Method and apparatus for providing energy dispersal using frequency diversity in a satellite communications system

• US 5,454,009 A
• Filed: 01/13/1994
• Issued: 09/26/1995
• Est. Priority Date: 01/13/1994
• Status: Expired due to Term

• Current Assignee: ViaSat Incorporated
• Original Assignee: Scientific-Atlanta Incorporated (Cisco Systems, Inc.)
• Inventors: Fruit, Larry J., Schachte, John A., Wilkerson, James A. Jr., Herbst, Charles A.
• Primary Examiner(s): Cain, David C.
• Application Number:
  US08/181,318
• Time in Patent Office: 621 Days
• Field of Search: 375/1, 380/34, 380/46, 380/49
• US Class Current: 375/130
• CPC Class Codes: H04B 7/18534 for enhancing link reliablility, e.g. satellites diversity
• CPC Class Codes: H04B 7/208 Frequency-division multiple access [FDMA]

Abstract

A satellite communications system for dispersing energy over a wide bandwidth includes a transmitter, a communication link, and a receiver. The transmitter takes a digital data signal and modulates that signal at a prescribed carrier frequency. The modulated digital data signal is then spread over M adjacent digital channels (M≧2 and being an integer multiple of 2), each channel containing the same information, to disperse the energy over a wide frequency range. The spectral bandwidth of the adjacent digital channels is chosen with compressed spacing to conserve bandwidth. Next, the spread modulated data signal is transmitted via the communication link to the receiver. In particular, a waveform generator at the transmitter generates a phase-aligned multichannel frequency diversity waveform according to a data clock at a predetermined phase relationship to the digital data. At the receiver, the spread modulated data signal received is mixed with a despreading waveform generated in a similar manner to the waveform spectrum generated at the transmitter to recover the modulated data signal. The despreading waveform is generated according to a symbol clock signal recovered from the received modulated data signal. A demodulator recovers the original digital data from the modulated data signal. To achieve higher spreading factors, multichannel frequency diversity may be utilized with known spread spectrum techniques to achieve high data recovery rates during adverse weather (fading) conditions at high radio frequencies in the microwave and higher regions of the radio spectrum.

Claims

1. An apparatus for dispersing energy over a wide bandwidth when communicating digital data having a predetermined data rate comprising:

means for modulating the digital data by a predetermined carrier frequency;

means for generating a plurality of M adjacent channels, M being an integer multiple of two less than or equal to sixteen, wherein each channel has a sinusoidal waveform in a mathematical relationship to each sinusoidal waveform of each other channel and each channel has a predetermined phase relationship to the digital data having the predetermined data rate; and

means for dispersing the modulated digital data on the M channels, wherein each channel contains the same modulated digital data.

2. The apparatus of claim 1 wherein M equals four.

3. The apparatus of claim 2 wherein said generating means includes means for compressing a spectral bandwidth of the four adjacent digital channels to be approximately equal to a spectral bandwidth of three adjacent digital channels.

4. The apparatus of claim 1 wherein the mathematical relationship between each sinusoidal waveform is a function of the predetermined data rate.

5. A communications system for transmitting and receiving digital data at a predetermined data rate comprising:

first channel generating means for generating a plurality of adjacent digital channels, each channel having a sinusoidal waveform in a mathematical relationship to each sinusoidal waveform of each other channel and a phase relationship to the predetermined data rate;

means for modulating the digital data by a carrier frequency;

means responsive to said first generating means and said modulating means for forming a composite signal by impressing the modulated digital data onto each channel;

means for transmitting the composite signal;

means for receiving the composite signal;

clock recovery means for recovering a data clock based on the predetermined data rate of the received composite signal;

second channel generating means, responsive to the data clock, for generating each channel generated by said first channel generating means, each channel having a sinusoidal waveform in the mathematical relationship to each sinusoidal waveform of each other channel and the phase relationship to the predetermined data rate prescribed by said first channel generating means;

means, responsive to said second channel generating means, for coherently combining channels generated by said second channel generating means with the composite signal to recover the modulated digital data; and

demodulating means for demodulating the digital data.

6. The communications system of claim 5 further comprising a satellite communication link for coupling said transmitting means and said receiving means.

7. The communications system of claim 5 wherein said first channel generating means includes a programmable read only memory which receives phases of a clock signal based on the predetermined data rate and generates digital word samples of digital waveforms.

