COHERENT DETECTION WITHOUT TRANSMISSION PREAMBLE

• US 20080095215A1
• Filed: 10/22/2007
• Published: 04/24/2008
• Est. Priority Date: 02/28/2000
• Status: Active Grant

• Current Assignee: Comtech Mobile Datacom Corporation (Comtech Telecommunications Corporation)
• Original Assignee: AeroAstro, Inc.
• Inventors: Jordan, Luis, Stafford, James, McDermott, Scott
• Granted Patent: US 7,596,170 B2
• Time in Patent Office: Days
• US Class Current: 375/146
• CPC Class Codes: H04B 1/70735 Code identification H04B1/7083 takes precedence
• CPC Class Codes: H04B 1/70752 Partial correlation
• CPC Class Codes: H04B 2001/70935 using a bank of matched fileters, e.g. Fast Hadamard Transform

Abstract

A receiving system allows for the coherent detection of a spread-spectrum transmission at any point in time during the transmission, thereby avoiding the need to identify the start of the transmission during the transmission-detection process. An input buffer captures the transmissions on a communication channel using a moving time-window. A detector processes a time-slice from the input buffer and identifies all of the simultaneously transmitting transmitters during that time-slice. As each transmitter is identified, the demodulator traces back-in-time to identify where the message can first be detected in the input buffer. The transmission includes suitable characteristics to facilitate detection and demodulation of the message content, but need not contain a preamble to allow the detection process.

Claims

1. A receiving system comprising:

a receiver that is configured to receive a composite signal from transmitters that provide transmitted signals at different code-phases of a spreading code;

a receive buffer that is configured to store the composite signal received during at least a buffer time interval;

a phase detector that is configured to;

receive a segment of the composite signal received during a sampling time interval within the buffer time interval,

applying a plurality of samples of the segment of the composite signal to a Fast M-Series transform that is configured based on the spreading code, and

identify one or more active code-phases corresponding to one or more of the transmitted signals based on outputs of the M-Series transform; and

a demodulator that is configured to;

search the receive buffer for a start time of each transmitted signal within the buffer time interval based on energy content within the composite signal at the corresponding active code-phase; and

demodulate each transmitted signal from the corresponding start time based on the corresponding active code-phase.

2. The receiving system of claim 1, wherein

the phase detector identifies the active code-phase independent of data content of the corresponding transmitted signal.

3. The receiving system of claim 1, wherein

the sampling time interval is substantially smaller than the buffer time interval.

4. The receiving system of claim 1, wherein

the receive buffer is configured to store the composited signal received during a subsequent buffer time interval,

the phase detector is configured to identify subsequent active code-phases based on subsequent samples from a subsequent segment of the composite signal, and

the demodulator is configured to demodulate subsequent transmitted signals and associate the demodulated subsequent transmitted signals to one or more prior demodulated transmitted signals having a common code-phase.

5. The receiving system of claim 1, including:

a plurality of queues that are configured to receive demodulated data from the demodulator, each queue being associated with an active code-phase; and

a queue controller that is configured to;

receive identification of the active code-phases and corresponding start times from the demodulator,

allocate a new queue to each identified code-phase having a start time that is substantially different from a start of the buffer time interval, and

deallocate each prior queue having an associated code-phase that does not correspond to an identified active code-phase.

6. The receiving system of claim 1, wherein

the demodulator is configured to

search the receive buffer for an end time of each transmitted signal within the buffer time interval based on energy content within the composite signal at the corresponding active code-phase, and

terminate demodulation of each transmitted signal at the corresponding end time.

7. The receiving system of claim 1, wherein

each transmitted signal includes a plurality of information bits, and

a duration of each information bit is at least as long as an epoch of the spreading code.

8. The receiving system of claim 1, wherein

each transmitted signal includes a plurality of information bits, and

the information bits are synchronous with the spreading code so as to reduce transitions of the information bits during an epoch of the spreading code.

9. The receiving system of claim 1, wherein

the phase detector includes a transition detector that is configured to identify transitions of information bits in the transmitted signal during an epoch of the spreading code, and

the demodulator is configured to demodulate the transmitted signal based on the identified transitions.

10. A receiving system comprising:

a receiver that is configured to receive a composite signal from a plurality of transmitters that provide transmitted signals of information bits that are encoded at different code-phases of a spreading code relative to the receiving system;

a receive buffer that is configured to store the composite signal received during at least a buffer time interval;

a phase detector that is configured to identify one or more active code-phases corresponding to one or more of the transmitted signals; and

a demodulator that is configured to demodulate each transmitted signal from the corresponding start time based on the corresponding active code-phase, without reliance on a sequence corresponding to predefined information bits in the transmitted signals.

11. The receiving system of claim 10, wherein

the information bits are encoded using a polarity-independent encoding.

12. The receiving system of claim 10, wherein

an error detection code is used to facilitate determination of a polarity of the information bits.

13. The receiving system of claim 10, wherein

the phase detector is configured to use at least one of an M-Sequence transform and a discrete Fourier transform to facilitate identification of the active code-phases.

14. A transmitter comprising:

a transducer that is configured to provide information bits,

a modulator that is configured to modulate the information bits based on a spreading code to provide a spread spectrum signal, and

a transmitter that is configured to independently and autonomously transmit the spread spectrum signal,

wherein

the transmitted spread spectrum signal does not include a sequence of bit values that are known to an intended receiver of the transmitted spread spectrum signal.

15. The transmitter of claim 14, wherein

a duration of each information bit is at least as long as an epoch of the spreading code.

16. The transmitter of claim 14, wherein

a duration of each information bit is substantially shorter than an epoch of the spreading code.

17. The transmitter of claim 14, wherein

each information bit is synchronous with the spreading code such that transitions of the information bits occur at one or more known code phases.

18. A transmitter comprising:

a transducer that is configured to provide information bits,

a modulator that is configured to modulate the information bits based on a spreading code to provide a spread spectrum signal, and

a transmitter that is configured to independently and autonomously transmit the spread spectrum signal,

wherein

the spread spectrum signal is configured to include multiple information bits per epoch of the spreading code, and

transitions of the information bit occur at predefined code-phases of the spreading code.

19. The transmitter of claim 18, wherein

the information bits are encoded using a polarity-independent encoding.

20. A method comprising:

storing a received signal in a buffer,

determining an active code phase of a transmitted signal within the received signal based on an M-Transform of a sample of the received signal taken at a substantially random time after a start of the transmitted signal,

determining a start of the transmitted signal in the buffer based on the active code phase, and

demodulating the transmitted signal from the start of the transmitted signal to an end of the transmitted signal in the buffer.

21. The method of claim 20, wherein

the demodulating is performed without reliance on a predefined sequence of information bits encoded within the transmitted signal.

22. The method of claim 20, wherein

the demodulating includes demodulating a polarity-independent encoding of the information bits.

23. The method of claim 20, including

determining an error measure corresponding to the demodulating, and

the demodulating includes determining a polarity of the information bits based on the error measure.

24. The method of claim 20, wherein

the received signal includes a plurality of transmitted signals,

the M-Transform of the system identifies an active code phase corresponding to each of the plurality of transmitted signals, and

the demodulating includes demodulating each of the plurality of transmitted signals based on the corresponding active code phase.

25. The method of claim 24, including

maintaining a plurality of queues for storing results of the demodulating of the plurality of transmitted signals.