radio resource management in multi-tier cellular wireless ...€¦ · \wireless communications,...

$: Radio Resource Management in Multi-tier Cellular Wireless ...€¦ · \Wireless Communications, Networks, and Services Research Group" at U. of Manitoba Current research interests:$
Radio Resource Management in Multi-tierCellular Wireless Networks

Ekram Hossain, Ph.D., P.Eng.

ProfessorDepartment of Electrical and Computer Engineering

University of Manitoba, Winnipeg, Canada

http://home.cc.umanitoba.ca/∼hossaina

PERUCON’1315 November 2013

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“Wireless Communications, Networks, and ServicesResearch Group” at U. of Manitoba

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“Wireless Communications, Networks, and ServicesResearch Group” at U. of Manitoba

Current research interests:

I Cognitive radio and dynamic spectrum access

I Hierarchical cellular wireless networks (small cell networks)

I Green cellular radio systems

I Applied game theory and network economics

Current group members:

I 3 PDF, 7 Ph.D. students, 2 M.Sc. students

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Outline

I Introduction

I Two-tier Macrocell-Femtocell Networks

I Technical Challenges in Femtocell Deployment

I Interference Management Schemes

I Standardization Activities

I Open Research Issues

I References

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Introduction

Evolution of the population of wireless devices:

Nu

mb

er o

f co

nn

ecte

d d

evic

es

2020

10b

20b

30b

40b

50b

2015 2010

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Introduction

I Global Mobile Data Traffic Forecast Report presented byCisco predicted 2.4 exabytes mobile data traffic per month forthe year 2013.

I M2M communications and IoT (Internet of Things)I Three evolution phases of user population:

1. connected consumer electronics phase (smart phones, tablets,computers, IPTVs)

2. connected industry phase (sensor networks, industry andbuildings automation, surveillance, and eHealth applications)

3. connected everything phase (IoT phase)

I A significant part of this traffic will be carried through cellularnetworks.

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Introduction

I Improvement of cell coverage, network capacity, and betterquality-of-service (QoS) provisioning are some of the majorchallenges for next generation cellular networks.

I Universal frequency reuse and make transmitters and receiverscloser

I Hierarchical layering of cells (referred to as HetNets), anefficient solution to improve cell coverage and networkcapacity.

I Adopted in the evolving Long Term Evolution(LTE)/LTE-Advanced (LTE-A) systems

I 3GPP Release-8 (LTE), 3GPP Release 10 onwards(LTE-Advanced)

7/80

Introduction

LTE/LTE-A HetNet:

I Long-Term Evolution (LTE) and LTE-Advanced systems aredesigned to support high-speed packet-switched services in 4Gcellular wireless networks.

I The cells or radio base stations in LTE/LTE-A can beclassified as: i) macrocell base station (referred as MeNB),and ii) small cells (e.g., microcells, picocells, femtocells).

I “Small cell” is an umbrella term for low-power radio accessnodes that operate in both licensed and unlicensed spectrumand have a range of 10 meter to several hundred meters.

I Small cells are deployed to improve the cell coverage and areaspectral-efficiency (capacity per unit area).

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Introduction

Motivations for small cells:

I High data rate and improved quality-of-services to subscribers

I Eliminate coverage holes in macrocell footprint

I Extended battery life of mobile phones

I Macrocell load reduced (hence more resources for macrocellusers)

I Mitigate spectrum underutilization problem

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Introduction

LTE/LTE-A HetNet:

Macrocell Base Station A (MeNB-A)

MeNB-A UE 1

HeNB-A1 UE 1MeNB-A UE 2

Picocell

PC-A1RN-A1 UE 1

Relay Node

RN-A1

MeNB-A UE 3

PC-A1 UE 1

PC-A1 UE 2

HeNB-A1

HeNB-A2 UE 1HeNB-A2

HeNB-A3 UE 1 HeNB-A3

X2Un

MeNB-B

HeNB-B1 UE 1

MeNB-B UE 2

PC-B1

RN-B1 UE 1

Relay Node

RN-B1

MeNB-B UE 1

PC-B1 UE 1

PC-B1 UE 2

HeNB-B1

HeNB-B2 UE 1

HeNB-B2

X2

Un

MeNB-C

MeNB-C UE 1

HeNB-C3 UE 1

MeNB-C UE 2

MeNB-C UE 3

RN-C1 UE 1

Relay Node

RN-C1

MeNB-C UE 4

HeNB-C3 HeNB-C2 UE 1

HeNB-C2

HeNB-C1 UE 1

HeNB-C1

X2

UnPicocell

PC-C1PC-C1 UE 1

PC-C1 UE 2

LTE Evolved Packet Core

HeNB Gateway

MME / S-GW

HeNB Gateway

X2

X2

X2

S1

S1

S1

S1 S1

S1

Internet

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Introduction

Comparison among different radio base stations in LTE/LTE-A:

