# Sixth-Generation (6G) Networks for Improved Machine-to-Machine (M2M) Communication in Industry 4.0
## 3 Results of the Review
### Key Issues to Be Addressed  
The key problem in 6G networks is the scarcity of spectrum, resulting from the unprecedented load generated by the large number of devices and communication between them. VLC is a viable high-speed alternative or complement to radio frequency (RF) technologies. This is important because VLC is cost-effective, energy-efficient, and safe, using current infrastructure, both in indoor environments (factory halls, offices) and underwater environments. VLCs face a number of limitations: restricted LED bandwidth, dimming, flicker, line-of-sight (LOS) requirements, impact of severe weather conditions, noise, light interference, shadows, transmitter and receiver alignment, signal decoding complexity, and mobility issues. Non-orthogonal multiple access (NOMA) has been introduced as an answer to these issues (Figure 6) [10]. The problem of optimizing energy consumption in IIoT must be solved in view of the limited energy resources of IIoT 6G devices. The total consumption of millions of devices, even of the energy-saving type, translates into significant energy use. The radio subsystem, properly managed, can significantly improve the energy efficiency of IoT networks [11]. Fleet networks are an extension of fixed infrastructure, using a vehicular ad hoc network (VANET) in which vehicles communicate in infrastructure mode via roadside units (RSUs). The services of disseminating emergency messages and vehicle updates about the traffic situation are performed using sparsely deployed RSUs by constantly switching between them despite frequent interruptions in RSU operation. A mobile RSU (mRSU) has been proposed to complement stationary sRSUs. This solution improves packet delivery rates, increases throughput, reduces end-to-end latency, and reduces hop counts but also leads to a 2% increase in energy consumption [12]. The areas of industrial application of 6G-based IoT include IIoT as well as the Internet of Vehicles (IoV) and Internet of Drones (IoD), offering intelligent communication between a huge number of objects around us, including M2M communication. This requires smooth data flow, uninterrupted communication capabilities, low latencies, and high reliability, based on a network of access points (APs) serving user nodes. Resource control is handled through pilot-based AP selection (PBAS). The PBAS algorithm improves spectral efficiency by 22% at the cell edge and 1.5% at the cell center [13]. Industrial activity also includes the management of vehicle fleets. Vehicle social networks (VSNs), a component of vehicle-to-everything (V2X) services, enable vehicles to create social networks on roads and in garages or warehouses to improve passenger comfort and safety and raise confidence in the transportation of goods and services. Their requirements include lower latency, higher connection density, and up to 100% coverage. Blockchain-based collaborative distributed intrusion detection (BCDID) and dynamic bandwidth throttling (DTS) prevent attacks in VSN networks (involving taking over one of the network devices) [14]. Ensuring high-speed communication between multiple applications and multiple interactive services simultaneously requires a stable propagation environment without connection drops, data loss, or high latency. Distributed IRSs increase the reliability of 6G transmission but cause increased overhead resulting from the use of multiple IRSs, which requires effective resource control (linking IRS sets to the user's equipment, based on the intelligent-reflecting surface-user  
Figure 6: Downlink NOMA-VLC [10]. equipment association based on pilots (IUABP) algorithm, for example). In tests, this resulted in a 30% improvement in the total transmission speed [15]. The error propagation and computational complexity of signal detection in multi-input, multi-output wireless orthogonal frequency division multiplexing (MIMO-OFDM) systems require the development of simpler and efficient iterative feedback signal detection.