Evaluation of Radio-over-Fiber technologies for Distributed MIMO

Licentiate thesis, 2024

The demand for wireless communication systems with better coverage, higher capacity and lower latency grows continuously. At the same time, the number of subscribers is increasing and has already surpassed the global population. In a cellular network, a centralized macro base station is responsible to serve the users within the cell area. However, it is difficult to reach the increasing requirements by the centralized cellular architecture. Instead, distributed multiple-input-multiple-output (D-MIMO) has been proposed as a solution, where several access points or remote radio heads (RRHs) are spread out in a cell. To achieve most gain from D-MIMO the RRHs should be processed coherently, which requires radio-frequency (RF) phase synchronization among them. The hardware implementation of a D-MIMO network is non-trivial and highly dependent on the level of synchronization that is required by the particular deployment. This thesis evaluates the use of radio-over-fiber (RoF) and centralized frequency up-conversion to achieve phase synchronized RRHs in D-MIMO downlink implementations.

The first part of the thesis describes the background and motivation. Specifically, the system models for the centralized cellular base station and the D-MIMO architecture are described.The second part of the thesis explains different technologies for modulating an RF-signal onto an optical carrier using RoF: digital-radio-over-fiber (DRoF), analog-radio-over-fiber (ARoF) and sigma-delta-over-fiber (SDoF). As the need for power hungry digital-to-analog converters (DACs) is avoided in ARoF and SDoF, they are proposed as primary candidates in D-MIMO implementations. Moreover, measurement results show that SDoF is more robust than ARoF towards non-linearities in an RoF transmitter and inter-channel interference in an RoF receiver.

Finally, a SDoF architecture for D-MIMO with serially connected RRHs is presented. By connecting the RRHs in series the scalability of the system is enhanced, as one fiber link can be used to transmit signals to all RRHs.The serial connection is implemented using wavelength-division multiplexing (WDM), addressing each RRH by a specific wavelength.Through measurement results it is shown that the system with serial connection can perform similarly to one with the RRHs connected in parallel.

To conclude, this thesis presents SDoF as a fronthaul alternative for building robust, scalable and energy-efficient downlinks for D-MIMO networks.The results provide insights into the practical implementation of D-MIMO, a promising architecture for wireless communication systems.