looking at 4g networks: solutions and perspectivelooking at 4g networks: solutions and perspective...

Looking at 4G networks: solutions and perspective

Ing. Giuseppe Piro, PhDDEI, Politecnico di Bari, Italy

[email protected]

May 18, 2016

IntroductionOverview of LTE Networks

LTE-A ImprovementsRel-12/13 Improvements

Scheduling in LTE systems

Outline

1 Introduction

2 Overview of LTE Networks

3 LTE-A Improvements

4 Rel-12/13 Improvements

5 Scheduling in LTE systems

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1 Introduction





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Why do we need new broadband wireless technologies?

[Capozzi et al., 2013, Capozzi et al., 2012, Piro et al., 2011b]

Problem

Expected exponential increase of data traffic from mobile devices

3G cellular systems are not able to support this massive increment of mobile datatraffic

More and more broadband services on mobile devices

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What do people need today?

!

Anytime, anywhere, always connected

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Emerging Challenges

!

Problem

Mobile users would be anytime, anywhere, and easily connected to the Internet

Cellular networks must handle a very large number of heterogeneous applicationflows

Mobile operators must increase the capacity and the coverage of broadbandcellular networks

Mobile voice services are not the core of the cellular system anymore

It is necessary to provide high-quality services even in mobile conditions

Other important needs: security, IP connectivity, enhanced QoS differentiation,indoor wireless broadband

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What’s the solution?

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Towards 4G systems

Answer

In particular, 3GPP introduced Long Term Evolution (LTE) and LTE-Advanced(LTE-A) specifications:

new architectures for radio access and core network

all-IP networks

packet-optimized architecture

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LTE main aspects

Goals

Support of a wide range of multimedia and Internet services

high mobility scenarios

Designed for..

high data rates

low latency

improved spectral efficiency

Some key aspects

OFDMA

Resource sharing

Channel Quality Indicators

Hybrid ARQ

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What’s about the Packet Scheduler?

Need for effective resource allocation

Efficient use of resources to meet user’s QoS/QoE requirements

Design of packet scheduler at base station, i.e., evolved NodeB (eNB)

Spectrum sharing among users, following specific policies

Counteract high variability of wireless channel quality (in time and frequencydomains), e.g., due to fading, multipath propagation, Doppler effect, and so on

Schedulers should maximize spectral efficiency using an effective resourceallocation policy

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1 Introduction





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Main design targets

Minimum requirements

Doubling spectral efficiency w.r.t. previous generation systems

Increasing network coverage in terms of bitrate for cell-edge users

Some new performance targets

Increased data rates: peak data rates for the downlink and uplink equal to 100Mbps and 50 Mbps, respectively

Very high user mobility (connection up to 350 km/h)

Scalable bandwidth occupation: from 1.4 to 20 MHz.

Significative novelty

Enhanced Quality of Service (QoS) support by means of new sophisticated RadioResource Management (RRM) techniques

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Summary of LTE features

Table: Main LTE Performance Targets

Peak Data Rate - Downlink: 100 Mbps- Uplink: 50 Mbps

Spectral Efficiency 2 - 4 times better than 3G systemsCell-Edge Bit-Rate Increased whilst maintaining same site locations as deployed todayUser Plane Latency Below 5 ms for 5 MHz bandwidth or higherMobility - Optimized for low mobility up to 15 km/h

- High performance for speed up to 120 km/h- Maintaining connection up to 350 km/h

Scalable Bandwidth From 1.4 to 20 MHzRRM - Enhanced support for end-to-end QoS

- Efficient transmission and operation of higher layer protocolsService Support - Efficient support of web-browsing, FTP, video-streaming, VoIP, etc.

