An Introduction to Multi-band OFDM Physical Layer Proposal for IEEE 802.15 Task Group 3a

NTU Confidential

An Introduction to Multi-band OFDM An Introduction to Multi-band OFDM Physical Layer Proposal for Physical Layer Proposal for IEEE 802.15 Task Group 3aIEEE 802.15 Task Group 3a

Presenter: Keng-Hsien LinPresenter: Keng-Hsien Lin

Advisor: Prof. Tzi-Dar ChiuehAdvisor: Prof. Tzi-Dar Chiueh

Date: Feb 16, 2004 Date: Feb 16, 2004

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OutlineOutline

• MotivationMotivation• MBOA History OverviewMBOA History Overview• Review of Some OFDM PrinciplesReview of Some OFDM Principles• PHY Specification of Multi-band OFDM ProposalPHY Specification of Multi-band OFDM Proposal• Responses to IssuesResponses to Issues• Conclusions & Future Work & ReferencesConclusions & Future Work & References

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Outline OverviewOutline Overview

• MotivationMotivation– Why I choose this topic?

• MBOA History Overview• Review of Some OFDM Principles• PHY Specification of Multi-band OFDM Proposal• Responses to Issues• Conclusions & Future Work & References

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Why I Choose This Topic?Why I Choose This Topic?

• First, according to Professor’s email saying:“When I say MS1 student, I actually mean all MS1 students who are free, that is except Mr. Wu and Mr. Jhuang.” I think that’s a hint of the topic of today’s presentation that I should choose.

• Second, after I read some background

materials of this topic, I really found many attractive things….

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PAN/LAN Standards PAN/LAN Standards Application RangeApplication Range

[1]

Can exceed 100 Can exceed 100 Mbps!Mbps!

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Broadband

wired &wireless

wired &wireless

wired &wireless

Long range deliverywired & wireless

wired &wireless

wired &wireless

Local high throughput delivery

UWB Application Range UWB Application Range

[2]

All this existing All this existing technologiestechnologies can can be replace with UWB techbe replace with UWB tech

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IEEE 802.15.3 and 802.15.3aIEEE 802.15.3 and 802.15.3afor Wireless Personal Area Networkfor Wireless Personal Area Network

• 802.15.3– High data rate (up to 55 Mbps)– Low power – Low cost

• 802.15.3a– Provide a higher speed (up to 480 Mbps) PHY

enhancement amendment to 802.15.3 for applications which involve imaging and multimedia.

– Now there are two proposals for this standard• MB-OFDM MB-OFDM Today’s Topic Today’s Topic• DS-UWB [3]

Use Use 3.1~10.6 GHz3.1~10.6 GHz band (Ultra-Wide-Band) band (Ultra-Wide-Band)

to pursue to pursue higher data ratehigher data rate

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• Motivation

• MOBA History OverviewMOBA History Overview– Let us see MBOA’s brief history until last year

• Review of Some OFDM Principles• PHY Specification of Multi-band OFDM Proposal• Responses to Issues• Conclusions & Future Work & References

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Multi-Band OFDM Alliance’s Multi-Band OFDM Alliance’s (MBOA) Brief History(MBOA) Brief History

Nov 10Nov 10

-03/268r2 draft 03/268r2 draft spec proposedspec proposed

[4]

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[4]

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• Motivation• MBOA History Overview

• Review of Some OFDM PrinciplesReview of Some OFDM Principles– Why and how we develop OFDM?– Typical baseband model of an OFDM transceiver– OFDM drawbacks

• PHY Specification of Multi-band OFDM Proposal• Responses to Issues• Conclusions & Future Work & References

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Why We Develop OFDM?Why We Develop OFDM?

