03-11-2016, 12:10 PM
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Abstract
Two important issues have arisen in 5G wireless communications: spectral efficiency
and energy efficiency. The existing wireless communication architectures and
technologies may not be able to address these two issues at the same time for 5G
multi-tier networks where users in different tiers have different priorities for channel
access. In this article, we first present the motivation for our proposal of a new
paradigm of integrated spectrum and energy harvesting for 5G wireless networks,
in which spectrum and energy resources are efficiently collected and intelligently
managed. Then we present an integrated spectrum and energy harvesting 5G
architecture for dealing with current challenges. After that, based on the proposed
architecture, we provide integrated spectrum and energy control mechanisms for
typical 5G networks. Finally, we propose a cooperative medium access control
scheme for heterogeneous 5G networks that attracts cooperative sensing participants
with heterogeneous energy harvesting capabilities. Illustrative results demonstrate
significant energy and spectrum efficiency for 5G wireless networks.
The fifth generation (5G) cellular wireless network is
treated as a promising solution for high traffic volume
demands spurred by the proliferation and penetration
of wireless services [1]. 5G wireless networks
are expected to be a mixture of network tiers of different
sizes, transmission powers, backhaul connections, and radio
access technologies accessed by unprecedented numbers of
intelligent and heterogeneous wireless devices. How to guarantee
the system capacity under various circumstances will be
a key research challenge in multi-tier and heterogeneous 5G
networks.
According to Shannon theory, channel capacity C, bandwidth
B, and received signal-to-noise ratio (SNR) have a relationship
of C = Blog2(1 + SNR). For the devices in 5G
networks, it is easy to note that the increment of energy at
transceivers can increase both bandwidth (by detecting more
available channels) and SNR (by enhancing the transmission
power), which will lead to improvement of channel capacity.
Meanwhile, the network capacity of 5G networks can be
improved by increasing the transmission power of a base station,
which can increase the communication coverage. Hence,
energy and spectrum resources are two vital factors influencing
5G networks’ capacity.
For a conventional wireless communication system, recent
research activities have been devoted to energy efficiency
improvement, including energy saving hardware and devices,
energy-efficient communication techniques, the design of energy-aware network architectures and protocols, energy-friendly
software and applications, and renewable energy sources.
Recently, interest has risen greatly with regard to wireless networks
with renewable energy sources, such as thermal, vibration,
solar, acoustic, wind, and even ambient radio power,
which are used to reduce energy costs or potentially harmful
effects on the environment caused by CO2 emissions [2].
Along with energy efficiency, another key design objective
in wireless networks is to maximize spectral efficiency. The
explosive increase of wireless devices and applications poses a
serious problem and a compelling need for numerous radio
spectra. On the contrary, a recent report published by the
Federal Communication Commission (FCC) reveals that most
of the licensed spectra are rarely utilized continuously across
time and space [3]. In order to address spectrum scarcity and
underutilization, cognitive radio (CR) technology has been
proposed to effectively utilize the spectrum [4]. In a CR network,
the CR users/devices are allowed to opportunistically
operate in the frequency bands originally allocated to the primary
users/devices when these bands are not occupied by primary
users. Secondary users are capable of sensing unused
bands and adjusting transmission parameters accordingly,
which makes CR an excellent candidate technology for
improving spectral efficiency [5].
However, the existing network architectures and technologies
have treated spectrum harvesting and energy harvesting
as two separate cases in conventional wireless systems. In this
article, we first motivate the design of an integrated energy
and spectrum harvesting paradigm for 5G wireless systems.
Then we present the integrated energy- and spectrum-harvesting-driven
5G architecture and model. After that, based on
the proposed architecture, we provide several integrated spectrum and energy control mechanisms for typical 5G systems,
including device-to-device (D2D) networks, small cell networks
(picocell and femtocell), and macrocell networks. Next,
considering a heterogeneous 5G network, we propose a new
medium access control (MAC) protocol, which attracts cooperative
sensing participants with heterogeneous sensing and
energy harvesting abilities. Illustrative results demonstrate
that the proposed protocol can obtain significant energy and
spectral efficiency for 5G wireless networks.
The remainder of this article is organized as follows. In the
following section, we first identify the motivations for exploiting
energy and spectrum harvesting for 5G wireless networks.
Then the spectrum and energy harvesting 5G architecture and
model are presented. The integrated spectrum and energy
control mechanisms for typical 5G systems are described following
that. Then we propose an energy-harvesting-based
cooperative sensing MAC protocol. The illustrative results
demonstrate the significant aggregate throughput and energy
saving of the proposed protocol. The conclusion and future
work are presented in the final section.