- VoIP should be supported with at least as voice traffic in UMTS

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LTE Architecture: Service Architecture Evolution, SAE

Seamless mobility supportHigh speed delivery for data and signalling

�

eNB

UE

MME

UE

eNB

UE

other

IP networks

PGW

SGW

Evolved Packet Core

E-UTRAN

E-UTRAN

��

��

��

Radio Access Network

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Service Architecture Evolution: the core network

Evolved Packet Core

Mobility Management Entity (MME)- handling of user mobility, intra-LTE handover, andtracking/paging procedures of UserEquipments (UEs)Serving Gateway (SGW)- routing and forwarding of user data packetsamong LTE nodes; managing of handover amongLTE and other 3GPP technologiesPacket Data Network Gateway (PGW)- gateway to the rest of the world

MME

other

IP networks

PGW

SGW

Evolved Packet Core

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Service Architecture Evolution: the radio access network

Evolved-Universal Terrestrial Radio AccessNetwork (E-UTRAN)

User Equipment- the end-userevolved NodeB- radio base station- eNBs directly connected to each other (speedingup signaling procedures) and to the MME gateway- differently from other cellular networks, eNB is theonly device in charge of performing both radioresource management and control procedures onthe radio interface.

eNB

UEUE

UE

eNB

UE

E-UTRAN

E-UTRAN

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LTE Radio Bearers

Definition

Logical channels established between UE and eNB. IP packets are mapped to a beareraccording to QoS requirements.

Default bearer

Created when an UE joins the network.

Used for basic connectivity and exchange of control messages.

It remains established during the entire lifetime of the connection.

Dedicated bearers

Set up every time a new specific service is issued.

Depend on QoS requirements.

Classified as Guaranteed bit-rate (GBR) or non-guaranteed bit rate (non-GBR)bearers.

A set of QoS parameters is associated to each bearer.

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QoS Management

Bearers differentiation/classification

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Standardized QoS Class Identifiers

The RRM module translates QoS requirements into: scheduling parameters, admissionpolicies, queue management thresholds, link layer protocol configurations, and so on.

QCI ResourceType

Priority Packet DelayBudget [ms]

PLR Example services

1 GBR 2 100 10−2 Conversational voice

2 GBR 4 150 10−3 Conversational video

3 GBR 5 300 10−6 Non-Conversational video

4 GBR 3 50 10−3 Real time gaming

5 non-GBR 1 100 10−6 IMS signaling

6 non-GBR 7 100 10−3 Voice, live video, interactivegaming

7 non-GBR 6 300 10−6 Video (buffered streaming)

8 non-GBR 8 300 10−6 TCP based

9 non-GBR 9 300 10−6

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LTE protocol stack

Radio Resource Control

It handles the establishment and management ofconnections, the broadcast of system information, themobility, the paging procedures, and the establishment,reconfiguration and management of radio bearers.

Packet Data Control Protocol

It operates header compression of upper layers before theMAC enqueueing.

Radio Link Control (RLC)

It provides interaction between the radio bearer and theMAC entity.

MAC

It provides all the most important procedures for the LTEradio interface, such as multiplexing/demultiplexing, randomaccess, radio resource allocation and scheduling requests.

PDCP

PDCP

RLC

RLC

MAC

MAC

PHY

PHY

User plane Control Plane

RLC

RLC

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Physical layer

Bandwidth

LTE supports several system configurations: from 1.4 MHz up to 20 MHz.

Radio Access

Based on Orthogonal Freq. Division Multiplexing (OFDM) scheme.

Uplink: Single Carrier Freq. Div. Mult. Access (SC-FDMA).

Downlink: Orthogonal Freq. Div. Mult. Access (OFDMA).

They allow multiple access by assigning sets of sub-carriers to each individualuser.

OFDMA can exploit sub-carriers distributed inside the entire spectrum.

SC-FDMA can use only adjacent sub-carriers.

OFDMA provides high scalability, simple equalization, and high robustnessagainst the time-frequency selective nature of radio channel fading.

SC-FDMA is used to increase the power efficiency of UEs (battery supplied).

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Radio resources

Radio resources allocated in Time/Frequency domain.

Time domain: distributed every Transmission Time Interval (TTI) (1 ms).

Time split in frames: 10 consecutive TTIs.

Each TTI made of 2 time slots with length 0.5 ms (corresponding to 7 OFDMsymbols in the default configuration with short cyclic prefix).

Frequency domain: total bandwidth divided in sub-channels of 180 kHz, each onewith 12 consecutive and equally spaced OFDM sub-carriers.

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Resource Block

A time/frequency radio resource spanning over 2 time slots in the Time domainand over 1 sub-channel in the Frequency domain.

