Distributed Multi-Antenna Systems Using 1-bit Radio-over-Fiber

Doktorsavhandling, 2025

Distributed multiple-input multiple-output (D-MIMO) is a promising approach to improve the wireless mobile network coverage and meet increasing capacity demands. Its foundation builds upon the ability to cooperatively utilize spatially distributed radio access nodes to exploit macro diversity. However, in order to implement spatial multiplexing, precise phase coherence at carrier frequency is required across all cooperating radio nodes. This poses a challenging implementation problem, since a radio typically uses a local oscillator to generate the carrier frequency, and each local oscillator is associated with a frequency offset and phase noise. In this thesis, we propose a D-MIMO architecture that eliminates local oscillators at the radio heads altogether, and implements instead digital frequency up- and down-conversion in a central processing unit, such that the radio frequency signals are phase-synchronized at the remote radio heads. This architecture relies on fiber-optic fronthaul, over which 1-bit signals are transferred.

First, we introduce a D-MIMO transceiver architecture that employs 1-bit quantization to reduce power consumption and facilitate efficient fiber-optic fronthaul. Phase-coherence is demonstrated in a wireless multi-user measurement implementing reciprocity-based precoding. Second, since this architecture relies on significant oversampling to battle the distortion introduced by the 1-bit converters, we investigate the tradeoff between oversampling in the spatial or temporal domain, when the total fronthaul rate is constrained. This sheds light on the minimum fronthaul rate required in a certain deployment for our D-MIMO architecture to outperform standard co-located MIMO architecture. Third, we present a testbed that we use to investigate the receiver architecture effects on multi-user scenarios. We find that our architecture shows greater uplink sensitivity to multi-user interference than a conventional receiver, and that user power control can mitigate this sensitivity.