34 research outputs found
Energy-efficient non-orthogonal multiple access for wireless communication system
Non-orthogonal multiple access (NOMA) has been recognized as a potential solution for enhancing the throughput of next-generation wireless communications. NOMA is a potential option for 5G networks due to its superiority in providing better spectrum efficiency (SE) compared to orthogonal multiple access (OMA). From the perspective of green communication, energy efficiency (EE) has become a new performance indicator. A systematic literature review is conducted to investigate the available energy efficient approach researchers have employed in NOMA. We identified 19 subcategories related to EE in NOMA out of 108 publications where 92 publications are from the IEEE website. To help the reader comprehend, a summary for each category is explained and elaborated in detail. From the literature review, it had been observed that NOMA can enhance the EE of wireless communication systems. At the end of this survey, future research particularly in machine learning algorithms such as reinforcement learning (RL) and deep reinforcement learning (DRL) for NOMA are also discussed
Performance enhancement of wireless communication systems through QoS optimisation
Providing quality of service (QoS) in a communication network is essential but challenging, especially when the complexities of wireless and mobile networks are added. The issues of how to achieve the intended performances, such as reliability and efficiency, at the minimal resource cost for wireless communications and networking have not been fully addressed. In this dissertation, we have investigated different data transmission schemes in different wireless communication systems such as wireless sensor network, device-to-device communications and vehicular networks. We have focused on cooperative communications through relaying and proposed a method to maximise the QoS performance by finding optimum transmission schemes. Furthermore, the performance trade-offs that we have identified show that both cooperative and non-cooperative transmission schemes could have advantages as well as disadvantages in offering QoS. In the analytical approach, we have derived the closed-form expressions of the outage probability, throughput and energy efficiency for different transmission schemes in wireless and mobile networks, in addition to applying other QoS metrics such as packet delivery ratio, packet loss rate and average end-to-end delay. We have shown that multi-hop relaying through cooperative communications can outperform non-cooperative transmission schemes in many cases. Furthermore, we have also analysed the optimum required transmission power for different transmission ranges to obtain the maximum energy efficiency or maximum achievable data rate with the minimum outage probability and bit error rate in cellular network. The proposed analytical and modelling approaches are used in wireless sensor networks, device-to-device communications and vehicular networks. The results generated have suggested an adaptive transmission strategy where the system can decide when and how each of transmission schemes should be adopted to achieve the best performance in varied conditions. In addition, the system can also choose proper transmitting power levels under the changing transmission distance to increase and maintain the network reliability and system efficiency accordingly. Consequently, these functions will lead to the optimized QoS in a given network
Physical layer security in 5G and beyond wireless networks enabling technologies
Information security has always been a critical concern for wireless communications due
to the broadcast nature of the open wireless medium. Commonly, security relies on cryptographic
encryption techniques at higher layers to ensure information security. However,
traditional cryptographic methods may be inadequate or inappropriate due to novel improvements
in the computational power of devices and optimization approaches. Therefore,
supplementary techniques are required to secure the transmission data. Physical layer
security (PLS) can improve the security of wireless communications by exploiting the characteristics
of wireless channels. Therefore, we study the PLS performance in the fifth generation
(5G) and beyond wireless networks enabling technologies in this thesis. The thesis
consists of three main parts.
In the first part, the PLS design and analysis for Device-to-Device (D2D) communication
is carried out for several scenarios. More specifically, in this part, we study the
underlay relay-aided D2D communications to improve the PLS of the cellular network. We
propose a cooperative scheme, whereby the D2D pair, in return for being allowed to share
the spectrum band of the cellular network, serves as a friendly jammer using full-duplex
(FD) and half-duplex (HD) transmissions and relay selection to degrade the wiretapped
signal at an eavesdropper. This part aims to show that spectrum sharing is advantageous
for both D2D communications and cellular networks concerning reliability and robustness
for the former and PLS enhancement for the latter. Closed-form expressions for the D2D
outage probability, the secrecy outage probability (SOP), and the probability of non-zero
secrecy capacity (PNSC) are derived to assess the proposed cooperative system model. The
results show enhancing the robustness and reliability of D2D communication while simultaneously
improving the cellular network’s PLS by generating jamming signals towards the
eavesdropper. Furthermore, intensive Monte-Carlo simulations and numerical results are
provided to verify the efficiency of the proposed schemes and validate the derived expressions’
accuracy.