8. The communications system of claim 7 wherein said first channel generating means further includes a digital to analog converter responsive to said programmable read only memory for converting the digital word samples to at least one analog waveform signal.

9. The communications system of claim 8 wherein the mathematical relationship between each sinusoidal waveform includes an odd integer multiplication factor,

10. The communications system of claim 9 wherein a first sinusoidal waveform is represented by sin(2π

*f₁ t+φ

) and a second sinusoidal waveform is represented by sin(2π

*3f₁ t+3φ

) wherein f₁ is a function of the predetermined symbol rate.

11. The communications system of claim 10 wherein the mathematical relationship further includes a sum of the first sinusoidal waveform and the second sinusoidal waveform.

12. The communications system of claim 5 wherein said second channel generating means includes a programmable read only memory which receives phases of the data clock and generates digital word samples of digital waveforms.

13. The communications system of claim 12 wherein said second channel generating means further includes a digital to analog converter responsive to said programmable read only memory for converting the digital word samples to at least one analog waveform signal.

14. The communications system of claim 5 wherein the phase relationship is within a range from 0°

to 90°

.

15. The communications system of claim 5 further including:

means for encoding the digital data with forward error correction information; and

error decoding means for detecting and correcting errors in the demodulated digital data based on the forward error correction information.

16. The communications system of claim 5 wherein said first channel generating means generates four adjacent digital channels.

17. The communications system of claim 16 wherein said first channel generating means compresses a spectral bandwidth of the four adjacent digital channels to be approximately equal to a spectral bandwidth of three adjacent digital channels.

18. The communications system of claim 17 wherein the phase relationship is approximately 45°

.

19. The communications system of claim 5 wherein said first channel generating means compress a spectral bandwidth of the adjacent digital channels by approximately 25%.

20. In a system for receiving a modulated data signal transmitted at a predetermined data rate via a communication link, a receiving apparatus comprising:

clock recovery means for recovering a data clock based on the predetermined data rate from the modulated data signal;

periodic waveform generating means, responsive to the data clock, for generating a plurality of adjacent digital channels, each channel having a sinusoidal waveform having a first mathematical relationship to each sinusoidal waveform of each other channel and a phase relationship to the predetermined data rate;

means, responsive to said waveform generating means, for recovering digital data from the modulated data signal based on each sinusoidal waveform of each channel; and

demodulating means for demodulating the digital data.

21. The receiving apparatus of claim 20 wherein the communication link includes a satellite link for coupling a transmitter and said receiving apparatus.

22. The receiving apparatus of claim 20 wherein said waveform generating means includes a programmable read only memory which receives phases of the data clock and generates digital word samples of digital waveforms.

23. The receiving apparatus of claim 22 wherein said waveform generating means further includes a digital to analog converter responsive to said programmable read only memory for converting the digital word samples to at least one analog waveform signal.

24. The receiving apparatus of claim 23 wherein the mathematical relationship between each sinusoidal waveform includes an odd integer multiplication factor.

25. The receiving apparatus of claim 24 wherein a first sinusoidal waveform is represented by sin(2π

*f₁ t+φ

) and a second sinusoidal waveform is represented by sin(2π

*3f₁ t+3φ

) wherein f₁ is a function of the symbol clock and φ

is a function of the phase relationship.

26. The receiving apparatus of claim 25 wherein the mathematical relationship further includes a sum of the first sinusoidal waveform and the second sinusoidal waveform.

27. The receiving apparatus of claim 20 wherein the phase relationship is within a range from 0°

to 90°

.

28. The receiving apparatus of claim 20 wherein said waveform generating means generates four adjacent digital channels.

29. The receiving apparatus of claim 28 wherein said waveform generating means compresses a spectral bandwidth of the four adjacent digital channels to be approximately equal to a spectral bandwidth of three adjacent digital channels.

30. The receiving apparatus of claim 29 wherein the phase relationship is approximately 45°

.

31. The receiving apparatus of claim 20 wherein said waveform generating means compresses a spectral bandwidth of the adjacent digital channels by approximately 25%.

32. In a system for transmitting digital data having a predetermined data rate via a communication link to a receiver, a transmitting apparatus comprising:

periodic waveform generating means for generating a plurality of adjacent digital channels, each channel having a sinusoidal waveform having a mathematical relationship to each sinusoidal waveform of each other channel and a phase relationship to the predetermined data rate;

means for modulating the digital data by a carrier frequency; and

means responsive to said waveform generating means and said modulating means for forming a composite signal by impressing the modulated digital data onto each channel.

