The standardization activities of wireless mobile telecommunications have begun with analog standards that were introduced in the 1980s, and a new generation develops almost every 10 years to meet the exponentially growing market demand. The leap from analog to digital started in second-generation (2G) systems, along with the use of mobile data services. The 3G digital evolution enabled video calls and global positioning system (GPS) services on mobile devices. The 4G systems pushed the limits of data services further by better exploiting the time-frequency resources using orthogonal frequency-division multiple access (OFDMA) as an air interface [1]. Recently, the International Telecommunications Union (ITU) has defined the expectations for 5G [2], and the study of the nextgeneration wireless system is in progress with a harmony between academia, industry, and standardization entities to accomplish its first deployment in 2020.5G is envisioned to improve major key performance indicators (KPIs), such as peak data rate, spectral efficiency, power consumption, complexity, connection density, latency, and mobility, significantly. Furthermore, the new standard should support a diverse range of services all under the same network [3]. The IMT-2020 vision defines the use cases into three main categories as enhanced mobile broadband (eMBB), massive machine-type communications (mMTC), and ultra-reliable low-latency communications (URLLC) featuring 20 Gb/s peak data rate, 10 6 /km 2 device density, and less than 1 ms latency, respectively [4]. A flexible air interface is required to meet these different requirements. As a result, the waveform, which is the main component of any air interface, has to be designed precisely to facilitate such flexibility [5].This chapter aims to provide a complete picture of the ongoing 5G waveform discussions and overviews the major candidates. The chapter is organized as follows: Section II provides a brief description of the waveform and reveals the 5G use cases and waveform design requirements. Also, this section presents the main features of CP-OFDM that is currently deployed in 4G LTE systems. CP-OFDM is the baseline of the 5G waveform discussions since the performance of a new waveform is usually compared with it. Section III examines the essential characteristics of the major waveform candidates along with the related advantages and disadvantages. Section IV summarizes and compares the key features of the waveforms. Finally, Section V concludes the chapter. This is the pre-peer reviewed version of the article which has been published in final form at [https://onlinelibrary.wiley.com/doi/abs/10.1002/9781119333142.ch2]. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.
