Public-Safety Use Cases
For a better assessment of emergencies, first-responders (e.g., police, fire, and medical emergency services), need broadband access in the next-generation public safety network . Using D2D technology, public safety networks en-able terminals to communicate directly without any support from the infras-tructure while being scalable to substantial group calls . It is worth noting that the standard LTE-D2D was initially developed for public safety use cases to provide the necessary functionalities: Push-To-Talk (PTT), Direct communi-cations between terminals and Group communications .
Locality and Context-Aware Services
Reliable discovery of nearby devices, using the D2D protocol, enables various use cases and services. Both fixed and mobile devices, (e.g., infrastructure sen-sors, beacons of transport and businesses, mobiles, and tablets) can interact with each other to provide locality and context-aware services. Typical exam-ples include: i) social discovery applications: e.g., finding nearby friends of per-sons with mutual interests in Facebook or LinkedIn, ii) local guidance and ad-vertisement: e.g., searching for nearby bus stations, ATMs, restaurants, and mu-seum guidance, and iii) transport information: e.g., notification of the arrival of the next bus, parking availability.
Local Content Sharing
UEs can use their D2D interfaces to exchange files rapidly while consuming lower energy than the conventional method involving the cellular connection. These interfaces also facilitate streaming video locally between users by form-ing clusters. Moreover, social applications can make use of D2D capabilities to share content between users in proximity.
Network Range Extension
A UE can reach a cellular BS through one or more UEs serving as relays to the network. An example of this scenario is devices that are either in weak con-nectivity areas (e.g., indoor or cell edge) or devoid of enough power to reach a distant BS (e.g., smartwatch). A neighboring UE with satisfactory connectivity or sufficient power source can connect with those devices in its vicinity, using its D2D interface, and forwards, then, their data to the BS through its cellular interface.
Traffic (Data) Offloading
D2D can be employed to enhance the networking of the future 5G networks in several ways. One way is to offload the traffic  from the cellular infras-tructure to the direct communication between two nearby UEs, which discover each other using D2D-based discovery protocol . Data offloading tech-niques  efficiently deal with the problem of congestion in next-generation cellular networks. In this context, a congestion-prone BS may take advantage of a secondary wireless technology to offload the circulating traffic between UEs and thus saving resources and bandwidth. A D2D-based protocol can provide the secondary mechanism to carry the offloaded traffic . Besides, the D2D communications can be either in the operator’s band (i.e., using 3GPP LTE-D2D) or in the unlicensed spectrum (e.g., based on Wi-Fi Direct ). The offloaded traffic can be either unicast or multicast, following BS-to-UE(s), UE-to-BS, or UE-to-UE models. Moreover, the offloading can be achieved using a multihop network of D2D links.
3GPP Architecture for D2D (LTE-D2D)
To support LTE-D2D, an enhanced user equipment (UE) implements an ad-ditional protocol stack besides the conventional one. This new LTE-D2D stack provides the so-called Proximity-based Services (ProSe) to the upper layer(s) . ProSe includes: i) Direct Discovery: a service whereby a UE can detect and iden-tify other UEs in its proximity, ii) Direct Communications: UEs can directly communicate with each other bypassing the cellular infrastructure, and iii) UE– to-Network Relay: remote UE uses another UE as a relay in the network.
From an upper-level perspective, ProSe is carried over a new type of wireless link beside the conventional ones: i.e., DownLink (DL), and UpLink (UL). This lateral link between UEs is called SideLink (SL). In LTE-D2D, SL is configured to use the same frequency resources as UL to increase the overall spectral effi-ciency . It also reuses much of UL structure and hardware to add another efficiency dimension. From the lower layers perspective, SL presents its direct communication services to the upper layers in terms of no-feedback SL Radio Bearers (SLRBs). This is done to present uniform support for both unicast and multicast IP communications .
LTE-D2D Protocol Stack for Direct Communications
Similar to their counterparts in the conventional LTE communication stack, Packet Data Convergence Protocol (PDCP), and Radio Link Control (RLC) layers provide IP packet segmentation, header compression, and security procedures. A single SLRB is identified by a pair of PDCP/RLC entities connected in tandem at the source UE and the corresponding pair(s) at the destination UE(s). At the interface with incoming packets from the upper IP layer resides an IP-flow clas-sifier that directs each IP packet to its corresponding SLRB PDCP/RLC entities.
