the United States. The first wave of public safety solutions with LTE has the idea of an overlay LTE network with the voice being carrier by the legacy public safety radio while data services would be provided using LTE network. The evolution step, however, also enables mission critical voice with LTE with the specification of group calls as well as D2D operation, latter known as direct mode in TETRA side. The LTE as such is well suited for voice communication but requires some protocol enhancements to enable group call (which is being done using the eMBMS) and enabling the D2D operation new physical channels are needed for the sidelink connection between UEs. The D2D operation may take place with two main different scenarios, as shown in Fig- ure 3.11. In-coverage case, where the eNodeB coverage is available (in the special case only one of the devices would be in coverage area). In this case, network can provide guidance when the UEs with D2D capability could page/search for other D2D UEs, known as D2D discovery. This allows less resource consuming search procedure for the signals from other D2D UEs. It remains also important to allow physical resource sharing between the regular communications and D2D communications. Discovery via the operator network Discovery via the direct radio links Figure 3.11 Discovery within network coverage and without network coverage LTE-Advanced Evolution in Releases 12-13 ((i)) Uplink/downlink Sidelink Figure 3.12 Sidelink with D2D communications This can be done as part of SIB broadcast or with dedicated RRC signalling. Out-of coverage case. In this case, no regular coverage is available, thus the devices need to manage on their own without timing reference. This is a critical requirement for the public safety operation. There are further different communication modes (after the actual discovery has taken place), with the actual channels between two UEs called as 'sidelink' to differentiate from the existing uplink and downlink channels, as shown in Figure 3.12. The sidelink oper