AbstractDevice-to-device (D2D) communication is rapidly evolving into a viable method of information exchange in a cellular network. It has a very low end-to-end latency and can increase spectral efficiency of a cellular network. The latest releases of 3GPP specification have given considerable attention to standardize this mode of communication and integrate it in the ecosystem of LTE advanced. This will give more impetus to the development of D2D technologies and their adoption by mobile operators. This paper presents a discussion and critical analysis of the main features of D2D communication as defined in Release 12 and subsequent releases of 3GPP specifications.
One of the biggest challenges faced by telecom operators is the continuous increase in demand for high data rate and low end-to-end delay. One way to achieve it is through device-to-device (D2D) communication. D2D communication enables direct communication between proximate devices bypassing the base station. As D2D communication promises ultra-low latency for communication among the users, it has attracted great attention from researchers working on the forthcoming 5th generation cellular networks [24, 25]. D2D communica-tion can operate using both licensed cellular spectrum and unlicensed spectrum; the two modes are known as in-band communication and out-band communication, respectively. In the first case, the D2D communication and the cellular communication can both operate over the entire licensed cellular spectrum; this is known as the underlay D2D com-munication mode. But it leads to heightened interference
across the D2D users and the cellular users. To avoid this problem, a new approach known as the overlay D2D com-munication mode has been proposed that allows the D2D users to use a certain fraction of cellular resources that is not assigned to the normal cellular users. The overlay mode focuses on the effective resource allocation, so the spec-trum wastage can be avoided. In terms of performance, D2D communication, when it is technically feasible, offers more benefits compared to the conventional cellular communica-tion. First, it is a transparent communication technique and is very efficient with a high spectral efficiency and low latency. Therefore, managing local traffic becomes easy for the user equipments (UEs) communicating directly in a given prox-imity. Computational offloading is one more benefit of D2D communication. D2D users under a static network environ-ment can use D2D links to offload computation-heavy tasks to nearby D2D users [34]. In D2D communication, the mode selection technique allows the devices to switch from the infrastructure path to direct path easily. This reduces the congestion in the network. From an econometric point of view, D2D communication can play a big role in commercial applications, social networking applications, e-commerce, etc., where users can directly share the required information locally [32, 31].
Researchers have proposed many novel applications and models for D2D communication. The 3rd Genera-tion Partnership Project (3GPP) has adopted a relatively simple architecture that fits easily in the existing long
term evolution-advanced (LTE-A) ecosystem so it can be adopted seamlessly by cellular operators. The integration of D2D communication into LTE-A began with the 3GPP Release 12. In the USA, this release has been adopted for the next-generation public safety network with a 700 MHz of dedicated spectrum allocated by Federal Communica-tions Commission (FCC) [29]. The following releases of 3GPP standards have enhanced the support for D2D communication.
This paper gives an overview of the standardization of D2D communication based on 3GPP Release 12 and sub-sequent 3GPP releases. We describe the general features of 3GPP releases in the context of D2D communication in Sect. 2. Section 3 illustrates the use case scenarios. We describe the reference architecture in Sect. 4, channel struc-ture in Sect. 5, and the core features of D2D communication as outlined in the 3GPP Release 12 in Sect. 6. We present a critical discussion in Sect. 7 and conclude in Sect. 8.
Features of 3GPP Releases
General Features
Standardization of technologies is very important as it ensures their interoperability across products and services manufactured by different vendors. Standardization makes the technology commercially viable [41]. The 3GPP stand-ards organization develops protocols for mobile telephony. It has made monumental contributions in developing and maintaining telecommunications standards from 2G to 5G. Figure 1 shows the current and upcoming 3GPP releases along with their year of release and current status. The initial LTE architecture was standardized as 3GPP LTE Release 8/9. Releases 8 and 9 both used a bandwidth of 1.4 MHz, 3 MHz, 5 MHz, 15 MHz, and 20 MHz to support various deployment scenarios. LTE-advanced or LTE-A arrived with 3GPP Release 10, which focused on enhancing the network capacity using carrier aggregation techniques [20]. It enhanced the overall bandwidth of existing cellular net-works by combining two or more component carriers for higher data rates [58]. Coordinated multipoint transmission and reception (CoMP), and heterogeneous deployments (HetNet) are supported since 3GPP Release 11. Network capacity enhancement, increase in coverage area, cell coordination, and cost reduction are some of the impor-tant enhancements in 3GPP Release 12. Along with that, 3GPP Release 12 introduced D2D communication in the LTE-A architecture; its major targeted application is public safety. Release 13 added support for other D2D functions and unveiled the first set of specifications covering mission critical services. Release 14 focused on mission critical enhancements and LTE support for vehicular (V2X) service.
The most important feature of Release 15 is the support for 5G radio systems. Work on Release 16 is in progress; it aims to increase support of vertical industries such as V2X, public safety, and Industrial Internet of Things (IIoT) [37]. Although the 3GPP releases specify a bevy of features for cellular communication, in the subsequent sections of this paper we concentrate on its D2D specifications exclusively.
Features Specific to D2D Communication
Key Motivations for D2D Communication
3GPP Release 12 focuses on developing a communication framework to meet public safety needs that can be used by police, fire fighters, ambulance drivers and other person-nel handling emergency situations. The goal is to reduce dependency on network infrastructure that can fail in times and areas of disaster, limit operational costs, and enable broadband communication. It supports two key technolo-gies as explained below [9].
1. Proximity services (ProSe): Proximity services in 3GPP Release 12 allow devices in physical proximity to dis-cover each other and communicate in an optimized manner. They are essentially supported with D2D com-munication that comprises D2D discovery and direct communication. In D2D discovery, a UE discovers another UE in its proximity. D2D discovery may be performed at the EPC level or done directly by the UEs. In direct communication, two UEs that have discovered each other communicate directly via the LTE air inter-face without routing the signal through the eNodeB
Fig. 1 3GPP Releases. The circles along the periphery show the 3GPP releases and the year (or tentative year) of publication of the corresponding release. Blue circles indicate frozen releases, while orange circles indicate open ones (color figure online)
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(eNB) and the core network. This avoids network con-gestion and allows devices to communicate even when the network coverage is absent. Note that device discov-ery is not a prerequisite to direct communication as the latter can also happen through broadcasting. Proximity services can be used for both public safety and com-mercial applications.
