UWA channels exhibit time-varying multipath characteristics. To this end, OFDM is well known for its robustness against multipath channels but is prone to ICI induced by time variation. More recently, inspired by spatial modulation, the so-called IM-OFDM has also been proposed to provide higher system throughput than plain OFDM under certain conditions. A key feature of IM-OFDM is that partial subcarriers are kept inactive. This could potentially improve system performance in the presence of ICI. Leveraging on this, we are the first to propose IM-OFDM for UWA communications. On the other hand, however, we realize that ICI could potentially lead to energy leakage from active subcarriers to inactive ones, and impair the demodulation of IM-OFDM. In this article, we introduce IM-OFDM for UWA communications and propose a hybrid IM-OFDM scheme with improved spectral efficiency. We then review existing ICI self-cancellation techniques for generic OFDM, and propose a new ICI cancellation method for IM-OFDM.
Bibliographical noteFunding Information:
This work was supported in part by the National Natural Science Foundation of China under Grants 61431005, 61501190, and 61571020, the National 973 Project under Grant 2013CB336700, the Major Project from Beijing Municipal Science and Technology Commission under Grant D151100000115004, the National Science Foundation under grant CNS-1343189, and the Natural Science Foundation of Guangdong Province under Grant 2014A030310389.
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