The increasing demand on user rates in the fifth generation (5G) requires network architectures
that can support high data rates with acceptable reliability. In order to increase
the data rates in the presence of the current spectrum crisis, shrinking cells and reusing the
spectrum is a proposed solution. Conventional implementation of dense cells requires a
large number of expensive BSs to locally process and decode users’ signals. Another limiting
factor that degrades the performance in a dense network is the inter-cell interference.
A cloud radio access network (CRAN) is a promising solution to those cost, complexity,
and interference challenges. A typical CRAN architecture consists of simplified low-cost
base stations (BSs), termed radio units (RUs), that collect the radio frequency (RF) user
equipments’ (UEs) signals and forward them over the fronthaul links to the central office
(CO) where signal processing is done over shared resources. Besides the reduced cost and
complexity of a CRAN, the joint processing at the CO enables joint interference mitigation
techniques. However, the performance of CRANs depends critically on the availability of
reliable fronthaul links with large bandwidth that may be expensive. Analog optical fronthaul
links provide high data rates at lower cost and complexity since UEs’ signals are
optically analog-modulated without digitalization, however, they suffer from other channel
impairments and nonlinearities.
In this thesis, analog optical fronthaul topologies are considered in which radio signals are forwarded over free-space optical (FSO) links, termed radio-over-free-space optical
(RoFSO) links, and optical fiber (OF) links, termed radio-over-fiber (RoF) links.
Firstly, a CRAN with mixed RF/RoFSO fronthaul is considered to investigate the performance
improvement when RF fronthaul links are replaced one-by-one by RoFSO links.
A novel joint optimization problem is introduced for the given architecture in which the
weighted sum of UEs’ rates is maximized by jointly designing RF and RoFSO links. The
optimization problem is solved over different numbers of RF and RoFSO links and under
various weather conditions. Under favorable weather conditions, the replacement of 1 RF
link by a RoFSO link is shown to increase the 50th percentile of UEs’ rates by 7 times.
Secondly, the reliability of a CRAN with two-hop RoFSO/RoF fronthaul links is derived
along with other performance metrics such as the average bit-error rate and the cumulative
distribution function of UEs’ rates. For the given architecture, the Gaussian noise
model of fiber nonlinearity is applied and an optimal OF average optical power is derived
to minimize the outage probability. Using the optimal power, and under favorable weather
conditions, the 50th percentile of user rate exceeds 1:5 Gbps.
Finally, a CRAN with passive all-optical two-hop fronthaul links is considered where
optical signals from the first RoFSO fronthaul hop are passively coupled into the RoF fronthaul
link. The fronthaul outage probability is derived in the context of network planning
to provide guidance on designing a set of system parameters. Those parameters include
coverage area radius, density of RUs, RoFSO gain, RoFSO optical power and RoF length. / Thesis / Doctor of Philosophy (PhD) / The upcoming generation of wireless communications, termed fifth generation (5G), promises
faster data rates and lower latency. In order to achieve this, more base stations (BSs) have
to be deployed which increases the cost and complexity of the network. A solution to
this challenge is to install simple BSs, i.e. radio units (RUs), that collect signals from
users and forward them to a central office (CO) for joint processing which is referred to
as a cloud radio access network (CRAN). The fronthaul network in a CRAN connects the
RUs to the CO and it can be implemented using different kinds of links. While there are
several fronthaul media (e.g., radio frequency (RF), free-space optical (FSO) links, copper
lines, satellite communications, and optical fiber (OF)), optical links provide high data
rates that are promising to achieve the 5G requirements. In this thesis, a novel architecture
of a CRAN is considered in which analog optical links, namely FSO links and OF
links, are used for fronthauling. Performance improvement in terms of rate and reliability
is investigated and optimized through different design tools. In response to the challenges
introduced by the proposed architecture, such as the nonlinearities of analog FSO and OF
links, various design parameters are proposed in the optimization problems to tackle those
challenges. Furthermore, a network planning framework is introduced to provide guidance
and insights on designing the network.
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/24831 |
| Date | January 2019 |
| Creators | Ahmed, Khaled |
| Contributors | Hranilovic, Steve, Electrical and Computer Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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