Robust Transmit Beamforming for SWIPT-Enabled Cooperative NOMA with Channel Uncertainties

In this paper, we study the robust beamforming design for a simultaneous wireless information and power transfer (SWIPT) enabled system, with cooperative nonorthogonal multiple access (NOMA) protocol applied. A novel cooperative NOMA scheme is proposed, where the strong user with better channel conditions adopts power splitting (PS) scheme and acts as an energy-harvesting relay to transmit information to the weak user. The presence of channel uncertainties is considered and incorporated in our formulations to improve the design robustness and communication reliability. Specifically, only imperfect channel state information (CSI) is assumed to be available at the base station (BS), due to the reason that the BS is far away from both users and suffers serious feedback delay. To comprehensively address the channel uncertainties, two major design criteria are adopted, which are the outage-based constraint design and the worst-case based optimization. Then, our aim is to maximize the strong user’s data rate, by optimally designing the robust transmit beamforming and PS ratio, while guaranteeing the correct decoding of the weak user. With two different channel uncertainty models respectively incorporated, the proposed formulations yield to challenging nonconvex optimization problems. For the outage-based constrained optimization, we first conservatively approximate the probabilistic constraints with the Bernstein-type inequalities, which are then globally solved by two-dimensional exhaustive search. To further reduce the complexity, an efficient low-complexity algorithm is then proposed with the aid of successive convex approximation (SCA). For the worst-case based scenario, we firstly apply semidefinite relaxation (SDR) method to relax the quadratic terms and prove the rank-one optimality. Then the nonconvex max-min optimization problem is readily transformed into convex approximations based on S-procedure and SCA. Simulation results show that for both channel uncertainty models, the proposed algorithms can converge within a few iterations, and the proposed SWIPT-enabled robust cooperative NOMA system achieves better system performance than existing protocols

Outage Constrained Robust Beamforming Design for SWIPT-Enabled Cooperative NOMA System

We investigate the robust beamforming design for a simultaneous wireless information and power transfer (SWIPT) enabled system, with the cooperative non-orthogonal multiple access (NOMA) protocol applied. A novel cooperative NOMA scheme is proposed, where a strong user with better channel conditions adopts power splitting (PS) scheme and acts as an energy-harvesting relay to forward the decoded signal to the weak user. The presence of channel uncertainties is considered by introducing the outage-based constraints of signal to interference plus noise ratio (SINR). Specifically, it is assumed that only imperfect channel state information (CSI) is known at the base station (BS), due to the reason that the BS is far away from both users and suffers serious feedback delay. Our aim is to maximize the strong user's data rate, by optimally designing the robust transmit beamforming and PS ratio, while guaranteeing the correct decoding of the weak user. The proposed formulation yields to a challenging nonconvex optimization problem. To solve it, we first approximate the probabilistic constraints with the Bernstein-type inequalities, which can then be globally solved by two-dimensional exhaustive search. To further reduce the complexity, an efficient low-complexity algorithm is proposed with the aid of successive convex approximation (SCA). Numerical results show that the proposed algorithm converges quickly, and the proposed SWIPT-enabled robust cooperative NOMA system achieves better performance than existing protocols

