An overview of device-to-device communication in

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An overview of device-to-device communication in cellular networks Udit Narayana Kar, Debarshi Kumar Sanyal ∗ School of Computer Engineering, KIIT University, Bhubaneswar-751024, India Received 10 May 2017; accepted 18 August 2017 Available online xxxx

Abstract Device-to-device (D2D) communication is expected to play a significant role in upcoming cellular networks as it promises ultra-low latency for communication among users. This new mode may operate in licensed or unlicensed spectrum. It is a novel addition to the traditional cellular communication paradigm. Its benefits are, however, accompanied by many technical and business issues that must be resolved before integrating it into the cellular ecosystem. This paper discusses the main characteristics of D2D communication including its usage scenarios, architecture, technical features, and areas of active research. c 2017 The Korean Institute of Communications Information Sciences. Publishing Services by Elsevier B.V. This is an open access article under ⃝ the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Device-to-device communication (D2D); Cellular network; 5G; Resource management; LTE direct

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Introduction ............................................................................................................................................................................................................................... Use cases ................................................................................................................................................................................................................................... 2.1. Local data services ......................................................................................................................................................................................................... 2.1.1. Information sharing ........................................................................................................................................................................................ 2.1.2. Data and computation offloading ...................................................................................................................................................................... 2.2. Coverage extension ........................................................................................................................................................................................................ 2.3. Machine-to-machine (M2M) communication..................................................................................................................................................................... Architecture ............................................................................................................................................................................................................................... 3.1. Spectrum allocation........................................................................................................................................................................................................ 3.1.1. Inband D2D communication ............................................................................................................................................................................ 3.1.2. Outband D2D communication.......................................................................................................................................................................... 3.2. D2D communication in LTE-Advanced ............................................................................................................................................................................ 3.3. Single-hop and multi-hop networks .................................................................................................................................................................................. Challenges and ongoing research .................................................................................................................................................................................................. 4.1. Synchronization ............................................................................................................................................................................................................. 4.2. Peer discovery ............................................................................................................................................................................................................... 4.3. Mode selection .............................................................................................................................................................................................................. 4.4. Resource allocation ........................................................................................................................................................................................................ 4.5. Interference management ................................................................................................................................................................................................ 4.6. D2D with mobility ......................................................................................................................................................................................................... 4.7. Pricing .......................................................................................................................................................................................................................... 4.8. Security ........................................................................................................................................................................................................................

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∗ Corresponding author.

E-mail addresses: [email protected] (U.N. Kar), [email protected], [email protected] (D.K. Sanyal). Peer review under responsibility of The Korean Institute of Communications Information Sciences. https://doi.org/10.1016/j.icte.2017.08.002 c 2017 The Korean Institute of Communications Information Sciences. Publishing Services by Elsevier B.V. This is an open access article under the 2405-9595/⃝ CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: U.N. Kar, D.K. Sanyal, An overview of device-to-device communication in cellular networks, ICT Express (2017), https://doi.org/10.1016/j.icte.2017.08.002.

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D2D communication and 5G networks .......................................................................................................................................................................................... 5 Conclusion................................................................................................................................................................................................................................. 6 References ................................................................................................................................................................................................................................. 6

1. Introduction Cellular network is now four generations old. Need for fast multimedia-rich data exchange along with high quality voice calls has been the primary motivation in this forward journey. As newer and more demanding applications arise and subscriber base increases exponentially, there is an urgent requirement for more novel techniques to boost data rates and reduce latency. D2D communication is a new paradigm in cellular networks [1]. It allows user equipments (UEs) in close proximity to communicate using a direct link rather than having their radio signal travel all the way through the base station (BS) or the core network. One of its main benefits is the ultra-low latency in communication due to a shorter signal traversal path. Various short-range wireless technologies like Bluetooth, WiFi Direct and LTE Direct (defined by the Third Generation Partnership Project (3GPP) [2]) can be used to enable D2D communication. They differ mostly in the data rates, distance between 1-hop devices, device discovery mechanisms and typical applications. For example, Bluetooth 5 supports a maximum data rate of 50 Mbps and a range close to 240 m, WiFi Direct allows up to 250 Mbps rate and 200 m range while LTE Direct provides rates up to 13.5 Mbps and a range of 500 m [3]. D2D connectivity will make operators more flexible in terms of offloading traffic from the core network, increase spectral efficiency and reduce the energy and the cost per bit. Fig. 1 illustrates how cellular communication and D2D communication function. Till recently D2D communication did not appear financially viable to cellular network providers. But the current boom in context-aware and location discovery services is bringing a rapid change to this situation [4]. Readers will find a list of authoritative surveys and original research on D2D communication in [5]. We do not attempt another survey here but only provide a high-level tutorial-style overview of the field. 2. Use cases Using D2D communication, a large amount of data can be transferred quickly between mobile devices in short range. We mention below some of the more common scenarios where D2D communication is an effective technique. 2.1. Local data services D2D communication can support local data services very efficiently through unicast, groupcast and broadcast transmissions. Example applications include the following.