• In limited bandwidth and technology, we must use bandwidth more efficiently to pursue higher data rate– OFDM is an elaborate solution

• But overlapped bandwidth may cause severe inter-channel (carrier) interference (ICI)

[5]

(a) Traditional FDM(a) Traditional FDM

(b) OFDM(b) OFDM

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How We Develop OFDM? How We Develop OFDM? – Guard Interval and Cyclic Prefix– Guard Interval and Cyclic Prefix

[5]

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How Cyclic Prefix Works?How Cyclic Prefix Works?

IFFTIFFT ChannelChannel FFTFFT

WWHH: : IDFT matrix (NxN)IDFT matrix (NxN)

TT

WWHHWWCC

F’F’T’T’FF

C: C: Discrete channel impulse Discrete channel impulse response matrix (NxN)response matrix (NxN)

W: W: DFT matrix (NxN)DFT matrix (NxN)

∴ ∴ F‘ = W * T’F‘ = W * T’ = W * (C * T) = W * (C * (W= W * (C * T) = W * (C * (WH H * F)) = W * C * W* F)) = W * C * WH H * F* F

= W * (W= W * (WH H * D * W) * W* D * W) * WH H * F* F = D * F= D * F FFkk’ = D’ = Dii * F * Fkk Easier FDE is possible!Easier FDE is possible!

F: F: Freq Domain Tx Data (Nx1)Freq Domain Tx Data (Nx1)

T: T: Time Domain Tx Data (Nx1)Time Domain Tx Data (Nx1)

F’: F’: Freq Domain Rx Data (Nx1)Freq Domain Rx Data (Nx1)

T’: T’: Time Domain Rx Data (Nx1)Time Domain Rx Data (Nx1)

∵∵Cyclic prefixCyclic prefix C is equivalent to a C is equivalent to a circulant matrixcirculant matrix

C can be decomposed to C can be decomposed to WWHH*D*W*D*W, where , where WWHH*W = I (orthogonal)*W = I (orthogonal),,

D is diagonal matrixD is diagonal matrix with with diagonal value = DFT{1st row of C}diagonal value = DFT{1st row of C} (proof in [6]) (proof in [6])

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Typical Baseband Model Typical Baseband Model of an OFDM Transceiverof an OFDM Transceiver

[5]

Prevent burst errors!

Provide Sync or Channel information

As previous mentioned

functionFit channel

and can reduce DAC

clock rate

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

• Capacity and power loss due to guard interval– The guard interval consumes 20% of the bandwidth and transmit

power in IEEE802.11a

• High sensitivity to synchronization errors– Interference and loss of orthogonality

[7]

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• A large peak-to-average ratio (PAPR) problem

2

2

( )

( )

Peak Power Peak AmplitudePAPR

Average Power RMS Amplitude

21

0

, 0,1,..., 1nkN jN

n kn

T F e n N

This term may be large, especially when n=0

[7]

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• Solution to PAP Problem– Distortion technique as clipping / peak windowing

– Forward error correction coding as convolutional coding

– Scrambling reduce probability of large PAPR occurrence [7]

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• Motivation• MBOA History Overview• Review of Some OFDM Principles

• PHY Spec. of Multi-band OFDM ProposalPHY Spec. of Multi-band OFDM Proposal– Overview of Multi-band OFDM– Band plan– PLCP frame format– Architecture discussion – Tx and Rx– System Parameters

• Responses to Issues• Conclusions & Future Work & References

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Overview of Multi-band OFDMOverview of Multi-band OFDM

• Basic idea: divide spectrum into several 528 MHz bands.

• Information is transmitted using OFDM modulation on each band.– OFDM carriers are efficiently generated using an 128-point IFFT/FFT.– Internal precision is reduced by limiting the constellation size to QPSK.

• Information bits are “interleaved across all bands” to exploit frequency diversity and provide robustness against multi-path and interference.

• 60.6 ns (32*1/528MHz) prefix provides robustness against multi-path even in the worst channel environments.

• 9.5 ns (5*1/528MHz) guard interval provides sufficient time for switching between bands.