Attributes MeNB Picocell HeNB Wi-Fi

BS Installation Mobile Operator Mobile Operator Customer Customer

Site Acquisition

Mobile Operator Mobile Operator Customer Customer

Transmission Range

300-2000 m 40-100 m 10-30 m 100-200 m

Transmission Power

40 W (approx.) 200 mW- 2 W 10-100 mW 100-200 mW

Band License Licensed band Licensed band Licensed band Unlicensed band

System Bandwidth

5, 10, 15, 20 MHz (with CA up to 100 MHz)



5, 10, 20 MHz

Transmission Rate

up to 1 Gbps up to 300 Mbps 100 Mbps-1 Gbps

up to 600 Mbps

Cost $ 60,000/yr $ 10,000/yr $ 200/yr $ 100-200/yr

Power Consumption

High Moderate Low Low

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Introduction

LTE-A HetNet:

I OFDM for downlink (DL) and single-carrier FDM (SC-FDM)waveform for uplink (UL) communications (20 MHzbandwidth)

I The eNB serving the RN (i.e., scheduling RN backhaul traffic)is denoted donor eNB (DeNB). The same eNB can be theDeNB for one RN and the regular serving node for UE.

I X2 interface, defined as a direct eNB-to-eNB interface, allowsfor inter-cell interference coordination (ICIC)

I S1 interface supports transfer of user and data traffic betweenthe corresponding nodes.

I Un interface refers to an air interface between DeNB and RN.Un is based on a modified interface between the eNB and UEin order to allow half duplex operation for the RN.

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Introduction

LTE/LTE-A HetNet:

I OFDM (SC-FDM) symbols are grouped in subframes of 1 msduration. Each subframe is composed of two 0.5 ms slots.

I Minimum scheduling unit for the DL and UL of LTE isreferred to as a resource block (RB)

I One RB consists of 12 subcarriers in the frequency domain(180 kHz) and one subframe in the time domain (1 ms)

I Subframes are further grouped in 10 ms radio frames.

I A reference or pilot signal, referred to as a common referencesignal (CRS), is used for mobility measurements as well as fordemodulation of the DL control and data channels

I CRS transmission is distributed in time and frequency

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Introduction

LTE-A HetNet:

#0 #1 #2 #3 ... #18 #19

Radio Frame = 10 ms

Sub-‐frame = 1ms

Slot = 0.5 ms

Resource Block (RB)

Resource Element

OFDM Symbol (Time)

Sub-‐carrier (Freq

uency)

#0 #1 ... #9

Figure: LTE-Advanced frame structure.

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Introduction

LTE/LTE-A HetNet:

I LTE Rel-10 (or LTE- Advanced), supports improved MIMOoperation as DL MIMO support is enhanced (8 Tx, 8 Rx issupported), and UL MIMO (4 Tx, 4 Rx) is introduced toimprove link spectral efficiency.

I Up to five 20-MHz component carriers can be aggregated,offering a peak data rate of more than 1 Gb/s

I Do not translate into significant improvements in terms ofsystem spectral efficiency in bits per second per Hertz

I System gains are only achievable through increased nodedensity and deployment of low-power nodes, such as pico,femto, and relay base stations.

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Two-tier Macrocell-Femtocell Networks

Femtocell

BSMobile Core

Network

Broadband

Router

Macrocell

BS

Femtocell

UE

Macrocell

UE

Macrocell

UE

Femtocell

BS

Femtocell

BS

Femtocell

UE

Internet

Macrocell

UE

Macrocell

UE

Macrocell

UE

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I Femtocells or femto access points (FAPs) are small,short-ranged (10∼30m) low-powered (10∼100 mW) accesspoints.

I FAPs improve indoor coverage and provide high-data-rateservices, enhance network capacity

I Femtocells operate in licensed spectrum owned by the mobileoperator and enable Fixed Mobile Convergence (FMC) serviceby connecting to the cellular network via broadbandcommunications (DSL/cable/Ethernet/WiMAX).

I Femtocell access via broadband Internet

I If wired backhaul is not available, relay nodes can be deployed.

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CDMA femtocells:

I The 3GPP refers to these femtocells as 3G femtocells orHome Node Bs (HNBs).

I 3G femtocells use Wideband Code-Division Multiple Access(WCDMA)-based air interface of Universal MobileTelecommunication system (UMTS) known as UMTSTerrestrial Radio Access (UTRA).

I Performance depends on the power control method(centralized power control in infeasible)

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OFDMA femtocells:

I LTE femtocells are referred as Home evolved Node Bs(HeNBs).

I Dynamic allocation of time and frequency slots (henceflexibility in resource allocation), but large amount ofcoordination may be necessary

I Semi-static allocation of frequency to interior, edge, andfemtocell users, along with power control

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Access modes:

I Closed access mode: A set of registered users belonging toClosed Subscriber Group (CGS) is allowed to access afemtocell (e.g. residential deployment scenario)

I Co-channel deployments of closed femtos cause coverageholes.

I Open access mode: Any user can access the femtocell andbenefit from it’s services. In public places like airports,shopping malls etc. open access mode of femtocells can beused.

I Hybrid access mode: A femtocell may allow up to Nnon-registered mobile users to access it (to limit the load onthe femtocell and its backhaul connection).

