le enough. In this section, we first give an over- view of the multiantenna solutions in general. Then we discuss the different MIMO variants in the context of LTE, also addressing the required resource management functions used for the antenna port configuration control. A MIMO system can be generally characterized by a set of input and output antennas, where there are N antennas on the transmit side and M antennas on the receiver side, which yields an N M MIMO system, as depicted in Figure 4.15. The input and the output should be interpreted from the radio channel perspective © 2009 by Taylor & Francis Group, LLC Architecture and Protocol Support for RRM Figure 4.15 A MIMO antenna system. (i.e., the N transmit antennas provide the input to the wireless channel, while the M receive antennas deliver the output of the channel). The transfer function of the multiantenna system can be characterized by the channel transfer matrix H, where the hij element of the matrix gives the channel coefficient between transmit antenna i and receive antenna j. This means that the system can be described by the following equation: z=Hxy+e; that is, (4.1) where vector Z contains the received signals on the M receive antennas (i.e., Z is the received signal on the ith receive antenna); vector y contains the N transmitted signals on the different transmit antennas (i.e., yj is the signal transmitted on the jth transmit antenna); and e contains the noise plus interference received on the different receive antennas. The preceding system equation is a frequency domain equation (i.e., Z and y are the frequency domain representation of the received and transmitted signals, respectively). In the special case of an OFDM system, where the modulated sig- nal is generated in the frequency domain, Z and y correspond to the received and C 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3 GPP LTE Radio and Cellular Technology transmitted modulation symbols directly (i.e., the received signal after the fast Fourier transform (FFT) operatio