Constellation multiplexed inband secondary channel for voiceband modem

• US 4,891,806 A
• Filed: 09/18/1987
• Issued: 01/02/1990
• Est. Priority Date: 09/18/1987
• Status: Expired due to Fees

• Current Assignee: Racal-Datacom, Inc. (Thales SA)
• Original Assignee: Racal Data Communications Incorporated
• Inventors: Wong, Chin-Pan, Krishnan, Vadavalli G., Farias, Luis V.
• Primary Examiner(s): Safourek, Benedict V.
• Assistant Examiner(s): Chin, Wellington
• Application Number:
  US07/098,547
• Time in Patent Office: 837 Days
• Field of Search: 370/20, 370/110.1, 370/119, 370/100, 375/13, 375/39, 375/106, 371/43
• US Class Current: 370/503
• CPC Class Codes: H04L 1/20 using signal quality detector
• CPC Class Codes: H04L 27/3472 by switching between alternative constellations
• CPC Class Codes: H04L 27/3477 by using the outer points of the constellation or of the constituent two-dimensional constellations

Abstract

A method of providing an inband secondary channel for a data modem includes encoding the main channel data using a convolutional encoder so that main channel data symbols are selected from a first constellation. The secondary channel data are encoded using a differential encoder and secondary channel data symbols are selected from a second constellation. The transmission of the main channel symbols takes place during 48 frames of a 49 frame master frame. The differentially encoded secondary channel data symbols are transmitted during the remaining frame. The symbols of the secondary channel constellation are selected to have a higher average energy than those of the main channel data so that the secondary channel is easily detected and may be used to obtain frame synchronization for the modem after brief dropouts without having to retrain the modem.

Claims

1. A method for providing a main channel and a secondary channel in a data modem, the method comprising the steps of:

receiving data to be transmitted over a main channel;

receiving data to be transmitted over a secondary channel;

generating main channel symbols from a first constellation corresponding to said main channel data;

generating secondary channel symbols from a second constellation corresponding to said secondary channel data; and

alternating transmission of said main channel symbols during a first predetermined time period with transmission of said secondary channel symbols during a second predetermined time period so that said main channel symbols from said first constellation and said secondary channel symbols from said second constellation are interleaved in time.

2. The method of claim 1, further including the step of encoding said main channel data using a redundancy generating encoding method.

3. The method of claim 2, wherein said redundancy generating encoding method includes convolutional encoding of said main channel data.

4. The method of claim 1, wherein said secondary channel data is modulated using a differential phase shift keying modulation method.

5. The method of claim 1, wherein symbols from said second constellation have a higher average energy than symbols from said first constellation.

6. The method of claim 1, wherein said main and secondary channel symbols are arranged in a master frame of 49 frames, with each frame including 4 symbols of primary or secondary channel data;

and wherein said master frame comprises 1 frame of secondary channel symbols and 48 frames of main channel symbols.

7. The method of claim 1, wherein said secondary channel has a higher average signal to noise ratio than said main channel.

8. The method of claim 1, wherein said first constellation has a greater number of points than said second constellation.

9. The method of claim 8, wherein said first constellation has 160 points and said second constellation has 4 points.

10. The method of claim 5, wherein said first constellation has a greater number of points than said second constellation.

11. A method of receiving main and secondary channel data in a synchronous data modem, the method comprising the steps of:

receiving a plurality of synchronously transmitted constellation symbols;

determining which of said plurality of constellation symbols are main channel symbols selected from a main channel constellation and which of said data symbols are secondary channel symbols selected from a secondary channel constellation, said secondary channel symbols and said main channel symbols being interleaved in time at periodic intervals so that said main channel symbols and said secondary channel symbols each carry synchronous data with the secondary channel data having a data rate which is a predetermined proportion of the data rate of said main channel data;

processing said main channel constellation symbols according to a method for extracting data from said main channel constellation symbols; and

processing said secondary channel constellation symbols according to a method for extracting data from said secondary channel constellation symbols.

