Synchronization system for time division multiple access relay communications system

• US 3,982,075 A
• Filed: 09/09/1974
• Issued: 09/21/1976
• Est. Priority Date: 09/09/1974
• Status: Expired due to Term

• Current Assignee: Post Office Limited (International Distributions Services plc)
• Original Assignee: Post Office Limited (International Distributions Services plc)
• Inventors: Hodson, Kevin, Jefferis, Alan Kenneth
• Primary Examiner(s): Stewart, David L.
• Application Number:
  US05/504,349
• Time in Patent Office: 743 Days
• Field of Search: 179/15 BS, 179/15 BP, 325/4, 178/69.5 R, 324/83 D, 324/83 FE
• US Class Current: 370/324
• CPC Class Codes: H04B 7/2126 using a reference station
• CPC Class Codes: H04L 2007/047 using a sine signal or unmodulated carrier

Abstract

A Time-Division Multiple Access (TDMA) synchronization system for a satellite relay system is provided by designating one of the ground stations a "Reference" station and arranging for a sinusoidal signal transmitted from the reference station to be received by other ground stations via the satellite and compared with their own sinusoidal synchronization signals after transmission to and from the satellite. The sinusoidal signal from the "Reference" station uses only a small proportion of the bandwidth of a satellite global beam transponder. Most of the traffic is transmitted to one or more spot beam transponders of the satellite. Each station derives from its symbol-rate clock a pulse indicating the start of each TDMA frame and a local synchronization sinusoidal signal at the frame repetition frequency. The phase of this local sinusoidal signal is such that its positive-going zero crossings coincide with the frame-start pulses. These local sinusoidal signals transmitted and received back through the satellite using high-index frequency modulation are compared in phase with the received Reference station sinusoidal signal. The phase difference signal derived from the comparison of the local and reference sinusoidal signals is used to adjust the station'"'"'s transmit frame timing to achieve synchronization with the reference station frame timing.

Claims

1. A synchronization system for a satellite communication Time Division Multiplex Access (TDMA) relay system having a predetermined frame period within which data bursts are transmitted and including a satellite relay and a plurality of ground stations, one of which is designated a reference station, said system comprising at each ground station:

transmitter means for transmitting a carrier signal having a frequency which is characteristic of the respectively corresponding ground station thereby defining a frequency division channel characteristic of the respectively corresponding ground station,

modulation means for modulating said carrier signal with a periodic signal having a period harmonically related to said frame period but which periodic signal is separate and distinct from said data bursts,

phase comparator means at each non-reference ground station for comparing the phase relationship of its respectively corresponding periodic signal after transmission to and return from the satellite relay relative to the periodic signal transmitted from the designated reference ground station via the satellite, and

control means at said non-reference ground stations responsive to said phase relationship to align the phase relationship of the compared periodic signals to achieve synchronization of the TDMA signals at the satellite.

2. A synchronization system as claimed in claim 1 wherein said modulation means contains timing means for determining the harmonic relationship between the frame period and the periodic signal so that unidirectional zero-crossings of the periodic signal coincide with the commencement of a frame period.

3. A synchronization system as claimed in claim 2 wherein said phase comparator means includes detection means for detecting positive-going zero-crossings of said periodic signals.

4. A synchronization system as claimed in claim 1 wherein said modulation means includes a sine wave generator the output of which comprises said periodic signal.

5. A synchronization system as claimed in claim 4 wherein said transmitter means includes a frequency modulation transmitter circuit for producing a signal frequency modulated by said periodic signal.

6. A synchronization system as claimed in claim 1 wherein said transmitter means at said designated reference ground station includes a power output circuit for said carrier signal to produce a carrier signal having a power output greater than the power output of carrier signals at the non-reference ground stations.

7. A synchronization system as claimed in claim 1 wherein the phase comparator means includes a counter the output of which represents the predetermined phase relationship.

8. A synchronization system as claimed in claim 7 wherein said phase comparator means includes a logic unit means arranged to receive the output from said counter and to determine the magnitude and direction of correction of the phase relationship to achieve alignment.

9. A synchronization system as claimed in claim 8 wherein each ground station contains the clock symbol generating means the output from which is fed to a divider circuit, the divisor of which is modified in accordance with the output from said logic unit means.

10. A synchronization system as claimed in claim 9 wherein the output from said divider circuit is fed to a TDMA burst control circuit which controls the timing bursts of said TDMA signals.

11. A synchronization system as claimed in claim 10 further including a time delay element at each non-reference ground station wherein the output from said divider is fed to said TDMA burst control arrangement by way of said time delay element which is arranged to equalize the time delay of the path followed by the periodic signal to and from the satellite and the path followed by the TDMA signals.

