Green Internet of Things (IoT) aims to enable a sustainable smart world by making energy efficiency (EE) the main performance indicator for IoT hardware and software. With respect to network design, this implies in developing energy-efficient communication protocols and network architectures adapted to the ubiquitousness of the IoT machine-type devices (MTDs) and the sporadic traffic generated by them, keeping a low power consumption at the MTDs-side. In this sense, reconfigurable intelligent surfaces (RISs) have presented the capacity of significantly improving the network coverage using mostly passive reflecting elements, drastically reducing the power expenditure. In this paper, we develop a realistic power consumption model and an expression for the overall system EE for RIS-aided IoT networks that adopt a two time-scale random access (RA) protocol to handle the uplink transmissions. Specifically, during each time slot of the RA protocol, the RIS covers a specific area of interest in the communication cell with a predefined set of phase-shift configurations, changing the channel qualities of the contending MTDs. Numerical results comparing the RA protocol performance reveal that access policies that exploit information of the channel qualities are suitable for green IoT networks, simultaneously attaining competitive EE and throughput combined with low power consumption at the MTDs-side.

翻译：绿色物联网旨在通过将能效作为物联网硬件和软件的主要性能指标，推动可持续智慧世界的实现。在网络设计层面，这意味着需要开发适用于物联网机器类型设备（MTD）普遍存在及其产生的零星流量的高能效通信协议和网络架构，同时保持MTD侧的低功耗。在此背景下，可重构智能表面（RIS）展现了利用无源反射元件显著提升网络覆盖能力，并大幅降低功耗的潜力。本文针对采用双时间尺度随机接入（RA）协议处理上行传输的RIS辅助物联网网络，建立了实际功耗模型并推导了系统整体能效表达式。具体而言，在RA协议的每个时隙内，RIS以预定义的相移配置集覆盖通信小区特定区域，从而改变竞争MTD的信道质量。RA协议性能对比数值结果表明，利用信道质量信息的接入策略适用于绿色物联网网络，能够在实现竞争性能效与吞吐量的同时，保持MTD侧的低功耗。