Pseudo-random noise detector for signals having a carrier frequency

Abstract

In the front end of a mobile phone receiver for operation in a network using DSSS signals, detection of a carrier frequency offset in a received PN-modulated signal is performed by first correlating the received signal with a local replica of the PN codes generated within the receiver. The resulting despread signal is integrated over a fixed period, or dwell, of time, then run through a square-law envelope detector. The integration is divided into a plurality of sub-dwells, the values of which are each provided as an input to a Fast Fourier Transform (FFT) to generate a plurality of frequency bins. The magnitude for each FFT frequency bin is computed and a maximum value is selected. The maximum value and its corresponding bin number are saved in the signal processor's memory. The next PN code phase is tested and, depending on the search scheme, this sequence continues until the decision algorithm terminates the search. In one embodiment, the magnitude is calculated for each bin for the collected samples by passing the data through a filter bank with one filter for each bin in the frequency domain, then operated on by a decision algorithm which compares the value to a pre-set threshold. After a match has been declared and the search is terminated, estimation of the frequency offset is achieved by averaging the bin indices for the highest magnitude per iteration following a number of iterations of the search process performed at the PN code phase at which the match was found.