It corresponds to the smallest radio resource unit that can be assigned to an UEfor data transmission.

As the sub-channel size is fixed, the number of Resource Blocks (RBs) variesaccording to the system bandwidth configuration (e.g., 25 and 50 RBs for systembandwidths of 5 and 10 MHz, respectively).

Time

1 TTI composed by

2 time slots of 0.5 s each

LTE frame composed by

10 consecutive TTI

sub-channel

of 180 kHz

Resource Block

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Duplexing

Frequency Division Duplex: bandwidth divided in two parts, allowingsimultaneous downlink and uplink data transmissions; the LTE frame is composedof 10 consecutive identical sub-frames.

Time Division Duplex (TDD): the LTE frame is divided into two consecutivehalf-frames, each one lasting 5 ms. Several frame configurations allow differentbalance of resources dedicated for downlink or uplink transmission.

Table: TDD Frame Configurations

sub-frame number1st half frame 2nd half frame

config. number 0 1 2 3 4 5 6 7 8 9

0 D S U U U D S U U U1 D S U U D D S U U D2 D S U D D D S U D D3 D S U U U D D D D D4 D S U U D D D D D D5 D S U D D D D D D D6 D S U U U D S U U D

D = downlink sub-frame; U = uplink sub-frame; S = Special sub-frame.

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Summary of Physical Layer Parameters

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Radio Resource Management

LTE makes massive useof RRM procedures, e.g.,link adaptation, HybridAutomatic RepeatRequest (HARQ), PowerControl, and ChannelQuality Indicator (CQI)reporting.

Placed at physical andMAC layers, and stronglyinteract with each otherto improve the usage ofavailable radio resources.

RRC

PHY

Interface

UPPER LAYERS

AMC

Packet

Scheduler

Radio bearer

MAC

queueRLC entity

QoS

paremeters

HARQ

RRC

AMC

Radio bearer

MAC

queueRLC entity

QoS

paremeters

HARQ

CQIMAC MAC

eNB UPPER LAYERSUE

PD

CC

H

PD

SC

H

PIL

OT

PU

SC

H

PU

CC

H

PHY

Interface

channel

mod /

demodulation

mod /

demodulation

Legend

AMC: Adaptive Modulation and Coding HARQ: Hybrid Automatic Repeat Request

RLC: Radio Link Control RRC: Radio Resource Control

PDCCH: Physical Downlink Control Channel PDSCH: Physical Downlink Shared Channel

PUSCH: Physical Uplink Schared Channel PUCCH: Physical Uplink Control Channel

CQI: Channel Quality Indicator

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Radio Resource Management: CQI reporting

Channel Quality Indicator reporting

Fundamental feature of LTE networks

It enables the estimation of the quality of the downlink channel at the eNB

Each CQI is calculated as a quantized and scaled measure of the experiencedSINR.

Main issue related to CQI reporting methods

To find a good tradeoff between

a precise channel quality estimation

a reduced signaling overhead

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Radio Resource Management: AMC

Adaptive Modulation and Coding

The CQI reporting procedure is strictly related to the Adaptive Modulation andCoding (AMC) module

It selects the proper Modulation and Coding Scheme (MCS)

Objective: maximize the supported throughput with a given target Block ErrorRate (BLER)

Limited number of allowed modulation and coding schemes, hence, systemthroughput is upper-bounded: over a certain threshold an increase in the Signalto Interference plus Noise Ratio (SINR) does not bring to any throughput gain.[Dahlman et al., 2008].

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Radio Resource Management: Power Control

Optimization: power control

Dynamic modification of transmission power to compensate for variations of theinstantaneous channel conditions:

saving energy while maintaining a constant bitrate (i.e., power reduction)

increasing bitrate by selecting a higher MCS (i.e., power boosting)

In both cases, the goal is obtained while keeping expected BLER below a targetthreshold.

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Radio Resource Management: Hybrid Automatic Repeat Request

Characteristics

Retransmission procedure at MAC layer

Based on the use of the well-known Stop-&-Wait algorithm

Performed by eNB and UE exchanging ACK/NACK messages

Procedure

NACK sent on the Physical Uplink Control Channel (PUCCH) if packettransmitted by eNB unsuccessfully decoded at UE

eNB retransmits packet

UE tries to decode packet combining retransmission with original received version

ACK message to the eNB upon a successfully decoding.

