 0, NSL symbol ,1} in a slot where the first and the second indices represent the frequency and time, respectively. The resource elements that are not used for transmission of a physical channel or a physical signal in a slot are set to zero. 822 Chapter 7 A physical resource block (PRB) is defined as Nsymbol or 6 (extended cyclic prefix) con- secutive SC-FDMA symbols in the time domain and NRB SC = consecutive subcarriers in the frequency domain. A PRB in the sidelink consists of NSL symbol X SC resource elements, corresponding to one slot in the time domain and 180 kHz in the frequency domain [3]. The relationship between the PRB number NPRB in the frequency domain and resource elements (k,1) in a slot is given by NPRB = k/12 A key concept in LTE-based sidelink communication is the resource pool, which defines a subset of available subframes and RBs for either sidelink transmission or reception. Sidelink communication is a half-duplex scheme and a UE can be configured with multiple transmit resource pools and multiple receive resource pools. The resource pools are config- ured semi-statically by RRC signaling. When data is sent using a resource pool, the actual transmission resources are selected dynamically from within the pool using one of the two following modes: Transmission mode 1, where the serving eNB identifies the resources via downlink con- trol information (DCI) format 5 that has to be sent to the transmitting UE. This mode requires the UE to be fully connected to the network; that is, in RRC_CONNECTED state. Transmission mode 2, where the transmitting UE self-selects the resources according to certain rules aimed at minimizing the risk of collision. This mode can be used when the UE is in connected state, idle state, or out of network coverage. There are two types of resource pools: (1) reception resource pools and (2) transmission resource pools. These are either signaled by the eNB for the in-coverage cases, or preconfi- gured for the out-of-coverage scenarios. Each transmission resource pool has 