4 g hetnets μακρής_dec15

4G Heterogeneous Networks (HetNets)
Πρόγραμμα Μεταπτυχιακών Σπουδών:
«ΕΠΙΣΤΗΜΗ ΚΑΙ ΤΕΧΝΟΛΟΓΙΑ ΥΠΟΛΟΓΙΣΤΩΝ
(ΕΤΥ)»
ΤΜΗΜΑ ΜΗΧΑΝΙΚΩΝ ΗΛΕΚΤΡΟΝΙΚΩΝ
ΥΠΟΛΟΓΙΣΤΩΝ ΚΑΙ ΠΛΗΡΟΦΟΡΙΚΗΣ
ΠΟΛΥΤΕΧΝΙΚΗ ΣΧΟΛΗ, ΠΑΝΕΠΙΣΤΗΜΙΟ ΠΑΤΡΩΝ

Presentation Outline
• Introduction
• Benefits and research challenges of 4G HetNets
• Traditional cellular networks vs. 4G Hetnets
• 2-tier HetNet (typical system model and analysis)
• Area Spectral Efficiency (ASE) and Area Green Efficiency (AGE)
• Comparison of various cell-edge related technologies
• Femtocells

Progression of cellular peak data rates

Reasons for traffic increase
Mobile User Side
• Number of subscribers due to
Smart phones and devices (18%
increase in sale of smart phones in
2013 and even greater in 2014-15)
• Video and Ultra High Definition
(UHD) movies
• E-facilities; online gaming, e-Gov
services, highly video content
exchange and peer-to-peer
networking
• Mobile broadband
Corporate Side
• Increase deployment of mobile
broadband
• Flat rate pricing and other package
incentives policy
• Launch of high speed protocols such
as HSDPA, VoIP and Small-cells
• Triple Play (broadband + IP Phone +
IP TV)
• New corporate services e.g.
Machine-Type communication

Addition of macro BSs? Probably not!

Small Cell solution is better!

Benefits of HetNets
Bringing the network close to the subscribers is one of the striking solution to:
• Offload the macrocells and reduce the coverage holes.
• Enhance the link quality by reducing the distance between TX and RX.
• Allow more efficient spectral reuse due to large number of small-cell deployments.
• Reduce expenditures due to no or limited upfront planning and lease costs.
• Reduced power consumption and thus achieves target rates with low power.
• Increase coverage especially for sparsely populated and rural areas.

Challenges of HetNets
The implementation of HetNets implies its own challenges:
• Co-tier and Cross tier interference: unplanned/ad hoc deployments, lack of
coordination among different tiers (e.g. macro and micro tiers).
• Subscription and Connectivity Issues: some cells may operate on restricted mode
and thus non-subscribers cannot connect and hand over to the nearest cells (access
policy conflicts).
• Handovers and traffic offloading: provide a seamless uniform service when users
move in or out of the coverage area at the expense of significant overhead.
• Self-Organization: each cell constantly monitors the network status and optimizes
its settings to improve coverage and reduce interference.
• Convergence and Integration: users desired to connect to multi heterogeneous
RATs in multi access and multi operator environment based on “Always Best
Connected (ABC)” rule (maintain and optimized quality).

What is a small cell?
• Small-cell is a low power, low cost user/operator deployed base station or access
point to complement macrocell setup.
• “Small-cells” concept includes femtocells (up to 100m), picocells (up to 200m),
microcells (up to 500m), etc.
• Small-cell can be deployed open access and closed access/restricted.
• In home use can support up 4-6 mobile users and for business use can support up to
16-20 users, (dependent on small-cell coverage).
• Small-cells connect with the main networks using digital subscribers line (DSL) or
cable broadband (wired or wireless front haul options).
Key features:
• Consumer side: coverage; capacity; superior indoor quality of service and improve
battery life.
• Operator side: Reduce CAPEX and OPEX of mobile networks.

Range expansion via small cells
Range Expansion is essentially a way to extend the reach of cells through small-cell
deployments so that they cover more and more users in their vicinity and their edge
particularly with improved link quality. It is critical for the following important
reasons:
1) More users who can be better served by small-cells (than the macro) are being
connected to them;
2) More users can be offloaded from macro network (to small-cells) freeing up
resources for the users on the macro.
3) More users can be transmitted with much less uplink power and thereby reducing co-
channel interferences.
• And all these are done without any additional spectrum or infrastructure.

