Investigation of Sigma-Delta-over-Fiber for High-Capacity Wireless Communication Systems

Sammanfattning: The advent of beyond-5G (B5G) and 6G technologies brings increases in wireless devices and their applications. Although massive multiple-input-multiple-output (MIMO) delivers high-capacity services using co-located MIMO (CMIMO) technology, distributed MIMO (D-MIMO) technology offers a more uniform user service. This thesis introduces an automatic D-MIMO testbed featuring a statistical MIMO capacity analysis for an indoor use case. Additionally, raytracing-based simulations are employed for predictions and comparisons in an indoor scenario. The statistical MIMO capacity analysis demonstrates that D-MIMO outperforms C-MIMO in terms of both higher and more uniform capacity, as observed in measurements and simulations. A promising solution for such future communication systems is radio-over-fiber (RoF). Achieving data rates in the range of several tens of Gbit/s necessitates the utilization of the millimeter-wave (mm-wave) frequency band in RoF. However, mm-wave signals exhibit high propagation loss. Overcoming these challenges requires the incorporation of beamforming and/or MIMO technology in mm-wave RoF systems. Subsequently, a mm-wave sigma-delta-over-fiber (SDoF) link architecture is proposed for MIMO applications. The first implementation utilizes bandpass sigma-delta modulation (SDM) between a central unit (CU) and a remote radio unit (RRU) through a commercially available QSFP28-based optical interconnect. The implementation achieves symbol rates of 700/500 Msym/s for single-input-single-output (SISO)/multi-user MIMO (MU-MIMO) cases at a 1 m over-the-air (OTA) distance. The second implementation employs lowpass SDM between a CU and a RRH, and reaches 1 Gsym/s with a 1024-quadrature amplitude modulation (1024-QAM) signal across a 5 m OTA distance. Furthermore, the proposed mm-wave link is extended to two SDoF DMIMO architectures, both incorporating a CU-inherited local oscillator for phase coherence verification. The bandpass SDoF-based D-MIMO system supports a 748 MHz bandwidth with orthogonal frequency-division multiplexing (OFDM) signals for multiple-input-single-output (MISO)/MU-MIMO cases, while the lowpass SDoF-based D-MIMO system operates in the W-band for MISO measurement cases. In conclusion, this thesis has shown that D-MIMO surpasses C-MIMO in both capacity and uniformity, as validated through statistical analyses from measurements and simulations. The proposed innovative mm-wave SDoF DMIMO architectures lay the foundation for future high-capacity wireless communication networks.