Abstract
In this paper, we propose a joint analog-digital (A/D) beamforming scheme for the point-to-point multiple-input-multiple-output system, where we exploit mutual coupling by optimizing the load impedances of the transmit antennas. Contrary to the common conception that mutual coupling strictly harms the system performance, we show that mutual coupling can be beneficial by exploiting the concept of constructive interference. By changing the value of each load impedance for the antenna array based on convex optimization, the mutual coupling effect can be manipulated so that the resulting interference aligns constructively to the useful signal vector. We first prove that the full elimination of mutual coupling effect is not achievable solely by tuning the values of the antenna load impedances. We then introduce the proposed A/D scheme for both PSK and QAM modulations, where performance gains with respect to conventional techniques are obtained. The implementation of the proposed schemes is also discussed, where a lookup table can be built to efficiently apply the calculated load impedances. The numerical results show that the proposed schemes can achieve an improved performance compared to systems with fixed mutual coupling, especially when the antenna spacing is small.
Highlights
T HE benefits of multiple-input-multiple-output (MIMO) systems have been widely acknowledged and extensively studied in recent years due to the performance gains over single-input-single-output (SISO) systems
Different from existing studies that aim at minimizing or compensating for the mutual coupling effect, in this paper we propose to exploit the mutual coupling effect to further improve the MIMO performance by exploiting the concept of constructive interference [6]
We summarize the contributions of this paper as: 1) We propose to exploit the mutual coupling effect for MIMO systems to further improve the system performance by means of analog processing with tunable antenna load impedances
Summary
T HE benefits of multiple-input-multiple-output (MIMO) systems have been widely acknowledged and extensively studied in recent years due to the performance gains over single-input-single-output (SISO) systems. Many existing studies on receiver structures and preprocessing approaches of MIMO assume an uncorrelated Rayleigh flat fading channel, where there is no spatial correlation or mutual coupling effect between antenna elements. Different from existing studies that aim at minimizing or compensating for the mutual coupling effect, in this paper we propose to exploit the mutual coupling effect to further improve the MIMO performance by exploiting the concept of constructive interference [6]. We summarize the contributions of this paper as: 1) We propose to exploit the mutual coupling effect for MIMO systems to further improve the system performance by means of analog processing with tunable antenna load impedances. C n×n represents an n × n matrix in the complex set and R(k, u) denotes the element of the kth-row and uth-column in R. di ag (·) denotes the conversion into a diagonal matrix with the values on its main diagonal and vec (·) is the operation of transforming a diagonal matrix into a column vector. (·) and (·) denote the real part and imaginary part of a complex number, respectively
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