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The Performance of Open AccessFemtocells in 4G MacrocellularNetworksAndy Jeffries
awj@nortel.com
WWRF 20, Ottawa 22.04.2008
THINK AHEADSTAY AHEAD
STRATEGIC VISIONSon future research directions
in the wireless field
Page 2Performance of Open Access Femtocells in 4G Macrocellular Networks WWRF#20 · Andy Jeffries · awj@nortel.com
Introduction
• Femtocells have the potential to offer significant performance benefits to future cellular networks
• RF interference issues are key risk area in the deployment of femtocells in macrocellular networks
• Paper addresses SNIR and resulting data rates that can be achieved- Downlink only RF performance using a generic 4G air interface protocol- Open Access femtocell operation
• Work conducted in collaboration with Vodafone and Multiple Access Communications under the MONOTAS* Programme with UK Department of Trade & Industry funding
- Public deliverable available at http://www.macltd.com/monotas/
* MONOTAS - MObile Network Optimisation Through Advanced Simulation
Page 3Performance of Open Access Femtocells in 4G Macrocellular Networks WWRF#20 · Andy Jeffries · awj@nortel.com
What are Femtocells?
ControlEntity
Femtocellsin homes
Femtocells in commercial/
industrial organisations
Macrocellboundary
Operator’s Network Management centre. May set some of the
femtocell parameters
Broadband (wired) backhaul to operator’s
network • Femtocells are low power basestations deployed in domestic and commercial environments to improve coverage (known as Home-eNodeB in 3GPP).
• These access points will be connected to the operator’s network through their wired broadband connections
• Operator may set power levels, carriers etc. on the femtocells• May use same carrier for all macro- & femto-cells in a network• Can operate in Open Access or Closed Access mode
• Femtocells are low power basestations deployed in domestic and commercial environments to improve coverage (known as Home-eNodeB in 3GPP).
• These access points will be connected to the operator’s network through their wired broadband connections
• Operator may set power levels, carriers etc. on the femtocells• May use same carrier for all macro- & femto-cells in a network• Can operate in Open Access or Closed Access mode
Femtocells extending range of the existing
macrocell
Page 4Performance of Open Access Femtocells in 4G Macrocellular Networks WWRF#20 · Andy Jeffries · awj@nortel.com
-2000
-1500
-1000
-500
0
500
1000
1500
2000
-2000 -1500 -1000 -500 0 500 1000 1500 2000x-coordinate (m)
y-co
ordi
nate
(m)
MacrocellsFemtocellsFemtocell BoundaryCentral Cell Boundary
Simulation Scenario - 1
Macrocellular network• 19 tri-sectored
basestations• Spacing 1km• TX power +43dBm• Beam pointing angles
at 30°, 150° and 270°(referenced to due E)
• Antenna peak gain ~14dB, backlobe level -6dB
Femtocells• TX power +24dBm• Omnidirectional
antenna pattern gain 0dB
• Located on a grid spaced by 15m in x-and y- directions covering an area of one tri-sectored macrocell BTS.
Users• Randomly distributed,
200 per macrocellsector
Macrocellular network• 19 tri-sectored
basestations• Spacing 1km• TX power +43dBm• Beam pointing angles
at 30°, 150° and 270°(referenced to due E)
• Antenna peak gain ~14dB, backlobe level -6dB
Femtocells• TX power +24dBm• Omnidirectional
antenna pattern gain 0dB
• Located on a grid spaced by 15m in x-and y- directions covering an area of one tri-sectored macrocell BTS.