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Physical Channels: Downlink

Downlink Data

Transmitted by the eNB on the Physical Downlink Shared Channel (PDSCH).

PDSCH shared among all users: no reservation.

PDSCH payloads transmitted only in given portion of the spectrum and timeintervals.

Downlink Control Signaling

Carried by 3 physical channels

Physical Downlink Control Channel important for scheduling

PDCCH carries assignments for downlink resources and uplink grants, includingthe used MCS.

Notes

Control overhead has influence on downlink performance

Every TTI a significant amount of radio resources is used for signaling

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Time/Frequency structure of downlink subframe

Case: 3 MHz bandwidth. Example with 3 OFDM symbols for control channels

Time

14 consecutive OFDM symbols

sub-channel

of 180 kHz

Control Region: 3 OFDM symbols dedicated to signalling information

Data Region: remaining 11 OFDM symbols used for data transmission

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Physical Channels: Uplink

Physical Uplink Shared Channel

Used for data transmission in uplink

Uplink control signals multiplexed on PUSCH when UE scheduled for datatransmission

Different control fields, e.g., ACK/NACK and CQI

Physical Uplink Control Channel

No data foreseen in a given TTI

Signaling, e.g., ACK/NACK related to downlink transmissions, downlink CQI,requests for uplink transmission

Due to single carrier limitations, simultaneous transmission on both channels is notallowed.

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1 Introduction





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Some Main Features of LTE-A

Enhanced Multiple Input Multiple Output (MIMO) techniques

Schemes for Coordinated Multi-Point (CoMP)

Carrier aggregation

Possibility to use Heterogeneous networks

Use of Relay Nodes

Massive use of Femtocells

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Enhanced MIMO techniques

Extension up to 8-layer transmission (increased from 4 layers in Rel-8/9) in thedownlink direction

Support for enhanced Multi-user MIMO (MU-MIMO) in downlink

Introduction of Single-user MIMO (SU-MIMO) up to 4-stream transmission inuplink

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CoMP

The transmission and/orreception is coordinated byusing multiple base stations

Decreases the co-channelinterference and improves thecell-edge performance

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Carrier Aggregation

Carrier aggregation is used in Long Term Evolution-Advanced (LTE-A) in orderto increase the bandwidth, i.e., to increase the bitrate

Since it is important to keep backward compatibility with UEs of Rel.8 and Rel.9,the aggregation is based on Rel.8/Rel.9 carriers.

Several LTE Rel.8 compatible component carriers are placed adjacent

The component carrier can have a bandwidth of 1.4, 3, 5, 10, 15 or 20 MHz anda maximum of five component carriers can be aggregated, hence the maximumaggregated bandwidth is 100 MHz

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How to arrange Carrier Aggregation

intra-band contiguous aggregation

Easiest way to arrange aggregation: use contiguous component carriers within thesame operating frequency band (intra-band contiguous).

Not always possible, due to operator frequency allocation scenarios.

Non-contiguous aggregation

Intra-band: same operating frequency band, but there are gaps among them

inter-band: different operating frequency bands

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Heterogeneous networks

A HetNet consists of a mix of macro cells, handled by a common LTE basestation (i.e., the eNB) and small-range cells managed by low-power nodes (i.e.,micro, pico, relay, and femto).Whereas micro, pico, and relay devices have been conceived for enhancingcoverage and capacity in some regions inside the macro cell, femto nodes havebeen conceived for offering broadband services in indoor (home and offices) andoutdoor scenarios with a very limited geographical coverage.