2. Group communication: To meet public safety demands, group communication is considered as one of the most important requirements. It provides services like one-to-many calling that can easily disseminate messages to a large group of people. 3GPP standardization and releases have focused heavily on the enhancement of group communications techniques. Generally in group communication, all UEs in a group receive a common downlink stream to communicate. Therefore, the overall resource utilization can be optimized. To enable effi-cient and flexible group communication, 3GPP stand-ardization activities have consistently paid attention to optimized broadcast and multicast techniques. Note that group communication can be achieved with D2D com-munication, or with existing or enhanced 3GPP mul-timedia broadcast/multicast services (MBMS) in LTE/LTE-A.
Scenarios for D2D Communication
3GPP Release 12 defines features that support D2D com-munication with or without the help of eNB and the core network. In particular, three coverage scenarios are consid-ered as shown in Fig. 2.
1. In-coverage: In this type of communication, all UEs are in the coverage of the eNB.
2. Out-of-coverage: In this scenario, none of the UEs are under the coverage of the eNB.
3. Partial-coverage: In partial-coverage, some UEs are in coverage of the eNB while other UEs are not. UEs in-coverage communicate with UEs not under the coverage of the eNB.
3GPP Release 12 supports D2D discovery for only in-coverage scenario in public safety and commercial use (i.e., non-public safety). Release 13 extends D2D discovery to all 3 scenarios in case of public safety. 3GPP Release 12 supports direct communication for all 3 scenarios. Note that direct communication between UEs occurs through broadcast (without feedback channels) at layer 1. At layer 2, destination UE identifier (in short, UE ID) is used for unicast transmission and Group ID is leveraged for groupcast transmission.
Other Characteristics of D2D Communication in 3GPP
3GPP Release 13 adds support for UE-to-network relay at layer 3 as shown in Fig. 3. It helps to increase the coverage of the network [30]. Release 13 also introduces ProSe per packet priority (PPPP) to support QoS across traffic streams. The subsequent 3GPP releases study how D2D links can be exploited for other applications like IIoT and how their quality can be improved.
D2D Use Cases
3GPP Release 12 discusses D2D communication as a mechanism for proximity services (ProSe) and formulates procedures for D2D discovery and D2D communication. As shown in Fig. 4, D2D use cases encompass both public safety use and commercial applications. Group communica-tion, a prominent feature of the public safety use case, is one
Fig. 2 D2D communication scenarios supported in 3GPP Release 12Fig. 3 Relay-aided D2D communication scenarios supported in 3GPP Release 13
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of the thrust areas in 3GPP standardization and D2D is one way to achieve it. D2D communication between UEs may be 1:many or 1:1. In 1:1 communication, a device trans-mits data directly to a specific receiver, whereas 1:many is a groupcast or group communication service between devices [41].
As far as D2D communication scenarios are concerned, various new terminologies are defined under 3GPP Release 12 [4, 51]. Some of the most important terms are
• ProSe-Enabled UEs: ProSe-enabled UEs are the UEs which support the requirements and procedures for proximity services as defined in 3GPP Release 12. They may be associated with both public safety and non-pub-lic safety applications. The ProSe-enabled public safety UEs can be used for both public safety and other ProSe services, whereas ProSe-enabled non-public safety UEs cannot be used for public safety ProSe services.
• ProSe discovery: ProSe discovery is a mechanism in which a UE discovers other UEs in a given proximity using Evolved UMTS Terrestrial Radio Access Net-work (E-UTRAN). This ProSe discovery can be either open discovery or restricted discovery. In open dis-covery mechanism, UEs discover each other without any authorization. In restricted discovery mechanism, UEs need to acquire permissions to discover the nearby devices. Similarly, network-assisted ProSe discovery is a mechanism where the mobile network operator verifies if the UE is authorized to discover another UE or not. The assistance is provided by the Evolved Packet Core (EPC) and the mechanism called EPC-level ProSe dis-covery since the EPC determines and informs the ProSe-
enabled UEs about their proximity. 3GPP Release 12 also allows network-independent discovery of UEs where they perform the discovery procedure autonomously, without assistance from the network. Therefore, it is useful when both UEs lie outside the network coverage.
• Switching between two different communication paths: This is a very important use case of 3GPP Release 12. In this case, the operator can easily switch the user traffic from an infrastructure path to a ProSe communication path. Therefore, all the proximity criteria can be dynami-cally controlled by the operator.
• ProSe-Based WLAN and WiFi-Direct: In this use case, there is a direct communication between ProSe-ena-bled UEs and WLAN under WiFi-Direct communica-tions. Here, the operator can switch the network session between infrastructure path and WLAN ProSe commu-nication path.
Apart from public safety, various other use cases for D2D communication are envisaged in the 3GPP standards. We enlist some of them below.
• Multiuser cooperative communication (MUCC) in Het-Nets: a simplified diagram of MUCC is given in Fig. 5. In this case, there are two users categorized as the ben-efited user and the supporting user. The benefited user lies under a weak network signal, whereas the supporting user is under a strong network signal; the latter helps the former in improving its signal. There are two paths defined, one is from the benefited user to the eNB and another from the supporting user to the small cell. The benefited user and the supporting user communicate with each other using LTE-A D2D communication.
• D2D offloading: D2D provides opportunistic data off-loading facility that can reduce the overall network over-
Fig. 4 Use cases for D2D communication in 3GPP Release 12
Fig. 5 MUCC communication (from [36])
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head. This also allows the network operators to save the spectrum resources and optimize the downlink transmis-sion overhead for the network operators. If one UE with a poor channel conditions identifies a neighboring UE in its close proximity with a good channel condition, the former can offload its data to the latter using D2D com-munication and data can be relayed further with the help of cellular communication [43, 44].
