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:

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 >.

http://standards.ieee.org/guides/bylaws/sect6-7.html#SC-FDMA%20in%20SISO%20(cont%E2%80%99d)

http://standards.ieee.org/guides/opman/sect6.html#6.3

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]


-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]


-30 -20 -10 0 10 20 30-110

-100

-90

-80

-70

-60

-50

-40

Frequency [MHz]

x

x(f)

[dB

m/H

z]


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]


-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]


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


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


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

Documents

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: