Adaptive Canceller Control and Signal Detection Algorithm for Full-Duplex Radio Transceiver

Abstract

Simultaneous transmission and reception at same frequency band is gaining increasing interest in military communications research. This is because simultaneous operating would enable combining different functionalities, such as communications, signals intelligence, and electronic warfare in a single transceiver, giving a tactical edge over adversaries. The main challenge in utilizing such full duplex capability is self-interference (SI), caused by transmitter. To make full-duplex system work, sufficient SI cancellation needs to be achieved. This requires combining cancellation methods at antenna level, in analog RF front end, and in digital domain. In analog RF front end, one method for cancelling SI is to take a low power sample of the transmitted signal and modify it such that it has equal amplitude and opposite phase compared to received SI signal. Signals are combined in the receiver, and due to opposite phases, they cancel each other out.

Much research has been done on the use of full duplex in commercial applications, where it has been proven that enough SI cancellation is achievable. However, research on implementing full-duplex technology to military applications has been minor until recently. Military applications bring many additional challenges for creating a full-duplex device, main ones being the use of lower frequencies, higher transmit powers, and lower receiver sensitivities. For these reasons, more SI cancellation is needed, and solutions used in commercial devices are not directly compatible. Before this thesis work, only a few working military full-duplex devices have been published, all operating at the 2.4 GHz ISM band.

In this thesis a full-duplex transceiver operating at lower 225 – 400 MHz frequency range is presented. Proposed transceiver uses already designed analog canceller board and can operate with 100 W transmit power. Focus of thesis was to create adaptive control algorithm, which constantly tunes analog cancellation to match channel conditions, and adding signal detection capability for the receiver. Control algorithm was implemented using least squares (LS) algorithm and data on how analog canceller modifies the cancellation signal, measured by the transceiver itself. LS is also utilized for calculating coefficient for compensating phase and amplitude errors between received signals, which would cause error on analog cancellation. Lastly, signal detection was implemented using a modification of cellaveraging constant false alarm rate (CA-CFAR), and a functionality for sweeping point of maximum cancellation over wanted frequency band.

Transceiver was demonstrated to provide 53 – 60 dB of analog domain SI cancellation when only direct path SI component is present, and can reach maximum cancellation within four iterations. Based on results transceiver could conduct jamming with 100 W transmit power and still be able to detect signal of interest (SOI) from ranges up to 15 km. Cancellation measurements were repeated in RF chamber to evaluate performance in multipath conditions. Multipath environment decreased cancellation to 29 dB due to large power reflections from chamber walls, and control algorithm’s relative slowness compared to the fast changes in multipath channel of RF chamber.