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Band Plan (1/2)Band Plan (1/2)

• Group the 528 MHz bands into 4 distinct groups.

• Group A: Intended for 1st generation devices (3.1 – 4.9 GHz) for Mode1, Mode2.

• Group B: Reserved for future use (4.9 – 6.0 GHz).• Group C: Intended for devices with improved “SOP” performance

(6.0 – 8.1 GHz) for Mode2.• Group D: Reserved for future use (8.1 – 10.6 GHz). [8]

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Band Plan (2/2)Band Plan (2/2)• The relationship between the center frequency fc and the band number nb is:

BAND_ID (nb) LowerFrequency

(fl)

CenterFrequency

(fc)

HigherFrequency

(fh)

BAND_ID (nb) LowerFrequency

(fl)

Center

Frequency(fc)

HigherFrequency

(fh)

1 3168 MHz 3432 MHz 3696 MHz 8 7128 MHz 7392 MHz 7656 MHz






7 6600 MHz 6864 MHz 7128 MHz

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Physical Layer Convergence Physical Layer Convergence Procedure (PLCP) Frame FormatProcedure (PLCP) Frame Format

[8]

[9]

(a) (a) Multi-band OFDMMulti-band OFDM

(b) (b) 802.11a packet format802.11a packet format

Compare later!Compare later!

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More Details on the PHY HeaderMore Details on the PHY Header

• PHY Header:

• Band Extension (3 bit field):– Indicates the mode of transmission for the payload (Mode 1 or Mode 2).

• Rate (4 bit field):– Indicates the rate of transmission for the payload. – Rate field also specifies the coding rate, puncturing pattern, and spreading

technique.

• Length (12 bit field):– Indicates the number of bytes in the payload (excludes the FEC).

• Scrambler (2 bit field):– Conveys information about the scrambler state.

RATE4 bits

LENGTH12 bits

Scrambler Init2 bits

Reserved1 bit

Band Extension3 bits

Reserved1 bit

Reserved1 bit

Reserved1 bit

Reserved3 bit

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Standard PLCP Standard PLCP Preamble Format (1/3)Preamble Format (1/3)

[10]

[9]

(a) (a) Multi-band OFDM (30 OFDM symbol time)Multi-band OFDM (30 OFDM symbol time)

(b) (b) 802.11a (4 symbol time)802.11a (4 symbol time)

9.375us

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• Multi-band OFDM preamble is composed of 3 sections:– Packet sync sequence: used for packet detection.– Frame sync sequence: used for boundary detection.– Channel estimation sequence: used for channel estimation

• Packet and frame sync sequences are constructed from the same hierarchical sequence.

• Correlators for hierarchical sequences can be implemented efficiently:

– Low gate count.– Extremely low power consumption.

• *Preamble sequences are designed to be extremely robust. * in this slides means the sentence was made by the proposer [8]

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• Basic idea: define 4 hierarchical preambles, with low cross-correlation values.

• Preambles are generated by spreading (multiply) a length 16 sequence by a length 8 sequence (total length 128 in freq. domain).

• *In the multiple overlapping piconet case, it is desirable to use different hierarchical preambles for each of the piconets

Preamble Pattern Sequence A

1 1 1 1 1 -1 -1 1 1 -1 -1 1 -1 1 -1 1 1

2 1 -1 -1 -1 -1 -1 1 -1 1 -1 -1 1 1 -1 -1 1

3 1 1 -1 -1 -1 1 -1 -1 -1 1 -1 -1 1 -1 1 1

4 1 -1 -1 1 -1 1 -1 -1 1 1 -1 -1 -1 -1 -1 1

Preamble Pattern Sequence B

1 1 -1 -1 -1 1 1 -1 1

2 1 -1 1 1 -1 -1 -1 1

3 1 1 -1 1 1 -1 -1 -1

4 1 1 1 -1 -1 1 -1 -1 [8]

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Proposed Tx Architecture (1/4)Proposed Tx Architecture (1/4)

• Block diagram of an example TX architecture:

• Architecture is similar to that of a conventional and proven OFDM system. Can leverage existing OFDM solutions for the development of the Multi-band OFDM physical layer.

