Multi-frequency differentially encoded digital communication for high data rate transmission through unequalized channels

• US 5,063,574 A
• Filed: 03/06/1990
• Issued: 11/05/1991
• Est. Priority Date: 03/06/1990
• Status: Expired due to Fees

• Current Assignee: Hmd Holdings, Inc. (Silver Scott Mines, Inc.)
• Original Assignee: Mercury Digital Communications, Inc.
• Inventors: Moose, Paul H.
• Primary Examiner(s): Safourek, Benedict V.
• Assistant Examiner(s): Tse, Young T.
• Application Number:
  US07/490,769
• Time in Patent Office: 609 Days
• Field of Search: 375/8, 375/9, 375/27, 375/39, 375/48, 375/51, 375/56, 375/58, 332/100, 332/103, 379/93, 379/97, 379/98, 364/576
• US Class Current: 375/244
• CPC Class Codes: H04L 27/2602 Signal structure

Abstract

A differentially encoded digital signal waveform is generated as a discrete time representation of a desired analog signal utilizing multi-frequency modulation techniques. The computational capability of present day, industry-standard microcomputers equipped with a floating point array processor or digital signal processor chip is utilized to perform digital frequency encoding and compute both discrete Fourier transforms and inverse discrete Fourier transforms to provide a transmitter and receiver system utilizing suitably programmed microcomputers coupled by a communications channel.

Claims

1. Apparatus for encoding a data signal on a plurality of carrier frequencies for transmission over a transmission channel, said apparatus comprising:

encoding means for differentially encoding a plurality of first digital words into a plurality of second digital words, each of said second digital words being a modulation value corresponding to one of said plurality of first digital words, each said modulation value being a predetermined function of said corresponding first digital word and a different one of said first digital words;

transform means coupled to said encoding means for transforming said plurality of modulation values into a digital signal representative of a plurality of predetermined digital carrier frequencies, each of said plurality of predetermined digital carrier frequencies modulated by a corresponding one of said plurality of modulation values; and

data transfer means coupled to said transform means for coupling said digital signal to a transmission channel and for converting said digital signal to an analog signal for transmission over a transmission channel, said analog signal being a sum of a plurality of modulated predetermined analog carrier frequencies corresponding to said plurality of predetermined digital carrier frequencies.

2. Apparatus as in claim 1 wherein said encoding means comprises:

differential encoding means for forming a plurality of modulation states, each of said modulation states associated with a different one of said first digital words, each of said modulation states being a predetermined function of the difference between said associated first digital word and a preceding one of said first digital words; and

modulation means coupled to said differential encoding means for generating a plurality of modulation values, each of said modulation states corresponding to a different one of said plurality of modulation values, each of said modulation values for modulating a corresponding different one of said plurality of predetermined digital carrier frequencies.

3. Apparatus as in claim 2 wherein said plurality of predetermined digital carrier frequencies form a set of orthogonal frequencies.

4. Apparatus as in claim 2 wherein said plurality of predetermined digital carrier frequencies are harmonic frequencies of a predetermined data sample frequency.

5. Apparatus as in claim 2 wherein said plurality of second digital words comprises an array of complex valued digital words, a first half of said array comprising said plurality of modulation states represented in complex form, a second half of said array comprising a plurality of complex valued digital words, each of said plurality of complex valued digital words of said second half representative of the complex conjugate of a different one of said plurality of modulation states.

6. Apparatus as in claim 5 wherein said digital signal comprises an array of real-valued discrete time samples corresponding to said analog signal.

7. Apparatus as in claim 2 further comprising means for receiving a digital information signal and segmenting said digital information signal into groups of said first digital words, each group having an equal number of said first digital words, the value of said modulation state associated with the first word of said first digital words in each group being a predetermined function of said first word and a predetermined reference modulation state, the value of each modulation state associated with the second word and each successive word of said first digital words in each group being a predetermined function of its associated first digital word and the modulation state associated with the immediately preceding first digital word in each group.

8. Apparatus as in claim 7 wherein said modulation means generates a reference modulation value corresponding to said predetermined reference modulation state, said reference modulation value for modulating a reference digital carrier frequency.

9. Apparatus as in claim 8 wherein the number of said predetermined digital carrier frequencies is at least K+1, where K is the number of first digital words in each of said groups.

10. Apparatus as in claim 8 wherein the number of said predetermined digital carrier frequencies is equal to K+1, where K is the number of first digital words in each of said groups.

11. Apparatus for encoding a signal for transmission over a transmission channel comprising:

receiving means for receiving a first signal representing digital information, said first signal comprising a plurality of first digital words of equal length;

differential encoding means coupled to said receiving means for forming a plurality of modulation states, each of said modulation states associated with a different one of said first digital words, each of said modulation states being a predetermined function of the difference between its associated first digital word and a preceding one of said first digital words;

modulating means coupled to said differential encoding means for generating a plurality of modulation values, each of said modulation values corresponding to a different one of said plurality of modulation states, each of said modulation values for modulating a corresponding different one of a plurality of predetermined digital carrier frequencies, said plurality of predetermined digital carrier frequencies being a set of orthogonal carrier frequencies;

transform means coupled to said modulating means for transforming said plurality of modulation values into a digital signal representative of said plurality of predetermined digital carrier frequencies, each of said predetermined digital carrier frequencies being modulated by a corresponding modulation value; and

data transfer means coupled to said transform means for coupling said digital signal to a transmission channel and for converting said digital signal to an analog signal for transmission over a transmission channel, said analog signal being a sum of a plurality of modulated predetermined analog frequencies corresponding to said plurality of predetermined digital carrier frequencies.