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Relay Nodes

Relays nodes pick up signals transmitted from a base station to a mobile deviceand resend an amplified or revised version of the signal to the mobile device

The eNB of the macro cell is called Donor eNB (DeNB)

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Frame Transmission with Relay Nodes

Due to self-interference, Relay Nodes cannot simultaneously transmit and receiveon both user and backhaul links

Multicast Broadcast Single-Frequency Network (MBSFN) frame structure isexploited to handle simultaneously both kinds of communications (i.e., with usersalong the user link and with the eNB along the backhaul link)

MBSFN classifies TTIs of the LTE frame in MBSFN sub-frames and non-MBSFNsub-frames. A relay node can exchange packets with the DeNB only during theformer kind of time slots, leaving the other ones for data transmissions with users

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Interference mitigation in HetNets

In a scenario with macro and pico/micro cells, the interference level may reallydowngrade the network performances

This effect is more evident for mobile operators having few frequencies andinterested in using the whole spectrum in each cell

The interference level is more disruptive for users attached to pico/micro cellsdue to the lower transmission power of their target base station

It is necessary to introduce enhanced schemes able to mitigate the impact of theinterference

To this aim, LTE-A uses enhanced Inter-cell interference coordination (eICIC)schemes: Range Expansion (RE) and Almost Blank Subframe (ABS)

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eICIC scheme: Range Expansion

The Range Expansion (RE)technique introduces a biasthat artificially increases theSINR of the pico/micro cell(suggested values [3-12] dB)

This would increase thenumber of UEs connected tothe small cell even if themacro cell SINR is stronger

All users will experience anincreased amount of availablebandwidth

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eICIC scheme: Almost Blank Subframe

Almost Blank Subframe (ABS), to further reduce the interference level generatedby the macro cell to all the other small cells, introduces the Time DomainMultiplexing inter-cell interference coordination

The base stations of macro cells, which cause severe interference to others basecells, are periodically muted for entire subframes, i.e., the ABS subframes

During ABS subframes, hence, only small cells can handle packet transmission

In this way, the chance to serve users suffering from severe interference levels(users at the cell edge) is given to the small cells

During not-ABS subframes, instead, all the base stations transmit data at thesame time.

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Femtocells

Femtocells have been devised for offering broadband services in indoor (i.e., homeand office) and outdoor scenarios with a very limited geographical coverage

They can be easily set up without any centralized coordination, but simplyenabling low-power and small-range radio base stations, that is, home evolvedNodeB (HeNB)

The HeNB has plug-and-play capabilities, is connected to the core networkthrough a DSL line, and operates in the spectrum licensed for cellular systems

!

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1 Introduction





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Rel-12/13 Improvements

New capabilities introduced with Release 12 and Release 13 (LTE-A Pro)

Full-Dimension MIMO

Extended Carrier Aggregation

Offload to unlicensed bands

Device-to-Device communication

Enhancements for MTC

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Full-Dimension MIMO

Use of large antenna arrays arranged in 2-dimensional panels

Allows simultaneous transmission to many users in different locations

Narrow beams can focus the energy only where it is needed

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Extended Carrier Aggregation

Up to 32 Component Carriers can be aggregated

Aggregation of FDD and TDD carriers is possible

Dual connectivity: UEs can associate to (and receive data from) eNodeBs locatedin different sites

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Offload to unlicensed bands

LTE+WiFi Aggregation

LTE in unlicensed bands with LAA (License-Assisted Access)

Limited interference to other technologies (e.g. WiFi) is needed

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Device-to-Device communication

Direct communication without going through eNodeB

Discovery and resource allocation with/without eNodeB assistance

Enables proximity gain, hop gain and location-based services

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Enhancements for MTC

Machine-Type Communications will involve a huge number of devices

Low cost, long battery life and wide coverage are needed for cellular IoT

LTE-M and NB-LTE-M: small bandwidth, simpler RACH access and longer sleeptimes for control channels

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1 Introduction





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Scheduling principles

Multi-user scheduling

One of the main features in LTE

Distribution of available resources among active users to satisfy QoS needs

Data channel (PDSCH) shared among users

Portions of spectrum assigned every TTI among users

Packet scheduling

Scheduling (for both the downlink and the uplink) deployed at eNB

Granularity: one TTI and one RB

Working rationale

Resource allocation for N UEs is usually based on the comparison of per-RB metrics:the k-th RB is allocated to the j-th user if its metric mj,k is the biggest one:

mj,k = maxi∈N{mi,k} . (1)

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Scheduling principles

Allocation Decision

Every TTI, decision of the scheduler for the next TTI

Information sent to UEs in the Physical Downlink Control Channel (PDCCH)

Downlink Control Information (DCI) messages in the PDCCH payload

To inform UEs about

RBs allocated for data transmission on the PDSCH in the downlink direction

dedicated radio resources for their data transmission on the Physical UplinkShared Channel (PUSCH) in the uplink direction.

