Device-to-device (D2D) communications
D2D communications provide direct links between the devices (users) without using any infrastructure node to transfer the data. There are various benefits of D2D communication such as increased spectral efficiency, improved user data rate and capacity per area, extended coverage, reduced latency, and enhanced cost and power efficiency. D2D communications are introduced in cellular networks with a motivation that
- There is a need for improvement in the use of resources of the network
- New cellular devices are enabled in which direct communication is required between devices.
With the use of direct connectivity between devices within the cell, the overall capacity of the network and the spectrum efficiency is increased. Hence, small cells known as cell densification has been introduced in the network. When there is a need to serve the identical content in the same localized area and reliable peer-to-peer link is possible for sharing in a licensed spectrum, the possibility of offloading the cellular infrastructure has been opened by the introduction of D2D communications.
It is necessary to discover the devices if they are in proximity of each other in the case of D2D. The network can support the proximity discovery. The proximity discovery is done in ad hoc manner if network support is lacked or the devices are outside the coverage of the cellular network. The direct link can be established by the devices only if they are in proximity with each other, no matter which proximity discovery approach is used. On the other hand, the cellular network infrastructure is used for communication if the devices are not proximal and are located in the coverage of the cellular network. The multi-hop relaying is used to provide connectivity when there is no coverage of the cellular network.
D2D from 4G to 5G
There is a prediction that the opportunity for local management of short-distance communication links is offered and separation of local traffic from the global network is allowed by the network-controlled D2D communication in the future 5G system. In this way the load burden on the backhaul and core network due to transfer of data and signalling will be removed and efforts to manage traffic at central network nodes will be reduced. With the incorporation of the end devices in the concept of network management, the idea of distributed network management in direct D2D communication extends. As a result, there is a dual role of user device of wireless with D2D capability which acts either as an infrastructure node or an end-to-end devices.
Moreover, low-latency communication is facilitated by the direct D2D as the users in proximity have the local communication link between them. In the future 5G systems, there are some additional features in which real time services are supported by the direct D2D. Also, the reliability can be increased through large extent of diversity by employing D2D links. Furthermore, there is a reduction in the consumption of the device power due to short-distance transmission. The use cases of communication using D2D link is shown in the fig. 1.
Fig. 1. Use cases of D2D communication in cellular networks
There are four D2D scenarios. First is local data sharing in which data caching is shared between the devices in proximity. Second is relaying in which D2D based relay can improve the availability of the network. The third one is single or multi-hop local proximity communication a peer-to-peer link or multicast link can be set up between the devices in proximity that does not use the infrastructure of the cellular network. D2D discovery is the last scenario in which identification of proximal UEs are done.
To simplify the design and implementation, the cellular air interface is used to derive the design of D2D air interface. Both licensed and unlicensed spectrum are used to operate by D2D links.
4G LTE D2D
3GPP has given various benefits of D2D in LTE out of which public safety is the main driver in Release 12 and 13 and there is a support of commercial discovery as well.
In 4G LTE system, an add-on feature is LTE D2D which allows UE to operate on the same carrier. A synchronous way is used by D2D to operate in LTE where eNode-B or UE may be the synchronization source. For D2D transmission, there is a use of UL spectrum or UL subframes. The management of interference between [email protected] and cellular links is one of the interesting features of D2D communication. The introduction of a new transmitter at UE can be avoided with the addition of the transmission signals based on the design of UL signal. Moreover, better coverage has been provided by SC-FDMA due to low Peak to Average Power Ratio (PAPR) as compared to OFDM signalling.
The time and frequency synchronization is provided by the sidelink synchronization signal that is transmitted by the D2D synchronization source in order to facilitate synchronous operation of D2D. There is a need for solving the following issues to achieve synchronization:
- synchronization signal design
- entities acting as synchronization source
- criteria to select/re-select the synchronization source
There are primary and secondary sidelink synchronization signals composed in the sidelink synchronization signal. When network coverage is achieved by the UE, both primary and secondary synchronization signals are transmitted by the eNode-B that can be used for D2D synchronization. UE transmits new sidelink synchronization sequences that act as a synchronization source.
There are two synchronization sources i.e. eNode-B and UE. At the cell edge, sinchronization signals are transmitted by the UEs with the network coverage. D2D UEs with network coverage provide synchronization signals to help the out-of-coverage synchronization to UEs which are without network coverage. Hence, there is a reduction in interference from D2D transmission to cellular links.
Different types of synchronization sources with different priority levels are specified to solve synchronization source selection and reselection issue. The highest priority is provided to eNodeB followed by UEs which are in network coverage and then UEs which are out-of-coverage but synchronized to the UEs which are in-coverage. The lowest priority is of the out-of-coverage UEs which are not synchronized to any in-coverage UEs.