12. Apparatus as in claim 11 wherein said plurality of modulation values comprise a Fourier transform representation of said analog signal.

13. Apparatus as in claim 12 wherein said transform means comprises means for transforming said plurality of modulation values by an inverse discrete Fourier transform.

14. Apparatus as in claim 11 wherein said plurality of first digital words are mapped into a plurality of digital modulation states, said plurality of digital modulation states being preceded by a first digital modulation state having a value of zero, the value of each successive digital modulation state being a function of its immediately preceding digital modulation state and the value of said associated first digital word, the value of each of said digital modulation states corresponding to different one of said modulation values.

15. Apparatus as in claim 14 wherein each said modulation state of said plurality of modulation states comprises its immediately preceding modulation state rotated by an integral multiple of π

/2 as a function of said associated first digital word.

16. Apparatus as in claim 11 wherein said plurality of predetermined digital carrier frequencies are harmonic frequencies of a predetermined data sample frequency.

17. A method for encoding a signal for transmission over a transmission channel comprising the steps of:

receiving a signal representative of digital information, said signal comprising a plurality of first digital words of equal word length;

differentially encoding each of said first digital words into a plurality of second digital words, each of said second digital words being a modulation value corresponding to a different one of said plurality of first digital words, each said modulation value being a predetermined function of said corresponding first digital word and a different one of said first digital words;

transforming said plurality of second digital words into a digital signal representative of a plurality of predetermined digital carrier frequencies, each of said plurality of predetermined digital carrier frequencies being modulated by a corresponding modulation value; and

converting said digital signal to an analog signal for transmission over a transmission signal, said analog signal being a sum of a plurality of modulated analog carrier frequencies corresponding to said plurality of predetermined digital carrier frequencies.

18. The method of claim 17 including the steps of:

receiving said transmitted analog signal;

converting said received analog signal to a serial string of a plurality of digital words of equal word length as said first digital words, said plurality of said digital words being a digital representation of said received analog signal; and

decoding said plurality of digital words forming a plurality of fourth digital words representative of said digital information.

19. The method of claim 18 further including the step of frame synchronization of said received analog signal, said transmitted analog signal including a synchronization signal, said frame synchronization of said received analog signal comprising detection of a digital representation of said synchronization signal and synchronization of a clock signal for clocking said plurality of digital words with said synchronizing signal.

20. The method of claim 17 including the steps of:

encoding the change in value from a first one of said first digital words to a second successive one of said first digital words forming a plurality of modulation values, each of said modulation values corresponding to the change in value between two successive ones of said first digital words; and

modulating each one of said plurality of predetermined digital frequencies with a different one of said modulation values forming said plurality of second digital words, each of said second digital words representative of a different one of said plurality of predetermined digital frequencies modulated by one of said plurality of modulation values.

21. The method of claim 20 wherein said step of transforming said plurality of second digital words comprises processing said second digital words by an inverse discrete Fourier transform.

22. A communications system for transmitting an analog encoded signal utilizing multi-frequency modulation techniques, comprising:

transmitter means for generating and transmitting a modulated, multi-frequency carrier signal including means for receiving an information signal and converting said information signal to a first digital signal comprising a plurality of first digital words of equal length, differential encoding means for differentially encoding said first digital signal into a plurality of modulation states, each of said modulation states associated with a different word of said first digital words, each of said modulation states being a predetermined function of said associated first digital word and a preceding first digital word, modulation means coupled to said differential encoding means for generating a plurality of modulation values, each of said modulation values corresponding to a different one of said plurality of modulation states, each of said modulation values for modulating a corresponding different one of a plurality of predetermined digital carrier frequencies, said plurality of modulation values forming a set of second digital words, first transform means coupled to said modulating means for transforming said set of second digital words into a second digital signal representative of said plurality of predetermined digital carrier frequencies, each of said predetermined digital carrier frequencies being modulated by a corresponding modulation value;

data transfer means coupled between said transmitter means and a transmission means for converting said second digital signal to an analog signal and for transferring said analog signal to said transmission means, said analog signal being a sum of a plurality of modulated predetermined analog carrier frequencies corresponding to said plurality of predetermined digital carrier frequencies, said analog signal transmitted by said transmission means;

signal acquisition means coupled to said transmission means for acquiring said transmitted analog signal and converting said transmitted analog signal to a third digital signal; and

receiver means coupled to said signal acquisition means for receiving said third digital signal, said receiver means including decoding means for decoding said third digital signal to a digital information signal representative of the information represented by said first digital signal.

23. A communications system as in claim 22 wherein said set of second digital words comprises a Fourier transform representation of said analog signal.

24. A communications system as in claim 23 wherein said first transform means comprises means for effecting an inverse discrete Fourier transform on said plurality of second digital words to form said second digital signal.

25. A communications system as in claim 24 wherein said transmitting means comprises a microcomputer.

26. A communications system as in claim 25 wherein said data transfer means comprises a digital-to-analog converter coupled to a DMA channel of said microcomputer, said second digital signal coupled to said digital-to-analog converter via said DMA channel.

27. A communications system as in claim 25 wherein said receiver means comprises a microcomputer.

28. A communications system as in claim 27 wherein said data acquisition means comprises an analog-to-digital converter coupled to said transmission means and a synchronization means for synchronizing said third digital signal with a clock of said receiver.

29. A communications system as in claim 24 wherein said receiver means includes second transform means coupled to said signal acquistion means for transforming said third digital signal into an array of third digital words, said decoding means for decoding said array of third digital words.

30. A communications system as in claim 29 wherein said second transform means comprises means for effecting a discrete Fourier transform on said third digital signal, said array of third digital words being the Fourier coefficients of said third digital signal.