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UBC Theses and Dissertations

Frequency offset estimation for GPS enabled Wi-Fi access points. Khan, Tausif Ezaj


A method to estimate the frequency offset between the internal oscillators of two or more GPS RF front-ends is described and evaluated. The purpose of this is to allow a Non-Line-of-Sight (NLOS) GPS receiver to acquire and track satellite signals with assistance data provided by a Line-of-Sight (LOS) GPS receiver. This facilitates indoor positioning with GPS, which is used to accurately position 802.11 WLAN access points that will be used in an integrated indoor positioning system. The assistance data consists of the Doppler frequency and code phase of GPS satellites that the LOS GPS receiver can clearly view. Knowing this information at the NLOS GPS receiver significantly reduces the Doppler search space and code phase search space resulting in faster signal acquisition. To effectively use this assistance information, the NLOS GPS receiver oscillator must be synchronized to that of the LOS GPS receiver to within 10 ns. This frequency synchronization is accomplished by estimating the frequency offset between the two oscillators with a signal transmitted between the two receivers via a WLAN channel. COCC (Carrier Only Cross Correlation) and CCCC (Code + Carrier Cross Correlation) are two proposed techniques to estimate this frequency offset to within ±30 Hz. In COCC, a single carrier signal is sent by the LOS receiver at 10.23 MHz and cross-correlated with a replica signal generated at the NLOS receiver. This technique’s best performance (frequency estimation within ±30 Hz) is achieved at SNR > -4 dB in an AWGN channel and at SNR > 22 dB in a Rayleigh fading channel at a separation distance of 25 m. The CCCC technique has a 1023 chip PRN code at 1.023 Mega chips/sec modulated on the transmitted carrier signal and is cross-correlated with the replica code-modulated signal generated at the NLOS receiver. The optimum code sampling frequency for CCCC was found to be 10 MHz. It is shown that CCCC can estimate a frequency offset to within ±30 Hz at SNR > -14 dB in an AWGN channel and SNR > -10 dB in a Rayleigh fading channel at a separation distance of 35 m.

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Attribution-NonCommercial-NoDerivs 2.5 Canada