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On the Multiple Access Schemes for IEEE 802.16m: Comparison of SC-FDMA and OFDMA
Document Number: C802.16m-08/045
Date Submitted: Jan 16, 2008
Source:Yang-Seok Choi Intel corp E-mail: [email protected] Yang Intel corp E-mail: [email protected] Wang Intel corp E-mail: [email protected] Harel Intel corp E-mail: [email protected] Lomnitz Intel corp E-mail: [email protected] Yin Intel corp E-mail: [email protected]
Venue:TGm Call for contribution on SDD, Levi, Finland
Base Contribution:C80216m-08/045Purpose:
For discussion of comparison between OFDMA and SC-FDMA, and approval of OFDMA system by IEEE 802.16 Working GroupNotice:
This document does not represent the agreed views of the IEEE 802.16 Working Group or any of its subgroups. It represents only the views of the participants listed in the “Source(s)” field above. It is offered as a basis for discussion. It is not binding on the contributor(s), who reserve(s) the right to add, amend or withdraw material contained herein.
Release:The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that
this contribution may be made public by IEEE 802.16.
Patent Policy:The contributor is familiar with the IEEE-SA Patent Policy and Procedures:
<http://standards.ieee.org/guides/bylaws/sect6-7.html#6> and <http://standards.ieee.org/guides/opman/sect6.html#6.3>.Further information is located at <http://standards.ieee.org/board/pat/pat-material.html> and <http://standards.ieee.org/board/pat >.
2
SC-FDMA structure and Link level comparison
3
SC-FDMA TX Structure• Spreading by DFT
– Signal at each subcarrier is a linear combination of all M symbols
– ~2 dB gain in PAPR
DFT
W
Sub-carrier Mapping
CP insertion
Size-M
Size-N
Coded symbol rate= R
M symbols
IFFT
Spreading
Low PAPR
Low PAPRHigh
PAPR
Signal at each subcarrier is a linear combination of all M symbols
dWx Md
Duality
4
OFDMA in SISO• Received signal after FFT
• The channel matrix H is “orthogonal” even in frequency selective channel – No inter-carrier interference– No ISI due to CP
• One-tap linear equalizer is sufficient
noiseHdr
noise
d
d
d
d
h
h
h
h
r
r
r
r
M
M
M
M
M
M 1
2
1
1
2
1
1
2
1
00
000
000
00
Channel Matrix : H
5
SC-FDMA in SISO
• Received signal after FFT
• The channel matrix is NOT “orthogonal” in frequency selective channel– Inter-subcarrier interference due to the spreading matrix– No ISI due to CP
noise
d
d
d
d
ehehehh
ehehehh
ehehehh
hhhh
M
noise
d
d
d
d
eee
eee
eee
h
h
h
h
M
r
r
r
r
M
MMMMj
MMMj
MMMj
MM
MMMjM
MMjM
MMjMM
MMjMjMj
M
M
MMMjMMjMMj
MMMjMMjMMj
MMjMjMj
M
M
M
M
1
2
1
/)1)(1(2/)1(22/)1(2
/)2)(1(21
/)2(221
/)2(211
/)1(22
/222
/222
1111
1
2
1
/)1)(1(2/)1(22/)1(2
/)2)(1(2/)2(22/)2(2
/)1(2/22/2
1
2
1
1
2
1
1
1
1
1
1111
00
000
000
00
1
Spreading Matrix :
H~
Channel Matrix : ~
MHWH
MW
6
SC-FDMA in SISO (cont’d)
• MMSE Equalizer• One tap equalizer followed by De-spreading
• Equalizer output :
1111~~~
M
HHHMM
HHH
SNRSNRIHHHWIHHHG
Sub-carrier Demapping
Discard CP
Size-N
FFT MMSE Eq.
