Pseudo-range measurements using OFDM channel estimation
Résumé
Mobile positioning using wireless telecommunication networks has been the focus of recent research as a potential, cost-effective solution for positioning a mobile user in urban environment, where the performances of Global Navigation Satellite System (e.g. availability, dilution of precision) may be reduced, while the availability of wireless telecommunication networks is important. Such research has been motivated by regulation incentives (e.g. E-911) and the explosion of location-based services. In this paper, a method for calculating pseudo-range measurements from DVB-T/-H/-SH signals is proposed. These signals are standardized for digital TV broadcasting and uses the spectrally-efficient and multipath-resistant OFDM modulation. The main challenge to be solved when using wireless telecommunication networks is the intense multipaths encountered in the urban propagation channel, leading to important bias in the pseudo-range measurements. The DVB-T family signals contain pilot symbols that are used for channel estimation and equalization necessary to demodulate the transmitted data, but which can also be used for pseudo-range measurement. The presented method makes use of these available pilot symbols. The proposed method comprises 3 steps: first, an estimation of the channel impulse response is obtained by calculating the correlation of the received signal with a pilot symbol-only local replica. Then, the matching pursuit algorithm is used to coarsely estimate the delay of several taps. Finally, these initial delay estimates are used to initialize several delay-lock loops (DLL), which refine the delay estimates and track their changes. Tracking several taps in the propagation channel is necessary to increase the chances of tracking the line-of-sight (LOS) tap in a multipath environment. Additionally, in case of Rayleigh fading usually encountered by mobile users, re-launching periodically the whole process can be useful to regain the LOS signal in case the DLL has diverged during a deep fading period. Finally, the proposed method can be used in a modified single frequency network, where several emitters transmit the same signal in a synchronized way and on the same carrier frequency, as foreseen for the deployment of DVB-SH systems. In that case, our method would not require modifications to the receiver up to the demodulator. This paper ends by illustrating the performances of the proposed method in AWGN, multipath and single frequency network scenarios.
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