The Road to Next-Generation Multiple Access: A 50-Year Tutorial Review

TL;DR

The paper presents a 50-year tutorial on MA, tracing the shift from orthogonal to non-orthogonal strategies and introducing NGMA concepts. It synthesizes information-theoretic foundations for power-domain NOMA, explores SDMA and near-field spatial-domain MA, and introduces MI-based ISAC frameworks to unify sensing and communications. It then outlines NGMA directions, including semantic-NGMA, RIS-enabled tuning, novel antennas, and ML-driven tools, highlighting open challenges and research opportunities. The work emphasizes flexibility, granularity, and maintainability as core properties for 6G MA frameworks, providing a comprehensive guide for researchers and engineers. The synthesis offers both theoretical insights and practical design pathways for future heterogeneous, intelligent, and integrated wireless networks.

Abstract

The evolution of wireless communications has been significantly influenced by remarkable advancements in multiple access (MA) technologies over the past five decades, shaping the landscape of modern connectivity. Within this context, a comprehensive tutorial review is presented, focusing on representative MA techniques developed over the past 50 years. The following areas are explored: i) The foundational principles and information-theoretic capacity limits of power-domain non-orthogonal multiple access (NOMA) are characterized, along with its extension to multiple-input multiple-output (MIMO)-NOMA. ii) Several MA transmission schemes exploiting the spatial domain are investigated, encompassing both conventional space-division multiple access (SDMA)/MIMO-NOMA systems and near-field MA systems utilizing spherical-wave propagation models. iii) The application of NOMA to integrated sensing and communications (ISAC) systems is studied. This includes an introduction to typical NOMA-based downlink/uplink ISAC frameworks, followed by an evaluation of their performance limits using a mutual information (MI)-based analytical framework. iv) Major issues and research opportunities associated with the integration of MA with other emerging technologies are identified to facilitate MA in next-generation networks, i.e., next-generation multiple access (NGMA). Throughout the paper, promising directions are highlighted to inspire future research endeavors in the realm of MA and NGMA.

Figures (38)

• Figure 1: Illustration of MAC and BC.
• Figure 2: Condensed overview of this tutorial review on MA.
• Figure 3: Illustration of the single-user DMC, where $p(y|x)$ denotes the transition probability density function of the channel for each channel use.
• Figure 4: Illustration of the discrete memoryless MAC, where $p(y|x_1,\ldots,x_K)$ denotes the transition probability density function of the channel for each channel use, $\mathcal{X}_k$ is the input alphabet of user $k$, and $\mathcal{Y}$ is the output alphabet.
• Figure 5: Illustration of SIC decoding for decoding order $1\rightarrow2\rightarrow\ldots\rightarrow K$.

Theorems & Definitions (8)

• Theorem 1
• Theorem 2
• Theorem 3
• Remark 1
• Theorem 4
• Theorem 5
• Theorem 6
• Lemma 1