33. The transmitting apparatus of claim 32 further comprising a satellite communications link for coupling said transmitting apparatus and said receiver.

34. The transmitting apparatus of claim 32 wherein said waveform generating means includes a programmable read only memory which receives phases of a clock signal based on the predetermined data rate and generates digital word samples of a digital waveform.

35. The transmitting apparatus of claim 34 wherein said waveform generating means further includes a digital to analog converter responsive to said programmable read only memory for converting the digital word samples to at least one analog waveform signal.

36. The transmitting apparatus of claim 35 wherein the mathematical relationship between each sinusoidal waveform includes an odd integer multiplication factor.

37. The transmitting apparatus of claim 36 wherein a first sinusoidal waveform is represented by sin(2π

*f₁ t+φ

) and a second sinusoidal waveform is represented by sin(2π

*3f₁ t+3φ

) wherein f₁ is a function of the predetermined symbol rate and φ

is a function of the phase relationship.

38. The transmitting apparatus of claim 37 wherein the mathematical relationship further includes a sum of the first sinusoidal waveform and the second sinusoidal waveform.

39. The transmitting apparatus of claim 32 wherein the phase relationship is within a range from 0°

to 90°

.

40. The transmitting apparatus of claim 32 wherein said waveform generating means generates four adjacent digital channels.

41. The transmitting apparatus of claim 40 wherein said waveform generating means compresses a spectral bandwidth of the four adjacent digital channels to be approximately equal to a spectral bandwidth of three adjacent digital channels.

42. The transmitting apparatus of claim 32 wherein said waveform generating means compresses a spectral bandwidth of the adjacent digital channels by approximately 25%.

43. A method of dispersing energy over a wide bandwidth when communicating digital data having a predetermined data rate, said method comprising the steps of:

modulating the digital data by a predetermined carrier frequency;

generating a plurality of M adjacent channels, M being an integer multiple of two less than or equal to sixteen, wherein each channel has a sinusoidal waveform mathematically related to each sinusoidal waveform of each other channel and each channel has a predetermined phase relationship to the digital data having the predetermined data rate; and

dispersing the modulated digital data on the M channels, wherein each channel contains the same modulated digital data.

44. The method of claim 43 wherein M equals four.

45. The method of claim 44 wherein said step of generating further includes compressing a spectral bandwidth of the four adjacent digital channels to be approximately equal to a spectral bandwidth of three adjacent digital channels.

46. In a communications system for transmitting and receiving digital data at a predetermined data rate a method comprising the steps of:

a) generating a plurality of adjacent digital channels, each channel having a sinusoidal waveform having a mathematical relationship to each sinusoidal waveform of each other channel and a phase relationship to the predetermined data rate;

b) modulating the digital data by a carrier frequency;

c) impressing the modulated digital data onto each channel to form a composite signal;

d) transmitting the composite signal from a first station;

e) receiving the composite signal at a second station;

f) recovering a data clock based on the predetermined data rate of the received composite signal;

g) generating sinusoidal waveforms based on the recovered data clock, the sinusoidal waveforms corresponding to each channel generated at said step a), each sinusoidal waveform having the mathematical relationship to each other sinusoidal waveform and the phase relationship prescribed at said step a);

h) recovering the modulated digital data from the composite signal based on the sinusoidal waveforms generated at said step g); and

i) demodulating the digital data.

47. The method of claim 46 further including the step of coupling the first station and the second station via a satellite communication link.

48. The method of claim 46 wherein said step a) includes the steps of:

receiving phases of a clock signal based on the predetermined data rate;

generating digital word samples of a digital waveform; and

converting the digital word samples to at least one analog waveform signal.

49. The method of claim 46 wherein said step g) includes the steps of:

receiving phases of the data clock;

generating digital word samples of a digital waveform; and

converting the digital word samples to at least one analog waveform signal.

50. The method of claim 49 wherein the mathematical relationship between each sinusoidal waveform includes an odd integer multiplication factor.

51. The method of claim 50 wherein a first sinusoidal waveform is represented by sin(2π

*f₁ t+φ

) and a second sinusoidal waveform is represented by sin(2π

*3f₁ t+3φ

) wherein f₁ is a function of the predetermined data rate and φ

is a function of the phase relationship.

