Time-Driven Sub-GHz Wireless Communication Protocol for Real-Time Cyber-Physical Systems
Tamás Kovácsházy, Samuel Varallyay · 2019
Application of wireless communication solutions grows in Cyber-Physical Systems (CPS), even if the CPS must fulfill real-time requirements. However, nearly all of these solutions use the ubiquitous 2.4/5 GHz ISM frequency bands (Wi-Fi, Bluetooth, etc.) or licensed frequency bands for mobile operators (2G/3G/4G, NB-IoT, etc.). In most applications these solutions are used even if they are not real-time, i.e., they cannot provide the synchronization, delay, and reliability requirements brought up by these applications. There are other wireless solutions for the lower 433 MHz or 868/933 MHz (depending on the geographic location) bands, but their use is limited, even if these frequencies provide much better propagation properties, longer range, can be operated using less power, etc. LoRa and Sigfox use these frequency bands, however, they provide only low data rate services, which are also without any guarantee for real-time operation. On the other hand, due to the simplicity and low complexity radio chips available, it is also possible to adapt these radios for specialized protocols. In the paper, we present a wireless radio protocol for 868 MHz ISM frequency band that provides microsecond range synchronization error, guaranteed delay, and clear message reliability schematics. The protocol is based on an IEEE 1588 derived clock synchronization and time-division multiple access communication scheme. The protocol is demonstrated in a simple application also, in which both time synchronization and low delay guaranteed message delivery are essential for successful operation.