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Correlating radio signals with light

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Researchers have devised an optical architecture to calculate the correlation of radio signals in an analogue manner. This operation would make it possible, for example, to locate radio transmitters with precision, or to improve the performance of optical interferometry in astronomy.

Signal correlation is a fundamental operation for many imaging and localisation applications. In astronomy, interferometry techniques provide high-resolution images from the correlation of signals received by different antennas or telescopes. Similarly, a radio transmitter can be located by correlating the relative delays of signals received at different locations. However, the digital techniques that usually perform this operation are inherently limited by the sampling rate of the converters, as well as by the real-time processing capacity. In practice, digital correlation cannot handle radio signals with a bandwidth of more than a few hundred MHz. However, bandwidth is an important parameter, as it is synonymous with more flux in astronomy and increased resolution in the case of transmitter location. Researchers at the Laboratoire Interdisciplinaire de Physique (LIPhy, CNRS/UGA) and the Institut de Planétologie et d’Astrophysique de Grenoble (IPAG, CNRS/UGA) have developed a new concept of photonic architecture that calculates the correlation function of radio signals in real time in an analogue manner and is suitable for signals with a bandwidth of a few MHz to a few GHz. These results are published in the journal Optica.

Figure : Measurement of the time difference of arrival of radio frequency signals by photonic correlation. The signal whose spectrum is shown in (a) is transmitted by the Tx antenna (b). Two receiving antennas (Rx1 and Rx2) pick up the signal and send it to the analogue optical correlator (b). The maximum of the correlation function gives the difference in signal propagation time between the transmitting antenna and the two receiving antennas. By moving Rx2 (in 1 inch steps), a translation of the correlation function is observed, due to the change in propagation time between the transmitter and Rx2 (c).

This concept is based on multi-heterodyne interferometry and consists of giving the full correlation function of two signals in real time by simultaneously calculating the cross-correlation coefficients between the signals for more than 200 relative delay values. To do this, the radio signals are transferred into the optical domain and then sent through a pair of loops whose function is to produce replicas of the input signals that are both time and frequency shifted. The main contribution of the method is this double loop architecture which allows the calculation of the correlation function to be parallelized. The time step of the correlator, which corresponds to the difference in travel times in the two loops, is adjusted from a few nanoseconds to a few picoseconds to process signals with a bandwidth ranging from MHz to GHz. The researchers have applied this architecture to the localisation of radio frequency transmitters by time difference of arrival (figure) and obtained an accuracy close to 10 ps for an integration time of 100 ms.

It now remains to characterise the performance of this architecture in order to apply it to the real-time localisation of transmitters, such as wifi and mobile phones. In astronomy, a preliminary experiment of imaging the sun by radio interferometry at 10 GHz will be carried out at IPAG in order to evaluate the interest of this technique for imaging.

Voir en ligne : Multi-delay photonic correlator for wideband RF signal processing. G. Bourdarot, J.-P. Berger, H. Guillet de Chatellus ;Optica, paru le 24 mars 2022.