l is copied to the beginning of the symbol. The OFDM symbol duration then is defined as Ts = Tu + T, where Tg is the guard interval or CP. When the guard interval is longer than the channel impulse response or the multipath delay, the ISI can be effectively eliminated. The ratio of the guard interval to useful OFDM symbol duration depends on the deployment scenario and the frequency band. Since the insertion of the guard intervals will reduce the system throughput, Tg is usually selected less than Tu/4. The CP should absorb most of the signal energy dispersed by the multipath channel. The entire the ISI energy is contained within the CP, if its length is greater than that of the channel RMS delay spread (Tg >TRMS). In general it is sufficient to have most of the delay spread energy absorbed by the guard interval, considering the inherent robustness of large OFDM symbols to time dispersion. Fig. 3.11 illustrate the OFDM modulation and demodulation process in the transmitter and the receiver, respectively. In practice, a win- dowing or filtering scheme is utilized in the OFDM transmitter side to reduce the OOB emissions of the OFDM signal (see Fig. 3.10). OFDM symbol 1 Corrupted due to ISI Received waveform Delayed OFDM symbol 1 IFFT output OFDM symbol 2 Weighting Weighting function Weighted received waveform function Interference region Overlapped Overlapped extension extension Overlap and add OFDM symbol 1 Delayed OFDM symbol 1 Transmitted waveform FFT input OFDM symbol 2 Interference region Cyclic prefix concept Weighted overlap and add at transmitter side Weighted overlap and add at receiver side Figure 3.10 Illustration of the effect of cyclic prefix for eliminating ISI (h(t) is the hypothetical channel impulse response) and practical implementation of CP insertion and removal 47]. New Radio Access Physical Layer Aspects (Part 1) 319 S-(N-1)/2 S-(N-1)/2+1 S-(N-1)/2+2 cos(w) Channel Serial to Parallel to coding and parallel serial Cyclic prefix Windowing conversion insertion and filtering modulation converter c