HIGH-POWER FULL-DUPLEX RADIO PROTOTYPE AT LOWER UHF BAND RF Self-Interference Canceller Design and Implementation

Miika Vuorenmaa: High-Power Full-Duplex Radio Prototype at Lower UHF Band — RF SelfInterference Canceller Design and Implementation Master of Science Thesis Tampere University Electrical Engineering – Electronics May 2022 It has been shown that simultaneous transmission and reception at the same frequency band is possible by cancelling the self-interference (SI) caused by a transmitter. One method for SI cancellation is to couple a low-power sample from the antenna feed and modify this signal to match the received SI signal with an inverse phase, and then combine it with the received signal. The amplitude and the phase of the sampled signal has to be exactly aligned with the received SI signal so that they cancel each other by superposition. Successful cancellation of the SI requires a combination of methods in the electromagnetic antenna domain, in the analog RF front end, and in the digital baseband to enable full-duplex (FD) operation. All of these methods suppress the SI and contribute toward the total cancellation. Several analog canceller circuits have already been implemented for power levels used in nonmilitary communications and commercial applications. Transmit power level is usually less than 40 dBm (or 10 watts) in these designs. The operating frequency of majority of the cancellers is centered around the 2.4 GHz industrial, scientific and medical (ISM) band and they are implemented using components with low power handling capability. The main challenges for the use of FD radio technology in military communications are to operate at lower frequencies, and to support significantly higher transmission powers from 100 W up to 1 kW. In particular, the power flowing through the components will increase compared to previous analog canceller designs. Therefore, solving these limitations, especially for an analog canceller, is one of the main topics of this thesis and one of the key challenges in the development of high-power full-duplex military radios. In this thesis, a new high-power analog RF canceller architecture is introduced, which is built on a PCB using discrete power-tolerant components. The new canceller consists of a delay bank in series with a vector modulator and it is designed for the lower UHF band from 225 MHz to 400 MHz which is relevant for military communications. Measurements show that the delay bank combined with the 90-degree adjustable tuning from a vector modulator is able to provide a full 360-degree phase coverage over the entire frequency band. Amplitude tuning range is about 40–50 dB. Cancellation performance is measured over both narrow and wide bandwidths while increasing the input power with different center frequencies from 225 MHz to 400 MHz. The results for a narrow band of 100 kHz show about 30–40 dB of cancellation with maximum available input power. The measurements also study a scenario where a desired signal is received while cancelling the interference. Link budget calculations are performed to estimate possible detection distances for the desired signal while cancelling the interference. The canceller is able to handle 100 watts of transmission power and can be scaled up to 1 kilowatt with slight modifications.