[8]

In order to change In order to change coding ratecoding rate! !

Proposer initially called this Proposer initially called this “Time-Frequency interleaved”“Time-Frequency interleaved” Change Change

frequencyfrequency!!

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Tx – Scrambler (2/4)Tx – Scrambler (2/4)

(a) (a) Multi-band OFDMMulti-band OFDM

(b) (b) 802.11a802.11a

X15 X14 X13 X1X2

Data In

Data Out

[9]

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Tx – Convolutional Encoder (3/4)Tx – Convolutional Encoder (3/4)

• Assume a mother convolutional code of R = 1/3, K = 7 (compare with 802.11a, R=1/2, K=7.) Having a single mother code simplifies the implementation.

• Generator polynomial: g0 = [1338], g1 = [1458], g2 = [1758].

• Higher rate codes are achieved by puncturing the mother code.

D D D D D DI nputData

Output Data A

Output Data B

Output Data C

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Tx – OFDM Modulation (4/4)Tx – OFDM Modulation (4/4)Use 128-point IFFT/FFTUse 128-point IFFT/FFT

[10]

128 = 100 + 12 + 10 + 6128 = 100 + 12 + 10 + 6

Total Total subcarrierssubcarriers

Data Data subcarrierssubcarriers

Pilot Pilot subcarrierssubcarriers

Guard Guard subcarrierssubcarriers

Null Null subcarrierssubcarriers

High-freq High-freq subcarrierssubcarriers

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Proposed Rx ArchitectureProposed Rx Architecture

• Block diagram of an example RX architecture:

• Architecture is similar to that of a conventional and proven OFDM system. Can leverage existing OFDM solutions for the development of the Multi-band OFDM physical layer.

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System Parameters System Parameters • System parameters for mandatory and optional data rates:

– Clock rate: 132 MHz The proposal used this to estimate The proposal used this to estimate power consumptionpower consumption– Sample rate: 1/528MHz Ex: 110 Mbps = (100)*(2)*(11/32)*(1/2)*(1/312.5n) Ex: 110 Mbps = (100)*(2)*(11/32)*(1/2)*(1/312.5n)

Info. Data Rate 55 Mbps* 80 Mbps** 110 Mbps* 160 Mbps**

200 Mbps* 320 Mbps**

480 Mbps**

Modulation/Constellation OFDM/QPSK

OFDM/QPSK

OFDM/QPSK

OFDM/QPSK

OFDM/QPSK

OFDM/QPSK

OFDM/QPSK

FFT Size 128 128 128 128 128 128 128

Coding Rate (K=7) R = 11/32 R = 1/2 R = 11/32 R = 1/2 R = 5/8 R = 1/2 R = 3/4

Frequency-domain Spreading

Yes Yes No No No No No

Time-domain Spreading Yes Yes Yes Yes Yes No No

Data Tones 100 100 100 100 100 100 100

Zero-padded Prefixor Cyclic Prefix

60.6 ns 60.6 ns 60.6 ns 60.6 ns 60.6 ns 60.6 ns 60.6 ns

Guard Interval 9.5 ns 9.5 ns 9.5 ns 9.5 ns 9.5 ns 9.5 ns 9.5 ns

Symbol Length 312.5 ns 312.5 ns 312.5 ns 312.5 ns 312.5 ns 312.5 ns 312.5 ns

Channel Bit Rate 640 Mbps 640 Mbps 640 Mbps 640 Mbps 640 Mbps 640 Mbps 640 Mbps

Multi-path Tolerance 60.6 ns 60.6 ns 60.6 ns 60.6 ns 60.6 ns 60.6 ns 60.6 ns

* Mandatory information data rate, ** Optional information data rate [8]

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Zero-padded Prefix (1/2)Zero-padded Prefix (1/2)

• Ripple in the transmitted spectrum (Guess that ∵CP duration isn’t a multiple of OFDM duration (excluding CP duration)) can be eliminated by using a zero-padded prefix.

