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In this paper, a ε-negative metasurface superstrate is proposed for mutual coupling reduction of large antenna arrays. Unlike the previous decoupling metasurface works that are mostly confined to two-ports antennas, the proposed decoupling superstrate can be extended to massive multiple-input multiple-output (MIMO) systems. A 4 × 4 antenna array is used as an example to illustrate the decoupling performance of the proposed metasurface. With the decoupling metasurface, the worst mutual coupling of the antenna array is improved by 8 dB over the operation bandwidth with a maximum mutual coupling reduction of 25 dB. Moreover, the decoupling metasurface also help restore the radiation patterns, bring down the active voltage sanding wave ratio, and broaden the bandwidth of the array.

The fifth generation (5G) millimeter-wave (mmWave) handset demands a cost-effective mmWave array antenna with beam steering capability to overcome the high-pass loss and to ensure seamless connectivity. Unlike sub-6-GHz handsets, emerging mmWave handsets usually employ phased array antennas with a reasonably large number of elements. Unfortunately, due to the legacy of a few antennas in sub-6-GHz handsets, the mutual coupling effect on the mmWave handset has not been thoroughly investigated. In this paper, we study the mutual coupling effect on the mmWave handset performance by comparing array antennas with different inter-element spacing and different configurations. It is found that mutual coupling tends to increase the active reflection (especially at large scanning angles), which in turn reduces the realized gain and maximum scanning angle of the phased array antenna. For a sub-6-GHz multiple-input multiple-output handset with two or four antenna ports and fully digital precoding/decoding, 10-dB isolation is usually regarded as good enough. It is shown in this paper, however, that the outage capacity of the mmWave handset can be clearly improved by reducing the mutual coupling.

This paper provides a systematic review of the mutual coupling in multiple-input multiple-output (MIMO) systems, including the effects on performances of MIMO systems and various decoupling techniques. The mutual coupling changes the antenna characteristics in an array, and therefore, degrades the system performance of the MIMO system and causes the spectral regrowth. Although the system performance can be partially improved by calibrating out the mutual coupling in the digital domain, it is more effective to use decoupling techniques (from the antenna point) to overcome the mutual coupling effects. Some popular decoupling techniques for MIMO systems (especially for massive MIMO base station antennas) are also presented.