Research On Real-time Performance And Clock Synchronization Of Wireless Distributed Measurement System Based On WLAN

The real-time performance and clock synchronization of Wireless local area network (WLAN)-based distributed measurement systems attract much attention from both academia and industry nowadays. The real-time performance of data transfer is one of the most important parameters in wireless distributed measurement systems. Maintaining reliable data transmission with minimum data transmission delay is a basic requirement in many wireless distributed systems. As a key technology in wireless distributed systems, clock synchronization allows all terminals to achieve data acquisition separately, which reduces the transmission of control information in data bus and significantly improves the efficiency and accuracy of the measurement. The main achievements and contributions of this dissertation are as follows:(1) The research on real-time performance of WLAN based distributed measurement system. The model of WLAN based distributed measurement system is proposed and the performance of the real-time communications is analyzed in this thesis. By using simulation for the real-time communication network, the performance variation of real-time communication through different MAC mechanisms with the gradually increase of the data traffic is investigated. In addition, the influence of different PHY layers on real-time performance and data packet miss probability of is studied. It provides reference data for further design of WLAN based wireless distributed system.(2) The research on real-time performance of distributed measurement system based on DCF of IEEE 802.11. DCF is a fundamental Media Access Control (MAC) mechanism in WLAN. Basic Access Mechanism, RTS/CTS mechanism and RTS/CTS-Fragment principle of DCF are all analyzed. Real-time performance of different wireless distributed MAC layer access mechanisms is studied. This thesis provides an improved MARKOV model and introduces Z transform to analyze and calculate the MAC delay of 802.11 DCF, which includes delay performance of the non-saturation and saturation transmission channel. The numerical analysis of MAC delay in MATLAB matches the network simulation in OPNET10.0, which proves accuracy of the analysis model in MAC delay predication. Furthermore, this thesis provides a theoretical basis for further system design.(3) The research on implementations of IEEE1588 for clock synchronization in wireless distributed system. This thesis analyzes effects of the network topology choice and domain partition of PTP,and studies the PTP clock synchronization accuracy in different network topology and different scheduling mechanisms. In addition, by using offset estimation and skew compensation to improve the accuracy of the PTP clock synchronization in wireless distributed measurement system. The research results are of most value in implementation of IEEE1588 in wireless distributed measurement system.(4) The research on SPTP clock synchronization mechanism. Focusing on problems of PTP clock synchronization in wireless distributed network applications, a new wireless distributed measurement system clock synchronization mechanism SPTP (Simplified Precision Time Protocol) is proposed. SPTP has the advantages of PTP, RBS and PBS methods. The clock synchronization accuracy of SPTP is compared with that of PTP by simulation. SPTP clock synchronization mechanism has obvious advantage in wireless distributed measurement system.(5) The research on MBBS clock synchronization mechanism. Focusing on rapid increasing of clock synchronization error and message amount in wireless distributed networks, Management and Boundary Broadcast Synchronization (MBBS) wireless distributed clock synchronization algorithm is presented. The main idea and realization steps of MBBS are described in detail. Comparing with PBS, MBBS adds a piece of message in each synchronization round, but solves another layer slave clocks'synchronization. It is demonstrated by simulation results that MBBS reduces the messages needed for global synchronization and reaches higher clock synchronization accuracy in multilayer topology. The proposition of MBBS algorithm is of most value in improving clock synchronization accuracy for wireless distributed measurement system.