  The aim of this paper is to analyze the array synthesis for 5 G massive MIMO
systems in the line-of-sight working condition. The main result of the
numerical investigation performed is that non-uniform arrays are the natural
choice in this kind of application. In particular, by using non-equispaced
arrays, we show that it is possible to achieve a better average condition
number of the channel matrix and a significantly higher spectral efficiency.
Furthermore, we verify that increasing the array size is beneficial also for
circular arrays, and we provide some useful rules-of-thumb for antenna array
design for massive MIMO applications. These results are in contrast to the
widely-accepted idea in the 5 G massive MIMO literature, in which the
half-wavelength linear uniform array is universally adopted.