12. The method of claim 11, wherein said method for processing said main channel constellation symbols includes a Viterbi decoding method for making soft decisions.

13. The method of claim 11, wherein said method for processing said secondary channel constellation symbols includes a differential decoding method.

14. The method of claim 11, wherein said constellation symbols of said secondary channel constellation have higher average energy than constellation symbols of said main channel constellation.

15. The method of claim 11, further comprising the step of adapting an adaptive equalizer during receipt of said secondary channel constellation symbols.

16. The method of claim 11, further comprising the step of adjusting the frequency of a carrier generator during receipt of said secondary channel constellation symbols.

17. The method of claim 11, further comprising the steps of:

adapting an adaptive equalizer;

monitoring the signal quality of received main channel constellation symbols;

comparing said signal quality of received main channel constellation symbols with a predetermined threshold of acceptable signal quality;

inhibiting adapting of said adaptive equalizer during receipt of main channel constellation symbols and continuing adapting of said adaptive equalizer during receipt of secondary channel constellation symbols if said signal quality is below said predetermined threshold.

18. The method of claim 11, further comprising the steps of:

adjusting the carrier frequency of a carrier generator;

monitoring the signal quality of received main channel constellation symbols;

comparing said signal quality of received main channel constellation symbols to a predetermined threshold of acceptable signal quality;

inhibiting adjusting of said carrier frequency during receipt of main channel constellation symbols and continuing adjusting of said carrier frequency during receipt of secondary channel constellation symbols if said signal quality is below said predetermined threshold.

19. A method for providing synchronization in a data modem, comprising the steps of:

receiving a segment of data having length equal to that of a master frame having predetermined length, said master frame including a designated time period for transmitting secondary channel symbols from a secondary channel constellation;

searching said segment for a first time period containing secondary channel symbols; and

synchronizing said data modem receiver by setting said designated time period equal to said first time period.

20. The method of claim 19, further comprising the step of:

after said searching step, verifying that said designated time period has been located by inspecting a successive segment for secondary channel symbols.

21. The method of claim 19, wherein said searching step includes passing a sliding window across said segment of data and determining the average energy of the symbols within said sliding window, wherein symbols having distinctive energy characteristics are secondary channel symbols.

22. The method of claim 21, wherein said master frame comprises 49 frames of data, each frame comprising 4 symbols and wherein said designated time period and said first time period each being equal to the duration of one frame.

23. An apparatus, comprising:

main channel mapping means for generating symbols from a main channel constellation corresponding to main channel data;

secondary channel mapping means for generating symbols from a secondary channel constellation corresponding to secondary channel data;

selecting means for alternately selecting main channel and secondary channel symbols in a predetermined arrangement for transmission over a data channel; and

transmitting means for transmitting said selected symbols.

24. The apparatus of claim 23, wherein said main channel mapping means further comprises means for convolutionally encoding said main channel data.

25. The apparatus of claim 23, wherein said main channel mapping means further comprises means for differentially encoding said main channel data.

26. The apparatus of claim 24, wherein said main channel mapping means further comprises means for differentially encoding said main channel data.

27. The apparatus of claim 23, wherein said secondary channel mapping means further comprises means for differentially encoding said secondary channel data.

28. The apparatus of claim 23, wherein said main channel constellation contains more symbols than said secondary channel constellation.

29. The apparatus of claim 28, wherein said main channel constellation contains approximately 160 points and said secondary channel constellation contains approximately 4 points.

30. The apparatus of claim 23, wherein said secondary channel constellation has a higher average energy than said main channel constellation.

31. The apparatus of claim 23, further comprising:

symbol receiving means for receiving main channel and secondary channel symbols;

separating means for separating main channel symbols taken from said first constellation from secondary channel symbols taken from said second constellation;

main channel decoding means for translating said main channel symbols to main channel data; and

secondary channel decoding means for translating said secondary channel symbols to secondary channel data.

32. The apparatus of claim 31, wherein said secondary channel constellation has a higher average energy than said main channel constellation.

