Method and apparatus for transmitting messages to and from remote locations

• US 4,030,033 A
• Filed: 06/13/1975
• Issued: 06/14/1977
• Est. Priority Date: 06/13/1975
• Status: Expired due to Term

• Current Assignee: Lowell Technological Institute Research Foundation
• Original Assignee: Lowell Technological Institute Research Foundation
• Inventors: Bibl, Klaus, Reinisch, Bodo W.
• Primary Examiner(s): Safourek, Benedict V.
• Application Number:
  US05/586,538
• Time in Patent Office: 732 Days
• Field of Search: 325/30, 325/35, 325/40, 325/41, 325/42, 325/56, 325/63, 325/66, 325/311, 178/66 R, 178/67
• US Class Current: 375/342
• CPC Class Codes: H04B 7/22 Scatter propagation systems , e.g. ionospheric, tropospheric or meteor scatter
• CPC Class Codes: H04L 23/02 adapted for orthogonal signalling

Abstract

A method and apparatus for transmitting a message, in digital form, to or from a remote location. In a transmitter, a sequence of predetermined multiple-bit phase codes corresponding to the individual characters in the message phase modulate a carrier wave which also shifts frequency. At a receiving station, the various modulated carrier waves propagated through a dispersive medium constitute incoming signals with both frequency and range diversity. The receiver transforms the incoming signals into a sequence of quadrature component signals which it correlates with the multiple-bit phase code sequences to yield the digital representation of a transmitted message character at the receiving station.

Claims

1. A communications system for transmitting a message including a plurality of characters through a highly noisy, variable delay propagation medium, said system comprising:

A. a modulated carrier wave transmitter including;

i. means responsive to each message character for establishing a corresponding multiple-bit phase code sequence,

ii. means for generating a carrier wave which shifts from one frequency to another, and

iii. means for phase-modulating the carrier wave in response to the phase code sequence thereby to transmit the modulated carrier wave with frequency diversity dependent upon the carrier wave frequency shifts to the propagation medium, and

B. a receiver for recovering from the propagation medium the transmitted message, said receiver including;

i. means for generating a carrier wave that shifts in frequency in synchronism with the carrier wave frequency shifts at said transmitter,

ii. input means for generating a demodulated signal in response to the signal from said incoming modulated carrier wave,

iii. detecting means for decoding the demodulated signal into a sequence of vector signals representing different predetermined range delays of the signal through the propagation medium, and

iv. correlation means for generating a message character based upon the correlation of the detected vector signals and the multiple bit phase code sequences.

2. A communication system as recited in claim 1 wherein each character is identified by a character code with a fewer number of bit positions than in the corresponding phase code sequence, said phase code sequence establishment means in said transmitter including:

i. means for storing the information for the plurality of phase code sequences, and

ii. means responsive to each character code for retrieving from said storage means the corresponding one of the phase code sequences.

3. A communications system as recited in claim 2 wherein said storage means stores four multiple bit sequences and each bit position in the sequence is stored in an addressable location, said retrieving means including:

i. a counter with a modulus corresponding to the number of bit positions in a phase code sequence,

ii. means responsive to predetermined character code bit positions for presetting said counter,

iii. means responsive to the other character code bits and the number in said counter for selecting a group of locations in said storage means, and

iv. means for advancing said counter through one iteration thereby to retrieve from said storage means a phase code sequence corresponding to the character code.

4. A communications system as recited in claim 2 wherein said storage means contains a plurality of sequences, each such sequence having a zero correlation for the first three shifted positions of that sequence.

5. A communications system as recited in claim 2 wherein said phase modulating means comprises:

a. a phase modulator for modulating the carrier wave in response to each bit in the phase code sequence, and

b. an output amplifier for transmitting to the propagation medium, a chain of pulses with the common carrier frequency where the sign of the phase in the sequence of the pulse chain corresponds to the bit values in a phase code sequence.

6. A communications system as recited in claim 1 wherein said carrier wave generator in said transmitter and in said receiver each include:

i. a frequency memory for generating a sequence of frequency selection signals, and

ii. a variable frequency oscillator for generating a carrier wave with a frequency that shifts in response to the sequence of the frequency selection signals.

7. A communications system as recited in claim 6 additionally comprising a timing control circuit in said transmitter and in said receiver, said timing control circuits generating, in synchronism, signals to control said frequency memories thereby to maintain the frequency shifts in synchronism.

8. A communications system as recited in claim 6 wherein said receiver input means includes

i. a carrier wave amplifier tuned in response to signals from said frequency memory, and

ii. a plurality of intermediate frequency circuits.

9. A communications system as recited in claim 1 wherein:

A. said transmitter modulating means includes pulsed amplifier means for transmitting a modulated carrier wave pulse to the propagation medium in response to each bit position in a phase code sequence,

B. the input signal from said receiver input means in response to each modulated carrier wave pulse represents a plurality of reflected signals produced by variable range delays,

C. said detecting means in said receiver obtains, for each modulated carrier wave pulse, vector signals for each range delay,

D. said transmitter additionally comprises means responsive to said carrier wave generating means for producing a reference phase code sequence each time said carrier wave generating means shifts frequency,

E. said receiver additionally comprises a detection circuit for determining the quality of the vector signals for a reference character corresponding to each range delay, thereby to preselect certain of the range delays, and

F. means for storing the vector signals for subsequent characters transmitted at that carrier wave frequency only for the preselected range delays.

10. A communications system as recited in claim 1 wherein said correlation means includes:

i. means for storing the phase code sequences,

ii. means responsive to all the vector signals for a character for obtaining, for each phase code sequence, an average correlation coefficient for all range delays, and

iii. means for selecting the phase code sequence from said memory which produces the greatest average correlation coefficient thereby to indicate the character.

11. A communications system as recited in claim 10 wherein said detector means transmits a plurality of sets of first and second quadrature signals, each set corresponding to a predetermined range delay for each bit in the phase code sequence.

12. A communications system as recited in claim 11 wherein said correlation means correlates all vector signals corresponding to a given range delay and averages the magnitude of the correlations for all range delays with respect to each phase code sequence.

13. A communications system as recited in claim 11 wherein said correlation means comprises:

i. means for establishing a signal-to-noise ratio for each carrier wave frequency, and

ii. means responsive to the signal-to-noise ratio for designating preselected range delays, and

iii. means for storing the sequence of vector signals in the preselected range delays only, and

iv. means for correlating, with respect to each phase code sequence, all the stored vectors corresponding to each predetermined range delay, thereby to obtain a range correlation coefficient for each range delay.

14. A communications system as recited in claim 13 additionally comprising means for obtaining the average of the magnitudes of each range correlation coefficient for each phase code sequence thereby to obtain an average correlation coefficient for each sequence.