The Role of Small Cells in an LTE Environment

Small Cells Americas 2012

December 3, 2012 Dallas

1

Ahmad Armand, Ph.D.Staff Vice President

CTO Office

The Role of Small Cells in an LTE

Environment

LTE Capacity Limits

LTE Advanced

Why Small Cells

Small Cell Architecture Options

Small cell Deployment Options

Interference Scenarios

Interference Mitigation

Conclusions

2

Agenda

3

LTE Capacity Limits:

Spectrum

Modulation order (QAM level)

MIMO order

Baseband Processing capability

Backhaul Capacity

UE capability

Traffic Mix

LTE Advanced Capacity Enhancements

Carrier Aggregation

Extension of MIMO techniques

- Up to 8 layer transmission in downlink

- Up to 4 layer transmission in uplink

Coordinated Multi-Point Transmission / Reception (CoMP)

HetNets

Relaying

SON techniques

Enhancing Data Capacity

I

Q

64-QAM, 6 bits/symbol

I

Q

16-QAM, 4 bits/symbol

MIMO order

4

LTE Advanced

Carrier Aggregation

Multi-Antenna Enhancements Relaying

5

Homogenous Networks

Base stations are in a planned layout

Base stations have similar transmit and receive characteristics (same transmit power

levels, antenna patterns, receivers , etc.)

Locations of macro base stations are primarily chosen to maximize the coverage

As the traffic demand grows and the RF environment changes, the network relies on

cell splitting or additional carriers to overcome capacity and link budget limitations

Site acquisition for macro base stations with towers becomes more difficult in dense

urban areas.

6

Why Small Cells (1/3)

7

Why Small Cells (2/3)

8

Why Small Cells (3/3)

Heterogeneous

Networks

9

Heterogeneous Networks

Macro Cell

60 W +

WiFI

DAS

Pico Cell

1-2 WMicro Cell

5-10 W

Femto Cell

10

Small Cell Architecture Options

S1

S1 S1

X2

X2 X2

S1/X2

RRH RRH

CPRI CPRI

Tight Coupling

Same vendor

Fiber interconnection

Loose Coupling

Same or different vendor??

Fiber/Microwave/Ethernet backhaul

No Coupling

Same or different vendor

Fiber/Microwave/Ethernet backhaul

11

HetNet Deployment Options

Microwave:

LOS or NLOS

Same Frequency OperationFocus of LTE Release 10 & 11

12

Interference

Select node with highest DL power

“Conventional” node selection

Not necessarily the cell with least path loss

Reduced uplink performance

Small pico uptake area

Select based on path loss

Not necessarily the cell with strongest downlink

Higher downlink interference

Expanded pico capture area

Extended off-loading

Enhanced uplink performance

Potentially severe DL interference from

macro site to pico UEs

Separate -Band OperationFocus of LTE Release 12

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Macro Small cell

Low band, e.g. AWS/PCS: macro

High band, e.g. 3.5 GHz: small cell

Small Cell Throughput Gains

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“A Survey on 3GPP Heterogeneous Networks,” Aleksandar Damnjanovic, et al, IEEE wireless Communications, June 2011

RP = Resource

Partitioning

Small Cell Interference Examples

15

(a) A macro user interfered by the small cell

(b) A macro user causes severe interference towards the small cell

(c) A small cell user is interfered by another small cell

(a)(b)

(c)

Cell Selection and Range Extension

16

Select node with highest DL power

“Conventional” node selection

Not necessarily the cell with least path loss

Reduced uplink performance

Small pico uptake area

Expanded pico capture area

Extended off-loading

Enhanced uplink performance

Potentially severe DL interference from

macro site to pico UEs

Interference

Select based on path loss

Not necessarily the cell with strongest downlink

Higher downlink interference

Cell Range Extension

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Cell range extension enables small cell to capture more traffic

Range extension is achieved by applying a bias to the RSRP of the small cell during

cell selection

The amount of bias is limited by the performance of control channels

Improved uplink speed, leveraging small cell link budget

Improved downlink speed, thanks to macro offload

Inter Cell Interference Coordination

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Load balancing between macro cell

and small cell

Improves control channel

performance

Requires perfect synchronization

between macro and small cell

X2 signaling used to exchange

information about protected

subframes

Pico

Micro

Regular

subframeABS

subframe

Protected

subframe

PDCCH PDCCH PDCCH PDCCH

Ma

cro

ce

ll PD

CC

H

Pic

oce

ll PD

CC

H

Ma

cro

ce

ll PD

CC

H

Pic

oce

ll PD

CC

H

Inter cell interference of control

channels avoided by coordinated

placement of ePDCCH between

macro and small cell layers

UE will get control information localized

within particular resource blocks

The existing PDCCH may remain

unchanged for the Release 10 and earlier

UEs

Placement of ePDCCH must be

coordinated between cells, not

specified in 3GPP

eICIC – Almost Blank Subframes (ABS)

ePDCCH – Enhanced DL Control Channel

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Reducing Interference via Cross Carrier Scheduling

Carrier aggregation with cross carrier scheduling

Avoids interference of PDCCH between macro and pico cells

Partition component carriers in each cell layer into two sets, one set used for data

and control and one set used mainly for data and possibly control signalling with

reduced transmission power

MacroPico

f1

f2

f1

f2

f1

f2

f1

f2

Macro UE

• Control signaling on f1• Data on f1 and/or f2

Pico UE

• Control signaling on f2• Data on f1 and/or f2

Macro UE

• Control signaling on f1 and/or f2• Data on f1 and/or f2

Conclusions

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Satisfying the ever-increasing demand for data requires continuous growth in

the overall LTE system throughput

LTE-Advanced multi-antenna and carrier aggregation techniques certainly

enhance the LTE throughput, but, may not always be feasible or cost effective

Small cells offer an alternative to pure macro cell splitting and play an

important role in addressing capacity limitations

A key challenge in small cell deployment is the management of interference

among different cell types

LTE Release 8/9 techniques along with small cell power management

techniques could provide adequate interference management

LTE-Advanced (Release 10 and beyond) provide further improvements in small

cell interference management and the overall system throughput