Towards circulator-free multi antenna transmitters for 5G

Multi-antenna transmitters based on Massive MIMO and beamforming will be one of the 5G enabler technologies. To have suitable commercial architectures for these transmitters, they must be scalable, cost-effective, energy efficient and present a high-level of integration. This paper presents a technique where the circulator (bulky and expensive) is no longer required in the architecture. This paper also addresses the mutual coupling between antennas as one of the main problems associated with the circulator removal and identifies the PA load impedance variation, efficiency degradation, distortion generation and EVM degradation as severe consequences. To mitigate these problems, a digital compensation technique is proposed and verified with measurements in a laboratorial setup using 6W ultra-compact 2-stages MMIC PAs. The obtained results show that it is possible to remove the circulator and keep almost similar performance as in the single antenna operation mode.