the functions described in FIG.  9 . Transmitter  1002  transmits the ranging and delayed communications signals to the subscriber. Receiver  1004  receives the signal timing and phase information from the intended subscriber. The signal timing, frequency, and phase are calculated respectively by time shift calculator  1006 , frequency shift calculator  1008 , and phase shift calculator  1010 . CDMA sequencer  1012  transmits the reference clock corrections to each subscriber to synchronize the signal phase of each subscriber with respect to gateway  1000 . 
   Other modifications, variations, and arrangements of the present invention may be made in accordance with the above teachings other than as specifically described to practice the invention within the scope of the following claims.