Subcarrier and Power Allocation for the Downlink of Multicarrier NOMA Systems

International audienceThis paper investigates the joint subcarrier and power allocation problem for the downlink of a multi-carrier non-orthogonal multiple access (MC-NOMA) system. A novel three-step resource allocation framework is designed to deal with the sum rate maximization problem. In Step 1, we relax the problem by assuming each of the users can use all subcarriers simultaneously. With this assumption, we prove the convexity of the resultant power control problem and solve it via convex programming tools to get a power vector for each user; In Step 2, we allocate subcarriers to users by a heuristic greedy manner with the obtained power vectors in Step 1; In Step 3, the proposed power control schemes used in Step 1 are applied once more to further improve the system performance with the obtained sub-carrier assignment of Step 2. To solve the maximization problem with fixed subcarrier assignments in both Step 1 and Step 3, a centralized power allocation method based on projected gradient descent algorithm and two distributed power control strategies based respectively on pseudo-gradient algorithm and iterative waterfilling algorithm are investigated. Numerical results show that our proposed three-step resource allocation algorithm could achieve comparable sum rate performance to the existing near-optimal solution with much lower computational complexity and outperforms power controlled OMA scheme. Besides, a tradeoff between user fairness and sum rate performance can be achieved via applying different user power constraint strategies in the proposed algorithm

Optimal Joint Subcarrier and Power Allocation in NOMA is Strongly NP-Hard

International audienceNon-orthogonal multiple access (NOMA) is a promising radio access technology for 5G. It allows several users to transmit on the same frequency and time resource by performing power-domain multiplexing. At the receiver side, successive interference cancellation (SIC) is applied to mitigate interference among the multiplexed signals. In this way, NOMA can outperform orthogonal multiple access schemes used in conventional cellular networks in terms of spectral efficiency and allows more simultaneous users. This paper investigates the computational complexity of joint subcarrier and power allocation problems in multi-carrier NOMA systems. We prove that these problems are strongly NP-hard for a large class of objective functions, namely the weighted generalized means of the individual data rates. This class covers the popular weighted sum-rate, proportional fairness, harmonic mean and max-min fairness utilities. Our results show that the optimal power and subcarrier allocation cannot be computed in polynomial time in the general case, unless P = NP. Nevertheless, we present some tractable special cases and we show that they can be solved efficiently

Weighted Sum-Rate Maximization in Multi-Carrier NOMA with Cellular Power Constraint

International audienceNon-orthogonal multiple access (NOMA) has received significant attention for future wireless networks. NOMA outperforms orthogonal schemes, such as OFDMA, in terms of spectral efficiency and massive connectivity. The joint subcarrier and power allocation problem in NOMA is NP-hard to solve in general, due to complex impacts of signal superposition on each user's achievable data rates, as well as combinatorial constraints on the number of multiplexed users per sub-carrier to mitigate error propagation. In this family of problems, weighted sum-rate (WSR) is an important objective function as it can achieve different tradeoffs between sum-rate performance and user fairness. We propose a novel approach to solve the WSR maximization problem in multi-carrier NOMA with cellular power constraint. The problem is divided into two polynomial time solvable sub-problems. First, the multi-carrier power control (given a fixed subcarrier allocation) is non-convex. By taking advantage of its separability property, we design an optimal and low complexity algorithm (MCPC) based on projected gradient descent. Secondly, the single-carrier user selection is a non-convex mixed-integer problem that we solve using dynamic programming (SCUS). This work also aims to give an understanding on how each sub-problem's particular structure can facilitate the algorithm design. In that respect, the above MCPC and SCUS are basic building blocks that can be applied in a wide range of resource allocation problems. Furthermore, we propose an efficient heuristic to solve the general WSR maximization problem by combining MCPC and SCUS. Numerical results show that it achieves near-optimal sum-rate with user fairness, as well as significant performance improvement over OMA

Cooperative Multi-Bitrate Video Caching and Transcoding in Multicarrier NOMA-Assisted Heterogeneous Virtualized MEC Networks

Cooperative video caching and transcoding in mobile edge computing (MEC) networks is a new paradigm for future wireless networks, e.g., 5G and 5G beyond, to reduce scarce and expensive backhaul resource usage by prefetching video files within radio access networks (RANs). Integration of this technique with other advent technologies, such as wireless network virtualization and multicarrier non-orthogonal multiple access (MC-NOMA), provides more flexible video delivery opportunities, which leads to enhancements both for the network's revenue and for the end-users' service experience. In this regard, we propose a two-phase RAF for a parallel cooperative joint multi-bitrate video caching and transcoding in heterogeneous virtualized MEC networks. In the cache placement phase, we propose novel proactive delivery-aware cache placement strategies (DACPSs) by jointly allocating physical and radio resources based on network stochastic information to exploit flexible delivery opportunities. Then, for the delivery phase, we propose a delivery policy based on the user requests and network channel conditions. The optimization problems corresponding to both phases aim to maximize the total revenue of network slices, i.e., virtual networks. Both problems are non-convex and suffer from high-computational complexities. For each phase, we show how the problem can be solved efficiently. We also propose a low-complexity RAF in which the complexity of the delivery algorithm is significantly reduced. A Delivery-aware cache refreshment strategy (DACRS) in the delivery phase is also proposed to tackle the dynamically changes of network stochastic information. Extensive numerical assessments demonstrate a performance improvement of up to 30% for our proposed DACPSs and DACRS over traditional approaches.Comment: 53 pages, 24 figure