We focus on..

the downlink scheduling

But, most of the considerations hold also for the uplink

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Metrics for resource allocation

Based on the desired performance requirement, metric computation is usuallyevaluated starting from information related to each flow

Status of transmission queues

Useful for minimizing packet delivery delays

E.g., the longer the queue, the higher the metric

Channel Quality

CQI values

Allocate resources to users experiencing better channel conditions

E.g., the higher the expected throughput, the higher the metric

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Metrics for resource allocation

Resource Allocation History

Information about past achieved performance

Used to improve fairness among users

E.g., the lower the past achieved throughput, the higher the metric.

Buffer State

Receiver-side buffer conditions

To avoid buffer overflows

E.g., the higher the available space in the receiving buffer, the higher the metric

Quality of Service Requirements

QoS Class Identifier value associated to each flow

To meet QoS requirements

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Packet scheduler model

Every TTI:

1 each UE decodes referencesignals, computes CQI, sendsit back to eNB

2 eNB uses CQI for allocationdecisions and fills up a RB“allocation mask”.

3 AMC module selects the bestMCS to be used for datatransmission by scheduledusers.

4 Information about this users,allocated RBs, selected MCSare sent to UEs on thePDCCH.

5 Each UE, if scheduled,accesses the proper PDSCHpayload.

Higher Layers

Information

UE

CQI

computation

PHY Layer

Information

AMC

PDCCH

RB Allocation

Map

MCS

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Simplified Taxonomy of schedulers

Channel-unaware

Based on the assumption of time-invariant and error-free media

Basic schemes (introduced in wired networks)

Application in LTE jointly with other approaches

Channel-aware/QoS-unaware

Knowledge of channel conditions

CQI feedbacks

Estimation of channel quality perceived by users

Channel-aware/QoS-aware

As previous class, but adding QoS differentiation

Not necessarly QoS provision

Decisions considering requirements of flows

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Notation

Expression Meaningmi,k Generic metric of the i-th user on the k-th RBri(t) Data-rate achieved by the i-th user at time t

Ri(t) Past average throughput achieved by the i-th user until timet

Risch(t) Average throughput achieved by data flow of the i-th user

when scheduledDHOL,i Head of Line Delay, i.e., delay of the first packet to be trans-

mitted by the i-th userτi Delay Threshold for the i-th userδi Acceptable packet loss rate for the i-th userdi(t) Wideband Expected data-rate for the i-th user at time tdik(t) Expected data-rate for the i-th user at time t on he k-th RBΓik Spectral efficiency for the i-th user over the k-th RB

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First In First Out

Description

Simplest case of channel unaware allocation policy

Users served according to the order of requests, exactly like a First In FirstOut (FIFO) queue.

LTE Metric

For the i-th user on the k-th RB

mFIFOi,k = t− Ti (2)

where

t: current time

Ti: time instant when the request was issued by the i-th user

Notes

Pros: very simple

Cons: inefficient and unfair

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Round Robin

Description

Fair sharing of time resources among users

LTE Metric

For the i-th user on the k-th RBmRR

i,k = t− Ti (3)

where

t: current timeTi: last time when the i-th user was served

Notes

Pros: fairness in terms of amount of time assigned to each userCons: Not fair in terms of throughput (which in wireless systems depends also onexperienced channel conditions)Cons: Not efficient due to the assignment of the same amount of time to users withvery different bitrates at application layer

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Blind Equal Throughput

Description

Storing of the past average throughput achieved by each userResources to flows served with lower average throughput in the past

LTE Metric

For the i-th user on the k-th RB [Kela et al., 2008]

mBETi,k = 1/Ri(t− 1) (4)

Ri(t) = βRi(t− 1) + (1− β)ri(t) (5)

where

Ri(t): past average throughput achieved by the i-th user until time tri(t): data-rate achieved by the i-th user at time t0 ≤ β ≤ 1