Fig. 1. Cellular communication and D2D communication. Both single-hop and multi-hop (including D2D relay) networks formed by D2D links are shown.

2.1.1. Information sharing UEs can leverage D2D links to transfer files, audios and videos with higher data rates and lower energy than those in conventional cellular channels. They facilitate streaming services like Google Chromecast, IPTV, etc. by forming clusters and groupcasting data within a cluster. They also aid in other proximity services like public safety. D2D links can operate unimpeded in a disaster-hit area where all BSs are paralyzed. 2.1.2. Data and computation offloading A device with a good Internet connectivity can act as a hotspot to which data is offloaded/cached from the BS and from which other devices may download data using D2D links. UEs having poor processing power or low energy budgets may also offload computation-heavy tasks to nearby more capable UEs using D2D links. Considerable research has gone into design of offloading techniques [6]. 2.2. Coverage extension A UE X (e.g., at the cell edge or in a disaster-hit area) may encounter poor signal quality while connecting to the BS. A UE Y close to it that has, however, a better link to the BS may act as a relay for it. Thus a D2D link X − Y followed by a cellular link Y − B S connects X to the BS. In Fig. 1, U E6 acts as a relay between the BS and U E9. Relays are used to extend the coverage of cellular service and enable multi-hop

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communication. Another way to boost signal strength at a receiver is to relay it via multiple parallel paths, each composed of collaborative devices. These techniques are often referred to as cooperative diversity techniques. Researchers have suggested a two-tier cellular architecture to support these applications: a macrocell tier comprising BS-to-device communication and a device tier comprising D2D communication [4]. 2.3. Machine-to-machine (M2M) communication M2M communication is an enabling technology for Internetof-Things (IoT). It involves autonomous connectivity and communication among devices ranging from embedded low-power devices to powerful compute-rich devices. D2D connections can be used to establish M2M communication in IoT since they afford ultra low latency and hence, real-time responses [7]. A particular application is vehicle-to-vehicle (V2V) communication where D2D links can be utilized to share information between neighboring vehicles quickly and offload traffic efficiently. They can also be harnessed for vehicle-to-infrastructure and vehicle-to-pedestrian communication.

Fig. 2. Simplified model for D2D communication based on ProSe architecture in 3GPP Rel. 12.

3. Architecture 3.1. Spectrum allocation

3.2. D2D communication in LTE-Advanced

In terms of spectrum usage, D2D communication is primarily classified into two types. They are inband and outband [1].

3GPP Rel. 12 of the LTE-Advanced standard specifies a general concept of proximity-based services (ProSe) that allows physically close devices to discover themselves and communicate via direct links [8]. ProSe is meant for public safety communication as well as commercial applications although the emphasis in Rel. 12 is on the public safety only. D2D discovery and D2D communication are defined as a support for ProSe. It is also known as LTE Direct since it supports direct communication between UEs using licensed spectrum and the global LTE ecosystem. Three scenarios for D2D communication are considered: (1) all UEs involved in D2D communication are within network coverage, (2) only some of the UEs in D2D communication are within network coverage, and (3) none of the UEs in D2D communication are within network coverage. A highly simplified model for D2D communication based on ProSe reference architecture (non-roaming case) is shown in Fig. 2. The BS or eNB, as it is called in 3GPP, connected to the Evolved Packet Core (EPC), can communicate with a UE directly using cellular communication. Additionally, UEs can communicate via direct D2D links. In terms of channel structure, the direct link between two UEs is called a sidelink which can operate by frequency division duplex or time division duplex. The UEs on being powered up first synchronize with the eNB or other UEs. For this purpose, several synchronization signals are defined in 3GPP Rel. 12. Coming back to the architecture in Fig. 2, various ProSe applications (APPs) can be installed in a UE and they may exchange data with the ProSe APPs in a remote ProSe APP server. When a UE wants to