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Technical Challenges in Femtocell Deployment

I Resource allocation and interference management

I Cell association and admission control

I Network performance analysis

I Handoff and mobility management

I Auto-configuration/self-organization, self-optimization,self-healing

I Security

I Timing and synchronization

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Resource allocation and interference management:

I Interference between neighboring femtocells, and betweenfemtocells and a macrocell.

I Co-tier interference: between same layer network elements,i.e. inter-femtocell interference or inter-macrocell interference.

I Cross-tier interference: between network elements that belongto the different tiers of the network, i.e. between femtocellsand macrocell.

I Distributed interference management scheme is requiredwhich satisfies the QoS requirements of the macrocell andfemtocell users and at the same time enhances the capacityand coverage of the network.

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Resource allocation and interference management:I Interference scenarios in OFDMA-based femtocell networks

Index Aggressor Victim Interference Type

Transmission Mode

Symbol

1 Macrocell UE Femtocell BS Cross-‐tier Uplink

2 Macrocell BS Femtocell UE Cross-‐tier Downlink

3 Femtocell UE Macrocell BS Cross-‐tier Uplink

4 Femtocell BS Macrocell UE Cross-‐tier Downlink

5 Femtocell UE Femtocell BS Co-‐tier Uplink

6 Femtocell BS Femtocell UE Co-‐tier Downlink

Femtocell BS

Internet

Mobile Core Network

Broadband Router

Macrocell BS

Femtocell UE

Macrocell UE

Macrocell UE

Femtocell BS

Femtocell BS

Index 1

Index 5

Index 6

Femtocell UE

Femtocell UE

Index 2 Index 4

Index 3

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I Precise characterization of interference in heterogeneous andmulti-tier networks

I With open-access and strongest cell selection,heterogeneous and multi-tier deployments may notworsen the overall interference conditions or evenchange the SINR statistics.

I Closed-access model may result in significant performancedegradation to femtocell (in uplink) or the cell-edge macrocelluser in the downlink which is near to a femtocell.

I In both open- and closed-access, signaling for coordinatingcross-tier interference may be difficult.

I Femtocells are not typically connected directly to theoperator’s core network (hence increased delay for backhaulsignaling)

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I Intercell interference coordination (ICIC) for femtocellnetworks is a major issue in 3GPP LTE-Advancedstandardization.

I In LTE femtocells, backhaul-based coordination, dynamicorthogonalization, subband scheduling, and adaptive fractionalfrequency reuse can be used for interference coordination.

I Advanced techniques, e.g., interference cancellation,cooperative communication among base stations

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I In a CDMA femtocell network, interference caused to themacrocell users need to be controlled such that QoSrequirements for macrocell users are satisfied.

I Support all macrocell users with guaranteed QoS requirementsat all times.

I Throughput-power tradeoff optimization for femtocell users

I Soft QoS provisioning for femtocell users

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Cell association and admission control:

I Simple approach: assign each user to the ‘strongest’ basestation

I Better approach is biasing: users pushed into small cells. InOFDMA femtocells, biased users can be assigned orthogonalresources to the macrocell

I Optimal cell association and load balancing/optimalbiasing/admission control is an open research issue

I Admission control decision should depend on: distribution ofusers and locations of base stations, traffic patterns,information available to the mobiles and femtocell accesspoints, etc.

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Network topology for a two-tier femtocell network

0 200 400 600 800 10000

100

200

300

400

500

600

700

800

900

1000Femtocell UserMacrocell UserBase Station

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Network performance analysis:

I Modeling aggregate interference in a multi-tier set up undergeneral fading scenarios and subsequent analysis of coverage,outage, capacity in the network

I Modeling and analysis of the effects of traffic burstiness(affects interference) and packet-level performance analysis(more useful)

I Analysis of uplink SINR (modeling user location and BSlocation simultaneously)

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Handoff and mobility management:

I An effective and efficient mobility management and handoverscheme (macrocell-to-femtocell, femtocell-to-macrocell andfemtocell-to-femtocell) is necessary for mass deployment offemtocells in UMTS and LTE networks.

I Also, vertical handoffs between femtocells and non-cellularaccess technologies such as Wi-Fi

I Lack of a low-delay connection to the core network may resultin significant handoff signaling delays.

I CDMA femtocells are typically unable to share a radionetwork controller (RNC) with a macrocell or other femtocellsfor coordinating soft-handoffs.

I Architectural changes required in the core network andfemtocell gateways functions

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Auto-configuration/self-organization, self-optimization, self-healing:

I Femtocells are randomly deployed by the subscribers and isnot location constrained.

I Femtocells must support a “plug-and-play” operation withautomatic configuration and adaptation (for scalabledeployment and maintenance)

I Self-organization reduces operational expenditure (OPEX)

I Intelligent algorithms to optimize network parameters fortransmit power, physical resources, access modes, admissioncontrol, handoff control etc.

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Self-organization:

I Automatic channel selection, power adjustment and frequencyassignment for autonomous interference coordination andcoverage optimization (cognitive radio concepts andcognitive femtocells)

I Cognitive femtocells should be able to dynamically sensespectrum usage by the macrocell and adapt theirtransmissions.