Zero-forcing Oriented Power Minimization for Multi-cell MISO-NOMA Systems: A Joint User Grouping, Beamforming and Power Control Perspective

International audienceFuture wireless communication systems have been imposed high requirement on power efficiency for operator's profitability as well as to alleviate information and communication technology (ICT) global carbon emission. To meet these challenges, the power consumption minimization problem for a generic multi-cell multiple input and single output non-orthogonal multiple access (MISO-NOMA) system is studied in this work. The associated joint user grouping, beamforming (BF) and power control problem is a mixed integer non-convex programming problem, which is tackled by an iterative distributed methodology. Towards this end, the near-optimal zero-forcing (ZF) BF is leveraged, wherein the semiorthogonal user selection (SUS) strategy is applied to select BF users. Based on these, the BF vectors and BF users are determined for each cell using only local information. Then, two distributed user grouping strategies are proposed. The first one, called channel condition based user clustering (CCUC), performs user grouping in each cell based on the channel conditions. This is conducted independently of the power control part and has low computational complexity. Another algorithm, called power consumption based user clustering (PCUC), uses both the channel conditions and inter-cell interference information to minimize each cell's power consumption. In contrary to CCUC, PCUC is optimized jointly with the power control. Finally, with the obtained user grouping and BF vectors, the resultant power allocation problem is optimally solved via an iterative algorithm, whose convergence is mathematically proven given that the problem is feasible. We perform Monte-Carlo simulation and numerical results show that the proposed resource management methods outperform various conventional MISO schemes and the non-clustered MISO-NOMA strategy in several aspects, including power consumption, outage probability, energy efficiency, and connectivity efficiency

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

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

Operating multi-user transmission for 5G and beyond cellular systems

Every decade, a new generation of cellular networks is released to keep up with the ever-growing demand for data and use cases. Traditionally, cellular networks rely on partitioning radio resources into a set of physical resource blocks (PRBs). Each PRB is used by the base-station to transmit exclusively to one user, which is referred to as single-user transmission. Recently, multi-user transmission has been introduced to enable the base-station to simultaneously serve multiple users using the same PRB. While multi-user transmission can be much more efficient than its single-user counterpart, it is significantly more challenging to operate. Thus, in this thesis we study the operation, i.e., the Radio Resource Management (RRM), for two popular multi-user transmission technologies; namely, 1) Non-Orthogonal Multiple Access (NOMA) and 2) Multi-User Multiple-Input Multiple-Output (MU-MIMO). For NOMA RRM, we study a multi-cell, multi-carrier downlink system. First, we formulate and solve a centralized proportional fair scheduling genie problem that jointly performs user selection, power allocation and power distribution, and Modulation and Coding Scheme (MCS) selection. While such a centralized schedule is practically infeasible, it upper bounds the achievable performance. Then, we propose a simple static coordinated power allocation scheme across all cells for NOMA using a simple power map that is easily calibrated offline. We find that using a simple static coordinated power allocation scheme improves performance by 80% compared to equal power allocation. Finally, we focus on online network operation and study practical schedulers that perform user-selection, power distribution, and MCS selection. We propose a family of practical scheduling algorithms, each of them exhibiting a different trade-off between complexity (i.e., run-time) and performance. The one we selected sacrifices a maximum of 10% performance while reducing the computation time by a factor of 45 with respect to the optimal user scheduler. For MU-MIMO RRM, we focus on the study of the downlink of an OFDMA massive MU-MIMO single cell assuming ZFT (Zero Forcing Transmission) precoding. An offline study is initiated with the goal of finding the best achievable performance by jointly optimizing user-selection, power distribution and MCS selection. The best performance is analyzed by using both Branch-Reduce-and-Bound (BRB) global optimization technique for upper-bounding the achievable performance and a set of different greedy searches for lower bounding the achievable performance to find good feasible solutions. The results suggest that a specific search strategy referred to as greedy-down-all-the-way (GDAW) with full-drop (FD) is quasi-optimal. Afterwards, we design a simple practical scheduler that achieves 97% of the performance to GDAW with FD and has comparable runtime to that of the state-of-the-art benchmark that selects all users, performs ZFT precoding followed by power distribution using water-filling. The proposed scheme performs a simple round robin grouping to select users, followed by ZFT precoding and joint power distribution and MCS selection via a novel greedy algorithm with a possible additional iteration to take zero-rate users into account. Our solution outperforms the benchmark by 281%

Double Iterative Waterfilling for Sum Rate Maximization in Multicarrier NOMA Systems

International audienceThis paper investigates the subcarrier and power allocation for the downlink of a multicarrier non-orthogonal multiple access (MC-NOMA) system. A three-step algorithm is proposed to deal with the sum rate maximization problem. In Step 1, we assume that each user can use all the subcarriers simultaneously and apply the synchronous iterative waterfilling algorithm (SIWA) to obtain a power vector for each user. In Step 2, subcarriers are assigned to users by a heuristic greedy method based on the achieved power allocation result of Step 1. In Step 3, SIWA is used once again to further improve the system performance with the obtained subcarrier assignment result of Step 2. The convergence of SIWA in Step 3 is proved when the number of multiplexed users is no more than two. Since SIWA is applied twice, we call our three-step method Double Iterative Waterfilling Algorithm (DIWA). Numerical results show that the proposed DIWA achieves comparable performance to an existing near-optimal solution but with much lower time complexity