 differ in the structure and size of the precoding matrices (codebooks), that is, Type I CSI and Type II CSI. Type I CSI primarily targets scenarios where a single user is scheduled within a given time/frequency resources potentially with transmission of a relatively large number of layers in parallel (high-order spatial multiplexing) and Type II CSI mainly targets MU-MIMO scenarios with multiple 552 Chapter 4 devices being scheduled simultaneously within the same time/frequency resources but with only a limited number of spatial layers (maximum of two layers) per scheduled device. The codebooks for Type I CSI are relatively simple and used to focus the transmit energy at the target receiver. Inter-layer interference is assumed to be primarily handled by utilizing multiple receive antennas and advanced receiver architectures. In contrast, the codebooks for Type II CSI are significantly more extensive, allowing the PMI to provide channel infor- mation with much higher spatial granularity. The more extensive channel information allows the network to select a downlink precoder that not only focuses the transmitted energy at the target device but also limits the interference to other devices simultaneously scheduled on the same time/frequency resources. The higher spatial granularity of the PMI feedback is made possible with significantly higher signaling overhead. While a PMI report for Type I CSI may consist of a few tens of bits, the PMI report for Type II CSI may con- sist of several hundred bits. Therefore, Type II CSI is mainly applicable for low-mobility scenarios where the feedback periodicity in time can be reduced. 4.1.9.1 Capacity of MIMO Channels A generic MIMO system consists of a MIMO transmitter with NTX transmit antennas, a MIMO receiver with NRX receive antennas, and NRX X NTX paths or channels between trans- mit and receive antennas. Let xk(t) denote the transmitted signal from the kth transmit antenna at time t then the received signal at the 1th antenna can be expressed as y1(t) = hik(t)*xx(t) + n