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
13
Macro Small cell
Low band, e.g. AWS/PCS: macro
High band, e.g. 3.5 GHz: small cell
Small Cell Throughput Gains
14
“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
17
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
18
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
19
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
20
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