1. Technologies based on Device-to-
device Communications
The 5th International Workshop LTE-
Advanced and Beyond
Hanbyul Seo, LG Electronics

2. D2D communications
 Data path of the evolved packet service*
 A UE directly communicates with the peer UE over-the-air.
 Advantages
Spatial reuse of time/frequency resources
Reduction of latency
Introduction of new services such as friend discovery and proximity-based
advertisement
* S1-120349, 3rd Generation Partnership Project; Technical Specification Group SA; Feasibility Study for Proximity Services
(ProSe)
SGW: Serving gateway, PGS: Packet data network gateway
UE
1
UE
2
eNB
eNB
SGW/PGW
UE
1
UE
2
eNB
eNB
SGW/PGW
Conventional data path setup An exemplary data path setup in D2D

3. D2D communications (cont’d)
 Introduction of D2D requires new features.
 For D2D, UE should to able to
Discover other UEs
 UE needs to know whether a certain UE is in its proximity or not.
Measure the channel from other UEs
 Synchronization between the peer UEs
 The measurement result needs be reported to other UEs or to eNB.
Receive signal via UL resource
 It is expected that D2D communication will take place in UL resource.
Maintain two different links
 One link with BS, another one with other UE(s)
 Coexistence should be guaranteed in terms of control signaling, HARQ operation,
handover, and so on.
 For D2D, eNB should be able to
Control D2D links
 Scheduling individual D2D transmission or high level control
Do interference coordination
 D2D transmission of a UE should not cause serious interference to the other
links.
Especially, the eNB-UE link should be protected.

4. UE discovery
 UE discovery?
UE1 measures some known signature transmitted by UE2.
 Two different approaches in terms of eNB control on the discovery
resources.
 Approach 1: Discovery under tight eNB control
eNB orders UE1 to receive a certain discovery signature transmitted by UE2.
eNB orders UE2 to transmit the signature in a given resource.
The measurement result at UE1 can be reported to eNB.
Mainly useful for UEs in the connected mode.
Fast and accurate discovery, invisible to the other UEs
 Approach 2: Discovery under loose eNB control
eNB broadcasts the set of resources that can be used for discovery signal
transmission.
Each UE generates the discovery signature to be used in the transmission (its own
signature) or reception (peer UE’s signature)
 Some hashing function from UE ID can be used.
Can be used for UEs in the idle mode.
Low control signaling overhead

5. Measurement of UE signal
 UE discovery is finished when eNB obtains the measurement
results, e.g., the received power of the interested discovery
signature.
eNB can initiate D2D data communications based on the measurement result.
This UE signal measurement can be treated as a new RRM measurement.
 This measurement is also needed to maintain the D2D link.
Needs to be reported to eNB or the peer UE to make a suitable decision on the link
establishment/termination, resource re-allocation, link adaptation, and so on.
 Location of the target UE?
Intra-cell UE measurement
 Relatively easy to get synchronized with the peer UE.
Inter-cell UE measurement
 More challenging especially when the two cells are not synchronized.
eNB
Intra-cell UE
measurement
eNB
eNB
Inter-cell UE
measurement

6. UE signal reception in UL resource
 Which resource is used for D2D signal transmission/reception?
UL resource is a better choice.
 Reuse the UE ability of UL transmission.
 Avoid severe interference from eNBs.
 Alleviate the impact on eNB-UE links.
The receiver in the UL resource (i.e., eNB) is usually far from the D2D UE location.
SINR comparison in DL resource and UL resource
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0.90
1.00
-150.00 -100.00 -50.00 0.00 50.00 100.00
CDF
D2D geometry [dB]
D2D in DL resource
D2D in UL resource

7. Maintaining the two links
 D2D UE still needs to communicate with eNB.
UE needs to maintain both UE-UE link and eNB-UE link simultaneously.
 D2D communication may have impact on eNB-UE communication.
For control signaling and data communication with the other devices
 An example of such impact
Is it possible for a D2D UE to transmit and receive UL signals at the same time?
 Expected to be difficult due to the self-interference => Half-duplex operation needs to be
considered in UL resources.
 Potential impact on the conventional UL communication
DL
UL
PDSCH
D2D signal Rx
HARQ-ACK?
SF #n SF #n+1 SF #n+2 SF #n+3

8. eNB control for D2D links
 eNB should be able to control D2D links.
At least for the interference coordination purpose
 The range of the eNB controllability?
Approach 1: eNB fully controls D2D transmission/reception.
 Including resource allocation, HARQ, link adaptation, power control, …
 Potential to have better interference coordination and coexistence with eNB-UE links
Approach 2: Some transmission attributes are determined by the UEs.
 For example, the transmit UE autonomously determines HARQ and link adaptation parts while
using time/frequency resources and transmit power the eNB indicated in a semi-static manner.
 Potential to reduce the control signaling overhead and D2D communication latency
eNB
D2D Comm.
Dynamic control
signaling
Time 0
eNB
D2D Comm.
Dynamic control
signaling
Time 1
eNB
D2D Comm.
Semi-static
control signaling
Time 0
eNB
D2D Comm.
Time 1
Approach 1
Approach 2

9. Interference management
 D2D signal is new source of interference.
New type of interference can be generated: UE-to-UE interference if UL resource is
used for D2D.
Transmit power control is needed for D2D
 To protect UEs receiving DL signal.
 To enable the spatial resource reuse as much as possible.
D2D transmit power control needs to be separated from that of eNB-UE link.
 The distance to the target reception point is different.
eNB
Intra-cell interference
coordination
Control eNB
Inter-cell interference
coordination
D2D

10. Advanced communication schemes based
on D2D
 D2D is a communication technology which requires a lot of new
functionalities.
 More advanced communication schemes can be built by using the
D2D functionalities.
 UE relay
UE receives other UE’s data and forwards it to the target UE.
 Dynamic resource adaptation
eNB transmits DL data in UL resource when the DL traffic is heavy.

