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Introduction to D2D Communications

Ghazanfar Ali Safdar1, Masood Ur Rehman2, and Mohammad Asad Rehman Chaudhry3

1 School of Computer Science and Technology, University of Bedfordshire, Luton, United Kingdom

2 James Watt School of Engineering, University of Glasgow, Glasgow, United Kingdom

3 Soptimizer, Canada

A perceptible upsurge has been discerned in wireless data traffic in recent years due to the occurrence of numerous new applications, thereby imposing an abrupt requirement for a large capacity network. Among them, multimedia applications such as high definition (HD) video streaming, HD video conferencing, and online gaming are most common [1]. Another factor impacting wireless data traffic is the primer of smart wearable technology such as watches, air pods and wrist band. In the near future, these devices would increase tenfold or even more, and their presence ensures more and more usage of smart wireless devices. For majority of the users, the wireless cellular network is the main access system for the internet due to its universal availability. According to Cisco white paper [2], there will be 12.3 billion mobile-connected devices by 2022 and mobile will represent 20 percent of total IP traffic, and global mobile data traffic will increase sevenfold between 2017 and 2022, reaching 77.5 exabytes per month by 2022. This pretense new contest to those working in the field of cellular communication and operators is deploying more and more wireless links between terminals with radio networking to meet the required demand.

1.1 D2D Communication

Existing device-to-device (D2D) technologies such as Bluetooth and Wi-Fi direct are available for thorough communication between two devices in close contiguity. They can associate different types of devices since the level of compatibility between modern devices is very high. The pairing procedure is considerably easy and both technologies simplify the entire pairing process by making enabled devices readily discoverable to one another [3]. Devices can communicate over a reliable communication channel with a moderate date rate and latency. Since the communication range is limited in these technologies, mobile device benefits from low power consumption that improves the battery life of the devices. In addition, it allows different devices to connect and communicate with each other without the need of an additional infrastructure.

Despite all the above benefits, both technologies have some limitations like single hop communication, limited range, latency, and moderate data. Both technologies are operated in the unlicensed band; hence, interference is eminent due to the number of devices already operated in that band [4]. Also, on an unlicensed band, the maximum transmit power constraint confines communication to a limited range. Both technologies have asynchronous nature, and the device receiver is supposed to continuously monitor the channel. The reason for this channel monitoring (idle listening) is not to miss the discovery signal from other devices, but this can drain the device battery significantly. Finally, to establish a reliable communication, the users are responsible for the discovery and initiation mechanism, and there is no centralized control. All of the above factors are the main obstacles in the integration of both technologies with next generation of mobile networks and they fail to act as a D2D on large scale. Another most common technology operated in the unlicensed band is Wi-Fi that has already assisted cellular networks in offloading local traffic and easing congestion. But its implementation requires a setup of access points connected to the internet and performance degrades as the number of user increases.

The diversity of radio accessing in mobile users provides lots of flexibility for D2D communication in terms of link establishment, resource allocation, energy efficiency, as well as applications and services. D2D communication enables researchers to merge together the achievements of long-term development in previously two disjoint networking techniques, i.e. ad-hoc networking and centralized networking. D2D users can form clusters or multihop routes, operates autonomously or under partial or full control of the operator. On the other hand, with the assistance of the operator, it is possible to achieve network-wide performance optimization after properly allocating frequency bands to D2D pairs and managing the interference there. D2D communication in cellular band is a very flexible communication technique with unique advantages over existing D2D techniques. D2D in cellular band can significantly enhance the cellular communication in terms of system capacity, coverage, throughput, latency, and user experience [1]. D2D communication can offload traffic from the serving base station by quickly exchanging a large amount of data among mobile devices in short range and alleviate congestion in the cellular core network.

In addition to that, D2D can also support local area services such as content distribution, local advertisement, and location aware services very effectively through unicast, groupcast, and broadcast transmission. A D2D in-coverage user can also act as a relay and extend the existing coverage of a serving base station and benefits the device that is on the edge of the cell boundary to experience better coverage [1]. The serving base station has the overall view of the entire cell, and it can facilitate D2D communication in different aspect, for example, finding the neighboring devices, reducing the transmitting power, and providing a suitable environment for communication. D2D implementation in cellular network does not require additional infrastructure and hence suppresses the high implementation costs. Undoubtedly, the reduction in transmission delay and offloading traffic from the existing base station to eases congestion in the cellular core network are the driving factors toward its addition in next generation of mobile networks [4].

In recent research, there are various attractive benefits of D2D in terms of spectral efficiency, energy efficiency, through put, and signal-to-interference and noise ratio (SINR). However, it is still unknown that if these gains could be fully utilized in the practice. In D2D communication, in spite of having many advantages, there are still many open challenges for the successful implementation of this technology. In particular, D2D communication will require efficient device discovery mechanisms (among devices in close proximity), intelligent mode selection (D2D or cellular mode) algorithms, complex resource management techniques (to avoid interference), and robust security protocols. Interference management is one of the compelling areas in D2D communication, and researchers have proposed different schemes, protocols, and methods to reduce interface. It is quite beneficial from a capacity and spectral efficiency point of view that D2D users reuse the same cellular resource for D2D communication. But it generates interference and requires different techniques to ensure that the quality of service (QoS) of a cellular link is not degraded because of D2D communication.

1.2 Evolution of D2D Communication

The wireless revolution in telecommunication begun in 1990s with the introduction of cellular network and has been driven by advances in radio frequency (RF) and microwave engineering [11]. In the past, infrared (IR) wireless technology was used for point-to-point links that use devices that are inexpensive, compact, lightweight, and consume low power. The limitations of IR technology were short distance, mobility of devices during transmission, and line-of-sight (LOS) requirement between transmitter and receiver. Unlike IR, the RF technology was effectively used for a long-distance communication that can penetrate through obstacles and does not require a LOS. RF technology operates in the range of 3 kHz-300 GHz and used for communication and broadcasting. The radio spectrum is divided into different segments and assigned to different technology industries, for example, very high frequency (VHF) band that ranges from 30 to 300 MHz is assigned to FM radio and TV broadcast, while ultra-high frequency (UHF) is dedicated to cellular, wireless local area network (WLAN), and Bluetooth, etc.

Current D2D technologies such as Bluetooth and Wi-Fi direct operate in unlicensed industrial, scientific, and medical (ISM) band; therefore, the interference with other devices operated in similar band is uncontrollable. Both suffers from limitations such as single hop communication, interference, limited range, latency, moderate data rate, and no centralized control. The burden of device discovery and communication initiation is also upon the user to establish a successful and reliable communication. The biggest obstacle is the inherit architecture of both technologies that does not allow a centralized controller to control and manage the D2D communication. Future cellular nodes would have the global view of the entire cell, and they can effectively manage the available resources for a successful D2D communication. The diversity of cellular communication provides lots of flexibility for D2D communication in terms of link establishment,...