LTE-Advanced Part 2: LTE-Advanced relaying

Jyri Hmlinen, 2015 Department of Communications and Networking

Contents

Part 2: LTE-Advanced relaying (Rel.10) 2.1 Relaying principles, need for relaying and use cases 2.2 LTE-Advanced relaying principles 2.3 LTE-Advanced Type 1 relaying: The Backhaul problem

2.1 Relaying principles, need for relaying and use cases

2/19/2010 Word template user guide

3

Wireless relay: Principle

I listen, modify and

retell

I have a message

I am only listening

Base Station (BS) Relay Station (RS) Mobile Station (MS)

Repeaters (amplify and forward relays) are well known and used in 2-3G networks.

Decode and forward relays in mobile communication systems: - IEEE802.16e admit amendment IEEE802.16j (relay specification) - DF relays form an integral part of IEEE802.16m - LTE-Advanced include DF relay specifications

Basic relaying types

2/19/2010 Word template user guide

5

In addition to DF and AF relays there are hybrid relay types but they are omitted in this course.

Decode and Forward (DF) Signal is detected and encoded

before retransmission. DF relay retransmit interference

free signal if detection is successful

The principle of DF relays have been well known for a long time but DF relays has been accepted to mobile communication standards only recently.

DF relays are feasible also for interference limited network scenarios.

Amplify and Forward (AF) Signal is received, amplified and

retransmitted as such. AF relay amplifies both desired

signal and noise. Conventionally used as gap fillers

(to fill coverage holes) Noise limited network scenarios

are more favorable for AF relaying.

AF admit simple structure but antenna implementation maybe costly if full duplex operation is assumed.

Duplexing approaches

Half Duplex (HD) No simultaneous reception and

retransmission of the signals.

Full Duplex (FD) Simultaneous reception and

retransmission of the signals.

RX

TX

1

1

2

2

1 2 3

1 2 3

Processing delay Processing delay

Relays in LTE

eNB

UE UE

UE

UE

IP network

High capacity wired backbone

Important: - RN is wireless => more location opportunities + lower site costs - Relays are used to boost cell coverage/capacity - RN is preferably simpler, smaller and cheaper than eNB

UE

BS signal is not received well but RN signal level is good

RN1

Direct connection to eNB possible but no high data rates without RN

RN2

RN above rooftop: Coverage increase

RN below rooftop: Local capacity/service boost

First hop

Link between BS and UE

LTE-Advanced relay?

Macro base station. - Antennas on towers or over the rooftop - High transmissions powers - High capacity backhaul and power backup needed.

Micro base station. - Wall installation possible. - Antennas under the rooftop but in relatively high locations - Few blocks coverage usual on urban areas

Pico base station - Indoor/outdoor installation. - Antennas can be integrated in the box - Coverage over an office area (floor)

Femto base station - Used in homes/offices etc - User may install - Indoor installation - Coverage for a very limited area

Wireless relay - LTE-A relays are actually small wireless base stations (Pico, Femto, Micro)

Why relays for LTE?

Some key requirements for LTE-Advanced

1 Gbps on the downlink and 500 Mbps on the uplink.

Higher peak and average spectral efficiencies than in LTE Rel8.

More homogeneous distribution of the user experience over the coverage area.

Expected properties of LTE-Advanced relays

Enhanced capacity in hotspots. Enhanced cell coverage. Overcome extensive shadowing. Enable more homogenous user

experience. Low total cost of operation

(TCO).

Proposed benefits from relaying

Extend coverage

Overcome excessive shadowing d-eNB

RN Increase throughput in hotspots

Relay link

Access link

UE Direct link

UE

UE

UE

RN

RN

Is this convincing? We need to dig little bit deeper.

But are relays really needed?

Claim is that relays will provide an easy and cost effective way to increase macrocell range, fill coverage holes in macrocells and improve indoor coverage. Counter argument: There are other efficient solutions like macrocell

antenna diversity and beamforming, and more importantly, micro and pico cells.

Fact: Among small access nodes only relays are wireless (no wired backhaul) and thus flexible to deploy. Therefore direct comparison against e.g. pico eNBs of the same size is not fair.

There seems not to be any inband DF relay products on the market. Outband relays (wireless pico base stations exists)

Use cases for LTE-Advanced relays

Relay use case

Fixed Infrastructure Usage

Outdoor Relay for Indoor Coverage

Enhancement

In-Building Relay for Coverage Enhancement

Temporary Usage

Coverage in case of emergency /

disaster

Coverage in case of events

Mobile Usage Coverage in trains, busses, ferries

This has not realized in Rel.10/11

2.2 LTE-Advanced relaying principles

LTE-Advanced relaying principles

In 3GPP Technical Report [TR 36.814] the following has been stated:

Relaying is considered for LTE-Advanced as a tool to improve e.g. the coverage of high data rates, group mobility, temporary network deployment, the cell-edge throughput and/or to provide coverage in new areas. LTE-A specifications support fixed relaying and nomadic relaying is

possible but relay (group) mobility is not yet part of the standards.

