# Wireless Communication Technologies for Safe Cooperative Cyber Physical Systems
## 4 Safety and Security for SafeCOP Co-CPSs
### Vehicle Control Loss Warning
Accordingly, the vehicle with high BER spends longer time on command retransmissions, which prolongs dissemination latency of the platoons. Therefore, allocating the transmit rate without a proper adaptivity leads to command dissemination latency performance degradation. In [70], we proposed a low-latency driving command dissemination (LCD) algorithm to adapt the transmit rate (i.e., modulation) allocation of vehicles as such that the latency of command dissemination in the platoon is minimized under guaranteed BER. We proved that the LCD algorithm achieves computation time complexity of O(NM2), where N and M are the number of vehicles and modulation levels, respectively. The simulation results show that LCD significantly improves the dissemination rate by 50.9% existing algorithms. Moreover, LCD also approximates the lower bound of dissemination latency with the maximum gap of up to 0.2 s.  
Regarding security, each node's wireless communications must also be encrypted. Data exchange among nodes must be properly secured to prevent unauthorized access and alteration of the message content for malicious purposes. Due to broadcast nature of radio channels, disseminating sensory data is vulnerable to eavesdropping, and message modification from an illegitimate eavesdropper. To improve communication security in CPS, using a shared secret key for data encryption/decryption is crucial to support data confidentiality, integrity, and sender authentication. Key generation based on the randomness in a wireless fading channel is a promising approach [71], where two sensor nodes extract secret bits from the inherently random spatial and temporal variations of the reciprocal wireless channel between them. However, while previous works on fading channel based secret key generation mainly focused on improving the secret bit generation rate between a pair of sensor nodes (by exploiting temporal and spatial variations of radio channel, multiple antenna diversity, or multiple frequencies), the problem of unanimity of the generated key for the real-time data dissemination remained a challenge. To address this, we presented in [72] a new data dissemination security protocol that quantizes the estimated received signal strength (RSS) measurements. The quantization intervals are cooperatively adapted to reduce secret bit mismatch rate (BMR). The secret key generated by our protocol is based on channel randomness over multiple hops, the eavesdropper at a different location experiences independent channel fading, which is not able to obtain the same key. In addition, the proposed protocol can be applied to more critical systems, as the secret key is generated in a distributed manner, eliminating a single point of failure. In addition to inter-vehicular communications, its wireless intra-vehicular counterpart is also being addressed. In-vehicle wireless networks have been recently proposed with the goal of reducing manufacturing and maintenance cost of a large amount of wiring harnesses within vehicles [73; 74]. The wiring harnesses used for the transmission of data and power delivery within current vehicles may have up to 4000 parts, weigh as much as 40 kg and contain up to 4 km of wiring. Eliminating these wires would additionally have the potential to improve fuel efficiency, greenhouse gas emission, and spur innovation by providing an open architecture to accommodate new systems and applications. Interestingly, in [75], Volvo group trucks technology presented a practical design of an in-vehicle WSN, using the IEEE 802.15.4 TSCH protocol as the MAC protocol. This work uses a network with only 10 nodes, while vehicles have the potential to use a much higher number of wireless sensors. DSME has better performance as the number of nodes increases and is probably the most flexible MAC behaviour from all the IEEE 802.15.4e proposals. Its multi-superframe structure allows for the transmission of both periodic as well sporadic traffic, while still supporting a fast reconfiguration of the DSME-GTS schedule.