• V2V communication and V2X communication (LTE-V): In 3GPP standardization, D2D-based V2V network is an active area of investigation. A feasibility study has been conducted in Release 13. A vehicle running at high speed can warn nearby vehicles using D2D links before it changes the lane. Depending on the received message, the nearby vehicles can slow down and avoid accidents. Based on the Release 12 sidelink communication pro-tocols, Release 14 specified V2X communication in the year 2017. V2X in 3GPP release 14 supports vehicle-to-vehicle (V2V), vehicle-to-pedestrian (V2P), vehicle-to-infrastructure (V2I) and vehicle-to-network (V2N) communication [14]. Both cellular communication and D2D communication are part of LTE-V services. Vehicle platooning, automated and remote driving are some of the extended features of V2X communication supported by 3GPP release 15. In comparison to conventional cel-lular communication, the LTE-V is somewhat different in terms of deployment and traffic characteristics. Inter-ference coordination, collision avoidance, and efficient resource allocation are some of the biggest challenges in LTE-V communication.
• Security in data communication: D2D communication allows uses to receive data that are saved in the nearby trusted UEs. This avoids accessing potentially insecure links to third-party databases like those in the cloud. Hence, it can provide a great deal of security.
• Machine type communication or MTC: D2D commu-nication protocols specified in LTE-A 3GPP standards provides a promising solution for MTC. 3GPP Release 13 and above allow cellular operators to multiplex the bandwidth between MTC devices and regular devices [45].
• Indoor installation and positioning: Indoor positioning is one of the key features in 3GPP Release 13. Earlier the accuracy level of indoor positioning was low and it was a great concern for the operators. Basically, in indoor conditions multipath propagation is difficult. Therefore, 3GPP is consistently working to facilitate both location service and mission critical voice service on LTE devices based on the ProSe D2D standard.
• Enhancement of D2D for UE-to-network relaying: The 3GPP release 15 focuses on the enhancements of D2D communication for network relaying.
The main aim is to retain the connectivity between the remote devices like sensors or smart wearables and the network through a relay-assisted D2D communication.
• Maritime communication: Activities on 3GPP Release 16 focus on the development of a maritime communica-tion system known as LTE-Maritime. The overall cov-erage area will be up to 100 kilometers and there will be seamless communication between the existing 3GPP system and the maritime radio communication system. It will also support voice communication and data com-munication between different vessels at sea [19]. This inter-vehicular communication can occur using D2D communication technology.
Reference Architecture for D2D Communication
Figure 6 gives a detailed architecture of 3GPP Release 12 that identifies the main components and the reference points of the system. Some of the most important entities of the given architecture are
1. ProSe App server: The ProSe App server consists of the basic capabilities of ProSe (e.g., public safety answering point or PSAP) used for public safety or various com-mercial use cases. The application server can directly communicate with an application defined in UE and it is generally defined outside of the 3GPP architecture.
2. ProSe UE App: It is an application in a UE that can use ProSe capability.
3. ProSe Function: The ProSe Function acts as the refer-ence point for ProSe App Server, EPC and UEs. Verifi-cation, authorization and configuration of UEs are the basic functionalities of ProSe Functions. It also allows EPC level discovery for direct communication between devices.
As discussed in Sect. 3, the use cases for D2D commu-nication not only support public safety services but also can be very helpful for social and commercial applications. Release 12 supports urgent D2D communications especially for public safety communications. Some of the important functionalities are
1. It gives the complete permission to the Home Public land mobile network (HPLMN) operator to authorize all the ProSe-enabled UEs to use the ProSe communica-tion path separately for HPLMNs and visited public land mobile networks (VPLMNs).
2. The architecture of 3GPP Release 12 is so well designed that it can control the ProSe-enabled UEs communicat-
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ing together, irrespective of whether they are being served by the same eNB or different eNBs.
3. It grants permission to the operator to authenticate and authorize the third-party applications before they use the ProSe features.
Therefore, to support the above conditions various reference points are given in the architecture shown in Fig. 6.
1. PC1: In D2D communication, various types of ProSe applications can be installed and used in a UE. The PC1 interface allows the ProSe applications to exchange data with the ProSe application server.
2. PC2: The PC2 interface normally allows ProSe Func-tion to update its ProSe application data for ProSe database. This will help the EPC to support all the ProSe functionalities used for direct communication.
3. PC3: The PC3 is the reference point between the UE and ProSe Function; it acts as an interface for D2D discovery and communication. If due to any reason two devices are connected to different PLMNs, the D2D discovery mechanism will completely rely upon PC3.
4. PC4: The communication between the ProSe Func-tion and the EPC occurs through PC4. Session control and mobility management are also handled by the PC4 interface.
5. Uu: The Uu interface supports uplink and downlink transmissions between the eNB and a UE.
6. PC5: To enable direct communication between a pair of UEs, the PC5 interface is used. It allows D2D communication in scenarios like in-coverage, out-of-
coverage and partial-coverage. PC5 supports sidelink transmission at layer 1.
7. PC6: If the UEs are registered under different PLMNs and with multiple ProSe Functions, then the D2D dis-covery mechanism can be done using the PC6 inter-face.
8. PC7: It is the interface between different ProSe func-tionalities in the visiting PLMN and the home PLMN. It also helps the home PLMN to handle the authoriza-tion of ProSe services.
9. PC8: The PC8 interface is used for ProSe Function in HPLMN to configure D2D communication of UEs. It acts as an interface between the ProSe Function of HPLMN and a roaming UE.
10. S1: It is the interface between the eNB and the EPC. 11. SGi: SGi is the interface used for exchange of applica-
tion level data and control information between the EPS and the ProSe application server.
Thus, UEs communication between a pair of UEs is executed over the PC5 interface. If a UE is out of the coverage area of eNB, but the signal strength is good enough for the UEs to communicate with each other, then they can exchange data directly among themselves by creating an ad hoc network; here, UEs communicate using a lower level interface called PC5ah [30].
Fig. 6 D2D architecture in 3GPP specification
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Basic Channel Structure
In the previous section, we have seen that UEs can com-municate directly with each other using the sidelink (SL) (which corresponds to the PC5 interface in terms of the ref-erence architecture). It is a new feature in 3GPP Release 12 and is distinct from the uplink (UL) and downlink (DL) on the Uu interface over which a UE communicates with the eNB.