Users• Randomly distributed,
200 per macrocellsector
Tri-cellular arrangement of
Macrocell sectors
1km Macrocellspacing
Baseline simulations use 20% Femtocell
density
Femtocellsonly in 1 house in N: typically
10-50%
Femtocell layout
Page 5Performance of Open Access Femtocells in 4G Macrocellular Networks WWRF#20 · Andy Jeffries · awj@nortel.com
Simulation Scenario - 2
0
1
2
3
4
5
6
-20 -10 0 10 20 30 40SNIR (dB)
Thro
ughp
ut (b
its/s
/Hz)
Implemented ThroughputCharacteristic
Shannon Throughput
Throughput Conversion Characteristic
-220
-200
-180
-160
-140
-120
-100
-80
-60
-40
-20
1 10 100 1000 10000Range (m)
Path
Los
s (d
B)
Macrocell to User Path Loss
Femtocell to User Path Loss
Downlink Path Loss
Minimum SNIR -10dB before any data can get through
• Because they are unplanned, femtocellscan be located close to windows, in an upper floor etc. This has a significant impact upon the propagation to users
• A single slope propagation model, with a significant loss, to reflect transmission from outside to inside, has been used to calculate initial system performance
• 2GHz• 10dB Log-Normal σ• Macrocell-to-user:
• COST Hata single slope path loss model with correction for UE height ~r3.5
• Femtocell-to-user:• Higher path loss to reflect wall
penetration losses• r3.5 slope
Maximum throughput capped at 4.4b/s/Hz
(ref. 3GPP)
Page 6Performance of Open Access Femtocells in 4G Macrocellular Networks WWRF#20 · Andy Jeffries · awj@nortel.com
Performance Evaluation Technique
• C/I for a user calculated from the set of RXed signals: best/ ∑rest
• Maximum RX power from best server calculated per user
Serving Basestation(Macro or Femtocell)
Basestation(Macro or Femtocell)
User
Users
Identify #users served by basestation
Raw data rate calculated from SNIR from serving
basestation and Shannon capacity equation
Effective data rate calculated from raw data rate scaled by #users for
serving basestationCDFs
calculated of SNIRs and
Effective Data Rates
• Path losses & power levels for Macro- & Femtocell basestations combined to determine user SNIR• User data rate determined by dividing raw data rate by number of users on a base station
• Path losses & power levels for Macro- & Femtocell basestations combined to determine user SNIR• User data rate determined by dividing raw data rate by number of users on a base station
Page 7Performance of Open Access Femtocells in 4G Macrocellular Networks WWRF#20 · Andy Jeffries · awj@nortel.com
SNIR Distribution (scale in dB) EDR Distribution (scale in bits/s/Hz/User)
• Maps show open access operation of femtocells in a macrocell network. One dot = one user; • Approximately 60% of users are served by femtocells, 40% by macrocells. • Users served by femtocells enjoy very high SNIRs• Macrocell users in the femtocell region often see lower SNIRs because of the increased
interference from the femtocells.
• Maps show open access operation of femtocells in a macrocell network. One dot = one user; • Approximately 60% of users are served by femtocells, 40% by macrocells. • Users served by femtocells enjoy very high SNIRs• Macrocell users in the femtocell region often see lower SNIRs because of the increased
interference from the femtocells.
For clarity, log normal shadowing is omitted
Open Access Performance - 1 SNIR-Based Cell Selection
Page 8Performance of Open Access Femtocells in 4G Macrocellular Networks WWRF#20 · Andy Jeffries · awj@nortel.com
Open Access Performance - 2 SNIR-Based Cell Selection
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
-10 -5 0 5 10 15 20 25 30 35 40 45
SNIR (dB)
Cum
ulat
ive
Prob
abili
ty
Macrocell only network - all users
Open Access Macrocell served users
Open Access Femtocell served users
• Femtocell served users enjoy >100x increase in effective data rates because they are the only user on that femtocell
• Macrocell served users see a 1.23x increase in data rates because of the lower macrocell loading despite the lower SNIR
• Femtocell served users enjoy >100x increase in effective data rates because they are the only user on that femtocell
• Macrocell served users see a 1.23x increase in data rates because of the lower macrocell loading despite the lower SNIR
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.0001 0.001 0.01 0.1 1 10
EDR (bits/s/Hz/User)
Cum
ulat
ive
Prob
abili
ty
Macrocell onlynetwork - all users
Open AccessMacrocell servedusersOpen AccessFemtocell servedusers
SNIR Distribution EDR Distribution
Page 9Performance of Open Access Femtocells in 4G Macrocellular Networks WWRF#20 · Andy Jeffries · awj@nortel.com
Improving Data Rates Using EDR Based Cell Selection
With SNIR-based cell selection, users very close to femtocells can remain served by the macrocell
EDR Distribution
• Preferentially associating users with Femtocells benefits all• Remaining macrocell users benefit by reduced cell
loading• Users moved over to the femtocells benefit from
higher data rates due to the low femtocell loading -even if their resulting SNIR is lower
• Best EDR cell selection technique proposed for improving the data rate distribution, rather than the highest SNIR cell selection as traditionally used
• Preferentially associating users with Femtocells benefits all• Remaining macrocell users benefit by reduced cell
loading• Users moved over to the femtocells benefit from
higher data rates due to the low femtocell loading -even if their resulting SNIR is lower
