# Modeling Network-Controlled Device-to-Device Communications in SimuLTE
## 2 Background on LTE-Advanced  
This section provides the necessary background on LTE-A and introduces the related terminology and abbreviations. We limit our description to those features that are relevant to D2D communications, namely protocol layering and resource allocation. At the end of this section, we introduce network-controlled D2D communications in LTE-A. The radio access network of LTE-A is depicted in Figure 1 (left). It is composed of UEs and base stations, called eNBs in the LTE terminology. At any time, a UE is connected to one eNB and sends/receives radio frames to/from it through the uplink (UL) and downlink (DL), respectively. LTE-A is a layer-2 technology. However, the LTE-A protocol stack consists of four distinct sublayers, as shown in Figure 1 (right). An IP packet sent down to the LTE-A interface is first handled by the Packet Data Convergence Protocol (PDCP). The latter creates the PDCP context for the (flow of) packet(s), i.e., ciphers the packet with a ciphering key, so that the latter can be deciphered only at the PDCP layer of the intended recipient, and assigns to it a progressive sequence number valid within that context. The packet is then passed down to the Radio Link Control (RLC) layer, in the form of an RLC Service Data Unit (SDU). The RLC can be configured in one of three modes: transparent (TM), unacknowledged (UM) and acknowledged (AM). The RLC TM adds nothing. The RLC UM, which is the one recommended by the standard for D2D, performs segmentation/concatenation of RLC SDUs on transmission, and reassembly, duplicate detection and reordering of RLC PDUs on reception. The RLC AM adds control messages (such as RLC STATUS PDUs) flowing in the opposite direction to confirm reception at the RLC level. RLC SDUs are buffered at the RLC. The underlying MAC fetches data from the RLC buffers, by requesting to it an RLC Protocol Data Unit (PDU) of a given size. The RLC complies by dequeueing from its buffer one or more (fragments of) RLC SDUs and combining them as necessary into RLC PDUs. The MAC adds its own header and forms a MAC PDU, also called Transmission Block (TB), which is then sent to the physical (PHY) layer for transmission over the radio interface. Access to the physical layer is regulated by the eNB, which decides which frequency resources are allocated to which transmission. In LTE-A, UL and DL transmissions occur periodically, at 1 ms  
Figure 1: LTE-A radio access network (**left**); and protocol layering (**right**). intervals called Transmission Time Intervals (TTI). With reference to a Frequency-division Duplexing (FDD) deployment, the UL and DL subframes are transmitted on non-overlapping frequency bands on every TTI. In the frequency domain, a subframe can be seen as a logical vector of Resource Blocks (RBs). A TB occupies an integer number of RBs. DL transmissions carry traffic buffered at the eNB to the UEs (Figure 2, left). The eNB selects the modulation and coding to be used (which determines how many information bits will fit in a RB), based on the Channel Quality Indicator (CQI) reported by the UE, which reflects its measured Signal to Interference and Noise Ratio (SINR). A Hybrid ARQ (H-ARQ) scheme, which allows a configurable number of retransmissions, provides reliability at the MAC layer: the receiving UE sends back either an ACK or a NACK four TTIs after the transmission. The eNB may then reschedule a retransmission at any future TTI (DL H-ARQ is asynchronous). The Orthogonal Frequency-division Multiple Access (OFDMA) scheme is used for physical transmissions in the DL. UL subframes carry UE traffic destined to the eNB (Figure 2, right). The latter issues transmission grants to the UEs, specifying on which RBs they can send their TB, using what transmission format. Single-Carrier Frequency-division Multiple Access (SC-FDMA) scheme is used for transmissions. Since UL buffers reside at the UEs, UEs must signal their backlog to the eNB through Buffer Status Reports (BSRs).