Resource allocation optimization for future wireless communication systems

To meet the ever-increasing requirements of high data rate, extremely low latency, and ubiquitous connectivity for the fifth generation (5G) and beyond 5G (B5G) wireless communications, there is imperious demands for advanced communication system design. Particularly, efficient resource allocation is regarded as the fundamental challenge whereas an effective way to improve system performance. The term ”resource” refers to scare quantities such as limited bandwidth, power and time in wireless communications. Moreover, the development of wireless communication systems is accompanied by the innovation of applied technologies. Motivated by the above observations, efficient resource allocation strategies for several promising 5G and B5G technologies in terms of non-orthogonal multiple access (NOMA), mobile edge computing (MEC) and Long Range (LoRa) are addressed and investigated in this thesis. Firstly, the strong user’s data rate maximization problem for simultaneous wireless information and power transfer (SWIPT)-enabled cooperative NOMA system, considering the presence of channnel uncertainties, is proposed and investigated. Two major channel uncertainty design criteria in terms of the outage-based constraint design and the worst-case based optimization are adopted. In addition to the high-complexity optimal two-dimensional exhaustive search method, the low-complexity suboptimal solution is further proposed. The advantages of SWIPT-enabled cooperation in robust NOMA are confirmed with simulations. Secondly, considering the application of NOMA and user cooperation (UC) in a wireless powered MEC under the non-linear energy harvesting model, a computation efficiency maximization problem subject to the quality of service (QoS) and power budget constraint, is studied and analyzed. The formulated problem is nonconvex, which is challenging to solve. The semidefinite relaxation (SDR) approach is first applied, then the sequential convex approximation (SCA)-based solution is further proposed to maximize the system computation efficiency. Finally, taking into consideration the aspect of energy efficiency (EE), this thesis investigates the energy efficient resource allocation in LoRa networks to maximize the system EE (SEE) and the minimal EE (MEE) of LoRa users, respectively. The energy efficient resource allocation is formulated as NP-hard problems. A low-complexity user scheduling scheme based on matching theory is proposed to allocate users to channels, then the heuristic SF assignment solution is designed for LoRa users scheduled on the same channel. The optimal power allocation strategy is further proposed to maximize the corresponding EE

Spectral, Energy and Computation Efficiency in Future 5G Wireless Networks

Wireless technology has revolutionized the way people communicate. From first generation, or 1G, in the 1980s to current, largely deployed 4G in the 2010s, we have witnessed not only a technological leap, but also the reformation of associated applications. It is expected that 5G will become commercially available in 2020. 5G is driven by ever-increasing demands for high mobile traffic, low transmission delay, and massive numbers of connected devices. Today, with the popularity of smart phones, intelligent appliances, autonomous cars, and tablets, communication demands are higher than ever, especially when it comes to low-cost and easy-access solutions. Existing communication architecture cannot fulfill 5G’s needs. For example, 5G requires connection speeds up to 1,000 times faster than current technology can provide. Also, from transmitter side to receiver side, 5G delays should be less than 1ms, while 4G targets a 5ms delay speed. To meet these requirements, 5G will apply several disruptive techniques. We focus on two of them: new radio and new scheme. As for the former, we study the non-orthogonal multiple access (NOMA) and as for the latter, we use mobile edge computing (MEC). Traditional communication systems allow users to communicate alternatively, which clearly avoids inter-user interference, but also caps the connection speed. NOMA, on the other hand, allows multiple users to transmit simultaneously. While NOMA will inevitably cause excessive interference, we prove such interference can be mitigated by an advanced receiver side technique. NOMA has existed on the research frontier since 2013. Since that time, both academics and industry professionals have extensively studied its performance. In this dissertation, our contribution is to incorporate NOMA with several potential schemes, such as relay, IoT, and cognitive radio networks. Furthermore, we reviewed various limitations on NOMA and proposed a more practical model. In the second part, MEC is considered. MEC is a transformation from the previous cloud computing system. In particular, MEC leverages powerful devices nearby and instead of sending information to distant cloud servers, the transmission occurs in closer range, which can effectively reduce communication delay. In this work, we have proposed a new evaluation metric for MEC which can more effectively leverage the trade-off between the amount of computation and the energy consumed thereby. A practical communication system for wearable devices is proposed in the last part, which combines all the techniques discussed above. The challenges for wearable communication are inherent in its diverse needs, as some devices may require low speed but high reliability (factory sensors), while others may need low delay (medical devices). We have addressed these challenges and validated our findings through simulations

Reconfigurable Intelligent Surface for Physical Layer Security in 6G-IoT: Designs, Issues, and Advances

