Enhanced Orthogonal Frequency Division Multiplexing with Index Modulation

Abstract

Index modulation concept has attracted considerable research interests in the past few years. As a realization of index modulation in the frequency domain, orthogonal frequency division multiplexing with index modulation (OFDM-IM) is recently proposed, which conveys information bits through both the subcarrier activation patterns and the amplitude phase modulation constellation points. This paper proposes two enhanced OFDM-IM schemes aiming at achieving higher spectral efficiency and diversity gain, respectively. The first one, termed OFDM with hybrid in-phase/quadrature index modulation (OFDMHIQ-IM), explores the I- and Q- dimensions jointly for index modulation, allowing transmission of more number of index modulation bits in each subcarrier group. The second one, termed linear constellation precoded OFDM-IQ-IM (LP-OFDMIQ- IM), spreads information symbols across two adjacent active subcarriers through linear constellation precoding to harvest additional diversity gain. By maximizing the minimum squared Euclidean distance, two different realizations of LP-OFDMIQ-IM are derived, which lead to a rotated and a diamond shaped constellation, respectively. The proposed OFDM-HIQ-IM and LP-OFDM-IQ-IM, as revealed by both theoretical analyses and computer simulations, enable low-complexity detection and exhibit superior error rate performance over the existing OFDMIM schemes.

Existing System

Spatial Modulation, Space-Time Block Coded Spatial Modulation.

Proposed System

In this paper, we have proposed OFDM-HIQ-IM and LPOFDM-IQ-IM to improve the SE and the asymptotic BER performance of OFDM-IM systems, respectively. The two proposed schemes can be further combined to harvest both of their advantages. OFDM-HIQ-IM explores the I- and Q- active components jointly for IM, doubling the IM bits of OFDMIM and transmitting one more IM bit than OFDM-IQ-IM in most cases. An asymptotically tight upper bound has been derived and two low-complexity near-optimal detectors have been designed for OFDM-HIQ-IM. LP-OFDM-IQ-IM combines a pair of I/Q- symbols linearly, achieving an additional diversity gain over OFDM-IM, OFDM-IQ-IM, and OFDMHIQ-IM. Two different optimal linear combination methods in the sense of maximizing the minimum CGD have been found, which result in the rotation and compression of theM2-QAM constellation, respectively. The precoding coefficients for both methods have been derived analytically and the low complexity LLR detector has been designed for LP-OFDMIQ-IM. Monte Carlo simulations have been conducted and results have verified the analysis and the advantages of OFDMHIQ-IM and LP-OFDM-IQ-IM over classical OFDM as well as existing OFDM-IM schemes.

Architecture

Block diagram of the OFDM-HIQ-IM transmitter