I Semi-distributed schemes for better convergence

I Autonomous shutting down and waking up of base stations(green femtocells)

I Procedures for automatic registration and authentication offemtocells, neighbor discovery, cell ID selection

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Security:

I Prevent unwanted users from accessing femtocells

I Femtocells are usually connected to the core network via DSLor broadband connection or wireless links (e.g. WiMAX) andvulnerable to a variant of malicious attack (e.g.masquerading, eavesdropping, man-in-the-middle attack).

I Enhanced authentication and key agreement mechanism forfemtocells is required to secure femtocell networks.

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Timing and synchronization:

I FAPs need to connect to the clock of the core network and usethat clock for synchronization with the rest of the network.

I Align packets for transmission

I Timing and synchronization is one of the major challenges forfemtocells since synchronization over IP backhaul is difficultand inconsistent delays may occur due to varying trafficcongestion.

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Interference Management Approaches

I Intercell interference coordination (ICIC) methods specified inRelease 8 and 9 of 3GPP do not specifically considerfemtocells.

I Enhanced ICIC (eICIC) methods techniques have beendeveloped for Release 10.

I Major categories: Time-domain techniques, frequency-domaintechniques, power control techniques, beamformingtechniques, interference cancellation techniques

I Time-domain methods: transmissions of victim users arescheduled in time domain to mitigate cross-tier interference(e.g., by subframe alignment, using almost blank subframes(ABSFs) in femtocells)

I Frequency-domain techniques use totally orthogonaltransmissions

I Power control techniques heavily discussed in 3GPPI Beamforming techniques for multi-antenna systems

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Interference management approaches

Time-domain techniques:

I Subframe alignment: When the subframes of macro eNB andhome eNB are aligned, their control and data channels overlapwith each other.

I To avoid interference to the control channel of MUEs, usealmost blank subframes (ABSFs) at femtocells where onlyreference signals, no control or data signals, are transmitted.

I When there are MUEs in the vicinity of a femtocell, they canbe scheduled within the subframes overlapping with theABSFs of the femtocell, which significantly mitigatescross-tier interference.

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Frequency-domain techniques:

I Femto-aware spectrum arrangement approach

I Graph-based clustering approach

I Collaborative frequency scheduling approach

I Cognitive radio-based approach

I Fractional frequency reuse (FFR) approach

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Frequency-domain techniques

Femto-aware spectrum arrangement:

Femtocell BS

Macrocell BS

Femtocell UE 1

Macrocell UE 1

Femtocell UE 2

Macrocell UE 6

Macrocell UE 5

Macrocell UE 2

Macrocell UE 4

Femtocell UE 3

Macrocell UE 3

Femtocell-interference Pool Macrocell UE 4 Macrocell UE 5 Macrocell UE 6

Macrocell UE 1Macrocell UE 2Macrocell UE 2Femtocell UE 1Femtocell UE 2Femtocell UE 3

Macrocell dedicated spectrum

Macrocell-Femtocell shared spectrum

I Does not consider inter-HeNB interference

I May be inefficient when the number of macrocell UEs nearthe HeNB increases

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Graph-based clustering approach:

I Reduces downlink interference (both cross-tier and co-tier)and enhances spectral-efficiency in OFDMA-basedclosed-access femtocell networks

I A portion of the entire frequency band may be dedicated formacrocell users and the rest is reused by macrocell andfemtocells.

I The clustering algorithm allocates HeNBs into differentfrequency reuse clusters and UEs of different HeNBs in thedifferent clusters use different sub-channels allocated from theshared frequency band.

I If the Euclidean distance between any two HeNBs is less thanthe threshold distance, then they are assigned to differentclusters

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Cognitive radio-based approach:

I Optimal channel and power allocation through environmentperception and interference recognition for interferenceminimization

I Each femtocell recognizes the macrocellular interferers andcognitively avoids to reuse the same channels marked as aninterference signature

!

through orthogonal channelization. By such means the uplinkinterference from the macrocell users can be well canceled oneach femtocell assuming that the signature information aboutIi,j and pI(i,j) is obtainable from the network environment.One can see that the uplink interference is managed in a cogni-tive and geographically distributed way based on each femto-cell’s individual radio environment.

frequency

time

Interference signature

reusable channelresource

Figure 3: Interference Recognition in an OFDMA Example

We discuss an example for cognitive channel reuse in anOFDMA cellular system where channels are allocated to usersin a two-dimensional time ! frequency matrix. We show sucha channel matrix managed by a femtocell j in Figure 3. Theinterference signature is illustrated by two red grids with dif-ferent power levels perceived from the environment. One canview this example in a way that two public users i and i! arein vicinity to the femtocell of interest. Due to the differentdistance to the femtocell, their interference signatures Ii,j andIi!,j are marked with different received power on the corre-sponding channel mode. Based on this recognition of the en-vironment, all the orthogonal channels can be safely reused inthe femtocell without being interfered by the macrocell users.