The relay node is wirelessly connected to the radio-access network via a donor cell.

Donor eNB Relay Node (RN)

Donor cell

Relay-eNB link

Inband operation/outband operation

In 3GPP terminology the relay nodes usage of spectrum can be classified into: Inband, in which case the eNB -relay link shares the same carrier

frequency with relay-UE links. LTE Rel-8 UEs should be able to connect to the donor cell in this case.

Outband, in which case the eNB-relay link does not operate in the same carrier frequency as relay-UE links. LTE Rel-8 UEs should be able to connect to the donor cell also in this case.

For both inband and outband relaying, it shall be possible to operate the eNB-to-relay link on the same carrier frequency as eNB-to-UE links

Inband operation/outband operation

Donor eNB Relay Node (RN)

Donor cell

Relay-eNB link

UE UE

Inband operation

Donor eNB Relay Node (RN)

Donor cell

Relay-eNB link

UE UE

Outband operation

UE-eNB link

UE-eNB link

Note: In outband operation RN-UE link do not need to be LTE Rel8 compatible if Rel8 terminals are not operating on this frequency carrier.

3GPP relays control their own resources

Relay control cells of its own

Donor eNB Relay Node (RN)

Donor eNB control resources in eNB relay link

UE

Relay control cell of its own: protocol terminations done mostly in RN

Relay control its own cell

Cell ID: A unique physical-layer cell identity (PCI) is assigned for each of the cells controlled by the relay. Thus, relay may control multiple sectors like eNodeB.

Radio Resource Management: The same RRM mechanisms as in eNodeB are used in relays and from a UE perspective there is no difference in accessing cells controlled by a relay and cells controlled by a normal eNodeB.

Backward compatibility: The cells controlled by the relay should support also LTE Rel-8 UEs.

Self-backhauling: Type 1 relay nodes, Type 1a relay nodes, and Type 1b relay nodes use this type of relaying (defined later).

Relaying types: Type 1

Type 1 relay node is an inband relaying node characterized by the following: It control it own cells, each of which appears to a UE as a

separate cell distinct from the donor cell The cells shall have their own Physical Cell ID (defined in LTE

Rel-8) and the relay node shall transmit its own synchronization channels, reference symbols, etc

Relay shall appear as a Rel-8 eNodeB to Rel-8 UEs (i.e. be backwards compatible)

To LTE-Advanced UEs, it is possible for a relay node to appear differently than Rel-8 eNodeB to allow for further performance enhancement.

Relaying types: Type 1a and Type 1b

Type 1a and Type 1b relays are defined as follows: Type 1a and Type 1b relay nodes are characterised by the same

set of features as the Type 1 relay node, except Type 1a operates outband and Type 1b operates inband with

adequate antenna isolation. Type 1a relay node is expected to have little or no impact on

LTE physical layer specifications.

Relaying types: Type 1a and Type 1b

Notes: Type 1a: outband operation means that RN-donor eNodeB link is

operated on different carrier frequency than link RN-UE. Then both links can be implemented according to existing Rel8 specifications since no time division between links is needed.

Type 1b: Similar inband operation is assumed for Type 1b like for Type 1 but the difference between Type 1 and Type 1b is that Type 1b assumes adequate antenna isolation between RN receive and transmit antennas =>

Type 1 RN is preferably a half duplex relay. Type 1a RN is preferably a full duplex relay Type 1b RN is preferably a full duplex relay

It is important to note that Type 1b requires antenna design that can be expensive due to assumed antenna isolation.

Type 2 relay

In 3GPP Technical Report [TR 36.814] A Type 2 relay node was defined to be an inband relaying node characterized by the following: It does not have a separate Physical Cell ID and thus would not

create any new cells. It is transparent to Rel-8 UEs; a Rel-8 UE is not aware of the

presence of a Type 2 relay node.

Yet, up to date such relay type has not been standardized for LTE.

2/19/2010 Word template user guide

22

2.3 LTE-Advanced Type 1 relaying: The Backhaul problem

Inband Type 1 relaying: the resource sharing needed between links In order to allow inband relaying, resources in the LTE

time-frequency space needs to be shared between backhaul and access links Backhaul link between RN and Donor eNodeB: The name of this

logical interface is Un (defined in LTE Rel.10) Backhaul resources cannot be used for the access link. The

name of this logical interface is Uu (as in LTE Rel. 8).

Donor eNB Relay Node (RN) UE Un Uu

Resource sharing should be compatible with LTE Rel8

Resource sharing: General principle

General principle for resource partitioning at the LTE-Advanced Type 1 relay: eNB RN and RN UE links are time division multiplexed in

a single carrier frequency RN eNB and UE RN links are time division multiplexed in

a single carrier frequency

(Donor) eNodeB RN RN UE

UE RN RN (Donor) eNodeB

DL

UL

Sounds simple but is it really straightforward?

