Information Guided Precoding for OFDM

Abstract

In conventional orthogonal frequency division multiplexing with index modulation (OFDM-IM), the M-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 M-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 L groups and modulates them using L distinguishable constellations. P-OFDM-IM-II partitions the set of all subcarriers into L 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.

Keywords

• OFDM
• index modulation
• precoding
• log-likelihood ratio (LLR)
• maximum-likelihood (ML)