Decentralized Interference Management for Two-Tier … · Decentralized Interference Management ... OFDMA femtocells: A roadmap on interference ... Nakata, A., Moessner.K., “Interference

AS Madhukumar

School of Computer Engineering

Nanyang Technological University, Singapore

Outline

February 18, 2013 School of Computer Engineering 1

Introduction

Femtocell Network Issues

Interference Management : A Review

Decentralized Interference Management

Through Femto-Relays

Through Reverse Frequency Allocation

Future Work

Conclusion

Introduction

The wireless capacity has doubled every 30 months since the last 104 years. This

translates into an approximately million-fold capacity increase since 1957.

Breaking down these gains shows the technology contributions as :

– Wider spectrum [25x]

– Spectrum slicing [5x]

– Better modulation and coding [5x]

– Topology and smaller cells [1600x]

M.-S Alouini and A. J. Goldsmith, “Area Spectral Efficiency of Cellular Mobile Radio Systems,” IEEE

Trans.Vehic. Tech., vol. 48, no. 4, July 1999, pp. 1047–66.


Introduction Femtocell Network Issues Interference Management Femto-Relays Reverse Frequency Allocation Future Work Conclusion

1,000,000=25x5x5x1600

Small cell seem to be the

key factor for improving

future network capacity

Introduction


This rapid increase in mobile data activity has raised the stakes on developing innovative new

technologies and cellular topologies that can meet these demands in an energy efficient manner


Introduction

Engineers brought in cellular hierarchy into the legacy cellular network to reduce load on the

macrocell base station and hence increase the coverage.

This resulted in reducing the cell size into still smaller cells creating a cellular pattern within

the existing cellular architecture.


Small cells provide coverage extension and boost local capacity with

minimal expense and planning.

2G 3G

4G

As throughput demand and usage

increased, cell size decreased


Femtocell Solution


– Small size cellular base stations for residential

or small business environments

– Use full strength mobile technology but with

simpler deployment

– Connects through internet grade backhaul.

– Operates in licensed spectrum.

– Typically support 2-6 concurrent users

– Available at prices compatible with Wi-Fi

access points.

– Alternative of Fixed Mobile Convergence.

– The concept is applicable to all wireless standards,

including UMTS, GSM, CDMA-2000 and WiMAX solutions.

Vikram Chandrasekhar and Jeffrey G. Andrews, “Femtocell Networks: A Survey,” IEEE Communications

Magazine, September 2008.


Femtocell Network




Network Architecture

Mobility Management and Handovers

Self Organization

Security

Timing and Synchronization

Interference Management

L. Perez, D. Valcarce, A.D. Roche, G.J. Zhang, OFDMA femtocells: A roadmap on interference avoidance IEEE Comm.. Mag, vol. 47, no. 9, pp. 41-48, Sept.2009.





Irregular deployment will incur inevitable interference

FC

FC

FC FC

FC

FC

FC

MC 1

FC

FC

FC FC

FC

FC

FC

FC

FC

FC

FC

MC 2

FC

FC

FC

FC

FC

FC

MC : Macro Cell

FC : Femto Cell




Types of Interference

– Co-tier Interference FBS to FBS Interference (relatively small

due to lower power and wall loss)

– Cross-Tier Interference • MBS to FBS Interference

• FBS to MBS Interference Femtocell

Macrocell

Femtocell

Femtocell




UE ASSOCIATION

A : MBS → FUE (DL)

B : MUE → FBS (UL)

C : FBS → MUE (DL)

D : FUE → MBS (UL)

E : FBS → FUE (DL)

F : FUE → FBS (UL)

A

B

C

D

E

F

Interference Scenarios


Interference Management Techniques


Zahir, T., Arshad, K., Nakata, A., Moessner.K., “Interference Management in Femtocells ,” IEEE Communications

Surveys & Tutorials, vol. PP, issue 99, pp. 1-19, February 2012.


