A downlink scheduler supporting real time services in LTE cellular networks (presentation)

IISA 2015
The 6th International Conference on Information, Intelligence, Systems and Applications
A downlink scheduler supporting
real time services in LTE cellular
networks
Emmanouil Skondras1, Angelos Michalas2,
Aggeliki Sgora1, Dimitrios D. Vergados1
1Department of Informatics, University of Piraeus, Piraeus, Greece
2Department of Informatics Engineering, Technological Education Institute of
Western Macedonia
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IISA 2015
Outline
• Introduction
• Resource Allocation Schemes & Algorithms
• The FLSA scheduler
• Simulation Results
– Simulation Setting
– Simulation Results
• Conclusions

IISA 2015
Introduction
• Proposition of a QoS aware scheduler meeting
the requirements of modern services in an LTE
networks.
• A three level scheduler - FLS-Advanced (FLSA).
• FLSA aims at QoS aware resource allocation.
– In order to satisfy the requirements of strict real
times services.
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IISA 2015
LTE frame
• The LTE frame length is 10 ms
– It contains 10 subframes with
length equal to 1 ms
– Each subfrace is consisted of 3
slots with 0.5 ms length
• 7 OFDM symbols per slot
• Bandwidth is distributed into
sub-channels with 180 Khz length
– 12 sub-carriers with length 15
KHz per sub-channel
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IISA 2015
Scheduling Strategies for LTE
• Several downlink packet schedulers have been
proposed in the current literature.
• They can be classified into two groups:
– Non-QoS aware
– QoS aware
• A non-QoS aware scheduler does not take into
account parameters that affect the service quality.
• A QoS aware distributes resources considering the
specific constraints of each service.
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IISA 2015
Non-QoS aware schedulers
• Maximum Throughput (MT)
• Proportional Fair (PF)
• Throughput to Average (TTA)
• Blind Equal Throughput (BET)
 di
k(t): Available throughput in the kth RB of the ithuser.
 𝑅i(t-1): Past average throughput.
 di(t): Available throughput in the ithuser. 6

IISA 2015
QoS aware schedulers
• Modified Largest Weighted Delay First (M-LWDF)
• Exponential/PF (EXP/PF)
 DHOL,i: Head of line delay.
 δi: Target packet loss ratio.
 τi: Delay constraint.
 Nrt: The number of active real time flows. 7

IISA 2015
• LOG RULE
• EXP RULE
 DHOL,i: Head of line delay.
 Nrt: The number of active real time flows.
 Γi
k: Spectral efficiency for the ith user on the kth subchannel.
 bi and c: Configurable parameters.
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IISA 2015
• Frame Level Scheduler (FLS)
– Two level QoS aware strategy.
– Upper level
• Estimates the ui(k) quota of
data that the ith real time
flow must transmit at the kth
frame to succeed its QoS
constraints.
• Coefficients are used in
order to guarantee the
required delay constraints
for real time flows.
 qi(k): Queue length in the kth frame.
 Mi: the number of coefficients used.
 ci(n): The nth coefficient value.
 τi: The target delay.
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IISA 2015
• Frame Level Scheduler (FLS)
– Lower level
• Uses the PF metric to allocate network resources to real
time flows for transmitting their quota of data.
• Whereas, the remaining resources are allocated to best
effort flows.
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IISA 2015
The Proposed Scheduler - FLSA
• It has been built on three distinct levels.
• The three levels cooperate each other.
– For dynamically assigning radio resources to users
in each TTI.
• Real time flows receive higher priority than
the best effort ones.
– Because of their strict service constraints that
must be fulfilled.
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IISA 2015
The Proposed Scheduler
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IISA 2015
The Upper Level of the Scheduler
• Uses the formula of FLS
– To estimate the quota ui(k) of data that the ith real
time flow should transmit in each kth TTI, to
succeed its QoS constraints.
• ui(k) quota is estimated in each kth TTI of a
frame.
– Whereas in FLS it is estimated once at the
beginning of each kth frame.
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IISA 2015
The Upper Level of the Scheduler
• Performance improvement has been observed.
– Due to the fact that:
• In FLS, when a real time flow transmits its ui(k) quota of
data, it loses the opportunity to continue the
transmission until the beginning of the next frame.
– By recalculating the formula in each TTI (instead of
estimating it only at the beginning of each frame):
• The FLSA provides more resources to real time flows that
have remaining data for transmission.
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IISA 2015
The Middle Level of the Scheduler
• The use of the QoS aware M-LWDF scheduler:
– Realizes improved resource distribution among the
real time flows.
• In comparison with the FLS scheduler which at the
second level uses the non-QoS aware PF algorithm.
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IISA 2015
The Lower Level of the Scheduler
• Allocates the remaining RBs of each TTI to both real
time and best effort flows using the M-LWDF algorithm.
• RBs are allocated to:
– Real time flows
• For transmitting their qi-ui(t) data.
• qi denotes the queue length for the flow i,
– Best effort flows
• Considering the fact they have no specific service constraints.
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IISA 2015
Performance Evaluation
• The performance of the FLSA was evaluated
against the schedulers:
– PF
– M-LWDF
– EXP/PF
– FLS
– EXP-RULE
– LOG-RULE
• Using the open source simulator LTE-Sim.
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IISA 2015
• The parameters considered in each scheduler:
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IISA 2015
• The simulation parameters:
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IISA 2015
• The simulated topology:
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IISA 2015
VoIP packet loss ratio using different
target delays (20 users/cell)
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• FLSA compared
to the rest
schemes has
the lowest PLR.

IISA 2015
Video packet loss ratio using different
target delays (20 users/cell)
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• FLSA achieves
the lowest
PLR.
• Compared to
FLS presents
up to 7%
lower values.

IISA 2015
VoIP packet loss ratio
25
• FLSA shows a
marginal
decrease of
PLR compared
to FLS.
• The rest
schemes
exhibit worse
PLR values.

IISA 2015
Video packet loss ratio
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• FLSA PLR is 10%
lower than that of
FLS.
• The rest schemes
exhibit worse PLR
values.

IISA 2015
VoIP throughput
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• FLSA succeeds up
to 330 kbps
higher throughput
than the rest of
the algorithms.

IISA 2015
Video throughput
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• FLSA succeeds up
to 4.7 Mbps higher
throughput than
the rest of the
algorithms.

IISA 2015
VoIP fairness index
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• The fairness for
all schemes is
close to 1.

IISA 2015
Video fairness index
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• FLSA improves the
fairness compared
to the rest of the
algorithms.

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Conclusions
• A three level scheduler - FLS-Advanced (FLSA) to
satisfy real time services.
• The performance of FLSA was tested against other
scheduling algorithms.
• The considered schedulers
– QoS-unaware: PF
– QoS-aware: M-LWDF, EXP/PF, LOG/EXP RULE, FLS
• QoS-aware schedulers support the QoS constraints for
real time flows in LTE.
• FLS-A scheduler achieves better performance in terms
of PLR, attainable throughput and fairness.

IISA 2015
Questions ?
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A downlink scheduler supporting real time services in LTE cellular networks (presentation)

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