OFDM Based Air Interfaces for Future Mobile Satellite Systemsinfo.ee.surrey.ac.uk/CCSR/Internal/Presentations/Sundarampillai... · S-UMTS systems-based on terrestrial ... The Payload

Mobile CommunicationsResearchCCSR

OFDM Based Air Interfaces for Future OFDM Based Air Interfaces for Future Mobile Satellite SystemsMobile Satellite Systems

PhD Viva PresentationPhD Viva Presentation

SundarampillaiSundarampillai

JanaaJanaaththananththanan

Supervisors: Supervisors: Prof. Barry G. EvansProf. Barry G. Evans Dr. Christos Dr. Christos KasparisKasparis

Mobile CommunicationsResearch

Jan. 2009 2

Outline of PresentationOutline of Presentation

••

Introduction :Introduction :--

((Sat. Comm. , OFDM)Sat. Comm. , OFDM)

••

Contributions made in this thesis:Contributions made in this thesis:

––

New Tone Reservation (TR) based PAPR Reduction TechniqueNew Tone Reservation (TR) based PAPR Reduction Technique

––

Improved Digital PreImproved Digital Pre--distortion algorithm for Satellite Applicationsdistortion algorithm for Satellite Applications

––

First comparison between OFDM based and HSPA based air interfaceFirst comparison between OFDM based and HSPA based air interfaces s both in the Forward and Return satellite linksboth in the Forward and Return satellite links


Jan. 2009 3

Mobile SatelliteMobile Satellite

Communications & OFDMCommunications & OFDM

•

Traditional Satellite systems -

niche geographical areas and broadcasting applications ( but not successful in capturing the global market)

•

Must follow the terrestrial based standards to capture the markets•

Several European Union Projects (SATIN, MODIS, MAESTRO) -

Investigated S-UMTS systems-based on terrestrial WCDMA

air interface•

Intermediate Module Repeaters (IMR)-

to penetrate urban and indoor areas

•

Due to the capacity issue in WCDMA based air interface•

Terrestrial systems are towards OFDM based air interface –

better cell capacityEg. WiMax, 3GPP LTE etc.

•

Satellite systems have recently started considering OFDM (commercial reason)For example : S-DMB systems based on DVB-SH standard (OFDM based standard)

• We mainly investigate OFDM based air interfaces for bi-directional future satellite systems.

1. commercial reason –

to capture the market.2. larger delay spread of the integrated terrestrial /satellite channel.


Jan. 2009 4

OFDM OFDM (1/3)(1/3)

•

Multicarrier technology with orthogonally overlapping

sub-carriers

•

Cyclic Prefix is used for:-

cope with channel delay spread or ISI.-

Low complexity

Frequency Domain Equalization at Rx.

.,,,,, 1N210neXN1x N

nk2j1N

0kkn

ＴＴ n=0n=0n=1n=1n=2n=2n=3n=3n=4n=4n=5n=5n=6n=6

•

Resulting in high Peak-to-Average Power Ratio (PAPR) => increases with the number of sub-carriers .i.e. high fluctuation in the envelope of the OFDM signal.

Where N-

number of sub-carriersi –

sample indexE[ ]-

Expectation operator


Jan. 2009 5

•

The on-board Power amplifiers are inherently non-linear.

•

The Payload Characteristics data from the Alcatel Space ( obtained from IST FP 6 MAESTRO project: S-band TWTA)

AM-AM AM-PM

−20 −15 −10 −5 0 5 10−14

−12

−10

−8

−6

−4

−2

0

IBO (dB)

OB

O (

dB

)

−20 −15 −10 −5 0 5 100

5

10

15

20

25

30

35

40

45

50

IBO (dB)

Pha

se (d

egre

e)

Compression Region

Non-Linear Region

Saturation Region

Satellite Payload Satellite Payload (2/3)(2/3)


Jan. 2009 6

•

Satellite systems are Power Limited due to on-board nature-

HPA must be operated near saturation region to have higher power efficiency

High PAPR + inherently non-linear TWTA = Severe Non-linear Distortion

•

Effect of non-linear distortion1.

In band distortion –

Bit Error Rate (BER) degradation2.

Out-of-band distortion –

Adjacent Channel Interference (ACI)

• Compensation techniques –

must be transmitter based in order to compensate ACI1

back-off (at the expense of power efficiency)2

PAPR Reduction => Tone Reservation Technique 3

Power Amplifier Linearization =>Digital

Pre-distortion Technique

NonNon--linear Distortion linear Distortion (3/3)(3/3)


Jan. 2009 7

Tone Reservation Techniques Tone Reservation Techniques (1/8) (1/8)

In discrete time-domain:

In discrete freq.-domain:

Reserved sub-carriers are orthogonal to data carriers

x

BCi1Ni0BCBC

xnxPAPRC minargmaxminarg][minarg*


Jan. 2009 8

Existing Algorithms Existing Algorithms (2/8) (2/8)

1.

