 of 1/\F. The FTN transmitter structure is depicted in Fig. 3.26. Since the time and frequency spacing varies for different FTN sys- items, direct implementation cannot provide sufficient implementation flexibility. An FTN mapper based on projection scheme has been designed and used in FTN signaling systems to provide flexibility. The FTN mapper is shown in Fig. 3.26. A cyclic extension is needed in the modulation block, with which the system can switch easily between FTN and Nyquist modes. As mentioned before, FTN signaling inevitably introduces ISI in the time domain and/or ICI in the frequency domain when its baud rate is over the Nyquist one (see Fig. 3.27). Therefore a very important issue is how to design a receiver with ISI and/or ICI suppression capability to recover the original transmitted data. 3.2.3.6 Comparison of the Candidate Waveforms A number of candidate waveforms were studied by 3GPP for NR, before CP-OFDM was selected as the default waveform for the downlink and uplink, which were based on FFT/IFFT processing, additional filtering, windowing, or precoding, in order to achieve higher time-frequency localization and lower OOB spectral leakage, and higher throughput. Filtering is a straightforward way to suppress OOB emissions by applying a digital filter with prespecified frequency response. Some waveforms like F-OFDM and UFDM belong to this category. However, the delay spread of the equivalent composite channel may exceed the CP size and guard period in TDD systems, which results in ISI and imposes restrictions on downlink-to-uplink switching time. Furthermore, the promised OOB emission performance may diminish significantly when PA nonlinearity and other non-ideal RF processing effects are taken into account. At the cost of increased PAPR, filtering techniques are generally known to be unfriendly to communication at high carrier frequencies. Windowing is used to prevent steep changes across two consecutive OFDM symbols in order to reduce the OOB emissions. Multiplying the time-domain samples l