Technique for mitigating rain fading in a satellite communications system using quadrature phase shift keying

• US 4,837,786 A
• Filed: 08/07/1986
• Issued: 06/06/1989
• Est. Priority Date: 08/07/1986
• Status: Expired due to Fees

• Current Assignee: ComStream Corp. (L3Harris Technologies, Inc.)
• Original Assignee: ComStream Corp. (L3Harris Technologies, Inc.)
• Inventors: Gurantz, Itzhak, Wright, David A.
• Primary Examiner(s): Griffin, Robert L.
• Assistant Examiner(s): Telesz, Jr., Andrew J.
• Application Number:
  US06/894,316
• Time in Patent Office: 1,034 Days
• Field of Search: 375/67, 375/39, 375/38, 375/102, 375/1, 370/19, 370/20, 370/21, 370/110.1, 370/85, 380/34
• US Class Current: 370/206
• CPC Class Codes: H04B 1/707 using direct sequence modulation
• CPC Class Codes: H04B 7/18513 Transmission in a satellite or space-based system
• CPC Class Codes: H04L 2027/0057 quadrature phase
• CPC Class Codes: H04L 2027/0087 Out-of-band signals, (e.g. pilots)
• CPC Class Codes: H04L 27/2071 in which the data are represented by the carrier phase, e.g. systems with differential coding
• CPC Class Codes: H04L 27/2273 associated with quadrature demodulation, e.g. Costas loop
• CPC Class Codes: Y10S 370/914 Rate converter

Abstract

A system for data communication between ground stations using an orbiting satellite as a relay includes a technique for avoiding the loss of signals from the satellite during periods of heavy precipitation. Data is communicated from an orbiting satellite to a plurality of earth stations on a single transmission frequency. The data is communicated on two separate transmission channels on that one frequency by using orthogonal channels of a coherent phase quadrature transmission system. The overall data rate on one of the two transmission channels is much lower than the overall data rate on the other of the two channels. The data on the lower data rate channel is less likely to be affected by heavy precipitation occurring in the line of sight between the satellite and the ground station, and the lower data rate channel is thus used to communicate data to a ground station located in an area of heavy precipitation. The lower data rate channel is further used to maintain phase synchronization so that the probability of losing phase synchronization with the signal transmitted by the satellite is significantly reduced.

Claims

1. A communications system for transmitting digital data from a first location to a second location, comprising:

means at said first location for providing to digital data in first and second input data streams at first and second data rates, respectively, wherein the second data rate is faster than the first data rate, and for synchronizing the first and second input data streams in a fixed timing relationship, with at least said first input data stream providing timing for the first and second input data streams;

means electrically connected to the means for providing digital data, for generating a first phase modulated output signal having a phase relative to a reference phase that is responsive to the data of the first and second input data streams, said phase relative to said reference phase having a first component responsive to the data of said first input data systems and having a second component responsive to the data of said second input data stream, said second component variable independent of said first component, said first component and said second component of said first phase modulated output signal providing two distinct, noninterfering data communications channels at the same time;

means responsive to the first phase modulated output signal for transmitting a radio frequency signal modulated by said first phase modulated output signal;

means at said second location for receiving a radio frequency signal corresponding to said transmitted radio frequency signal and for generating a second phase modulated output signal corresponding to the first phase modulated output signal; and

means for demodulating the second phase modulated output signal to provide first and second output data streams at said first and second data rates, respectively, wherein said first and second output data streams correspond, respectively, to said first and second input data streams, and wherein the means for demodulating provides the first output data stream independently of the second output data stream so that timing information encoded on said first output data stream can be decoded independent of the second output data stream, said first data rate selected to be sufficiently slower than said second data rate so that said means for demodulating continues to provide said first output data stream in the presence of signal attenuation between said first and second locations that prevents said means for demodulating from providing said second output data stream, said first output data stream providing timing for said second output data stream.

