Joint de-spreading and frequency correction using a correlator
AssigneeFreescale Semiconductor, Inc.
InventorsRobert Mark Gorday, Jorge Ivonnet
Life of this patent
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Abstract
A correlator (140) for de-spreading a spread-spectrum signal includes a state machine (205), a frequency correction look-up table (207), a pseudorandom code generator (209), and a correlator structure (301 and 801). The spread-spectrum signal includes symbols, and each symbol includes a plurality of chips. The correlator structure includes a plurality of taps (309 and 809) at which a coordinate rotation digital computer (CORDIC) operation is performed to determine an offset from a nominal carrier frequency of the spread-spectrum signal and to change a phase of each chip of a received symbol, in order to correct a carrier frequency of the spread-spectrum signal while de-spreading the spread-spectrum signal.
Claims20
A correlator for de-spreading a digitally-modulated signal including a plurality of symbols, each symbol spread among a plurality of chips, the digitally-modulated signal having a nominal carrier frequency, the correlator comprising:
- a tapped delay line including a plurality of registers, each register for holding a chip of a symbol of the digitally-modulated signal as a function of time;
- a frequency correction generator for outputting a plurality of sets of phase rotation tap weights to the tapped delay line, each set of phase rotation tap weights corresponding to a frequency offset of an actual carrier frequency of the digitally-modulated signal from the nominal carrier frequency;
- and
- taps operatively coupled to registers for performing a tap weight operation on each chip to rotate a phase of each chip to correct the frequency offset from the nominal carrier frequency and for performing another tap weight operation on each chip to de-spread the digitally-modulated signal.
A method of operating a correlator in a receiver, the receiver receiving a coded-modulation spread spectrum signal including a plurality of symbols, the coded-modulation spread spectrum signal having a nominal carrier frequency and a packet structure comprising a preamble followed by a payload, comprising the steps of:
- during reception of a preamble, correlating a received symbol with a same de-spreading code a plurality of times, each correlation performed at a different frequency offset from the nominal carrier frequency, to determine an indication of an actual carrier frequency of the received coded-modulation spread spectrum signal;
- and
- based upon the actual carrier frequency determined, selecting a plurality of tap weights for the correlator to compensate for the actual carrier frequency of the coded-modulation spread spectrum signal.
A correlator for de-spreading a direct sequence spread spectrum (DSSS) signal, the DSSS signal including symbols, the DSSS signal having a nominal carrier frequency, the correlator comprising:
- a correlator structure having a plurality of taps, each tap having a tap weight;
- a frequency correction generator for outputting a plurality of sets of phase rotation tap weights, each set of phase rotation tap weights corresponding to a pre-determined offset of an actual carrier frequency of the DSSS signal from the nominal carrier frequency;
- a pseudorandom code generator for outputting a plurality of sets of de-spreading tap weights, each set of de-spreading tap weights corresponding to a pre-determined pseudorandom code for de-spreading the DSSS signal;
- and
- a controller for selectively controlling application of the sets of de-spreading tap weights and the phase rotation tap weights to the taps.
Dependencies are inferred from the claim text.