Photonic technique for unambiguous AOA measurement without knowing incoming RF signal power

Abstract

A technique that is capable of determining an RF signal angle-of-arrival (AOA) unambiguously over a wide measurement range without knowing the power of the incoming RF signal is presented. It is implemented in a microwave photonic system using a dual-polarisation Mach Zehnder modulator followed by optical filters, and a pair of polarisation beam splitters and low-speed balanced photodetectors. Two DC voltages are generated at the balanced detector outputs. The DC voltages as a function of the input RF signal phase difference have a sine–cosine behaviour. This enables the RF signal phase difference in the range of ±180°, which corresponds to a full ±90° AOA measurement range, to be determined unambiguously simply using the ratio of the two system output DC voltages. Hence, knowing the power of the incoming RF signal is not required but is needed in many reported structures for AOA measurement. The proposed photonic-assisted AOA measurement system is free of microwave components and can measure an RF signal AOA instantaneously. Experiments have been conducted to demonstrate the concept of the proposed technique. Measured results demonstrate AOA measurement in the range of −70.8° to 70.8° with errors less than ±2.2°, for different input RF signal frequencies, without requiring knowledge of the input RF signal power.