Research On The Key Technology Of FSO System

The wireless optical communication (WOC), also called as the free space optical (FSO) communication, is a bi-direction communication system which uses the laser as the carrier wave, and transmits laser from source to over distance through atmosphere without fiber cable. It has wideband, good security, immunity to electromagnetic interference, no spectrum licensing, small size and lower power consumption, etc .It widely applies to broadband access, wide-area networks (WANs) and metropolitan area networks (MANs) extension, enterprise local area networks (LANs) connection and deep space communication, etc. It is one of the hot points in the field of researching the optical communication technology today.The basic concept, advantages and defects, the applied fields of FSO, and the developing in the world are discussed in this paper. The traditional Rytov method and the Markov approximation method are summarized in the weakly turbulent and the strong one. Based on the fundamental work conducted, the key technology of the FSO system is researched; the major innovation points are summarized below.(1) The mathematic model of the MIMO-FSO is established. This paper researched how to apply the MIMO (Multiple-Input Multiple-Out-put) technology of the mobile communications to the FSO communications. The physics meaning of this model is very clear, and its format is very simple. Mathematic results show that MIMO is a very effective way for intensity fluctuation reduction induced by turbulence, thus reduce the bit-error-rate of the system.(2) The modified space-time coding is put forward to apply to FSO communications. The techniques of space-time coding and diversity under the atmospheric turbulence channel are studied. The feasibility of applying space-time coding to atmospheric laser communication with intensity modulation and direct detection (IM/DD) is analyzed. A new diversity coding scheme is put forward. Numerical simulations and bit error probability analysis show that the new scheme can improve the performance of the FSO system obviously. When the channel BER equal to 10-3, the coding gain is about 4dB.(3) A new modulation method DH-PIM (dual-header pulse interval modulation) is put forward. There are many modulation methods in the atmospheric laser communication system with intensity modulation and direct detection (IM/DD), such as OOK(On-off Keying), PPM (Pulse Position Modulation), DPPM ( Differential Pulse Position Modulation), PIM (pulse interval modulation) and DH-PIM, etc.. After analyzing these modulation methods, a new modulation method DH-PIM is put forward. The symbol structure, the average transmission power, the power spectrum and bandwidth efficiency are analyzed in detail. The simulations results show that DH-PIM with higher information transmission rate and the better power performance, it is very fit for the real-time communications.(4) A combined axis ATP control system is put forward. The ATP (Acquisition, Tracking, and Pointing) subsystem is a very important part of the FSO system, the ATP technology of FSO communication system is analyzed in this paper. The paper discusses the influence of the building movement or vibration, then its mathematic model is put forward and the simulation results are given out. The basic concept and the main structure, integral parts and detailed work process of the ATP subsystem are introduced in detail. In FSO communication system, in order to realize exact acquisition and high accuracy tracking, combined axis control technique is applied in ATP system. The main parameters of the ATP subsystem are also introduced in this paper. Simulation results show that the tracking accuracy of the main axis control system is much better and this fulfills the demands of ATP subsystem.The project of this paper is the Innovation Fund for Small Technology-based Firms (Innofund), the aim of this project is to study and develop a new wireless laser communication system with our own intellectual property. On the base of study the key technology of the FSO system, such as the channel performance, MIMO, the space-time coding, the modulation methods, and the ATP technology, we designed the sample device, and given the experiment results and the main parameters of the sample device. The achievements of study have some guiding significance to development research new products.