2. The communications system as defined in claim 1 wherein said first phase modulated output signal has a first modulation power level responsive to said first input data stream and a second modulation power level responsive to said second input data stream.

3. The communications system as defined in claim 2 wherein one of said first and second modulation power levels is greater than the other of said first and second modulation power levels.

4. A communications system for transmitting digital data from a first location to a second location, comprising:

means at said first location for providing the digital data in first and second input data streams at first and second data rates, respectively, wherein the second data rate is faster than the first data rate and said second data rate is an integer multiple of said first data rate, and for synchronizing the first and second input data streams in a fixed timing relationship, with at least said first input data stream providing timing for the first and second input data streams;

means electrically connected to the means for providing digital data, for generating a first phase modulated output signal having a phase relative to a reference phase that is responsive to the data of the first and second input data streams, said means for generating said first phase modulated output signal including means for spreading the data in said first input data stream to provide a spread data stream at said second data rate, said first phase modulated output signal responsive to said spread data stream so that said first phase modulated output signal has substantially the same modulation bandwidth for each of said first and second input data streams;

means responsive to the first phase modulated output signal for transmitting a radio frequency signal modulated by said first phase modulated output signal;

means at said second location for receiving a radio frequency signal corresponding to said transmitted radio frequency signal and for generating a second phase modulated output signal corresponding to the first phase modulated output signal; and

means for demodulating the second phase modulated output signal to provide first and second output data streams at said first and second data rates, respectively, wherein said first and second output data streams correspond, respectively, to said first and second input data streams, and wherein the means for demodulating provides the first output data stream independently of the second output data stream so that timing information encoded on said first output data stream can be decoded independent of the second output data stream.

5. The communications system as defined in claim 4 wherein said means for demodulating said second phase modulated output signal comprises:

means for separating said second phase modulated output signal into first and second unstabilized output data streams; and

means for converting said first and second unstabilized output data streams into first and second stabilized output data streams corresponding to said first and second output data streams.

6. The communications systems as defined in claim 5 wherein said first stabilized output data stream corresponds to said spread data stream at said second data rate, and wherein said demodulating means includes means for converting said first stabilized output data stream to said first output data stream at said first data rate.

7. A communication system for reliably transmitting digital data from a first location to a second location, comprising:

means for generating first and second source data streams having first and second data rates, said second data rate higher than said first data rate;

means for synchronizing the first and second source data streams in a fixed timing relationship with each other, said means for synchronizing further providing data synchronization information as part of said first source data stream that synchronized said first source data stream with said second source data stream;

means for quadrature phase modulating a radio frequency carrier with said first and second source data streams, said first source data stream shifting the phase of said radio frequency carrier to provide a first phase shift component, said second source data stream shifting the phase of said radio frequency carrier to provide a second phase shift component, said first and second phase shift components selected to be non-interfering to provide two simultaneous distinct communications channels on said radio frequency carrier;

means for transmitting said carrier from the first location to the second location;

means at said second location for receiving a signal corresponding to said transmitted signal; and

means for demodulating said received signal to generate first and second received data streams, said first received data stream corresponding to said first source data stream and including said data synchronization information, said second received data stream corresponding to said second source data stream,

wherein said lower data rate of said first source data stream is selected to be sufficiently lower than said second data rate to enable said first received data stream to be demodulated in the presence of signal attenuation that prevents demodulation of said second received data stream, said first received data stream continuing to provide said synchronization information for said second received data stream regardless of whether said second received data stream is demodulated.

