Next-Generation Multiple Access: From Basic Principles to Modern Architectures

The pressure to develop new network architectures and multiple access technologies is driven by increasing demands on network performance, number of devices, network traffic, and use cases. Recent advances in open radio access networks (RANs) with open interfaces and software-defined network functionalities allow adaptability in terms of medium access control and physical layer, but also flexibility in terms of network architectures. The aim of this tutorial is to provide a comprehensive overview of the current set of network architectures for wireless access together with next-generation multiple access technologies. It starts with the classical models for multiple access channel (MAC), broadcast channel (BC), and interference channel (IC) from network information theory and derives the fundamental results on capacity regions and their coding and signal processing schemes. Extensions to multicarrier, multiantenna, and multicell scenarios are discussed. The evolution from orthogonal to spatial-division multiple access (SDMA), nonorthogonal multiple access (NOMA), and rate splitting multiple access (RSMA) techniques and their performance guarantees are carefully explained. Recent advances toward multiconnectivity, cloud-RAN (C-RAN), and cell-free multiple access (CFMA) are explained. The data rate benefits of an anecdotal open RAN network are developed and the corresponding user data rates are calculated. Massive random and grant-free access schemes are also discussed. The tutorial concludes with a list of open research questions.