Outage probability analysis and robust power allocation for downlink Non-Orthogonal Multiple Access (NOMA) system

Abstract

Due to massive connectivity and increasing demands of various services and data-hungry applications, a full-scale implementation of the fifth generation (5G) and future-generation wireless systems requires more effective radio access techniques. In this regard, non-orthogonal multiple access (NOMA) has recently gained ever-growing attention from both academics and industry. NOMA has been recognized as an effective multiple access (MA) technique that significantly improves the spectral efficiency. Different from conventional orthogonal multiple access (OMA) technologies,NOMA allows multiple users to share time and frequency resources in the same spatial layer via power domain multiplexing, and to apply successive interferencecancellation (SIC) in order to achieve multi-user detection.Taking into account the quality-of-service (QoS) requirements of users, the outage probability becomes a key performance measure in NOMA systems to ensure that the data rate of each user does not fall a required target with a guaranteedhigh probability due to SIC imperfections, imperfect channel state information (CSI) or incomplete CSI knowledge. This thesis aims to provide a comprehensive analysis of the outage probability of downlink NOMA systems considering variouseffects including SIC imperfections and statistical CSI.First, under imperfect SIC assumption, we analyze the outage probability in a downlink multi-carrier NOMA (MC-NOMA) system modelling and taking into account the possible decoding error propagation in the SIC process. Moreover,subject to the outage probability constraints for QoS provisioning for individual users, we develope a robust power allocation algorithm aiming to minimize the total transmit power of system.Secondly, since the perfect CSI assumptions are unrealistic to be achieved with development of wireless network due to channel estimation and quantization errors, or inevitable delays in feedback links, the closed-form outage expressions for each user of a downlink MC-NOMA system are derived when only the statistical CSI is available at the transmitter. The calculation of outage probability for the distant user is challenging, since the total rate expression is given by the sum of logarithmic functions of the ratio between two shifted exponential random variables, which are independent. In order to derive the closed-form outage probability expressions bothfor two subcarriers and for a general case of multiple subcarriers, efficient approximations are proposed. The probability density function (PDF) of the product shifteddistributions can be determined for the near user by the Mellin transform and the generalized upper incomplete Fox’s H function.Thirdly, motivated by excellent benefits of multi-antenna aided NOMA design, we concentrate on the analytical study of the behavior of outage probability for individual users in downlink multiple-input single-output NOMA (MISO-NOMA) system. We derive tractable closed-form outage probability expressions given a minimum target rate for the individual users for the case of two antennas, by modeling cumulative distribution function (CDF) of received signal-to interference plus noise ratio (SINR).