Fast frequency/code search

• US 4,485,477 A
• Filed: 07/19/1982
• Issued: 11/27/1984
• Est. Priority Date: 07/19/1982
• Status: Expired due to Term

• Current Assignee: L-3 Communications Corporation (L3Harris Technologies, Inc.)
• Original Assignee: RCA Corporation
• Inventors: Nossen, Edward J.
• Primary Examiner(s): Safourek, Benedict V.
• Application Number:
  US06/399,879
• Time in Patent Office: 862 Days
• Field of Search: 364/604, 364/724, 364/728, 364/819, 343/5 AF, 343/5 PN, 375/1, 375/96, 375/97, 455/257
• US Class Current: 375/150
• CPC Class Codes: H04B 1/69 Spread spectrum techniques
• CPC Class Codes: H04B 1/709 Correlator structure

Abstract

A decoder for decoding received pseudo random sequence (PRS) codes modulated upon a carrier signal and having an original chip rate f_c, and for correcting for unknown frequency shifts in the received PRS codes. The decoder includes a local oscillator for demodulating the carrier signal and detecting the received PRS codes and a signal source for generating X correction PRS codes each having a pattern of chips similar to the chip pattern of a predetermined one of the received PRS codes and each shifted in chip rate from f_c by amounts which will provide a spectrum of frequencies spanning any expected frequency shift of a received PRS code. Also provided is a correlator for correlating each of the X correction PRS codes with each received PRS code at a rate f_r to determine which particular correction PRS code produces the greatest correlation signal, where f_r >>f_c. Further logic is responsive to the chip rate of the particular correction PRS code for correcting the output frequency of the local oscillator to compensate for the unknown frequency shift during the reception of subsequently received PRS codes.

Claims

1. In a decoder for decoding received pseudo random sequence (PRS) codes modulated upon a carrier signal, having an original chip rate f_c and an unknown frequency shift thereon, frequency compensating logic for compensating for said frequency shift to improve correlation of locally generated reference PRS codes of chip rate f_c with said received PRS codes and comprising:

means including local oscillating means for demodulating said carrier signal and detecting said received PRS codes;

means for generating X correction PRS codes each having a pattern of chips similar to the chip pattern of a predetermined one of said received PRS codes and each shifted in chip rate from f_c by amounts which will provide a spectrum of frequencies spanning any expected frequency shift of a received PRS code;

means for correlating each of said X correction PRS codes with a predetermined received PRS code at a rate f_x to determine which particular correction PRS code produces the greatest correlation signal, where f_x >

>

f_c ; and

means responsive to the chip rate of said particular correction PRS code for correcting the frequency of the output of said local oscillator means to compensate for said unknown frequency shift during the reception of subsequently received PRS codes.

2. A decoder for decoding received pseudo random sequence (PRS) codes modulated upon a carrier signal, having an original chip rate f_c and having an unknown frequency shift f_d thereon, means for compensating for said frequency shift f_d to improve correlation of a locally generated reference PRS code of chip rate f_c with said received PRS code and comprising:

means for demodulating said carrier signal and generating I and Q quadrature signals of said predetermined received PRS code;

means for generating X correction PRS codes each having a pattern of chips similar to the pattern of chips of a predetermined one of said received PRS codes and shifted in chip rate from f_c by amounts to provide a spectrum of frequencies spanning any expected frequency shift of the predetermined received PRS code;

means for correlating each of said X correction PRS codes with each new group of received chips each time a new chip is received and where a new group of chips is formed upon the reception of each new chip;

means for determining which particular correction PRS code produces the greatest correlation with said predetermined received PRS code; and

means responsive to the determination of said particular correction code signal to offset the received PRS code by a frequency equal and opposite to the unknown frequency shift f_d.

3. In a decoder for decoding any one of M possible N chip pseudo random sequence (PRS) codes received serially as a stream of successive chips having an original chip rate f_c and having an unknown frequency shift thereon, a method for compensating for said unknown frequency shift to enable improved correlation of locally generated reference PRS codes of chip rate f_c with said received PRS code and comprising the steps of:

generating I and Q quadrature signals of said received PRS code;

generating X correction PRS codes each having a pattern of N chips similar to the pattern of chips of a predetermined one of said M possible received PRS codes and each shifted in chip rate from f_c by an amount to provide a spectrum of frequencies spanning any expected frequency shift of the received PRS codes;

correlating each of said X correction PRS codes with each new group of N chips of the received serial stream of chips, and with each new group being formed by the reception of a new chip to determine which particular correction PRS code and which group of N chips provides the greatest correlation over a predetermined number of correlations; and

altering the frequency of said I and Q quadrature signals in accordance with the chip rate of the determined particular correction PRS code to compensate for said unknown frequency shift.

4. In a communication system employing a transmitted PRS signal consisting of a series of M possible received pseudo random sequence (PRS) codes each consisting of a group of N chips and each of which has a frequency shift superimposed thereon when received at a receiver, a decoder for receiving and decoding such received PRS codes comprising:

first logic means comprising a local oscillator for generating I and Q quadrature components of said received PRS codes;

first generating means for generating I and Q components of X correction PRS codes each having the same pattern as a predetermined one of said M possible received PRS codes but each shifted in frequency by predetermined amounts from said predetermined received PRS code to provide a spectrum of frequencies covering the largest expected amount of frequency shift on any received PRS code;

correlation means for correlating each of said X correction PRS codes with each successive group of N chips of the received PRS code with a new group being formed upon the reception of each succeeding chip;

second logic means for detecting and identifying the specific one of said X correction PRS codes having the greatest correlation with any group of N received chips over the reception of N chips of said received PRS signal; and

means responsive to the identified correction PRS code for altering the frequency of said local oscillator to compensate for the frequency shift defined by said identified correction PRS code for received PRS codes received subsequent to the alteration of the frequency of said local oscillator.

5. A decoder as in claim 4 and further comprising:

second generating means for successively generating and supplying M reference PRS codes to said correlation means for correlation with said received signal each time a new chip is received, where each of said M reference PRS codes corresponds to one of said M possible received PRS codes; and

wherein said second logic means is responsive to the correlations of said M reference PRS codes and said received PRS codes to identify which reference PRS code produces the greatest correlation with each of said received PRS codes.