NTUST - 4G HetNet

8/12/2019 NTUST - 4G HetNet

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4G HeterogeneousNetworks (HetNet)


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G Heterogeneous Networks Supporting high data rates has been the most essentialrequirement for the next generation cellular system.

1G bps for static; 100M bps for mobility (definition of 4G by IMT-Advanced)

How to significantly enhance the data rate? Enhancing multiple access, technology

Shortening the distance between the transmitter and the receiver Extending the bandwidth by allowing more concurrent transmissions.

Reduce the cell size The cost of re-deployment is too high

Deploying cells with smaller coverage overlaying conventional

Macrocells Femtocells (indoor) Picocells (outdoor) Relay nodes (RNs)

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4G Heterogeneous Networks

Multi-tier network

eNB

eNB

eNB

eNB

eNB

eNB

eNB

Pico-eNB

Pico-eNB

Pico-eNB

Macrocell

MacrocellMacrocell

Macrocell

Macrocell

Macrocell

Picocell

Picocell

Picocell

UEUE

UEUE

UE

UE

Interference

RN

RN

HeNBHeNB

HeNB

Femtocell

Femtocell

HeNB

Femtocell

HeNBHeNB

FemtocellHeNB

UEUE

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Femtocell

HeNB

HeNB

S1

S1

S1

S1S1

S1

S1 MME / S-GW

P-GW

S5S6a

Evolved

Packet

Core (EPC)

Macrocell

Femtocell

HeNB GW

SeGWAHR

AAA

ServerD/Gr HSS

Wx

Wm

X2

X2

X2 eNBa

eNBc

eNBb

HeNB

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HetNet architecture

Femtocell enhance the indoor signal strength backhaul: usersdigital subscription line (DSL),

delivery delay is around the level of milliseconds.

a real-time communication between a femto-BS and amacro-BS is unavailable.

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HetNet architecture

Picocell share the traffic load of the macrocell on a hopspot area.

identical to a macro-BS, except a smaller

transmission. direct interface (known as X2) between a pico-BS and a macro-BS a real-time communication between pico-BS and amacro-BS is possible.

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Relay Station

MME / S-GW

RS

RS

Type II

Type IeNBRS

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HetNet architecture

RNs deployed at the coverage edge of a macrocellto enhance the signal strength of a macro-BS atthe coverage edge.

Type I RN has a cell identity, Type II RN does not

a RN is more than just a signal repeater in

Layer 1, but may also be capable ofscheduling and resource allocation.

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HetNet Challenges

Haphazard deployment femto-BSs are deployed by users in a fullydynamic fashion

Restricted/closed access femto-BSs are paid by the customers, only users defined by the owners are allowed toaccess femto-BSs.

No coordination between macro-BS andfemto-BS

Backward compatibility:

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HetNet Technical Issues

Interference management Access control

Security/QoS

Time Synchronization

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Interference Management

Severe cross-tierandintra-tierinterferenceoccurs

An effective interferencemitigation scheme

Centralized (global) radioresource allocationschemes

Macrocell

femto-MS a

macro transmission

femto transmission intra-tier interference

cross-tier interference

macro-MS c

femto-BS 2

macro-BS

femto-MS b

Wired

Backhual

femto-BS 1

High computational complexity No scalability

Distributed radio resource allocation schemes withinformation exchange A large amount of information exchanges among users/cells Interfaces for information exchange may not be available

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Inter-cell/intra-tier interference

macro-BS x0

macro-BS

macro-BS

macro-BS

macro-BS

macro-BS

macro-BS

macrocell

macrocell

macro-UE macro-UE

macro-UE

macro-UE

macro-UE

macro-UE macro transmission

inter-cell (intra-tier) interference

x

y

The typical macro-UE

located at origin

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Intra-tier and cross-tier interference

pico-BS

pico-BS

pico-BS

femto-BSfemto-BS

femto-BS

femto-BS

macro-BS x0

macro-BS

macrocell

macrocell

macro transmission

intra-tier interference

x

y

The typical macro-UE

located at origin

pico-BS

pico transmission femto transmission

cross-tier interference

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Orthogonal /separate channel assignment

time

frequencey

femto macro

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Co/common channel assignment

time

frequencey

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Location Locking

Emergency Call Location: Spectrum Use

People may bring the femto-BS to a newlocation without the operator's awareness and

permission to enjoy the free transmission withhigh quality everywhere.