33. The apparatus of claim 32, further comprising synchronization means for detecting said higher average energy with relative phase characteristics indicative of secondary channel symbols and for synchronizing said separating means so that said separating means properly separates said main channel symbols from said secondary channel symbols.

34. An apparatus for receiving main channel and secondary channel data in a data modem, comprising:

symbol receiving means for receiving main channel and secondary channel symbols;

separating means for separating main channel symbols taken from a first constellation from secondary channel symbols taken from a second constellation, said main channel symbols and said secondary channel symbols being periodically interleaved in time;

main channel decoding means for translating said main channel symbols to main channel data; and

secondary channel decoding means for translating said secondary channel symbols to secondary channel data.

35. The apparatus claim 34, wherein said secondary channel constellation has a higher average energy than said main channel constellation.

36. The apparatus of claim 35, further comprising synchronization means for detecting said higher average energy and for synchronizing said separating means so that said separating means properly separates said main channel symbols from said secondary channel symbols.

37. The apparatus of claim 34, wherein said main channel decoding means further comprises a Viterbi decoder.

38. The apparatus of claim 34, wherein said main channel decoding means further comprises a differential decoder.

39. The apparatus of claim 37, wherein said main channel decoding means further comprises a differential decoder.

40. The apparatus of claim 34, wherein said secondary channel decoding means further comprises a differential decoder.

41. The apparatus of claim 34, wherein said symbol receiving means further comprises an adaptive equalizer.

42. The apparatus of claim 41, further comprising means for monitoring received signal quality and means for adapting said adaptive equalizer only during periods of receipt of secondary channel symbols when said received signal quality is judged to be poor.

43. The apparatus of claim 34, wherein said symbol receiving means includes a demodulator for converting said received symbols to baseband, and a carrier generator for generating a carrier signal having frequency for use by said demodulator;

and further comprising means for adaptively adjusting said carrier generator'"'"'s frequency.

44. The apparatus of claim 43, further comprising:

signal quality monitoring means for monitoring received signal quality and for permitting adaption of said carrier generator'"'"'s frequency only during periods of receipt of secondary channel symbols when said received signal quality is judged to be poor.

45. An apparatus for adjusting a characteristic of a data modem having a secondary channel and a main channel, said secondary channel being more immune to noise and other line disturbances than said main channel, said main and secondary channels being transmitted during main and secondary time intervals respectively, the apparatus comprising in combination;

receiving means for receiving signals carrying said main and secondary channel;

timing means for determining when said main and seconary channels respectively are being received;

monitoring means for monitoring the quality of said received signals; and

adjusting means, coupled to said monitoring means, for adjusting said characteristic of said receiving means while receiving either main or secondary channel when said signal quality is good and for adjusting said characteristic only during the time interval of receipt of said secondary channel when said signal quality is poor.

46. The apparatus of claim 45, wherein said monitoring means includes means for monitoring an indication of mean-square error computed by a Viterbi decoder.

47. The apparatus of claim 46, wherein said adjusting means includes means for receiving said mean-square error and comparing said mean-square error to a threshold, and wherein signal quality is judged poor if said mean-square error is greater than said threshold and good if said mean-square error is smaller than said threshold.

48. The apparatus of claim 45, wherein said receiving means further comprises an adaptive equalizer having tap values and wherein said adjusting means adjusts the tap values of said adaptive equalizer.

49. The apparatus of claim 45, wherein said receiving means further comprises a demodulator for demodulating incoming signals to baseband, and a carrier generator coupled to said demodulator, and wherein said adjusting means adjusts the frequency of said carrier generator.

50. An apparatus for synchronizing a modem receiver to a modem transmitter, comprising in combination:

receiving means for receiving one or more segments of data having length equal to that a master frame including a designated time period for transmitting secondary channel constellation symbols from a secondary channel constellation;

searching means for searching said segment for a first time period containing a group of symbols separated by a predetermined phase characteristic and having average energy within a predetermined range; and

synchronizing means for synchronizing said data modem receiver by setting said designated time period equal to said first time period.