3.1.1. Inband D2D communication Here, cellular communication and D2D communication use the same spectrum licensed to the cellular operator. The licensed spectrum may be either divided into non-overlapping portions for D2D and cellular communication respectively (overlay) or may not be divided at all (underlay). Overlay scheme is easier to implement but underlay scheme leads to opportunistic and hence, more efficient spectrum use and more profit to operators. 3.1.2. Outband D2D communication Here, D2D communication uses unlicensed spectrum (e.g., the free 2.4 GHz ISM band or 38 GHz mm Wave band) where cellular communication does not occur. It helps in eliminating the interference between D2D and cellular users although interference is still present from other electronic devices (like Bluetooth and WiFi) operating in this band. In fact, operators can control interference when using licensed spectrum but that is infeasible for outband scheme. Outband technology is further divided into controlled and autonomous types. In the former, the radio interface for D2D communication is controlled by the cellular network while in the latter, the cellular network controls only the cellular communication leaving the control of D2D communication to the users.


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communicate with its peer UE, the ProSe APP in it requests for expression codes of itself and its target peer from the server. Alternatively, a UE can obtain the expression codes from the Proximity Function in the eNB. After the expression codes are retrieved, the UE initiates the discovery procedure by announcing its own expression code or inquiring if the target UE (identified by the retrieved expression code) is present. After device discovery, the UEs can communicate directly. In terms of air interface for D2D signal and data transmission, resources are allocated either by the eNB or by the UEs randomly from a preconfigured pool of resources. D2D communication occurs using open-loop communication in layer 1, i.e., a D2D receiver does not send any feedback (including channel state information and acknowledgments) to a D2D transmitter [8]. 3.3. Single-hop and multi-hop networks Generally a D2D link connects a transmitter UE with its intended receiver UE resulting in a single-hop communication. One can also have a multi-hop network composed of D2D links, reminiscent of a mobile ad hoc network (MANET). In a multihop D2D network, the intermediate UEs act as relays either between a BS and a UE or between two UEs (refer to Fig. 1). A variant of the first scenario could be a cooperative cluster of UEs in which the BS transmits a data item to the cluster head which then groupcasts it to other UEs in the cluster (perhaps with network coding to improve throughput). 3GPP Rel. 13 enables UE-to-network relay while 3GPP Rel. 14 adds support for vehicular communication (i.e., high speed and high density of nodes) based on D2D technology. 4. Challenges and ongoing research We will now discuss the various technical aspects and the corresponding challenges of D2D communication in wireless networks. 4.1. Synchronization In a typical cellular network, UEs achieve time and frequency synchronization using periodic broadcasts from the BS. Devices in D2D communication can also synchronize with the same broadcasts so long as they belong to the same BS. The situation gets complicated in the following cases: (1) UEs belong to different BSs that may not be themselves synchronized, or (2) some of the UEs are in the coverage of the network and some outside the coverage, and (3) all UEs lie outside network coverage [9]. Synchronization among UEs is beneficial for D2D communication because it helps a UE to use the right time slot and frequency for discovering and communicating with its peer and thus engage in more energy-efficient communication. Note that global synchronization among all UEs in a network may not be required for D2D communication; rather local synchronization among neighboring devices is sufficient. Although one may adopt the synchronization protocols proposed for MANETs and wireless sensor networks, D2D communication usually requires more accurate synchronization and also allows