Serving the network edges
• LTE Advanced (LTE-A) should target the cell-edge user throughput to be as high as
possible, given a reasonable system complexity
• An intelligent interference management control is required to handle cell-edge
interferers
• A more homogeneous distribution of the user experience over the coverage
area is highly desirable and therefore a special focus should be put on improving
the cell-edge performance

Traditional cellular vs. 4G Hetnets (1/7)
Traditional Cellular:
• Outage/coverage probability distribution in terms of SINR (signal-to-interference
plus noise ratio) or spectral efficiency (bps/Hz)
4G HetNet:
• Outage/coverage probability distribution in terms of Rate or Area spectral
efficiency(bps/Hz/m2
)
Change (performance metrics):
Stop measuring performance with BER (bit error rate) or SINR distribution, or with
spectral efficiency. These metrics are no longer very relevant. Instead, use the rate
distribution (user-perceived, i.e., accounting for load), area spectral efficiency, area
green efficiency, etc.

• BSs spaced out, have distinct coverage areas. Hexagonal grid is an ubiquitous
model foe BS locations.
4G HetNet:
• Nested cells (pico/femto) inside macrocell. BSs are placed opportunistically and
their locations are better modeled as a random process.
Change (network topology):
• Phase out the grid model for BS locations, which is neither tractable nor realistic for
HetNets. Instead adopt a random spatial model for the BS locations.

• Usually connect to strongest BS, or perhaps two strongest during soft handover.
4G HetNet:
• Connect to BS(s) able to provide the highest data rate rather than signal strength.
Use biasing for small BSs (i.e. predetermined and modeled conditions).
Change (cell association):
• Initial work shows that load balancing through cell range extension is very valuable
in a HetNet, and that biasing is nearly optimum compared to a centralized
optimization, which is perhaps surprising.

• Downlink and Uplink to a given BS have approximately the same SINR. The best
DL BS is usually the best in UL too.
4G HetNet:
• Downlink and Uplink can have very dierent SINRs; should not necessarily use the
same BS in each direction.
Change (downlink-uplink relationship):
• The DL and UL need to be considered as two different networks, and will require
different models for interference, cell association, and throughput.

• BSs have heav-duty wired backhaul, are connected into the core network. BS to MS
connection is the bottleneck.
4G HetNet:
• BSs often will not have high speed wired connections. BS to core network
(backhaul) link is often the bottleneck in terms of performance and cost.
Change (backhaul bottleneck):
• One clever approach is the idea of caching popular content such as video clips or
other common downloads at the small cells. Such content can be updated
periodically at a time of low backhaul load. The gain of such innovations on
network performance can be large.

• Handoff to a stronger BS when entering its coverage area, involves signaling over
wired core network.
4G HetNet:
• Handoffs and dropped calls may be too frequent if use small cells when highly
mobile, communication overhead is a major concern.
Change (mobility):
• Improved mobility modeling, handover optimization, and mobility-aware
interference management are all challenging topics for future work.

• Employ (fractional) frequency reuse and/or simply tolerate very poor cell edge
rates. All BSs are available for connection, i.e. open access.
4G HetNet:
• Manage closed access interference through resource allocation; users may be in one
cell while communicating with a different BS; interference management is hard due
to irregular backhaul and sheer number of BSs.
Change (interference management):
• Efficient interference management in a HetNet relies on reasonable models for all
the previous topics discussed until now. Interference management is also another
challenging topic for research and future work.

2-Tier HetNet (typical system model)
Macrocell network Small cell network

Macrocell-Only network (Monet)

Uniformly Distributed Small Cell (UDC)

Is UDC setup an efficient choice? (1/2)

Is UDC setup an efficient choice? (2/2)

Energy efficiency problem and targets

Research towards green edges
• Energy efficiency has been recently marked as one
of the alarming bottleneck in the telecommunication
growth paradigm mainly due to two major reasons:
1.Slowly progressing battery technology
2.Dramatically varying global climate
What is a “green edge”?