• Interference Cancellation

– Successive Interference

Cancellation

– Parallel Interference

Cancellation

– Multi-Stage Interference

Cancellation

– Multi-User Detection

• Interference Avoidance

– Spectrum Splitting

– Power Control

– Time Hopping

– Spectrum Arrangement

• Fractional frequency

Reuse

• Soft frequency Reuse

– Resource Allocation

– Spectrum Sensing

• Cognitive Femtocells

– Inter-cell coordination

• Distributed Interference

Management

– Distributed power control

algorithm

– Distributed dynamic inter-

cell interference avoidance

scheme

– Geo-static power control

scheme

– Adaptive power control

scheme

– Pilot power minimization

scheme

Decentralized Interference Management

– Minimal involvement of a centralized controller

– No disturbance to the legacy macrocell network

– Base stations will self-configure, self-optimize and self-

heal.

– Rely more on existing architecture, framework and

resources to improve the system performance.



Femto-Relays Challenges

– Ever increasing demand for data intensive applications.

– Excessive load on the macrocell network.

– Increase in call drops due to limited macrocell backhaul.

– Dead-zones created in the macrocell coverage area.

– Extensive high density deployment of femtocells which seem to be

under-utilized.

– Limited coverage area of femtocells.



Femto-Relays Solutions – How to offload the heavy traffic from the macrocell network

• Increasing base station density through small cells or femtocells.

• Cisco expects that by 2015, over 800 million terabytes of mobile data traffic will be offloaded to the fixed network by means of femtocells.

• Telecoms & Media expects the small cell market to experience significant growth over the next few years, reaching just under 60 million femtocell access points in the market by 2015.

– Is the capacity of these femtocells fully exploited? • Obviously NO.

– How to make use of the large number of under utilized femtocells? • Extending the coverage of femtocells beyond the home environment to serve

macrocell users as well.

– How to extend femtocell coverage beyond the home environment? • Through relay nodes a.k.a FEMTO-RELAYS



Femto-Relays A novel air interface solution which integrates multihop

transmission into femtocell networks.

– Extends the coverage further, even beyond the home

environment.

– Accommodate more users.

– Higher capacity gains in both Uplink and Downlink direction.

– Improved data rate offered by the femtocells.

– Power efficiency offered by multi-hop transmission.



Femto-Relays : System Model



Femto-Relays : System Architecture

Assumptions

– Each user is capable of sensing the pilot signal channels from the Macro Cell

Base Station [ MBS] as well as from the nearby femtocell which helps it in

updating its Neighboring Cell List [NCL].

– MBS always maintains information about the femtocells and the UEs

associated with the femtocells, within its coverage area.

– The voice calls being highly delay sensitive will be served by the MBS itself

except for that of the registered femtocell users

– When the UE needs to make a high data rate request, it notifies the same to the

MBSby sending the NCL (Neighboring Cell List) along with its request.



Femto-Relays : System Architecture

– On reception of request from the UE, MBS makes a decision on whether to

serve it by itself or through the femtocell based on:

• Available Macrocell Backhaul.

• Average Backhaul Utilization greater than the Backhaul Utilization Threshold.

– If the MBS does not have sufficient resources, it initiates the handover of the

request to a femtocell chosen from the NCL based on two factors:

• Number of idle users attached to the femtocell.

• Largest value for available uplink data rate to average uplink data rate usage ratio.

– The femtocell now connects with the UE through multiple hops, the first hop

being to its associated user and the next hop from the associated user to the

destination UE.



Femto-Relays : Performance Analysis



Assuming multi-hop routes support the worst hop SIR and that; an optimal SIR and hence

capacity is achieved when relay node is located half way between the FBS and MUE:

Femto-Relays : Performance Analysis



Simulation Parameters



Femto-Relays: Results Uplink Capacity Gain


• Uplink capacity gain

increases with decrease in

femtocell transmit power

• Largest multihop capacity

gains are experienced at the

lower SNRs, where the

SNR improvements due to

relaying have the greatest

effect.


