D) from MeNB and SeNB should be within a certain threshold. These two metrics measure the relative difference for transmission and reception between each of the eNBs (primary and secondary). In asynchronous operation, the UE can perform the operation without a specific MRTD or MTTD. The MRTD and MTTD synchronization requirement considers relative propagation delay, transmit time difference, and multipath delay propagation of radio signals from MeNB and SeNB antenna ports and requires phase accuracy as outline in Table 10-8. Table 10-8 Time Synchronization Requirement for LTE Dual Connectivity Application MRTD/MTTD Error Reference LTE Synchronous Dual Connectivity 33 us/32.47 us 3GPP TS 36.133 LTE Asynchronous Dual Connectivity 10.3.1 Multimedia Broadcast Multicast Service (MBMS) Another use case in 4G LTE that requires accurate phase/time synchronization between eNBs is the Multimedia Broadcast Multicast Service (MBMS). MBMS transmits the same content to multiple users located within a predefined MBMS service area, letting all the users subscribed to MBMS simultaneously receive the same multimedia content. MBMS services are frequently used to provide (TV-like) broadcast multimedia services via the LTE network in, for example, venues such as sporting arenas. Using the OFDM modulation, multiple cells can transmit truly identical and mutually time-aligned signals into the MBMS service area. In this case (as shown in Figure 10-20), the transmission re- ceived from multiple cells will appear as a single multipath transmission to the subscribed UE devices. This kind of transmission is referred to as an MBMS Single Frequency Network (MBSFN). Not only can an MBSFN area consist of multiple cells, but a single cell can also be part of multiple MBSFN areas. MBSFN area overlap (A,B) Area A Area B Area C ((co)) synchronized transmission Transmission from different cells looks like a multipath from same cell Cells (a) MBSFN area (b) MBMS / eMBMS transmission Figure 10-20 MBMS Area and Parallel Transmission of the Same Data Str