The function of radio cognitivity plays an important rolefor autonomous interference management. A femtocell isable to cognitively recognize the network interference and au-tonomously reuse the appropriate channels. The factor of Pj

actually determines the radio sensitivity of a femtocell accesspoint j. The smaller this factor is, more sensitively a femtocellbehaves to its interference environment.

IV ANALYSIS OF COGNITIVE CHANNEL REUSEEFFICIENCY

The idea of cognitive channel reuse based on orthogonal chan-nelization is to have each femtocell recognize the macrocellu-lar interferers and cognitively avoid to reuse the same channelsmarked as an interference signature. In other words, a partialnumber of channels have to be sacrificed in a femtocell whensome macrocell users come close with strong interference. Itturns out to be interesting to study the reusability of the cellularchannels in a femtocell, namely the channel reuse efficiency.

Suppose that the number of hi uplink channels are allocatedto a macrocell user i by the macrocell base station. Accordingto a specific sensitivity threshold of femtocell access points, Ni

femtocells are involved for orthogonal channelization. Becauseeach of the Ni femtocells will lose a number of hi uplink chan-nels, the aggregate loss of the femtocell uplink channels in a

macrocell can be written as:

hloss =

Nm!

i=1

hi · Ni (3)

Now suppose that the same number of total channels C arereused in both macrocell and femtocells, then a channel reuseefficiency parameter ! can be found as:

! = 1 " hloss

C · Nf= 1 "

"Nm

i=1 hi · Ni

C · Nf(4)

! represents the average percentage of reusable channels ina femtocell. Because the aggregate loss of femtocell channelshloss is bounded by the total number of femtocell channels C ·Nf in a macrocell, we have: ! # 1. Note that better femtocellsensitivity leads to bigger number of channel sacrifice Ni andthus causes a lower efficiency !.

V AN OPPORTUNISTIC CHANNEL REUSE SCHEDULER

The above discussion presented a cognitive channel reusemethod based on orthogonal channelization. In practical im-plementation, an opportunistic scheduler can be designed to re-alize the above resource allocation procedure. Suppose a fem-tocell j has the number of Mj users and manages a total num-ber of C channels. The interference signature of each channel1 # c # C can be cognitively characterized from the environ-ment for user m (m = 1, 2, ..., Mj):

"(c)j,m =

|h(c)j,m|2

I(c)j,m

, 1 # c # C (5)

where |h(c)j,m|2 and I

(c)j,m respectively denote the channel gain

and the aggregate interference level on channel c when user mis served in femtocell j. Noise is neglected in this case becausethe communication is considered as interference-limited. Ourscheduler works to allocate the optimal channel c" such that:

c" = arg maxc

("(1)j,m, ...,"

(c)j,m, ...,"

(C)j,m) (6)

Equation (6) produces an opportunistic identifier c" that al-locates the best channel mode to user m based on its own inter-ference signature. Note that the scheduler represents the basicidea of the cognitive channel reuse discussed in the previoussection. Based on the instantaneous interference signature per-ceived from the environment, a femtocell j cognitively deter-mines the channel reuse pattern which produces the best signa-ture benefit:

"(c")j,m =

|h(c")j,m |2

I(c")j,m

(7)

One can view that a femtocell manages a candidate channelpool for each user. The order of channels in the pool is uniquefor different users based on their individual interference signa-ture perception. At a particular time when a femtocell j hasmultiusers to serve, the transmit power has to be autonomouslyconfigured in order to minimize the interference to the network.We propose a simple optimization criterion as follows:

1569

Y.-Y. Li and E. S. Sousa, “Cognitive uplink interference management in 4G cellular femtocells,” in Proc. of IEEE

PIMRC’10. 40/80


Interference management through cognitive approach:

CC1 CC2

CC1 CC2 CC1 CC2

CC1 CC2

CC3 CC4

CC3 CC4

Component Carriers (CC): CC3 CC4CC1 CC2

CC3 CC4

CC3 CC4

CC1 CC2

(a) (b)

HeNB1

HeNB3

HeNB2

HeNB4HeNB4

HeNB3

HeNB1

HeNB2

HeNB5

Femtocell : HeNB

41/80


Fractional frequency reuse (FFR) andresource partitioning-based approach:

I The basic mechanism of this method is to divide the entirefrequency spectrum into several sub-bands.

I Each sub-band is differently assigned to each macrocell orsub-area of the macrocell.

I When a femtocell is turned on, it senses the pilot signals fromthe macrocell BSs and discards the sub-band/sub-bands withthe very high received signal power.

I Since the resource for MeNB and HeNB is not overlapped,interference between MeNB and HeNB can be mitigated.

I Mitigates both co-tier and cross-tier interference

I Static FFR scheme and dynamic FFR scheme

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FFR-based approach – static FFR:

I Strict FFR, soft-FFR, and sectored-FFR schemes

I Strict-FFR: apply frequency reuse factor (FRF) of 1 to thecenter-zone MUEs and FRF of N (e.g. 3) to the edge-zoneMUEs.

