NTU Confidential

Packet Format Report of OFDM-based Packet Format Report of OFDM-based Agile Baseband Transceiver for Agile Baseband Transceiver for

Future Spectrum Pooling Wireless SystemsFuture Spectrum Pooling Wireless Systems

Advisor : Tzi-Dar ChiuehStudent : Jui-Ping Lien Date : Sept 13th , 2004

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OutlineOutline

• Motivation• Spectrum pooling• OFDM-based agile baseband transceiver

– Specifications– Band allocation

• Packet Format Design– Short/Long preamble– Pilot/ data sub-carrier allocation

• Conclusion and Future Work• Reference

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MotivationMotivation

• The demand for more bandwidth is increasing but spectrum is scarce.

• Large chunks of frequency bands are used rarely, which is inefficient.

[1]

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Spectrum PoolingSpectrum Pooling

• Basic idea: merging spectral ranges from different spectrum owners into a common pool.

• The licensed system do not need to be changed.

[1]

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Introduction of OFDM-based Introduction of OFDM-based Agile Baseband TransceiverAgile Baseband Transceiver

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OverviewOverview

• Point-to-point (initially one way with return channel), fixed to slowly mobile terminals

• OFDM structure: frequency as basis

data

return channel

58155735

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Band allocationBand allocation

sub-band No. #1 #2 #3 #4

RULULU LULULU RURULU RU

• Divided into 4 sub-bands• Totally 15 combinations• The available bandwidth is variable.

LU/RU: licensed/rental user

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FeaturesFeatures

• Capable of sensing spectral occupancy.– sub-carrier scanning

• Capable of using the spectrum flexibly.– Select sub-carrier adaptively to avoid sub-carriers oc

cupied by active 802.11a systems– Reconfigurable number of FFT points, range of point

s– Adaptive and dynamic power allocation and/or varia

ble modulation/coding among sub-carriers – to achieve QoS as often as possible.

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System specificationsSystem specifications

FFT size 256

Bandwidth 100MHz (use 80MHz actually)

Sampling rate 80MHz (4 times faster than 802.11a)

Subcarrier spacing 312.5KHz (=80 MHz/256)

FFT period 3.2us (=1/312.5 KHz)

Guard interval duration 0.8us

OFDM symbol duration 4.0us (=3.2+0.8 us)

Sub-carrier in use variable

Modulation type BPSK, QPSK, 16QAM, 64QAM

Code rate 1/2, 2/3, 3/4

Operating frequency Upper UNII bands

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Packet Format ReportPacket Format Report

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Packet FormatPacket Format

• It is basically the same as 802.11a.

GI Data 1 GI Data 2GI SIGNALGI2 T1 T2t9t8t7t6t5t4t3t2t1

Signal Detect, AGC, Diversity Selection

Channel and Fine Freq. Offset Estimation

RATE LENGTH

SERVICE + DATA DATA

10*0.8=8 us 2*0.8+2*3.2 = 8.0 us 0.8+3.2 us = 4.0 us 0.8+3.2 us = 4.0 us 0.8+3.2 us = 4.0 us

8 + 8 = 16 us

Coarse Freq. Offset Estimation

Timing Synchronize

t0

Preamble 12 Symbols

SIGNAL One OFDM symbol

DATAVariable Number of OFDM Symbols

RATE4 bits

Reserved1 bit

LENGTH12 bits

Parity 1bit

Tail 6 bits

SERVICE16 bits

PSDUTail 6 bits

Pad Bits

PLCP Header

Coded /OFDM(BPSK, r=1/2)

Coded/OFDM(RATE is indicated in SIGNAL)

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Short Preamble (1/3)Short Preamble (1/3)

• Requirement– Not interfere to LU: can only use part or all the

available sub-bands.– Coarse symbol boundary/CFO : good auto-

correlation property– Low PAPR (peak to average power ratio)

• One possible approach– Modify 802.11a short preamble

LU RU

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Short Preamble (2/3)Short Preamble (2/3)