51. The apparatus of claim 50, further comprising:

verifying means for verifying that said designated time period has been located by inspecting a successive segment for average energy within said predetermined range and symbol separation of 0°

, 90°

, 180°

or 270°

between successive symbols in said first time period.

52. The apparatus of claim 50, wherein said searching means includes means for passing a sliding window across said segment of data and means for determining the average energy of the symbols within said sliding window.

53. The apparatus of claim 50, wherein said master frame comprises 49 frames of data, each frame comprising 4 symbols and wherein said designated time period and said first time period each being equal to the duration of one frame.

54. A modulation-demodulation apparatus having a transmitter and a receiver comprising:

said transmitter including a first and second mapping means,

said first mapping means for grouping main-channel digital data into a plurality of main-channel bit groups and for selecting in response to each of said main-channel bit groups a main-channel complex-valued symbol from a main-channel symbol constellation having M complex-valued symbols so as to define a sequence of said selected main-channel symbols;

said second mapping means for grouping secondary channel digital data into a plurality of secondary-channel bit groups and for selecting in response to each of said secondary-channel bit groups a secondary-channel complex-valued symbol from a secondary-channel constellation having N complex-valued symbols so as to define a sequence of said selected secondary-channel symbols, where M>

N;

said transmitter further including modulated carrier generating means for modulating a carrier signal by one of said selected symbols during each one of a plurality of symbol intervals;

said transmitter further including a first switching means for repeatedly alternating between providing at least one said selected main channel symbol during a main-channel time period having at least one of said symbol intervals and at least one said selected secondary-channel symbol during a secondary channel time period having at least one of said symbol intervals;

said receiver including symbol extracting means for receiving a modulated carrier signal having pairs of alternating carrier signal portions with one of said pair having been modulated by at least one received selected symbol from said main-channel symbol constellation of M complex-valued symbols and the other one of said pair having been modulated by at least one received selected symbol from said secondary channel symbol constellation of N complex-valued symbols, where M>

N;

said symbol extracting means including a second switching means for switching to a main receiver channel to receive said carrier signal portions modulated by received selected symbols from said main channel constellation and to a secondary receiver channel to receive said carrier signal portions modulated by said received selected symbols from said secondary channel constellation, said receiver channels each having signal extracting components suitable for operating on said carrier signal portions switched to said each receiver channel to extract said at least one selected symbol.

55. The modulation-demodulation apparatus according to claim 54, wherein said transmitter further includes means for sending a synchronization signal indicative of the state of said first switching means, said receiver includes means for receiving a received synchronization signal and for using said received synchronization signal for synchronization of said second switching means with said carrier signal portions.

56. The modulation-demodulation apparatus according to claim 55, wherein said receiver further includes synchronization means for detecting loss of said synchronization of said second switch means with said carrier signal portions.

57. The modulation-demodulation apparatus according to claim 56, wherein said synchronization means further includes means for using detection of the occurrence of said carrier signals portions to reestablish said synchronization.

58. The modulation-demodulation apparatus according to claim 57, wherein said first mapping means includes a multidimensional convolutional encoder, said first mapping means is operable to provide frames of a plurality of said selected main-channel symbols with each of said frames defining a multidimensional symbol of greater than two dimensions, said carrier signal portion modulated by at least one of said received selected symbols from said main-channel constellation includes having been modulated by a plurality of said received selected symbols, said plurality of said received selected symbols including at least one received frame of said received selected symbols that define a multidimensional symbol of greater than two dimensions, said synchronization means being operable for using said detection of the occurrence of said carrier signal portions to ascertain beginnings of said frames.

59. A modulation-demodulation apparatus according to claim 58, wherein said first mapping means includes a multiplexer for time multiplexing a plurality of streams of data to form said main-channel digital data, said main-channel digital data defining a plurality of masterframes, said synchronization means being operable for using said detection of the occurrence of said carrier signal portions to ascertain beginnings of said masterframes.

