Spread spectrum detector

• US 4,122,393 A
• Filed: 05/11/1977
• Issued: 10/24/1978
• Est. Priority Date: 05/11/1977
• Status: Expired due to Term

• Current Assignee: E-Systems Incorporated (Rtx Corporation)
• Original Assignee: NCR Corporation (NCR Voyix Corporation)
• Inventors: Sanders, David E., Chambers, Ramon P., Gordy, Robert S.
• Primary Examiner(s): Griffin, Robert L.
• Assistant Examiner(s): Masinick, Michael A.
• Application Number:
  US05/795,697
• Time in Patent Office: 531 Days
• Field of Search: 325/321, 325/323, 325/324, 325/42, 325/65, 325/474, 329/110, 329/50, 328/133
• US Class Current: 375/343
• CPC Class Codes: H04B 1/70712 with demodulation by means of convolvers, e.g. of the SAW type SAW convolvers in general G06G7/195
• CPC Class Codes: H04B 1/709 Correlator structure
• CPC Class Codes: H04B 7/216 Code division or spread-spectrum multiple access [CDMA, SSMA]
• CPC Class Codes: H04L 7/043 Pseudo-noise [PN] codes variable during transmission synchronisation of spread spectrum receivers H04B1/69

Abstract

The detector of the present invention includes a first and a second signal channel each of which is adapted to receive a spread spectrum signal, or any other similar type multi-modulated carrier signal. In the preferred embodiment of the invention, the first signal channel is provided with a correlator for partially demodulating the received signal, as to one modulation signal component, with a demodulation signal derived from the second signal channel. The first signal channel is also provided with an additional correlator for demodulating the partially demodulated signal as to another modulation component. The second signal channel is comprised of a means for generating a dithered demodulation signal which signal is driven into synchronization with a corresponding modulation signal component in the received signal so as to enable the accurate generation of the demodulation signal utilized by the first signal channel.

Claims

1. A detector for use in a communication receiver in which a received coded signal is to be detected utilizing a locally generated similarly coded signal, comprising:

a first channel and a second channel;

means for applying said received coded signal to said first channel and to said second channel;

first correlation means included in said first channel for correlating said received coded signal with a generated coded signal received from said second channel to provide a detected received coded signal;

means responsive to said detected received coded signal for providing a lock indication signal when the degree of synchronization between said generated coded signal and said received coded signal is within a desired range;

local means included in said second channel for generating a coded signal and a dithered coded signal in response to a control signal;

second correlation means adapted to receive said received coded signal and said generated dithered coded signal, said second correlation means providing a signal indicative of the degree of synchronization between said received coded signal and said generated dithered coded signal; and

means responsive to said provided signal from said second correlation means for generating said control signal and for providing said control signal to said local means so as to drive said dithered coded signal into synchronization with said received coded signal.

2. The detector in accordance with claim 1 wherein said local means is a pseudo-random signal generator for generating a pseudo-random signal which corresponds to a pseudo-random signal component in the received coded signal.

3. A detector according to claim 1 wherein said local means is comprised of:

a signal generator responsive to said control signal for generating a local coded signal the phase of which is a function of said control signal;

a dither modulator receiving as one input thereof said local coded signal, said modulator comprised of, a first and a second serially connected delay means, each of said delay means providing a signal delay to the signal present at its inputs, which delay corresponds to a fixed time interval of said local coded signal; and

switch means responsive to a periodic signal for alternately connecting the output of said second delay means and the input to said first delay means to an input of said second correlator means, the output from said first delay means being applied to an input of said first correlator means.

4. A detector in accordance with claim 1 wherein said second channel is further comprised of:

lock detection means operatively connected to said second correlation means for providing a signal indicating the state of synchronization between the signals received by said second correlation means; and

switch means responsive to said provided signal for operatively connecting said local means to a search signal for driving said dithered coded signal to search for synchronization with said received coded signal when said provided signal indicates a lack of synchronization, and for operatively connecting said local means to receive said control signal when said provided signal indicates the presence of a selected degree of synchronization between the received coded signal and the dithered coded signal.

5. A detector in accordance with claim 4 wherein said lock detection means is comprised of:

a first filter means for filtering the signal from said second correlation means;

rectifying means for rectifying the filtered signal from said first filter means;

threshold detector means for providing a first output signal when said rectified signal is above a fixed threshold and a second output signal when said rectified signal is below said fixed threshold; and

means for converting said first and said second output signals into said provided signal and for applying said provided signal to said switch means.

6. A detector in accordance with claim 1 and further comprising:

a demodulator means operatively connected to receive the signal provided by said second correlation means, said demodulator means having first and second outputs and providing at said first output a signal indicative of the degree of dither of said dithered coded signal in a first direction and at said second output a signal indicative of the degree of dither in a second direction; and

summer means operatively connected to said first and said second outputs for providing said control signal, said control signal being proportional to the degree of difference between the signals at said first and said second output of said demodulator means.

