Simplified transversal correlator for MSK and MSK related waveforms

• US 4,392,232 A
• Filed: 09/28/1981
• Issued: 07/05/1983
• Est. Priority Date: 09/28/1981
• Status: Expired due to Fees

• Current Assignee: E-Systems LLC
• Original Assignee: B-Systems Incorporated
• Inventors: Mosley, William H. Jr., Andren, Carl F.
• Primary Examiner(s): Safourek, Benedict V.
• Application Number:
  US06/306,490
• Time in Patent Office: 645 Days
• Field of Search: 375/39, 375/53, 375/54, 375/77, 375/86, 375/96, 375/1, 375/2.1, 375/2.2, 375/57, 375/58, 364/486, 364/728, 329/112, 329/124, 370/12, 370/18
• US Class Current: 375/332
• CPC Class Codes: H04L 27/227 using coherent demodulation
• CPC Class Codes: H04L 27/2332 using a non-coherent carrier

Abstract

A transversal correlator is disclosed for demodulating a phase shift keyed (PSK) signal. The PSK signal is mixed with a local oscillator signal to produce quadrature I-channel and Q-channel signals. The I-channel and Q-channel signals are sampled at a periodic rate and the samples are propagated through a shift register for each channel. A preselected pseudo noise signal is stored in a local register which has an output tap for each stage. Each of the analog shift registers has a stage corresponding to one of the stages of the local register. Each of the I-channel and Q-channel samples is multiplied by the binary state in the corresponding stage of the local register. This multiplying operation produces a plurality of product signals. Product signals for alternate samples of the I-channel signal are summed with alternate but offset samples of the Q-channel signal to produce a first summation signal. The remaining alternate samples of the I-channel and Q-channel signals are also summed to produce a second summation signal. The first and second summation signals are full wave rectified then summed to produce a correlation output signal.

Claims

1. A transversal correlator for demodulating a phase shift keyed (PSK) signal, comprising:

means for producing quadrature I-channel and Q-channel signals derived from said PSK signal;

means for sampling said I-channel and said Q-channel signals to produce sequential samples;

means for storing a plurality of said samples for each of said I and Q-channel signals;

means for storing a plurality of bits comprising a selected digital code sequence;

means for correlating said digital code sequence with alternate samples of said I and Q-channel signals in said means for storing to produce a first summation signal;

means for correlating said digital sequence with the remaining alternate samples in said means for storing to produce a second summation signal; and

means for combining said first and said second summation signals to produce a correlation output signal.

2. The transversal correlator recited in claim 1 wherein said means for sampling produces analog samples and said means for storing comprises an analog shift register for storing said analog samples.

3. The transversal correlator recited in claim 1 wherein said means for sampling produces digital samples and said means for storing comprises a plurality of serially connected latches for storing said digital samples.

4. A transversal correlator for demodulating an alternate phase shift keyed (PSK) signal, comprising:

means for mixing the PSK signal with a first reference signal to produce an I-channel signal;

means for mixing the PSK signal with a second reference signal, which is phase shifted from said first reference signal, to produce a Q-channel signal;

a first analog shift register connected to receive said I-channel signal and shift sequential samples of said I-channel signal through the cells thereof;

a second analog shift register connected to receive said Q-channel signal and shift sequential samples of said Q-channel signal through, the cells thereof;

means for producing a pseudorandem noise (PN) digital sequence;

a local register for storing at least a part of said PN digital sequence, said local register having cells, each of which corresponds to a cell of said first analog shift register and a cell of said second analog shift register;

means for multiplying the analog samples in each cell of said analog shift register by the digital quantity in the corresponding cell of said local register to produce a product quantity for each of the cells of the analog shift registers;

means for summing the product quantities corresponding to alternate cells of each of said first and second analog shift register to produce a first summation signal;

means for summing the product quantities corresponding to the remaining alternate cells of each of said first and second analog shift registers to produce a second summation signal; and

means for summing said first and said second summation signals to produce a correlation output signal.

5. The transversal correlator recited in claim 4 wherein the even cells of said first analog shift register are summed with the odd cells of said second analog shift register to produce said first summation signal and the odd cells of said first analog shift register are summed with the even cells of said second analog shift register to produce said second summation signal.

6. The transversal correlator recited in claim 4 including a first full wave rectifier connected to receive and rectify said first summation signal and supply the rectified first summation signal to said means for summing said first and second summation signals and a second full wave rectifier connected to receive and rectify said second summation signal and supply the rectified second summation signal to said means for summing said first and second summation signals.

7. The transversal correlator recited in claim 4 wherein said means for multiplying comprises a mixer for each cell of each of said analog shift registers, each mixer having the sample in the corresponding cell of the analog shift register as a first input and the digital state in the corresponding cell of the local register as the second input and producing said product quantity at the output thereof.

8. A method for demodulating a phase shift keyed (PSK) signal, comprising the steps of:

producing quadrature I-channel and Q-channel signals derived from said PSK signal;

sampling each of said I-channel and Q-channel signals to produce sequential samples;

storing a plurality of said samples for said I-channel and said Q-channel signals;

storing a selected digital code sequence comprising a series of bits;

correlating said digital code sequence with alternate samples of said I and Q-channel signals to produce a first summation signal;

correlating said digital code sequence with the remaining alternate samples of said I and Q-channel signals to produce a second summation signal; and

summing said first and second summation signals to produce a correlation output signal.

9. The method recited in claim 8 wherein the even samples of said I-channel signal and the odd samples of said Q-channel signal are correlated with the digital code sequence to produce said first summation signal and the odd samples of said I-channel signal and the even samples of said Q-channel signal are correlated with said digital code sequence to produce said second summation signal.

10. The method recited in claim 8 wherein the steps of correlating comprise adding or subtracting each of the samples within each group of alternate samples according to the states of the bits in said digital code sequence.

11. The method recited in claim 8 wherein the step of sampling comprises producing analog samples and the step of storing said samples comprises storing said analog samples in an analog shift register.

12. The method recited in claim 8 wherein the step of sampling comprises producing digital samples and the step of storing a plurality of said samples comprises storing said digital samples in a digital shift register.

13. A method for demodulating a carrier signal which has been phase modulated with a binary signal which has a predetermined code sequence, comprising the steps of:

generating a local oscillator signal;

phase shifting said local oscillator signal to generate a quadrature local oscillator signal;

mixing said carrier signal with said local oscillator signal to produce an in-phase, I-channel, signal;

mixing said carrier signal with said quadrature local oscillator signal to produce a quadrature, Q-channel, signal;

sampling said I-channel signal and supplying the samples produced thereby to a first analog shift register having an output tap for each stage;

sampling said Q-channel signal and supplying the samples produced thereby to a second analog shift register having an output tap for each stage;

locally generating a digital signal comprising at least a part of said predetermined code sequence;

storing said locally generated digital signal in a digital shift register having an output tap for each stage of the register;

multiplying the quantity in each stage of the digital shift register by the quantity in each corresponding stage of the first analog shift register to produce a plurality of first register product signals each corresponding to a stage of said first analog shift register;

multiplying the quantity in each stage of the digital shift register by the quantity in each corresponding stage of the second analog shift register to produce a plurality of second register product signals each corresponding to a stage of said second analog shift register;

summing the product signals derived from alternate taps of said analog shift registers to produce a first summation signal;

summing the product signals derived from the remaining alternate taps of said analog shift registers to produce a second summation signal; and

summing said first and second summation signals to produce a correlation output signal.

14. The method recited in claim 14 including the step of full wave rectifying said first and said second summation signals prior to the step of summing said first and second summation signals.