Carrier Phase Tracking of OFDM-Based DVB-T Signals for Precision Ranging
Résumé
In general, the carrier phase of orthogonal frequency division multiplexing (OFDM) signals is not tracked for at least two reasons. First, the dc component of most baseband OFDM symbols is a null subcarrier so as to avoid the effect of dc bias at reception. Second, generation and transmission of OFDM symbols are independent from one symbol to next. As a result, no phase continuity is required to be maintained at any subcarriers. For communications, demodulation of OFDM symbols with cyclic prefix (CP) is tolerant to small timing errors and insensitive to the absolute phase but relative phase at data subcarriers, which can be easily calibrated with the help of pilot subcarriers. However, the OFDM signaling adopted by the Terrestrial Digital Video Broadcast (DVB-T) retains the dc component. Besides, the CP duration is specified in such a way that a whole number of cycles is ensured for the middle carrier. It happens in DVB- T that the middle carrier is assigned as a continual pilot subcarrier, which has a constant value across OFDM symbols. As a result, the baseband center frequency (dc component) has no phase discontinuity, which gives rise to the opportunity for carrier phase tracking. Carrier phase tracking has the potential to provide more accurate timing for ranging and ultimately for positioning than cross-correlation of CP and pilot subcarriers in coarse and fine time of arrival (TOA) estimation, respectively. In this paper, in-the air DVB-T signals collected in experimental tests are used to show the functionality and performance of DVB-T signal carrier phase tracking.