12. A synchronized time division multiple access relay communications system comprising:

a plurality of ground communications stations each including means for transmitting and for receiving information bearing radio frequency signals,

a remote relay means for receiving said information bearing radio frequency signals transmitted from any one of said ground stations and for retransmitting corresponding information bearing radio frequency signals to at least one of said ground stations,

said relay means including at least one transponder means for simultaneously servicing all said ground stations on distinct frequency divided communications channels, each ground station being uniquely associated with a respectively associated one of said channels,

said means for transmitting and for receiving at said ground stations including time division multiple access means for defining successive synchronized time frames within which each ground station transmits a burst of its respectively associated information bearing radio frequency signals during a corresponding predetermined portion of said time frame,

a predetermined one of said ground stations constituting a reference station and including means for transmitting a periodic reference synchronization signal over its respectively associated distinct communications channel to all other ground stations, said periodic reference synchronization signal being harmonically related to the repetition rate of said time frames,

each of the remaining ground stations including means for locally generating a periodic local synchronization signal and for transmitting and receiving same over its respectively associated distinct communications channel, said local synchronization signal having a nominal harmonic relationship to the expected repetition rate of said time frames,

each of said remaining ground stations also including phase comparison means for comparing the phase of the received reference synchronization signal and the received local synchronization signal and means for determining the sense and magnitude of any detected phase difference therebetween, and

each of said remaining ground stations also including phase control means for controllably advancing and retarding the phase of said local synchronization signal in response to said detected phase difference to reduce any such difference towards zero.

13. A synchronized time division multiple access relay communications system as in claim 12 wherein all said ground stations each include frequency modulation means for frequency modulating a predetermined carrier frequency signal with its respectively associated periodic synchronization signals and wherein each of said remaining ground stations include frequency demodulation means for recovering said received reference synchronization signal and said received local synchronization signal from the frequency modulated carrier signals corresponding to said reference ground station and to the local ground station respectively.

14. A synchronized time division multiple access relay communications system as in claim 12 wherein said means for transmitting said reference synchronization signal at said ground station includes means for transmitting a higher powered output than the output emanating from the means for transmitting the local synchronization signals from each of said remaining ground stations.

15. A synchronized time division multiple access relay communications system as in claim 12 wherein each of said phase comparison means comprises:

a clock for producing regularly recurring clock pulses,

a counter connected for counting said clock pulses when enabled, and

phase sensitive counter control means for enabling said counter for the time duration between the detection of phase similarities in said received reference synchronization signal and said received local synchronization signal whereby the magnitude of the counter contents is indicative of both sense and magnitude of any phase difference between said received signals.

16. A synchronized time division multiple access relay communications system as in claim 15 further comprising logic means connected to receive the contents of said counter and to produce phase control output signals representative of the phase correction needed to reduce any detected phase difference towards zero.

17. A synchronized time division multiple access relay communications system as in claim 16 wherein each of said means for locally generating a periodic local synchronization signal includes a controllable divider means connected to divide said clock pulses by a divisor dependent upon said phase control output signals.

18. A synchronizing time division multiple access relay communications system as in claim 17 wherein said divider means normally divides by the nominal number of clock pulses included in each time frame but which is controlled to add a predetermined amount to the nominal divisor to retard the phase of the local synchronization signal and to subtract a predetermined amount from the nominal divisor to advance the phase of the local synchronization signal.

19. A synchronization time division multiple access relay communications system as in claim 18 wherein said logic means comprises:

sign register means for storing data representing the sense of any detected phase difference,

value register means for storing data representing the magnitude of any detected phase difference,

comparator means for determining if said counter contents is less than or greater than substantially one-half said nominal number and for storing data in said sign register means and said value register means representative of the sense and magnitude respectively of any detected phase difference,

detector output means connected to detect a non-zero contents of said value register and to produce said phase control output signals as required by the contents of said sign register means upon such detection,

subtraction means for subtracting a predetermined value from the contents of said value register for each successive time frame thereby reducing the value register means contents to zero after sufficient time frames have elapsed to allow the magnitude of needed phase correction represented by the original contents of said value register means, and

inhibition control means connected to inhibit data flow from said counter to said logic means while said value register means maintains a non-zero contents and until after the effected phase correction has had sufficient time to become reflected in said counter contents.

20. A synchronization time division multiple access relay communications system as in claim 19 including a time division multiple access burst timing control subsystem means controlled by the output of said divider means.

