Abstract:
A multiple-access, time-division multiplex, satellite communications systemn which the uplink and downlink frequencies are the same. 
     In each ground station, the outgoing information (be it data, encoded data or digitized samples of the voice frequency signals) are assembled into a sequence of pulses to be transmitted as a burst in the time slot assigned to that particular earth station. If there are n ground stations in the system, n≧2, then the duration of each ground station sequence is, on the average, 1/2n times the duration of a complete frame or epoch, because for half of the epoch the satellite is in the receiving mode and the other half of the epoch the satellite is in the transmitting mode.

Description:
GOVERNMENT LICENSE 
     The invention described herein may be manufactured and used by or for the government, for governmental purposes, without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     Broadly speaking, this invention relates to satellite communications systems. More particularly, in a preferred embodiment, this invention relates to a multiple-access, time-division satellite communications system using a common radio frequency for both the up and downlinks to the satellite. 
     (b) Discussion of the Prior Art 
     Satellite communications systems have proved to be an effective means of establishing communications between distant geographical areas. The growth of such systems has been limited, however, by the cost of the satellite launch vehicle as well as the cost of the satellite and the ground stations themselves. 
     In a typical prior art system, the frequency used for the uplink transmission to the satellite differs from the frequency used for the downlink transmission to the ground station, 4 GHZ and 6 GHZ being respectively the transmitting and receiving frequencies most commonly employed for this purpose. This arrangement inevitably complicates the design and implementation of both the satellite and the ground stations, adding to their cost and complexity. It would clearly be preferable to use a common frequency for both the up and downlinks, but heretofore this has not been possible. 
     Further, while various approaches have been tried to establish multiple-access, satellite communications systems, the fact that different frequencies must be used in the satellite&#39;s transmitter and receiver greatly complicates the design of the multiple-access antennas, as well as the switching and multiplexing equipment in the satellite. 
     Moreover, the coordination of the earth stations, for frequency assignment purposes, has to be considered with respect to two bands; e.g. 4 GHZ and 6 GHZ. This greatly complicates frequency sharing between different satellite communications systems, and between satellite communications systems and terrestrial communications systems because the frequency translation in the satellite systems is relatively fixed, forcing a unique separation on up and down link frequencies. 
     SUMMARY OF THE INVENTION 
     The problem, then, is to design a multiple-access, satellite communications system which utilizes a common transmit and receive frequency at both the satellite and ground stations and which is less complicated and more reliable than those heretofore in use. 
     Fortunately, the above and other problems have been solved by the instant invention which, in a preferred embodiment, comprises a time-division, multiple-access satellite communications system comprising at least one orbiting satellite repeater and n ground stations, n≧2, communicating with each other via the satellite, each of the ground stations including a transmitter, a receiver and an antenna connected thereto, characterized in that the transmitter and receiver operate on the same frequency. Each of the ground stations further comprises means for processing a plurality of information channels for transmission as a compressed pulse sequence to the remaining (n-1) ground stations in the system, each of the information channels having priorly been sampled and digitized; first means for multiplexing the plurality of information channels into a sequence of digital samples for transmission in the one or more time slots assigned to that ground station in a transmission frame or epoch having a duration, on the average, which is n times greater than the duration of the sequence, the sequence generated by each multiplexing means in the system occupying a unique, non-overlapping segment of the transmission frame or epoch; means for modulating the transmitter with the output of the multiplexing means; and means, interposed between the antenna and the output of the transmitter, for enabling the transmitter for transmission to the satellite only during the time duration of the corresponding sequence of digital samples. The invention and its mode of operation will be more fully understood from the following detailed description when taken with the appended drawings in which: 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially schematic, partially diagrammatic view of an illustrative, multiple access, satellite communications system according to the invention; 
     FIG. 2 is a block schematic drawing of an illustrative ground station for use with the system shown in FIG. 1. 