• *Using a zero-padded (ZP) prefix instead of a cyclic prefix is a well-known and well-analyzed technique.

• *Almost no ripple in PSD.

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Zero-padded Prefix (2/2)Zero-padded Prefix (2/2)• A Zero-padded Multi-band OFDM has the same multi-path robustness as a

system that uses a cyclic prefix (60.6 ns of protection).

• *The receiver architecture for a zero-padded multi-band OFDM system requires ONLY a minor modification (less than < 200 gates).

• Added flexibility to implementer: multi-path robustness can be dynamically controlled at the receiver, from 1.9 ns up to 60.6 ns.

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• Motivation• MBOA History Overview• Review of Some OFDM Principles• PHY Specification of Multi-band OFDM Proposal

• Responses to Issues Responses to Issues (Proposed “(Proposed “Self Evaluation MatrixSelf Evaluation Matrix”)”)

– General aspects– PHY protocol aspects

• Conclusions & Future Work & References

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General Aspects General Aspects (Proposed “General Solution Criteria”)(Proposed “General Solution Criteria”)

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Interference and SusceptibilityInterference and Susceptibility

• Minimum tolerable separation Minimum tolerable separation

when interfered by other systemwhen interfered by other system

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ScalabilityScalability

• Data rate scaling: – Data rates from 55 Mb/s to 480 Mb/s has been defined in the

current proposal.

• Frequency scaling:– Mode 1 (3-bands) and optional Mode 2 (7-band) devices.

– *Guaranteed interoperability between different mode devices.

• Power scaling:– Implementers could always trade-off power consumption for range

and information data rate.

• Complexity scaling:– Digital section will scale with future CMOS process improvements.

– Implementers could always trade-off complexity for performance.

[8]

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PHY Protocol AspectsPHY Protocol Aspects(Proposed “PHY Protocol Criteria”)(Proposed “PHY Protocol Criteria”)

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System PerformanceSystem Performance

• RangeRange vs vs link success probabilitylink success probability and and channel channel environmentenvironment is plotted below (at 110 Mbps of mode1) is plotted below (at 110 Mbps of mode1)

[10]

CM1~4: Channel model that was estimated

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SensitivitySensitivity

• Receiver performance requirementsReceiver performance requirements

(Target: PER < 8% with 1024 bytes payload )(Target: PER < 8% with 1024 bytes payload )

[10]

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• Motivation• MBOA History Overview• Review of Some OFDM Principles• PHY Specification of Multi-band OFDM Proposal• Responses to Issues

• Conclusions & Future Work & ReferencesConclusions & Future Work & References– Conclusions– Future work– References

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ConclusionsConclusions

• MB-OFDM is still under revision, and materials of today’s presentation is up to 2004/02/16.

• Let us wait and see what will happen in the future

• This proposal doesn’t explain many issues clearly, especially in Rx implemetation

• Nevertheless, some nice ideas in MB-OFDM such as time-frequency interleaved (time-frequency codes) is worthy of our further discussion or research

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Future WorkFuture Work

• I will often open IE to browse and see whether there are new things in MBOA and IEEE802.15.3a websites

• If needed, I will do further investigation and find out more materials of implementation issues to understand or even simulation

• Welcome to discuss with me and thanks for your attention

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References (1/2)References (1/2)• [1] Intel Labs, Jeff Foerster, “Ultra-wideband Technology for Short-

Range, High-Rate Wireless Communications”