I In a cluster of N cells, the center-zone MUEs in eachmacrocell are allocated with a common sub-band offrequencies while the rest of the sub-bands of frequencies areequally partitioned into sub-bands according to the FRF of theedge-zone and assigned separately to each cell edge-zone ofthe cluster.

I A total number of N+1 sub-bands are required.

I One important design parameter is the radius of thecenter-zone of the macrocell.

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Interference management using strict FFR (with N = 3):

A

B

C

D

X1Macro: BFemto: A,C,D

X3Macro: DFemto: A,B,C

X2Macro: CFemto: A,B, D

Macro: A

C1Femto: C,D

C3Femto: B,C

C2Femto: B,D

1

2

3

4

5

7

6

X1Macro: GFemto: A,B,C

D,E,F X2Macro: BFemto: A,C,D

E,F,G

X3Macro: C

Femto: A,B,DE,FGX4

Macro: DFemto: A,B,C

E,F,G

X5Macro: EFemto: A,B,C

D,F,G

X6Macro: F

Femto: A,B,CD,E,G

C1Fem:C,D

E

C4Fem: B,F

G

C3Fem: E,F

G

C2Fem: D,E

F

C5Fem: B,C

G

C6Fem: B,C

D Mac:A

A

G

F

E

D

C

B1

2

3

4

5

6

7

B

A

B

C

B+C

A+C

A+B

Macro: C

Macro: A

Macro: B

Macro: A,B

Macro: B,C

Macro: A,C1

7

B B

B4

3

2

5

6

Femto: A

Femto: B,C

Femto: A,C

Femto: B

Femto: C

Femto: A,B

(a) (b)

(d)(c)

A

B

C

D

B

1

2

7

B6

B4

35

B

Macro: D

Macro: A

Femto: B,C Macro: C

Femto: B,C Macro: A

Femto: B,D

Femto: B,D

Macro: B

Femto:C,D

Macro: A

Femto:C,D

(ii)(i) (ii)(i)

(ii)(i)(ii)(i)

44/80


FFR-based approach – soft-FFR:

I Uses a cell partitioning technique similar to that of strict FFRscheme.

I The center-zone MUEs of any cell are allowed to use thesub-bands of cell edge-zone MUEs within the cluster.

I Example: A soft-FFR scheme with FRF of N = 3 for theedge-zone MUEs. The entire frequency is divided intosub-bands A, B, and C, and assigned to the cell edge-zoneMUEs of macrocell 1, macrocell 2, and macrocell 7,respectively.

I FFR = N =⇒ N bands

45/80


Interference management using soft-FFR:

A

B

C

D


X3Macro: DFemto: A,B,C

X2Macro: CFemto: A,B, D

Macro: A

C1Femto: C,D

C3Femto: B,C

C2Femto: B,D

1

2

3

4

5

7

6



E,F,G

X3Macro: C

Femto: A,B,DE,FGX4


E,F,G


D,F,G

X6Macro: F

Femto: A,B,CD,E,G

C1Fem:C,D

E

C4Fem: B,F

G

C3Fem: E,F

G

C2Fem: D,E

F

C5Fem: B,C

G

C6Fem: B,C

D Mac:A

A

G

F

E

D

C

B1

2

3

4

5

6

7

B

A

B

C

B+C

A+C

A+B

Macro: C

Macro: A

Macro: B

Macro: A,B

Macro: B,C

Macro: A,C1

7

B B

B4

3

2

5

6

Femto: A

Femto: B,C

Femto: A,C

Femto: B

Femto: C

Femto: A,B

(a) (b)

(d)(c)

A

B

C

D

B

1

2

7

B6

B4

35

B

Macro: D

Macro: A

Femto: B,C Macro: C

Femto: B,C Macro: A

Femto: B,D

Femto: B,D

Macro: B

Femto:C,D

Macro: A

Femto:C,D

(ii)(i) (ii)(i)

(ii)(i)(ii)(i)

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FFR-based approach – soft-FFR:

I The center-zone MUEs of macrocell 1 are allowed to usesub-band B and sub-band C, i.e. the sub-bands of celledge-zone MUEs of macrocell 2 and macrocell 7, respectively.Therefore, soft FFR is more bandwidth-efficient than strictFFR.

I An HeNB located in the center-zone may select the sub-bandthat is used by the MUEs in the edge-zone, and if the HeNBis located in the edge-zone, it chooses the sub-bands that areused by the MUEs in the center-zone

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FFR-based approach – sectored FFR (FFR-3):

I The macrocell coverage area is partitioned into center-zoneand edge-zone including three sectors per each.

I The entire frequency band is divided into two parts – one partis solely assigned to the center-zone (sub-band A) and theother part is partitioned into three sub-bands (sub-bands B,C, and D) and assigned to the three edge-zones.

I An HeNB chooses a sub-band which is not used in themacrocell sub-area. When the HeNB is located in thecenter-zone, it also excludes the sub-band that is used by theMUEs in the edge-zone of the current sector.

I All cells use all frequency bands.

