# A Controller Switching Mechanism for Resilient Wireless Sensor-Actuator Networks
## 3 System Overview
### Wireless Communication System  
We focus on the LWB protocol for our CPS design because it was designed as a low-power multihop wireless solution. LWB provides a higher-layer protocol and shared-bus application abstraction. This is based on IEEE 802.15.4 radios with O-QPSK modulation in the 2.4 GHz band. The LWB protocol enables reliable communication among sensors, actuator, and controller. In every slot, as shown in Figure 2, one initiator node transmits a message to all the others using a Glossy flood [8]. Since it employs synchronous transmission, it provides reliability above 99.9% in real-world scenarios. The external interference and node failures only marginally affect LWB performance [8]. In addition, LWB is a suitable protocol for controller switching. Unlike WirelessHART's TDMA, the backup controller can overhear the information between plant and main controller due to the  
Figure 1: System model of industrial CPSs. characteristic of flooding communication. In addition, it is unnecessary to consider the independent wireless link status for any time-varying network topology. The LWB network consists of a host node and several source nodes. Similar to the network manager in WirelessHART, the host node schedules and manages the source nodes. In every slot, one initiator node transmits a message to all others using a Glossy flood. Glossy [38] exploits the constructive interference of IEEE 802.15.4 symbols for fast network flooding and implicit time synchronization. Glossy affords network flooding and time synchronization. It executes a minimal, deterministic sequence of operations between the packet receptions and retransmissions in order to ensure a constant retransmission delay. Glossy floods are confined to reserved time slots where no other tasks (such as the application or operating system) are allowed to execute. This avoids interference from shared resources, which can cause unpredictable jitter. Consequently, Glossy also enables the nodes to time-synchronize with the initiator. When a node becomes an initiator and broadcasts a packet, surrounding nodes relay the packet. The receiving side then detects the superposition of multiple signals at the same frequency from multiple transmitters. These synchronous transmissions utilize the constructive interference and the capture effect. The capture effect refers to the phenomenon where a receiver receives one of the synchronously transmitted packets. However, to achieve this effect, the following two conditions must be met:  
1. The difference between the receiving power of one signal and the others, and noise is greater than the capture threshold. 2. A weak signal arrives first and a strong signal arrives thereafter; the strong signal can be received if it arrives within the capture window. Because the receiver no longer performs a preamble search upon receipt, it can receive the strong signal if it arrives before the complete preamble. Both capture threshold and capture window depend on the physical layer. In the case of IEEE 802.15.4, the capture threshold was reported to be 2-3 dB [39], and the capture window was 128 us [40]. Another characteristic is constructive interference, which implies that signals overlap perfectly without time or phase offsets. However, the phase offset varies during reception, although the transmitter can compensate for different path delays owing to the different carrier offsets caused by the asynchronous operation of the transmitter's radio oscillator. Under IEEE 802.15.4, the oscillators are allowed to drift by \(\pm\)40 ppm, which translates to a maximal frequency offset of 192 kHz between sender and receiver over a 2.4 GHz band [41]. Owing to the carrier frequency offset, there are alternating periods of constructive  
Figure 2: Communication rounds in LWB protocol. and destructive interference. Destructive interference can cause bit errors at the receiver. However, a receiver can still successfully receive an entire packet if the signal's time offset is less than 0.5 \(\upmu\)s [39].