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

Licentiate thesis, 2019

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.