Research On Transmission Quality Enhancement Technology For Long-Distance Link Of Internet Of Things

Long-distance link transmission of Internet of Things usually refers to the wireless transmission technology with a single hop transmission distance up to kilometers or even more than ten kilometers.With the demand for wide coverage of the Internet of Things,long-distance link transmission has become an important research direction for the Internet of Things,and it has important applications in ocean monitoring,wild environment sensing and emergency relief.However,in the long-distance link transmission of the Internet of Things,there are still problems such as difficult accurate synchronization,low transmission rate,and insecure transmission,which become the bottleneck restricting the long-distance transmission capability of the Internet of Things.This thesis studies the enhancement of synchronization accuracy of long-distance links,the increase of transmission rate of long-distance links,and the security guarantee of long-distance links.The research achievements have been made in time synchronization,link anti-interference and continuous authentication.The main contributions of this thesis are summarized as follows:(1)Time synchronization of long-distance IoT nodes.Time synchronization is the basis of long-distance link control and link scheduling.Target for the complex application scenarios of long-distance Internet of Things,the existing time synchronization methods oriented to the Internet of Things cannot meet the requirements of long-distance and high-precision heterogeneous node synchronization.This thesis proposes a time synchronization method for IoT nodes with long-distance based on aircraft ADS-B signals.Taking advantage of the characteristics of wide coverage and strong penetration of ADS-B signal,it is used as the trigger source of time synchronization,and the position information and velocity information are used to perform linear regression to obtain the clock error and clock drift of the two nodes.A multi-aircraft joint regression algorithm is proposed to avoid the difference in signal quality of different aircraft.When two nodes are far apart and can not receive enough matching packets,we design a transitive synchronization mechanism for relay nodes to improve the robustness of the system.Finally,referring to the neighbor discovery technique in low-power nodes of the Internet of Things,a synchronization method for low-power nodes is designed.We deploy and verify the long-distance time synchronization system in the field.The results show that,compared with the commonly used NTP method,which can only achieve millisecond-level time synchronization between nodes,this method can achieve sub-millisecond time synchronization between long-distance heterogeneous nodes.(2)Anti-interference MAC protocol for long-distance links.The increase in communication distance reduces the ability of the link to access the channel,making it more susceptible to interference,which becomes the main factor limiting the transmission rate of the long-distance links.Specifically,before the short-distance 802.11 link accesses the channel,the node performs channel sensing within the DIFS time of 50us.The increase in WiLDNet link transmission distance brings inevitable transmission delay,which makes it difficult for interfering nodes to perceive that the channel is busy and cause interference during DIFS time.We define it as a time-hidden terminal problem,and the existing WiLDNet mainly controls the transmission time of different links through a TDMA method.But the scalability of the TDMA method is limited.In this thesis,we use the asymmetry of short-distance links to longdistance links to send and receive ends,and propose a protective interference MAC protocol-CSMA/PJ that conforms to the 802.11 standards.By predicting the arrival time of data packets,and sending them in advance.The way of protecting packets realizes the coexistence of long and short-distance links.At the same time,the dynamic sending mechanism of the protection packet is designed,so that the uncontrolled short-distance link can perceive the existence of the long-distance link as much as possible.We implement the CSMA/PJ method on a real device and verify it on an emulated long-distance WiLD link.Compared with the existing CSMA method,it increases the transmission rate of long-distance links by 6 times in the coexistence scenario.(3)FM signal-based parasitic coding authentication technology.The increase in communication distance reduces the link’s ability to access the channel,making forged signals a greater threat.At the same time,the larger transmission range makes it more difficult for attackers to distinguish.For the longdistance link based on 802.11 protocol(such as the transmission link of a drone),which only performs authentication once when connecting.Unable to achieve continuous and reliable authentication,vulnerable to de-authentication attacks.However,existing methods cannot resist replay attacks or adapt to the mobility scenarios of the Internet of Things.In this thesis,we propose a parasitic coding authentication technology,which realizes the coupling of the communication channel and the host channel by encoding the information to be authenticated into the host signal,and realizes secure coding in long-distance scenarios.We select the widely existing FM signal as the host signal and propose preprocessing and signal alignment methods to ensure the consistency of the signal.Finally,a parasitic coding strategy is designed by referring to the vector coding method in the field of audio and video compression.We deploy and verify the parasitic coding authentication system on real devices.The results show that the FM signal can maintain a high degree of consistency in the long-distance complex reception scenario,and the parasitic coding method can achieve secure and accurate continuous authentication of long-distance links.Compared with the key-based authentication method,this method can effectively resist the threat of replay attack.