     FIG. 3 is a diagram which follows the operation of the satellite communications system shown in FIG. 1 through two successive time frames; and 
     FIG. 4 is a graph illustrating the duty cycles of the ground stations and satellite repeater in the system shown in FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 depicts an illustrative satellite communications system according to the invention. As shown, satellite repeater 10 is in communication with three ground stations A, B and C, respectively, each of which includes an antenna 11, an rf switch 12 and a radio frequency transmitter and receiver 13 and 14, respectively. Satellite 10 is advantageously synchronous, i.e. in a stationary orbit, but asynchronous operation is also encompassed by this invention. Also, one skilled in the art will appreciate that the arrangement shown in FIG. 1 is only illustrative and that a practical embodiment of the invention may have more than one satellite repeater and more or less than three ground stations. 
     It should be pointed out that to simplify the explanation, it has been assumed that the information channels in the illustrative system are all voice-frequency channels and that all n stations in the system have the same number of information channels. However, the information channels can carry digital or analog data, or encoded data with equal facility. Moreover, different ground stations may, in a practical system, have unequal numbers of channels. Further, while the system antenna is normally a parabolic dish at microwave frequencies, the use of a phased array, at lower frequencies, is also possible. If a phased array were used, it could operate simultaneously in more than one satellite communications network, provided, of course, that all time networks were synchronized and coordinated as to relative epoch. 
     FIG. 2 depicts ground-station A in greater detail. Grounds stations B and C are essentially identical to ground station A; hence need not be discussed in detail. As shown, antenna 11a is connected to an rf switch 12a, thence to either transmitter 13a or receiver 14a. Transmitter 13a is connected to the output of an rf generator 16a and to the output of a first multiplexer 17a which has as its input the several voice-frequency channels to be transmitted to ground stations B and C, via satellite 10. In like manner, the receiver 14a is also connected to generator 16a and to the output of a second multiplexer 18a the output of which comprises the several voice-frequency channels received from ground stations B and C, via the satellite. A system clock 19a is connected to both of the muliplexers and to the rf generator to insure synchronous operation, as will be explained. Clock 19a, also is connected to a control circuit 21a which, inter alia, controls rf switch 12a and both of the multiplexers. 
     As previously discussed, in the above arrangement transmitter 13a and receiver 14a both operate on the same frequency. The same then must be true for the transmitter and receiver in the satellite, not shown in the drawing. Rf generator 16a, thus supplies a signal for the power amplifier in transmitter 13a and for the heterodyne detector (not shown) in receiver 14a. 
     Obviously, in such a system to prevent damage the receiver must be shut-off while the transmitter is transmitting. This is accomplished by switch 12a in the ground station and by a similar arrangement (not shown) in the satellite. In the satellite, means are provided to delay or store the information received from the ground stations by a period of time T x  so that the satellite receiver operates on an approximately 50% duty cycle (less a very small guard interval to avoid overlap). 
     Advantageously, the n ground stations using the satellite use time division multiple access to the satellite. At each ground station, the receiver will be open for about a 50% duty cycle but the transmitters will be energized for only a (1/2n) duty cycle. 
     It is clearly a waste of a valuable resource to have the satellite transmitter in use only 50% of the time. Accordingly, a second embodiment of the invention proposes that the satellite transmitter be switched back and forth between two bands, each having almost a 50% duty cycle for transmission. 
     In operation, assume that each voice channel to be transmitted has priorly undergone a conventional analog-to-digital conversion. Thus, multiplexer 17a will output a series of frames each comprising a plurality of sample words themselves comprising a sequence of binary digits. If a PCM process is used for the A/D conversion, each sample word may comprise several address bits plus a PCM code representating a Nyquist note amplitude sample of the voice frequency signal in the channel. 
     In the illustrative embodiment, there are three ground stations, thus n=3. To simplify the following discussion, assume that each ground station transmits only four voice frequency channels, two to each of the other ground stations. 
