Spread spectrum correlation receiver

• US 4,653,069 A
• Filed: 11/06/1975
• Issued: 03/24/1987
• Est. Priority Date: 11/06/1975
• Status: Expired due to Term

• Current Assignee: Lockheed Martin Corporation (Martin Marietta Corporation)
• Original Assignee: General Electric Company
• Inventors: Roeder, Allan W.
• Primary Examiner(s): Cangialosi, Salvatore
• Application Number:
  US05/629,248
• Time in Patent Office: 4,156 Days
• Field of Search: 325/30, 325/42, 325/65, 325/320, 325/321, 325/323, 325/324, 325/473, 325/474, 325/476, 375/1, 375/2.1, 375/2.2
• US Class Current: 380/31
• CPC Class Codes: H04B 1/70752 Partial correlation
• CPC Class Codes: H04B 1/70775 Multi-dwell schemes, i.e. multiple accumulation times
• CPC Class Codes: H04B 1/709 Correlator structure

Abstract

A spread spectrum communication system having a correlation receiver for decoding an information signal transmitted on a pseudo-noise (PN) carrier. The receiver synchronizes to the transmitted signal by performing a continuous sequence of correlations until a correlation output exceeding a predetermined threshold level is detected. Thereafter, the receiver performs a plurality of correlations during a sampling interval which is timed to occur at approximately the time when high subsequent correlation output signals are likely to appear. After enhancement through adaptive filter processing, the correlation outputs generated during each sampling interval are accumulated (integrated and the summed output present at the end of the sampling interval is representative of the transmitted data message (a binary bit). The adaptive filter develops and stores weighting values representing the expected signal strength of the correlation outputs and the actual correlation outputs are multiplied by the weighting values. Received signal energy representing atmospheric and specular multipath signal components are detected and channeled to the accumulator along with the main signal component. The accumulator thus realigns the multipath signals with the main signal, resulting in a significant increase in processing gain. An audio version of the system is also disclosed.

Claims

1. In a spread spectrum communication system, a receiver for decoding a binary data signal transmitted on a pseudo-noise (PN) coded carrier, comprising, in combination:

correlation means for performing an N-point correlation of said data signal against a reference signal representing said PN coded carrier by simultaneously comparing the amplitude of N samples of said data signal with N samples of said reference signal;

means for controlling said correlation means to serially perform a plurality of said correlations during each of a series of sampling intervals when a high correlation output is likely to occur;

an adaptive filter for processing the correlation outputs generated during each of said sampling intervals, said filter including first signal storage means, second signal storage means for storing the correlation outputs from the last preceding sampling interval, means for combining the signals stored in said first and second signal storage means to derive a control signal representing a smoothed correlation function in accordance with the expression F_i =F_i-1 +d_i-1 K(C_i-1 -d_i-1 F_i-1) where F_i represents the smoothed correlation function and F_i-1, C_i-1 and d_i-1 represent, respectively, the smoothed correlation function, correlation output and decoded binary data output derived during the preceding sampling interval, means for multiplying said correlation outputs by said control signal to produce a filtered correlation output signal, and means for transferring said control signal to said first signal storage means;

means for integrating said filtered correlation output signal over the period of said sampling interval; and

detection means for generating a decoded binary output signal in accordance with the value of said integrated output signal.

2. The system set forth in claim 1 wherein said means for combining the signals stored in said first and second signal storage means comprises:

a first adder-subtractor connected to receive the signals from said first and second storage means and operable to generate a first sum/difference output signal;

a scaling circuit for multiplying said first sum/difference output signal by a scaling constant;

a second adder-subtractor connected to receive the signal from said first storage means and from said scaling circuit and operable to generate a second sum/difference output signal representing said control signal; and

means for selectively switching said first adder-subtractor into a predetermined add/subtract state and for switching said second adder/subtractor into the opposite state in response to the output from said detection means.

3. The system set forth in claim 2 further comprising a deadband circuit connected to the output of said second adder-subtractor for passing said control signal to said multiplying means when said control signal exceeds a predetermined threshold level and for passing a zero level signal to said multiplying means when said control signal does not exceed said threshold level.

