i(t) where hik(t) and ni(t) are the channel impulse response between the kth transmit antenna and 1th receive antenna and the additive noise at the 1th receive antenna port, respectively. The preceding equation can be written in the frequency- domain as Y1(w)=Hik(w)Xk(w)+N1(w).If x(w) = (X1(w),X2(w), y(w) Y1(w), Y2(w), , YNRx(w)]T, and = = transform vectors of Xk(t), y1(t), and n/(t), respectively then y(w) H(w)x(w) + n(w) where H(w) is an NRX X NTX channel matrix with entries. Assuming a linear time-invariant MIMO channel, the channel input-output relationship can be further described in the discrete time-domain as follows: OnM - 1</NRX where xx(n)lk=1,2,...N, and yl(n)|1=1,2,...NRX represent the channel input and output time- domain signals, respectively. In the case of time-varying channels, the preceding equation can be written as y1(t) = hik(t,T)XK(T) + ni(t) where hik(t,T) denotes the time- varying impulse response of the 1kth channel. The matrix form of MIMO channel output in frequency-domain sampled at single frequency Wm can be written as y(wm) = H(wm)X(wm) + n(wm). In an OFDM system, the signal New Radio Access Physical Layer Aspects (Part 2) 553 processing is inherently performed in frequency domain. The OFDM transforms a frequency-selective fading channel to a flat-fading channel when considering narrowband orthogonal subcarriers. In such system, the MIMO signal processing can be performed at each subcarrier. This is the main reason for suitability of MIMO extension to an OFDM sys- tem. When MIMO processing is performed at each subcarrier, the MIMO channel input-output relationship can be demonstrated as y = Hx + n, where the channel between the transmitter and the receiver is typically modeled as a finite impulse response (FIR) fil- ter. In this case, each tap is typically a complex-valued Gaussian random variable with exponentially decaying magnitudes. The tap delays correspond to the RMS delay spread and the channel type (e.g., low-delay spread or flat fading, high-delay spread or frequency- select