52. The method of claim 51 wherein the mathematical relationship further includes a sum of the first sinusoidal waveform and the second sinusoidal waveform.

53. The method of claim 46 wherein the phase relationship is within a range from 0°

to 90°

.

54. The method of claim 46 further including the steps of:

encoding the digital data with forward error correction information before said step d) ; and

detecting and correcting errors in the demodulated digital data based on the forward error correction information.

55. The method of claim 46 wherein said step a) includes the step of generating four adjacent digital channels.

56. The method of claim 55 wherein said step a) further includes the step of compressing a spectral bandwidth of the four adjacent digital channels to be approximately equal to a spectral bandwidth of three adjacent digital channels.

57. The method of claim 56 wherein the phase relationship is approximately 45°

.

58. The method of claim 46 wherein said step a) includes the step of compressing a spectral bandwidth of the adjacent digital channels by approximately 25%.

59. A method of receiving a modulated data signal transmitted at a predetermined data rate via a communication link, comprising the steps of:

recovering a data clock from the modulated data signal based on the predetermined data rate;

in response to the data clock, generating sinusoidal waveforms, each sinusoidal waveform corresponding to at least one of a plurality of adjacent digital channels, each sinusoidal waveform having a mathematical relationship to each other sinusoidal waveform and a phase relationship to the predetermined data rate;

recovering modulated digital data from the modulated data signal based on the generated sinusoidal waveforms; and

demodulating the digital data.

60. The method of claim 59 wherein said step of generating includes the steps of receiving phases of the data clock;

generating digital word samples of a digital waveform; and

converting the digital word samples to at least one analog waveform signal.

61. The method of claim 60 wherein the mathematical relationship between each sinusoidal waveform includes an odd integer multiplication factor.

62. The method of claim 61 wherein said step of generating includes generating a first sinusoidal waveform represented by sin(2π

*f₁ t+φ

) and a second sinusoidal waveform represented by sin(2π

*3f₁ t+3φ

), wherein f₁ is a function of the symbol clock and φ

is a function of the phase relationship.

63. The method of claim 62 wherein the mathematical relationship further includes a sum of the first sinusoidal waveform and the second sinusoidal waveform.

64. The method of claim 59 wherein the phase relationship is within a range from 0°

to 90°

.

65. The method of claim 59 wherein said step of generating includes generating four adjacent digital channels.

66. The method of claim 65 wherein said step of generating further includes compressing a spectral bandwidth of the four adjacent digital channels to be approximately equal to a spectral bandwidth of three adjacent digital channels.

67. The method of claim 66 wherein the phase relationship is approximately 45°

.

68. The method of claim 59 wherein said step of generating includes compressing a spectral bandwidth of the adjacent digital channels by approximately 25%.

69. A method of transmitting digital data having a predetermined data rate via a communication link to a receiver comprising the steps of:

generating a plurality of adjacent digital channels, each channel having a sinusoidal waveform having a mathematical relationship to each sinusoidal waveform of each other channel and a phase relationship to the predetermined data rate;

modulating the digital data by a carrier frequency; and

impressing the modulated digital data onto each channel to form a composite signal.

70. The method of claim 69 wherein said step of generating includes the steps of:

receiving phases of a clock signal based on the predetermined data rate;

generating digital word samples of a digital waveform; and

converting the digital word samples to at least one analog waveform signal.

71. The method of claim 70 wherein the mathematical relationship between each sinusoidal waveform includes an odd integer multiplication factor.

72. The method of claim 71 wherein said step of generating includes the step of generating a first sinusoidal waveform represented by sin(2π

*f₁ t+φ

) and a second sinusoidal waveform represented by sin(2π

*3f₁ t+3φ

), wherein f₁ is a function of the predetermined data rate and φ

is a function of the phase relationship.

73. The method of claim 72 wherein the mathematical relationship further includes a sum of the first sinusoidal waveform and the second sinusoidal waveform.

74. The method of claim 69 wherein the phase relationship is within a range from 0°

to 90°

.

75. The method of claim 69 wherein said step of generating includes the step of generating four adjacent digital channels.

76. The method of claim 75 wherein said step of generating further includes compressing a spectral bandwidth of the four adjacent digital channels to be approximately equal to a spectral bandwidth of three adjacent digital channels.

77. The method of claim 76 wherein said step of generating includes compressing a spectral bandwidth of the adjacent digital channels by approximately 25%.