1. Device to Device Communication in LTE Network
Arjun Nanjundappa
Illinois Institute of Technology
ananjun1@hawk.iit.edu
Abstract
Over recent years, Mobile communication world has been one of the fastest growing industries
witnessing rapid changes in the services offered by it. The transition from third Generation (3G) to
Generations beyond Fourth Generation (LTE) is a clear indication that satisfying the demands of the
mobile users by offering improved services and generating more revenue for the operator has been the
main area of focus in this industry. Though LTE has evolved considerably there are challenges on Higher
Network Coverage, Spectral efficiency and at the same time keeping the mobile operator, end user
satisfied in their requirements. There is a requirement for not just commercial communication system but
also for a critical communication system during critical and adverse situations. This paper gives an
hindsight on device to device communication in LTE network which is focused on network coverage, last
mile connectivity ,congestion control and public safety mechanism. It provides direct communications
between user equipments in LTE mobile network by bypassing the traversing of the base station called
eNodeB. Unlike existing device to device communication technologies such as Wifi-direct, Bluetooth, NFC(
Near field communication) , here the operator controls the communication process to provide better user
experience and make profits accordingly. It addresses public safety which is of prime worldwide through
Direct Communication mechanism. It further dwells on the Commercial aspect and End user convenience
through Direct Discovery mechanism. Final sections of this paper focuses on the idea and proposal to
improve on the existing D2D Communication to standardize them into 3GPP for further deployment .

2. 2. Introduction
There is a growing necessity of network functioning
over a period of time which requires reliability and
resilience, direct communication between terminals,
group communication and off-network
communication. The National Public Safety
Telecommunications Council (NPTSC), the
Association of Public Safety Communication Officials
(APCO) and the National Emergency Number
Association (NENA) in the US decided to endorse LTE
as a the platform for the next-generation public
safety network with broadband capabilities. Below
are the Performance metrics of LTE Network.
Peak data rate: 100 Mbps (Downlink) DL/ 50
Mbps (Uplink) UL within 20 MHz
bandwidth. Up to 200 active users in a cell
(5 MHz)
Reduce latency to 10 ms round-trip time
between user equipment and the base
station
Spectrum flexibility: 1.25, 2.5, 5, 10, 15, and
20 MHz
Enhanced multimedia broadcast multicast
service (E-MBMS)
Mobility: Up to 500 km/hr – Up to 15 km/h
optimized. Slide degradation after 120
km/h.
Coverage (Cell size): 5 to 100 km with slight
degradation after 30 km
E-UTRA<->UTRA and E-UTRA<->GERAN
Handoff interruption time
o < 300 ms for real-time service
o < 500 ms for non-real time service
[1]
3. D2D Communication
Commercial LTE networks target 95% population
coverage but US public safety is targeting 99%. Last
4% requires a 60% larger coverage area [5]. Solving
this with a denser network would be very
expensive as the number of subscribers have
grown to 2.7 billion in 2014 and it only increases by
hereon. An alternative would be for device to
communicate with each other by bypassing the
traversing of base stations ( Evolved NodeB in LTE
Network) or core network. Existing wireless
technologies provide D2D communication but they
operate in unlicensed spectrum and lesser range.
Due to this factor, they suffer from collisions while
transmitting and receiving as guaranteed time slots
are not available. Hence, device suffer from high
battery consumption and at the same time
Network operator doesn't derive any income.
To Solve these shortcomings, D2D
communication is developed in LTE network which
is the mission critical communication during
adverse or critical situations and also operates in
licensed frequency band that results in guaranteed
slots of access transmissions from devices thereby
mitigating collision and hence device have a longer
battery life and operator too is benefited. Network
Overload during Peak traffic is limited and also
increasing the spectral efficiency. The Scenarios
and use cases cover are following.
3.1 D2D Use Cases [2]
Partial or no coverage due to mobility of the user devices.
UE1 UE2
Figure 1: Partial or No Coverage
During Network disaster such as Power Outage or Equipment failures.
UE1 UE2
Figure 2: Power Outage

3. Network Overload or Congestion Reached in Access( eNodeB) and Core Network (MME).
Network can be congested due to either of limitation in Air link and backhaul link has reached its
maximum permissible limit, maximum number of devices per Cell per eNB ( Evolved Node B),
number of data radio bearers per cell has reached its maximum permissible limit, maximum PRB(
physical resource block) usage has reached.
UE1
UE2
Figure 3: Communication during Congestion
Communication requirements such as devices partially within network coverage or outside of the
coverage or maintaining concurrent device to device and LTE connectivity regardless of the
network coverage for spectrum efficiency.
UE1 UE2
Figure 4: Multiple Cells
3.2 D2D Mechansim [2]
Figure 5: D2D Mechanism [2][11]

