Grant-free access schemes are steadily gaining momentum for emerging machine type communications and internet of things applications. Relying on variations of the classical ALOHA protocol to allow a vast population of users share a common channel in an uncoordinated fashion, these solutions have recently been embedded in international standards as well as proprietary commercial products. In this context, the use of multiple low-complexity receivers that act as relays, collecting packets from users and forwarding them towards a central unit represents an interesting approach to increase the efficiency of the system, which finds natural application in both terrestrial and non-terrestrial settings. To gain insights on the potential of such a configuration, this paper investigates a two-tier slotted ALOHA system under an erasure channel model. Closed form expressions for both end-to-end throughput and packet loss rate are derived for an arbitrary number of receivers, and the role of correlation among wireless channels connecting users to relays is thoroughly discussed. Non-trivial tradeoffs and systems design hints are highlighted, pinpointing some fundamental benefits and limitations of receiver diversity for grant-free based machine type communications.

对于新兴的机器类型通信和物联网应用，免授权访问方案正在稳步增长。依靠经典ALOHA协议的变体，以允许大量用户以不协调的方式共享一个公共频道，这些解决方案最近已嵌入国际标准以及专有的商业产品中。在这种情况下，使用多个低复杂度的接收器作为中继，从用户收集数据包并将其转发到中央单元代表了一种提高系统效率的有趣方法，该方法自然适用于地面和非地面系统。地面设置。为了深入了解这种配置的潜​​力，本文研究了在擦除信道模型下的两层时隙式ALOHA系统。对于任意数量的接收器，导出了端到端吞吐量和丢包率的闭式表达式，并详细讨论了将用户连接到中继的无线信道之间的相关性。非平凡的权衡和系统设计提示被突出显示，指出了基于无授权的机器类型通信的接收机多样性的一些基本好处和局限性。