antenna-specific reference symbols. As has been mentioned also in Section 4.2.2, certain time-frequency resource elements in each resource block are allocated for the transmission of reference symbols in order to enable channel estimation. Each antenna port has its own reference symbol, where the time-fre- quency resource element used to transmit the reference symbol differs for different antenna ports. This is actually what defines and identifies an antenna port. By observing the receiver Equation (4.2), it becomes obvious that the inverse transform of the channel transfer matrix, and thereby the separation of the spatial channels, is possible only if the inverse of the matrix H exists. It is known from © 2009 by Taylor & Francis Group, LLC Architecture and Protocol Support for RRM elementary matrix calculus that the inverse of a matrix A exists if and only if det A # 0-that is, when the rows and also the columns of the matrix are independent. (The matrix analogy is used only for illustration purposes; in a real MIMO system, the perfect matching of the analogy may not be possible.) The number of indepen- dent rows and columns of a matrix is defined as the rank of the matrix and, for an arbitrary matrix A of size Nx M, it holds that rank (A) min(N,M). We define the channel rank (vch) of a MIMO system as the maximum number of independent streams that can be transmitted in parallel (i.e., using the same time- frequency resource element). The transmission rank (rii) is defined as the number of independent streams actually transmitted. (Obviously, in all feasible configurations it should always hold that Vtr <rch). Transferring the preceding matrix calculus anal- ogy into the context of MIMO systems yields the intuitive interpretation of the channel rank as the rank of the channel transfer matrix H-that is, rch=rank(H)<min(N,M). The MIMO use case that we have assumed in the preceding discussion is the spatial multiplexing case when different data streams are transmitted from the dif- ferent antennas, using the same t