# Modeling Network-Controlled Device-to-Device Communications in SimuLTE
## 2 Background on LTE-Advanced
### Network-Controlled Device-to-Device Communications  
Network-controlled D2D communications have been added to the LTE-A standard in release 12 [17]. The standard deals with point-to-multipoint (P2 MP), or one-to-many, communications. The SL is often physically allocated in the UL spectrum in an FDD deployment, since UL frequencies are less likely to be congested than DL ones. This requires D2D-enabled UEs to be able to receive SC-FDMA transmissions besides OFDMA ones [7]. Resource allocation on the SL is done by the eNB, and it can be either static or on demand. In a static approach, called Autonomous Resource Selection (ARS), the eNB configures a static resource pool, e.g., \(k\) RBs every \(T\) TTIs, and UEs can use these resources without any signaling. This implies some latency, since a UE must wait for the next TTI where resources are available, and does not regulate collisions, since two nearby UEs may decide to use the same resources simultaneously. The Scheduled Resource Allocation (SRA), on the other hand, is an on-demand scheme which mirrors resource allocation in the UL for standard communications: the UE sends a Random Access (RAC) request to the eNB, which grants enough space for it to send its BSR. Then, the eNB schedules SL resources accordingly and issues the grant to the UE for D2D communications. SRA allows the eNB to schedule interference-free transmissions, at the price of more control traffic.  
Figure 2: Procedures for DL (**left**) and UL (**right**) data transmissions.  
P2 MP D2D transmissions are sent as multicast MAC-layer transmissions. This is done by using a multicast group ID as a MAC ID in the message. They are heard by whichever UE is within hearing distance of the transmitter, and they are not acknowledged. Therefore, a sender has no way to know which neighboring UEs, if any, actually received a message.  
Point-to-point (P2P) or unicast D2D communications have not been standardized yet. However, there is a large body of scientific literature on the subject, which substantially agrees on what the transmission paradigm should be: with P2P D2D, a sender UE \(a\) can send a packet to one (and only one) receiver UE \(b\), when the eNB allows them to use the SL. This is done by granting \(a\) one or more RBs for transmission and instructing \(b\) to listen on the same RBs for reception, still assuming that \(b\) is equipped with a SC-FDMA receiver. UEs \(a\) and \(b\) form a unidirectional flow, whose data are not relayed at the eNB. A key requirement of P2P D2D communications is that they should be transparent to the upper layers, so that the decision to route a flow to the SL or through the traditional UL/DL cellular data path can be taken dynamically by the eNB, without the sender/receiver applications being aware of it (save, possibly, for experiencing different performance in the two modes). The above decision, called mode selection, may be taken according to different policies (channel quality on the UL/DL/SL, congestion, possibility of frequency reuse, etc.), and it entails a set of non-trivial decisions to be made at the two endpoints and the eNB--e.g., whether to drop or reschedule buffered traffic--which go by the name of mode switching (MS) (see [18] for a comprehensive discussion of the subject).  
The above discussion assumed that D2D communications are arranged in licensed spectrum. However, D2D communications can also occur in unlicensed spectrum. The latter can be added to the licensed one to increase the capacity of the system. In such deployments, additional mechanisms for managing the interference with other wireless technologies (e.g., Wi-Fi) must be put in place [19; 20; 21]. In this paper, we focus on D2D communications in the licensed spectrum only.