Information Guided Precoding for OFDM

In conventional orthogonal frequency division multiplexing with index modulation (OFDM-IM), the <inline-formula> <tex-math notation="LaTeX">$M$ </tex-math></inline-formula>-ary modulated symbols are transmitted on a subset of subcarriers under the guidance of information bits. In this paper, a novel information guided precoding technique, called precoding aided (P-) OFDM-IM, is proposed to improve the spectral efficiency (SE) of OFDM-IM. In P-OFDM-IM, the information bits are jointly conveyed through the conventional <inline-formula> <tex-math notation="LaTeX">$M$ </tex-math></inline-formula>-ary modulated symbols and the indices of precoding matrices and vectors. The P-OFDM-IM principle is embodied in two different implementations, P-OFDM-IM-I and P-OFDM-IM-II. Specifically, P-OFDM-IM-I divides all subcarriers into <inline-formula> <tex-math notation="LaTeX">$L$ </tex-math></inline-formula> groups and modulates them using <inline-formula> <tex-math notation="LaTeX">$L$ </tex-math></inline-formula> distinguishable constellations. P-OFDM-IM-II partitions the set of all subcarriers into <inline-formula> <tex-math notation="LaTeX">$L$ </tex-math></inline-formula> overlapping layers and performs IM layer by layer, where distinguishable constellations are employed across layers. A practical precoding strategy is designed for P-OFDM-IM under the phase shift keying/quadrature amplitude modulation constraint. A low-complexity log-likelihood ratio detector is proposed to ease the computational burden on the receiver. To evaluate the performance of P-OFDM-IM analytically, an upper bound on the bit error rate and the achievable rate is studied. Computer simulation results show that P-OFDM-IM-I outperforms the existing OFDM-IM-related schemes at high SE, while P-OFDM-IM-II performs the best at low SE.