Mobile Relay Enhancements For LTE-A - Oulu

Mobile Relay Enhancements For LTE-A

University of Oulu Wireless

Communications Research Seminar 2012

Simon Scott

LOCON - Local connectivity and cross layer design for future broadband mobile systems

© Centre for Wireless Communications, University of Oulu

Introduction

• Support for fixed cellular relays was introduced in 3GPP

release 10 (LTE-A).

• Relays may be used to extend the coverage of cells, or

improve spectral efficiency by decreasing user to

infrastructure distance.

• Wireless backhaul makes relays cheaper and faster to

deploy than eNodeBs, particularly in built up areas.

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Introduction: mobile relays

• In 3GPP release 10 handover is not supported for relays.

• The 3GPP has approved a study item (36.416) on mobile

relays for release 11.

• The mobile relay study item focuses on a high speed train

scenario characterised by:

• High speed ~300Km/h

• High penetration loss through carriage walls.

• Known path

• UE are stationary, or at pedestrian speeds relative to

the relay nodes.

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Motivation for Mobile Relays

• High speed train networks are being deployed at

increasing rate worldwide.

• Providing cellular services to onboard passengers is

important but more challenging than in typical

environments:

• High penetration loss, may be upto 24dB on some

more modern trains.

• Severe doppler shift.

• Reduced handover success rate, reduced spectral

efficiency.

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Motivation for Mobile Relays

• The use of mobile relays can overcome these problems:

• Penetration loss eliminated, backhaul link antennas

on exterior of train, access link antennas on interior.

• Group mobility increases succesful handover

probability, and reduces handover signalling

overhead.

• More advanced signal processing algorithms may be

utilised in relays than in UE to combat doppler shift.

• UE may use reduced transmit power prolonging

battery life.

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Other solutions

• One solution is to deploy dedicated macro eNodeBs with

(or without) L1 repeaters onboard:

• Dedicated macro eNodeB deployment with

directional antennas provides path for all train traffic,

with larger cell overlap to increase handover success

rate.

• Such systems have been successfully deployed in for

example metro train systems.

• In addition L1 repeaters can be used to overcome the

penetration loss.

• Several proposed and implemented systems relying on

WiFi for access link.

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Advantages of mobile relays over

existing solutions

• Improved quality of service for all cellular services.

• Can use existing cellular infrastructure, reducing cost over

solutions requiring dedicated eNodeBs.

• Improved spectral efficiency over repeaters.

• Onboard services.

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Some challenges

• Backhaul link and access links may operate in shared

spectrum, duplexing operation may be required.

• Resource allocation at the DeNodeB in order to provide

sufficiently high data-rates for the the backhaul links, while

maintaining fair sharing of resources with normal macro

users.

• The backhaul link is the capacity bottleneck of the whole

system, high speed poses challenges to MIMO-OFDMA

link adaptation when considering maximizing the backhaul

link spectral efficiency.

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System Model

9 16.2.2012

ce

ll bo

rde

r

crX2 interface

moving train

L meters

RN#1

Donor eNB#X

Donor eNB#Y

X2 interface

RN#2RN#N-1RN#N

moving cooperative cell group


System level simulations

• LTE compliant system simulator modified to simulate high

speed train scenario.

• Backhaul link (DeNB -> MRN) is modelled accurately.

• Multiple backhaul links.

• Direct link from eNodeB to UE onboard train is also

modelled.

• Access link is not modelled at present. It is assumed that

the backhaul link is the capacity bottleneck, and that the

access link is always better than the backhaul link.

• Static 2:2 half-duplex operation of the MRN backhaul link.

10 16.2.2012


Simulator layout

• Trains are dropped on

a track with radius of

4Km.

• Train dropped along

the track such that the

whole train is inside the

central 57 cell layout.

• A train consists of 8

carriages 30m long,

each with an MRN.

11 16.2.2012


Simulator layout

• 20 active train users are evenly distributed amongst the

carriages of a train.

• Train users are paired with the MRN access point located

in the same carriage.

• Normal macro users are evenly distributed throughout the

cell layout.

• MRNs are paired independently with base-stations, the

serving link is determined by path-loss.

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Resource allocation

• MRNs are guaranteed a proportion of resources based on

the number of users they are serving (half duplex <= 50%).

• Proportional fair scheduling with a rate constraint is used

for normal macro users.

• Macro users are scheduled while the MRN backhaul link is

inactive (access link active).

• When users are connected through a MRN, it is assumed

that all MRNs may cooperate to share resources perfectly

fairly.

• Proportional fair scheduling is used for the users onboard

the train in the case of being connected directly to the

eNodeB.

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Simulation parameters

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System Bandwidth: 10MHz (50PRB)

Propagation Environment Urban Macro

Number of Macro Users ~10 per cell

Number of Train Users 20

Base-station Tx Antennas 1

UE Rx Antennas 2

MRN Rx Antennas 4

Carriage Penetration Loss 20dB

Traffic Model (Train users) Full Buffer

Traffic Model (Normal

Macro users) 250Kbits/s Rate Constraint


Results: Train user throughput

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Results: Normal macro user throughput

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Further work

• Extend to multiple transmit antennas in MRN backhaul.

• Access link model:

• Not a fair assumption that access link will be

bottleneck in full duplex-case.

• Allow for dynamic half-duplex scheme to be

implemented (1:3, 2:2, 3:1).

17 16.2.2012

Mobile Relay Enhancements For LTE-A - Oulu

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