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more complex algorithms (in terms of computational and power budgets) in UEs unlike the case in resource-constrained sensors. Researchers have proposed some physical layer [9] and MAC layer schemes [10] for synchronization in D2D communication. 4.2. Peer discovery Looking at the demand of D2D network, there should be an efficient method for discovering peers. This means a UE should be able to discover other nearby UEs quickly and with low power consumption. From user perspective, there are two types of peer discovery techniques, restricted and open [11]. In the first case, devices cannot be discovered by the end users without their permissions. In the second case, devices can be discovered whenever they lie in the proximity of other users. From network perspective, peer discovery can be controlled lightly or tightly by the BS [11]. In a multicell network, it is very difficult to get cooperation from adjacent BSs, making peer discovery a challenging job [12]. Incentive-based schemes (i.e., game-theoretic frameworks) may be investigated as a probable solution. 4.3. Mode selection A pair of UEs that have discovered themselves are potential candidates for a D2D communication. But performance-wise, cellular communication may be more preferable if, for example, the direct channel is more noisy. Mode selection is concerned with choosing the right mode – cellular or D2D – for communication between two UEs to achieve some performance objective like high spectral efficiency, low latency, or low transmit power. Mode selection can be done by the network or by the UEs. To formulate the mode selection problem, one may associate a decision variable with each UE, that captures the selected mode and then add a variety of objectives and constraints. A simple objective could be that the channel gain of the selected mode should be higher than that of other possible modes. More sophisticated objectives like optimal spectrum reuse, weightedsum-rate maximization, etc. could also be used. Constraints could be minimum QoS at receiver, maximum transmit power, etc. Thus mode selection is generally coupled with power control. The analysis could be done using instantaneous system information (which may be difficult to acquire in practice) or statistical system information leading to decisions that are respectively optimal at a given instant or optimal in an average sense over a longer duration [13]. 4.4. Resource allocation Radio resource (e.g., subcarriers) allocation is an important step (especially, in inband mode) in creating and maintaining direct links between D2D pairs in a cellular network. A simple but general resource allocation framework is proposed in [14] for inband multicell architecture: in overlay, uplink spectrum is divided into two orthogonal portions with fraction η assigned to D2D communication and 1 − η to cellular communication; in underlay, the spectrum is divided into B bands and D2D



UEs can randomly and independently access β B (β ∈ [0, 1]) of them. The optimal values of η and β are computed by assuming that the UEs are distributed according to a random spatial Poisson point process and mode selection is based on a UE’s distance from its intended receiver, and then optimizing some performance objective like a joint function of the rates achieved by cellular and potential D2D users. Different resource allocation schemes can be designed by changing the optimization objectives and adding various constraints. 4.5. Interference management In inband communication, cellular and D2D links may interfere with each other based on how they share the frequencies. In outband communication, D2D links suffer interference from each other as well as from other devices operating in the same band [15]. Interference can be reduced if UEs transmit at lower power levels which might, however, affect the QoS at the receiver. Thus interference-aware resource management is an involved optimization problem. Often it is cast as a weighted-sum-rate maximization problem subject to maximum transmit power and minimum QoS constraints or as a transmit power minimization problem subject to a minimum QoS constraint [16]. Power control, in addition to interference mitigation, leads to energy-efficient operation which is one of the goals of next generation wireless networks. Careful scheduling of transmissions also helps to minimize interference. Suitable modulation and coding schemes (adaptively chosen based on channel quality) and hybrid automatic repeat request (which is the combination of automatic repeat request and forward error correction) increase the robustness of the transmitted signal against noise. Mode selection, resource allocation and interference minimization are closely related and often jointly optimized. For these three kinds of problems, several centralized, distributed and hybrid algorithms have been proposed but research is still active [16]. 4.6. D2D with mobility Most D2D-related research has focussed on static users while cellular networks essentially cater to mobile users. More analysis is needed to understand how the performance gains auger in dynamic scenarios (from pedestrian to vehicular speeds) and what interference handling and handover mechanisms are needed as UEs move within and across cells [17]. Multi-hop D2D communication also poses many challenges [18]. 4.7. Pricing This is one of the most pressing issues to cellular operators. The difficult question is how to control the direct link between the devices and how to charge the users. Various pricing models are explained in [4]. For example, operators may use UEs as relays for other users and may give financial incentives to the relay UEs [4]. Operators can also provide chargeable services like security during D2D communication [4]. Other economic

