Varying early-late spacing in a delay locked loop
A rake receiver tracks a component of a multipath information signal transmitted over a communication network. The rake receiver aligns and synchronized the component with a locally generated replica of the code sequence that was originally used to spread the information signal at the transmitter side. The rake receiver derives an early de-spread signal using an early shifted version of the replica of the code sequence. The replica is early shifted by a variable delay. The rake receiver also derives a late shifted de-spread signal using a late shifted version of the replica of the code sequence. The replica is late shifted by the same variable delay. The variable delay may be chosen arbitrarily or among a selection of predetermined values. The predetermined values may be selected so that the tracking process is optimized.
1. A rake receiver comprising a plurality of finger circuits each for tracking a resolved component of a multipath signal, each of a plurality of finger circuits comprising:
an early shifting circuit that derives an early code sequence by advancing a locally generated code sequence by a first variable delay;
a first correlating circuit that de-spreads the resolved component using the early code sequence resulting in an early de-spread signal;
a late shifting circuit that derives a late code sequence by retarding the locally generated code sequence by a second variable delay that is substantially equal to twice the delay of the first variable delay;
a second correlating circuit that de-spreads the resolved component using the late code sequence resulting in a late de-spread signal;
a circuit that derives a quality indicator based on at least one of a plurality of signals in the multipath signal, wherein the quality indicator indicates a signal-to-noise ratio of at least one of the plurality of signals in the multipath signal; and
an adjustment circuit that generates a correction signal from the early and late de-spread signals to control the tracking of the resolved component of the multipath signal.
2. The rake receiver of claim 1 , comprising a circuit that generates the first and secondly delays.
3. An apparatus comprising:
a rake receiver comprising a plurality of finger circuits each for tracking a resolved component of a multipath signal, each of a plurality of finger circuits comprising:
an early shifting circuit that derives an early code sequence by advancing a locally generated code sequence by a first variable delay;
a first correlating circuit that de-spreads the resolved component using the early code sequence resulting in an early de-spread signal;
a late shifting circuit that derives a late code sequence by retarding the locally generated code sequence by a second variable delay that is substantially equal to twice the first delay;
a second correlating circuit that de-spreads the resolved component using the late code sequence resulting in a late de-spread signal;
a circuit that derives a quality indicator based on at least one of a plurality of signals in the multipath signal, wherein the quality indicator indicates a signal-to-noise ratio of at least one of the plurality of signals in the multipath signal; and
an adjustment circuit that derives a correction signal from the early and late de-spread signals to control the tracking of the resolved component of the multipath signal.
4. The apparatus of claim 3 , wherein the apparatus is a cellular phone.
5. The apparatus of claim 3 , further comprising a circuit that generates the first and second variable delays.
6. A communication receiver that receives an information signal comprised of a multipath signal from a transmitter, the information signal spread by a code sequence in the transmitter, the communication receiver comprising:
a rake receiver circuit having a plurality of finger circuits, each finger circuit tracking a resolved component signal of the received multipath signal, each finger circuit comprising:
an input to receive the respective component signal of the received multipath signal;
a synchronizer circuit that synchronizes the component signal with a replica of an original code sequence used to generate the information signal;
a correlator circuit that de-spreads the component signal by correlating the component signal with the replica of the original code sequence; and
a correction signal circuit that generates a correction signal used to generate the replica of the original code sequence, the correction signal circuit comprising an early de-spread signal correlator that receives the component signal and uses a first time-shifted replica of the original code sequence having a first delay to de-spread the component signal into an early de-spread signal, and a late de-spread signal correlator that receives the component signal and uses a second time-shifted replica of the original code sequence having a second delay that is varied from the first delay to de-spread the component signal into a late de-spread signal, and a series-coupled digital processing unit and loop filter that process the early and late de-spread signals to generate the correction signal.
7. The communication receiver of claim 6 , comprising a delay value generator circuit to generate the first and second delays of the early and late de-spread signal correlators, respectively.
8. The communication receiver of claim 7 , wherein the first and second delays are substantially equal.
9. The communication receiver of claim 7 , wherein the second delay is substantially equal to twice the first delay.
10. The communication receiver of claim 7 , wherein the delay value generator randomly generates the first and second delays.
11. The communication receiver of claim 10 , wherein the random delay generator of the first and second delays randomly selects the first and second delays from a look-up table of values.