Out-of-Band Radiation from Large Antenna Arrays
Journal article, 2018
The radiation pattern of the out-of-band (oob) radiation from large arrays with nonlinear hardware is different from the radiation pattern of the beamformed inband signal. This is the main difference between the oob radiation from large arrays and from well-studied legacy systems. Beamforming might focus the oob radiation in certain directions but also significantly reduce the total power that has to be transmitted. For cost and power-consumption reasons, large arrays might have to be built from low-complexity hardware without advanced precompensation for linearization, which increases the relative amount of oob radiation. Given that large arrays will be used in future base stations, a correct understanding of the oob radiation is crucial to specify appropriate linearity requirements for the hardware. We show that the oob radiation from large arrays varies little between coherence times; it is isotropic in many cases; and, when it is beamformed, it is directed towards the served user in a very narrow beam with an array gain equal to or less than that of the in-band signal. We draw the conclusion that, compared to legacy systems, less stringent linearity requirements can be used in many systems with large arrays by virtue of the lower transmit power needed to upkeep the same received signal-to-noise ratio.