60. A modulation-demodulation apparatus according to claim 56, wherein said synchronization means includes means for ascertaining and comparing detectable characteristics of a signal derived from a part of said modulated carrier signal occurring within each of a sequence of time intervals, each of said time intervals having a fixed duration of at least one of said symbol intervals, and being offset from a previous one of said time intervals by one of said symbol intervals.

61. The modulation-demodulation apparatus according to claim 54, wherein said first switch means repeatedly alternates between providing a plurality of said selected main-channel symbols during said main-channel time period having a first number of said synbol intervals and a plurality of said selected secondary-channel symbols during said secondary-channel time period of a second number of said symbol intervals, wherein said first number of said symbol intervals is greater than said second number of said symbol intervals.

62. The modulation-demodulation apparatus according to claim 61, wherein said plurality of said selected main-channel symbols during each said main-channel time period includes a plurality of frames of said selected main-channel symbols and said plurality of said selected secondary-channel symbols during each said secondary-channel time period includes a single frame of said selected secondary-channel symbols, the number of said selected main-channel symbols in each said frame thereof being equal to the number of said selected secondary-channel symbols in each said frame thereof.

63. The modulation-demodulation apparatus according to claim 62, further including synchronization means for detecting loss of synchronization and for reestablishing synchronization, said synchronization means including means for ascertaining and comparing detectable characteristics of signals derived from said modulated carrier signal occurring within each of a sequence of time intervals, each of said time intervals having a duration equal to that of said frames and being offset from the previous one of said time intervals of said sequence by one of said symbol intervals.

64. The modulation-demodulation apparatus according to claim 63, wherein said detectable charcteristics of said signals comprise average energy levels.

65. The modulation-demodulation apparatus according to claim 54, wherein said signal extracting components for each said receiver channel include a demodulator means for demodulating its respective said carrier signal portions to produce a demodulated signal.

66. The modulation-demodulation apparatus according to claim 65, wherein said signal extracting components for each said receiver channel further include an adaptive equalizer means for producing an equalized signal.

67. The modulation-demodulation apparatus according to claim 65, wherein said demodulator means for said secondary channel updates carrier phase determinations for said main channel when line conditions exceed a threshold level of acceptability and does not use said carrier phase determinations of said main channel when the line conditions do not exceed a threshold level of acceptability.

68. The modulation-demodulation apparatus according to claim 66, wherein said equalizer means updates a plurality of tap coefficient settings when line conditions exceed a threshold level of acceptability and does not update said tap coefficient settings during receipt of main channel data when said line conditions do not exceed said threshold level.

69. A modulation-demodulation apparatus having a transmitter and a receiver, comprising:

a plurality of mapping means, each being operable to receive one of a plurality of sequences of binary data;

each of said mapping means being operable for grouping said binary data allocated to said mapping means into a plurality of bit groups and for selecting in response to each said bit group a complex-valued symbol from a symbol constellation having a number of complex-valued symbols, said number being dependent upon a particular said mapping means;

at least one of said constellations of one of said mapping means having a different said number of said complex-valued symbols than said number of said complex-valued symbols for one of the other said constellation of one of the other said mapping means;

70. said transmitter further including modulated carrier generating means for modulationg a carrier signal by one of said selected symbols during each one of a plurality of symbol intervals;

said transmitter further including a first switching means for sequentially providing during each one of a plurality of repetitive time cycles at least one said selected symbol from each of said mapping means to said modulated carrier generating means;

said receiver including symbol extracting means for receiving a modulated carrier signal transmitted over a transmission medium, said modulated carrier signal having a repetitive cycle of a plurality of carrier signal portions with each of said carrier signal portions having modulated therein at least one selected symbol from one of a plurality of constellations of complex-valued symbols; and

said symbol extracting means including a second switching means for switching between a plurality of receiver channels in response to said carrier signal portions, each of said receiver channels having signal extracting components suitable for operating on said carrier signal portions switched to said each receiver channel to extract said at least one selected symbol.