• [2] Intel Corporation, Jeff Foerster, “Ultra-wideband Standards and

Technology Development,” Mar. 2003

• [3] Meng-Hau Wu, “Introduction to 802.15.3a and its two standard candidates,

MB-OFDM and DS-UWB,” Feb. 2004

• [4] MBOA, Evening Public Session, Nov. 2003

• [5] Prof. Tzi-Dar Chiueh, “OFDM and Its Application to High-Speed Wireless

LAN”

• [6] Robert M. Gray, Stanford University, “Toeplitz and Circulant Matrices: A

review”

• [7] MES, Darmstadt University of Technology, “OFDM Basics for Wireless

Communications”

• [8] MBOA, “Multi-band OFDM Physical Layer Proposal (267r6 ppt file),”

Sep. 2003

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References (2/2)References (2/2)

• [9] IEEE802.11a, “Wireless LAN MAC and PHY specifications,” Sep. 1999

• [10] MBOA, “Multi-band OFDM Physical Layer Proposal (268r2 pdf file),”

Nov. 2003

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Backup SlidesBackup Slides

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PiconetPiconet

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This allows a single transmit and receive analog chain at the receiver at all times,

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Derivation of SC-FD-LEDerivation of SC-FD-LE

2vAssume channel length , FFT block size N=5

0

1

2

3

4

021

102

210

210

210

3

4

0

1

2

3

4

210

210

210

210

210

0

1

2

3

4

00

00

00

00

00

0000

0000

0000

0000

0000

x

x

x

x

x

ppp

ppp

ppp

ppp

ppp

x

x

x

x

x

x

x

ppp

ppp

ppp

ppp

ppp

y

y

y

y

y

Py xCyclic prefix

, and channel doesn’t change during N samples.

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Strengths of OFDMStrengths of OFDM

• OFDM is spectrally efficient.– IFFT/FFT operation ensures that sub-carriers do not interfere with

each other.• OFDM has an inherent robustness against narrowband interference.

– Narrowband interference will affect at most a couple of tones.– Information from the affected tones can be erased and recovered

via the forward error correction (FEC) codes.• OFDM has excellent robustness in multi-path environments.

– Cyclic prefix preserves orthogonality between sub-carriers.– Cyclic prefix allows the receiver to capture multi-path energy more

efficiently.

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• Preamble structure for Mode 1 (3-band) device:

• Preamble needs to be designed to allow both Mode 1 (3-band) and Mode 2 (7-band) devices to operate in the same piconet. All devices in the same piconet must be able to detect the preamble and

demodulate PHY/MAC header. But the proposal doesn’t say how to But the proposal doesn’t say how to implementimplement

• End of synchronization pattern [p1,p2,p3] is used to indicate that the interleaving sequence remains constant throughout the packet.

Time

Band #1

Synchronizaton(24 symbols)

11.5625 usec (Preamble + Header)

End of SynchIndicator

Band #2

Band #3

Band #6

Band #7

Band #8

Band #9

ChannelEstimation

Header PayloadSynchronization

Channel Estimation

Header

Payload

p

p

p

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• Preamble structure for Mode 2 (7-band) device:

• Preamble/header are transmitted on bands 1–3 using length 6 interleaving sequences, so Mode 1 (3-band) devices can correctly decode the header.

• End of synchronization pattern [p4,p5,p6] is used to indicate the transition to length 7 interleaving sequence.

p

p

p

Time

Band #1

Synchronizaton(24 symbols)

14.0625 usec (Preamble + Header)

End of SynchIndicator

Band #2

Band #3

Band #6

Band #7

Band #8

Band #9

Channel EstimationLower 3 Bands

Synchronization

Channel Estimation

Header

Payload

Channel Estimation Upper 4 Bands+

Header on Lower 3 Bands Payload

[9]

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Power ConsumptionPower Consumption

[10]

An Introduction to Multi-band OFDM Physical Layer Proposal for IEEE 802.15 Task Group 3a

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Transcript of An Introduction to Multi-band OFDM Physical Layer Proposal for IEEE 802.15 Task Group 3a