D2D communication is based on the communication of physical layer broadcast in Release 12 of LTE. Multicast and unicast is supported by indicating the targeted group ID or user ID in the higher layer message. For D2D links do not support physical layer feedback, link adaptation and HARQ. Uu interface is used for the air interface and the channel structure for UL is also extended for D2D communication. There is a reuse of the PUSCH structure as physical channels for D2D data communication. A resource pool concept is used by D2D communication as shown in fig. 2 where there is a configuration of certain time/frequency resources for D2D usage. One cell is needed for configuring D2D resource pool and for D2D control information and D2D data transmission, separate resources are available. The broadcast messages are used to carry the resource pool information.
A control signal is sent by every transmitter with information on the data transmission format before the transmission of D2D data. At receiver, there is no need to locate the D2D data, the location of relevant resource can be found from the content of the D2D control channel.
Fig. 2. D2D resource pool
For D2D communication, there are two modes for the usage of the resource, such as:
Mode 1: For D2D data and D2D control information transmission, the exact resources used by a UE are scheduled by an eNode-B or relay node.
Mode 2: The resources are selected by UE by itself from the configured resource pools for D2D data and D2D control information transmission.
The UEs within the coverage of network are allowed or discovery in Release 12 of LTE. The D2D discovery resources are also arranged as resource pools similar to D2D communication resources. There are various parameters to define the resource pool such as discoveryPeriod, discoveryOffsetIndicator and subframeBitmap. The parameters startPRB, endPRB and numPRB give the frequency resources within a D2D subframe. To get the resources for transmission of the discovery message, two ways are specified for the transmitting UE:
- The transmission resource can be autonomously selected from the discovery pools by the UE.
- The transmission is done by the UE on the resources allocated by the network.
In Release 13, further D2D enhancements are specified by the 3GPP RAN WGs. But the challenges of 5G D2D aim to address use cases at a wider range.
Public safety is the main focus for D2D communication in 4G LTE, the D2D operation provides the potential improvements which are not fully exploited. These restrictions do not exist in the 5G system anymore and 5G systems have integrated the operations of D2D. The main gains achieved by using D2D links are:
- Capacity/throughput gain: There is an improvement in the link throughput due to better Modulation and Coding Scheme (MCS) level because there is a close proximity of the involved devices with better conditions of propagation as compared to the the propagation conditions toward the Base Station (BS).
- Latency gain: There is a reduction in the End-to-end (E2E) latency due to a short distance with less propagation delay and infrastructure network entities also do not involve that results in reduction of transport and processing delays.
- Availability and reliability gain: The network coverage can also be extended by using one hop or multi-hop D2D link. To enhance the quality of the link D2D network coding and cooperative diversity can be used. In case of infrastructure failure or the infrastructure establishment is not easy, a fall back solution is provided by D2D ad hoc network.
- Enabling new services: To enable new services and applications vertical industries and telecommunication area use full-blown D2D. In LTE Release 14, Vehicle-to-vehicle (V2V) communication has been used as an extension of D2D solutions.
New challenges are posed in order to utilize D2D gains completely in terms of device discovery, communication mode selection, co-existence, interference management, efficient multi-hop communication support and multi-operator support among others.
- Device discovery: A key element to determine proximal devices and establishment of direct D2D link is efficient network-assisted D2D discovery that can enable D2D communication and new applications.
- Communication mode selection: The another core function to control whether the communication between two devices is in direct D2D mode or in regular cellular mode is mode selection. The proximity advantage has been taken by the devices in the direct D2D communication and cellular resources can be reused for communication using direct link. A common or separate serving serving BS is used by the devices to communicate with each other in cellular mode.
- Co-existence and interference management: There are two different aspects while considering co-existence and interference issues:
- Co-existence among a large number of D2D links
- co-existence among D2D links and regular cellular links
To achieve the benefits of D2D, efficient schemes can be made to handle the interference.
- Multi-operator or inter-operator D2D operation: It is essential to support inter-operator D2D operation for the D2D concept of 5G. There will be limited applicability of the future D2D solution without support of multi-operator D2D. Various issues such as spectrum usage and control and coordination of UEs across multi-operator networks can be solved by considering inter-operator D2D operation.
RRM for mobile broadband D2D
The new challenges can be posed by the addition of the D2D layer in terms of management of interference in comparison with traditional cellular communication. These challenges come from the resource reuse between the users of cellular and D2D due to which inter-cell interference can be created. The essential management of resources has been taken to exploit D2D communication benefits and achieve improved performance of the system. Spectral efficiency, minimization of power and Quality of Service (QoS) performance are the most common objectives of RRM algorithms and techniques. Mode selection, resource allocation
and power control are the basic toolbox of RRM techniques.
Multi-operator D2D communication
To meet the requirements of V2V communication, there is a need for inter-operator D2D support. In a multi-operator setting, the D2D discovery should rely on both ends of the D2D pair and on the networks of both operators. It is easy to implement an overlay multi-operator D2D scheme. The mode of communication can be selected by getting the level of the received signal at the D2D receiver without incurring excessive communication signaling overhead.