50 100 150 200 250 300

5

5.1

5.2

5.3

5.4

5.5

5.6

5.7

5.8

5.9

Number of femtocells

Uplin

k C

apacity g

ain

of

Fem

to w

ith R

ela

yin

g

Capacity analysis with respect to number of femtocells

-25 dB

-20 dB

-15 dB

-10 dB

-5 dB

Femto-Relays: Results

Downlink Capacity Gain


50 100 150 200 250 300

5

5.2

5.4

5.6

5.8

6

Number of femtocells

Dow

nlin

k C

apacity G

ain

of

Fem

to w

ith R

ela

yin

g

Capacity analysis with respect to number of femtocells

-20 db

-10 db

0 db

10 db Capacity gain increases with

decrease in femtocell transmit

power and reaches almost a stable

state at about -20 dB.



Coverage Extension Outage Probability


-100 -80 -60 -40 -20 0 200

50

100

150

200

250

Femtocell maximum transmit power (dBm)

Covera

ge r

adiu

s (

m)

Coverage of femtocells

With dual hop

Direct transmission

-150 -140 -130 -120 -110 -100 -90 -80 -70 -60

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Tsnr (dB)

Outa

ge P

robability

Outage Probability Analysis

Direct mode

Femto-relay mode



Dead Zone Control


-90 -85 -80 -75 -70 -65 -60 -55 -50 -45 -40

5

10

15

20

25

30

35

40

45

Macrocell RSSI

Deadzone R

adiu

s

Deadzone radius analysis with respect to femtocell transmit power

-20 db

-10 db

0 db

10 db

20db

• Dead zone radius is dependent on FBS

transmit power for constant MCBS RSSI

• Dead zone radius can be decreased by

reducing the femtocell transmit power.


Femto-Relays: Benefits Power efficient

Extends the coverage of femtocells to accommodate more users.

Reduction in interference to macrocell network.

Offloads the load on macrocell network.

Provides better capacity gains in uplink and downlink.

Improvised dead zone control.

Reduced outage probability.



Challenges

– Decrease in cell size and increase in BS density followed by the need for more

frequency carriers.

– The available radio spectrum is a finite, scarce and expensive resource.

– Femtocells operating on same licensed spectrum as that of macrocells resulting

in interference.

– Demand for high data rate multimedia applications requiring larger bandwidth

increasing day by day.

– Higher bandwidth requirement in the downlink than in the uplink.



Reverse Frequency Allocation (RFA)


Solutions

• Shared spectrum usage

• Results in higher cross-tier interference

• Dedicated spectrum usage

• Results in spectral inefficiency

• A hybrid spectrum allocation scheme incorporating

advantages of both is the need of the hour.

• Hence the Reverse Frequency Allocation scheme was

proposed.



Frequencyf1 f2

Femtocell

Macrocell

Frequencyf1 f2

Femtocell

Macrocell


• A novel spectrum utilization method that assures increased spectral efficiency and reduced interference in FDD operation.

• It does not need any dedicated spectrum allocation for femtocells.

• The entire macrocell spectrum is made available to the femtocell in the reversed fashion.

• “Reverse Frequency” means that the UL frequency of the MUE is allocated as the DL frequency for the FBS and the DL frequency of the MBS is allocated as the UL frequency for the FUE.

• To bring about better interference avoidance, we partition the cell into cell-center region (inner region) and cell-edge region (outer region) and allocate complementary frequency spectrum in both these regions.