Notes

Widely used in most of the state of the art schedulerPros: user experiencing the lowest throughput performs, in practice, resource preemptionCons: fair only in terms of throughput (no control on delays)

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Guaranteed Delay Policies: EDF

Description

Each packet has to be received within a deadline to avoid packet dropsDefined mostly for real-time operating systems and wired networks to avoid deadlineexpirationEarliest Deadline First schedules the packet with the closest deadline expirationLargest Weighted Delay First is based on system parameter δi, representing acceptableprobability for the i-th user that a packet is dropped due to deadline expiration

LTE Metrics

For the i-th user on the k-th RB [Liu and Lee, 2003]

mEDFi,k =

1

(τi −DHOL,i)(6)

mLWDFi,k = αi ·DHOL,i (7)

where

τi: Delay Threshold for the i-th userDHOL,i: Head of Line Delay, i.e., delay of the first packet to be transmitted by the i-thuser

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From CQI feedbacks to the maximum achievable throughput

General aspects

The scheduler can estimate the channel quality perceived by each UE

The maximum achievable throughput can be predicted

How to proceed

di(t): achievable throughput expected for the i-th user at the t-th TTI over allthe bandwidth

dik(t): achievable throughput expected for the i-th user at the t-th TTI over thek-th RB

Calculation by using Adaptive Modulation and Coding module

Estimation by considering the well-known Shannon expression for the channelcapacity

dik(t) = log[1 + SINRik(t)] (8)

This gives a numerical explanation of the relevance of channel-awareness in wirelesscontexts

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Maximum Throughput Scheduler

Description

Maximization of the overall throughputAssignment of each RB to the user that can achieve the maximum throughput in thecurrent TTI

LTE Metric

For the i-th user on the k-th RBmMT

i,k = dik(t) (9)

where dik(t): expected data-rate for the i-th user at time t on the k-th RB

Notes

Pros: Maximum Throughput (MT) is obviously able to maximize cell throughputCons: unfair resource sharing since users with poor channel conditions (e.g., cell-edgeusers) will only get a low percentage of the available resources (or in extreme case theymay suffer of starvation)A practical scheduler should be intermediate between MT, that maximizes the cellthroughput, and Blind Equal Throughput (BET), that guarantees fair throughputdistribution among users, to exploit fast variations in channel conditions as much aspossible while still satisfying some degrees of fairness.

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Proportional Fair Scheduler

Description

Tradeoff between requirements on fairness and spectral efficiency

Merging MT and BET metrics

Past average throughput as a weighting factor of the expected data rate, so thatusers in bad conditions will be surely served within a certain amount of time

LTE Metric


mPFi,k = mMT

i,k ·mBETi,k = dik(t)/Ri(t− 1) (10)

where

Ri(t): past average throughput achieved by the i-th user until time t

dik(t): expected data-rate for the i-th user at time t on the k-th RB

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Schedulers for Guaranteed Delay Requirements: M-LWDF

Description

Strategies to guarantee bounded delayThe Modified LWDF (M-LWDF) [Andrews et al., 2001] is a channel-aware extension ofLargest Weighted Delay First (LWDF)

LTE Metric


mM−LDWFi,k = αiDHOL,i ·mPF

i,k = αiDHOL,i ·dik(t)

Ri(t− 1)(11)

where

Ri(t): past average throughput achieved by the i-th user until time tdik(t): expected data-rate for the i-th user at time t on the k-th RBDHOL,i: the delay of the head of line packetαi: weighting parameter of LWDF

Notes

With respect to its channel unaware version, M-LWDF uses information about theaccumulated delay for shaping the behavior of Proportional Fair (PF)Good balance among spectral efficiency, fairness, and QoS provisioning

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Schedulers for Guaranteed Delay Requirements: EXP/PF

Description

Exponential rule for modifying PF [Basukala et al., 2009]

LTE Metric


mEXP/PFi,k = exp

(αiDHOL,i − χ

1 +√χ

)·

dik(t)

