gardless of the modulation scheme applied to different subcarriers [30,46]. 3.2.3 Other Waveform Candidates In the study phase of 3GPP NR, a number of waveforms promising to improve upon OFDM waveform and to overcome the limitation of the latter were proposed and evaluated. However, when the practical aspects of implementation complexity and analog RF proces- sing were considered, many of those candidate waveforms fail to provide any significant improvement over the status-quo, and thus 3GPP agreed to specify the OFDM as the base- line waveform for the new radio. In the following sections, we briefly describe those candi- date waveforms and their respective advantages and disadvantages over OFDM. 336 Chapter 3 3.2.3.1 Filtered-OFDM To mitigate the limitations of OFDM waveform, filtered-OFDM (F-OFDM) waveform was proposed wherein subband-based splitting and filtering were used to allow independent OFDM systems operate in the assigned bandwidth. In this way, F-OFDM can overcome the drawbacks of OFDM while retaining the advantages of it. With subband-based filtering, the requirement on system-wide synchronization is relaxed and inter-subband asynchronous transmission can be supported. Furthermore, with suitably designed filters to suppress the OOB emissions, the guard band size can be reduced to a minimum. Within each subband, optimized numerology can be applied to suit the needs of certain type of services. Fig. 3.18 shows the block diagram of a frequency-localized OFDM-based waveform. As shown in the figure, the baseband OFDM signal of each subband with its specific numerol- ogy is independently generated by processing through a spectrum shaping filter. The main purpose of this filtering is to avoid interference to the neighboring subbands. There are vari- ous approaches to the design of the spectrum shaping filter. In subcarrier filtering, the sinc(.) pulse shape of each individual subcarrier within the subband is filtered to make it more localized in frequency. An example of this method is the windowed OFDM wher