Research On Asynchronous Parallel Transmission Based On Direct Spread

With the development of mobile communication,especially after entering the 5G era,communication system has put forward higher and higher requirements for transmission rate and transmission delay.With the increasing of transmission rate,communication system works in frequency selective fading channel,which makes data recovery more and more difficult.In order to solve this problem,the idea of parallel transmission is proposed.The frequency selective fading problem is simplified to the flat fading problem,which greatly simplifies the complexity of channel processing and improves the transmission quality.In parallel transmission,Orthogonal Frequency Division Multiplexing(OFDM)is the most typical representative.It has the advantages of strong anti-fading ability and relatively simple implementation,so it has been widely used in 4G.However,the delay of this modulation method is relatively large because it requires the parallel transmission branches to keep synchronization.In order to solve the problem of transmission delay,many asynchronous parallel transmission technologies have been deeply studied,such as Filter Bank Multi-Carrier(FBMC),Universal Filtered Multi-Carrier(UFMC)and Generalized Frequency Division Multiplexing(GFDM),etc.But these methods are difficult to be popularized in practical application due to their relatively high complexity.Based on the above reasons,this paper applies the idea of Code Division Multiple Access(CDMA)to parallel transmission,and gives an asynchronous parallel transmission scheme based on direct spread.For OFDM system,the idea of synchronous parallel is to transform the serial parallel into multi-channel,and the multichannel alignment is required when each channel is widened,and then the transformation and transmission can be carried out.Asynchronous transmission system also needs to carry out serial parallel transformation,but each channel is broadened without alignment.After the transmission symbol of each branch arrives,the transformation transmission can be carried out directly.This asynchronous transmission technology has well anti-multipath ability and is easy to implement.However,due to the asynchronous parallel transmission mode,the transmission delay can be greatly reduced.In this paper,the basic principle,system characteristics and implementation method of the system are described in detail.The characteristics of common pseudo-random sequences are analyzed and compared.According to the characteristics of the system,the selection basis and use method of pseudo-random code are given.At the same time,the spectrum characteristics,Peak-to-Average Power Ratio,anti-interference and anti-fading ability,system implementation complexity and other indicators are simulated and analyzed,and compared with OFDM system in detail.The analysis results show that its transmission delay is only half of that of OFDM system,and its implementation complexity is much lower than that of OFDM system.Moreover,it can flexibly trade-off between frequency band utilization and transmission performance,and it is easy to realize transmission of variable rate services.