Indicating and detecting the start of signal transmission employing frequency division multiplexing

1. A method of transmitting information on a transmission line comprising:

configuring and operating at least one digital signal processor to transmit information on a transmission line as components of a burst of carriers of different frequencies,

wherein the burst is preceded by a start symbol signal in predetermined timing relationship with the burst and said start symbol signal comprises a cyclically time-varying signal envelope-modulated with a substantially Gaussian waveform.

2. A method according to claim 1 in which the burst is a set of OFDM signals.

3. A method according to claim 1 in which the cyclically time-varying signal is a single sinusoid.

4. A method of detecting, in a signal received from a transmission line, a burst of information-bearing carrier signals of different frequencies, wherein the burst is preceded in predetermined temporal relationship by a start symbol comprising plural cycles of a cyclically time-varying signal envelope-modulated with a substantially Gaussian waveform, said method comprising:

repetitively sampling said received signal from a transmission line including a burst of information-bearing carrier signals of different frequencies to provide a succession of samples;

computing for successive intervals, each interval corresponding to an integral number of cycles of the cyclically time-varying signal, power in said received signal; and

determining if the computed power for a particular interval is above a threshold and is less than the computed power for an immediately previous interval.

5. A method according to claim 4 in which integral the number of cycles is unity.

6. A method according to claim 4 in which the burst is a set of OFDM signals.

7. A method according to claim 4 in which the cyclically time-varying signal is a single sinusoid.

8. A modem for use with a subsea transmission line, said modem comprising:

input and output circuits coupled to a digital signal processor organized to respond to a succession of segments of a data signal to (a) produce a respective set of orthogonal frequency division multiplexed signals, (b) transmit those signals as a burst on the subsea transmission line, and (c) prepend the burst with a start signal comprising a cyclically time-varying signal envelope modulated with a Gaussian waveform.

9. A modem for detecting in a signal received from a transmission tine a burst of information-bearing carrier signals of different frequencies, in which the burst is preceded in predetermined temporal relationship by a start symbol comprising plurality of cycles of a cyclically time-varying signal envelope-modulated with a substantially Gaussian waveform, the modem comprising:

means for repetitively sampling said received signal from a transmission line to provide a succession of samples;

means for computing for successive intervals, each interval corresponding to an integral number of cycles of the cyclically time-varying signal, power in said received signal; and

means for determining if the computed power for a particular interval is above a threshold and is less than the computed power for an immediately previous interval.

10. A modem according to claim 9 in which the integral number of cycles is unity.

11. A modem according to claim 10 in which the means for computing and determining is a digital signal processor.

12. A modem according to claim 8 , further comprising a digital signal processor and circuit controlled thereby configured to:

(a) repetitively sample a received signal from a transmission line to provide a succession of samples;

(b) compute for successive intervals, each interval corresponding to an integral number of cycles of the cyclically time-varying signal, the power in said received signal; and

(c) determine if the computed power for a particular interval is above a threshold and is less than the computed power for an immediately previous interval.

13. A method of detecting according to claim 4 , in combination with a method for coding information to be transmitted on a transmission line as components of a burst of carriers of different frequencies, wherein the burst is preceded by a start symbol in predetermined timing relationship with the burst and comprising a cyclically time-varying signal envelope-modulated with a substantially Gaussian waveform.

14. A method of communicating information on a subsea transmission line, said method comprising:

generating an information-bearing signal including components of a burst of carriers of different frequencies;

generating a start symbol preceding said burst and in predetermined timing relationship with the burst, said start symbol including a cyclically time-varying signal envelope-modulated with a substantially Gaussian waveform;

transmitting said information-bearing signal preceded by said start symbol on a subsea transmission line;

receiving said cyclically time-varying signal and said information-bearing signal from said subsea transmission line; and

determining the occurrence of a maximum value of said cyclically time-varying signal to provide a datum for decoding said burst.

15. The method of claim 14 , wherein the determining step comprises:

computing for successive intervals, each interval corresponding to an integral number of cycles of said cyclically time-varying signal, power in said cyclically time-varying signal; and

determining if the computed power for a particular one of said successive intervals is above a threshold and is less than the computed power for an immediately previous one of said successive intervals.