An experimental performance evaluation of a novel radio-transmitter identification system under diverse environmental conditions

In this paper, the effects of environmental conditions on a radio-transmitter identification system are investigated. Transmissions from 10 commercial VHF FM transmitters operating under varying environmental conditions are analyzed using an experimental setup. It is observed that a change in environmental conditions such as battery voltage or ambient temperature causes the feature vectors to spread over the feature space, resulting in classification performance degradation. It is experimentally shown that performance degradation resulting from changing environmental conditions can be compensated for by training the system with data collected in a range of battery-voltage and ambient temperature levels. The signal-to-noise ratio versus probability of correct classification is also determined for systems operating in noisy channels in order to obtain a confidence measure for the classification decision to be made. The transient features are extracted using a technique based on the amplitudes and phase changes of the received signals. This technique is well suited to radio-transmitter turn-on transient signals, and classification with these features, as shown, outperforms current techniques.

I. Introduction

When a radio transmitter is activated or “keyed,” the RF signal emitted from the transmitter generally shows a transient behaviour with respect to the instantaneous frequency and amplitude. The duration of this transient behaviour may range from a few microseconds to a few tens of milliseconds, depending on the type and the model of the transmitter. Transient signal behaviour has been attributed to a variety of sources, such as the acquisition characteristics of phase-locked-loop frequency synthesis systems, modulator subsystems, RF amplifiers’ antenna, switch and relay characteristics. Typically, differences are observable even for transmitters of the same type, and can be used to differentiate between transmitters. Consequently, the turn-on transient signal behaviour is sometimes considered to be a “fingerprint” of the radio. A transmitter identification system would be a valuable tool for spectrum management operations in both military and civil-ian situations where unauthorized use of the electromagnetic spectrum may occur.