Sigma-Delta-over-Fiber for High-Speed Wireless Communication Systems

Sammanfattning: Future mobile communication networks aim to increase the communication speed, provide better reliability and improve the coverage. It needs to achieve all of these enhancements, while the number of users are increasing drastically. As a result, new base-station (BS) architectures where the signal processing is centralized and wireless access is provided through multiple, carefully coordinated remote radio units are needed. The sigma-delta-over-fiber (SDoF) is a communication technique that can address both requirements and enable very low-complexity, phase coherent remote radio transmission, while transmitting wide-band communication signals with high quality. This thesis investigates the potential and limitations of SDoF communication links as an enabler of future mobile networks. In the first part of the thesis, a multiple-input-multiple-output (MIMO) communication testbed with physically separated antenna elements, distributed- MIMO, is formed by multiple SDoF links. It is shown that the digital upconversion, performed with a shared local-oscillator/clock at the central unit, provides excellent phase coherency between the physically distributed antenna elements. Moreover, the same approach decreases the complexity and the package size of the antenna units significantly by moving the complexity of the BSs to a central unit. The implemented testbed is evaluated through various communication experiments. The results show that distributing the antenna units of a MIMO communication system can increase the coverage and signal quality. In the second part of the thesis, an ultra-high-speed SDoF (UHS-SDoF) link is realized by using the state-of-the-art vertical-cavity surface-emitting-lasers (VCSEL). The effects of VCSEL characteristics on such links in terms of signal quality, energy efficiency and potential lifespan is investigated. Furthermore, the potential and limitations of UHS-SDoF are evaluated with signals having various parameters. The results show that, low-cost, reliable, energy efficient, high signal quality SDoF links can be formed by using emerging VCSELs. Therefore, UHS-SDoF is a very promising technique for beyond 10 GHz communication systems. In conclusion, this thesis has showed that low-complexity, low-cost, and energy efficient ultra-high speed communication links and distributed MIMO systems can be implemented by employing SDoF.