Massive machine-type communications (mMTC) is one of the main three focus areas in the 5th generation (5G) of wireless communications technologies to enable connectivity of a massive number of Internet of things (IoT) devices with little or no human intervention. In conventional human-type communications (HTC), due to the limited number of available channel resources and orthogonal resource allocation techniques, users get a transmission slot by making scheduling/connection requests. The involved control channel signaling, negligible with respect to the huge transmit data, is not a major issue. However, this may turn into a potential performance bottleneck in mMTC, where huge number of devices transmit short packet data in a sporadic way. To tackle the limited radio resources and massive connectivity challenges, non-orthogonal multiple access (NOMA) has emerged as a promising technology that allows multiple users to simultaneously transmit their data over the same channel resource. This is achieved by employing user-specific signature sequences at the transmitting devices, which are exploited by the receiver for multi-user data detection. Due to its massive connectivity potential, NOMA has also been considered to enable grant-free transmissions especially in mMTC, where devices can transmit their data whenever they need without the scheduling requests. The existing surveys majorly discuss different NOMA schemes, and exploit their potential, in typical grant-based HTC scenarios, where users are connected with the base station, and various system parameters are pre-defined in the scheduling phase. Different from these works, this survey provides a comprehensive review of the recent advances in NOMA from a grant-free connectivity perspective. Various grant-free NOMA schemes are presented, their potential and related practical challenges are highlighted, and possible future directions are thoroughly discussed at the end.

中文翻译：

---

物联网的免授权非正交多路访问：一项调查

大规模机器类型通信 (mMTC) 是第 5 代 (5G) 无线通信技术的三个主要重点领域之一，可在很少或无需人工干预的情况下实现大量物联网 (IoT) 设备的连接。在传统的人机通信（HTC）中，由于可用信道资源和正交资源分配技术的数量有限，用户通过发出调度/连接请求来获得传输时隙。所涉及的控制信道信令对于庞大的传输数据可以忽略不计，并不是主要问题。然而，这可能会变成 mMTC 中潜在的性能瓶颈，其中大量设备以零星的方式传输短数据包数据。为了应对有限的无线电资源和巨大的连接挑战，非正交多路访问 (NOMA) 已成为一种有前途的技术，它允许多个用户通过相同的信道资源同时传输他们的数据。这是通过在发送设备上使用用户特定的签名序列来实现的，接收器利用这些签名序列进行多用户数据检测。由于其巨大的连接潜力，NOMA 还被认为可以实现免授权传输，尤其是在 mMTC 中，设备可以在不需要调度请求的情况下随时传输数据。现有的调查主要讨论不同的 NOMA 方案，并在典型的基于授权的 HTC 场景中开发它们的潜力，其中用户与基站连接，并且在调度阶段预定义各种系统参数。与这些作品不同的是，本次调查从无赠款连接的角度全面回顾了 NOMA 的最新进展。介绍了各种无赠款的 NOMA 计划，强调了它们的潜力和相关的实际挑战，并在最后深入讨论了可能的未来方向。