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models in the context of D2D communication include how D2D UEs in a cluster may buy or sell data items [19], how cellular users can sell their bandwidth to D2D UEs [20] and how D2D-capable UE pairs can auction their cellular resources to other waiting cellular users and themselves opt for D2D communication [21]. 4.8. Security D2D communication affords stronger anonymity and data privacy compared to conventional cellular communication since the data are not stored at a central location. However, various common attacks like eavesdropping, denial of service, manin-the-middle, node impersonification, IP spoofing, malware attack, etc. can paralyze D2D links. Users would also like to protect their privacy, e.g., by restricting the availability of their sensitive personal data. The same lack of a central authority makes it difficult to implement security and privacy measures. Authors in [3] model threats in a three-dimensional space: (1) whether the attacker is internal or external, (2) whether the attacker is active (e.g., it modifies in-transit data) or passive (e.g., it only snoops on data), and (3) whether the attack is local or extended across the network. Several proposals to safeguard D2D networks are reviewed in [3,22]. 5. D2D communication and 5G networks The upcoming 5G network is expected to support aggregate data rates (i.e., total amount of data the network can serve, measured in bits/sec/area) 1000 times that of the current 4G network [23]. The spectral efficiency (number of bits transmitted per Hz) and energy efficiency (number of bits per Joule) should be 10 times higher than those of 4G network, data rates for mobile users should be multi-Gbps and end-to-end latencies around 1 ms. 5G network will essentially contain an umbrella of technologies including heterogeneous network (HetNet) (i.e., use of various radio access technologies, multiple backhaul techniques and a hierarchy of cells—macro, pico, femto), massive MIMO (i.e., large antenna arrays at BS to serve many users concurrently), cognitive radio network (CRN) (where secondary users opportunistically use the primary users’ spectrum), millimeter wave (mmWave) spectrum (works over 30–300 GHz frequency) and D2D communication [23]. The last one is fundamentally important in 5G despite the challenges of implementation [4]. The mmWave spectrum in 5G can be used to form short-range D2D links between UEs. Since mmWave suffers low multi-user interference, many mmWave D2D links can operate concurrently, thus improving network capacity. Secondary users in a CRN can also use D2D communication to avoid interference to primary users. D2D communication complements HetNets and massive-MIMO-enabled BSs in improving spectral efficiency and data rates. Additionally, MIMO antennas embedded in UEs will increase noise resilience and system capacity by exploiting diversity and multiplexing gains. D2D-based relays with MIMO-enabled devices can also enhance system capacity significantly [1].


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6. Conclusion We have presented a brief overview of D2D communication in terms of its use cases, architecture and the main technical challenges to its implementation. We have also seen that D2D communication can play a pivotal role in realizing the ambitious goals of 5G wireless networks. Preliminary systems employing D2D communication have already started to arrive. More developments and industry standards are in the pipeline. References [1] A. Asadi, Q. Wang, V. Mancuso, A survey on device-to-device communication in cellular networks, IEEE Commun. Surv. Tutor. 16 (4) (2014) 1801–1819. [2] 3GPP, 3rd generation partnership project (3GPP). Available: http://www. 3gpp.org/ (Accessed 18 September 2017). [3] M. Haus, M. Waqas, A.Y. Ding, Y. Li, S. Tarkoma, J. Ott, Security and privacy in device-to-device (D2D) communication: a review, IEEE Commun. Surv. Tutor. 19 (2) (2017) 1054–1079. [4] M.N. Tehrani, M. Uysal, H. Yanikomeroglu, Device-to-device communication in 5G cellular networks: challenges, solutions, and future directions, IEEE Commun. Mag. 52 (5) (2014) 86–92. [5] IEEE ComSoc, Best reading topics on device-to-device communications. Available: http://www.comsoc.org/best-readings/topics/devicedevice-communications (Accessed 18 September 2017). [6] A. Aijaz, H. Aghvami, M. Amani, A survey on mobile data offloading: technical and business perspectives, IEEE Wirel. Commun. 20 (2) (2013) 104–112. [7] O. Bello, S. Zeadally, Intelligent device-to-device communication in the internet of things, IEEE Syst. J. 10 (3) (2016) 1172–1182. [8] S.-Y. Lien, C.-C. Chien, F.-M. Tseng, T.-C. Ho, 3GPP device-to-device communications for beyond 4G cellular networks, IEEE Commun. Mag. 54 (3) (2016) 29–35. [9] N. Abedini, S. Tavildar, J. Li, T. Richardson, Distributed synchronization for device-to-device communications in an lte network, IEEE Trans. Wireless Commun. 15 (2) (2016) 1547–1561. [10] W. Sun, F. Brännström, E.G. Ström, Network synchronization for mobile device-to-device systems, IEEE Trans. Commun. 65 (3) (2017) 1193–1206.

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