Protection scheme of subcarrier index in OFDM with index modulation aided by LDPC coding

基于LDPC编码辅助的OFDM-IM子载波索引保护方案

¹ School of Marine Science and Technology, Northwestern Polytechnical University, Xi’an 710072, China
² School of Electronics and Information, Northwestern Polytechnical University, Xi'an 710129, China

Received: 17 December 2020

Abstract

The receiver of OFDM with Index Modulation (OFDM-IM) usually adopts a Log Likelihood Ratio (LLR) detection algorithm based on the activation state of subcarriers. However, the LLR detection algorithm will cause detection errors in subcarrier activation pattern (SAP) or get illegal SAP. Consequently, further errors occur in demodulation, increasing the bit error rate (BER). To solve this problem, we propose the protection scheme of subcarrier index aided by LDPC coding, which reduces the SAP detection errors by encoding the index information bits. On the receiver, the LDPC Coding Aided (LA) detection algorithm is designed, and the formula of LLR of index information bits is derived in detail. Monte Carlo simulation is carried out over multi-path fading channel by MATLAB software. The results show that under the condition that the spectrum efficiency is not lower than the classical OFDM-IM scheme, the proposed protection scheme can obtain a gain of about 5~9 dB when the BER is 10^-4, effectively improving the BER performance of OFDM-IM scheme.

摘要

索引调制OFDM(OFDM with index modulation，OFDM-IM)的接收端常采用基于子载波激活状态的对数似然比(log likelihood ratio，LLR)检测算法。然而，LLR检测算法会出现子载波激活模式(subcarrier activation pattern，SAP)判决错误或判决出非法的SAP，不仅引起对索引信息的估计错误，还严重影响符号解调的正确率，导致整体的误码率增大。针对上述问题，提出了一种基于低密度奇偶校验(low density parity check，LDPC)编码辅助的子载波索引保护方案，通过对索引信息比特编码降低子载波激活模式的判决错误率。在接收端设计了LDPC编码辅助(LDPC coding aided，LA)检测算法，并详细推导了索引信息比特LLR的闭合表达式。利用MATLAB软件在多径衰落信道下进行蒙特卡洛仿真，结果表明在频谱效率不低于传统OFDM-IM方案的条件下，所提方案在误码率为10^-4时可取得约5~9 dB的增益，能够有效提高OFDM-IM传输方案的误码率性能。