Green Cellular Networks, Heterogeneous Networks, Cognitive Radio

The millimeter wave (mmWave) frequency band is seen as a key enabler of multi-gigabit wireless access in future cellular networks. In order to overcome the propagation challenges, mmWave systems use a large number of antenna elements both at the base station and at the user equipment, which lead to high directivity gains, fully-directional communications, and possible noise-limited operations. The fundamental differences between mmWave networks and traditional ones challenge the classical design constraints, objectives, and available degrees of freedom. This paper addresses the implications that highly directional communication has on the design of an efficient medium access control (MAC) layer. The paper discusses key MAC layer issues, such as synchronization, random access, handover, channelization, interference management, scheduling, and association. The paper provides an integrated view on MAC layer issues for cellular networks, identifies new challenges and tradeoffs, and provides novel insights and solution approaches.

Wireless access networks today consume 0.5 percent of the global energy. Rapidly growing demand for capacity will further increase the energy consumption. Thus, improving energy efficiency has a great importance not only for environmental awareness but also to lower the operational cost of network operators. However, current networks which are optimized based
on non-energy related objectives introduce challenges towards green wireless access networks. In this thesis we investigate the solutions at the deployment level and handle energy efficiency assessment issues in wireless access networks.

The precise characterization of the power consumption of the whole network has a crucial importance in order to obtain consistent conclusions from any proposed solution at the network level. For this purpose, we propose a novel power consumption model  considering  the impact of backhaul for two established
technologies, i.e., fiber and microwave, which is often ignored in the literature. We show that there is a tradeoff between the power saved by using low power base stations and the excess power that has to be spent for backhauling their traffic which therefore needs to carefully be included into energy efficiency analysis. Furthermore, among the solutions that are analyzed, fiber-based backhaul solution is identified to outperform microwave regardless of the considered topology. The proposed model is then used to gain a general insight regarding the important design parameters and their possible impact on energy- and cost oriented network design. To this end, we present a  high-level framework to see the main tradeoffs between energy, infrastructure cost, spectrum and show that future high-capacity systems are increasingly limited by infrastructure and energy costs where spectrum has a strong positive impact on both.

We then investigate different network deployment strategies to improve the energy efficiency where we focus on the impact of various base station types, cell size, power consumption parameters and the capacity demand. We propose a refined power consumption model where the parameters are determined in accordance with cell size. We show that network densification can only be justified when capacity expansion is anticipated and over-provisioning of the network is not plausible for greener network. The improvement through heterogeneous networks is indicated to be highly related to the traffic demand where up to 30\% improvement is feasible for high area throughput targets.

Furthermore, we consider the problem of energy efficiency assessment at the network level in order to allow operators to know their current status and quantify the potential energy savings of different solutions to establish future strategies. We propose elaborate metric forms that can characterize the efficiency and a methodology that indicate how to perform a reliable and accurate measurement considering the complexity of wireless networks. We show the weakness of the current metrics reporting the "effectiveness" and how these might indicate disputable improvement directions unless they are properly revised. This illustrates the need for a standardized network level energy efficiency evaluation methodology towards green wireless access.

The cognitive radio is an emerging technology that enables dynamic spectrum access in wireless networks. The cognitive radio is capable of opportunistically using the available portions of a licensed spectrum to improve the application performance for unlicensed users. The opportunistic use of the available channels in the wireless environment requires dynamic channel assignment to efficiently utilize the available resources while minimizing the interference in the network. A challenging aspect of such algorithms is the incorporation of the channels’ diverse characteristics, highly dynamic network conditions with
respect to primary users’ activity, and different fragmented
sizes of the available channels. This paper presents
a comprehensive survey on the state-of-the-art channel
assignment algorithms in cognitive radio networks. We
also classify the algorithms by presenting a thematic
taxonomy of the current channel assignment algorithms
in cognitive radio networks. Moreover, the critical aspects
of the current channel assignment algorithms in cognitive
radio networks are analyzed to determine the strengths
and weaknesses of such algorithms. The similarities and
differences of the algorithms based on the important
parameters, such as routing dependencies, channel models,
assignment methods, execution model, and optimization
objectives, are also investigated. We also discuss open
research issues and challenges of channel assignment in
the cognitive radio networks. (Accepted in IEEE Communication Surveys and Tutorials IF 6.4)

Limited spectrum resources and dramatic growth of high data rate applications have motivated opportunistic spectrum access utilizing the promising concept of cognitive networks. Although this concept has emerged primarily to enhance spectrum utilization and to allow the coexistence of heterogeneous network technologies, the importance of energy consumption imposes additional challenges, because energy consumption and communication performance can be at odds. In this paper, the approaches for energy efficient spectrum sensing and spectrum handoff, fundamental building blocks of cognitive networks is investigated. The tradeoff between energy consumption and throughput, under local as well as under cooperative sensing are characterized, and what further aspects need to be investigated to achieve energy efficient cognitive operation under various application requirements are discussed.

Positioning techniques have been a common objective since the early development of wireless networks. However, current positioning methods in cellular networks, for instance, are still primarily focused on the use of the Global Navigation Satellite System (GNSS), which has several limitations, like high power drainage and failure in indoor scenarios. This study introduces a novel approach employing standard LTE signaling in order to provide high accuracy positioning estimation. The proposed technique is designed in analogy to the human sound localization system, eliminating the need of having information from three spatially diverse Base Stations (BSs). This is inspired by the perfect human 3D sound localization with two ears. A field study is carried out in a dense urban city to verify the accuracy of the proposed technique, with more than 20 thousand measurement samples collected. The achieved positioning accuracy is meeting the latest Federal Communications Commission (FCC) requirements in the planner dimension.

Abstract: A cellular network is radio network distributed over land through cells where each cells include a fixed
location transceiver known as a Base Station (BS). The major concern for cellular operators is achieving energy
efficiency in cellular network. They have to maintain profitability and at the same time reduce the negative impact on
environment. A lot of research has been carried out concerning energy efficient or green cellular networks. In a cellular
network, the BSs consume the highest amount of energy compared to other components. The focus of this paper is on
reducing the energy consumption in the BS. A cellular network can be made more energy efficient by using a design
based on switching BSs. Significant energy savings can be obtained by considering the uplink and downlink traffic for
the switching purpose.
Keywords: Green cellular network, energy saving, base station switching, green communication