Packet Format Report of OFDM-based Agile Baseband Transceiver for Future Spectrum Pooling Wireless Systems

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Packet Format Report of OFDM-based

Agile Baseband Transceiver for

Future Spectrum Pooling Wireless Systems

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

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Outline

• Motivation

• Spectrum pooling

• OFDM-based agile baseband transceiver

– Specifications – Band allocation

• Packet Format Design

– Short/Long preamble

– Pilot/ data sub-carrier allocation

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Motivation

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

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

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Spectrum Pooling

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

• The licensed system do not need to be chang

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Introduction of OFDM-based

Agile Baseband Transceiver

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Overview

• Point-to-point (initially one way with return channel), fixed to slowly mobile terminals • OFDM structure: frequency as basis

data return channel

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Band allocation

sub-band No. #1 #2 #3 #4 RU LU LU LU LU LULU RU RU RU

• Divided into 4 sub-bands • Totally 15 combinations

• The available bandwidth is variable.

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Features

• Capable of sensing spectral occupancy.

– sub-carrier scanning

• Capable of using the spectrum flexibly.

– Select sub-carrier adaptively to avoid sub-carriers occu pied by active 802.11a systems

– Reconfigurable number of FFT points, range of points – Adaptive and dynamic power allocation and/or variable

modulation/coding among sub-carriers – to achieve Qo S as often as possible.

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System 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

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Packet Format Report

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Packet Format

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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

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Short Preamble (2/3)

• Use the lowest available sub-band • Generation method

802.11a padding zero

shift

padding zeros in freq. domain  interpolation in time domain

X(f-Δf)

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Short Preamble (3/3)

• Length = 64 pt

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

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Long Preamble (1/2)

• Requirement

– Fine symbol boundary/CFO: good auto-correlation prop erty

– 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

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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

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Rate & 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) Modulati on 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 field

• SIGNAL field: use the lowest available sub-band to transmit.

• Separate RATE1-4: for variable

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Pilot allocation & Data Interleaver

• 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 / ( ) 12 mod )( 12 / (     N_CBPS k floor k k N_CBPS i  (12 / ) mod 0,1,... 1 ) / (         s floor i s i N_CBPS floor i N_CBPS s i N_CBPS j ) 1 , 2 / max(N_BPSC s 

N_CBPS: coded bit in one OFDM symbol

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Conclusion

• OFDM-based agile baseband transceiver for fu ture spectrum pooling wireless systems has b een introduced.

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Future Work

• Sub-carrier scanning design

• Survey about variable modulation/coding rate issue

• Survey about reconfigurable FFT • Construct transmitter & receiver

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Reference

• [1] Spectrum pooling: an innovative strategy for the enhancement of spectrum efficiency

Weiss, 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/r ef/pnsequencegenerator.html