4. Device to Device communication is divided into 3
steps:
Synchronization: A D2D Synchronization
source transmits at least a D2D
synchronization signal( D2DSS). Here, the
transmitted D2DSS may be used by a UE(
User Equipment) to obtain time and
frequency synchronization and
Synchronization Source is an eNodeB which
transmits Primary D2D Synchronization
Signal according to the Zadoff Chu sequence
and a Secondary D2D Synchronization
Signal according to the M sequence. These
signals are used by D2D UE for aligning
transmitter timing offset and parameters
when transmitting D2D channels and for
aligning receiver window and frequency
correction when detecting D2D channels.
Discovery: Discovery is a process in which
D2D UE tries to detect the presence of
other D2D UEs in the close proximity.
Discovery procedure are of two types.
Type 1: Resources ( subframes/physical
resource blocks) for discovery signal
transmission are allocated on a non UE
specific basis. For transmission and
reception, eNodeB provides the
resources through Cell Information
Broadcast called System Information
Broadcast(SIB19). D2D UEs gain the
access through First come First serve
basis.
Type 2: Resources for discovery signal
transmission are allocated on a per UE
specific basis where type 2A resources
are allocated for specific transmission
instance of discovery signals and type 2B
resources are semi-persistently
allocated for discovery signal
transmission. For D2D discovery
reception, eNodeB provides resources
through SIB19. For D2D discovery
transmission, Once the D2D UEs are in
connected state to the Network should
make access requests to eNodeB for
further transmission.
Communication: Discovery process is
required for Unicast but for groupcast ad
broadcast communication, Direct
Communication is sufficient. Broadcast and
groupcast are for Public Safety where
communication should be possible
irrespective of infrastructure coverage. D2D
Communication are of two types. [2]
Mode1: This kind of communication is
for in-coverage where the location
resources for transmission of scheduling
assignment and data by the
broadcasting D2D UE comes from the
eNodeB through SIB message.
Mode2: This kind of communication is
for edge-of-coverage and out-of-
coverage UEs where the resource pool
for scheduling assignment and data is
pre-configured or semi-statically
allocated and UE selects from the
resource pool for transmission and
reception.[2]
3.3 ProSe D2D Network Architecture
UE B
ProSe
application
LTE-Uu
E-UTRAN
UE A
S1
ProSe Function
MME
S/PGW
HSS
ProSe
application
HSS SLP
ProSe
Application
Server
S6a
PC3
PC4a PC4b
PC2
PC1
PC3
LTE-Uu
PC5
PC1
Figure 6: ProSe D2D Network Architecture[3]

5. Devices communicate with each other based on
proximity and the range of the proximity is 1km.
Hence the name Proximity based Service(ProSe).
So, to support the proximity alert, type of service
and resource allocation, following network
architecture is deployed. PC5 is the interface
between the D2D devices. PC3 is the interface
between each D2D device and Proximity Service
(ProSe) Function. PC2 is the interface between
ProSe Function and ProSe Application Server.
ProSe Function. The ProSe Function provides three
functionalities namely Direct Provisioning Function
(DPF), Direct Discovery Name Management
Function and EPC-level Discovery Function. As the
name implies the DPF provisions the UE with all
required parameters for Direct Discovery and
Direct Communication for this particular network
(PLMN). The task for the Direct Discovery Name
Management Function is to allocate and process
ProSe Application ID’s and ProSe Application Codes
that are used for Direct Discovery. This sub-
function maintains a table that lists the mapping
between the Application Identity (App ID) and
Prose Application Code (ProSe App Code). An
additional task for this sub-function is to contact
the Home Subscriber Server (HSS) to check if the
device is authorized to do Direct Discovery and if
so, authorizes the device by synchronizing a ProSe
Policy. Finally, it provides the device with integrity
parameters to protect discovery messages that are
transmitted over the air.
ProSe Application Server is the one which provides
content delivery to the network, registration for
type of service.
SLP is Secure User Plane loation which provides
location updates of D2D UEs to ProSe Function.[3]
3.4 Direct Discovery[3][4]
UE
ProSe
Function
HPLMN
ProSe
Function(s)
1. Service authorisation
2a. Discovery Request(announce)
2b. Discovery Request(monitor)
4b. Match report
3a. Discovery
announce on PC5
3b. Discovery
monitor on PC5
For the announcing UE
For the monitoring UE
Other PLMNs (VPLMN or local)
Figure 7: Direct Discovery [3]
Direct Discovery and Direct Communication are
independent services. Direct Discovery is for
Commercial point of view and therefore they run
when there is In-coverage of the network and
hence full control under the serving network.
Network is informed in the initial attach procedure
between eNodeB and MME if the D2D UE supports
Proximity Services and whether Direct Discovery
and Communication are supported or not.
For Service Authorization, D2D UE needs
to send authorization request to ProSe function
through PC3 interface explained in Figure 7. IP
Address to connect to the ProSe function could be
available through DNS lookup. ProSe Function
provisions UE for the Service. Two types of
Discovery, Open and Close Discovery is defined.
Open discovery is without permit which has a
announcement similar to "John is here!" where a
device broadcasts information about itself that
contains it's discovery id. In this model a
monitoring device is only looking for certain
information of interest in its proximity which
receives all discovery messages in L1, filters its
interested discovery messages in L2 and forwards