The Medium Access Control (MAC) layer serves the upper layers by transmit-ting Transport Block (TB) composed of RLC Protocol Data Units (PDU) from possibly several SLRB bearers as long as they have the same destination. Each TB is identified by its layer L2 identifiers, namely, i) source ProSe-UE-ID, and ii) the destination ProSe-L2-Destination-ID. A TB is transmitted when a new SL transmission opportunity arrives while Hybrid Automatic ReQuest (HARQ) op-erations in LTE-D2D are restricted to blind retransmissions (i.e., with no feed-back) to increase the reliability. Hence, each TB is further retransmitted three times in the subsequent transmission opportunities with different redundancy versions.
Similar to UL, SL transmission, at the Physical (PHY) layer, uses the Single Car-rier Orthogonal Frequency Modulation (SC-OFDM) format using the grid of re-source blocks (RBs). The latter occupies a subframe, i.e., a Transmissions Time Interval (TTI), which lasts 1 ms and is characterized by a bandwidth of 12 sub-carriers (180 kHz) in the frequency domain. However, unlike UL, SL allocations are organized in longer periodic intervals called SideLink Control Periods (SC-Periods), which can be configured between 40 and 320 subframes in length. As depicted in Figure 1.5, a SC-Period starts with a control part followed by a data part. However, the information, on which subframes and RBs are available for the operation, is conveyed by a configuration parameter called a resource pool.
Table of contents :
1.1 Device-to-Device Communications (D2D)
1.2 Advantages and Use Cases of D2D
1.2.1 Public-Safety Use Cases
1.2.2 Locality and Context-Aware Services
1.2.3 Local Content Sharing
1.2.4 Network Range Extension
1.2.5 Traffic (Data) Offloading
1.2.6 IoT and V2X Communications
1.3 D2D Architecture
1.3.1 Spectrum Allocation
1.3.2 3GPP Architecture for D2D (LTE-D2D)
1.3.3 LTE-D2D Protocol Stack for Direct Communications
1.4 D2D Challenges
1.4.1 Peer Discovery
1.4.2 Mode Selection
1.4.3 Resource Allocation and InterferenceManagement
1.4.4 Routing over D2D Links
1.5 Problem Statement
1.7 Thesis Outline
2.1 Literature on Unicast D2D Systems
2.2 Literature onMulticast D2D Systems
2.3 Literature on Other RoutingModels in D2D Systems
2.4 Literature on Energy-Aware D2D Routing
2.5 Comparative Summary and Remarks on Literature
3 Joint Unicast Routing and Wireless Resource Allocation in Multihop LTE-D2D Communications
3.2 SystemModel and Problem Formulation
3.3 Proposal: JRW-D2D
3.4 Performance Evaluation
3.4.1 Network Simulation Environment
3.4.2 Network Simulation Setup
3.4.4 Simulation Results
4 A Scalable Joint Routing and Resource Allocation Scheme: D2D-based Unicast andMulticast Data Offloading
4.2 SystemModel and Problem Formulations
4.2.1 Initial Link-Based Formulation
4.2.2 Path-Based Formulation
4.4 Performance Evaluation
4.4.1 General Scenario Parameters
4.4.2 Baselines for Comparison
4.4.3 Collected PerformanceMetrics
4.4.4 Unicast Applications Scenario
4.4.5 Simulation Results in the Unicast Scenario
4.4.6 Multicast Applications Scenario
4.4.7 Simulation Results in theMulticast Scenario
5 D2D-BasedCellular Traffic Offloading: An Energy-Aware ScalableHeuristic Scheme
5.3.1 Exact Resolution Proposal: JRRA-EE
5.3.2 Novel Heuristic-Based Proposal: HERRA
5.4 Performance Evaluation
5.4.1 General Scenario Parameters
5.4.2 Simulated Traffic Parameters
5.4.4 Simulation Results
6.1 Summary of Contributions
6.2 FutureWork and Perspectives