Research On Joint Optimization Of Finite Length Coded Modulation For Ultra Reliable Low Latency Communications

The fifth generation(5G)of mobile communications propose Ultra-Reliable Low Latency Communication(URLLC)to meet the needs of ultra-high reliability and low latency wireless communications,such as autonomous driving,telemedicine and smart grid,etc.The general requirement of URLLC is that the Block Error Rate(BLER)of a 32-byte encoded packet is lower than10^-5,and the communication delay is less than 1 millisecond.Therefore,the physical layer encoding and modulation technology for URLLC is a key problem in 5G technology,which has attracted extensive researches and attentions.On one hand,the physical layer needs finite block length channel coding to transmit data,but the reduction of the coding block length will cause serious loss of channel coding gain.Therefore,in order to ensure the ultra-high reliability of communication,it is necessary to use channel coding technology with strong error correction capabilities.However,the current standard encoding methods are all fixed code rate,which have low flexibility.It is necessary to accurately estimate the channel state information and select the optimal code rate and modulation scheme,and combine with the error correction mechanism with retransmission for highly reliable transmission.This greatly increases the communication delay,and leads to exceed the requirement of low latency.On the other hand,the current physical layer modulation methods are optimized for moderate and long block length codes,and have not been combined with short block length coding.This leads to a decreasing coding gain under short block length conditions and limits the system performance.Considering the above two issues,this article introduces a rateless code with“online”encoding and decoding characteristics and without feedback link,i.e.,analog fountain code.Combined with the Amplitude Phase Shift Keying(APSK)modulation technology,a highly reliable and low-delay coding modulation scheme under short block length conditions is proposed,and the theoretical performance under short block length conditions is analyzed,and the joint optimized coded modulation is further studied.The specific research content is as follows:Based on the theoretical analysis of the error performance under finite length code,the minimum average Euclidean distance formula with only one bit error is derived,and the minimum average Euclidean distance formula with two or more bits error is proposed.Considering that the minimum average Euclidean distance increases with the number of errors increases,and the corresponding probability decreases,an APSK mapping scheme based on constellation location optimization is proposed.By utlizing the tabu search algorithm,the complexity of the mapping scheme is reduced,and obtain more constellation point mapping schemes.The simulation results show that the above-mentioned APSK optimized placement scheme improves the BLER performance up to 3 d B compared with the traditional APSK modulation scheme under low signal-to-noise ratio.We maximize the mutual information of the above optimized APSK constellation placement scheme to further improve the BLER performance.The BLER performance of this optimized scheme is improved than the aforementioned APSK mapping.Finally,in order to further improve the mutual information,this paper optimizes the radius and angle of the constellation diagram to maximize the mutual information.In order to reduce the computational complexity,a genetic algorithm-based APSK mapping scheme to maximize mutual information is proposed.The simulation results show that the BLER performance of the optimized scheme is 4 d B higher than that of the traditional APSK modulation schemes.