11. UE relay
 UE relaying for throughput enhancement
UE receives other UE’s data and forwards it to the target UE.
Throughput improvement by reinforcing the weak channel between eNB and the target UE
 Type 2 relay
A terminology introduced during the study on relaying in 3GPP*
A relay node does not create any new cell. It appears as a group of antennas to the destination UE.
 The destination UE thinks that it is connected to the eNB and controlled by the eNB.
 More suitable for the UE relay
UE relaying can be operated within a HARQ process.
Control
* TR 36.814, 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Further Advancements
for E-UTRA Physical Layer Aspects.
Target UE
Relay UE
Data
Overhear
Time 1
HARQ ACK
Target UE
Relay UE
Time 2
HARQ NACK
Target UE
Relay UE
Time 4
HARQ ACK
Control
Target UE
Relay UE
Data as
retransmission
Time 3

12. UE relay (cont’d)
 Information flooding
UEs are involved in broadcasting the information relevant to all the other UEs.
 E.g., public warning system
The broadcast coverage can be improved.
eNB
Broadcast
information
Broadcast
information
Broadcast
information
Broadcast
information

13. UE relay (cont’d)
 Commonality among UEs can be found based on UE discovery procedure
eNB-UE communication can be enhanced if eNB knows which UEs are close to each other.
 Information sharing
UEs in the close proximity can share the common information.
An example is the group handover.
 A UE is selected and performs the handover procedure for a group of UEs.
 The handover result (e.g., the new cell identification, system information, timing information,
and so on) is forwarded to the UEs.
 Signaling overhead and handover latency can be reduced.
Group HO

14. UE relay (cont’d)
 Collaborative transmission/reception
UEs in close proximity exchange the signal to transmit and a UE transmits another UE’s data signal.
A UE receives another UE’s data signal and exchange it.
This effectively increases the number of transmit/receive antennas.
 For higher data rank or more diversity order
Time 1
Transmit signal
exchange
Time 2
Data transmission
Virtual MIMO
operation
Time 0
Transmit signal
S1
Transmit signal
S2 S1+S2
S1+S2

15. Dynamic resource adaptation
 The current cellular system statically divides the whole resource for the use
of DL and UL transmission.
DL and UL band in FDD, DL and UL subframe in TDD
 Dynamic resource adaptation in consideration of the traffic load
eNB transmits DL data in UL resource when the DL traffic is heavy.
 UE transmission in DL resource seems difficult due to the implementation cost and heavier
inter-cell interference.
A D2D UE is already equipped with the ability to receive data in UL resource.
Symmetric Traffic Situation
…
Time 1 Time 2
Resource used for DL/UL
Buffer Status
(Time 1)
UL traffic
DL buffer
UL buffer
Heavy DL Traffic Situation
UL traffic
DL buffer
UL buffer
Buffer Status
(Time 2)

16. Dynamic resource adaptation (cont’d)
 eNB transmission in UL resource?
Can be seen as D2D between a normal UE and a special UE which is possessed by
eNB?
Interference issues
 eNB-eNB interference management by eNB transmit power control, resource
coordination, …
 UE-UE interference is similar to the conventional D2D operation
Transmission
in UL resource
Wireline connection
eNB transmission
in UL resource
A cell operating eNB-
to-UE transmission
A cell operating UE-
to-eNB transmission
UE transmission
in UL resource
eNB-to-eNB
interference
UE-to-UE
interference

17. Dynamic resource adaptation (cont’d)
 A study item has been completed for TDD in 3GPP LTE*
Significant performance gain was observed in the dynamic resource adaptation.
A couple of issues were identified in control signaling, HARQ, interference
management, …
* RP-101427, Further Enhancements to LTE TDD for DL-UL Interference Management and Traffic Adaptation
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UEAvg.DLPktThpt[Mbps]
DLArrival Rate
'Infinity' Resource Reconfiguration Period
'10ms' Resource Reconfiguration Period
'640ms' Resource Reconfiguration Period
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0 0.25 0.5 0.75 1UEAvg.ULPktThpt[Mbps]
ULArrival Rate
'Infinity' Resource Reconfiguration Period
'10ms' Resource Reconfiguration Period
'640ms' Resource Reconfiguration Period

18. Summary
 Required functions for D2D
UE discovery
Signal reception in UL resource
Maintaining UE-UE link and eNB-UE link
Interference management
 The functions introduced for D2D can be
important enablers of advanced communication
schemes including
UE relay
Dynamic resource adaptation