Chapters 2 and 3 discuss how index modulation operates in the space and space-time domains, respectively. In this chapter, we focus on the implementation of index modulation in the frequency domain. First, the orthogonal frequency division multiplexing with index modulation (OFDM-IM), which is a representative of this sort, is introduced. Then, design guidelines for improving the diversity gain, coding gain, and spectral efficiency of OFDM-IM systems are provided. Finally, we present the idea of integrating intercarrier interference (ICI) self-cancellation techniques into the OFDM-IM framework to combat the severe ICI in the Vehicle to X (V2X) communications.