Reuse of Uplink Resources
The SL supports a new set of physical signals, physical chan-nels, transport channels, logical channels, and messages. Since a UE transmits on the SL, the latter closely resembles the UL. Before we discuss the details of SL, we note that broadcast in D2D communication reuses the Physical Uplink Shared Channel (PUSCH) through the eNB-UE (Uu) link in LTE. The reasons are as follows. When the eNB is receiv-ing data in PUSCH from a UE, another UE in proximity can engage in direct communication with its peer if the interfer-ence caused at the latter is not too high. The interference that the transmitter UE in D2D communication causes at the eNB can be handled by the latter as it is considerably more powerful. Instead, if D2D communication reuses the Physi-cal Downlink Shared Channel (PDSCH), the D2D UE trans-mitter will produce high co-channel interference to the UE receiving data on the downlink from the eNB. Moreover, UL frames are usually less utilized than DL frames. So using the UL will increase spectral efficiency. PUSCH uses single car-rier frequency division multiple access (SC-FDMA) whereas PDSCH uses orthogonal frequency division multiple access (OFDMA) that has a higher PAPR than that of SC-FDMA. Hence, using PUSCH incurs lower power consumption [30]. The downside is that constructing an SC-FDMA receiver is more difficult than constructing an OFDMA transmitter as the former needs a complex equalization. An important type of signal is the DeModulation Reference Signal (DMRS), transmitted on a fixed symbol position in each slot (in time domain) of PUSCH, that a receiver UE uses to estimate the channel. DMRS signals are also associated with the SL channels defined below (Fig. 7).
Physical Channels
The physical channels or layer 1 channels that are part of the SL are
1. Physical Sidelink Broadcast Channel (PSBCH): If the eNB and the UEs in a cellular network are synchronized to the same timing reference, the network usually per-
forms more efficiently (e.g., power consumption and interference would be low) than if they are not synchro-nized. The primary/secondary synchronization signal (PSS/SSS) is the synchronization signal sent out by the eNB in LTE, and is also used as synchronization sig-nal for D2D communication if the UEs are within the eNB coverage. Instead, if both the UEs are not inside the eNB coverage, they can still synchronize to the Primary/Secondary Sidelink Synchronization Signals (PSSS/SSSS) transmitted by one of the UEs; if one of the UEs is in network coverage, it can send PSSS/SSSS based on the eNB synchronization signal. The eNB or the UE that transmits its own synchronization signal is called a D2D synchronization source. The PSSS/SSSS is sent out on the PSBCH. The PSBCH from a UE is also used to broadcast information like the ID and type (i.e., whether it is eNB or UE) of the synchronization source, the system bandwidth, the TDD or FDD configuration, the frame and subframe number of the synchronization signal, and whether the UE lies in network coverage.
2. Physical Sidelink Discovery Channel (PSDCH): A UE discovers another UE in its proximity with the discovery signal in layer 1, transmitted via the PSDCH. Note that the discovery procedure is initiated by the application layer of a UE. The radio resources for transmitting the discovery signal may be shared by all UEs or may be allocated on a per-UE basis by the eNB. Only the first of the two methods can be used when the UEs are outside the eNB coverage.
3. Physical Sidelink Control Channel (PSCCH): PSCCH is responsible for uplink transmission control. The D2D transmitter transmits a sidelink control information (SCI) that includes modulation and coding schemes, the frequency hopping flag, the resource block allocation,
Fig. 7 D2D channel structure
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the hopping resource allocation, and the timing assistant to differentiate between the uplink time and downlink time [40].
4. Physical Sidelink Shared Channel (PSSCH): A UE sends data to other UEs in D2D communication via the PSSCH. The PSSCH follows the PSCCH in time domain. The SCI message transmitted in the PSCCH announces the PSSCH resources where the data trans-mission will occur. For in-coverage UEs, PSSCH resources may be allocated by the eNB, or selected ran-domly and autonomously by the UEs from a resource pool pre-configured by the eNB for this purpose. In case of out-of-coverage UEs, PSSCH resources are chosen randomly by the UEs from a resource pool. There is a possibility of conflict when resources are chosen autono-mously by the UEs.
Transport Channels
1. Sidelink Broadcast Channel (SL-BCH): SL-BCH is the transport channel that maps to the PSBCH in the physi-cal layer. The SL-BCH carries some important informa-tion needed for out-of-coverage devices to setup side-link connectivity. The required information is carried through Sidelink Master Information Block (SL-MIB). The information carried out in SL-MIB consists of car-rier bandwidth, TDD configuration, transmitted frame number and in-coverage indicator [8].
2. Sidelink Shared Channel (SL-SCH): SL-SCH follows the same techniques as downlink shared channel. It is a transport that may have chances of collision depending upon the resources assigned through eNB. It also inter-faces physical sidelink shared channel, that transports data from air.
3. Sidelink Discovery Channel (SL-DCH): The SL-DCH belongs to the transport layer of basic channel structure of sidelink communications and is used for discovery announcements. It is directly mapped with PSDCH in physical layer [8]. Depending on the sidelink transport format, the sidelink physical layer processing is con-figured and processed by the UE. The basic function of SL-DCH is the scheduling of resource allocation. If the scheduled resources allocation of SL-DCH is non-autonomous, then the complete scheduling is done by the network side, else the scheduling is done by the UE side [11].
4. Sidelink Control Information (SCI): The SCI gener-ally carries all the relevant information required for the receiving UE. It also helps the UE in demodulating the physical sidelink shared channel (PSSCH).
Logical Channels
1. Sidelink Traffic Channel (STCH): Side link traffic chan-nel is a point-to-multipoint logical channel that is used to transfer user information from one UE to other UE.
2. Sidelink Broadcast Control Channel (SBCCH): SBCCH is a logical channel that broadcasts the sidelink system information from one UE to another UE.
Important Features of D2D Communication in 3GPP
To enable efficient direct transmission between two UEs, it is very important that they are synchronized and can dis-cover each other easily. Hence, D2D synchronization, D2D discovery, and direct communication are the most important features in 3GPP specification. In this section, we explain them in detail.
