Article Highlight | 30-Mar-2023

MIMO performance enhancement of MIMO arrays using PCS-based near-field optimization technique

Science China Press

Extensive efforts have been made in designing large multiple-input multiple-output (MIMO) arrays. However, there is still a gap between array antenna design and MIMO performance evaluation in realistic multipath scenarios. As a result, improvements in conventional antenna characteristics (such as isolation enhancement) cannot ensure significant MIMO performance improvement in real-life multipath environments. Due to the space limitation and the limited angular spread of the propagation channel, correlations inevitably occur between the MIMO array elements, which can significantly degrade the MIMO performance. Several array decorrelation techniques have been proposed, where correlation reductions have been achieved by either tilting the antenna beams or shifting the phase centers away from each other. Hence, these methods are mainly limited to small arrays for single-cell scenarios.

The paper titled: "MIMO Performance Enhancement of MIMO Arrays Using PCS-based Near-field Optimization Technique" mainly proposes a decorrelation optimization technique based on phase correcting surface (PCS) that can be applied to large existing MIMO arrays, enhancing their MIMO performances by reducing antenna correlation in a realistic (non-isotropic) multipath environment. Furthermore, the PCS structure can function as a radome or billboard, enhancing the performance of the existing base station (BS) and access point (AP) arrays.

The main idea of the PCS-based near-field optimization technique is to enhance the MIMO capacity of the MIMO array by improving its degree of freedom (DoF). MIMO capacity is the ultimate metric evaluated in the objective function during the optimization process. It also subjects to the improvement of DoF, a metric evaluating the antenna correlation of an array in multipath scenarios. These metrics can be calculated using the array’s embedded radiation patterns and channel coefficients. There is a performance gap between optimal and existing arrays in multipath scenarios. Thus, it is possible to change the near-field phase distribution on the array’s aperture, adding effects on far-field patterns, and providing better MIMO performances, say capacity and DoF, in multipath scenarios. The PCS-based near-field optimization technique is composed of two major steps. First, a near-field phase distribution that improves MIMO capacity is obtained using a near-field channel model and an optimization algorithm. Under the assumption of a spherical wave, the planer near-field channel model describes the planer near-field phase distribution on the array's aperture. The near- to far-field transformation algorithm also connects planer near-field and far-field radiation patterns. Thanks to the orthogonal basis representation of planar near-field, the workload of the optimization process can be significantly reduced. Second, the PCS (composed of square elements with desired transmission coefficients) is used to cover the array's aperture, resulting in the desired near-field phase distribution obtained in the first step. Two examples are introduced and have demonstrated the effectiveness of this PCS-based near-field optimization technique. One is a 1 ×  4 dual-polarized patch array (working at 2.4 GHz) covered by a 2 ×  4 PCS with 0.6λ  center-to-center distance. The other is a 2 ×  8 dual-polarized dipole array, for which a 4 ×  8 PCS with 0.4 λ  center-to-center distance is designed. Their MIMO capacities and DoF can both be effectively enhanced by about 8% in the single-cell scenario, which is a simple multipath scenario mainly considering the effects of angular spread. Moreover, the MIMO performance of the 2 ×  8 dual-polarized dipole array can be improved by 10% in multi-cell scenarios, which is a more realistic multipath scenario, including path losses of the UEs and intra- and inter-cell interferences. Furthermore, the 1 ×  4 dual-polarized patch array with the PCS has been fabricated and measured to demonstrate the conventional antenna characteristics. Results show that the PCS has insignificant effects on mutual coupling, matching, and the average radiation efficiency of the patch array, and changes in radiation patterns benefit the diversity and MIMO capacity of the array. Unlike the previous decorrelation works mainly confined to small MIMO arrays, the PCS-based near-field optimization technique can be applied to large BS arrays, effectively improving MIMO performance for it can reduce the correlations in a large MIMO array while keeping broadside radiations to ensure good cellular coverage.

This work was funded partly by National Natural Science Foundation of China (Grant No. 62171362).

See the article: Wang Y P, Chen X M, Pei H L, et al. MIMO Performance Enhancement of MIMO Arrays Using PCS-based Near-field Optimization Technique. Sci China Inf Sci, 2023.

http://engine.scichina.com/doi/10.1007/s11432-022-3595-y

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