4G Heterogeneous Networks (HetNet

4G Heterogeneous Networks (HetNet)

4G Heterogeneous Networks › Supporting high data rates has been the most essential

requirement 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

UEUE

Interference

RN

RN

HeNBHeNB

HeNB

Femtocell

Femtocell

HeNB

Femtocell

HeNBHeNB

FemtocellHeNB

UEUE

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Femtocell

HeNBHeNB

S1

S1

S1

S1S1

S1

S1 MME / S-GW

P-GW

S5S6a

Evolved

Packet

Core (EPC)

Macrocell

Femtocell

HeNB GW

SeGWAHR

AAA Server

D/Gr’ HSS

Wx

Wm

X2

X2

X2 eNBa

eNBc

eNBb

HeNB

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HetNet architecture

› Femtocell – enhance the indoor signal strength

– backhaul: users’ digital subscription line (DSL), › delivery delay is around the level of milliseconds.

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

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HetNet architecture

› Picocell – share the traffic load of the macrocell on a hop spot 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 a macro-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 macrocell to enhance the signal strength of a macro-BS at the 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 of scheduling and resource allocation.

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HetNet Challenges

› Haphazard deployment – femto-BSs are deployed by users in a fully dynamic fashion

› Restricted/closed access – femto-BSs are paid by the customers,

– only users defined by the owners are allowed to access femto-BSs.

› No coordination between macro-BS and femto-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-tier and intra-tier interference occurs

› An effective interference mitigation scheme

› Centralized (global) radio resource allocation schemes

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 with information exchange – A large amount of information exchanges among users/cells – Interfaces for information exchange may not be available

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Interference Management

› 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

freq

uen

cey

femto macro

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› Co/common channel assignment

time

freq

uen

cey

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Location Locking

› Emergency Call Location:

› Spectrum Use – People may bring the femto-BS to a new location without the operator's awareness and permission to enjoy the free transmission with high quality everywhere.

– incur heavy cross-tier and intra-tier interference

› Commercial – Operators may decide they can justify charging an additional fee to process a femtocell relocation

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Location Locking

HeNBb

eNB

Macrocell

Evolved Packet Core (EPC)

Move to a

new location

UE 2UE 3

UE 1

HeNBa

UE 4

eNBHeNBb

Cross-tier interference

Intra-tier interference

HeNB Management System (HMS) Internet

P-GW

HeNB GWSeGW

MME S-GW

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empty RB

RB occupied by small cells

RB occupied by macrocells

RB occupied by both

macrocells and small cells

freq

uen

cey

time

macrocell BS macrocell users

small cell BS small cell users

cross-tier interference from

small cells to macrocell

orthogonality at time-frequency

no orthogonality at time-frequency

orthogonality at location/spatiality

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Target of Orthogonality › Location/Spatiality

– two nodes physically locating closely to each other do not suggest severe interference to/from each other even if a small transmission power is adopted in both nodes

› Time-Frequency – Resource blocks (RB)

› Antenna Orthogonality

› Multiple Input Multiple Output – (MIMO) form different transmissions spatial paths.

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Methods of Information Acquisition › Exchanging at the BS side

– By leveraging wireless/wired interfaces, small cell BSs and macro-BS could directly exchange information

– 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 to its 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 RB exceeds a certain threshold, femto-BS identifies that this RB is occupied and retrieves activity

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Information required Inference mitigation techniques

Type

Activity Channel Codebook Message

Coordination Domain of

orthogonality

Time-frequency

o

Space/ locaction

o

Antenna spatiality

o

Cancellation Coding techniques o o o

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Interference Management Theoretical Analysis – Stochastic Geometry

› Consider a network snapshot with a receiver at the origin

– All other simultaneous transmissions have randomly located transmitters

– Assume all nodes have same transmit power

– Nodes are randomly distributed according to a point process Π

– Fading power is ℎ𝑖𝑗

› Aggregate interference

SIR0 =ℎ0𝑟

−𝛼

ℎ𝑖0|𝑋𝑖|−𝛼

𝑖∈Π(𝜆)

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r

Π={Xi}

Interference Management Theoretical Analysis – Stochastic Geometry › Reliability: outage probability relative to a SIR threshold 𝜂

› Shot noise interference: Z = ℎ𝑖0|𝑋𝑖|−𝛼

𝑖∈Π(𝜆)

› Assume exponential ℎ𝑖𝑗 with unit mean

– Signal is Rayleigh faded with unit average power

• Assume Poisson point process (PPP) Π – The probability that there are 𝑛 nodes in 𝐴 is given by the Poisson

distribution and thus equal to (𝜆𝐴)𝑛𝑒−𝜆𝐴/𝑛!

› Outage probability for fetmo-UE receiver (FUE):

𝑝FUE = Pr SIRFUE > 𝜂 = Pr ℎ0 > 𝜂𝑟𝛼Z

= 𝑒−𝜂𝑟𝛼z𝑓𝑍 𝑧 𝑑𝑧

∞

0= E 𝑒−𝑠𝑍 |𝑠=𝜂𝑟𝛼

= exp(−𝑟2𝜆𝑒2/𝛼𝐶𝛼)

where 𝐶𝛼 = 2π2/𝛼 csc(2𝜋

𝛼)

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Access Control

› Closed Access Mode – Only emergency calls if not in closed subscriber group (CSG)

› Open Access Mode – All UEs are treated equally in the cell

› Hybrid Access Mode – UEs not part of CSG may camp and acquire some level of service.

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Access Control

› Closed Access – Nonsubscribers have strong interference behind the femtocell.

– High probability of outage.

› Open Access – High frequency of handover.

– Not enough cell IDs for femtocells in a macrocell.

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Security/QoS

› 3GPP was not able to complete the security aspect. – Device Authentication

– Encryption/Ciphering

› QoS control – QoS for delay sensitive traffic, voice

– Operator’s backhaul system

– Third party entity (DSL, Cable Network)

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Synchronization

› Distributed interference management methods – based on an ideal assumption of perfect timing synchronization between femtocells and Macrocells/picocells, and among all femtocells.

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Synchronization

› Synchronization and timing are very critical

› What makes it difficult to achieve in Femtocells – Timing and synchronization depends on reliable receipt and delivery of RF signals. › There can be areas where there are no macro cells nearby.

› Macro cell with very bad radio condition.

– Number of Femtocells and location of each femtocell is unpredictable.

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Synchronization

› IEEE1588 : A Precision Time Protocol – deliver timing information from a synchronization serverto all femtocell BSs through the wired backhaul,

– each femtocell BS can measure the timing difference between the femtocell BS itself and the synchronization server.

› Air interface Based : GPS – GPS makes use of multiple continuously moving satellite for synchronization

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Synchronization

› TV Receiver – Femto-BSs equipped with TV receiver are proposed to achieve synchronization by receiving broadcasted TV signals

– penetration loss of building walls

› User Equipment (UE): – UEs are leveraged to assist the serving BS to synchronize with neighboring synchronized BSs

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Synchronization › Solution: inter femto-BS communications

› Gossip algorithm – fast averaging problem with continuous changes of connections among nodes and the taken values

› Voter algorithm – Femto-BS updates its timing to that of only one randomly selected femto-BS.

› Asynchronous radio resource management ?

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Mobility Management

› Random deployment

› Load balancing

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4G Heterogeneous Networks (HetNet

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