ons 1 ms Sub-frame 0.5 ms Slot Frequency 180 kHz System Bandwidth Figure 2.7 Uplink PUCCH allocations LTE and LTE Advanced in Releases 8-11 other band edge. Same PUCCH resource can be assigned to multiple devices as UEs have their own code sequence thus allowing multiple simultaneous PUCCH transmissions using the same resources. The LTE physical layer included already from Release 8 full use of Multiple Input Multiple Output (MIMO) transmission operation, with the support for two antenna reception being mandatory for the typical UE classes. Similarly, the first LTE networks also supported the dual-stream transmission from the start of the LTE deployments. Release 8 includes the support for up to four antenna MIMO operation, but going beyond two antennas is foreseen to be implemented based on Release 10 as discussed later in this chapter. 2.2.2 LTE Architecture The design principle with LTE architecture was to enable well scalable solution, without too many layers. The architecture from radio point of view concluded to have all radio protocols in the eNodeB, thus resulting to the single element radio access network. The S1 interface towards core network was divided to two parts, S1-U for the user plane data and S1-MME for control plane information between the Mobility Management Entity (MME) and eNodeB. None of the user data goes through MME, thus MME processing load is depending only on the amount of users and their frequency of signalling events such as connection setups (attach), paging or mobility events like handovers. The MME does not make any decisions on handovers, as those are done by the eNodeB, but the MME keeps track where the UE is going in the networks and guides the Evolved Packet Core (EPC) gateways to set up the data connection to the correct eNodeB. In the case of idle mode, the MME keeps track of the location of the UE and knows which tracking areas (TA) to be used when there is a need to page a UE. The radio-related architecture is shown in Figure 2.8, with the X2-interface also visible between