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Interference management using FFR-3:

A

B

C

D


X3Macro: CFemto: A,B,D

X2Macro: DFemto: A,B, C

Macro: A

C1Femto: C,D

C3Femto: B,D

C2Femto: B,C

1

2

3

4

5

7

6



E,F,G

X3Macro: C

Femto: A,B,DE,FGX4


E,F,G


D,F,G

X6Macro: F

Femto: A,B,CD,E,G

C1Fem:C,D

E

C4Fem: B,F

G

C3Fem: E,F

G

C2Fem: D,E

F

C5Fem: B,C

G

C6Fem: B,C

D Mac:A

A

G

F

E

D

C

B1

2

3

4

5

6

7

B

A

B

C

B+C

A+C

A+B

Macro: C

Macro: A

Macro: B

Macro: A,B

Macro: B,C

Macro: A,C1

7

B B

B4

3

2

5

6

Femto: A

Femto: B,C

Femto: A,C

Femto: B

Femto: C

Femto: A,B

(a) (b)

(d)(c)

A

B

C

D

B

1

2

7

B6

B4

35

B

Macro: D

Macro: A

Femto: B,C Macro: C

Femto: B,C Macro: A

Femto: B,D

Femto: B,D

Macro: B

Femto:C,D

Macro: A

Femto:C,D

(ii)(i) (ii)(i)

(ii)(i)(ii)(i)

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Power control techniques:

I Proactive or reactive power control

I Cluster-based power control

I Game-theoretic power control

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Power control techniques

Proactive or reactive power control:

I Generally focus on reducing transmission power of HeNBs

I Dynamic or adjustable power setting is preferred over fixedHeNB power setting

I Performed either in proactive or in reactive manner. Also,either in open loop power setting (OLPS) or closed-looppower setting (CLPS) mode

I In the OLPS mode, the HeNB adjusts its transmission powerbased on its measurement results or predetermined systemparameters (i.e. in a proactive manner).

I In the CLPS mode, the HeNB adjusts its transmission powerbased on the coordination with MeNB (i.e. in a reactivemanner).

I A hybrid mode can also be used where the HeNB switchesbetween the two modes.

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Cluster-based power control approach:

I Power control for HeNBs on a cluster basis in which the initialpower setting for the HeNBs is done opportunistically basedon the number of active femtocells in a cluster.

I Centralized sensing can be used by which an MeNB canestimate the number of active femto cells per cluster andbroadcast the interference allowance information to femtocellsfor their initial power setting.

I Alternatively, distributed sensing can be used where eachfemtocell senses if the others are active in the same clusterand adjusts its initial power setting accordingly.

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Game-theoretic power control:

I Game theoretic models can be used to design and analyzedistributed power control methods in femtocell networks

I Problem of downlink transmit power control – femto accesspoints (HeNBs) and macro base stations (MeNBs) competewith each other to maximize capacity under power constraints

I Model as Stackelberg game where MeNBs are leaders andHeNBs are followers

I Two sub-games: sub-game comprised of the set of leadersreferred to as the upper sub-game, and sub-game comprisedof the set of followers referred to as the lower sub-game

I Players in each sub-game compete with each other in anon-cooperative manner to reach a sub-game Nash equilibrium

I Throughput of each station in the network is maximized underpower constraints.

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Transmit beamforming approach:

!Fig. 1. Two-tier Femtocell Network.

tocell base station is equipped with Nf antennas. Moreover, weassume that both the macrocell user and the femtocell usersare equipped with a single antenna. Within each macrocell,each macrocell user accesses the channel orthogonally and thetransmissions within each macrocell are synchronized so thatno intra-cell interference exists between macrocell users. Assuch, cross tier interference exits between the macrocell andfemtocell users.

At the macrocell and the kth femtocell , the received signalsare

ym = h†mxm +

!

k!Kh†

f,kxf,k + zm, (1)

yf,k = g†k,kxf,k + g†

m,kxm +!

i!Ki"=k

g†k,ixf,i + zf,k (2)

where K ! {1, 2, . . . ,K}, xm ! CNm#1 and xf,k ! CNf#1

are the transmitted signal vectors of the macrocell and thekth femtocell users, respectively, such that E

"x†

mxm

#" Pm,

E$x†

f,kxf,k

%" Pf and zm # CN (0,!2

m), and zf,k #CN (0, !2

f,k) are zero-mean circularly symmetric complexGaussian random variables (r.v.’s). The channel vectors fromthe macrocell base station to the macrocell users and thekth femtocell users are denoted by hm ! CNm#1 andgm,k ! CNm#1, respectively. The channel vectors from thekth femtocell base station to the macrocell user are denoted

by hf,k ! CNf#1, whereas the channel vectors from the ithfemtocell base station to the kth femtocell user is denoted bygk,i ! CNf#1. The channel vector will include both small andlarge scale fading.

At each transmitter, we assume a transmit beamform-ing strategy, such that xm = dm

$pmum and xf,k =

df,k$

pf,kuf,k, where dm and df,k are the normalized informa-tion signals for the macrocell and the kth femtocell users, pm

and pf,k are the transmit power allocated to the macrocell andkth femtocell users, and um ! CNm#1 and uf,k ! CNf#1 arethe normalized transmit beamforming vectors of the macrocelland kth femtocell users, i.e. u†

mum = 1 and u†f,kuf,k = 1.