In the second part, we consider a secure underlay cognitive radio (CR) network in the
presence of a primary passive eavesdropper. Herein, a secondary multi-antenna full-duplex
destination node acts as a jammer to the primary eavesdropper to improve the PLS of the
primary network. In return for this favor, the energy-constrained secondary source gets
access to the primary network to transmit its information so long as the interference to the
latter is below a certain level. As revealed in our analysis and simulation, the reliability and
robustness of the CR network are improved, while the security level of the primary network
is enhanced concurrently.
Finally, we investigate the PLS design and analysis of reconfigurable intelligent surface
(RIS)-aided wireless communication systems in an inband underlay D2D communication
and the CR network. An RIS is used to adjust its reflecting elements to enhance the data
transmission while improving the PLS concurrently. Furthermore, we investigate the design
of active elements in RIS to overcome the double-fading problem introduced in the RISaided
link in a wireless communications system. Towards this end, each active RIS element
amplifies the reflected incident signal rather than only reflecting it as done in passive RIS
modules. As revealed in our analysis and simulation, the use of active elements leads to a
drastic reduction in the size of RIS to achieve a given performance level. Furthermore, a
practical design for active RIS is proposed
Stochastic Geometry for Modeling, Analysis and Design of Future Wireless Networks
This thesis focuses on the modeling, analysis and design of
future wireless networks with smart devices, i.e., devices with
intelligence and ability to communicate with one another
with/without the control of base stations (BSs). Using stochastic
geometry, we develop realistic yet tractable frameworks to model
and analyze the performance of such networks, while incorporating
the intelligence features of smart devices.
In the first half of the thesis, we develop stochastic geometry
tools to study arbitrarily shaped network regions. Current
techniques in the literature assume the network regions to be
infinite, while practical network regions tend to be arbitrary.
Two well-known networks are considered, where devices have the
ability to: (i) communicate with others without the control of
BSs (i.e., ad-hoc networks), and (ii) opportunistically access
spectrum (i.e., cognitive networks). First, we propose a general
algorithm to derive the distribution of the distance between the
reference node and a random node inside an arbitrarily shaped
ad-hoc network region, which helps to compute the outage
probability. We then study the impact of boundary effects and
show that the outage probability in infinite regions may not be a
meaningful bound for arbitrarily shaped regions. By extending the
developed techniques, we further analyze the performance of
underlay cognitive networks, where different secondary users
(SUs) activity protocols are employed to limit the interference
at a primary user. Leveraging the information exchange among SUs,
we propose a cooperation-based protocol. We show that, in the
short-term sensing scenario, this protocol improves the network's
performance compared to the existing threshold-based protocol.
In the second half of the thesis, we study two recently emerged
networks, where devices have the ability to: (i) communicate
directly with nearby devices under the control of BSs (i.e.,
device-to-device (D2D) communication), and (ii) harvest radio
frequency energy (i.e., energy harvesting networks). We first
analyze the intra-cell interference in a finite cellular region
underlaid with D2D communication, by incorporating a mode
selection scheme to reduce the interference. We derive the outage
probability at the BS and a D2D receiver, and propose a spectrum
reuse ratio metric to assess the overall D2D communication
performance. We demonstrate that, without impairing the
performance at the BS, if the path-loss exponent on cellular link
is slightly lower than that on D2D link, the spectrum reuse ratio
can have negligible decrease while the average number of
successful D2D transmissions increases with the increasing D2D
node density. This indicates that an increasing level of D2D
communication is beneficial in future networks. Then we study an
ad-hoc network with simultaneous wireless information and power
transfer in an infinite region, where transmitters are wirelessly
charged by power beacons. We formulate the total outage
probability in terms of the power and channel outage
probabilities. The former incorporates a power activation
threshold at transmitters, which is a key practical factor that
has been largely ignored in previous work. We show that, although
increasing power beacon's density or transmit power is not always
beneficial for channel outage probability, it improves the
overall network performance