n and the transmitted signal before the IFFT operation). For simplicity, we assume a non-frequency-selective channel, which means that the matrix H is independent of the OFDM carrier on which the signal is transmitted. Obviously, the received signal on any of the receiver antenna ports will contain components from the transmitted signals of all the transmitter antenna ports. That is, the signals transmitted from the different transmit antennas will cause interfer- ence with each other and the receiver needs somehow to separate out the different spatial "channels" in order to be able to demodulate the different data streams transmitted from the different antennas. Although different receiver algorithms are used in practice, it is common to all methods that the receiver needs to esti- mate the channel transfer matrix H and apply an inverse of the channel transfer on the received signal in order to separate out the different streams transmitted from the different antennas. One of the methods often used for this purpose is the so-called zero-forcing MIMO technique, where the receiver tries to null out the channel from the direction of the interferer. In what follows, we illustrate MIMO spatial multiplexing for the zero-forcing technique, where the following operation is applied at the receiver: V: =H-1xz=H-1xHy+H-1xe = (4.2) =y+H-1ee (4.3) where V denotes the received signals after performing the inverse channel operation. (In order to ensure that the matrix H is quadratic and thereby the inverse opera- tion is meaningful, we assume an Nx N MIMO system for illustration purposes.) After the different streams have been separated out via the inverse operation, it is possible to use one of the well-known receiver decision algorithms on the obtained vector V, such as the minimum mean square error (MMSE) algorithm (also called interference rejection combining [IRC] in the literature), to decide for the modula- tion symbols. The channel estimation (i.e., the estimation of the matrix H) can be done based on the reception of 