e general principle of a MIMO system 10.4 Multi-antenna transmission schemes Assuming a linear time-invariant MIMO channel, the channel input-output relationship can be further described in the discrete time-domain as follows: )<nM-1;11<Nr (10-23) k=1m=0 where xk(n)|k=1,2,....N, and y1(n)|i=1,2,...,N, represent the channel input and output time-domain signals, respectively. In the case of time-varying channels, Equation (10-23) can be written as y1(t) = +ni(t) where hik(t,T) denotes the time-varying impulse response of the 1kth channel. The matrix form of Equation (10-22) in a frequency-domain sampled at single frequency Wm can be written as follows: y(wm) = H(wm)x(wm) n(wm) (10-24) In an OFDM system the signal processing is inherently performed in the frequency-domain. Furthermore, OFDM transforms a frequency-selective fading channel to a flat-fading channel when considering narrowband orthogonal sub-carriers. In such a system, the MIMO signal processing can be performed at each sub-carrier. This is the main reason for the suitability of the extension of MIMO to an OFDM system. When MIMO processing is done at each sub-carrier, 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) filter. 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 selective fading). There is a new realization of the channel at every transmitted packet, if the channel remains invariant for the duration of the packet; otherwise, the variation of the channel is explicitly modeled in the signal detection. As mentioned earlier, there are N N t paths between the transmitter and the receiver, where each channel is the sum of several FIR filters with different delay spreads. The channels may or may not be correlat