• Use the lowest available sub-band• Generation method

- 32 3126- 26

#1

- 128 127DC

- 128 127DC

Padding zero

26- 26

(a)

(b)

(c)

802.11a

padding zero

shift

padding zeros in freq. domain interpolation in time domain

X(f-Δf) x[n] ej2pi*Δf*nTs

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Short Preamble (3/3)Short Preamble (3/3)

• Length = 64 pt• PAPR is about 2.24 dB (2.08 dB in 802.11a)• Auto-correlation property

– Time resolution is the same as 802.11a

802.11a result

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Long Preamble (1/2)Long Preamble (1/2)

• Requirement– Fine symbol boundary/CFO: good auto-correlation p

roperty– Channel estimation: use all the available sub-bands.– Low PAPR

• One possible approach– Use M-sequence, r=8

primitive polynomial p(x)= =x8+x6+x5+x4+1

LU RU

[3]

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Long Preamble (2/2)Long Preamble (2/2)

• Length = 256 point• PAPR range from 4.55-6.8 dB (3.16dB in 802.11a system)• Good auto-correlation property

RU at sub-band #1

RU at sub-band #1-2

RU at sub-band #1-3

RU at sub-band #1-4

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Rate & Data sub-carriersRate & Data sub-carriers

• The number of data sub-carriers is variable, so the system can offer more different kinds of rate (6M-256.5Mbps).

available band No.

available band number

data sub-carrier number

#1 or #2 or #3 or #4 1 48

#13 or #24 or #14 2 96 (=48*2)

#12 or #34 2 108 (=48*2+12)

#23 2 108 (=48*2+11+1)

#134 or #124 3 156 (=48*3+12)

#123 or #234 3 168 (=48*3+12+11+1)

#1234 4 228 (=48*4+12*3)

Modulation Coding rate

BPSK 1/2

BPSK 3/4

QPSK 1/2

QPSK 3/4

16-QAM 1/2

16-QAM 3/4

64-QAM 2/3

64-QAM 3/4

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SIGNAL fieldSIGNAL field

• SIGNAL field: use the lowest available sub-band to transmit.• Separate RATE1-4: for variable modulation/coding among

sub-bands

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 230

RATE(4 bits)

RLENGTH(12 bits)

LSB MSBP

SIGNAL TAIL(6 bits)

Transmit Order

#1 #2 #3 #4sub- band No.

RATE1(4 bits)

RATE2(4 bits)

RATE3(4bits)

RATE4(4bits)

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Pilot allocation & Data InterleavPilot allocation & Data Interleaverer

• Pilot spacing: 16 pt (while 14 pt in 802.11a)• Data interleaving

• Service field (SERVICE), tail bit field (TAIL), Pad bits (PAD), DATA scrambler and descrambler, convolutional encoder are remained the same as 802.11a system.

1,...1,0)12/()12mod)(12/( CBPSCBPS NkkfloorkNi

1,...1,0mod)/12()/( CBPSCBPSCBPS NisNifloorNisifloorsj

)1,2/max( BPSCNs

NCBPS: coded bit in one OFDM symbolNBPSC: the number of coded bit per sub-carrier

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ConclusionConclusion

• OFDM-based agile baseband transceiver for future spectrum pooling wireless systems has been introduced.

• Packet format has been proposed.

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Future WorkFuture Work

• Sub-carrier scanning design• Survey about variable modulation/coding rate issue• Survey about reconfigurable FFT• Construct transmitter & receiver

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ReferenceReference

• [1] Spectrum pooling: an innovative strategy for the enhancement of spectrum efficiencyWeiss, T.A.; Jondral, F.K.;Communications Magazine, IEEE , Volume: 42 , Issue: 3 , Mar 2004 Pages:S8 – 14

• [2] IEEE Std 802.11a-1999• [3]

http://www.mathworks.com/access/helpdesk/help/toolbox/commblks/ref/pnsequencegenerator.html