Energy consumption at the downlink

Energy consumption at the uplink

Power Control (PC) techniques
Open Loop PC Closed Loop PC

Uplink power adaptation based on PC
Mobile users are considered to
be able to estimate/ compensate
their path loss while adjusting
their transmit power accordingly.
Uplink receiver at the BS
estimates the SINR of the
received signal and compares it
with the target SINR value. If the
received SINR is below the target
SINR, a TPC command is
transmitted to the mobile user to
request an increase in transmit
power. Otherwise, the TPC
command will request a decrease
in transmit power.

Area Spectral Efficiency (ASE) of
HetNets
The Area Spectral Efficiency (ASE) of the 2-tier
HetNet is defined as the sum of the maximum
achievable rates (of all UEs) per unit bandwidth
per unit area [bit rate/Hz/macrocell area] jointly
supported by the small-cell and macrocell Base
Stations.

ASE vs. macrocell radius
It is clear that the ASE
of the COE model has
been significantly
improved when Small-
cells are active in the
macrocell compared to
the MoNet and UDC
models. This is due to
the fact that COE
deployment restricts
only the cell-edge
mobile users to
communicate with the
Small-cells which
enhances the overall
network ASE compared
to UDC and MoNet
configurations.

Relative ASE gain vs. number of small cells
Due to reduced link distance
between the edge mobile
users and their respective
small-cell BSs, higher
relative ASE gain is
observed even with only few
active Small-cells in COE
configuration. This gain,
however, tends to reduce
with the further increase in
the number of small-cells as
the level of interference
increases with the population
of Small-cells.

Interference problem in 4G Hetnets

4G Hetnets’ interference mitigation
It can be seen that the received
interference power at
macrocell BS has been
significantly reduced due to
the presence of Small-cells
arranged on the edge of the
macrocell. Moreover, it is also
noted that the interference
received at a Small-cell BS
from N-1 small-cells is almost
same compared to the
interference experienced from
two adjacent Small-cells.

Standardization for Hetnet eICIC
• The ICIC methods specified in 3GPP Rel. 8 and Rel. 9 do not specifically consider
HetNet settings and may not be effective for dominant HetNet interference
scenarios (see previous figure)
• In order to address such dominant interference scenarios, enhanced Inter-Cell
Interference Coordination (eICIC) techniques were developed for Rel. 10, which
can be grouped under three major categories according to:
•
– Time-domain techniques.
– Frequency-domain techniques.
– Power control techniques.

Time domain techniques for eICIC
• Almost Blank Subframes (ABSFs) at femtocells
• As shown in next figure, in the ABSFs, no control or data signals, but
only reference signals are transmitted.
• When there are MUEs in the vicinity of a femtocell, they can be
scheduled within the subframes overlapping with the ABSFs of the
femtocell, which significantly mitigates cross-tier interference.
• Similar eICIC approach using ABSFs can also be used to mitigate
interference problems in picocells (and relays) that implement
range-expansion.
• When no interference coordination is used for range-expanded
picocell users, they observe large DL interference from the macrocell.
The interference problem can be mitigated through using ABSFs at
the macrocell, and scheduling range-expanded picocell users
within the subframes that are overlapping with the ABSFs of the
macrocell.

Frequency domain techniques (eICIC)
• In frequency-domain eICIC solutions, control channels and physical signals (i.e.,
synchronization signals and reference signals) of different cells are scheduled in
reduced bandwidths in order to have totally orthogonal transmission of these
signals at different cells. While frequency-domain orthogonalization may be
achieved in a static manner, it may also be implemented dynamically through
victim UE detection.
• For instance, victim MUEs can be determined by the macro eNBs by utilizing the
measurement reports of the MUEs, and their identity may be signaled by the macro
eNB to the home eNB(s) through the backhaul. Alternatively, victim MUEs may
also be sensed by the home eNBs.

Power control techniques (eICIC)
• Apply different power control techniques at femtocells.
• While reducing the radiated power at a femtocell also reduces the total throughput
of femtocell users, it may significantly improve the performance of victim MUEs.

Area Green Efficiency (AGE) of HetNets

AGE vs. macrocell radius
It is illustrated clearly that the
COE configuration outperforms
the UDC configuration since the
deployment of small-cells at the
macrocell edge mandates a
reduction in the number of edge
mobile users transmitting with
the maximum power. The AGE
improvement is due to the fact
that the number of energy
efficient users increase in both
UDC and COE deployments
with the increase in macrocell
radius.