Resource sharing: FDD and TDD principles Multiplexing of backhaul links in FDD:

eNB RN transmissions are done in the DL frequency band RN eNB transmissions are done in the UL frequency band

Multiplexing of backhaul links in TDD: eNB RN transmissions are done in the DL subframes of the

eNB and RN RN eNB transmissions are done in the UL subframes of the

eNB and RN

This is still simple and straightforward

BH backward compatibility: A problem

Backward compatibility requirement with LTE Rel.8 creates a problem: Rel.8 UE expects continuous pilot/control transmission in DL

from eNodeB. In case of Type 1 relay, RN represents the eNodeB for Rel.8 terminal.

RN should be able to receive backhaul (Un) transmissions on the same frequency.

Problem: Reception and transmission on the same frequency carrier is possible only for Type 1b relay that admit physical separation between RX and TX antennas. Yet, this is costly solution.

This problem does not occur in uplink. Type 1 relaying is the most attractive relaying option. Yet, due to above problem there was a threat in the beginning of the LTE-Advanced standardization that relaying will be dropped out.

BH backward compatibility: The solution

Recall: LTE Frame consists of 10 subframes of 1 ms each. Part of the LTE DL subframes can be configured as MBSFN

subframes

What this means?

MBSFN what?

MBSFN refers to term Multi-Media Broadcast over a Single Frequency Network.

In LTE Rel.8 MBSFN subframes are designed to carry MBMS (Multimedia Broadcast Multicast System) information.

Before LTE MBMS was introduced for WCDMA/HSPA Rel.6 and it supports multicast/broadcast services over a single frequency network. MBMS service area typically covers multiple cells.

Example application is Mobile TV.

MBSFN subframe assignment

LTE Rel.8 terminals in Donor eNodeB cell can be informed regarding the set of applied MBSFN subframes. LTE Rel.8 terminals in RN cell will have reception gap during

MBSFN subframe but RN will be able to listen backhaul transmission from Donor eNodeB. Thus, backward compatibility problem is solved.

The set of MBSFN subframes is semi-statically assigned; a maximum of 6 subframes can be configured out of the subframes 1, 2, 3, 6, 7, and 8 [*].

MBSFN subframe assignment

It is important to note that UEs that are directly connected to eNodeB and RNs can both be co-scheduled on MBSFN subframes.

Number of MBSFN subframes defines the backhaul resources for relays in the Donor eNodeB cell. If there are too few MBSFN subframes, then RN cell capacity

will be limited (backhaul becomes bottleneck). If there are too many MBSFN subframes, then RN cells enjoy of

good data rates but in cost of reduced rates among users that are directly connected to Donor eNodeB.

Some details

A new physical control channel (referred as the R-PDCCH) is used to dynamically or semi-persistently assign resources, within the semi-statically assigned sub-frames, for the downlink backhaul data (corresponding to the R-PDSCH physical channel). The R-PDCCH may assign downlink resources in the same and/or in one or more later subframes.

The R-PDCCH is also used to dynamically or semi-persistently assign resources for the uplink backhaul data (the R-PUSCH physical channel). The R-PDCCH may assign uplink resources in one or more later subframes.

More information: Physical layer for relaying operation (Release 10), 3GPP TS 36.216

2.4 Relay deployments and performance

2/19/2010 Word template user guide

33

Outdoor to indoor coverage provision

200 m

Donor eNodeB

Height: 25 m

30 m

Important: Relay should be placed in location where - Link to donor eNodeB is good - Target area is covered - Interference is minimized

RN

Feasible indoor Internet service range without RN

RN is outdoors => 20dB better link budget than for indoor terminals

Relay height e.g. 5m 1000m 600m 200m

RLB example on use of relaying

Assume the (previous CA example) 10MHz band, 800MHz/2GHz component carriers, 35 meter base station antenna height, 1.5 meter UE height and 5 meter relay height (lamp post relay).

Compute the relay link rate (on 2GHz carrier) when eNodeB allocates 60% of the 1 MBSFN subframe for the relay backhaul. Relay is placed outdoors, distance from eNodeB is 600 meters.

2/19/2010 Word template user guide

35

Macro eNodeB

Macro indoor coverage for 800MHz carrier

Macro indoor coverage for 2GHz carrier

Relay coverage on 2GHz carrier

BH link to relay (on 2GHz carrier)

RLB example on use of relaying

According to previous CA example, the 800MHz component carrier defines the cell range:

Indoor user maximum distance from eNodeB = 730 meters = cell range Relay is located 600 meters from eNodeB

Relay backhaul link assumptions: Relay is outdoors, on 5 meter height, applies 2 antennas and consumes 1 MBSFN subframe

(10% of eNodeB resources) in backhaul link Relay applies 2x2 MIMO, admit no receive antenna gain and NF = 5dB

Relay BH rate: If relay link uses 29PRBs on one subframe it reaches 38 Mbit/s instantaneous rate and in

average 0.2*38Mbit/s = 7.6 Mbit/s rate which is available for users connected to the relay. Remarks:

In this example only very few resources are allocated to relay but since it operates close to maximum efficiency (7.28bit/s/Hz) the BH rate is quite good.

In general outdoor relays operate close to maximum efficiency and take only small amount of BH resources

2/19/2010 Word template user guide

36