Generalized OFDM Index Modulation Performance Analysis and Error Propagation Mitigation Using LDPC

Index modulation (IM) techniques have the potential to achieve better error performance, spectral efficiency (SE), and/or power efficiency (PE) enhancements for next-generation communications. In this paper, we study the generalized orthogonal frequency division multiplexing (OFDM) index modulation (G-OFDM-IM) system whose subcarrier activation pattern is completely controlled by the information bits without the need for a predefined number of active subcarriers, the so-called free-form OFDM-IM. Although G-OFDM-IM promises better spectral efficiency, the variable length of data transmitted brings in error propagation as a main challenge in this system, which causes considerable performance degradation, especially in low SNR. In this paper, the SE, PE, and BER of this system are analyzed. Our analysis is supported by numerical simulation for the system performance compared to the conventional OFDM. With the effect of error propagation, it's found that the spectral efficiency of G-OFDM-IM decreases compared to OFDM for higher modulation orders under the same power constraint. Checking out how far the G-OFDM-IM system performance can go, correct indices detection is assumed to get the lower limit BER. The lower limit BER shows a great enhancement in BER by more than 3dB in the case of QPSK modulation and full power reallocation and, hence, the SE and/or PE can be enhanced. Taking one step forward, the indices bits are coded using LDPC to mitigate the effect of error propagation caused by incorrect indices detection. Depending on the code rate, the BER will eventually meet the BER lower limit at the expense of less SE.