ennas eNodeB eNodeB (a) Spatial diversity (b) Spatial multiplexing Improved reliability Improved data rate Figure 10-14 MIMO: Spatial Diversity and Spatial Multiplexing In a MIMO system, it is necessary to discover the signal characteristics of each radio path between each combination of transmit and receive antennas. This requires an accurate phase/time alignment at each antenna to identify the required spatial alignment of the paths. 3GPP 36.104 and 3GPP 38.104 define the time alignment error (TAE) budget required for transmission diversity in LTE and 5G as shown in Table 10-4. Table 10-4 Time Synchronization Requirements for Transmission Diversity or MIMO E-UTRA (LTE) Time Alignment Error LTE MIMO or transmission (Tx) diversity transmissions, at each carrier frequency Maximum 65 ns Although this is a very low value for phase alignment, it is not a significant problem because it is a relative alignment-it applies only between the member of an antenna array connected to a single radio. "Chapter 11. 5G Timing Solutions", provides more discussion on relative time errors and cluster-based timing. 10.3.1 Inter-cell Interference Coordination At the network level, interference is caused when UEs in neighboring cells attempt to use the same resource at the same time. For example, as shown in Figure 10-15, at the edge of the cell, two UEs ( UEA and UEB) might experience interference because 1) both are using the same frequency (f2) and high transmit power to communicate; and 2) they do not have any knowledge about the neigh- boring UE; and 3) the eNBs are independently scheduling their radio resources. Low power High power High power Low power interference interference Cell A Cell B f={{1,f2.....fn} : fx = Sub-carrier Figure 10-15 Inter-cell Interference at the Cell Edge To mitigate such problems, 4G LTE and LTE-A adopted several technologies known as inter-cell interference coordination (ICIC) and enhanced inter-cell interference coordination (eICIC). ICIC, defined in 3GPP Release 8, reduces inter-cell interference by 