8. A communications system for reliably transmitting digital data from a first location to a second location, comprising:

means for generating first and second source data streams having first and second data rates, said second data rate higher than said first data rate;

means for synchronizing the first and second source data streams in a fixed timing relationship with each other;

means for quadrature phase modulating a radio frequency carrier with said first and second source data streams;

means for transmitting said carrier from the first location to the second location;

means at said second location for receiving a signal corresponding to said transmitted signal; and

means for demodulating said received signal to generate first and second received data streams, said first received data stream corresponding to said first source data stream and said second received data stream corresponding to said second source data stream, said means for demodulating said received signal comprising;

a translation circuit for converting said received signal into first and second digital data signals each having a magnitude that varies in response to said first and second source data streams, said translation circuit comprising;

a controllable oscillator for generating first and second mixing signals at substantially the same frequency as said received signal;

first and second frequency mixers responsive to said first and second mixing signals, respectively, and responsive to said received signal for generating first and second analog signals; and

first and second analog-to-digital convertors for receiving said first and second analog signals and for generating said first and second digital data signals at a sampling rate that is at least twice the second data rate; and

a set of tracking circuits for receiving said first and second digital data signals from said translation circuit and for converting said first and second digital data signals into first and second digital reproductions of said first and second source data streams, respectively, said first digital reproduction comprising data at said first data rate and said second digital reproduction comprising data at said second data rate, said set of tracking circuits further providing a frequency control signal to said translation circuit to control the frequency of said controllable oscillator and a sampling signal to control the sampling rate of said analog-to-digital convertors,

wherein said lower data rate of said first data stream enable s said first data stream to be demodulated in the presence of signal attenuation that prevents demodulation of said second data stream.

9. The communications system as defined in claim 8, wherein said set of tracking circuits comprises:

a coordinate conversion circuit for receiving said first and second digital data signals and for converting said first and second digital data signals into first and second intermediate data streams at said second data rate, said second intermediate data stream corresponding to said second digital reproduction of said second source data stream, said coordinate conversion block further producing an angular output signal having a magnitude proportional to an angular phase difference between said received signal and one of said first and second mixing signals;

a despread circuit for receiving said first and second intermediate data streams at said second data rate and for converting said first and second intermediate data streams to first and second despread data streams at said first data rate, said first despread data stream corresponding to said first digital reproduction of said first source data stream;

a frequency tracking circuit for receiving said angular output signal from said coordinate conversion circuit and for generating an angular correction signal to said coordinate conversion circuit to correct for angular phase differences between said received signal and said one of said first and second mixing signals, said circuit further generating said frequency control signal to control the frequency of said controllable oscillator in said translation circuit; and

a clock oscillator circuit for generating said sampling signal to control the time at which said first and second analog-to-digital convertors sample said first and second analog signals, said time controlled so that said first and second analog signals are samples at times corresponding to maximum and minimum magnitudes of said first and second analog signals.

10. A method of transmitting digital data from a first location to a second location, comprising the steps of:

generating first and second source data streams, said first source data stream having a first data rate and said second source data stream having a second data rate higher than said first data rate

synchronizing said second source data stream with said first source data stream so that said second source data stream has a fixed timing relationship with said first source data stream;

providing timing information in said first source data stream for said first source data stream and said second source data stream;

quadrature phase modulating a radio frequency carrier with said first and second source data streams such that said radio frequency carrier is shifted in phase by a first component responsive to said first source data stream and shifted in phase by a second component responsive to said second source data stream, said second component independent of said first component so that two independent data communications channels are provided on said radio frequency carrier;

transmitting said carrier form a first location to a second location;

receiving said transmitted signal at second location as a received signal;

demodulating said received signal to generate first and second received data streams, said first received data stream corresponding to said first source data stream and said second received data stream corresponding to said second source data stream, said first received data stream including timing information for said first received data stream and said second received data stream;

selecting said lower data rate of said first source data stream to be sufficiently lower than said higher data rate of said second source data stream so that said first received data stream can be demodulated in said demodulating step in the presence of signal attenuation that prevents demodulation of said second received data stream.