4. A system for correlating an input signal against an N-bit reference signal, said system comprising, in combination:

N correlation circuits, each said circuit having first storage means for storing a sample of said input signal, second storage means for storing a bit of said reference signal and means for providing a correlation output, said second storage means being serially interconnected so that said correlation circuits are arranged in an N-stage sequence;

scan means for serially loading samples of said input signal into said first storage means, said scan means operating to load said storage means in order according to said N-stage sequence;

loading means for entering the bits of said reference signal into said second storage means; and

control means for serially shifting the bits of said reference signal through said second storage means in synchronism with the loading of said input signal samples, the sequence of said reference signal bits being arranged such that the last bit is kept in alignment with the newest sample of said input signal and the first bit is kept in alignment with the oldest stored sample of said input signal.

5. In a spread spectrum communication system including a plurality of transmitters operating to transmit independent audio signals impressed on a pseudo-noise (PN) carrier signal, a receiver comprising in combination:

correlation means for receiving a composite of said audio signals and for performing a continuous sequence of N-point correlations on said composite signal against a reference signal representing N bits of said PN carrier signal;

accumulation means for storing the sum of the outputs generated by said correlation means;

control means for resetting the level stored in said accumulation means to a predetermined reference value after each N correlation operations; and

output means operable prior to each said resetting operation for feeding the output of said accumulation means into an audio output channel to generate an audio signal representing a composite of said transmitted signals.

6. In an rf communication system, a receiver which provides detection of all multipath components of a signal transmitted in rf form on a pseudo-noise (PN) carrier, said detection being provided independent of phase differences that exist between said components, said receiver comprising:

synchronous demodulation means for converting the received rf signal into I and Q signal components representing baseband demodulations of said signal at orthogonally related phase-detection angles;

I correlation means for performing an N-point correlation of said I signal component against a reference signal representing said PN coded carrier;

Q correlation means for performing an N-point correlation of said Q signal component against a reference signal representing said PN coded carrier;

means for controlling said I and Q correlation means to serially perform a plurality of said correlations during a sample interval when a high correlation output is likely to occur;

means for accumulating the outputs from said correlations;

means for generating a decoded output signal in accordance with the sum of said accumulated correlation outputs; and

a pair of adaptive filter circuits for processing the outputs from said I and Q correlations prior to the presentation thereof to said accumulating means, each said filter circuit including means for multiplying its respective correlation output by a signal function representing an estimation of said correlation output over the duration of said sampling interval.

7. In a communication system, a receiver for decoding an information signal transmitted on a psuedo-noise (PN) coded carrier comprising, in combination:

correlation means for performing an N-point correlation of said received signal against a reference signal representing said PN coded carrier by simultaneously comparing the amplitude of N samples of said received signal with N samples of said reference signal;

means for controlling said correlation means to serially perform a plurality of said correlations during a sampling interval when a high correlation output is likely to occur;

means for accumulating the output from said correlations;

means for generating a decoded output signal in accordance with the sum of said accumulated correlation outputs; and

an adaptive filter for processing said correlation outputs prior to the presentation thereof to said accumulating means, said filter comprising means for developing a smoothed correlation function including first signal storage means, second signal storage means for storing the correlation outputs from the last preceding sampling interval, means for combining the signals stored in said first and second signal storage means in accordance with a predetermined filtering function to derive a control signal representing said smoothed correlation function, means for multiplying said correlation outputs by said control signal, and means for transferring the latter for said first signal storage means.

8. In a communication system, a receiver for decoding an information signal transmitted on a psuedo-noise (PN) coded carrier compring, in combination:

correlation means for performing an N-point correlation of said received signal against a reference signal representing said PN coded carrier by simultaneously comparing the amplitude of N samples of said received signal with N samples of said reference signal;

means for controlling said correlation means to serially perform a plurality of said correlation during a sampling interval when a high correlation output is likely to occur, said controlling means further includes synchronization means for controlling said correlation means to perform a continuous sequence of said N-point correlations;

cycle timing means; and

means responsive to a correlation output of a predetermined level generated during said continuous sequence of correlations for causing said cycle timing means to generate a series of clock signals for timing said sampling interval;

means for accumulating the outputs from said correlations; and

means for generating a decoded output signal in accordance with the sum of said accumulated correlation outputs.