21. A synchronization time division multiple access relay communications system as in claim 20 further comprising a time delay means connected between said divider means and said timing control subsystem for equalizing the time delay in the communications circuits followed by the local synchronization signal including to and from the relay with the time delay in the communications circuits followed by the time division multiple access information bearing bursts.

22. A synchronized communication station for use in a synchronized time division multiple access relay communications system which includes a plurality of communications stations each including means for transmitting and for receiving information bearing radio frequency signals, a remote relay means for receiving said information bearing radio frequency signals transmitted from any one of said communications stations and for retransmitting corresponding information bearing radio frequency signals to at least one of said communication stations, said relay means including at least one transponder means for simultaneously servicing all said communications stations on distinct frequency divided communications channels, each communications station being uniquely associated with a respectively associated one of said channels, said means for transmitting and for receiving at said communications stations including time division multiple access means for defining successive synchronized time frames within which each ground station transmits a burst of its respectively associated information bearing radio frequency signals during a corresponding predetermined portion of said time frame, a predetermined one of said communications stations constituting a reference station and including means for transmitting a periodic reference synchronization signal over its respectively associated distinct communications channel to all other stations, said periodic reference synchronization signal being harmonically related to the repetition rate of said time frames;

said synchronized communication station comprising;

means for locally generating a periodic local synchronization signal and for transmitting and receiving same over its respectively associated distinct communications channel, said local synchronization signal having a nominal harmonic relationship to the expected repetition rate of said time frames,

phase comparison means for comparing the phase of the received reference synchronization signal and the received local synchronization signal and means for determining the sense and magnitude of any detected phase difference therebetween, and

phase control means for controllably advancing and retarding the phase of said local synchronization signal in response to said detected phase difference to reduce any such difference towards zero.

23. A synchronized communication station as in claim 22 including frequency modulation means for frequency modulating a predetermined carrier frequency signal with its respectively associated periodic synchronization signals and frequency demodulation means for recovering said received reference synchronization signal and said received local synchronization signal from the frequency modulated carrier signals corresponding to said reference ground station and to the local station respectively.

24. A synchronized communication station as in claim 22 wherein said means for transmitting the local synchronization signal includes means for transmitting a lower powered output than the output emanating from the means for transmitting said reference synchronization signal at said predetermined station.

25. A synchronized communication station as in claim 22 wherein said phase comparison means comprises:

a clock for producing regularly recurring clock pulses,

a counter connected for counting said clock pulses when enabled, and

phase sensitive counter control means for enabling said counter for the time duration between the detection of phase similarities in said received reference synchronization signal and said received local synchronization signal whereby the magnitude of the counter contents is indicative of both sense and magnitude of any phase difference between said received signals.

26. A synchronized communication station as in claim 25 further comprising logic means connected to receive the contents of said counter and to produce phase control output signals representative of the phase correction needed to reduce any detected phase difference towards zero.

27. A synchronized communication station as in claim 26 wherein each of said means for locally generating a periodic local synchronization signal includes a controllable divider means connected to divide said clock pulses by a divisor dependent upon said phase control output signals.

28. A synchronized communication station as in claim 27 wherein said divider means normally divides by the nominal number of clock pulses included in each time frame but which is controlled to add a predetermined amount to the nominal divisor to retard the phase of the local synchronization signal and to subtract a predetermined amount from the nominal divisor to advance the phase of the local synchronization signal.

29. A synchronized communication station as in claim 28 wherein said logic means comprises:

sign register means for storing data representing the sense of any detected phase difference,

value register means for storing data representing the magnitude of any detected phase difference,

comparator means for determining if said counter contents is less than or greater than substantially one-half said nominal number and for storing data in said sign register means and said value register means representative of the sense and magnitude respectively of any detected phase difference,

detector output means connected to detect a non-zero contents of said value register and to produce said phase control output signals as required by the contents of said sign register means upon such detection,

subtraction means for subtracting a predetermined value from the contents of said value register for each successive time frame thereby reducing the value register means contents to zero after sufficient time frames have elapsed to allow the magnitude of needed phase correction represented by the original contents of said value register means, and

inhibition control means connected to inhibit data flow from said counter to said logic means while said value register means maintains a non-zero contents and until after the effected phase correction has had sufficient time to become reflected in said counter contents.

30. A synchronized communication station as in claim 29 including a time division multiple access burst timing control a subsystem means controlled by the output of said divider means.

31. A synchronized communication station as in claim 30 further comprising a time delay means connected between said divider means and said timing control subsystem for equalizing the time delay in the communications circuits followed by the local synchronization signal including to and from the relay with the time delay in the communications circuits followed by the time division multiple access information bearing bursts.