Influence of Transmission Rank on EMF Exposure Measured With Provoked Data Traffic Around 5G Massive MIMO Base Stations

Abstract

The introduction of 5G New Radio networks with massive MIMO technology has complicated electromagnetic field exposure assessments for radiation protection. Massive MIMO transmission enables beamforming, beam steering, and spatial multiplexing across multiple transmission layers, with the number of simultaneous transmission paths depending on the rank of the radio channel, further named 'transmission rank'. Since the total transmission power of a base station is shared among these layers, rank variations affect the measured exposure levels, e.g., when assessments use provoked traffic via user equipment. This study investigates the impact of the transmission rank on the measured maximum exposure in the 3.6 GHz (n78) band of a German 5G network employing massive MIMO technology. Field measurements were performed using a spectrum analyzer with isotropic probe, to capture maximum field strengths under full-load traffic conditions. The transmission rank was manipulated by artificially degrading the reception quality of the user equipment with a shielding bag, forcing a single transmission layer (rank-1). The results were compared with unshielded operation allowing up to the maximum number of four independent transmission layers (rank-4). The data reveal exposure differences ranging from 1.7 dB to 5.4 dB, with a median of 4.3 dB at the measurement points studied. These findings highlight the necessity of considering the transmission rank in exposure assessments to electromagnetic fields.