Resource Allocation for Non-orthogonal Multiple Access (NOMA) Systems

非正交多址接入系統資源分配算法的研究

Student thesis: Doctoral Thesis

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Award date12 Mar 2018

Abstract

Non-orthogonal multiple access (NOMA) with the use of successive interference cancellation (SIC) has been regarded as a promising multiple access technique for the fifth generation cellular systems (5G) as it allows multiple users to share the same time and frequency resource simultaneously, which brings a higher spectrum efficiency. Because of the entirely different principle to that of the conventional orthogonal multiple access (OMA) techniques where different users are allocated to orthogonal resources in terms of time, frequency, or code domain, the resource allocation algorithms that are applied in conventional OMA systems cannot be invoked in NOMA networks directly.

In this thesis, a number of resource allocation algorithms are devised and analyzed for the NOMA networks, which can be divided into two categories: power minimization (PM) and rate maximization (RM). Specifically, the objective of PM is to minimize the total power consumption with user's quality of service (QoS) requirement taken into account. Meanwhile, the objective of RM is to maximize the system throughput under the resource constraint.

For the PM approach, the total power minimization problem for the downlink of a multi-cell NOMA system subject to the data rate requirements of the users is investigated. The feasibility of the problem, called P-OPT, is characterized by the Perron-Frobenius eigenvalues of the matrices arising from the power control subproblems that constitute another optimization problem called Q-OPT. We have shown that the optimal solution to P-OPT could be obtained by solving the corresponding Q-OPT.Therefore, a distributed power control algorithm to solve Q-OPT is proposed and its convergence is proved by showing that the underlying iterative function is a standard interference function. Computer simulations are included to validate the convergence of the distributed power control algorithm and quantify the improvement of the proposed method over fractional transmit power control (FTPC) and OMA schemes in terms of power consumption and outage probability.

For the RM approach, we investigate the joint subcarrier and power allocation problem for the downlink of a multi-carrier NOMA (MC-NOMA) system. The sum rate maximization problem was shown NP-hard and solved by an existing near-optimal solution based on Lagrangian duality and dynamic programming (LDDP). However, the computational complexity of LDDP is very high. Therefore, in this thesis, we design a time-efficient resource allocation framework with three steps. 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 proposed. Simulation results depict that the proposed three-step resource allocation strategies could achieve comparable performance to LDDP with much lower computational complexity.