6. upper layer (L3) that includes the discovery id.
Closed model can be described with the phrase
“Who is out there?” or “Are you out there?” The
‘Discoverer UE’ sends out a request containing
certain information about what it is interested in
to discover. The ‘Discoveree UE’, that receives this
message, can respond with some info related to
the discoverers request. In this Match Report, the
device shall indicate if it wishes to receive meta
data about the related ProSe Application. The
ProSe Function uses the provided information for
validation and verification and in case this
operation is successful, it sends an
acknowledgement to both the devices for data
communication. [3][4]
Discovery Resources for D2D UE is
provided by eNB through Cell broadcast using
Session Information Broadcast 19( SIB19) message
to UE. This contains resources for Transmission
and as well as for reception. D2D UEs need to be
connected to the eNB to avail those resources as it
is for In-coverage. Transmission across devices is
in Uplink spectrum. In the Context of ProSe, the
name is changed to "Sidelink" to refer to ProSe's
PC5 interface. [6].
3.5 Direct Communication
UE-1
1. UEis configured
withgroup
information
1. UE is configured
with group
information
1. UEis configured
with group
information
UE-2 UE-3
2. Access radio
resource to send
group
communication
3. One-to-manydirect traffic
2. Listens to radio
resourcesto
receive group
communication
2. Listens to radio
resourcesto
receive group
communication
Figure 8: Direct Communication [3]
Direct Communication is for devices within close
proximity which covers all 3 coverage point of view
for in-coverage , partial coverage and no-coverage
of the network. This communication is for public
safety purpose.
Provisioning of type of services to the UE is pre-
requisite for configuration. All the D2D UEs are
provisioned as a pre-requisite for communication.
Provisioning information include type of service
and Group ID (L2) information. So, One UE who has
access to radio resources and other UEs listen to
the group communication in the form of One-to-
many direct traffic. Main difference compared to
ProSe discovery is to use IP-Layer message in
Direct Communication. Here, A D2D transmitter
broadcasts scheduling information and data for a
destined group. D2D receivers receives its
interested scheduling information, receives the
data according to L1, filters its interested group
data in L2, checks the security and decompress the
IP Header in L3 and then forwards upper layer
data.[3][4]
Communication Resources for D2D UE is
provided by eNB through Cell broadcast using

7. Session Information Broadcast 18( SIB18) message
to UE.[6] This contains resources for Transmission
and as well as for reception. UE can avail these
resource based on autonomous resource selection
by UEs in First Come First Serve basis from the
resource pool or could be provided through eNB
scheduled resource allocation. The resource
allocation is available in dual modes . Mode 1 is
applicable for "In-coverage" and "Partial coverage"
and the resource allocation is scheduled by eNB.
Mode 2 is applicable for "In-coverage", "Partial
coverage" and "Out of coverage" in which UE on its
own selects resources from resource pools.
3.6 Comparison with Peer technologies
Here is how D2D fares with it's peer technologies.
4. Idea/Proposal
IP Address for D2D UE:: For Service
Authorization, D2D UE IP Address could be
pre-configured in the device as an
alternative to do DNS lookup before
sending the request for Authorization to
ProSe Function. This will be useful
particularly when there is no network
coverage or in case of traffic congestion in
network. Analysis is when D2D UE IP
Address could be programmed in the boot-
loader of the device as part of boot prompt.
When the Software Image boots up, it could
by default pick up the address from the
Environment variables of the Image.
QOS for Triple Play Services::For
Supporting Triple Play Services in D2D
Applications, separate Quality of Service
Identifiers (QCI) should be used namely QCI
#1 for Voice, QCI#2 for Video and QCI#3 for
data. These QCI's could be used during
bearer establishment between UEs for Call
Establishment, Video transfer and FTP
service. This could prioritize traffic among
D2D UEs and provide better customer
service.
eNB Resource Allocation for Direct
Communication:: For Direct
Communication, UE can autonomously
select resources from the available resource
pool on First Come first serve basis.
Alternatively to provide more control to the
eNB, eNB can schedule resource allocation

