Transcript Document

Proportional Fair FrequencyDomain Packet Scheduling for
3GPP LTE Uplink
Suk-Bok Lee, Ioannis Pefkianakis, Adam Meyerson, Shugong Xu, Songwu Lu
IEEE INFOCOM 2009 proceedings.
Speaker：Tsung-Yin Lee
Outline

Introduction
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The Model
Problem Formulation
Heuristic Algorithm
Simulation
Conclusion
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OFDMA for LTE
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Orthogonal Frequency Division Multiple
Access (OFDMA) has been considered as a
strong candidate for the broadband air
interface
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robustness to multipath fading
higher spectral efficiency
bandwidth scalability
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Disadvantage of OFDMA
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one major disadvantage of OFDMA is that
the instantaneous transmitted RF power can
vary dramatically within a single OFDM
symbol
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high peak-to-average power ratio (PAPR)
(decrease battery life)
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Single-Carrier FDMA
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selected for LTE uplink multiple access
scheme
keeping most of the advantages of OFDMA
SC-FDMA has significantly lower PAPR
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benefits the mobile terminal in terms of
transmit power efficiency
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LTE Uplink Scheduler
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a scheduler needs to know the instantaneous
radio channel conditions across all users
and all resource blocks (RBs)
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LTE UL each user transmits a Sounding
Reference Signal (SRS) to the BS
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channel quality indicator (CQI)
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Proportional Fair (PF) algorithm
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PF algorithm as a basic scheduling principle
and apply the PF algorithm directly over
each RB one-by-one independently
SC-FDMA requires that all the RBs
allocated to a single user must be
contiguous in frequency within each time
slot [5][6]
[5] Moray Rumney. 3GPP LTE: Introducing SIngle-Carrier FDMA Agilent MeasurementJournal, 2008.
[6] 3GPP TSG-RAN WG2 Meeting #57, R2-070585, “Resource fragmentation in LTEuplink”, St. Louis,
USA, Feb, 2007.
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The Model
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The base station can allocate m RBs to a set
of n users
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At each time slot multiple RBs (with the
contiguity constraint) can be assigned to a
single user
indicate whether or not RB c is assigned to
user i at time slot t
denote the instantaneous channel rate for
user i on RB c at time t
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Problem Formulation (1/3)
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the well known PF algorithm aims to maximize
the logarithmic utility function
In order to maximize
, one should
maximize
where di(t) is total data
transmitted to user i at time t ( this paper change
di(t) to
) [7][10][14]
[7] M. Andrews. A survey of scheduling theory in wireless data networks. IMA, 2005.
[10] H. Kushner and P. Whiting. Asymptotic properties of proportional-fair sharing
algorithms. Allerton, 2002.
[14] D. Tse. Multiuser diversity in wireless networks.
http://www.eecs.berkeley.edu/ dtse/stanford416.ps , 2002.
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Problem Formulation (2/3)
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Let
be the PF metric
value that user i has on RB c at time slot t
We can establish PF objective function
when scheduling time slot t as follows:
(1)
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Problem Formulation (3/3)
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for LTE UL we need to incorporate the contiguous
RB constraint into this objective (1) due to the
physical layer requirement of SC-FDMA
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serve users with suboptimal PF metric value
for some RBs so as to optimize the PF objective (1)
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Hardness Result
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Theorem 1
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LTE UL PF-FDPS problem (i.e. maximizing
objective (1) with the contiguous RB constraint)
is NP-hard [11]
[11] S.-B. Lee, I. Pefkianakis, A. Meyerson, S. Xu, and S. Lu. Proportional Fair
Frequency-Domain Packet Scheduling for 3GPP LTE Uplink. UCLA TR-090001, 2009.
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Heuristic Algorithm

Paper’s heuristics do not give guaranteed
error bound, and moreover we believe that
no practical greedy algorithms can give an
approximation to this particular problem [11]
[11] S.-B. Lee, I. Pefkianakis, A. Meyerson, S. Xu, and S. Lu. Proportional Fair
Frequency-Domain Packet Scheduling for 3GPP LTE Uplink. UCLA TR-090001, 2009.
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carrier-by-carrier in turn (1/2)
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As a starter, our first greedy heuristic Alg1
schedules data from RB1 to RBm in
sequence, and for each RB c it assigns the
best user i who
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1) has the maximum PF metric value
2) satisfies the contiguity constraint.
on c
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carrier-by-carrier in turn (2/2)
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largest-metric-value-RB-first (1/2)
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Assign all the “in-between” RBs to a
candidate user
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it assigns RB5 to i, which as a result comes
with assignment of RB4 to i, since i is already
assigned RB3
RB3 Already Assigned
Assigned RB
Between RB3 and RB5
RB3
RB4
RB5 Assigned Now
RB5
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largest-metric-value-RB-first (2/2)
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riding peaks (1/3)
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Seeing the drawback of Alg2, we would like
to utilize each user’s high valued RBs as
much as possible
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Fundamental physical layer characteristic is
that in multi-carrier systems the channel SNR
values (i.e. CQI) are correlated in both time and
frequency
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riding peaks (2/3)
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if for each user i RB c has good channel rate, then
the neighboring RBs (c−1, c+1) have high channel
rate as well with high probability
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riding peaks (3/3)
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RB grouping (1/2)
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Alg3 relies on the strong frequency-domain correlation,
it is easily cheated by the small-scale variation
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RB grouping (2/2)
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This RB grouping might be helpful to catch
a bit large-scale fluctuation
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divide m RBs into n groups
apply the “peak riding” over those RB groups
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Simulation Parameter (1/2)
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use traces generated as specified in 3GPP
deployment evaluation [2]
[2] Technical specification
group radio access networks Deployment aspects. 3GPP
TR 25.943
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Simulation Parameter (2/2)
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paper use an algorithm that optimizes objective (1)
without the constraint as our reference, and we
refer to this algorithm as OPT∗ (upper bound of
the optimum)
Jain’s fairness index [9], measured by the data-rate
fairness criterion
[9] R. Jain, D. M. Chiu, and W. Hawe. A Quantitative Measure of Fairness and Discrimination for
Resource Allocation in Shared Systems. DEC Research Report TR-301.
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System throughput and fairness
with varying number of users
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Average number of users
scheduled per 1 TTI
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Conclusion

Due to its single carrier property of SC-FDMA,
LTE UL requires the RBs allocated to a single user
to be contiguous in frequency
 NP-hard nature of this problem
 Four Heuristic Algorithm to solve this problem
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Comment
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In LTE scheduling problem, we will handle
uplink and downlink from different scheme

We should combine uplink and downlink to
increase network performance
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