Sixth-generation (6G) networks pose substantial security risks because confidential information is transmitted over wireless channels with a broadcast nature, and various attack vectors emerge. Physical layer security (PLS) exploits the dynamic characteristics of wireless environments to provide secure communications, while reconfigurable intelligent surfaces (RISs) can facilitate PLS by controlling wireless transmissions. With RIS-aided PLS, a lightweight security solution can be designed for low-end Internet of Things (IoT) devices, depending on the design scenario and communication objective. This article discusses RIS-aided PLS designs for 6G-IoT networks against eavesdropping and jamming attacks. The theoretical background and literature review of RIS-aided PLS are discussed, and design solutions related to resource allocation, beamforming, artificial noise, and cooperative communication are presented. We provide simulation results to show the effectiveness of RIS in terms of PLS. In addition, we examine the research issues and possible solutions for RIS modeling, channel modeling and estimation, optimization, and machine learning. Finally, we discuss recent advances, including STAR-RIS and malicious RIS.Comment: Accepted for IEEE Internet of Things Journa

Determination of optically stimulated luminescence dosimetric characteristics and suitability for entrance surface dose assessement in diagnostic x-ray examinations

The availability of Optically Stimulated Luminescence (OSL) dosimeter system developed by Landauer Inc. (Glenwood IL) has greatly improved radiation dosimetry application in the medical field. Recent studies with OSL dosimeters (nanoDots) gave much emphases to patient radiation exposure in radiotherapy but ignoring the potential risks from radiographic examinations. This study focused on the measurement of entrance surface dose (ESD) resulting from radiographic examination. Monitoring procedures have been developed by the International Atomic Energy Agency (IAEA) to estimate ESD, while considering exposure parameters and patient’s characteristics. However, dosimetric properties of the OSL system must be characterized to ascertain its suitability for ESD measurements in medical radiography due to energy dependence and over-response factors of the Al2O3 material. This thesis consists of three phases: 1) evaluating stability of the new OSL dosimetry system, 2) characterizing the nanoDots in radiographic energy range from 40 kV to 150 kV with typical doses ranging from 0 to 20 mGy, and 3) assessing suitability of the nanoDots for ESD measurement in routine X-ray examinations. The dosimetric characteristics of the nanoDots in the above energy range are presented in this study, including repeatability, reproducibility, signal depletion, element correction factor, linearity, angular and energy dependence, and dose measurement accuracy. Experimental results showed repeatability of below 5% and reproducibility of less than 2%. OSL signals after sequential readouts were reduced by approximately 0.5% per readout and having good linearity for doses between 5 – 20 mGy. The nanoDots OSL dosimeter showed significant angular and energy dependence in this energy range, and corresponding energy correction factors were determined in the range of 0.76 – 1.12. ESDs were determined in common diagnostic X-ray examinations using three different methods including direct (measured on phantom/patient) and indirect (without phantom) measurements with nanoDots OSL dosimeters, and CALDose_X 5.0 software calculations. Results from direct and indirect ESD measurements showed good agreement within relative uncertainties of 5.9% and 12%, respectively, in accordance with the International Electrotechnical Commission (IEC) 61674 specifications. However, the measured results were below ESDs calculated with CALDose_X 5.0 software. Measured eye and gonad doses were found to be significant compared to ESDs during anterior-posterior (AP) abdomen and AP skull examinations, respectively. The results obtained in this research work indicate the suitability of utilizing nanoDots OSL dosimeter for entrance surface dose assessment during diagnostic X-ray examinations

Transmitter Optimization Techniques for Physical Layer Security

Information security is one of the most critical issues in wireless networks as the signals transmitted through wireless medium are more vulnerable for interception. Although the existing conventional security techniques are proven to be safe, the broadcast nature of wireless communications introduces different challenges in terms of key exchange and distributions. As a result, information theoretic physical layer security has been proposed to complement the conventional security techniques for enhancing security in wireless transmissions. On the other hand, the rapid growth of data rates introduces different challenges on power limited mobile devices in terms of energy requirements. Recently, research work on wireless power transfer claimed that it has been considered as a potential technique to extend the battery lifetime of wireless networks. However, the algorithms developed based on the conventional optimization approaches often require iterative techniques, which poses challenges for real-time processing. To meet the demanding requirements of future ultra-low latency and reliable networks, neural network (NN) based approach can be employed to determine the resource allocations in wireless communications. This thesis developed different transmission strategies for secure transmission in wireless communications. Firstly, transmitter designs are focused in a multiple-input single-output simultaneous wireless information and power transfer system with unknown eavesdroppers. To improve the performance of physical layer security and the harvested energy, artificial noise is incorporated into the network to mask the secret information between the legitimate terminals. Then, different secrecy energy efficiency designs are considered for a MISO underlay cognitive radio network, in the presence of an energy harvesting receiver. In particular, these designs are developed with different channel state information assumptions at the transmitter. Finally, two different power allocation designs are investigated for a cognitive radio network to maximize the secrecy rate of the secondary receiver: conventional convex optimization framework and NN based algorithm

Beamforming and non-orthogonal multiple access for rate and secrecy enhancement of fifth generation communication system

The fifth-generation (5G) communication systems have many anticipated functionalities and requirements such as high data rate, massive connectivity, wide coverage area, low latency and enhanced secrecy performance. In order to meet these criteria, communication schemes that combine 5G key enabling technologies need to be investigated. In this thesis, a novel communication system that merges non-orthogonal multiple access (NOMA), energy harvesting, beamforming, and full-duplex (FD) techniques in order to enhance both capacity and secrecy of 5G system is introduced. In the capacity improving scheme, NOMA is first combined with beamforming to serve more than one user in each beamforming vector. Next, simultaneous wireless information and power transfer (SWIPT) technique is exploited to encourage the strong user (user with better channel condition) to relay the information messages of the weak user (user with poor channel condition) in FD manner. The total sum rate maximisation problem is formulated and solved by means of convex-concave procedure. The system performance is also analysed by deriving the outage probability of both users. Additionally, the model is extended to a more general case wherein the users are moving, and the outage probability of this dynamic topology is provided by means of the stochastic geometry framework. Novel secure schemes are also introduced to safeguard legitimate users’ information from internal and external eavesdroppers. In the internal eavesdropper’s case, artificial signal concept is adopted to protect NOMA’s weak user’s information from being intercepted by the strong user. The secrecy outage probability of theweak user is derived and validated. In addition, game theory discipline is exploited to provide an efficient eavesdropping avoidance algorithm. Null-steering beamforming is adopted in the external eavesdropper’s case in two different schemes namely self and nonself-cooperative jamming. In self-cooperative strategy, the base station applies the null-steering jamming to impair the eavesdropper channel, while sending the information-bearing signals to the intended legitimate users. Whereas in the nonself-cooperative jamming scheme, the base station provides the helpers with the required information and power by means of SWIPT technique in the first phase. The helpers deploy null-steering beamforming to jam the eavesdropper during the information exchange between the base station and the intended users in the second phase. The secrecy outage probability of the legitimate users is derived in both jamming schemes. Game theory is also introduced to the nonself-cooperative jamming scheme for further improvements on the secrecy outage behaviour and the economic revenue of the system. The proposed capacity enhancing scheme demonstrates about 200% higher sum rate when compared with the non-cooperative and half-duplex cooperative NOMA systems. In addition, the novel secure scheme in the internal eavesdropper case is proven to enhance the information security of the weak user without compromising the functionalities of the strong user or NOMA superiority over orthogonal multiple access systems. Null-steering based jamming system also illustrates improved secrecy performance in the external eavesdropper case when compared to the conventional jamming schemes. Numerical simulations are carried out in order to validate the derived closed-form expressions and to illustrate the performance enhancement achieved by the proposed schemes where the rate is increased by 200% and the secrecy outage probability is decreased by 33% when compared to the baseline systems