D2D Synchronization
The synchronization procedure for D2D communication in 3GPP Release 12 consists of a few simple steps [30]: (1) After a UE powers on, if it detects a synchronization signal (PSS/SSS) from an eNB, it synchronizes to it. (2) Otherwise, if it detects synchronization signals (PSSS/SSSS) and PSBCH from other UEs, it synchronizes to the signals from one of them. (3) Otherwise, it becomes a D2D syn-chronization source and transmits synchronization signals and PSBCH. The first case occurs when the UE is under the coverage of an eNB while the other cases occur if the UE is not under the coverage of an eNB. In case (1), if a UE has synchronized to an eNB but has detected synchronization signal and PSBCH from a UE (as indicated by the source type in the synchronization signal), it understands that there is an out-of-coverage UE that is acting as a synchronization source. Hence, it becomes a D2D synchronization source itself so the out-of-coverage UEs can synchronize to the eNB. The above cases are illustrated in Fig. 8.
D2D Discovery
A UE has to discover a neighboring UE in close proximity before it can communicate with it directly. Alternatively, D2D discovery can be used as a standalone service enabler, e.g., it can be used to get information like whether there is a vehicle nearby; once the appropriate information is discov-ered from a nearby UE, the discoverer can use it for other applications provided it is permitted to do so. Device dis-covery is essentially an application layer procedure where specific messages are exchanged between two UEs. At layer 1, UEs use PSDCH for device discovery. The device
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discovery techniques are categorized into the following two types based on whether the eNB assists it.
1. Network-assisted discovery or EPC-level discovery: In EPC-level discovery, the eNB acts an entrusted facilita-tor in determining the proximity of UEs. The proximity of the UEs is derived from the frequent location updates.
As shown in the Fig. 9, UE A makes a proximity request, i.e., a request to be alerted with the proximity of UE B. In response to the request of UE A, ProSe Function A requests the Secure User Plane Location Platform (SLP)-A about the location updates of both UE A and B. To generate the location of UE B, ProSe Function A contacts ProSe Function B. The generated report is then forwarded to the EPC. The EPC informs the requesting devices about their proximity and initiates a proximity alert in PC3 interface [12, 40]. Clearly, the main task here is to analyze the location of UEs in the network and alert the corresponding the interested users. This task is managed by the EPC. Thus, EPC-level device discov-ery reduces periodical announcements for the UEs and, hence, their power consumption. The above network-assisted discovery mechanism can work only if network coverage is available. When coverage is not available, direct discovery may be performed.
2. Direct discovery: The network-assisted discovery mech-anism can work only if network coverage available. If the network access fails the D2D discovery procedure cannot sustain [60]. Hence, to overcome such type of problem direct discovery mechanism has been provided in 3GPP Release 12. The direct discovery procedure can work for both in-coverage and out-of coverage scenarios. It is the procedure by which a UE detects and identifies
Fig. 8 D2D synchronization
Fig. 9 D2D EPC-level discov-ery (from [40])
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another UE physically close to it, using E-UTRA direct radio signals. Depending on whether the discoverer needs explicit permission from the discovery, it is of two types:
(a) Open discovery: No special authorization is needed by the discoverer UE from the remote UE being discovered.
(b) Restricted discovery: For discovering the remote UE, explicit permission is needed by the discov-erer from the remote UE.
In direct discovery, there are two discovery models—Model A and Model B—and corresponding messages for device discovery.
(a) Model A (“I am here”): In this model, the announcing UE broadcasts the discovery mes-sages at predetermined intervals of time. Monitor-ing and announcement are two important steps in direct discovery. In the announcement procedure, the UEs intended to discover the nearby devices announce the required information. The other UEs in the given proximity with discovery authoriza-tion can use this information. The interested users can monitor and process them as shown in Fig. 10. Model A supports both open and restricted discov-ery mechanisms.
(b) Model B (“who is there?” / “are you there?”): In this model, the UE that needs to discover informa-
tion broadcasts a message containing the infor-mation it needs. Another UE that receives the message can respond to the discoverer with the relevant information as shown in Fig. 11.
Generally, the transmission and reception of the discovery packets in D2D communication are very complex. This is because of the size of the discovery packets. If the size of the discovery packet is small, the UEs may be able to transmit certain sequences of discovery information. Even though for the smaller packets, the information regarding the UEs may be limited but the complexity is relatively low. If the discovery information packet is too large, sending periodical beacons may be too difficult for the UEs. Hence, the trans-mission of larger discovery packets is difficult task.
Direct Communication
3GPP standards describe the procedures for direct communi-cation for in-coverage, partial-coverage, and out-of-coverage scenarios. The control signals for direct communication are exchanged using PSCCH, while the data are transmitted using PSSCH via the PC5 interface at layer 1. The communication can be 1:1 (i.e, unicast) or 1:many (i.e., broadcast and group-cast). In 1:many communication, groupcast is supported with a destination Group ID at layer 2. Further, 3GPP Release 13 supports UE-to-network relay at layer 3; this two-hop com-munication helps extend the coverage of a network. UE pro-visioning for direct communication requires several network
Fig. 10 D2D direct discovery Model A for in-coverage sce-nario (from [40])
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parameters like ProSe layer-2 Group ID, ProSe Group IP mul-ticast address, security parameter for group communication, whether the UE is authorized to perform direct communica-tion when out-of-coverage, and the radio parameters to use when the UE is out-of-coverage. The network performs the provisioning when the UEs are in-coverage. To handle out-of-coverage scenarios, values for these parameters may be stored in the device or the USIM (Fig. 12).
Critical Discussion
The technical specifications provided in 3GPP standards are very well organized and globally approved. After a deep survey on recent standardization efforts, we find that they are many critical issues and scope of improvement in D2D com-munication. We also highlight the benefits that D2D com-munication can afford in the upcoming 5G cellular network.
Important Issues in D2D Communication
In this subsection, we discuss various challenges in D2D communication [6, 50].
