Research On FPGA-based Rate-adaptive Coding And Modulation Technology

Free space optical communication has outstanding advantages such as large transmission capacity,good security and confidentiality,simple equipment installation,and strong anti-electromagnetic interference.It can realize the"last mile"wireless access in the ground link,and has strong Practicality.However,free-space optical communication is greatly affected by atmospheric channels,making the bit error rate very high when the channel quality is poor.Therefore,this paper proposes a rate-adaptive transmission scheme to reduce the impact of channel quality on communication.At the same time to further improve communication quality,bandwidth utilization and cost reduction,this paper proposes a parallel rate adaptive coding and modulation scheme based on the Field Programmable Gate Array(FPGA)platform.The idea of this scheme is to use the bit error rate as a parameter of channel estimation,by performing error detection at the receiving end and generating a rate switching signal,and then changing the data transmission rate at the transmitting end and adaptively assigning the number of channels to achieve rate-adaptive coding modulation.This article includes the following work:(1)Based on the FPGA platform,the Turbo code encoding and decoding,16QAM modulation and demodulation functions are realized.The design selects the Turbo code as the encoding method,and the high-order 16QAM as the modulation method,which realizes the Turbo code encoding based on the QPP interleaver and the recursive system convolutional code and decoding of the simplified MAX-Log-MAP algorithm,carrier synchronization based on Costas loop and DD algorithm,and bit synchronization based on Gardner algorithm and other key technology demodulation.Among them,one-dimensional Euclidean distance is used in decoding to realize the simplification of MAX-Log-MAP algorithm.The frequency offset test of the Costas loop and bit synchronization loop shows that the loop can still lock and converge quickly when the frequency offset is 2k Hz.(2)The design has realized a single-carrier single-channel low-speed four-channel parallel rate adaptive coding and modulation system.The system has designed a four-channel parallel rate adaptive coding and modulation system.Through real-time error detection at the receiving end,the expected value of the transmitting end transmission rate is generated according to the error detection result and fed back to the transmitting end as the rate switching signal of the adaptive system to make the transmission The transmission rate of the terminal is automatically increased or decreased,and the number of data channels between the transmitting terminal and the receiving terminal is adaptively allocated at the same time,and the coding and modulation system under three transmission rates is adaptively adjusted according to channel quality.Experiments have proved that the system can reduce the transmission rate and reduce the number of parallel channels when the bit error rate at the receiving end is higher than10^-7,and increase the transmission rate and increase the number of parallel channels when the bit error rate is lower than 10^-8.This design can be improved to a certain extent.The influence of atmospheric channel quality on the performance of the communication system enables the system to maintain the stability of the communication link,and also to better perform error control and frequency band utilization.Finally,this parallel low-speed system can adjust the system rate and increase the rate adaptive gear by increasing the number of channels.At the same time,the parallel adaptive coding and modulation mode of single carrier and single channel also provides a possibly adaptive solution for multi-carrier and multi-channel systems.