Improper Signaling for SISO Two-user Interference Channels with Asymmetric Hardware Impairment

Hardware non-idealities are among the main performance restrictions for upcoming wireless communication systems. Asymmetric hardware impairments (HWI) happen when the impairments of the I/Q branches are correlated or imbalanced. In this paper, we investigate the benefits of improper Gaussian signaling (IGS) in a two-user interference channel (IC) with asymmetric HWI when interference is treated as noise. The real and imaginary parts of improper or asymmetric Gaussian signals are correlated and/or have unequal powers. IGS has been recently shown to improve the performance of different interference-limited systems. We propose two iterative algorithms to optimize the parameters of the improper transmit signals. We first rewrite the rate region as an energy region and employ fractional programming and the generalized Dinkelbach algorithm to find a suboptimal solution for the achievable rate pairs. Then, we propose a simplified algorithm based on a separate optimization of the powers and complementary variances of the users, which exhibits lower computational complexity. We show that IGS can improve the performance of the two-user IC with HWI. Our proposed algorithms outperform proper Gaussian signaling as well as competing IGS algorithms in the literature that do not consider asymmetric HWI.