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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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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Interference Management
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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Interference Management
Orthogonal /separate channel assignment
time
frequencey
femto macro
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Interference Management
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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Interference Management
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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Interference Management
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