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Abstract. In this paper, the characteristics of radio transmitter fingerprints will be examined by analyzing both the amplitude and phase information obtained from complex envelope recordings of transmitter turn-on transients. The interest in the analysis of such transients is related to the identification of malfunctioning or illegally operated radio transmitters in support of radio spectrum management practices. Of the 28 VHF radios considered in this study, many were found to produce fingerprints having uniquely distinctive features which could be used for identification purposes. Unfortunately, some of these radios were found to have fingerprints that were virtually indistinguishable from each other, making the identification process more difficult, if not impossible. Details of the equipment, analyses, and data collection procedures will be presented along with a discussion of the experimental results. The merits of this technique over others currently in use will also be presented. IntroductionWhen a radio transmitter is activated or "keyed," it goes through a relatively short transient phase during which the signal emanating from the unit displays characteristics that are believed to be unique to the extent that they can be used to unambiguously identify an individual transmitter [Toonstra and Kinsner, 1995]. These signatures or transients are often referred to as "fingerprints" and have been attributed to a number of potential causes (which will not be dealt with in this paper) such as the lock-in times associated with phase-locked loop ( Experimental Setup and Data CollectionOne advantage of the complex envelope technique is that amplitude, phase, and, consequently, frequency information can be extracted from the recorded fingerprints. By virtue of this additional information it is likely that a greater number of features can be found, which may aid in the identification process. To obtain the complex envelope, a quadra-

A transient detection technique is presented for the detection of the turn-on transients of Wi-Fi radios. The turn-on transients are detected by a Bayesian change detector, which estimates the time instant when the transmitter starts to power up. The proposed technique is verified with the transient data collected from a number of Wi-Fi radios and it is shown that the ramp detector outperforms the abrupt change detector in detecting the turn-on transients of Wi-Fi transmitters.

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