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-SIM-OFDM, 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 SS-SIM-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.

本文提出了一种新颖的扩频和子载波索引调制 (SS-SIM) 方案，该方案集成到正交频分复用 (OFDM) 框架中，以增强传统 IM 方案的多样性。特别是，得到的方案，称为 SS-SIM-OFDM，联合使用扩频和子载波索引调制来形成预编码向量，然后使用该向量在所有活动子载波上扩展M元复数符号。因此，所提出的方案实现了三个信号域的新颖传输：SS 和子载波索引，以及单个M进制符号。对于实际实现，提出了两个降低复杂度的近似最优检测器，它们的复杂性较少取决于M-ary 调制大小。然后，分析误码概率及其上限以深入了解分集增益，这表明分集增益受到子载波索引顺序的强烈影响。基于这一观察，我们提出了两种新的子载波索引映射方法，它们显着增加了 SS-SIM-OFDM 的分集增益。最后，仿真结果表明，与可以携带多个M进制符号的经典 OFDM 和 OFDM-IM 相比，我们的方案以较低的频谱效率为代价实现了比基准更好的错误性能。