FPGA Implementation Of Turbo Codec In Direct Spread Communication System

Spread spectrum communication technology is the first to transmit the information multiplied by the spread spectrum function(spread spectrum code),multiple expansion of the transmission of information signal spectrum,and then in the receiving end of the corresponding de-expansion operation,and finally restore the original information transmitted.Due to its advantages of strong anti-interference ability,good addressing ability,high security performance and low frequency spectrum density,it first appeared in the war as a military purpose,and was later widely used in the field of civil mobile communications.The main spread spectrum technologies are: direct sequence spread spectrum communication(DSSS),frequency hopping(FH),time hopping(TH),hybrid spread spectrum communication.Direct sequence spread spectrum communication(DSSS)is the most common spread spectrum method with the best technology accumulation.Channel coding is one of the key technologies in current digital communication system.It is a technique to solve the error-free transmission of signals in the channel and to ensure the correctness of receiving information at the receiving end.In the field of channel coding,researchers constantly propose excellent coding and decoding schemes.From the initial BCH code,RS code and convolution code to the mesh-coded modulation(TCM)which combines modulation and channel coding,although it has excellent performance in various communication standards,it is still far from shannon limit.Until the introduction of Turbo code,channel coding theory has been on the road of approaching shannon limit,and countless researchers have been working towards the goal.Turbo is regarded as the greatest technical achievement in the research of information and coding theory since the advent of grid coding modulation technology,and plays an important role in the field of digital communication.The communication system model designed in this paper is a direct sequence spread spectrum communication system combined with Turbo coding and decoding,and implemented based on FPGA hardware.Firstly,the 63-bit Gold sequence was used as the pseudo-code sequence to conduct the direct sequence spread spectrum of 63-bit spread spectrum,and the pseudo-code synchronization was carried out by sliding correlation acquisition algorithm in the process of demodulation at the receiving end.Then,a Turbo code encoder using RSC component code encoding algorithm is designed,and the truncated multiplexer in the encoder is improved to obtain better decoding performance,in which the coding structure is PCCC code structure,the frame length is 1248 code block length,and finally the Turbo code output 1/2 bit rate is encoded.Next,a Turbo code decoder using max-log-map algorithm for iterative decoding is designed,and its decoding structure is also PCCC.The design process of each part of the communication system is to conduct performance simulation of Matlab algorithm first,then map to FPGA through Verilog HDL,and finally complete functional simulation,data capture of the upper board and result verification.This design has completed the FPGA design of the entire communication system model,defined the module interface,built the communication circuit model,and completed the result verification.