Spread spectrum receiver and method for carrier frequency offset compensation in such a spread spectrum receiver

1. A spread spectrum receiver for receiving and decoding an input signal r(k) consisting of a sequence of chips, the receiver comprising:

a correlator for processing the input signal r(k) and providing at its output a signal y(n) which consists of a sequence of symbols;

a phase rotator, coupled to an output of the correlator, for rotating the phase of the signal y(n) based on a filtered phase error signal θ(n) in order to compensate a carrier frequency offset Ω contained in the input signal r(k);

a detector, coupled to an output of the phase rotator, for receiving the rotated signal z(n) and for taking a decision on the symbols and providing on an output an output signal c(n) which can subsequently be demapped to a sequence of bits,

an error signal generator, coupled to the outputs of the phase rotator and the detector, for generating a phase error signal e(n) which depends on the carrier frequency offset of the input signal r(k),

a loop filter, coupled to an output of the error signal generator, for filtering the phase error signal e(n) and feeding the filtered phase error signal θ(n) to the phase rotator, and

a phase pre-compensation block which comprises a phase pre-compensation estimator, a pre-compensation signal generator and a pre-compensation phase rotator.

2. A receiver according to claim 1 , wherein the pre-compensation estimator is coupled to an output ψ(n) of the loop filter and provides an estimate value φ for the carrier frequency offset Ω.

3. A receiver according to claim 2 , wherein the pre-compensation signal generator is coupled to an output of the pre-compensation estimator-and generates a pre-compensation phase signal φ.k based on the estimate value φ.

4. A receiver according to claim 2 , wherein the pre-compensation phase rotator is coupled to the pre-compensation signal generator and also receives a receiver signal x(k) and applies a phase rotation to the receiver signal x(k) based on the pre-compensation phase signal φ.k in order to coarsely compensate the carrier frequency offset Ω contained in the receiver signal x(k).

5. A receiver according to claim 1 , wherein the pre-compensation signal generator is coupled to an output of the pre-compensation estimator and generates a pre-compensation phase signal φ.k based on the estimate value φ.

6. A receiver according to claim 5 , wherein the pre-compensation phase rotator is coupled to the pre-compensation signal generator and also receives a receiver signal x(k) and applies a phase rotation to the receiver signal x(k) based on the pre-compensation phase signal φ.k in order to coarsely compensate the carrier frequency offset Ω contained in the receiver signal x(k).

7. A receiver according to claim 1 , wherein the pre-compensation phase rotator is coupled to the pre-compensation signal generator and also receives a receiver signal x(k) and applies a phase rotation to the receiver signal x(k) based on the pre-compensation phase signal φ.k in order to coarsely compensate the carrier frequency offset Ω contained in the receiver signal x(k).

8. A receiver according to claim 1 , wherein the loop filter comprises a reset input for resetting the value ψ(n) to zero.

9. A receiver according to claim 8 , wherein the pre-compensation estimator comprises a reset output which is coupled to the reset input of the loop filter.

10. A method for carrier frequency offset compensation in spread spectrum receivers for receiving and decoding an input signal r(k) consisting of a sequence of chips, the method comprising the steps of:

applying the input signal r(k) to a correlator which processes the input signal and provides an output signal y(n) which consists of a sequence of symbols;

applying the signal y(n) to a phase rotator which rotates the phase of the signal y(n) based on a filtered phase error signal θ(n) in order to compensate a carrier frequency offset Ω contained in the input signal r(k);

applying the output signal z(n) from the phase rotator to a detector which takes a decision on the symbols and provides an output signal c(n) which is subsequently demapped to a sequence of bits,

applying the output signals of the phase rotator and the detector to an error signal generator which generates a phase error signal e(n) which depends on the carrier frequency offset of the input signal r(k),

applying the phase error signal e(n) to a loop filter which filters the phase error signal e(n) and provides a filtered phase error signal θ(n) which is applied to the phase rotator, and

applying an output signal ψ(n) of the loop filter to a pre-compensation estimator which provides an estimate value φ for the carrier frequency offset Ω.

11. A method according to claim 10 , wherein the received signal r(k) passes through the correlator before it passes through the phase rotator.

12. A method according to claim 10 , including the step of applying the estimate value φ to a pre-compensation signal generator which generates a pre-compensation phase signal φ.k based on the estimate value φ.

13. A method according to claim 10 , including the step of applying a pre-compensation phase signal φ.k to a pre-compensation phase rotator which also receives a receiver signal x(k) and applies a phase rotation to the receiver signal x(k) based on the pre-compensation phase signal φ.k in order to coarsely compensate the carrier frequency offset Ω contained in the receiver signal x(k), whereby the phase rotated receiver signal r(k) is fed to an input of the correlator.

14. A method according to claim 10 , wherein at the time when pre-compensation has started, a reset signal is generated in the pre-compensation estimator and applied to the loop filter as to reset the value ψ(n) to zero.