We use the SINR as the QoS measure. Given instantaneouschannel coefficients, p ! [pm, pf,1, pf,2, . . . , pf,K ]T , and U ![um,uf,1,uf,2, . . . ,uf,K ], the instantaneous received SINR atthe macrocell and the kth femtocell users can be expressed as

SINRm (p,U) =pm|h†

mum|2!

k!Kpf,k|h†

f,kuf,k|2 + !2m

, (3)

SINRf,k (p,U) =pf,k|g†

k,kuf,k|2!

i"=k

pf,i|g†k,iuf,i|2 + pm|g†

m,kum|2 + !2f,k

.

(4)

Since there is an increasing trend to limit the averagetransmission power due to radio pollution, regulatory agencieswill limit the total transmission power so as to reduce cross-tier interference subject that the QoS are satisfied at both themacrocell and femtocell users. Using (3) and (4), we can thenformulate the following optimization problem:

P :

&'''(''')

minp,U

1T p

s.t. SINRm (p,U) % "m,SINRf,k (p,U) % "f,k, &k ! K,p " P

(5)

where P ! [Pm, Pf,1, Pf,2, . . . , Pf,K ]T , "m ! R+ and"f,k ! R+ are the SINR thresholds of the macrocell andthe kth femtocell users, respectively, and #m ! R++ and#f,k ! R++ are positive weights assigned to the macrocelland the kth femtocell users to reflect long-term priority,respectively. Although we can solve P using the centralizedapproach in [12], distributed interference management willbe preferred to maintain cross-tier interference to acceptablelevels in femtocell networks to reduce network overhead.

III. PROPOSED POWER CONTROL AND BEAMFORMERDESIGN ALGORITHM

In this section, we propose to decompose the power con-trol and beamformer design problem into two subproblems.We will present our proposed algorithm and reformulate Paccording to our proposed approach.

R. Mo, T. Q. S. Quek, and R. W. Heath Jr., “Robust beamforming and power control for two-tier femtocell

networks,” in Proc. IEEE VTC11-Spring.

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Transmit beamforming approach:

I Beamforming-based cross-tier interference reduction schemein closed-access femtocell networks

I The MeNB selects the beam subset and the users for eachchannel based on the signal-to-interference-plus-noise (SINR)information for all the channels which is fedback by themacrocell UEs.

I The throughput of the macrocell can be maximized byadaptive selection of optimal number of beams.

I The adaptive selection of the number of beams decreasescross-tier interference, and provides spatial opportunity toHeNBs to access the spectrum in an opportunistic manner.

I Distributed power control mechanism for HeNBs integratedwith this scheme reduces cross-tier interference significantly.

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Interference cancellation:

I Successive interference cancellation, parallel interferencecancellation, multiuser detection

I Complex and could be used for uplink interferencemanagement only (e.g. in MeNB, HeNB)

I Selection of an interference management scheme depends onthe desired trade-off between complexity and efficiency.

I Should require minimal/no coordination among HeNBs andMeNB and effectively solve the problem of cross-tier andco-tier interferences in uplink and downlink transmission fordifference access modes of HeNBs

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Standardization Activities

RF requirements for HNB and HeNB

I 3GPP Release-8 provides guidelines on access mode control,power control, maximum transmit power setting, measurementof ambient environment, interference management/control,and other RF functionalities for HNB and HeNB

I 3GPP Release-10, Release-11 for LTE-Advanced

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Standardization Activities

Deployment configurations for HNB and HeNB

I Dedicated deployment, co-channel deployment, partialco-channel deployment

I Fixed or dynamic power allocation

I Fixed and adaptive resource partitioning

I Intercell interference coordination in time, frequency, andspatial domain (ICIC in LTE system and eICIC inLTE-Adavnced)

58/80

Open Research Issues

I Distributed resource management schemes – satisfy the QoSrequirements of macrocell and femtocell UEs and at the sametime enhances the capacity and coverage of the network

I Low overhead for coordination among macrocell BSs (i.e.MeNBs), and should be able to integrate mobilitymanagement with different access modes

I Adaptive admission control, power control, and advancedcommunication strategies

I Hybrid interference management schemes which combinepower control with resource partitioning are promising

I Cooperative and cognitive resource management techniquesfor femtocell networks

I Interference modeling and effect of mobility (temporalcorrelation)

59/80

Open Research Issues

I Joint distributed cell association and resource allocationdesign where users associate with the “best” cell of either tier

I Design an overflow strategy jointly with cognitive resourceallocation and mobility management in two-tier networks

I Distributed beamforming algorithms using finite beamformingcodebook with limited signaling over backhaul when BSs areequipped with multiple antennas

I Admission control integrating decentralized load balancingamong different network tiers and its performance analysis

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Gracious!

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References - Introduction

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References - Frequency scheduling

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References - Graph-based approach

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[43] S. Park et al.,“Beam subset selection strategy for interference reduction in two-tier femtocell networks,” IEEE

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