Fourier Projection Algorithm (FPA)• Based on the Projection onto Convex Sets (POCS) technique• Iteratively clips and filters the signal

2.

Controlled Clipper Algorithm• Based on a peak cancelling kernel signal• Amplitude scaled version of the kernel is subtracted from the peaks.

3.

Active Set algorithm <= applicable only in real domain • Most efficient existing algorithm• Minimizes the first peak to second peak and then minimizes both of them to

third peak, so on.• Uses the same kernel signal as in the Controlled Clipper Algorithm.

-

convergence is slow.-

take large number of iterations to obtain a sub-optimal solution.

-

convergence is faster -

complexity increases with the iteration

-

becomes very complex for complex baseband signalsTherefore , there is a need for an efficient optimization algorithm.


Jan. 2009 9

Proposed Algorithm Proposed Algorithm (3/8)(3/8)

• We propose a gradient decent algorithm due to its computational efficiency.

• Optimization function is not smooth –

approximated to obtain the gradient asymtotically

(p-norm approximation)

original cost function

• Asymptotic gradient of the cost function :

• Update rule with constant step size

b1lb

1b

p xxJ

v

*,

F

C

b1lb

1b

1ii xxv

*, FCC

ppJ xx

lim

In the Frequency Domain


Jan. 2009 10

Proposed Algorithm Proposed Algorithm (4/8)(4/8)

• In practice C-

must be limited due to the regulatory constraints -

Power Spectral Mask

• The gradient projection algorithm

is used ( Gradient + Projection onto constraint signal set)

• Projection step:

Power Spectral Density of OFDM based HyperLAN2


Jan. 2009 11

Positions of Reserved SubPositions of Reserved Sub--carriers carriers (Pilots)(Pilots)(5/8)(5/8)

Data sub-carriers

Reserved sub-carriers

Position 1-

Uniformly spaced locations

Position 2-

continuous locations (Edge of the frequency band)

Position 3-

Randomly spaced locations

frequency


Jan. 2009 12

Results: Unconstrained PerformanceResults: Unconstrained Performance (6/8)(6/8)

• Better Performance than Active set method

• PAPR reduction depends on pilot locations Random > Equal Spacing > continuous

Possibility of different peak cancelling signal with different locations.

unconstrained case TR compared with Active set algorithm

3 4 5 6 7 8 9 10 11 12 1310

-4

10-3

10-2

10-1

100

PAPRo [dB]

Pr(P

APR

>PA

PRo)

Original OFDMTR-Pos. 1TR-Pos. 2TR-Pos. 3active-set [BRIA04]


Jan. 2009 13

3 4 5 6 7 8 9 10 11 12 1310

-4

10-3

10-2

10-1

100

PAPRo [dB]

Pr(P

APR

>PA

PRo)

Original OFDMTR-Pos. 1TR-Pos. 2TR-Pos. 3Cosntrained TR-Pos. 1Constrained TR-Pos. 2Constrained TR-Pos. 3

Peak Reduction Tones are constrained to the spectral mask level

Results: PSDResults: PSD--constrained Performanceconstrained Performance(7/8)(7/8)

• Reduction performance is significantly affected in all cases.

• Reserved tones are frozen at constraint values, therefore, limit the performance.


Jan. 2009 14

SummarySummary--

Tone Reservation Tone Reservation (8/8)(8/8)

• Proposed algorithm

• Low complexity:

L-oversampling factor, N-

FFT size, M-

No. of reserved sub-carriersI-

iteration number in active set algorithm, G-

Polygon approx (≥4)

•Improved performance compared to the active set algorithm.

• Spectral constraints can be applied by simply limiting the reserved sub-carriers –

Frequency domain iterations.

Algorithms Real Add. Real Multip. Real Div.

Proposed 3NL+2M 4NL+8M -

Active set 2GLN(I-1)+8GLN 2GLN+2GLNI 4GLN


Jan. 2009 15

Digital PreDigital Pre--distortion Techniquedistortion Technique(1/6) (1/6)

• Most efficient Existing approaches are:1.

Look UP Table (LUT)

-

Popular Technique2.

