Asynchronous Space-Time Coding Scheme For Distributed Satellite Cooperative Systems

With the capability of providing large-scale coverage and reliable communications,distributed satellite cooperative systems show great advantages in multiple communication scenarios such as remote area communications and global broadcasting.The application of space-time coding technology in the distributed satellite cooperative systems can provide spatial cooperative diversity and effectively resist fading.Since different satellites have different physical positions in a large scale,the signal transmission has large relative delay,which constitutes a asynchronous transmission.The asynchronous transmission will destroy the structure of space-time codes,leading to severe inter-symbol-interference.To solve this problem,this paper investigate the asynchronous space-time coding scheme in the distributed satellite cooperative systems.The major contributions are summarized as follows:In the two-satellite cooperative systems,one of the existing asynchronous space-time coding schemes performs time-reversal on the transmission sequence and uses zero suffix as the guard interval.However,the zero suffix requires frequent switching process of the transmitting antenna resulting in power fluctuation problem.To address this issue,this paper proposes a time-reversal space-time coding scheme based on employing the cyclic prefix rather than the zero suffix to remove the zero element of the transmission matrix.In addition,considering the large degree of relative delay difference and the requirement of long guard interval in the two-satellite cooperative system,this paper proposes a time-reversal space-time coding scheme based on guard interval superposition.Aiming to improve the symbol rate,the proposed scheme superimposes two consecutively data block suffixes and take the superposition block as an inter-block guard interval,where the length of the guard interval is half of the original length.The simulation results reveal that the time-reversal space-time scheme based on cyclic prefix has delay tolerance and can avoid the power fluctuation of the system.The results also indicate that the time-reversal space-time coding scheme based on guard interval superposition guarantees the diversity gain nearly without reducing the error performance,and obtains the effective symbol rate gain of nearly 20%.In the multi-satellite cooperative systems,the existing asynchronous quasi-orthogonal spacetime coding scheme based on orthogonal frequency division multiplexing utilizes the cyclic prefix and the periodic cycle characteristics of discrete Fourier transform and inverse transform to combat the effect of time asynchronism from different satellites.Due to the quasiorthogonal characteristic of the coding matrix in the scheme,the receiver needs to perform pairwise joint maximum likelihood decoding while the complexity of decoding is high.Motivated by this,in this paper,we propose an asynchronous quasi-orthogonal space-time coding scheme based on feedback factor.By introducing feedback factors related to delay and channel coefficients into some satellites,the equivalent coding matrix becomes orthogonal.Then the receiver can perform linear maximum likelihood decoding which can greatly reduce the decoding complexity.The simulation results show that the asynchronous quasiorthogonal space-time coding scheme based on feedback factors obtains the delay tolerant capability.