This paper is a tutorial introduction to the field of unsourced multiple access (UMAC) protocols. We first provide a historical survey of the evolution of random access protocols, focusing specifically on the case in which uncoordinated users share a wireless broadcasting medium. Next, we highlight the change of perspective originated by the UMAC model, in which the physical and medium access layer's protocols cooperate, thus reframing random access as a novel coding-theoretic problem. By now, a large variety of UMAC protocols (codes) emerged, necessitating a certain classification that we indeed propose here. Although some random access schemes require a radical change of the physical layer, others can be implemented with minimal changes to existing industry standards. As an example, we discuss a simple modification to the 5GNR Release 16 random access channel that builds on the UMAC theory and that dramatically improves energy efficiency for systems with even moderate number of simultaneous users (e.g., $5-10$ dB gain for $10-50$ users), and also enables handling of high number of users, something completely out of reach of the state-of-the-art.

翻译：本文是关于无源多址接入协议领域的教程式导论。我们首先回顾了随机接入协议的发展历程，特别聚焦于无协调用户共享无线广播信道的情形。接着，我们重点阐述了UMAC模型带来的视角转变——物理层与媒体接入控制层协议协同工作，从而将随机接入重构为一个新颖的编码理论问题。迄今为止，已涌现出大量UMAC协议（编码方案），亟需建立分类体系，本文正致力于提出这样的分类框架。尽管部分随机接入方案需要对物理层进行根本性改造，但其他方案仅需对现有工业标准进行最小改动即可实现。作为示例，我们讨论了对5GNR Release 16随机接入信道的简易改进方案：该方案基于UMAC理论构建，能显著提升中等规模并发用户系统的能效（例如10-50用户场景下可获得5-10 dB增益），同时还能支持海量用户接入——这是现有技术完全无法企及的。