Recursive Least Squares Self-Interference Mitigation in Broadband MIMO Full-Duplex Relays

In-band full-duplex transmission allows a relay station to theoretically double its spectral efficiency by simultaneously receiving and transmitting in the same frequency band, when compared to the traditional half-duplex or outof-band full-duplex counterpart. Consequently, the induced self-interference suffered by the relay may reach considerable levels of power, which decreases the signal-to-interferenceplus-noise ratio (SINR) in a decode-and-forward (DF) relay, leading to a degradation of the relay performance. Thus, this paper presents a technique to cope with the problem of self-interference in broadband multiple-input multiple-output (MIMO) relays. The proposed method uses a time-domain cancellation in a DF relay, where a replica of the interfering signal is created with the help of a recursive least squares (RLS) algorithm that estimates the interference frequency-selective channel. The algorithm properties are analyzed, considering the filter asymptotic covariance error matrix expression. Its convergence mean time is shown to be negligible by simulation results, when compared to the length of typical orthogonalfrequency division multiplexing (OFDM) sequences. Moreover, the bit-error-rate (BER) and the SINR in a OFDM transmission are evaluated, which confirm that, under low limited dynamic range, the proposed method provides a significant performance gain when compared with reference mitigation schemes. Index terms Decode-and-forward relay, in-band full-duplex, broadband MIMO, frequency-selective channel, adaptive filtering, recursive least squares, self-interference cancellation.