Angling for angles

Abstract

Researchers from Australia have presented their novel technique to estimate incoming radio-frequency (RF) signal angles of arrival (AOA), irrespective of the RF signal amplitude. Experimental results show that the AOA measurement can be achieved between 0°–63° with less than 3° of errors for a 15 GHz signal at varying power levels. Identifying the location of an object is extremely important for a number of different applications, including navigation, wireless communication and electronic warfare. The process of identifying an object involves a transmitter, for example a mobile phone or a radar unit, that transmits an RF signal, and a detector device to measure the RF signal. The RF signal reflects off an object and towards the detector. The detector unit can identify the difference in time or phase for the arriving RF signal to when it was transmitted – the angle of arrival. The AOA can then be used to identify the object's location. Chan (left) and Chen (right), authors of the published submission. Correlation between estimated (Y) and actual (X) AOA. The deviation above 60° can clearly be seen. Schematic of the proposed system. Using microwave photonics, the AOA of high-frequency RF signals can be determined, with a reduction in electromagnetic interference as well as system size, weight and power consumption. There are drawbacks to using microwave photonics, however, in that a calibration process involving measuring either system output power at a 0° AOA, or measuring the incoming RF signal amplitude, is necessary to eliminate the RF signal amplitude dependence in the system output prior to measuring the AOA. The work of Chan and Chen is useful because it allows for a measurement of the AOA without the need for measuring the incoming RF signal amplitude, simplifying the process. The new technique works by using a continuously laser-generated wave of light, which is split equally between two modules. Each module comprises one dual-drive Mach Zehnder modulator, an optical bandpass filter and a photodetector. After some phase shifting, the photodetectors eventually generate two DC photocurrents, which can be used to calculate a K value which is a function of the RF signal phase difference. K can then be used to derive the AOA without the need for information about the input RF signal amplitude. Further experiments were carried out using the system. An RF signal of 15 GHz was generated by a microwave signal generator, producing two RF signals with 3.2 dBm power level after the initial splitting between the two modules. It was found that the methodology performed well when the actual AOA was between 0° and ∼60°. The experiment was also repeated with different signal powers of −0.3 dBm and −3.6 dBm, with accurate results being obtained, proving that the AOA can be estimated independent of the RF signal amplitude. With a larger optical carrier suppression, it should be possible to accurately measure the RF signal AOA beyond 60°.