Active Signal Injection for Self-Interference-Cancellation

Abstract

A common problem in full-duplex operation is the existence of a large self-interference (SI) at the receiver input, which prevents the proper detection of the intended signal. This chapter presents a new active signal injection (ASI) approach for full-duplex multiple-input multiple-output (MIMO) OFDM systems to reduce the SI prior to the receiver low-noise amplifier (LNA) and the analog-to-digital converter (ADC) to avoid overloading them while ensuring a proper signal detection. Basically, the ASI approach aims to add to the transmitted signal, an appropriate pre-cancelling signal that can reduce the SI at the receiver input of the same transceiver, but does not affect the detection process of the other intended receiver. To that end, two methods to realize the ASI approach are proposed and studied. In the first one, called tone reservation, the pre-cancelling signal only occupies some reserved subcarriers that are not used for data transmission, so that the subcarrier orthogonality can separate the transmitted data signal from the pre-cancelling signal in data detection. In the second method, called constellation relaxation, the data signal constellation points are dynamically extended to include the pre-cancelling signal in such a way that it minimizes the received SI but does not affect the intended data detection. The designs of the effective pre-cancelling signal in two methods are formulated as the optimization problems with derived solutions. A combination of both methods is also proposed to further enhance the SI-cancellation capability. Illustrative simulation results indicate that the proposed methods can offer a very good SI-cancellation performance with simple additional baseband digital signal processing implementation in the transmitter and without any required change in the receiver.