Technique for determining signal dispersion characteristics in communications systems

• US 4,980,767 A
• Filed: 03/14/1990
• Issued: 12/25/1990
• Est. Priority Date: 11/30/1989
• Status: Expired due to Term

• Current Assignee: American Telephone & Telegraph Company (AT&T, Inc.)
• Original Assignee: AT&T, Inc.
• Inventors: Chao, Tzy-Hong, Wang, Jin-Der, Saltzberg, Burton R.
• Primary Examiner(s): Britton, Howard W.
• Application Number:
  US07/494,941
• Time in Patent Office: 286 Days
• Field of Search: 358/187, 358/905, 358/139, 358/21 V, 375/13, 375/15, 375/39
• US Class Current: 348/613
• CPC Class Codes: H04L 25/03133 with a non-recursive structure H04L25/03127 takes precedence
• CPC Class Codes: H04L 27/2331 wherein the received signal is demodulated using one or more delayed versions of itself
• CPC Class Codes: H04L 27/38 Demodulator circuits; Receiver circuits
• CPC Class Codes: H04N 5/211 Ghost signal cancellation H04N23/81 takes precedence
• CPC Class Codes: H04N 7/015 High-definition television systems

Abstract

The present invention covers the notion of determining signal dispersion characteristics, such as amplitude, phase and delay, by transmitting training sequences. More specifically, each sequence comprises a plurality of predetermined symbols and is transmitted two or more times. Detection of each training sequence is provided by a correlator at the receiver. This detection creates a signal peak for each training sequence at the output of the correlator. Between the peaks, a "quiet zone" is created which permits signal dispersion in this zone to be characterizerd with respect to amplitude, phase and delay. Two different quiet zones are utilized to resolve ambiguities which arise in determining the aforesaid signal dispersion characteristics.

Claims

1. Transmitter apparatus for use in a communications system, said apparatus comprising

means for generating a first training sequence at least two times in succession and for generating a second training sequence at least two times in succession, said first training sequence including a first number of predetermined symbols and said second training sequence including a second number of predetermined symbols, said first and second numbers being different, and

means for transmitting each of said first and second training sequences in associated time intervals.

2. The transmitter apparatus of claim 1 wherein said first and second sequences generated by said generating means are modified pseudorandom sequences in which a constant value is added to each symbol in at least one pseudorandom sequence.

3. The transmitter apparatus of claim 1 wherein said first and second sequences generated by said generating means are polyphase sequences.

4. The transmitter apparatus of claim 1 wherein said generating means generates said first training sequence a first number of times in succession and generates said second training sequence a second number of times in succession, said first and second number of times being different.

5. The transmitter apparatus of claim 1 wherein said transmitting means also transmits information signals in time intervals surrounding said associated time intervals.

6. The transmitter apparatus of claim 1 wherein said generating means generates a substantially zero dc signal in a predetermined time period immediately preceding the successive generation of said first training sequence.

7. The transmitter apparatus of claim 1 wherein said generating means generates a substantially zero dc signal in a predetermined time period immediately succeeding the successive generation of said first training sequence.

8. The transmitter apparatus of claim 1 wherein said generating means generates a substantially zero dc signal in a predetermined time period immediately preceding the successive generation of said second training sequence.

9. The transmitter apparatus of claim 1 wherein said generating means generates a substantially zero dc signal in a predetermined time period immediately succeeding the successive generation of said second training sequence.

10. The transmitter apparatus of claim 1 wherein said generating means generates a predetermined portion of said first training sequence comprising less than said first number in a first time period immediately preceding the successive generation of said first training sequence.

11. The transmitter apparatus of claim 1 wherein said generating means generates a predetermined portion of said first training sequence comprising less than said first number in a time period immediately succeeding the successive generation of said first training sequence.

12. The transmitter of claim 1 wherein said generating means generates a predetermined portion of said second training sequence comprising less than said second number in a first time period immediately succeeding the successive generation of said second training sequence.

13. The transmitter apparatus of claim 1 wherein said transmitting means further includes means for spectrally shaping said first and said second training sequences prior to transmission.

14. The transmitter apparatus of claim 13 wherein the predetermined symbols in said first and second training sequences occur at a predetermined symbol rate and said spectral shaping means includes a tapped delay line clocked at a multiple of said predetermined symbol rate.

15. The transmitter apparatus of claim 1 wherein said first and second sequences generated by said generating means are pseudorandom sequences. sequences.

16. The transmitter apparatus of claim 15 wherein said predetermined symbols in said pseudorandom sequence have equal positive and negative values.

17. The transmitter apparatus of claim 15 wherein said predetermined symbols in said pseudorandom sequence have values which are zero and non-zero.

18. Receiver apparatus for use in a communication system wherein any received signal has propagated through a communications medium in which signal dispersion may be present, said receiver apparatus comprising

means for correlating each of a plurality of received first training sequences with a predetermined set of coefficients and for correlating each of a plurality of received second training sequences with another predetermined set of coefficients, each received first sequence including a first number of predetermined symbols and each received second training sequence including a second number of predetermined symbols, said first number being different from said second number, and

means responsive to the correlated first and second training sequences for determining at least one characteristic of said signal dispersion.