1. User mobility: To handle the mobility in D2D com-munication, a lot of time has been invested in profi-cient handoff (or handover) selection. Handoff can be categorized into two types: (1) horizontal handoff and (2) vertical Handoff. Horizontal handoff takes place within the same type of network and is dependent on the signal strength of the eNB. If the signal strength goes below a given threshold value, horizontal hando-ver takes place. Vertical handoff takes place when user moves across networks, for example, from WLAN to cellular connectivity to access the Internet. It can either be either network initiated or user initiated [21]. Under a dense heterogeneous network where mobility is very high, vertical handover depends on the controlling and signalling frequencies that a UE needs to operate. For UEs to function using controlling and signalling fre-quencies quickly deteriorates the signal to noise ratio
Fig. 11 D2D direct discovery Model B for in-coverage sce-nario (from [40])
Fig. 12 1: many direct traffic in D2D Communication (from [12])
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(SNR) and so is the performance. Hence, selection of appropriate handover criteria is still a question. The standardization mechanism should focus on developing appropriate handover criteria for D2D communication. Generally in D2D communication, UE mobility pos-sesses significant challenges. This is because constant mobility increases the Doppler spread. It can also affect the temporal correlation of shadowing as well as the fast fading techniques used for communication. Therefore, 3GPP standardization techniques should focus on user mobility for efficient D2D communica-tion where the energy consumption can be minimized and the lifetime of devices can be enhanced.
2. D2D synchronization: In the in-coverage scenario, it is sometimes difficult to achieve complete synchroniza-tion. This is because there may be multiple D2D links available and it is confusing to choose between uplink time and downlink time for D2D transmission. Due to different distances of various UEs from an eNB and the availability of multiple eNBs, none of the uplink time and downlink time guarantees a complete synchroni-zation. Synchronization in out-of-coverage scenario is even more difficult as it needs periodic transmission of synchronization signals from UEs, which increases the power consumption of the UEs. There are chances of UEs in out-of-coverage not being synchronized; if a UE that is being searched for by a discoverer UE is not synchronized with the discoverer, i.e., the synchroniza-tion information is not available before discovery, the discovery is likely to fail.
3. D2D discovery signal: D2D discovery signal can be sequence-based in which limited information is con-veyed, or packet-based in which synchronization infor-mation is not limited but the transmission scheme is very complicated. In terms of the discovery mecha-nism, device discovery is classified into two types: EPC-level discovery and direct discovery. EPC-level discovery is a centralized discovery mechanism where the message flow is handled by the eNB. In direct dis-covery which is a distributed approach, the message passing scheme is handled by the devices directly and there is no involvement of eNB [21]. In direct discov-ery, a pilot signal is transmitted by the UEs to dis-cover the nearby UEs in given proximity. For network-assisted discovery, the transmission power of the pilot signal depends on the range of the D2D users inside a given cell [36]. However, continuous transmission of discovery messages affects the performance of D2D communication as it consumes power and may lead to interference with other devices present in the same cell. Standardization mechanisms should focus on appropriate scheduling of pilot signals so that other devices in the cell are not affected.
4. Hybrid Automatic Repeat Request (HARQ) proto-col: HARQ protocol is the combination of Automatic Repeat Request (ARQ) and forward error correction. In D2D communication, both direct and indirect HARQ can be used [35]. In indirect HARQ, the D2D receiver transmits the ACK to the eNB. The eNB relays the same message to the D2D transmitter. This increases the overhead of the network. In case of direct HARQ, the D2D receiver transmits the ACK to the D2D trans-mitter directly; however, the uplink and downlink channels cannot be utilized. D2D standardization ini-tiatives should focus on incorporating HARQ protocol in D2D communication so that the robustness of D2D communication is enhanced.
5. Resource allocation: Generally efficient resource allo-cation schemes and the reuse of resources enhance the scalability of D2D network. If resources can be managed effectively, the power consumption can be minimized, the throughput can be enhanced, and the interference can be reduced. Both orthogonal and non-orthogonal resource allocation schemes are easier to manage in static conditions than in dynamic condi-tions. Public safety communication typically uses dedicated resources (spectrum), while commercial D2D services share the spectrum with cellular users. The problem of resource allocation gets more com-plicated in a dense network where many base stations are involved. Involvement of multiple number of base stations makes it more difficult for D2D users to share the underlay spectrum [21, 38]. Therefore, upcom-ing releases in 3GPP standardization should focus on developing a versatile resource allocation strategy that can be adopted under various network conditions.
6. Mode selection: D2D-capable users in a cellular net-work can communicate using D2D mode or infrastruc-ture mode (i.e., conventional cellular communication using eNB) [16]. In D2D mode, cellular and D2D users can share resources (reuse mode) or can have their own dedicated resources (dedicated mode). Mode selec-tion is a mechanism by which D2D users choose an appropriate mode of communication. Mode selection criteria for D2D communication are not clearly defined in 3GPP standards. Mode selection possesses several challenges. We detail two of them below:
(a) High overhead: The selection of the mode of com-munication depends upon various factors like channel quality, distance, and channel state infor-mation. Every time the channel quality alters, the mode of communication could potentially change. But frequent alteration of modes generates an enormous amount of overhead in the network;
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therefore, mode switch is generally activated infrequently [21, 38].
(b) Mode selection in dynamic conditions: Most of the existing literature focuses on mode selection in a static networking environment. The selec-tion of the mode of communication under highly dynamic conditions is more complex and needs to be adequately explored [38].
7. Interference management: In in-band communication, cellular and D2D links can interfere with each other, depending on the frequency sharing mechanisms [38]. We illustrate the situation in Fig. 13 where UE1 (D2D transmitter) and UE2 (D2D receiver) are in close prox-imity and intend to communicate using a direct link. For communication, the devices can reuse the licensed spectrum allocated for the cellular user. In this type of scenario, the connection establishment between UE1 and UE2 will be controlled by the eNB. Higher spectral efficiency and lower power consumption are some of the benefits in a scenario. We describe the interference scenario depicted in Fig. 13. Interference is caused by D2D users to cellular user and vice versa. When UE1 transmits data to UE2, the eNB may also broadcast data to the cellular UE. In such a condition, two types of interference arise
• Case 1: There will be interference from UE1 to cel-lular UE. It is also quite possible that the UE2 may face interference due to eNB. This affects the overall performance of the network. Both D2D users and cellular user suffer due to interference. The transmis-sion power of the UE1 is much lower as compared to eNB; therefore, the interference from UE1 to cellular UE will result in inconsequential loss in terms of performance but UE2 will be significantly affected.
• Case 2: While sharing the downlink resources, the eNB acts as an aggressor. As eNB is more powerful
compared to the D2D users, there is a strong interfer-ence to D2D users.
