Low-Complexity LLR Calculation for OFDM With Index Modulation

Orthogonal frequency division multiplexing with index modulation (OFDM-IM) is a promising multicarrier transmission technique. In OFDM-IM, only a subset of subcarriers are activated to transmit modulated symbols and the indices of the activated subcarriers are used to convey information bits implicitly. In the coded OFDM-IM system, we need to calculate the log likelihood ratio (LLR) value of each bit, whose computational complexity is extremely high due to the dependence between the subcarriers caused by the subcarrier-index modulation. In this letter, we propose a novel simplified LLR calculation algorithm for OFDM-IM. Simulation results show that the proposed algorithm achieves near-optimal coded bit error rate performance with considerably low computational complexity.

I. Introduction

Orthogonal frequency division multiplexing with index modulation (OFDM-IM) is a recently proposed multicarrier transmission technique for the next generation wireless communication system [1]–[7]. By introducing the principle of spatial modulation (SM) into OFDM to control the status of the subcarriers, additional information bits can be implicitly transmitted by the active indices combinations (AICs) of the subcarriers [4], [8]. The uncoded bit error rate (BER) performance of OFDM-IM based on the maximum likelihood (ML) detector and the log-likelihood ratio (LLR) based low-complexity detector is studied in [4]. The achievable rates of OFDM-IM systems under Gaussian and finite-alphabet input, respectively, are investigated in [9] and [10].