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We study a multifunction transceiver whose operation is based on the frequency-modulated continuous-waveform concept to facilitate self-interference suppression. This causes the received signals after downconversion to move across the spectrum according to the frequency-modulation function. It is critical to accurately compensate these frequency deviations in order to process the received information. In this paper, we introduce a method for estimating these deviations in terms of the relative time delay between the signals used for downconversion and compensation. Estimation of this delay relies on statistics of the received signal, and only requires the aid of a generic auxiliary signal. Our method is supported by analytic derivation of the estimation formula, and is validated by means of simulation results. acceptedVersion

Malicious operators in the shared spectral resources may threaten the security of legitimate users. Their illegal operations can take the form of trespassing drones or the eavesdropping of communications that can lead to hijacking of systems or spoofing of legitimate devices, such as Internet access points. More defence mechanisms against such usage need to be created. One method to defend against malicious acts in the physical layer is to counter-attack them by utilizing jamming that prevents other devices, such as radio-controlled drones, from using a frequency band in a geographic area. This traditionally also affects the device sending the jamming signal. A device operating with full-duplex capability could be utilized to enable the jamming device to perform data reception, spectrum sensing, radar functions or positioning operations. Full-duplex means simultaneous transmission and reception utilizing the same spectral resources. Traditionally such operation has been deemed impossible due to the high-powered transmit signal being received as self-interference, but recent research into its cancellation has shown that full-duplex operation is indeed plausible. We call a device that transmits protective jamming while simultaneously receiving on the same frequencies a radio shield. In this thesis, a novel full-duplex radio shield device inspired by the frequency-modulated continuous-wave radar is proposed. The transmitted frequency-sweeping jamming signal is used in the downmixing operation of the received signal, which enables the self-interference signal to be attenuated simply-but-effectively by a DC block. Unfortunately, such operation also causes sweeping attenuation of some subcarriers in the signal-of-interest. Through theoretical analysis and practical measurements, the effects of different jamming signal parameters on the signal-to-interference-plus-noise ratio of a received WLAN signal-of-interest are investigated. In the measurement system, the DC block is emulated digitally to study the effects of different stopband widths in the absence of an electronic variable DC block, and the transmission and reception electronics are connected with a coaxial cable. The system is shown to provide a maximum self-interference cancelation of 47 decibels. This value is measured without a signal-of-interest. From the experiments, the following three observations are made: (1) The DC block is found to be unable to mitigate all of the distortions created by transmission and reception electronics operating non-linearly when the transmit power is set too high, and increasing the DC block stopband width improves the situation; (2) The WLAN signal is found to be robust enough to withstand some symbol errors caused by the DC block without erroneously interpreted bits, and increasing the DC block stopband width deteriorates this performance; (3) The jamming signal parameters, namely sweep frequency and sweep bandwidth, are shown to cause a small effect on the signal-of-interest reception performance, while increasing these parameters necessitates the increase of the DC block stopband width for the system to stay robust against reflections from the radio channel. In addition, an equation to determine theoretical minimum DC block stopband width for a certain jamming signal and deployment environment is derived for the design of radio shield devices. The analog-domain cancelation performance of the proposed system is found to be close to the reported state-of-the-art in research literature. Additionally, full-duplex WLAN reception is shown to be possible with the system. These results prove that the proposed system is a viable candidate for a radio shield device.