In-band full-duplex (FD) relays are useful for extending coverage areas and increasing overall throughput in wireless\nnetworks. The main technical difficulty hindering their implementation and use is their inherent self-interference (SI),\ngenerated due to simultaneous in-band reception and forwarding. Efficient SI mitigation is a practical necessity, and\nthe imperfections in transceiver electronics, from which power amplifier (PA) non-linearity is one of the most serious\nphenomena, have to be taken into account in order to not limit the performance of such techniques. The magnitude\nof the distortion introduced by the PA depends on the relay input back-off (IBO) whose optimization for alleviating\nthe effect of PA non-linearity is the main research objective in this paper. In particular, although plain signal-to-noise\nratio (SNR) at the destination obviously increases when the IBO decreases, increased transmit power also strengthens\nthe non-linear distortion leading to decreasing overall signal-to-interference-plus-noise ratio (SINR). We develop\nexpressions for bounding the optimal IBO setting that maximizes the SINR at the destination, considering all relevant\nhardware impairments and SI cancellation with I/Q imbalance compensation. We provide closed-form solutions for\nthe soft-limiter PA model and numerical results for more general PA models. Finally, the derived IBO bounds are\ncompared with the numerical maximization of the SINR and the minimization of the bit-error rate (BER) to\ndemonstrate that the theoretical bound settings provide good approximations to the optimal one.
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