Polynomial functions

GES

OFD

M

MOD

PD

Training Sequence

Noisy Feedback sequence

TWTA

• New problem in satellite :-

Remote Adaptation of Pre-distorters

• Assumptions: • The feedback signal from the satellite is

only degraded by AWGN noise.• Low SNR conditions are more realistic


Jan. 2009 16

LUT PreLUT Pre--distorter distorter (2/6)(2/6)

• The Secant algorithm is used for Training the LUT

• Transmit a ‘ramp-up’

sequence of L samples for training the LUT


Jan. 2009 17

Instability of the Secant algorithmInstability of the Secant algorithm(3/6)(3/6)

Adaptation becomes unstable over noisy channel (some LUT entries converge to infinity)

0 10 20 30 40 50 600

0.2

0.4

0.6

0.8

1

Table Location

Am

plitu

de o

f PD

coe

ffici

ents

SNR= 10dB

0 10 20 30 40 50 600

0.2

0.4

0.6

0.8

1SNR= 0 dB

Table Location

Am

plitu

de o

f PD

coe

ffici

ents


Jan. 2009 18

Modified Modified ––Secant AlgorithmSecant Algorithm(4/6)(4/6)

• Based on the recently proposed Modified Newton -Raphson Algorithm

• r=1 conventional Secant Algorithmr>1 uses a curvature instead of straight line.

r1

k

k1r

krk1k xf

xfrxxx

'

1kk

1kkk xx

xfxfxf

'where

Faster convergence than Secant but increased complexity with r.


Jan. 2009 19

Modified Modified ––Secant AlgorithmSecant Algorithm(5/6)(5/6)

0 2 4 6 8 10 12 14 1610

-6

10-5

10-4

10-3

10-2

10-1

100

Eb/N0 [dB]

BER

Mod. Secant SNR= 30, inf dBMod. Secant SNR= 10, 20dBSecant SNR= infdBSecant SNR= 25dBSecant SNR= 20dBSecant SNR= 10dBOFDM AWGN

In low SNR region, Modified-Secant is stable compared to conventional secant.

When SNR ≤

20dBModified Secant Algorithm

Else ( SNR > 20dB)Classical Secant Algorithm


Jan. 2009 20

Summary Summary ––

Digital PreDigital Pre--distortiondistortion(6/6)(6/6)

• LUT Pre-distorter

• Secant algorithm • Becomes unstable in lower SNR , but stable in higher SNR.

• Modified Secant algorithm• Becomes stable in lower SNR. (approx. <20dB)• No improvement at larger SNR.

• Proposal:

an adaptive approach can be used by adapting the r according to the SNR in the feedback channel.


Jan. 2009 21

Air Interface Comparisons Air Interface Comparisons (1/7)(1/7)

•Single user scenario in all cases•Comparison is performed based on 3GPP standards.•Performance comparison –

Block Error Rate Vs. Eb/No


Jan. 2009 22

Multipath Channel ProfilesMultipath Channel Profiles(2/7)(2/7)

10 20 30 40 50 60 700

0.5

1

No of Samples

Nor

. Pow

erchannel case 1

10 20 30 40 50 60 700

0.5

1channel case 3

No of Samples

Nor

. Pow

er

0 10 20 30 40 50 60 700

0.5

1channel case 5

No of Samples

Nor

. Pow

er Satellite with 3 terrestrial repeaters (urban environment)

Satellite only with Non Line of Sight (NLOS)

Satellite only with Line of Sight (LOS)

Multipath channel models from measurements in IST FP6 MAESTRO project.


Jan. 2009 23

OFDM Vs. HSDPAOFDM Vs. HSDPA--

Simulation Simulation Results Results (3/7)(3/7)

0 2 4 6 8 10 1210

-4

10-3

10-2

10-1

100

Eb/N0 [dB]

BLE

R

HSDPA CH 1HSDPA CH 3HSDPA CH 5OFDM CH 1OFDM CH 3OFDM CH 5OFDM CH 5 CP=68

• 6 Rake Finger receiver in HSPDA, ZF Equalization in OFDM• HSDPA performs better in “Sat. + IMR channel”

(case 5)

Performance of HSDPA and OFDM for different mobile channel profiles, assuming no amplifier distortion and perfect channel

estimation


Jan. 2009 24

Return LinkReturn Link--

SCSC--FDMA FDMA (4/7)(4/7)

Considered in LTE for Return link.

Multi user version of SC-FDE.

DFT spreading at the Tx-

Low PAPR compared to OFDMA.