Ri(t− 1)(12)

with

χ =1

Nrt

Nrt∑i=1

αiDHOL,i (13)

where

Ri(t): past average throughput achieved by the i-th user until time tdik(t): expected data-rate for the i-th user at time t on the k-th RBDHOL,i: the delay of the head of line packetαi: weighting parameter of LWDFNrt: number of active downlink real-time flows

Notes

Also in this case, Proportional Fair handles non real-time flowsIn M-LWDF and Exponential/PF (EXP/PF) a strictly positive probability of discardingpackets is acceptable

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Schedulers for Guaranteed Delay Requirements: LOG and EXP rules

Description

Very promising strategies: LOG and EXP rules [Sadiq et al., 2009]EXP rule enhancement of aforementioned EXP/PF

LTE Metrics


mLOGrulei,k = bi log

(c+ aiDHOL,i

)· Γi

k (14)

mEXPrulei,k = bi exp

aiDHOL,i

c+√

(1/Nrt)∑

j DHOL,j

· Γik (15)

where

DHOL,i: delay of the head of line packetbi, c, and ai: tunable parametersΓik: spectral efficiency for the i-th user on the k-th sub-channelNrt: number of active downlink real-time flows

Notes

EXP rule more robust solution since the head of line delay is weighted exponentiallyEXP rule takes into account overall network status: delay of considered user is somehownormalized over the sum of experienced delays of all users.

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The two-level scheduler algorithm [Piro et al., 2011a]

Main details

Resource allocation procedure work on different time granularity at two levels

At the highest level, a discrete time linear control law applied every frame (i.e.,10 ms) to calculate the total amount of data that real-time flows should transmitin the following frame

At the lowest layer, RBs assigned to each flow every TTI.

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References I

Andrews, M., Kumaran, K., Ramanan, K., Stolyar, A., Whiting, P., andVijayakumar, R. (2001).Providing quality of service over a shared wireless link.IEEE Commun. Mag., 39(2):150 –154.

Basukala, R., Mohd Ramli, H., and Sandrasegaran, K. (2009).Performance analysis of EXP/PF and M-LWDF in downlink 3GPP LTE system.In Proc. of First Asian Himalayas International Conf. on Internet, AH-ICI, pages1 –5, Kathmundu, Nepal.

Capozzi, F., Piro, G., Grieco, L. A., Boggia, G., and Camarda, P. (2012).On accurate simulations of LTE femtocells using an open source simulator.EURASIP Journal on Wireless Communications and Networking, 2012(328).doi:10.1186/1687-1499-2012-328.

Capozzi, F., Piro, G., Grieco, L. A., Boggia, G., and Camarda, P. (2013).Downlink packet scheduling in LTE cellular networks: Key design issues and asurvey.IEEE Commun. Surveys and Tutorials, 15(2):678–700.doi:10.1109/SURV.2012.060912.00100.

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References II

Dahlman, E., Parkvall, S., Skold, J., and Beming, P. (2008).3G Evolution HSPA and LTE for Mobile Broadband.Academic Press.

Kela, P., Puttonen, J., Kolehmainen, N., Ristaniemi, T., Henttonen, T., andMoisio, M. (2008).Dynamic packet scheduling performance in UTRA Long Term Evolutiondownlink.In Proc. of International Symposium on Wireless Pervasive Comput.,, pages 308–313, Santorini, Greece.

Liu, D. and Lee, Y.-H. (2003).An efficient scheduling discipline for packet switching networks using EarliestDeadline First Round Robin.In Proc. of Interntional Conf. on Computer Commun. and Net., ICCCN, pages 5– 10, Dallas, USA.

Piro, G., Grieco, L., Boggia, G., Fortuna, R., and Camarda, P. (2011a).Two-level Downlink Scheduling for Real-Time Multimedia Services in LTENetworks.In IEEE Trans. Multimedia, to be published, volume 13, pages 1052 –1065.

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References III

Piro, G., Grieco, L. A., Boggia, G., Capozzi, F., and Camarda, P. (2011b).Simulating LTE cellular systems: an open source framework.IEEE Trans. Veh. Technol., 60(2):498–513.

Sadiq, B., Madan, R., and Sampath, A. (2009).Downlink scheduling for multiclass traffic in lte.EURASIP J. Wirel. Commun. Netw., 2009:9–9.

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