On The Adaptive LDPC Coding Scheme

LDPC code is one kind of good code whose performance is very close to Shannon’s limit,and also has parallel decoding ability and high throughput advantage.Besides Turbo code,LDPC code becomes another research hotspot in channel coding field.In the transmission process of communication system,due to the uncertainty of the transmitted data,that the communication system completes adaptive LDPC coding by dynamic adjusting its code length and code rate based on different channel conditions can significantly improve the transmission efficiency and reliability of the communication system.There is a simple method to get adaptive LDPC coding,one can use multiple encoders and decoders,but this method is unacceptable for very high implementation complexity.While the application of shortening,puncturing and extending technologies is a reasonable method to achieve adaptive LDPC coding.In the IEEE 802.11ac standard,puncturing technique and shortening technique have been utilized to derive the adaptive LDPC codes.However,the current design is simple and did not take into account the characteristics of the utilized parity check matrix,so the performance loss is noticeable in some settings.In the next generation evolution standard of IEEE 802.11ad,Channel Bonding(CB)technology will be introduced to increase the bandwidth.Thus longer LDPC code becomes a natural choice to improve the performance.But the existing extending method can not satisfy the constructions of arbitrary code lengths.In this paper,we first study the existing adaptive LDPC coding methods of WLAN systems(IEEE 802.11ac,IEEE 802.11 ad).Then we propose the concept of noise-free decoding and reliabilty,and propose an adaptive coding scheme based on the reliability criterion.The proposed adaptive coding scheme can be applied to shortening,puncturing and extending methods.Compared to the existing shortening and puncturing schemes,such as the puncturing scheme based on the recovery tree,the puncturing scheme based on the NG ordering criterion,the shortening scheme based on the external information,the shortening scheme based on the minimum row variance priority,the shortening and puncturing schemes based on destructive force and the puncturing and shortening methods of IEEE802.11ac standard,both the shortening scheme and puncturing scheme based on the reliabilty criterion have the advantages of simple realization and strong flexibility.In this paper,we use the LDPC code of IEEE 802.11ac standard as an example to analyze and compare all the mentioned shortening schemes and puncturing schemes.The simulation results show that both the proposed shortening scheme and puncturing scheme based on the reliabilty criterion are better than the existing schemes,and can flexibly satisfy to the requirements of any adaptive LDPC coding.In addition,the paper achieves another adaptive LDPC coding method which combines extending and shortening based on the reliabilty criterion.Simulation results show that the LDPC codes based on the extending and shortening can make full use of the advantages of long code when compared with the existing direct extending schemes.Thus,the LDPC coding scheme based on the extending and shortening is better than the existing direct extending schemes,and can satisfy the requirements of constructions of arbitrary code length.The research of the paper has some reference value for further research and improvement of adaptive LDPC coding.