W
IDFT
W
Size-M
Despreading
rGd Hˆ
rd̂
7
SC-FDMA in SISO (cont’d)
• Post-MMSE SINR• OFDMA:
• SC-FDMA :
• From above
11
,
1
kkH
M
kSNR
SINRHHI
1
1
1~~1
,
1
,
1
kkMH
MH
M
kkH
M
k
SNR
SNRSINR
WHHIW
HHI
M
lOFDMAl
FDMASCm
SINRM
SINR
1 111
11
8
SC-FDMA in SISO (cont’d)• Note is a harmonic mean of
• Thus,
– where the equality holds if and only if is constant regardless of l (i.e. flat fading)
– is constant irrespective of m “Steeper PER curve”, “Diversity gain”– As delay spread increases, “PER curve moves to right”
• Longer delay spread at Cell edge
– As M increases, becomes smaller in frequency selective channel
– Note
OFDMAm
m
OFDMAl
M
l
FDMASCk
OFDMAm
mSINRSINRSINRSINR max
M
1min
FDMASCmSINR 1
OFDMAlSINR1
OFDMAlSINR1
FDMASCmSINRE
OFDMAm
FDMASCm SINRSINR min
OFDMAl
FDMASCk SINRESINRE
FDMASCmSINR
As delay spread and/or M increase, the loss of SC-FDMA in link level will be more evident
9
Link-Level Simulation Results• In frequency-selective fading, the loss due to loss of
orthogonality is noticeable– Delay spread=CP, rms delay spread=CP/4, exponential
decaying delay profile
4 6 8 10 12 14 16 18 20 22 2410
-2
10-1
100
av. SNR per subcarrier(dB)
PE
R16 QAM 1/2, Red: OFDMA, Blue:IFDMA, FFT size:1024, M=128
3 dB loss
SC-FDMA
OFDMA
10
MIMO
• Use of maximum likelihood detector (MLD) receiver– In large eigen-value spread channel, MMSE does not provide MIMO gain– fast MLD for 2x2 and 4x4 MIMO available– Virtual MIMO, MU-MIMO in UL
20 21 22 23 24 25 26 27 28 29 3010
-2
10-1
100
av. SNR (dB)
PE
R
16 QAM 1/2, Conv. code, 120 bytes, Ricean K=10, flat
MMSENortel MLD
Fast MLD I
QRD
full MLD(max-log)full MLD
Significant Gain
Other fast MLD
11
KxK MIMO (cont’d)• OFDMA : block diagonal matrix
• No inter-carrier interference —KxK MLD per subcarrier
• SC-FDMA: Not a block diagonal matrix • Inter-carrier interference
• Can’t apply per-subcarrier MLD — Need KMxKM MLD (not feasible)
M
M
H
H
H
H
H
00
000
000
00
1
2
1
MMMjM
MMjMM
MMjMj
ee
ee
M/)1)(1(2/)1(2
/)1(22
/222
111
1~
HHH
HHH
HHH
H
K x K matrix
12
Large PAPR in Frequency Domain
• After spreading :– x can be modeled as Gaussian random variable– means high PAPR in frequency domain
• SC-FDMA has larger PAPR in frequency domain– Out-of-band emission
Though Average ICI power and OOBE are the same as in OFDMA, Larger fluctuation of instantaneous OOB Emission causes worse interference to adjacent carrier
– In-band fluctuation Larger ICI power variance in time-varying channel
dWx M
13
ICI• ICI component at k-th subcarrier :
• ICI power :
• 4th order moment (assuming flat channel)
• Variance of ICI power
1
0
/)(2)(1 N
n
Nkmnjm
kmm
mk enHdN
)(1 2
2
2kmXdE
NE
kmm
mk
2
2121
22
,
424
4
4),()()(
1)(
221
1
1
21
2
2
kmkmYkmXkmXddEN
kmXdEN
E mm
kmm
mmkm
mkm
mmk
1
0
/))((21
0
210
1
21
2
)(2)( where
N
n
NkmnnjN
n
d eN
nnTfJkmX
1
0
/)(2/)(21
0
21021
1
2211
2
)(2),( where
N
n
NkmnjNkmnjN
n
d eN
nnTfJkmkmY
224
kk EEVar OFDMA
SC-FDMA
QPSK 1 2
16QAM 1.32 2
64QAM 1.381 2
4
mdE
1 of assumptionw/ 2 mdE
14
ICI (cont’d)
• Normalized variance of ICI power• SC-FDMA exhibits higher fluctuation of ICI power
22
224
k
kk
E
EENormVar
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
Normalized Doppler freq. (fdT)
Nor
mal
ized
Var
ianc
e
QPSK(OFDMA)
16QAM(OFDMA)
64QAM(OFDMA)
SC-FDMA
15
TX power improvement of SC-FDMA
16
Simulation assumptions
• QPSK modulation• WiMAX frequency assignment (BW=10 MHz, Nfft=1024,
Nused=841, SamplingFactor=28/25)• PA model: Rapp-3, saturation power 31dBm• Spectral mask FCC BRS (absolute) and ETSI Mobile
