Actual Out-of-Band Emissions from Massive MIMO Antennas

Abstract:
In this paper, we address the issue of out-of-band (OOB) emissions in 5G Massive Multiple Input Multiple Output (MIMO) antennas. Due to the limited available spectrum, signals in different allocated bands are at risk of causing interference in adjacent frequency bands. The spatial behavior of OOB emissions depends on the operating regime of the power amplifiers (PAs). In the nonlinear regime, and in the absence of digital predistortion (DPD), the radiation is coherent across subarrays, causing the OOB emissions to be beamformed in the direction of the desired signal. Conversely, in the linear regime, with DPD applied, the radiation remains coherent for inband components, but the OOB emissions become decorrelated across subarrays, effectively turning the radiating elements into uncorrelated noise sources that radiate incoherently. We investigated this phenomenon experimentally in an anechoic chamber using a 32TX/32RX n78-band Massive MIMO antenna provided by Andrew-Amphenol. The results aligned well with theoretical expectations and confirmed the predicted benefits of spatial averaging, particularly its positive impact on the adjacent channel leakage ratio (ACLR), with radiated measurements showing better performance than conducted ones. This study highlights the spatial filtering advantages of Massive MIMO and supports its integration into standardization efforts and network planning.