Multipath tracking algorithm for reducing the code alignment error

It is well known that multipath errors become particularly significant in urban or indoor navigation, where the presence of many scatterers around the receiver may be relevant. When a SIS replica, delayed within one chip with respect to Line Of Sight (LOS), arrives at the receiver the S-curve of the DLL is distorted. The punctual local code replica of the DLL can not be exactly aligned to the LOS and a systematic error in the pseudorange estimation is experienced. The architecture implemented (introduced in Dovis et al., 2004) in the reconfigurable hardware is depicted in Figure 6. A key role is played by the multipath Monitoring Unit which is in charge of activating the multistage architecture according to the scenario. In particular it has to:-
- estimate the number of replicas impinging at the antenna employing    an algorithm
- compare them with a predefined threshold to roughly determine their    relevance to the distortion of the S-curve
- activate the multistage procedure for the tracking loop

Figure 6 shows the innovative architecture in the case of one multipath arriving at the receiver, but it can be easily extendable to the general situation in which the input signal arrives at the antenna via n-th different paths.

The local code replica of the DLL0 is usually delayed with respect to the LOS code due to the distortion of the S-curve. Such a code instance is subtracted from the filtered incoming code r(t), giving a signal r1(t) as input for the DLL1, which is therefore able to track the multipath. After a transient phase, the first DLL tracks the incoming LOS and the second multipath replica. The local code of DLL1, that follows the evolution of multipath, is then subtracted from the incoming code r(t) and the resulted signal is sent as input for the DLLNAV. The local code of this last DLL can be used for pseudorange estimation.

Figure 7 shows the result obtained by simulation of the module in case the multipath and LOS signals have the same phase rotation, the multipath is 0.7 chip delayed with respect to LOS and its amplitude is half the LOS amplitude. It can be noted that using a single DLL narrow correlator (0.125 chip spacing), no reasonable tracking is possible; in fact, the mean of the tracking jitter is about 0.07 chip apart from the correct zero crossing (dashed line in Figure 7) for all the C/N0 values considered. Figure 7 also reports the performance of a single DLL when no multipath is present (dotted line), and the performance of the innovative architecture (solid line). It can be observed from the picture that employing the new scheme, the systematic error can be significantly reduced for all C/N0 values.

Figure 8 shows the theoretical multipath envelops of a single DLL narrow correlator and of the whole simil-RAKE architecture compared to the systematic error obtained by simulation. Both simulation results and theoretical envelopes show that using the innovative architecture, it is possible to drastically reduce the systematic error for all the multipath delays considered.

Conclusions

This paper has discussed the implementation of innovative functionalities for the realization of GNSS receiver according to the SDR philosophy. The study has been conducted highlighting novel aspects of the architecture which allows several degrees of reconfigurability. In particular, the implementation of the Navigation Unit has been analyzed and finally, the adaptive multipath rejection algorithm has been discussed presenting the simulation results of the implemented architecture.

Akos D M (2003) The role of Global Navigation Satellite Systems (GNSS) software radios in embedded systems, GPS solution, vol. 7, no. 1

Braasch M. W. (1996) Performance Comparison of Multipath Mitigating Receiver Architecture, IEEE PLANS 1996

Buracchini E (2000) The Software Radio Concept, Commun. Mag., vol.38, no.9, pp. 138-143

Dovis F, Gramazio A, Mulassano P (2002) SDR Technology Applied to Galileo Receivers, ION GPS 2002

Dovis F, Mulassano P, Pini M (2004) Multiple DLL Architecture for Multipath Recovery in Navigation Receivers, IEEE VTC 2004

Laxton M C, De Vilbiss S L (1977) GPS Multipath Mitigation During Code Tracking, in Proc. of the American Control Conference

Mitola J (1995) The Software Radio Architecture, IEEE Commun. Mag., vol.33, no.5, pp. 26-38

Mulassano P, Dovis F, Avagnina D, Gramazio A (2001) REGAL: A Reconfigurable Receiver for GPS and Galileo, GNSS 2001