(relative)• WiMAX UL permutation:
• Distributed (WiMAX-I PUSC), 3 subchannels• Localized (WiMAX-I AMC)
• SC-FDMA modes• Distributed diversity mode • Localized (adopted in LTE)
– TX power shown is the maximum TX power that can be attained with the above PA parameters while obeying FCC masks
17
Transmit power and consumed power
• Higher TX power with same 1dB compression point, implies higher power consumption
• Thus, even if with same PA settings, a certain TX power improvement is shown, it is not always feasible due to power consumption
• A possible fair normalization is to keep the consumed power constant (by changing Vcc of the PA), and measure the improvement in TX power for the same consumed power – With Class-AB amplifier the consumed power is approximately
proportional to
– Therefore if results show N-dB improvement, there is also N/2-dB penalty in consumed power
– In order to normalize to the same consumed power while keeping a constant backoff, the improvement is halved (i.e. the improvement will be N/2-dB)
TXncompressiodBconsumed PPcP _1
18
TX power improvement
-30 -20 -10 0 10 20 30-110
-100
-90
-80
-70
-60
-50
-40
-30
Frequency [MHz]
x
x(f)
[dB
m/H
z]
Spectral density and masks. TX power = 23.30
-30 -20 -10 0 10 20 30-110
-100
-90
-80
-70
-60
-50
-40
-30
Frequency [MHz]
x
x(f)
[dB
m/H
z]
Spectral density and masks. TX power = 26.14
-30 -20 -10 0 10 20 30-110
-100
-90
-80
-70
-60
-50
-40
-30
Frequency [MHz]
x
x(f)
[dB
m/H
z]
Spectral density and masks. TX power = 25.15
-30 -20 -10 0 10 20 30-110
-100
-90
-80
-70
-60
-50
-40
Frequency [MHz]
x
x(f)
[dB
m/H
z]
Spectral density and masks. TX power = 25.69
PUSC (distributed OFDMA)
AMC (localized OFDMA)
DistributedSC-FDMA
LocalizedSC-FDMA
23.3 dBm
25.2 dBm
25.7 dBm
26.1 dBm
19
TX power improvement (contd.)
• No difference in maximum TX power if resource is allocated at band center
-30 -20 -10 0 10 20 30-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
Frequency [MHz]
x
x(f)
[dB
m/H
z]
Spectral density and masks. TX power = 30.57
-30 -20 -10 0 10 20 30-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
Frequency [MHz]
x
x(f)
[dB
m/H
z]
Spectral density and masks. TX power = 30.53
Localized SC-FDMA and OFDMACentered in band
30.53 dBm
30.57 dBm
Subcarrier Mapping Gain of SC-FDMA over OFDMA under same PA size
Gain of SC-FDMA over OFDMA under same power consumption and backoff
Distributed 2.4 dB 1.2 dB
Localized @Band edge 0.9 dB 0.45 dB
Localized @Band center 0.04 dB 0.02 dB
20
Modeling
• PAPR/CM not accurate metric– Indirect method– Need to consider OOBE, EVM requirement, power consumption,
and Multipath Effect together
• Rather,– Pass to RF filter (Tx Mask)– PA – Check OOBE and EVM requirement– Adjust Tx power– Channel – Check PER/Coverage
21
Block Diagram of Joint Simulation
Random source bit
CTC encoded
QAM modulation
32/64/128 DFT(SC-FDMA only)
Subcarrier mapping
OFDMA symbol generation
(IFFT, Add-CP)
16m EVM channel model
OFDMA demodulation (De-CP, FFT)
Sub-carrier demapping
MMSE/ML equalization
32/64/128 IDFT(SC-FDMA only)
Soft demodulation
CTC decoding
Rapp Power amplifier model
Low pass filter
Joint simulation (PA model+link level) : automatically include the non-linear distortion
22
Path loss
• Received signal power
1dB compression point backoff
Tx antenna gainRx antenna gainPath loss
• With 90% availability of shadow fading ( 8dB standard deviation)