incur heavy cross-tier and intra-tier interference

Commercial Operators may decide they can justify chargingan additional fee to process a femtocellrelocation

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empty RB

RB occupied by small cells

RB occupied by macrocells

RB occupied by both

macrocells and small cells

frequencey

time

macrocell BS macrocell users

small cell BS small cell users

cross-tier interference from

small cells to macrocell

orthogonality attime-frequency

no orthogonality attime-frequency

orthogonality at

location/spatiality

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Target of Orthogonality

Location/Spatiality two nodes physically locating closely to eachother do not suggest severe interferenceto/from each other even if a small transmissionpower is adopted in both nodes

Time-Frequency Resource blocks (RB)

Antenna Orthogonality

Multiple Input Multiple Output (MIMO) form different transmissions spatialpaths.

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Methods of Information Acquisition

Exchanging at the BS side By leveraging wireless/wired interfaces, smallcell BSs and macro-BS could directly exchangeinformation

no direct interface between a macro-BS and a

femto-BS heavy communication overheads

Measuring at the UE side performing periodical channel measurement in

UEs and reporting the measurement results toits serving BS consume power in UEs

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Methods of Information Acquisition

Measuring at the BS side BS has the capability of measuring interference if the received interference power on a RBexceeds a certain threshold, femto-BS identifies

that this RB is occupied and retrieves activity

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Information requiredInference mitigation techniques TypeActivity Channel Codebook Message

CoordinationDomain of

orthogonality

Time-frequency

oSpace/locaction

oAntennaspatiality

oCancellation Coding techniques o o o

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Interference ManagementTheoretical Analysis Stochastic Geometry

Consider a network snapshotwith a receiver at the origin All other simultaneous

transmissions have randomlylocated transmitters

Assume all nodes have same

transmit power Nodes are randomly

distributed according to apoint process

Fading power is

Aggregate interferenceSIR0

||()

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r

={Xi}


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Interference ManagementTheoretical Analysis Stochastic Geometry

Reliability:outage probability relative to a SIR threshold

Shot noise interference: Z 0||

()

Assume exponential with unit mean Signal is Rayleigh faded with unit average power

Assume Poisson point process (PPP)

The probability that there are nodes in is given by the Poissondistribution and thus equal to ()/!

Outage probability for fetmo-UE receiver (FUE):

FU Pr SIRFU> Pr 0> Z

z

0 E

|=

exp(/)

where 2/ csc(

)

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Access Control

Closed Access Mode Only emergency calls if not in closed subscribergroup (CSG)

Open Access Mode

All UEs are treated equally in the cell

Hybrid Access Mode UEs not part of CSG may camp and acquiresome level of service.

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Access Control

Closed Access Nonsubscribers have strong interference behindthe femtocell.

High probability of outage.

Open Access High frequency of handover. Not enough cell IDs for femtocells in amacrocell.

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Security/QoS

3GPP was not able to complete the securityaspect. Device Authentication Encryption/Ciphering

QoS control QoS for delay sensitive traffic, voice

Operators backhaul system Third party entity (DSL, Cable Network)

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Synchronization

Synchronization and timing are very critical

What makes it difficult to achieve inFemtocells Timing and synchronization depends on reliablereceipt and delivery of RF signals. There can be areas where there are no macro cellsnearby.

Macro cell with very bad radio condition.

Number of Femtocells and location of eachfemtocell is unpredictable.

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Synchronization

IEEE1588 : A Precision Time Protocol deliver timing information from asynchronization serverto all femtocell BSsthrough the wired backhaul,

each femtocell BS can measure the timingdifference between the femtocell BS itself andthe synchronization server.

Air interface Based : GPS

GPS makes use of multiple continuously movingsatellite for synchronization

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Synchronization

Solution: inter femto-BS communications Gossip algorithm

fast averaging problem with continuous changesof connections among nodes and the takenvalues

Voter algorithm Femto-BS updates its timing to that of only onerandomly selected femto-BS.

Asynchronous radio resource management?

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Mobility Management

Random deployment Load balancing

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NTUST - 4G HetNet