11. A system for transmitting digital data from a first ground location to a second ground location via an orbiting relay satellite, comprising:

a source of digital data, said digital data comprising a first input data stream having a first data rate, and a second input data stream having a second data rate higher than said first data rate, said first input data stream and said second input data stream synchronized so that said first input data stream and said second input data stream have a fixed timing relationship, said first input data stream providing timing information for said first input data stream and said second input data stream;

a quadrature phase modulator electrically connected to receive said first and second input data stream, said modulator generating a first phase modulated output signal having a phase relative to a reference phase that is responsive to the data of said first and second input data streams;

a transmitter electrically coupled to said modulated output signal for generating a first radio frequency signal modulated by said modulated output signal, and an antenna electrically connected to said transmitter for radiating said first radio frequency signal toward said satellite wherein a second radio frequency signal responsive to said first radio frequency signal is transmitted toward said second ground location;

an antenna and receiver combination at said second ground location for receiving said second radio frequency signal transmitted by said satellite, said receiver generating a second phase modulated output signal corresponding to said first phase modulated output signal; and

a demodulator electrically connected to receive said second phase modulated output signal from said receiver, said demodulator generating a first output data stream at said first data rate, said first output data stream corresponding to said first input data stream, and a second output data stream at said second data rate, said second output data stream corresponding to said second input data stream, wherein said first output data stream is demodulated independently of said second output data steam so that timing information provided by said first input data stream can be reproduced independent of whether said second output data stream is demodulated, said first data rate selected to be sufficiently lower than said second data rate so that said first output data stream can be demodulated in the presence of signal attenuation that prevents the demodulation of said second output data stream.

12. In a satellite communications system, a method of mitigating the loss of signals caused by atmospheric attenuation, comprising the steps of:

generating first and second source data streams having first and second data rates, said second data rate higher than said first data rate, said first source data stream including data synchronization information, said second source data stream having a fixed timing relationship to said first source data stream so that said second source data stream is synchronized with said data synchronization information of said first source data stream;

quadrature phase modulating a radio frequency carrier with said first and second source data streams to provide two independent communications channels on said radio frequency carrier for said first and second source data streams;

transmitting said carrier from a first location to a second location via an orbiting relay satellite;

receiving said transmitted signal at said second location as a received signal;

demodulating said received signal to generate first and second received data streams, said first received data stream corresponding to said first source data stream and said second received data stream corresponding to said second source data stream, said first received data stream including said data synchronization information for said first and second received data streams; and

selecting said lower data rate of said first source data stream to be sufficiently lower than said higher data rate of said second source data stream such that said first received data stream can be demodulated in said demodulating step in the presence of signal attenuation that prevents demodulation of said second received data stream, said first received data stream providing synchronization for both said first received data stream and said second received data stream regardless of whether said second data stream is demodulated.

13. A method for transmitting digital data through the earth'"'"'s atmosphere at a frequency susceptible to attenuation by precipitation, comprising the steps of:

providing first and second source digital data streams at first and second digital data rates, said second digital data rate higher than said first digital data rate;

synchronizing said first source digital data stream with said second source digital data stream so that said second source digital data stream has a fixed timing relationship to said first source digital data stream;

providing timing information in said first source digital data stream for said first source digital data stream and said second source digital data stream;

phase modulating a radio frequency carrier with said first source digital data stream, to provide a first phase modulated signal;

phase modulating said radio frequency carrier with said second source digital data stream, to provide a second phase modulated signal in quadrature with said first phase modulated signal;

combining said first phase modulated signal and said second phase modulated signal to provide a combined quadrature phase shift modulated signal;

transmitting said combined signal;

receiving a combined signal responsive to said transmitted signal;

demodulating said received signal to generate a first output signal at said first digital data rate, said first output signal corresponding to said first source digital data stream and including said synchronization information, and to generate a second output signal at said second digital data rate, said second output signal corresponding to said second source data stream and synchronized with said synchronization information;

selecting said first source digital data rate to be sufficiently lower than said second source digital data rate such that said first output signal can be demodulated in said demodulating step to provide said synchronization information in the presence of precipitation attenuation that prevents the demodulation of said second output signal.