spectively. Let us further assume that the pilot subcarriers are transmitted at integer multiples of subcarrier spacing and OFDM symbol duration in frequency and time directions, respectively (i.e., fp = m/Tu and Tp = nT symbol where m and n are integers). The (m,n) pair represents the pilots' separation in terms of subcarrier spacing and OFDM symbol duration. From the sampling theorem point of view, the channel's two-dimensional delay-Doppler response h(T,v) can be fully reconstructed, if the two-dimensional transform function H(t,f) is sampled greater than or equal to the Nyquist rate across time and fre- quency dimensions. Hence, the time-domain sampling rate must be greater than or equal to the channel's maximum Doppler spread, that is, Tp < 1/Vmax (the sampling rate in time must be less than the coherence time) and the frequency-domain sampling rate must be greater than or equal to the channel's maximum delay spread, that is, fp < 1/Tmax (the sam- pling rate in frequency must be less than coherence bandwidth). Assuming a wide-sense sta- tionary uncorrelated scattering channel model and further assuming the channel to be constant over one OFDM symbol, the frequency response H(t,f) of an L-path channel is given H(t,f) = where V1, V1, and T1 denote the phase, Doppler frequency, and delay of the 1th path, respectively. All these parameters are independent random variables. In general, the pilot signals are oversampled to ensure a good trade-off between performance and overhead. Therefore the choice of (m,n) depends on the channel's maximum delay spread and maximum Doppler spread and must satisfy the following equation according to two-dimensional sampling theorem: ms27" New Radio Access Physical Layer Aspects (Part 2) 415 It should be noted that the pilot density for a regular pattern can be calculated using the pre- ceding equation. For large values of Vmax (i.e., large Doppler spread, or alternatively small channel coherence time means that channel is time-varying), n should be small to appropri- ately track c