19. The receiver apparatus of claim 18 wherein each of said first and second training sequences is received two times and said correlating means correlates each of said first and second training sequences two times.

20. The receiver apparatus of claim 18 wherein each of said first and second received training sequences are pseudorandom sequences and said correlating means correlates these pseudorandom sequences.

21. The receiver apparatus of claim 18 wherein said determining means determines the amplitude and delay characteristics of said signal dispersion.

22. The receiver apparatus of claim 18 further including a source of at least one carrier signal and said determining means determines the phase characteristic of said signal dispersion and controls said carrier signal source.

23. The receiver apparatus of claim 18 further including an equalizer and said determining means adjusts the operation of said equalizer.

24. The receiver apparatus of claim 18 further including an automatic gain control circuit through which each of said plurality of received first and second training sequences are coupled and said determining means includes means for controlling the gain of said automatic gain control circuit.

25. The apparatus of claim 18 wherein the predetermined symbols in said first and said second training sequences occur at a predetermined symbol rate and said correlating means provides correlation using a clock signal which is n times said predetermined symbol rate, where n is an integer≧

1.

26. The apparatus of claim 18 wherein said predetermined set and said another predetermined set of coefficients are binary numbers.

27. The apparatus of claim 26 wherein the binary numbers have equal positive and negative values.

28. The apparatus of claim 26 wherein the binary numbers have values which are zero and non-zero.

29. A method of determining at least one characteristic of signal dispersion in a communications system, said method comprising the steps of

generating a first training sequence at least two times in succession and generating a second training sequence at least two times in succession, said first training sequence including a first number of predetermined symbols and said second training sequence including a second number of predetermined symbols, said first and second numbers being different,

transmitting each of said first and second training sequences in associated time intervals,

correlating each received first training sequence with a predetermined set of coefficients and for correlating each received second training sequence with another set of predetermined set of coefficients, and

determining at least one characteristic of said signal dispersion in response to the correlated first and second training sequences.

30. A communications system comprising a transmitter and a receiver which are separated by a communications medium in which signal dispersion may be present, said transmitter comprising

means for generating a first training sequence at least two times in succession and for generating a second training sequence at least two times in succession, said first training sequence including a first number of predetermined symbols and said second training sequence including a second number of predetermined symbols, said first and second numbers being different, and

means for transmitting each of said first and second training sequences in associated time intervals,

and said receiver comprising

means for correlating each received first training sequence with a predetermined set of coefficients and for correlating each received second training sequence with another set of predetermined set of coefficients, and

means responsive to the correlated first and second training sequences for determining at least one characteristic of said signal dispersion.

31. Apparatus for use in a television signal transmitter wherein the transmitted television signal incorporates a vertical blanking interval in a video frame, said apparatus comprising

means for generating a first training sequence at least two times in succession and for generating a second training sequence at least two times in succession, said first training sequence including a first number of predetermined symbols and said second training sequence including a second number of predetermined symbols, said first and second numbers being different, and

means for transmitting said successively generated first and second training sequences in at least one vertical blanking interval.

32. The transmitter apparatus of claim 31 wherein said transmitting means also transmits video and audio signals.

33. The transmitter apparatus of claim 31 wherein said transmitting means transmits said video and audio signals in a high definition television signal format.

34. The transmitter apparatus of claim 31 wherein said transmitting means further includes means for spectrally shaping said first and said second training sequences prior to transmission.

35. The transmitter apparatus of claim 34 wherein the predetermined symbols in said first and said second training sequences occur at a predetermined symbol rate and said spectral shaping means includes a tapped delay line clocked at a multiple of said predetermined symbol rate.

36. The transmitter apparatus of claim 34 wherein said spectral shaping means provides raised cosine spectral shaping.

37. Apparatus for use in a television receiver wherein the transmitted television signal incorporates a vertical blanking interval and wherein the transmitted television signal propagates through a communications medium in which signal dispersion may be present, said receiver apparatus comprising

means for correlating each of a plurality of received first training sequences with a predetermined set of coefficients and for correlating each of a plurality of received second training sequences with another predetermined set of coefficients, each first training sequence including a first number of predetermined symbols and each second training sequence including a second number of predetermined symbols, said first number being different from said second number, and said first and second training sequences being received during at least one of said vertical blanking intervals, and

means responsive to the correlated first and second training sequences for determining at least one characteristic of said signal dispersion.

38. The receiver apparatus of claim 37 wherein said determining means determines the amplitude, delay and phase characteristics of said signal dispersion.

39. The transmitter apparatus of claim 37 wherein said spectral shaping means provides raised cosine spectral shaping.

40. The apparatus of claim 37 wherein the predetermined symbols in said first and said second training sequences occur at a predetermined symbol rate and said correlating means provides correlation using a clock signal which is n times said predetermined symbol rate, where n is an integer ≧

1.