UPLINK

FCFC

FC

FC

FC

FC

DOWNLINK

FCFC

FC

FC

FC

FC

System Model

MC FC

FCFC

FC

FCFC

FC

FCFC

FC

FCFC

MC


Comparison

Region Interferers In

Conventional

Method

Interferers In Soft

Frequency Reuse

Method

Interferers In RFA

Method

Inner Region

Femtocells

MBS and FBS in

the inner and outer

region

MBS and FBS in the

inner region

FBS in the inner region

and MUEs in the outer

region

Outer Region

Femtocells

MBS and FBS in

the inner and outer

region

MBS and FBS in the

outer region

FBS in the outer region

and MUEs in the inner

region






RFA: Performance Analysis


𝑆𝐼𝑁𝑅𝐷𝐿_Fi= 𝑃𝑖𝑙

𝐼j𝑙𝐹𝑖

𝑗=1,𝑗≠𝑖 + 𝐼k 𝑙 + 𝜎2𝑀𝑜

𝑘=1

𝑆𝐼𝑁𝑅𝐷𝐿_Fo= 𝑃𝑖𝑙

𝐼j𝑙𝐹𝑜

𝑗=1,𝑗≠𝑖 + 𝐼k 𝑙 + 𝜎2𝑀𝑖

𝑘=1

𝑆𝐼𝑁𝑅𝑈𝐿_Fi = 𝑃𝑙𝑖

𝐼j𝑖𝐹𝑖

𝑗=1,𝑗≠𝑙 + 𝐼k𝑖 + 𝜎2𝑀𝑜

𝑘=1

𝑆𝐼𝑁𝑅𝑈𝐿_Fo = 𝑃𝑙𝑖

𝐼j𝑖𝐹𝑜

𝑗=1,𝑗≠𝑙 + 𝐼k𝑖𝑀𝑖

𝑘=1 + 𝜎2


RFA: Performance Analysis



Outage probability

RFA: Results DL Capacity Comparison


150 200 250 300 350 400 450 500 550

5.2

5.3

5.4

5.5

5.6

5.7

5.8

5.9

6

6.1

x 107

Distance from the Macro cell

Thro

ughput

Downlink Capacity analysis of femtocells

Reverse UL-DL Allocation

Soft frequency

Conventional Method

Inner region Outer region


150 200 250 300 350 400 450 500 5500.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

x 108


Thro

ughput

Downlink Capacity analysis of Femto-Macro System

Reverse Frequency Allocation

Soft Frequency Allocation

Conventional Method

Inner region Outer region

RFA: Results Outage Probability



150 200 250 300 350 400 450 500 550

0.17

0.175

0.18

0.185

0.19

0.195


Outa

ge P

robabili

ty

Outage Probability Analysis of femtocells

Reverse Frequency Allocation

Soft Frequency Allocation

Conventional Method

RFA Benefits

Femtocells and macrocells making use of independent frequency resource in a

given direction (say UL /DL), assures minimal interference.

Increase in Downlink throughput helps to meet the ever increasing consumer

demands.

Doubles the spectral efficiency.

Requires neither any complex power control scheme nor any signal exchange

between the FBS and the MBS.

FBS coverage area is not restricted due to power control even if the femtocell is

located closer to the MBS.



Future Work Advanced Decision Algorithm for Coverage Management and Intelligent Handovers

– Enable the femtocells to dynamically adapt their coverage radius through cell breathing

– Carry out intelligent handoff management to load balance users across the tiers. • Minimize frequent handoffs through proper maintenance of coverage

radius and thereby conserve FAP power

• Reduce traffic in the backbone network

• Introduce cluster algorithm that groups femtocell into different frequency reuse clusters.

• Handoffs will be initiated among clusters.

• Figure out the algorithm for cluster head selection and optimal resource allocation for each cluster.



Future Work


Femtocell Clusters that can initiate intelligent handovers


Future Work Self Organizing Femtocells with Adaptive Access and

Adaptive Power Control – Smart planning algorithms to enable femtocells in making their own decision

based on the channel conditions and information from its neighbors.



Future Work Distributed Interference Management for dense

deployment of closed subscriber femtocell groups

– Involves the development of a hybrid scheme that ensures

• Minimize additional load on the legacy operator infrastructure

• Scalability to millions of femtocell units in the same network

• Suitable admission control mechanisms

• Introducing orthogonally polarized transmissions in two-tier networks

for interference mitigation.

• Open-standard management interface, with reduced interactions with

the femtocell management systems.

• Complexity as minimal as possible



Conclusion

Technical challenges confronting tiered cellular wireless systems were addressed which include :

– Coverage extension

– Interference reduction

– Power control

– Accommodating more users with the existing cellular architecture

– Intelligent allocation of spectrum in both the tiers

– Improving system capacity

Femto-Relays helps in coverage extension and power savings along with interference management

Reverse Frequency Allocation maximizes the spatial reuse in two-tier networks employing OFDMA, while guaranteeing a minimum desirable quality-of-service to users in either tier.



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Decentralized Interference Management for Two-Tier … · Decentralized Interference Management ... OFDMA femtocells: A roadmap on interference ... Nakata, A., Moessner.K., “Interference