Other cell-edge related technologies

Distributed Antennas on Edge (DOE)

D2D deployment at the edge (DCOE)

Small cell deployment: Comparison

Femtocells (some basics)
• A Femtocell is a low-power access point based on mobile technology which
provides wireless voice and broadband services.
• The Femtocell connects to the mobile operator’s network via a standard broadband
connection, including ADSL, cable or fibre.
• Data to and from the femtocell is carried over the Internet.
• Femtocells use standard wireless protocols over the air to communicate with
standard mobile devices, including mobile phones and a wide range of other
mobile-enabled devices.
• Standard protocols include GSM, WCDMA, LTE, Mobile WiMAX, CDMA and
other current and future protocols standardised by 3GPP and 3GPP2
• The use of such protocols allows femtocells to provide services to several billion
existing mobile devices worldwide.

Types of Femtocell – Class 1
• Similar transmit power and deployment view to Wi-Fi access points,
typically 20 dBm (100 mW) of radiated power or less.
– For residential or enterprise application.
– They typically support 4–8 users.
– Installed by the end-user.
• Class 1 femtocells may be stand-alone devices or integrated with
other technology (residential gateways).
• Access will often be closed, restricted to a specified group of users,
but may also be open.

• Higher power typically up to 24 dBm (250 mW) of radiated power,
– Class 2 femtocells may be installed in small-office, in large
enterprise buildings or in a corporate campus.
– Support longer range or more users (8–16)
– May be installed by the end-user or the operator.
• Will typically support additional functionality such as handover
between femtocells, integration with PBX and local call routing.
• Access may be closed or open.

• Still higher power (+24 dBm) and can be deployed:
– Indoors (e.g. in public buildings) for localised capacity, Outdoors
in built-up areas to deliver distributed capacity or in rural areas for
specific coverage needs
– longer range or more users (e.g. 16 or greater).
– Installed by the operator.
• Class 3 femtocells are used to solve specific coverage, capacity or
service issues.
• Typically they are open access.

Deployment scenarios for open access
femtocells

Femtocell characteristics (1/3)
• Generates coverage and capacity.
– Provide reliable coverage throughout the home. This allows users
to rely on their mobiles as a prime means of making and receiving
calls.
– Femtocells also create extra network capacity, serving a greater
number of users.
• Permits low prices.
– The large volumes envisaged for femtocells will allow substantial
economies of scale.
– Some femtocell operators include special tariffs for calls made or
received on the femtocell

• Operate in licensed spectrum.
– By operating in licensed spectrum femtocells provide assured
quality of service to customers over the air, free from harmful
interference.
• Fully managed by licensed operators.
– Femtocells only operate within parameters set by the licensed
operator. The operator is always able to create or deny service to
individual femtocells or users.

• Self-organising and self-managing.
– Class 1 and Class 2 Femtocells can be installed by the end
customer. They set themselves up to operate with high
performance, with no need for intervention by the customer or
operator.
• Over Internet-grade backhaul.
– Femtocells backhaul their data over Internet-grade broadband
connections, including DSL and cable, using standard Internet
protocols.

Femtocell vs. WiFi (1/2)
• Wi-Fi Access Points always operate in unlicensed spectrum.
– Interfering devices can legally appear close to any given user.
Most Wi-Fi devices operate in the 2.4 GHz frequency band, where
only three non-overlapping channels are available.
• Femtocells, operate in licensed spectrum.
– The operator is in control of every transmitting device and can
manage interference.

Femtocell vs. WiFi (2/2)
• Transmit Power Control in Wi-Fi networks
– Wi-Fi access points and client devices all transmit at a power of around
100 mW, which does not change even when far less power is required,
increasing the incidence of interference and draining batteries.
• Transmit Power Control in cellular technology
– Both the mobile devices and the femtocells continually adjust their
transmit power to the minimum necessary, reducing interference,
increasing capacity and increasing battery life.