Similarly, if multiple D2D users under a given cell share the same resources, there will be co-channel interference from the transmitting D2D user of one pair to the receiving D2D user of another pair [33]. There-fore, interference between cellular users and D2D users holds a great risk. In in-band D2D communications, there are several approaches proposed by the research-ers to avoid the interference. We have enlisted some of them below.
(a) Power control: At higher transmission power, the D2D users can cause significant interference to cellular communication. If there is interference from D2D UEs to cellular UEs, the power of the D2D UEs can be reduced to mitigate the interfer-ence [33, 54]. The power control mechanisms can be used in both static and dynamic manner. In the static case, the power and signal-to-interference-plus-noise-ratio (SINR) stay fixed for D2D users, whereas in dynamic state, open loop power con-trol and closed-loop power control schemes are provided. In the initial scheme, the D2D users can fine-tune the power based on some of the prede-fined system parameters as mentioned in [53]. In closed loop power control scheme, the D2D users can adjust their power in coordination with the eNB. An in-depth study of power control schemes provided in [38]. The power control scheme com-bined with efficient mode selection technique and resource allocation mechanisms can be more effective in avoiding the interference in D2D ena-bled cellular communications.
(b) Mode selection: Mode selection can play a vital role in avoiding the interference from D2D UEs to cellular UEs and vice versa. Although the D2D users may be in close proximity, looking at the level of interference, operation in the D2D mode may not be the best idea. The mode selection scheme allows the D2D users to choose the best mode of communication under a cellular net-work. The multiple D2D pairs operating under the same cell using D2D mode of communication use the same frequency resources. Hence, there is an interference between the D2D users. To over-come such a problem, the D2D users can evaluate the D2D spectrum and fix an energy consump-tion threshold. If the evaluated energy goes below the threshold, the D2D users can communicate using D2D mode, else they can go to the cellular mode of communication [7]. Mode selection is Fig. 13 Interference between D2D and cellular transmissions
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an integral part of D2D communication. Various approaches for selection of an appropriate mode of communication, interference avoidance using modes selection and recent advances of mode selection in D2D communication are presented in [16, 21, 38].
(c) Spectrum splitting: In a D2D enabled cellular communication, spectrum splitting is one of the simplest mechanisms to avoid interference. Divid-ing the spectrum into two parts where one part is dedicated to the D2D users and another part is dedicated to cellular users can reduce the interfer-ence among the cellular users and D2D users [46]. Spectrum splitting mechanism does not work well when multiple D2D pairs communicate under the same cell using the same resources.
Apart from power control and mode selection schemes, several other approaches like radio resource allocation, implementation of multiple input multiple output (MIMO) and enhanced antenna schemes, joint mode selection and power allocation, and channel con-trol are available to avoid interference in in-band D2D communication. [16, 21, 33, 38]. In out-band com-munication, D2D users operate using unlicensed spec-trum. Here, the communication between the D2D users can either be network assisted or autonomous. One of the major benefits of out-band D2D communication is that there is no interference between D2D users and cellular users. However, WiFi and Bluetooth operate in the same unlicensed spectrum. Simultaneous opera-tion of multiple entities at the same frequency band generates interference. Under LTE-Unlicensed (LTE-U) spectrum, interference avoidance using clear chan-nel assessment (CCA) or listen before talk (LBT) has been presented in [52]. The suggested mechanism says that an unlicensed D2D UE before transmitting should perform a carrier sensing mechanism. By doing this, the unlicensed UE can identify the other users operat-ing on the same channel. The UE measures the energy level of the channel due to other unlicensed UEs. If the energy level is lower than a threshold, the UE transmits a channel occupancy time (COT), else it waits for a random period of time after which it retries.
8. LTE-U or LTE Licence-Assisted Access (LAA): Licensed spectrum and operation control provide advanced services and good user experience in mobile communications. However, D2D communication in licensed band using the cellular UL/DL resources cannot operate close to the eNB as the eNB transmits with high power on the DL, and near the cell edge where the UEs transmit with high power on the UL. In contrast, if D2D communication uses LTE-U, it
can operate almost over the whole coverage area of the eNB (except where other devices using same unli-censed band are in operation). At the same time, it can achieve an improved network-wide capacity [52]. 3GPP Release 13 includes study item (SI) to study the feasibility in LTE-A to apply and enable LAA applica-tion in unlicensed spectrum, and work item (WI) that specifies the LTE enhancements to be applied in unli-censed spectrum [1, 13, 26].
9. MIMO: MIMO can help enhance the spectral efficiency in LTE-A. Although MIMO with up to 8 antenna ports is common, 3GPP Release 13 introduced Full Dimen-sional MIMO (FD-MIMO) which utilizes up to 64 antenna ports at the transmitter side [22]. FD-MIMO is hailed as an important enabling technology in the evolution from 4G to 5G [28]. Implementation of large antenna arrays at eNB can allow the D2D users and the cellular users to transmit simultaneously at the same time–frequency resources without interference [15].
10. Superposition coding: wireless systems have tradition-ally used orthogonal multiple access techniques like frequency division multiple access, time division mul-tiple access, etc. In non-orthogonal multiple access, UEs transmit using non-orthogonal resources. This increases the spectral efficiency but there is a chance of interference at the receiver. 3GPP Release 13 has identified LTE enhancements to enable downlink mul-tiuser superposition transmission [10], which uses the concept of superposition coding. For example, if there are two UEs, one at the cell center and the other at the cell boundary, the downlink transmissions to both of them can be scheduled using the same beam. However, the transmit power allocated in the first case is lower than that in the second case. One way to use superpo-sition coding in D2D communication is to make the D2D link act as an in-band relay to cellular link [47], i.e., the D2D link shares the radio resources with a cel-lular downlink. A D2D transmitter acting as an in-band relay can transmit its own data linearly combined with the data from the eNB for a remote cellular user who is far away from the eNB. This technique can improve both spectral efficiency and network coverage.
