Exploitation of Triangular Lattice Arrays for Improved Spectral Efficiency in Massive MIMO 5G Systems

Abstract

A thorough analysis of the performance of planar arrays with a regular periodic lattice is carried out and applied to massive multiple-input-multiple-output (MIMO) systems operating within 5G NR n257 and n258 frequency band. It is shown that, among different arrangements with uniform spacing, a triangular lattice guarantees the reduction of the Average Side Lobe Level (ASLL), a better angular scan resolution of the main beam within a predefined angular sector and a lower mutual coupling level among elements. Moreover, single beam and multibeam application scenarios are considered for the performance comparison and both cases assess the remarkable features offered by a triangular arrangement. Particular attention is paid to illustrate, for different propagation channel scenarios, the effects of the array lattice on overall system performance including average gain as well as Signal-to-Interference plus Noise Ratio (SINR) and Sum Spectral Efficiency (SSE). The obtained results prove that a regular and periodic triangular lattice is appealing for arrays to be adopted in massive MIMO 5G systems. INDEX TERMS Antenna array, 5G, massive MIMO, multibeam antenna, broadband array.

Spectral Efficiency Improvement of 5G Massive MIMO Systems for High-Altitude Platform Stations by Using Triangular Lattice Arrays

“…The HAPS scenario differs from that of a 5G ground BS—whose coverage spans 30° in elevation and 120° in azimuth [ 37 , 40 ]—since a circular scan area turns out to be more appropriate [ 18 ]. Therefore, it is assumed to cover an angular sector with a maximum steering angle of 60° off broadside (–60 θ 0 60°) for all ϕ angles (0 ϕ 0 180°), as shown in Figure 2 in the u–v plane.…”

“…The received signal-to-interference-plus-noise ratio (SINR) for the n th user in case of a multi-user scenario can be written as [ 40 , 50 , 51 ]: where G ( θ n , ϕ n ) represents the HAPS array’s gain toward the n th user located at ( θ n , ϕ n ); δ represents the ratio between the transmitted power ( P t ) and the noise power ( σ 0 ); h n ϵ 1× M ( n = 1,2,.., K ) corresponds to the channel propagation between the n th user and the HAPS antenna array; and w n ϵ M ×1 consists of the precoding vector that, in general, depends on the selected beamforming method. It is worth noting that, differently from Reference [ 37 ], the simulated matching efficiency ( η Γ ) was included in (9) in order to achieve a more accurate SINR estimate. To evaluate the system quality of service, a maximum ratio (MR) precoding technique [ 2 ] with a perfect channel state information (CSI) was assumed in the following analysis.…”

“…Additionally, since array elements’ mutual coupling (MC) undermines the MIMO performance [ 53 ] due to the unwanted correlation among elements, the channel matrix H is modelled as [ 54 ]: where H 0 ϵ K × M denotes the complex channel gain among the users and the HAPS antenna elements in the absence of antenna mutual coupling; I m×m is an identity matrix; and S ϵ M × M corresponds to the scattering parameters matrix of the HAPS planar array. Unlike [ 37 ], the channel matrix was evaluated by taking into account in (10) the full-wave simulation of the planar array, and not just the antenna element distance. Once the SINR is known, the maximum achievable bitrate over 1 Hz of bandwidth for the n th user, which is the spectral efficiency (SE) per user, is: …”

“…Few examples of triangular lattice 5G phased arrays are reported in the literature [ 35 , 36 ]. However, a comprehensive analysis addressing the overall performance of massive MIMO systems, including SE and SIR, has not been yet presented, although some recent studies have proven the advantages of triangular lattice arrays over square and rectangular ones for 5G massive MIMO system ground stations [ 37 ].…”

“…Section 3 is devoted to highlighting the superior robustness of triangular lattices for the antenna elements’ impedance variation during beam steering inside the sector when compared to square lattices, via full-wave electromagnetic simulations. This represents a step forward with respect to Reference [ 37 ], since it better quantifies the effects of mutual coupling and matching efficiency ( η Γ ). Massive MIMO metrics, such as SE and SIR, are evaluated in Section 4 , whilst conclusions are summarized in Section 5 .…”