Simulation Results

The simulations are conducted to compare the proposed ACC algorithm with the DCC and the NCC schemes. Each technique is executed with 200 runs to acquire the averaged simulation results. Figs. 4.2 and 4.3 illustrate the performance comparison between these three schemes

under different SNR values. Under the same 95% of detection probability (as shown in Fig. 4.2), the proposed ACC scheme results in comparably less integration time for signal acquisition, which is around 56 ms and 16 ms less than that from the NCC and the DCC schemes under SNR = -39 dB. As illustrated in Fig. 4.3, the detection probability obtained from the ACC scheme can achieve up to 68% at SNR = -39 dB (with 50 ms of integration time); while only 50% and 20% of the detection probabilities are obtained by using the DCC and the NCC scheme. To make the comparison more fair, we also compare the detection probability and the integration of DCC and NCC with the adaptive detection threshold.

As shown in Fig. 4.2 and 4.3, the results of DCC and NCC with ADT(adaptive detection threshold) excel the ordinary DCC and NCC with pre-defined detection threshold.

It is also noted that the pure CC scheme is not utilized for comparison since it is compara-bly the worst scheme (which can not even achieve 10% of detection probability under various SNR values) due to the navigation bit-transition. The merits of using the ACC schemes can therefore be seen from the simulation results. To consider the more realistic situation, we add a un-deterministic term to the received center frequency, make it varies around the center frequency about 100 Hz for each 1-ms signals. We use the same criterion to compare the ACC with the NCC and DCC scheme. The simulation results can be shown in Fig. 4.4 and 4.5.

The variations of the received center frequency decreases the probability of all three schemes, and increases the necessary integration length to achieve the 95% detection probability, but the advantage of ACC scheme is still obvious comparing with other two schemes.

We add a feedback loop in the ordinary C/N0 estimator and the simulation results are as shown in Fig. 4.6, 4.7, and 4.8. As we can see in these results, the modified C/N0 estimator outperforms the ordinary estimator in convergence speed and accuracy.

To investigate the performance of the C/N0 estimator in the whole system, the detec-tion probabilities of the system with the SNR estimated and the SNR previously known are compared in Fig. 4.9.

−330 −34 −35 −36 −37 −38 −39

Figure 4.2: The Total Integration Time vs the SNR Value (with 95% of Detection Probability)

−33 −34 −35 −36 −37 −38 −39

Figure 4.3: The Detection Probability vs the SNR Value (with 50 ms of Total Integration Time)

−330 −34 −35 −36 −37 −38 −39

Figure 4.4: The Total Integration Time vs the SNR Value with Center Frequency Uncertainty (with 95% of Detection Probability)

Figure 4.5: The Detection Probability vs the SNR Value with Center Frequency Uncertainty (with 50 ms of Total Integration Time)

0 50 100 150 200 250

Figure 4.6: The Estimation Curve of Open Loop Estimator and Close Loop Estimator in -30dB

Figure 4.7: The Estimation Curve of Open Loop Estimator and Close Loop Estimator in -35dB

0 50 100 150 200 250

Figure 4.8: The Estimation Curve of Open Loop Estimator and Close Loop Estimator in -39dB

Figure 4.9: The Detection Probability of Known SNR and Estimated SNR

Chapter 5

Conclusions

In this thesis, the related techniques for the GPS receiver acquisition process has been inves-tigated. The acquisition in time domain and frequency domain are introduces individually.

The concept of A-GPS and the indoor acquisition methods, including three combining meth-ods, even-odd multi-period integration method, are also introduced. An Adaptive Coherent Combining (ACC) acquisition technique is proposed. The ACC scheme is adaptively adjusted between the coherent or the differential coherent combining method based on the detection of the navigation bit-transition. The peak detection threshold is adaptively adjusted accord-ing to the environmental SNR estimated form the C/N0 estimator. The total integration time and the detection threshold are also adjustable based on the SNR values. As shown in the simulations, the proposed ACC scheme can provide better performance comparing with both the differential coherent and the non-coherent methods, especially under weak signal environment.

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