7. A detector in accordance with claim 6 and further comprising:

envelope detector means interposed in the signal path between said second correlation means and said demodulator means for amplitude detecting the signal present at its input and for providing at its output a signal which is a function of the detected amplitude.

8. A detector in accordance with claim 7 and further comprising:

filter means interposed in the signal path between said second correlation means and said envelope detector means for filtering out all signal components outside of a desired intermediate frequency range.

9. In a receiver for a spread spectrum coded signal which signal is comprised of a carrier signal that has been phase modulated by a data signal and by a pseudo-random sequence signal, a detector comprising:

a first and a second signal channel;

means for receiving and applying said received spread spectrum coded signal to said first channel and to said second channel;

first correlation means included in said first channel for correlating the pseudo-random sequence component of said received signal with a pseudo-random signal received from said second signal channel so as to provide a partially detected received signal;

carrier demodulation means responsive to said partially detected received signal for demodulating the carrier signal component from said partially detected signal so as to provide a detected received signal;

local means included in said second channel for generating a pseudo-random signal and a dithered pseudo-random coded signal the phasing of which is a function of a control signal;

second correlation means adapted to receive said spread spectrum coded signal and said generated dithered pseudo-random coded signal for providing an error signal indicative of the degree of synchronization between said signals; and

means operatively connected to said second correlation means for providing said control signal to said local means as a function of said error signal so as to drive the phasing of said generated dithered pseudo-random coded signal to a minimum.

10. A detector in accordance with claim 9 wherein said local means is comprised of:

a signal generator responsive to said control signal for generating a pseudo-random coded signal the phase of which is a function of said control signal;

a dither modulator receiving as one input thereof said local coded signal, said modulator comprised of a first and a second serially connected delay means, each of said delay means providing a signal delay to the signal present at its inputs, which delay corresponds to a fixed time interval of said pseudo-random coded signal; and

switch means responsive to a periodic signal for alternately connecting the output of said second delay means and the input to said first delay means to an input of said second correlator means, the output from said first delay means being applied to an input of said first correlator means.

11. The detector in accordance with claim 9 wherein said carrier demodulation means is comprised of:

a third correlator having one input connected to the output of said first correlator and another input connected to receive a locally generated carrier signal so as to provide at an output a detected received signal; and

a carrier recovery loop having an input connected to the output of said first correlator for generating and providing to said third correlator a local carrier signal in synchronism with the carrier signal component of said received spread spectrum coded signal and for also providing an indication signal indicating synchronization or the lack of synchronization between said carrier signal component and said generated local carrier signal.

12. The detector in accordance with claim 11 and further comprising:

data rate detection means operatively connected to said third correlator means for providing a rate signal indicative of the rate of the data signal component of said received signal, said rate signal being directed to said local means for controlling the rate of said dithered pseudo-random coded signal.

13. A detector in accordance with claim 11 wherein said second channel is further comprised of:

lock detection means operatively connected to said second correlation means for providing a signal indicating the state of synchronization between the signals received by said second correlation means; and

switch means responsive to said provided signal for operatively connecting said local means to receive said control signal when said provided signal indicates the presence of a selected degree of synchronization between the pseudo-random sequence component of said received signal and said dithered pseudo-random coded signal.

14. A detector in accordance with claim 13 wherein said lock detection means is comprised of:

a first filter means for filtering the signal from said second correlation means;

rectifying means for rectifying the filtered signal from said first filter means;

threshold detector means for providing a first output signal when said rectified signal is above a fixed threshold and a second output signal when said rectified signal is below said fixed threshold; and

means for connecting said first and said second output signals into said provided signal and for applying said provided signal to said switch means.

15. A detector in accordance with claim 9 and further comprising:

a demodulator means operatively connected to receive the error signal provided by said second correlation means, said demodulator means having first and second outputs and providing at said first output a signal indicative of the degree of dither of said dithered pseudo-random coded signal in a first direction and at said second output a signal indicative of the degree of dither of said dithered pseudo-random coded in a second direction; and

summer means operatively connected to said first and said second outputs for providing said control signal, said control signal being proportional to the degree of difference between the signal at said first and said second output of said demodulator means.

16. A detector in accordance with claim 15 and further comprising:

envelope detector means interposed in the signal path between said second correlation means and said demodulator means for amplitude detecting the signal present at its input and for providing at its output a signal which is a function of the detected amplitude.

17. A detector in accordance with claim 16 and further comprising:

filter means interposed in the signal path between said second correlation means and said envelope detector means for filtering out all signal components outside of a desired intermediate frequency range.