     Referring now to FIG. 3, the duration of each complete transmit frame is T t . T t  is the total time that all earth station transmitters are on and is therefore half the total time frame of epoch. As previously discussed, each ground station transmitter will be connected to the antenna for the interval T t  /n or T t  /3 in the illustrative example. This is illustrated in Graphs (a), (b) and (c) of FIG. 3. 
     During each interval T t , sample words S 1 , S 2 , S 3 , S 4 , S 5 , S 6 , S 7 , S 8 , S 9 , S 10 , S 11  and S 12  will be transmitted to the satellite according to the following scheme. 
     TRANSMITTER 13a TRANSMITS 
     S 1  S 2  S 3  S 4  ;S 1  +S 2  to receiver 14b, S 30  +S 4  to receiver 14c 
     TRANSMITTER 13b TRANSMITS 
     S 5  S 6  S 7  S 8  ;S 5  +S 6  to receiver 14a, S 7  +S 8  to receiver 14c 
     TRANSMITTER 13c TRANSMITS 
     S 9  S 10  S 11  S 12  ; S 9  +S 10  to receiver 14a; S 11  +S 12  to receiver 14b 
     After an uplink delay of T u , the satellite receiver is opened to receive the sequence S 1 , S 2  . . . S 12 . The receiver then shuts off at T u  +T t , as shown in graph (d) in FIG. 3. 
     After a short delay T x , the satellite transmitter is turned on, then transmits the sequence S 1 , S 2  . . S 12  to all ground stations, finally the transmitter is turned off at T d  ×T t , as shown in graph (e) in FIG. 3. 
     On the ground, receiver 14a in ground station A is turned on to receive the sequence S 1  S 2  S 3  S 4  S 5  S 6  S 9  and S 10 , as shown in graph (g) of FIG. 3. The sequence S 1  S 2  S 3  S 4  is used to synchronize the multiplexer 18a, whereas the sequences S 5  S 6  and S 9  S 10  are decoded as transmissions originating in ground stations B and C, respectively. 
     In like manner, receiver 14b in ground station B is turned on to receive the sequence S 1  S 2  S 5  S 6  S 7  S 8  S 11  S 12 , as shown in graph (h) of FIG. 3. The sequence S 5  S 6  S 7  S 8  is used to synchronize the multiplexer 18b and the sequences S 1  S 2  and S 11  S 12  are decoded as transmissions originating in ground stations A and C, respectively. 
     In like manner, receiver 14c in ground station C is turned on to receive the sequence S 3  S 4  S 7  S 8  S 9  S 10  S 11  S 12 , as shown in graph (i) of FIG. 3. The sequence S 9  S 10  S 11  S 12  is used to synchronize multiplexer 18c and the sequences S 3  S 4  and S 7  S 8  are decoded as transmissions originating in ground stations A and B, respectively. 
     FIG. 4 depicts the duty cycles, for both transmitter and receiver, for al three ground stations as well as the satellite repeater. It will be observed that the duty cycle of receiver 14a is exactly 50% of T u  +T d  +T x  =kT t , where K=1, 3, 5 . . . T u  +T d , the round trip delay from ground station A to the satellite, is normally in the range of 240-260 milliseconds, depending upon the particular orbit of the satellite. T t , the duration of a frame, is normally a fraction of a millisecond. T x , the delay before the satellite transmitter is energized after receipt of the last sequence from the ground, is selected so that in the satellite, both the transmitter and receiver have a 50% duty cycle. This is done by setting 
     T x  =mT t  where m is an odd integer. 
     All of the above assumes that no guard times are needed to protect the receivers. If this is not the case, these ideal duty cycles would be reduced from the 50% figure to something less than 50%. Further, where it is desired to reduce the instantaneous heating of the earth station or satellite transmitter, it is possible to distribute the bursts across the epoch 2T t , although this complicates system management. 
     The principles of the instant invention can, of course, be applied to cover terrestrial or airborne radio systems. Further, one skilled in the art may make various changes and substitutions without departing from the spirit and the scope of the invention.