Method and apparatus for improved auto-correlation and range correlation in pseudo-random noise coded systems
First Claim
1. A method for improving autocorrelation in a pseudo-random noise coded system, comprising the steps of:
- phase modulating a carrier frequency with a pseudo-random noise code sequence to produce a phase modulated carrier frequency, said phase modulated carrier frequency having two phase states, said phase states having a relative phase difference of approximately one-hundred eighty degrees;
transmitting a transmit signal that includes said phase modulated carrier frequency;
adjusting said relative phase difference between said two phase states of said phase modulated carrier frequency based on a magnitude of a center spectral line of said transmit signal to maintain a predetermined phase offset from said relative phase difference; and
correlating a received signal with a reference signal that has spectral characteristics of said pseudo-random noise code sequence.
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Abstract
A method and apparatus is disclosed for improved out-of-range rejection in pseudo-random noise (PN) coded systems by adjusting the relative phase difference between the two phase states of a bi-phase modulation. The energy in the spectral lines of the bi-phase modulated RF signal to generate a signal to be fedback to a voltage-controlled bi-phase modulator to adjust the relative phase difference between the two phase states. Sampled signals are mixed down to baseband using single sideband suppressed carrier modulation of the center frequency of the transmitted signal and the offset frequency. The offset frequency is chosen to be less than the PN code repetition frequency. The baseband signal is filtered and envelop detected producing a DC voltage proportional to the amplitude of the center line of the PN spectrum which is compared to either a fixed reference or to the amplitude of one or more of the remaining lines in the spectrum. The comparison provides an error signal adjusting the phase of the bi-phase modulator providing improved out-of-range rejection.
43 Citations
26 Claims
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1. A method for improving autocorrelation in a pseudo-random noise coded system, comprising the steps of:
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phase modulating a carrier frequency with a pseudo-random noise code sequence to produce a phase modulated carrier frequency, said phase modulated carrier frequency having two phase states, said phase states having a relative phase difference of approximately one-hundred eighty degrees; transmitting a transmit signal that includes said phase modulated carrier frequency; adjusting said relative phase difference between said two phase states of said phase modulated carrier frequency based on a magnitude of a center spectral line of said transmit signal to maintain a predetermined phase offset from said relative phase difference; and correlating a received signal with a reference signal that has spectral characteristics of said pseudo-random noise code sequence. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13)
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14. A method of improving out of range rejection in a pseudo-random noise coded system comprising the steps of:
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sampling a transmitted signal comprised of a carrier frequency phase modulated with a first pseudo-random noise code sequence, said first pseudo-random noise code sequence having two phase states with a first relative phase difference, the sampling step producing a sampled transmitted signal; generating a first error signal based on said sampled transmitted signal; adjusting said first relative phase difference between said two phase states of said first pseudo-random noise code sequence based on said first error signal to maintain a first predetermined phase offset from said first relative phase difference, said first predetermined phase offset based on a code length of said first pseudo-random noise code sequence; and combining a received signal with a reference signal having a spectral characteristic of said first pseudo-random noise code sequence to obtain a correlation range response. - View Dependent Claims (15, 16)
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17. A pseudo-random noise coded system for improving discrimination between returns from a target and returns from clutter comprising;
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an radio frequency generator for generating a carrier frequency; a transmit bi-phase modulator for generating a transmit signal comprised of said carrier frequency phase modulated with a first pseudo-random noise coded sequence, said transmit bi-phase modulator coupled to said radio frequency generator and having a transmit phase control port and a transmit output port; a transmit antenna for transmitting said transmit signal and coupled to said transmit output port of said bi-phase modulator; a receive bi-phase modulator for generating a reference signal comprised of said carrier frequency phase modulated with a second pseudo-random noise coded sequence, said second pseudo-random noise coded sequence being a delayed version of said first pseudo-random noise coded sequence, said receive bi-phase modulator coupled to said radio frequency generator and having a receive phase control port and a receive output port; a receive antenna for receiving a received signal, said received signal being a Doppler shifted version of said transmit signal; and a correlator for producing a Doppler frequency from said received signal and said reference signal, said correlator having first, second and third ports, said first port coupled to said receive bi-phase modulator, said receive antenna coupled to said second port of said correlator. - View Dependent Claims (18, 19, 20, 21, 22, 23, 24)
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25. A pseudo-random noise coded system for improving discrimination between returns from a target and returns from clutter comprising:
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a transmit bi-phase modulator for generating a transmit signal comprised of a carrier frequency phase modulated with a first pseudo-random noise (PN) code sequence, said first PN code sequence having two code states resulting in two phase states of said transmit signal that have a first relative phase difference of approximately one-hundred eighty degrees; a transmit feedback control loop for sampling said transmit signal and producing a transmit feedback signal based on a magnitude of spectral lines of said transmit signal, and wherein said transmit bi-phase modulator has means for adjusting said first relative phase difference based on said transmit feedback signal to maintain a first predetermined phase offset from said first relative phase difference of said two phase states; and a correlator for combining a received signal with a reference signal to obtaining a correlation range response, said reference signal including said carrier frequency phase modulated with a second PN code sequence, said second PN code sequence being a delayed version of said first PN code sequence. - View Dependent Claims (26)
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Specification