**1. Introduction**

Starting from Marconi's first transatlantic wireless transmission in 1899, wireless communication has been a crucial technology for developing today's modern lifestyle. There is a wide range of potential applications in wireless communication and sensing areas, such as cellular devices [1], wireless local area networks (WLANs) [2], vehicular communications [3], security scanner, biological diagnosis, non-destructive detection, and radar imaging [4]. From a technological perspective, a converged system is expected to provide enormous benefits in terms of both spectrum efficiency and cost-effectiveness [5–7]. In the past, different waveforms have been used independently for implementing most wireless communication, and sensing functionalities [8,9]; consequently, the systems are bulky, energy consumable, and uneconomical. In this context, a unified waveform simultaneously serving communication and sensing has gained substantial interest [10]. So far, the orthogonal frequency division multiplexing (OFDM) technique is well known for its benefits for wireless communications, and has not only been adopted in numerous standards but is also considered as a strong candidate for future wireless communication systems (5G and beyond) [11,12]. More interestingly, the OFDM waveform has also been well documented for its effectiveness in radar applications [13–15]. Therefore, OFDM waveforms are promising for the convergence of communication and sensing [16–20].

The OFDM wireless communication technically requires inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT) operations to transmit and receive data. The cyclic prefix interval (CPI), also known as a guard interval, makes OFDM transmission robust against multi-path radio channel. However, under the channel impulse response longer

**Citation:** Idrees, N.M.; Lu, Z.; Saqlain, M.; Zhang, H.; Wang, S.; Zhang, L.; Yu, X. A W-Band Communication and Sensing Convergence System Enabled by Single OFDM Waveform. *Micromachines* **2022**, *13*, 312. https:// doi.org/10.3390/mi13020312

Academic Editor: Jeonghyun Kim

Received: 23 January 2022 Accepted: 15 February 2022 Published: 17 February 2022

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than the CPI, inter-symbol interference (ISI) degrades communication performance, and in mobility scenarios, inter-carrier interference (ICI) causes orthogonality loss among the subcarriers and ISI as a consequence. There are some approaches to equalize this issue in communication, for instance, basis-expansion-model-based channel transformation [21], iterative finite length-equalization technique [22], and adjusting the CPI length according to the channel length [23].