• Best EDR cell selection technique proposed for improving the data rate distribution, rather than the highest SNIR cell selection as traditionally used
• Femtocells are small due to low Txpower and building penetration losses
• Cell loading thus generally low enabling very high data rates to be offered to users
• Femtocells are small due to low Txpower and building penetration losses
• Cell loading thus generally low enabling very high data rates to be offered to users
Page 10Performance of Open Access Femtocells in 4G Macrocellular Networks WWRF#20 · Andy Jeffries · awj@nortel.com
Comparison of SNIR and EDR Based Cell Selection
0
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0.4
0.5
0.6
0.7
0.8
0.9
1
0.0001 0.001 0.01 0.1 1 10
Effective Data Rate (bits/s/Hz/User)
Cum
ulat
ive
Prob
abili
ty
Best SNIR cell selection Macro onlyBest SNIR cell selection Macro servedBest SNIR cell selection Femto servedBest EDR cell selection Macro servedBest EDR cell selection Femto served
EDR Distributions with users split by server type
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.0001 0.001 0.01 0.1 1 10Effective Data Rate (bits/s/Hz/User)
Cum
ulat
ive
Prob
abili
ty
Best SNIR cell selection Macro only
Best SNIR cell selection all servers
Best EDR cell selection all servers
EDR Distributions across all users in scenario
Best EDR cell selection improves data rates by ~3.7 times for users at the 10% level of the
distribution. These are macrocell served users seeing the benefit of lower macrocell loading
• Femtocell TX power +24dBm
• 20% Femtocelldensity
• Femtocell TX power +24dBm
• 20% Femtocelldensity
Macrocell users see improved EDRs
Femtocell users see slightly degraded EDRs, but still a substantial improvement
over macrocell users
Macrocellonly
baseline
EDR Cell selection increases proportion of femtocell users from 60% (SNIR-based) to 80%
Page 11Performance of Open Access Femtocells in 4G Macrocellular Networks WWRF#20 · Andy Jeffries · awj@nortel.com
Performance Impact of Femtocell Density
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 10 20 30 40
Active Femtocell Density (%)
Prop
ortio
n of
Use
rs s
erve
d (%
)
Femtocell Users Best SNIR cell selection LN 10dB
Macrocell Users Best SNIR cell selection LN 10dB
Femtocell Users Best EDR cell selection LN 10dB
Macrocell Users Best EDR cell selection LN 10dB
Proportion of Users Served for scenario with varying femtocell density
To off-load 50% of users to femtocells,
require ~10% active femtocelldensity, or ~7% if ‘Best EDR’ cell selection used
• A relatively small femtocell penetration is sufficient to reduce macrocell loading by 50%• EDR-based cell selection off-loads users onto femtocells faster than SNIR-based cell selection
• A relatively small femtocell penetration is sufficient to reduce macrocell loading by 50%• EDR-based cell selection off-loads users onto femtocells faster than SNIR-based cell selection
FemtocellTX power +24dBm
FemtocellTX power +24dBm
Page 12Performance of Open Access Femtocells in 4G Macrocellular Networks WWRF#20 · Andy Jeffries · awj@nortel.com
Impact of EDR-based Cell Selection in Real DeploymentsEDR distribution for real user distribution in a major city,
10dB LN hexagon network data for comparison
0
0.1
0.2
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0.5
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0.9
1
0.0001 0.001 0.01 0.1 1
Effective Data Rate (bits/s/Hz/User)
Cum
ulat
ive
Prob
abili
ty
Best SNIR cell selected Macrocell hexagon network 10dB LN
Best SNIR cell selected data from major city
Best EDR cell selected data from major city
The use of ‘Best EDR’ cell selection increases the EDRs for users in the lowest percentiles of the EDR distribution
Higher EDRs than regular hexagon network reflect the
higher SNIRs seen in the traffic distribution for this city
• Use of an EDR-based cell selection mechanism is effective at improving performance in current deployments as well as in macro- / femtocell networks
• Use of an EDR-based cell selection mechanism is effective at improving performance in current deployments as well as in macro- / femtocell networks
Best EDR-based cell selection improves data rates by 30% at 10% level c.f. best SNIR-based Cell Selection
Page 13Performance of Open Access Femtocells in 4G Macrocellular Networks WWRF#20 · Andy Jeffries · awj@nortel.com
• Introducing open-access Femtocells into Macrocell networks can significantly improve network capacity and user data rates
- Data rates for users attached to femtocells can be improved by 2-3 orders of magnitude
- Macrocell users also benefit due to the reduced loading on the cell despite a general increase in interference
• Femtocells can provide significant network performance improvements with relatively low penetration rates
- a femtocell density of ~10% of households can displace ~50% of the macrocellload
• A Best-EDR based cell selection technique is very effective at promoting macrocell users onto femtocells
- It is particularly beneficial to poorer served users, with improvements in data rates ~3.7x seen by poorest 10% of users
• Best EDR-based cell selection also offers benefits in macrocell-only networks- ~30% improvement in EDRs for the worst 10% of users in a real network
Conclusions
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