Asynchronous Space-time Cooperative Coding Rearch Based On LEO Satellite System

With the development of technology and the rise in communications needs, people’s lives arebeing extended from ground to space. Every country is creating an integrated space-groundinformation network, providing a full range of communication services stereo. And satellitescommunication occupies a pivotal position in the construction of the space-ground network,providing remote communication and seamless access to it. This paper studies encoding anddecoding techniques of satellite mobile communication on the basis of applications and transmitcharacteristics in inter-grated space-ground network system. Terrestrial transmitters send signalswith different coding and modulation to different satellites relays and satellites re-transmit signalsto terrestrial receivers. This paper is to study a new distributed space time cooperative coding todiminish the affect of large different delays on the conventional synchronous space time coding anddecoding technology in satellite communications, obtain higher coding gain and lower bit error ratio,increase the reliability of the system and improve transmission efficiency under condition of timingerrors. In the context of applications, this paper studies theoretical research and simulation of spacetime coding in low orbit satellite communication systems to make a foundation of the practicalapplication of distributed space time coding in satellite environment and inter-grated space-groundnetwork system.Cooperative communication provides a virtual multi-antenna relay with single-antenna relays,relays mutually cooperate with each other to achieve cooperative diversity and spatial diversity withdistributed space time coding improving transmission efficiency and reliability. In satellitecommunications, the cooperative relays transmit signals in different channels and signals mayarrive at the receiver with timing errors and synchronization errors. Delays may change dynamically,so bit error rate is high in asynchronous large scaled distributed systems.To deal with the problems above, this paper firstly supposing channel characteristics anddifferent delays can be accurately estimated by receivers. To merge into high rate of transmittingcoding, linear combination constructed by several consecutive symbols is radio or directlytransmitted to relays with orthogonal frequency division multiplexing modulated formation. Relaysprocess signals into conjugate or time-reversed formation before amplification forward in timedomain and receivers decode signals with maximum likelihood decoding technique. Simulationresults show that the linear combination has a better BER performance and the coding rate isimproved. Then, supposing dynamic channel characteristics and different delays cannot be accurately estimated by receivers, differential space time decoding is considered. As code divisionmultiple access system distinguish between different users with orthogonal spreading codes to sharewith limited spectrum and time resources, signals from different satellite relays can be consideredas different users similarly. Signals from different relays can be marked with different spreadingcodes. With codebooks of spreading codes, receivers detect correlation to extract useful signals fordecoding easily. From the simulation results, it can be seen that combination of spreading codes anddifferential space time coding can be a good fight to the influence of random delay difference andreceivers do not need estimation of channel state information and reduce the complexity.