Method and apparatus for carrier recovery and compensation in spread spectrum communications
First Claim
1. In a direct sequence spread spectrum communication system, wherein digital spread spectrum signals modulated by a pseudo-random sequence have a carrier frequency, wherein the carrier frequency includes an expected carrier frequency and a carrier frequency offset, a carrier offset estimation method for recovering the carrier offset, comprising the steps of:
- receiving digital spread spectrum signals modulated with a pseudo-random sequence, wherein said digital spread spectrum signals have a carrier frequency, wherein the carrier frequency includes an expected carrier frequency and a carrier frequency offset;
demodulating said digital spread spectrum signals to remove said expected carrier frequency, wherein said demodulating also includes demodulating said digital spread spectrum signals to remove said pseudo-random sequence, wherein said demodulating produces demodulated received signals;
computing a result containing phase change information for each of a plurality of pairs of said demodulated received signals wherein elements of said plurality of pairs have a predetermined L-chip interval;
calculating the average of said results to compute the carrier offset estimate; and
normalizing said average by a factor of L to compute the carrier offset estimate, wherein said normalizing is performed after said calculating said average.
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Abstract
An improved system and method for determining and tracking the carrier offset in a communications system. In an exemplary implementation, sampled spread spectrum digital signals are compensated with an expected carrier offset and stripped of pseudo-noise codes to produce demodulated signals. The demodulated signals are applied to a carrier offset estimator. The carrier offset estimator divides pairs of demodulated signals with a predetermined interval to produce results containing phase change information. The results are then averaged, and the average is normalized with respect to the predetermined interval to yield a new estimate of the carrier offset. The expected carrier frequency is adjusted based on the new offset estimate for frequency compensation in subsequent reception and transmission operations.
19 Citations
11 Claims
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1. In a direct sequence spread spectrum communication system, wherein digital spread spectrum signals modulated by a pseudo-random sequence have a carrier frequency, wherein the carrier frequency includes an expected carrier frequency and a carrier frequency offset, a carrier offset estimation method for recovering the carrier offset, comprising the steps of:
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receiving digital spread spectrum signals modulated with a pseudo-random sequence, wherein said digital spread spectrum signals have a carrier frequency, wherein the carrier frequency includes an expected carrier frequency and a carrier frequency offset; demodulating said digital spread spectrum signals to remove said expected carrier frequency, wherein said demodulating also includes demodulating said digital spread spectrum signals to remove said pseudo-random sequence, wherein said demodulating produces demodulated received signals; computing a result containing phase change information for each of a plurality of pairs of said demodulated received signals wherein elements of said plurality of pairs have a predetermined L-chip interval; calculating the average of said results to compute the carrier offset estimate; and normalizing said average by a factor of L to compute the carrier offset estimate, wherein said normalizing is performed after said calculating said average. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
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9. A carrier offset estimation system for recovering carrier offset in a direct sequence spread spectrum communication system, wherein the communication system receives digital spread spectrum signals modulated with a pseudo-random sequence, wherein the digital spread spectrum signals have a carrier frequency, wherein the carrier frequency includes an expected carrier frequency and a carrier frequency offset, the carrier offset estimation system comprising:
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a demodulator for demodulating said digital spread spectrum signals to remove said expected carrier frequency, wherein said demodulator also demodulates said digital spread spectrum signals to remove said pseudo-random sequence, wherein said demodulator produces demodulated received signals; a carrier offset estimator, comprising; means for computing a result containing phase change information for each of a plurality of pairs of said demodulated received signals including first and second registers for storing respective elements of said pairs of said demodulated received signals and a divider coupled to said first and second registers for dividing an element of a pair stored in said first register by an element of said pair stored in said second register; means for calculating the average of said results to compute the carrier offset estimate including an averager coupled to said divider for calculating an average of at least a subset of said results containing phase change information to compute the carrier offset estimate; and a normalizer coupled to said averager for normalizing said average to compute the carrier offset estimate. - View Dependent Claims (10, 11)
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Specification