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where is the NCO gain, is the discriminator gain, is the loop filter transfer function. Depending on the dynamics on the received signal the order of the filter is adjusted. However, due to carrier aiding, a first order delay locked loop is usually employed.

The Delay Lock Loop is characterised by Code Generator, Code NCO, and Discriminator algorithms. The Code Generator generates three different phases in phase quadrature – Early, Prompt and Late which are typically separated by ˝ chip spacing. The Early phase is ˝ chip advanced with respect to the prompt phase, whereas the late phase is ˝ chip retarded with respect to the prompt phase. These three signals are multiplied with the incoming composite signal to identify the code phase starting point in the composite signal. In an ideal scenario, when the prompt phase gets exactly aligned to the incoming code phase, the prompt arm correlation output results in a triangle with a sharp peak at zero offset and linearly ramping down to zero at 1 chip offset, as shown in Figure 3. Under this condition, the early and late arms will have 3 dB less power than in the prompt arm.


Figure 3. Code Autocorrelation

However, due to finite front-end bandwidth and correlated errors such as multipath, the correlation peak is not sharp but distorted, which degrades the pseudo-range measurements, and eventually the navigation solution. The offsets in the locally-generated code phases with respect to the incoming signal are determined by the code discriminator functions. Ward (1998) proposes four different discriminators, tabulated in Table 1, with each having their own advantages and disadvantages.

The performance of the discriminator transfer functions are shown in Figure 4. As the Normalized envelope is insensitive to amplitude fluctuations, it is the preferred one, especially for Software Receivers. Correlator spacing is an optimal parameter which affects the DLL performance; while wider spacing allows faster acquisition the narrow spacing improves interference and multipath effects (Van, 1992; Gustafson, 2003).


Figure 4. Discriminators output

As a Software Receiver is used for the analysis and experiments in this paper, an E-L Normalized discriminator algorithm is used. The early minus late normalized envelope removes amplitude sensitivity, which provides better performance against pulse-type interference, but demands the highest computational load.


Table 1

3.0 Integration of Mobile phone signals with GPS receiver tracking loops

A simulation experiment has been conducted in Matlab to evaluate the performance of the GPS receiver tracking loops in the mobile phone environment. In particular, the code tracking loop was analysed for its performance. With an interference signal at 1.5GHz and with a code spacing of 0.5 chips with a code tracking loop bandwidth of 2Hz, the code tracking loop was analysed and the results are plotted in Figure 5.


Figure 5. Carrier BW = 13 Hz, Code bandwidth = 2 Hz, Code spacing = 0.5 chip The Doppler signals were then removed from the composite GPS signal and then the same experiment was then performed with code tracking loop bandwidth of 1Hz and chip spacing of 0.25 chips. It can be seen from the results plotted in Figure 6 that the code tracking loop performance has improved significantly by optimizing the loop parameters. However, it should be understood that this is possible by having a prior knowledge of Doppler information.


Figure 6. Carrier BW =13 Hz, code BW = 1 Hz, chip spacing = 0. 25 chips 4.0 Conclusion
It is shown from the simulation experiments that the receiver tracking loop performance can be improved substantially if the loop parameters, in particular the code tracking loop, can be optimized. In this experiment, both the bandwidth of the chip spacings have been optimized for enhanced performance. This paper also provided a comprehensive overview of the code and carrier tracking loops. More experiments have been planned for the future to demonstrate the integration issues.

References

Cahn, R.C., Leimer, D.K., Marsh, C.L., Huntowski, F.J., & Larue, G.D., (1977) Software Implementation of a PN Spread Spectrum Receiver to Accommodate Dynamics. IEEE Transactions on communications, 25 (8) pp.832-839.

Gustafson, D., & Dowdle, J., (2003) Deeply Integrated Code Tracking: Comparative Performance Analysis. 16th Int. Tech. Meeting of the Satellite Division of the U.S. Inst. of Navigation, Portland, OR, 9-12 September, 2553-2561.

Jwo D.-J., (2001) Optimization and Sensitivity Analysis of GPS Receiver Tracking Loops in Dynamic Environments. IEE Proceedings of Radar, Sonar Navigation, 148, 241-250.

Kaplan, E. D., (1996) Understanding GPS Principles and Applications. Mobile Communications Series, Artech House, Boston, London.

Tsui, J.B.Y., “Fundamentals of Global Positioning Receivers – A Software Approach”, John Wiley & Sons, 2000.

Van, D.A.J., Fenton, P., & Ford, T., (1992) Theory and Performance of Narrow Correlator Spacing in a GPS Receiver. Navigation: Journal of the Institute of Navigation, 39, 265-283.

Ward, P., (1998) Performance Comparisons Between FLL, PLL and a Novel FLL-Assisted PLL Carrier Tracking Loop Under RF Interference Conditions, 11th Int. Tech. Meeting of the Satellite Division of the U.S. Inst. of Navigation, Nashville, Tennessee, 15-18 September, 783-795.