0 ns LTE inter-band CA with or without MIMO or Tx diversity 260 ns LTE intra-band contiguous CA with or without MIMO or Tx diversity 130 ns LTE MIMO or Tx diversity at each carrier frequency 65 ns 36.104 Maximum transmission time difference in intra-band noncontiguous and 30.26 us inter-band CA 10.3.1 Dual Connectivity 3GPP Release 12 introduces the concept of Dual Connectivity (DC), which allows the use of two eNBs to communicate with the UE. Whereas CA allows the UE to simultaneously transmit and re- ceive data on multiple carriers from a single eNB, DC extends it to allow the UE to transmit and re- ceive data simultaneously from two cell groups or eNBs. These consist of a master eNB (MeNB) and a secondary eNB (SeNB), as shown in Figure 10-19. The MeNB and SeNB coordinate the scheduling between themselves using the X2 interface. This technique is widely used to improve overall performance when introducing small cells into heterogeneous networks. However, when using DC, each eNB handles its own scheduling and maintains its own timing rela- tions with the UE. Hence, the X2 interface latency and synchronization requirements are somewhat relaxed in contrast to the CA use cases. However, there can still be a requirement to coordinate between the two eNBs, depending on the type of operation. Dual Connectivity operations are either synchronous or asynchronous. In the synchronous mode of operation, the maximum absolute timing mismatch between data transmitted by MeNB and SeNB is critical. For a given UE, the two eNBs must align a data transmission within a finite window offset from each other. You cannot have a situation where one eNB is sending current data and the other sending related data many seconds later. In an asynchronous mode, there is little or no relation between the two streams, SO there are no strict limits tying the two separate transfers together. In technical terms we define this as follows: In synchronous operation, the maximum receive time difference (MRTD) and maximum trans- mit time difference (MTT