8. to the D2D UE on a need basis. Providing
resource grants on need basis is the
proposal to improve the D2D architecture.
When D2D UE #1wishes to send
data to another UE, it needs to request
resource to eNB. eNB provides 1 bit request
for UL grant in PDCCH( Physical Downlink
Control channel) channel for UE to send
Buffer status report (BSR). UE requests for
the resources through Buffer status report
to eNB. eNB grants UL grant resource to
UE#1. UE#1 is the transmitter and wishes to
communicate to UE#2 who is in the
reception state. So, UE#1 after receiving the
UL grant forwards the Scheduling
Assignment to UE#2. UE#2 through
Scheduling assignment determines when
UE#1 will transmit on its physical channel.
Analysis is this minimizes signaling
overhead for eNB to avoid sending resource
grant to UE#2 and also useful in Partial-
coverage scenario where One UE is in-
coverage and other UE out-of-covergae.
Through the resources granted, both UEs
communicate with each other.
Figure 9: eNB Scheduled Resource Allocation
5. Conclusion
This paper stresses on the evolving nature of the wireless industry. Ever since the mobile cellular
communication became commercially available to the customers, we have seen tremendous
advances in this field, in terms of technology, coverage, services, handsets and standards. But due to
the growing need of public safety with broadband capabilities in critical circumstances, D2D
communication is standardized in LTE. We studied the Network architecture, mechanism of D2D and
later the potential use cases and technical design considerations in the Device-To-Device
Communication in LTE networks. Every use case has its own market requirements namely coverage
aspect, commercial point of view, overload control and should take these factors into consideration
for resource allocation, usable spectrum, connection establishment. Few Proposed enhancements
which were yet to be standardized in 3gpp for D2D were discussed to improve D2D performance.
Further notable additions to ProSe D2D Applications are expected to be Mission Critical Push to Talk (
MCPTT) and Isolated E-UTRAN Operation for Public Safety(IOPS) with low latency time in 3gpp
Release 13 ( 2016) time frame.

9. 6. References.
1. 3gpp TS 36.300 Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description
( Release 12)
2. 3gpp TR 36.843 Study on LTE device to device proximity services; Radio aspects(Release 12)
3. 3gpp TS 23.303 Proximity based Services ( ProSe); Stage 2( Release 12).
4. 3gpp TR 23.703 Study on Architecture enhancements to support Proximity based Services
(ProSe);Release 12.
5. 3gpp TR 36.837 Public Safety high power User equipment (UE) for band 14.( Release 11 )
6. 3gpp Report R2-145417 TS 36.300 Evolved Universal Terrestrial Radio Access Network (E-UTRAN);
Overall description CR0681.
7. 3gpp Report R2-150699 TS 36.304 Evolved Universal Terrestrial Radio Access Network (E-UTRAN);
User Equipment (UE) procedures in Idle Mode. CR0264.
8. 3gpp Report R2-150717 TS 36.321 Evolved Universal Terrestrial Radio Access Network (E-UTRAN);
Medium Access Control Protocol. CR0755.
9. 3gpp Report R2-150734 TS 36.331 Evolved Universal Terrestrial Radio Access Network (E-UTRAN);
Radio Resource Control Protocol. CR1770.
10. IEEE Paper "Device-to-Device Communications Underlaying Cellular Networks" by Daquan Feng, Lu
Lu, Yi Yuan-Wu, Geoffrey Ye Li, Gang Fend and Shaoqian Li.
11. Andreas Roessler "LTE for Critical Communications APCO Broadband Submit 2014" Whitepaper.
12. IEEE Paper "Operator Controlled Device-To-Device Communications in LTE-Advanced Networks" by
Lei Lei, Zhangdui Zhong, Chuang Lin.
13. IEEE Paper " A Survey on Device-to-Device Communication in Cellular Networks" by Arash Asadi, Qing
Wang, Vincenzo Mancuso.
14. IEEE Paper " Device-to-Device Communications for National Security and Public Safety" by Gabor
Fodor, Stefan parkvall, Stefano Sorrentino, Pontus Wallentin, Qianxi lu and Nadia Brahmi.
15. IEEE paper "Resource Allocation for Device-To-Device Communications Underlaying LTE -Advanced
Networks" by Phond Phunchongharn, Ekram Hossain and Dong In Kim.