ofdma 1
TRANSCRIPT
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QoS Aware Adaptive QoS Aware Adaptive Subcarrier Allocation Subcarrier Allocation in OFDMA Systemsin OFDMA Systems
Mustafa Ergen & Sinem Coleri
{ergen,csinem}@eecs.berkeley.edu
University of California Berkeley
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Introduction
Motivation Orthogonal Frequency Division Multiple Access(OFDMA) OFDMA System Resource Allocation Problem Algorithms
Optimal Suboptimal
Simulation Conclusion
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Motivation
Broadband Wireless AccessEx: IEEE 802.16, Wireless MAN
OFDMEliminates InterSymbol Interference
OFDMA
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OFDM Diagram
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Multiuser OFDM
OFDM-TDMA OFDM-FDMA OFDMA
User 1
User 2
User 3
Subcarrier
Time
OFDM-TDMA
Subcarrier
Time
OFDM-FDMA
Subcarrier
Time
OFDMA
…
…
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Resource Allocation
Goals: Dynamic subcarrier selection Improve system performance with adaptive
modulation More bits transmitted in large channel gain carriers
Provide QoS Rate and BER
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Resource Allocation
Assumptions: Base station knows
the channel Base station informs
the mobiles for allocation
BaseStation
subcarrier
user
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System
Application
Network
Resource Allocation
Physical Layer
rQoS=[rR,rBER] oQoS=[oR,oBER,oCoS]
oCoS=Ptotal for downlinkoCoS=Pu for uplink
[User x Subcarrier]
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OFDMA
AWGNw(n)
Adaptive Modulation
IDFTGuard
InsertionP/S
Channel
+S/PGuard
RemovalDFT
Adaptive Demodulation
X(k)
Y(k)
x(n)
y(n)
h(n)
xf(n)
yf(n)
PathLoss
Channel Informationfrom
user kResouce Allocation Module
Subcarrierallocation
withDifferent
Modulation
SubcarrierExtraction
foruser
k
Adaptive Modulation
Adaptive Modulation
User 1 (Rate R1, BER1)
User 2 (Rate R2, BER2)
User K (Rate RK, BERK)
Maximum TotalPower
.
.
.
.
.
.
.
.
.
.
.
User k Adaptive Demodulation
Adaptive Demodulation
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Resource Allocation
64-QAM
16-QAM
4-QAM
Channel
Subcarrier
Use
r
RATE: [12 6 6 8 ] BER: [1e-2 1e-2 1e-4 1e-4]
QoS
Resource Allocation
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Notation
)12()4
(3
)(:
2,
:
},...,1,0{,
:
},...,1{:
},...,1{:
)(:
2
1
2,
,,
c
nk
nkkcnk
BERQ
NocfQAMM
nkgainchannel
Mnk
cbitassigned
Nnsubcarrier
Kkuser
cfPPowerTransmit
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Optimal Integer Programming
K
k
M
ccnk
cnk
N
n
M
cnkk
cnkcnk
K
k
N
n
M
c nk
nk
nallforand
kallforcRtosubject
forcf
cnk
1 1,,
,,1 1
,
,,,,1 1 1
2,
,
.,10
,.
}1,0{)(
min,,
Subcarrier
Use
r
Subcarrier
Use
r
Subcarrier
Use
r
Subcarrier
Use
r
Pc2
Pc3
Pc1
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Motivation for Sub-optimal Algorithms
IP is complex Allocation should be done within the
coherence time Time increases exponentially with the
number of constraints
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Current Suboptimal Algorithms
2-step: Subcarrier Allocation
Assume the data rate for all subcarriersAssume modulation rate is fixedAssign the subcarriers
Bit LoadingGreedy approach to assign the bits of user
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Current Suboptimal Algorithms Subcarrier Allocation
Hungarian algorithm Optimal, very complex
LP approximation to IP problem
Close to optimal
Bit Loading
Subcarrier
Use
r
Subcarrier
Use
r
Subcarrier
Use
r).(
1
)(minarg
:,
,,
,,
,,
nknk
nknk
nknkSn
k
cPevaluate
cc
cPn
timesRfollowingtherepeatkeachFor
k
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Problems in Current Suboptimal Algorithms
Subcarrier assignment and bit loading are separatedUsers with bad channels may need higher
number of subcarriers Not iterative subcarrier assignment
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Iterative Algorithm
Iterative algorithm based on Assignment of bits according to highest
modulation Finding the best places
Distributing the assigned bits to other subcarriers or to non-assigned subcarriers
Exchanging the subcarriers among user pairs for power reduction.
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Iterative Algorithm
Fair Selection(FS) Greedy Release(GR) Horizontal Swaping(HS) Vertical Swaping(VS)
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Iterative Algorithm
FAIR SELECTIONPtotal<Pmax
GREEDYRELEASE
Start
Modulation--
HO
RIZ
ON
TA
L S
WA
P
VE
RT
ICA
L S
WA
P
ASSIGNMENT ITERATION
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Simulation Environment
Build the OFDMA system Modulations:4-QAM,16-QAM,64-QAM Independent Rayleigh fading channel
to each user Number of subcarriers =128 Nodes are perfectly synchronized
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CDF of total transmit power without Pmax constraint
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CDF of total transmit power with Pmax constraint
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Average bit SNR vs. RMS delay spread
As RMS delay spread increases, the fading variation increases hence higher gains are obtained by adaptive allocation
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Average bit SNR vs. number of users
As the number of users increases, the probability of obtaininggood channel at a subcarrier increases
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Instantaneous Average bit SNR vs Time
Iterative Algorithm improves its Average Bit SNR by the time.
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Conclusion
OFDMA Broadband Wireless Access
Resource Allocation Channel Information QoS Requirement
Optimal Algorithms complex
Iterative Algorithms