NOMA schemes with symbol level processing

Requirements for KPI in 6G networks are significantly higher than in 5G networks. Non-orthogonal multiple access (NOMA) technology has been proposed as a solution to meet the requirements of future communication networks. NOMA technology aims to improve the spectral efficiency of wireless communication systems and support the simultaneous operation of large number of subscribers (especially in mMTC scenarios) and provide flexible resource use. This article discusses the popular symbol-level processing NOMA schemes recently proposed as part of the 3GPP standardization, such as SCMA, PDMA, WSMA, MUSA, and other varieties of NOMA. The main advantages and principles of non-orthogonal access technology are illustrated using the NOMA system with division in power domain (PD-NOMA) for uplink and downlink. Characteristics of various symbol-level processing NOMA schemes, their advantages and disadvantages, as well as problems in the way of their implementation in future wireless networks are analyzed. In addition, this article discusses block diagrams of NOMA systems and mathematical models illustrating the formation of baseband signals in such systems. In conclusion, the noise immunity characteristics of the most popular NOMA schemes with symbol-level processing are given for various loading factors. From the analysis performed, it follows that no NOMA scheme has yet been proposed that is effective in various scenarios of future wireless communication systems and has an acceptable processing complexity in combination with flexible resource allocation for mass connection of subscriber devices.