Enhancing Diversity of OFDM with Joint Spread Spectrum and Subcarrier Index Modulations

TL;DR

This paper introduces SS-SIMOFDM, a novel OFDM scheme combining spread spectrum and sub-carrier index modulation to enhance diversity, with reduced-complexity detection and improved error performance over benchmarks.

Contribution

The paper proposes a new SS-SIMOFDM scheme with joint spread spectrum and index modulation, along with low-complexity detectors and novel index mapping methods to improve diversity gain.

Findings

Achieves better error performance than benchmarks.

Provides reduced-complexity near-optimal detectors.

Enhances diversity gain through novel index mapping methods.

Abstract

This paper proposes a novel spread spectrum and sub-carrier index modulation (SS-SIM) scheme, which is integrated to orthogonal frequency division multiplexing (OFDM) framework to enhance the diversity over the conventional IM schemes. Particularly, the resulting scheme, called SS-SIMOFDM, jointly employs both spread spectrum and sub-carrier index modulations to form a precoding vector which is then used to spread an M-ary complex symbol across all active sub-carriers. As a result, the proposed scheme enables a novel transmission of three signal domains: SS and sub-carrier indices, and a single M-ary symbol. For practical implementations, two reduced-complexity near-optimal detectors are proposed, which have complexities less depending on the M-ary modulation size. Then, the bit error probability and its upper bound are analyzed to gain an insight into the diversity gain, which is shown to be strongly affected by the order of sub-carrier indices. Based on this observation, we propose two novel sub-carrier index mapping methods, which significantly increase the diversity gain of SSSIM-OFDM. Finally, simulation results show that our scheme achieves better error performance than the benchmarks at the cost of lower spectral efficiency compared to classical OFDM and OFDM-IM, which can carry multiple M-ary symbols.