Looking at 4G networks: solutions and perspective
Ing. Giuseppe Piro, PhDDEI, Politecnico di Bari, Italy
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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IntroductionOverview of LTE Networks
LTE-A ImprovementsRel-12/13 Improvements
Scheduling in LTE systems
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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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?
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Anytime, anywhere, always connected
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Emerging Challenges
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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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Scheduling in LTE systems
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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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
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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
2 Overview of LTE Networks
3 LTE-A Improvements
4 Rel-12/13 Improvements
5 Scheduling in LTE systems
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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
2 Overview of LTE Networks
3 LTE-A Improvements
4 Rel-12/13 Improvements
5 Scheduling in LTE systems
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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
2 Overview of LTE Networks
3 LTE-A Improvements
4 Rel-12/13 Improvements
5 Scheduling in LTE systems
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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
For the i-th user on the k-th RB
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
For the i-th user on the k-th RB
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
For the i-th user on the k-th RB
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
For the i-th user on the k-th RB
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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