Massive MIMO beamforming
With every new generation, mobile networks must increase their capacity while keeping operational costs under control. One way to increase capacity is by increasing the number of physical base stations. This is, however, associated with site acquisition, higher power demands and problems with long-term maintenance. A more practical approach is to use technologies that allow existing base stations to deliver higher performance.
Massive MIMO (M-MIMO) has emerged as the primary technology to increase the capacity of mobile networks. M-MIMO uses large, precisely controlled antenna arrays. These arrays can serve multiple users simultaneously or direct focused beams toward individual devices. This significantly increases spectral efficiency and reduces unnecessary energy consumption without the need for new base stations.
The growing importance of new frequency ranges is key to the evolution of M-MIMO. The rise of the FR3 spectrum (7.125 GHz to 24.25 GHz) is reshaping how next-generation capacity layers are designed. FR3 is expected to combine high bandwidth with excellent propagation characteristics. Within FR3, the lower range (around 7.1 GHz to 8.5 GHz) behaves similarly to extended mid-band, with more favorable propagation. The higher range (around 12 GHz to 15 GHz) introduces greater directionality and path loss, leading to denser antenna arrays and more advanced M-MIMO processing to deliver the desired capacity.
M-MIMO is a central technology for 5G and future networks. Through the use of large antenna arrays and beamforming, M-MIMO enables base stations to simultaneously serve many users with higher efficiency and improved signal quality.
In Open RAN architectures, these capabilities are brought into a disaggregated system. The Open RAN radio unit (O-RU) must reliably execute beamforming commands received from the Open RAN distributed unit (O-DU), while maintaining tight timing, amplitude and phase alignment across many antenna paths. To ensure this, the O-RAN ALLIANCE Working Group 4 defines conformance requirements for O-RU that support M-MIMO with beamforming. These test cases cover both analog (time domain) and digital (frequency domain) beamforming control. They verify that the O-RU correctly interprets and applies beamforming instructions through the control plane interface. The purpose of this is to confirm protocol and functional behavior, not to evaluate the quality of the radiated beam.
Many modern O-RUs integrate radio electronics directly with the antenna array. The O-RU’s internal RF and digital processing can be validated using conducted testing, which confirms signal generation quality and ensures that the beamforming control paths behave as intended. Once signals reach the antenna array, however, the system can only be evaluated through over-the-air (OTA) testing, which characterizes the overall antenna behavior under realistic propagation conditions. Conducted and OTA testing together form a complete validation approach, ensuring that both the RU and the integrated antenna system perform correctly when deployed in a live M-MIMO network.