Motivation of Using Energy and Spectrum
Harvesting in 5G Networks
Energy-Efficient Communication
One of the main challenges in 5G wireless networks is to
improve the energy efficiency of battery-equipped wireless
devices. In 5G networks, energy saving is extremely important
for optimizing devices’ operation, and ultimately prolonging
the lifetime of the devices and networks. Energy harvesting
technology is an appealing solution that can harvest energy
from environmental energy sources (e.g., solar and wind energy)
and even from ambient radio signals (i.e., RF energy harvesting)
with reasonable efficiency over small distances. Due
to the reconfigurability of a CR device, the specific circuit for
harvesting energy is easily equipped in a conventional receiver
that is designed for information transfer only. Also, a combination
of different energy harvesting technologies can be utilized
in energy harvesting CR assisted 5G wireless networks.
Prioritized Spectrum Access
The notions of both traffic-based and tier-based priorities will
exist in 5G multi-tier networks. Traffic-based priority arises
from different requirements of the users (e.g., reliability and
latency requirements, energy constraints), whereas tier-based
priority is for users belonging to different network tiers. By
leveraging the software reconfigurability of CR technology,
users are able to rapidly switch among different wireless
modes, satisfy different service requirements, and fit in different
network tiers.
Interference Management
There is increasingly intensive interference in 5G multi-tier
networks due to heterogeneity and dense deployment of wireless
devices, coverage and traffic load imbalance due to varying
transmit powers of different base stations in the downlink,
public or private access restrictions in different tiers that lead
to diverse interference levels, and so on. The performance of
5G networks may be seriously degraded due to intra-tier and
inter-tier interference. By employing the environmental sensing
ability of energy harvesting CR, users in 5G networks are
able to persistently detect the spectrum usage situation and
ongoing transmissions of other users or networks. Hence,
energy harvesting CR assisted 5G devices/networks can potentially
reduce or even avoid interference caused by other
devices/networks.
Data Rate and Latency
For dense urban areas, 5G wireless networks are envisioned
to enable a data rate of 300 and 60 Mb/s in downlink and
uplink, respectively, in 95 percent of locations and time [1].
The end-to-end latencies are expected to be on the order of
2–5 ms. One of the most revolutionary applications of energy
harvesting CR is to address the spectrum scarcity issue in 5G
wireless communications. Therefore, providing efficient spectrum
utilization is one of the primary reasons for applying
spectrum harvesting in 5G networks. In addition, energy harvesting
CR also provides sufficient energy by harvesting energy
from an ambient environment to improve data rate while
decreasing transmission latency.
Energy and Spectrum Harvesting Driven 5G
Architecture and Model
Network Architecture
The main components of an energy and spectrum harvesting
driven 5G system (Fig. 1) include energy harvesting CR
devices, picocell networks, femtocell networks, and a macrocell
network. All devices and networks in the proposed architecture
are equipped with an energy harvesting CR module
which gives the devices and networks the ability of energy harvesting
and spectrum sensing. Generally, the devices and networks
are allowed to use the unlicensed channels and also to
access the licensed channels in an opportunistic way, without
causing interference to the licensed users. With spectrum and
energy harvesting abilities, the main components of a 5G system
are explained in detail as follows.
Energy harvesting CR devices communicate with others by
periodically sensing and accessing the available licensed channels.
Meanwhile, an energy harvesting CR device equipped
with an energy harvester is able to convert ambient energy
into electricity.
D2D networks are an underlaying network with sensing
ability. D2D networks not only use the spectrum resources
provided by a base station (BS), but also use the available
licensed spectrum discovered by distributed sensing.
Picocell networks consist of a BS and energy harvesting CR
devices. This network is operated and managed by the network
operator. The picocell BS manages the spectrum and
energy harvesting within its coverage in a centralized manner.
Femtocell networks are installed, powered, and connected
by the end users with less active remote management. The
network includes a femtocell access point (AP) and energy
harvesting CR based end users that are semi-autonomous,
sensing from their immediate environment to find the best
spectrum channels and radio parameters to use. The femtocell
AP harvests energy from renewable sources, and controls the
spectrum and energy usage of all users.
Macrocell networks consist of a BS with an energy and
spectrum harvesting (E&SH) controller. The E&SH controller
is able to collect the spectrum and energy information
from the devices, low-level networks, and energy sources;
hence, it can jointly manage the harvesting and usage of both
spectrum and energy in the entire macrocell network.
Multi-Tier Energy Harvesting Model
D2D Networks — Energy can be harvested from ambient
radio signals (i.e., RF energy harvesting) with reasonable efficiency
over small distances. The harvested energy may be
used for D2D communication or communication within a
small cell. Considering the D2D network with CR technology,
[6] proposed a method for a D2D network where devices