Support of many different services, approximately 1000x increase of current data rates, ultra-low latency and energy/cost efficiency are among the expectations from the upcoming 5G standards. In order to meet these expectations, researchers investigate various potential technologies involving different network layers and discuss their tradeoffs for possible 5G scenarios. As one of the most critical components of communication systems, waveform design plays a vital role here to achieve the aforementioned goals. Basic features of the 5G waveform can be given in a nutshell as more flexibility, support of multiple access, the ability to co-exist with different waveforms, low latency and compatibility with promising future technologies such as massive MIMO and mmWave communications. Orthogonal frequency division multiplexing (OFDM) has been the dominant technology in many existing standards and is still considered as one of the favorites for broadband communications in 5G radio access network (RAN). Considering the current interest of industry and academia on enhancing OFDM, this paper drafts the merits and shortcomings of OFDM for 5G RAN scenarios and discusses the various approaches for its improvement. What is addressed in this paper includes not only enhancing the waveform characteristics, out of band leakage and peak to average power ratio in particular, but also methods to reduce the time and frequency redundancies of OFDM such as cyclic prefix and pilot signals. We present how the requirements of different 5G RAN scenarios reflect on waveform parameters, and explore the motivations behind designing frames that include multiple waveforms with different parameters, referred to as numerologies by the 3GPP community, as well as the problems that arise with such coexistence. In addition, recently proposed OFDM-based signaling schemes will also be discussed along with a brief comparison.
