Technologies based on Device-to-device Communications

The 5th International Workshop LTE-

Advanced and Beyond Hanbyul Seo, LG Electronics

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

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UE

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SGW/PGW

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Conventional data path setup An exemplary data path setup in D2D

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.

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

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

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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D2D geometry [dB]

D2D in DL resource

D2D in UL resource

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

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

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eNB D2D Comm.

Semi-static

control signaling

Time 0 eNB

D2D Comm.

Time 1

Approach 1

Approach 2

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

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.

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

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HARQ ACK

Target UE

Relay UE

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HARQ NACK

Target UE

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HARQ ACK Control

Target UE

Relay UE

Data as retransmission

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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

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

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

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Transmit signal exchange

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Data transmission

Virtual MIMO operation

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Transmit signal S2 S1+S2

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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

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Resource used for DL/UL

Buffer Status

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UL traffic

DL buffer

UL buffer

Heavy DL Traffic Situation

UL traffic

DL buffer

UL buffer

Buffer Status

(Time 2)

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

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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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