Based on the sub-carrier mapping•Localized SC-FDMA

(considered)•Distributed SC-FDMA


Jan. 2009 25

SCSC--FDMA Vs. HSUPA FDMA Vs. HSUPA ––

Results Results (5/7)(5/7)

0 1 2 3 4 5 6 7 8 9 1010

-4

10-3

10-2

10-1

100

Eb/No [dB]

BLE

RSC-FDMA-Ch.1SC-FDMA-Ch.3SC-FDMA-Ch.5HSUPA-Ch.1HSUPA-Ch.3HSUPA-Ch.5

Performance of HSUPA and SC-FDMA for different mobile channel profiles, assuming no amplifier distortion and perfect channel estimation

• 6 Fingers Rake Receiver in HSUPA• MMSE Equalization in SC-FDMA


Jan. 2009 26

SCSC--FDMA + CAZACFDMA + CAZAC––

Results Results (6/7)(6/7)

4 5 6 7 8 9 10 11 12 13 1410

-4

10-3

10-2

10-1

100

Eb/N0 [dB]

BLE

R

SC-FDMA Ch.1 (amp)SC-FDMA Ch.3 (amp)SC-FDMA Ch.5 (amp)SC-FDMA Ch.1 (amp+CAZAC)SC-FDMA Ch.3 (amp+CAZAC)SC-FDMA Ch.5 (amp+CAZAC)

Performance of SC-FDMA for different mobile channel profiles with realistic channel estimation using CAZAC sequence

• Constant Amplitude Zero Auto-Correlation (CAZAC) sequence based channel Estimation provides improved performance.

NOTE: other results are based on the best random pilot sequence obtained via trial-and-error.


Jan. 2009 27

Summary Summary --Air Interface ComparisonsAir Interface Comparisons

(7/7)(7/7)

• Forward Link• HSDPA performs better in terms of BLER Vs. Eb/No performance

-

Rake receiver utilizes larger window to detect all the delayed signals.

• Return Link• HSUPA performs better in terms of BLER Vs. Eb/No performance• CAZAC sequence provides improved performance in the non-linear channel.

The results are not necessarily surprising.(multicarrier air interfaces do not necessarily perform best)

More work needs to be done to provide final conclusion. ( Throughput , Spectral Efficiency)


Jan. 2009 28

Journal Paper:

1.

Janaaththanan

S., Kasparis

C., Evans B.G.,“A Performance Comparison Study between HSPA based and OFDM based Air Interfaces for Future Mobile Satellite Communications”, to be submitted to Internal Journal on Satellite Communications and Networking. Feb. 2009.

Conference Papers:

2

Janaaththanan

S., Kasparis

C., Evans B.G., ”Performance Comparison between Adaptive LUT and Polynomial based

Pre-distorters in the Forward Link of Mobile Satellite Systems”, submitted to 27th

AIAA International Communications Satellite Systems Conference (ICSSC).

3

Janaaththanan

S., Kasparis

C., Evans B.G., ”Improved Adaptation Algorithm for LUT-based Pre-distorter with Noisy Training Phase in OFDM based Satellite System”, 26th

AIAA International Communications Satellite Systems Conference (ICSSC), San Diego, California, U.S.A, 11-14 June 2008.

4

Janaaththanan

S., Kasparis

C., Evans B.G., ”A Gradient Based Algorithm for PAPR Reduction of OFDM using Tone

Reservation Technique”, Vehicular Technology Conference, 2008. VTC Spring 2008. IEEE, pp.2977-2980, 11-14 May 2008.

5

Janaaththanan

S., Kasparis

C., Evans B.G., ”Comparison of SC-FDMA and HSUPA in the Return Link of Evolved S-

UMTS”, International Workshop on Satellite and Space Communications (IWSSC), pp.56-60, Austria, 13-14 Sept. 2007.

6

Janaaththanan

S., Kasparis

C., Evans B.G., ”Feasibility Study of Adaptive LUT-based Pre-distorter for OFDM in Non-linear Satellite Downlink Channel”, International Workshop on Satellite and Space Communications (IWSSC), pp.126-129, Madrid, Sept. 2006.

7

Janaaththanan

S., Kasparis

C., Evans B.G., ”A Comparison between OFDM and SC-FDE over Wideband Satellite Downlinks”, 24th

AIAA International Communications Satellite Systems Conference (ICSSC), San Diego, California, U.S.A, 11-14 June 2006.

List of PublicationsList of Publications


Jan. 2009 29

Thank You Thank You

Any Questions ??Any Questions ??

Documents

OFDM Based Air Interfaces for Future Mobile Satellite Systemsinfo.ee.surrey.ac.uk/CCSR/Internal/Presentations/Sundarampillai... · S-UMTS systems-based on terrestrial ... The Payload