Challenge 1: Spectrum Allocation
• How will a Femtocell adapt to its
surrounding environment and allocate
spectrum in the presence of Intra-cell and
Inter-cell Interference?
• Due to the absence of coordination
between the macrocell and femtocells, and
between femtocells, decentralized
spectrum allocation is an open research
problem.
• Can fractional frequency reuse be used for
overlapping macrocell and femtocells?
• Should interference between macrocell
and femtocell users be minimized through
bandwidth splitting?
• Which is the best scheme in various
configurations?
F1, F2, and F3 are different sets of sub-channels

Challenge 2: IP backhaul bottleneck
• How Will Backhaul Provide Acceptable QoS?
• IP backhaul should provide sufficient capacity to avoid creating a traffic bottleneck.
• Trials reveal that when users employed Wi-Fi, femtocells experience difficulty
transferring data and even low-bandwidth services like voice.
• This is a very important challenge to meet as improved voice coverage is the main
driver for femtocells.
• The Femtocell and Wi-Fi architectures should be extended in order to incorporate a
unified bandwidth controlling mechanism responsible for the dynamic allocation of
the backhaul bandwidth between the Femtocell, the Wi-Fi and the local wire line
network.
• A unified Call Admission Control could also improve the QoS by admitting a new
service request either to the Femtocell or to the Wi-Fi network depending on the
available respective capacity.

Challenge 3: Open or Closed Access?
• Should Femtocells Provide Open or Closed Access?
• A closed access femtocell has a fixed set of subscribed home users that, for privacy
and security, are licensed to use the femtocell.
• Open access femtocells, on the other hand, provide service to macrocell users if
they pass nearby. Radio interference is managed by allowing strong macrocell
interferers to communicate with nearby femtocells.
• Although open access reduces the macrocell load, the higher numbers of users
communicating with each femtocell will strain the backhaul to provide sufficient
capacity and raise privacy concerns for home users.
• Ethical and legal dilemmas arise on whether a femtocell should service macrocell
users for making emergency calls if they are located within its radio range.
• Operators are looking at hybrid models where some of the femto’s resources are
reserved for registered family members while others are open for roamers.

Challenge 4: Mobility Management
• How Will Handoff be Performed?
• Current cellular systems broadcast a neighbour list of potential handover cells to
be used by a mobile attached to the current cell.
• In general, handoff from a femtocell to the macrocell network is significantly easier
(as there is only one macro BS) than handoffs from the macrocell to the femtocell.
•
• Such a handoff protocol does not scale to the large numbers of femtocells that
“neighbor” (actually underlay) the macrocell.
• The underlying network equipment is not designed to rapidly change the lists as
femtocells come and go.
• In open access, channel fluctuations may cause a passing macrocell user to perform
multiple handovers.

Challenge 5: Femtocell location
• Can Subscribers Carry Their Femtocells for Use Outside the Home Area?
• Unlike Wi-Fi networks that operate in unlicensed spectrum in which radio
interference is not actively managed, femtocell networks will operate in licensed
spectrum.
• Femtocell mobility can cause problems when a subscriber with operator A carries
their femtocell to another location where the only service provider is rival operator
B.
• Should the femtocell be allowed to transmit on operator B’s
spectrum? How will Femtocells Provide Location Tracking for
Emergency calls?
• Femtocell location may be obtained by either:
– using GPS inside femtocells (added cost with possibly poor indoor
coverage),
– gathering information from the macrocell providing the femtocell falls
within the macrocell radio range,

Small cells in 5G era
• Objective is NOT just the link spectral efficiency (as in 2G, 3G, 4G)
–Area Spectral Efficiency and Energy Efficiency
• 4G is suitable for low density cells, NOT high density cells (small-cells)
– Minimum network management control overheads
– Sub-millisecond Air-Interface latency
– Flexible for carrier aggregation across highly fragmented spectrum including
license-exempt band
– Highly energy efficient (at least 10 times)
– Support fast and reliable spectrum sensing for spectrum sharing
– Simple Wi-Fi-like MAC
– Support of device to device communications
– Scalable for Machine type communications

Managing overheads for small cells
Before After

Split Data/Control plane  SDN
technology in 5G?
Splitting the data and control functionality – scalable deployment
solution for small-cells in 5G cellular networks

Open issues
• Cooperative wireless communications/Relays
• Mobile cloud computing (MCC)
• M2M/IoT
• Self-organized networks (SON)
• Context aware mobile and wireless networking (CAMoWiN)
• Software-defined networking (SDN)
• Network function virtualization (NFV)

4 g hetnets μακρής_dec15

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