• Path loss : Urban Macro
PLGGPAPP rxtxbackoffncompressiodBrx _1
ncompressiodBP _1
BackoffPA
txG
rxGPL
10_1 PLGGPAPP rxtxbackoffncompressiodBrx
)2/(log26)(log352.35 1010 fdPL
23
Simulation conditions
• 1dB compression: 31dBm (assume the same PA size)• Power amplifier backoff: depend on DFT size, subcarrier location, etc. • Tx antenna gain: 0dBi• Rx antenna gain (include cable loss): 15dBi• Carrier frequency: 2.5 GHz• System bandwidth: 10MHz• Noise figure: 5dB• FFT size: 1024• ½ CTC QPSK• Channel Model: Urban Macro-cell in 16m EVM document• 1x1 SISO/2x2 MIMO (SM, vertical coding)• Antenna spacing: Tx = 0.5 lambda, Rx = 4.0 lambda• Packet length = 120 bytes• SC-FDMA, localized, 32/64/128 DFT• OFDMA, localized, 32/64/128 used sub-carriers• Band edge and center• Rapp power amplifier model, p=2.0• 8 times over-sampling• 193-order low-pass filter, cut-off frequency = 0.9• Equalization: MMSE (MLD for 2x2 MIMO, OFDMA)
24
OFDMA vs. SC-FDMA: 1x1 SISO, band edge• Equalizer loss in SC-FDMA is more dominant than the effect of larger EVM noise and backoff in OFDMA • SC-FDMA has higher power consumption
200 400 600 800 1000 1200 1400 160010
-3
10-2
10-1
100
Distance
PE
R
OFDMA vs. SC-FDMA, 1x1 SISO, 1/2 CTC QPSK, Urban Macrocell, MMSE, band edge
OFDMA, 128, PA backoff = 5.72dBSC-FDMA, 128, PA backoff = 4.23dBOFDMA, 64, PA backoff = 4.90dBSC-FDMA, 64, PA backoff = 4.08dBOFDMA, 32, PA backoff = 2.56dBSC-FDMA, 32, PA backoff = 2.02dB
25
OFDMA vs. SC-FDMA: PER CDF @ SNR = 5dB
•
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10.75
0.8
0.85
0.9
0.95
1
PER
CD
F
PER CDF @ SNR = 5dB, 1x1 SISO, 1/2 CTC QPSK, Urban Macrocell, band edge
OFDMA, 256, PA backoff = 6.76dBSC-FDMA, 256, PA backoff = 5.36dBOFDMA, 128, PA backoff = 5.72dBSC-FDMA, 128, PA backoff = 4.23dBOFDMA, 64, PA backoff = 4.90dBSC-FDMA, 64, PA backoff = 4.08dB
26
OFDMA vs. SC-FDMA: 2x2 MIMO, band edge• MMSE : with larger M, SC-FDMA is better
– In correlated MIMO channel the Interstream interference effect is more dominant than the equalizer loss in SC-FDMA
– EVM noise effect is more dominant than noise (note operating SNR is higher at MMSE)– The crossing point moves to higher SNR
• MLD : the gain of MLD is noticeable
200 400 600 800 1000 1200 1400 160010
-4
10-3
10-2
10-1
100
Distance
PE
R
OFDMA vs. SC-FDMA, 2x2 MIMO, 1/2 CTC QPSK, Urban Macrocell, band edge
OFDMA, 128, MMSE, PA backoff = 5.72dBSC-FDMA, 128, MMSE, PA backoff = 4.23dBOFDMA, 128, ML, PA backoff = 5.72dBOFDMA, 64, MMSE, PA backoff = 4.90dBSC-FDMA, 64, MMSE, PA backoff = 4.08dBOFDMA, 64, ML, PA backoff = 4.90dBOFDMA, 32, MMSE, PA backoff = 2.56dBSC-FDMA, 32, MMSE, PA backoff = 2.02dBOFDMA, 32, ML, PA backoff = 2.56dB
In cell edge, STBC will be chosen highly likely. In this case, OFDMA will be better than SC-FDMA as in SISO case
27
OFDMA vs. SC-FDMA: 1x1 SISO, band center
•
400 600 800 1000 1200 1400 1600 180010
-3
10-2
10-1
100
Distance
PE
R
OFDMA vs. SC-FDMA, 1x1 SISO, 1/2 CTC QPSK, Urban Macrocell, MMSE, band center
OFDMA, 128, PA backoff = 0.42dBSC-FDMA, 128, PA backoff = 0.15dBOFDMA, 64, PA backoff = 0.42dBSC-FDMA, 64, PA backoff = 0.15dBOFDMA, 32, PA backoff = 0.41dBSC-FDMA, 32, PA backoff = 0.15dB
28
OFDMA vs. SC-FDMA: 2x2 MIMO, band center•
200 400 600 800 1000 1200 1400 1600 180010
-4
10-3
10-2
10-1
100
Distance
PE
R
OFDMA vs. SC-FDMA, 1x1 SISO, 1/2 CTC QPSK, Urban Macrocell, MMSE, band center
