A Bi-Directional Distributed Multi-User MIMO Testbed Using Digital Sigma-Delta-Over-Fiber

Sigma-Delta modulated signals over fiber is proposed for fronthauling of distributed signals. Analog Radioover- Fiber (ARoF) provides perfect synchronization, but linearity is directly impacted by the required circuit elements. Digital Radio-over-Fiber (DRoF) has a low cost of implementation but complicates synchronization. Sigma-Delta modulated signals over fiber combines the synchronization benefit from ARoF and the low cost of DRoF. In this work, we show a simplified and synchronous distributed multiple-input multiple-output (MIMO) communication system for both up-link and down-link transmission, based on our real-time sigma-delta modulated signals over fiber front-haul network. The testbed uses a carrier frequency of 3.686 GHz and is composed of 4 distributed units with 4 antennas each, totaling 16 antennas. The received clock is used to provide synchronization between the central and distributed units. With this implementation, we demonstrate distributed multi-user MIMO. The synchronicity of the system is demonstrated by performing over-the-air channel reciprocity calibration and reciprocity-based uplink transmission. This is verified using 64-QAM OFDM symbols. Measurement results for 2 users/4 users show an average error vector magnitude for up-link of 2.51% (−32.02 dB)/2.89% (−30.79 dB) and for down-link of 4.23% (−27.48 dB)/6.09% (−24.31 dB) respectively.