11. Security: Security in D2D communication is one of the most important aspects that need a consistent focus. In comparison to traditional cellular communication, D2D communication is highly vulnerable to security threats because D2D communication bypasses the core network. It is prone to various security attacks like eavesdropping, man-in-middle, and impersonation. In network-assisted D2D communication, cellular opera-tors can provide security to the D2D users. But for D2D users outside the coverage area, it is difficult to provide desired security measures. In such scenarios,
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relay-assisted D2D communication can be used and the security signals can be relayed from the eNB to the D2D users. However, the relayed signals might be vulnerable to security attacks. In 3GPP standardiza-tion, the discussion related to security is very limited although many researchers have advanced various schemes to address the security needs of D2D com-munication. In particular, D2D users must satisfy the basic security requirements such as confidential-ity, authorization, non-repudiation, availability, and dependability [18, 55, 56]. We have enlisted some of the proposed schemes below.
(a) Key management: Key management scheme is one of the common cryptographic mechanisms used in network security. To attain a secured data communication between two D2D users in an LTE-A cellular network, a data sharing protocol is designed in [55]. The protocol uses both public key cryptography and symmetric encryption. Uti-lization of public key cryptography ensured UE authentication, data authority and integrity while using symmetric key encryption guaranteed data confidentiality. The man-in-middle attack is a common security threat in case of out-of-coverage D2D communication. In such situations, efficient key management schemes can be very helpful for secured communication. A secret key can be shared between two D2D users and authentication can be attained by verbal or visual comparison of the secret key [48].
(b) Physical layer security: To ensure a secured com-munication between D2D users, it is very impor-tant to secure the physical layer of D2D commu-nication. The physical layer deals with coding and modulation, antennas, and characteristics of wire-less channels [17].The wireless channel proper-ties can be known from channel state information (CSI). A secret key generation mechanism using CSI has been proposed in [2]. To set up the shared secret schemes in between two D2D users in the physical layer, a cooperative key generation mech-anism has been presented in [27]. By securing the physical layer, the confidentiality and integrity of D2D communication can be enhanced.
(c) Secured routing: To provide security measures to out-of-coverage D2D users, relay-assisted D2D communication can be used. As relay assisted D2D communication is more prone to security threats, it is important to choose a secure routing mechanism between the source and the destina-tion. The secure message delivery protocol pre-sented in [42] can detect the malicious messages
in each route. The protocol also provides support for non-repudiation and user authentication. For a secured unlicensed D2D communication in 5G communication, a combined operation of routing control and group key management is presented in [23]. The proposed approach uses the home IP address of the ad hoc nodes to manage the group key and performs authentication using the public key infrastructure of cellular networks.
Most of the D2D security-related proposals in the literature deal with specific security issues and are not integrated into a single protocol. Especially for out-of-coverage scenario, most of the algorithms pro-posed are quite equivalent to the security algorithms for MANETs and wireless sensor networks [17]. Pri-vacy preservation and appropriate security measures for multi-hop D2D communication still need proper investigation and analysis. As D2D communication is likely to be an integral part of 5G cellular networks, it is important to design appropriate security protocols for D2D users [16, 39, 59].
D2D Communication in 5G Networks
Since 3GPP Release 12, 3GPP is publishing a release roughly once a year. As far as 5G mobile systems are con-cerned, 3GPP is working according to the timeline of Inter-national Telecommunication Union—Radio Communication (ITU-R) to complete the standardization process. Currently, 3GPP is focusing on the Radio Access Network (RAN) and the core aspects of 5G. The standardization phase of 5G is divided into two parts. The first part focuses on the deploy-ment requirements and second part focuses on enhancing the capabilities of 5G wireless communication systems. After significant efforts, the first complete set of 5G standards was published in 3GPP Release 15 in the year 2018. It cov-ers the 5G New Radio (NR) system. 3GPP’s focus has now shifted to the first stage of Release 16 because Release 15 is mature and near closure. Release 17 work will proceed mainly through the years 2020 and 2021. Some important areas of interaction between 5G and D2D communication are the following.
1. mmWave communications: In upcoming 5G cellular networks, mmWave communications is a promising and one of the most emerging technologies. It can oper-ate between 30 GHz and 300 GHz frequency band and thereby offer higher data rate to 5G enabled mobile devices. One of the major demerits of mmWave com-munications is its shorter wavelength. Due to the shorter wavelength, the difficulty in penetrating the nearby obstacle increases. In such a scenario, however, relay-
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assisted D2D communication can be very helpful. Estab-lishment of a D2D relay between two mobile devices and between mobile devices and a mmWave enabled base station can be done to extend the overall coverage area even if these devices do not have a clear connecting line of sight (LOS) [49]. If D2D communication can be integrated properly with mmWave communication, the overall network capacity can be enhanced.
2. D2D communication in social networks: The volume of mobile data will be way higher in 5G communication as of now. The majority of data will be carried on by social networks. Analysis of the social ties, users’ simi-larities and their interest can be helpful in identifying their geographical locations [5]. D2D communication together with the social network can ensure a trusted connection setup [57]. Data dissemination which is one of the important features of D2D communication can be used in social networks for effective content sharing between the users with similar interest. This can weigh down the load of the BS and can increase the spectral efficiency as well [57]. The combination of D2D net-works and social networks can be a great boon for the proximity services in 5G cellular communication [3]. For that purpose, D2D discovery mechanisms could be designed exploit social networks at the application layer.
3. D2D communication in ultra-dense networks: Ultra-den-sification of nodes is expected in 5G cellular networks. Hence, it is considered as one of the major paradigms of 5G communication. In an ultra-dense network, a large number of active small cells will be deployed to increase communication efficiency. To have convenient traffic offloading, D2D communication can be integrated with small cells to reduce the load on the eNB [49].
Conclusion
D2D communication is going to play a major role in the upcoming 5G cellular network. In this article, we have pre-sented a brief overview on the standardization of D2D com-munication. The standardization is based on 3GPP Release 12 and subsequent releases. We have discussed in detail the architecture, discovery procedure, synchronization, and direct communication technique of D2D communication. We have also highlighted some critical issues in D2D com-munication. It is expected that D2D communication will play a pivotal role in both public safety and various com-mercial use cases in future cellular networks.
Funding This study is partially funded by the Ph.D. scholarship offered by KIIT Deemed University to the first author.
Compliance with Ethical Standards
Conflict of Interest The authors declare that they have no conflict of interest.
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