%0 Journal Article %1 Yang2000a %A Yang, C. %D 2000 %J Proc. of the ION National Technical Meeting %K FFT, GPS, correlation, sampling %P 724-730 %T GPS Code Correlation with FFT under Pseudo Quadrature Sampling %X A Global Positioning System (GPS) signal at its baseband is usually represented by an in-phase and quadrature pair. Such a representation is unique in that it removes the ambiguity as to whether the signal is down converted from the upper or lower side of the bandpass center frequency. There are two basic ways to perform the down-conversion to baseband: (1) analog down-conversion with two channels for sampling at baseband and (2) sampling at an intermediate frequency (IF) followed by digital downconversion. The latter is predominantly used in today's digital receivers because it requires less hardware and eliminates the need for analog channels matching and balancing. This paper will consider a special case of the second type of down-conversion in which the sampling rate fs is selected to be exactly four times of the last intermediate frequency f0 (typically chosen as the signal bandwidth plus anticipated Doppler shift). The advantage of such an arrangement is that the local in-phase and quadrature components become sequences of alternative 0 and +/-1. As a result, the consecutive incoming samples can be simply split into odd and even samples and paired as the in-phase and quadrature components without the need for actual mixing (i.e., multiplication with local references) except for a simple sign reversal. A Fast Fourier Transform (FFT)-implemented circular correlation can then be applied at a data rate of fs/2 for GPS signal acquisition. This quadrature sampling scheme, though making the down-conversion extremely simple, faces a difficulty in that two consecutive samples have a one-sample delay and the in-phase and quadrature components may correspond to code chips of different polarity. Because of this, it is called ''pseudo quadrature sampling'' according to van Dierendonck Parkinson and Spilker, 1996. To solve this problem, a technique will be described in this paper that first takes the complex FFT of the delayed sample pairs and then extracts the original signal spectrum from it before a frequency-domain code correlation is performed. Simulation results as well as analyses will be presented in this paper to demonstrate the technique as applied to fast direct GPS P-code acquisition.

@article{Yang2000a, abstract = {A Global Positioning System (GPS) signal at its baseband is usually represented by an in-phase and quadrature pair. Such a representation is unique in that it removes the ambiguity as to whether the signal is down converted from the upper or lower side of the bandpass center frequency. There are two basic ways to perform the down-conversion to baseband: (1) analog down-conversion with two channels for sampling at baseband and (2) sampling at an intermediate frequency (IF) followed by digital downconversion. The latter is predominantly used in today's digital receivers because it requires less hardware and eliminates the need for analog channels matching and balancing. This paper will consider a special case of the second type of down-conversion in which the sampling rate fs is selected to be exactly four times of the last intermediate frequency f0 (typically chosen as the signal bandwidth plus anticipated Doppler shift). The advantage of such an arrangement is that the local in-phase and quadrature components become sequences of alternative 0 and +/-1. As a result, the consecutive incoming samples can be simply split into odd and even samples and paired as the in-phase and quadrature components without the need for actual mixing (i.e., multiplication with local references) except for a simple sign reversal. A Fast Fourier Transform (FFT)-implemented circular correlation can then be applied at a data rate of fs/2 for GPS signal acquisition. This quadrature sampling scheme, though making the down-conversion extremely simple, faces a difficulty in that two consecutive samples have a one-sample delay and the in-phase and quadrature components may correspond to code chips of different polarity. Because of this, it is called ''pseudo quadrature sampling'' according to van Dierendonck [Parkinson and Spilker, 1996]. To solve this problem, a technique will be described in this paper that first takes the complex FFT of the delayed sample pairs and then extracts the original signal spectrum from it before a frequency-domain code correlation is performed. Simulation results as well as analyses will be presented in this paper to demonstrate the technique as applied to fast direct GPS P-code acquisition.}, added-at = {2011-05-30T10:41:10.000+0200}, author = {Yang, C.}, biburl = {https://www.bibsonomy.org/bibtex/2a283ae822e2a1af65b44133d59e51c80/bmuth}, groups = {private}, interhash = {2a01ab732759f2b96c12cabe13ac6273}, intrahash = {a283ae822e2a1af65b44133d59e51c80}, journal = {Proc. of the ION National Technical Meeting}, keywords = {FFT, GPS, correlation, sampling}, owner = {bmuth}, pages = {724-730}, timestamp = {2014-08-11T22:37:44.000+0200}, title = {{GPS Code Correlation with FFT under Pseudo Quadrature Sampling}}, username = {bmuth}, year = 2000 }