OFDMA, 128, MMSE, PA backoff = 0.42dBSC-FDMA, 128, MMSE, PA backoff = 0.15dBOFDMA, 128, ML, PA backoff = 0.42dBOFDMA, 64, MMSE, PA backoff = 0.42dBSC-FDMA, 64, MMSE, PA backoff = 0.15dBOFDMA, 64, ML, PA backoff = 0.42dBOFDMA, 32, MMSE, PA backoff = 0.41dBSC-FDMA, 32, MMSE, PA backoff = 0.15dBOFDMA, 32, ML, PA backoff = 0.41dB
29
Duality
OFDMA SC-FDMA
PAPR High in TimeLow in Frequency
Low in TimeHigh in Frequency
Spreading
Data spread in TimeData localized in Frequency
Data spread in FrequencyData localized in Time
30
Duality (cont’d) - PAPR
• PAPR in TD– High in OFDMA
• Smaller Tx power : Due to higher PAPR, more back-off needed. However, by scheduling the resource at the center of band, no difference compared with SC-FDMA is observed
• Higher EVM noise : Due to higher PAPR, more non-linear distortion observed. However, in cell edge the thermal noise is dominant. In addition, the operating SNR of OFDMA is lower due to no equalizer loss and advanced receiver such as MLD. Thus, the impact of EVM noise is negligible.
– Low in SC-FDMA• Larger Tx power
• PAPR in FD– Low in OFDMA
• Smaller fluctuation of OOBE• Smaller fluctuation of ICI power
– High in SC-FDMA• Higher instantaneous OOBE
– Larger variance of out-of-band power : More ACI to neighboring systems
• Higher instantaneous ICI power in time varying channel– Larger variance of ICI power
31
Duality (cont’d) - Spreading
• Spreading in TD– Data spread in TD in OFDMA
• More robust to impulse noise and nonlinear distortion
– Data localized in TD in SC-FDMA• More susceptible to impulse noise and nonlinear distortion
• Spreading in FD– Data localized in FD in OFDMA
• Less frequency diversity
– Data spread in FD in SC-FDMA• More frequency diversity
– Steeper PER curve
• Equalizer loss– PER curve moves to right
32
Pilot design
• OFDMA– Two dimensional pilot allocation : Time and Frequency– More flexible and potentially lower pilot overhead
• SCFDMA– Only Time domain :
• pilot subcarriers@ dedicated symbol • Does not allow mix of data and pilot subcarrier• Can’t optimize the pilot design
7 SC-FDMA symbols
12 s
ubca
rrie
rs
Resource block (12x7)
Pilots
33
TDD Duplex Scheme
• OFDMA can be straightforwardly used to exploit the TDD reciprocal DL/UL channel properties
• By applying the DL common pilots and UL dedicated pilots or sounding symbol
• Enable many channel-aware transmission techniques and allow the implementation based enhancement
• Such as beam-formed MIMO, SDMA
• SC-FDMA makes it difficult to explore the TDD application/advantage if not possible
34
Conclusions and Remarks
• SC-FDMA exhibits 0 to 1.2 dB gain in max TX power owing to smaller PAPR– However, by proper scheduling the resource, OFDMA shows no degradation– The typical Tone Reservation/clipping algorithms can achieve similar PAPR of
SC-FDMA
• SC-FDMA exhibits equalizer loss in frequency selective channel– Especially when the delay spread is large and/or the number of subcarriers is
large• Note that in cell edge the delay spread is larger
• No future proof for Higher order MIMO in SC-FDMA– Practical MLD for MIMO is not feasible even in 2x2 MIMO– Higher order MIMO is not possible– Practical MLD receiver in OFDMA significantly outperforms SC-FDMA receiver– The limitation of SC-FDMA to evolve for future UL MIMO Capability is clear
• In SC-FDMA asymmetric resource allocation in UL/DL– OFDMA can be used to exploit the TDD reciprocal DL/UL channel properties
• SC-FDMA should be ruled out for 16m Multiple access discussion– Adopt OFDMA system in both UL and DL