IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K....

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IEEE Symp./ IISc - 2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering Telecom and Computer Networks (TeNeT) Group IIT Madras, Chennai 600036 http://www.tenet.res.in tructional Workshop on Wireless Networks : Physical Layer Asp DRDO-IISc Program on Mathematical Engineering, Feb. 14, 2003

Transcript of IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K....

Page 1: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 1

OFDM Physical Layer -- Fundamentals, Standards, &

Advances

K. Giridhar

Associate Professor of Electrical EngineeringTelecom and Computer Networks (TeNeT) Group

IIT Madras, Chennai 600036http://www.tenet.res.in

Instructional Workshop on Wireless Networks : Physical Layer AspectsDRDO-IISc Program on Mathematical Engineering, Feb. 14, 2003

Page 2: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 2

Contents

Wireless Propagation -- Overview OFDM Fundamentals Comparing TDMA, CDMA, and OFDM OFDM Standards Case Study: IEEE 802.11a OFDM WLAN Key Advances in Wireless Technology Space-Time Processing for OFDM Summary

Page 3: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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Basics of Radio Propagation

Distance

Pow

er

10-100 m(1-10 secs)

0.1 -1 m(10-100 msecs)

Exponential

Long-term Fading

Short-term Fading

Page 4: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 4

Multi-path Propagation

r(t) = 0 s(t-0) + 1 s(t-1) + 2 s(t-2) + 3 s(t-3)

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Multi-path Propagation -- contd.

r(t) = 0 s(t-0) + 1 s(t-1) + 2 s(t-2) + 3 s(t-3)

channelInput (Tx signal)

Output(Rx signal)

ImpulseResponse h(t)

3 - 0

time

3

0

freq.

Frequency Response H(f)

Page 6: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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Frequency Selective Fading

Time2.0 secs 2.5 secs 3.0 secs

Delay Spread

rms =

5secs

Gai

n (in

vol

ts)

Fading

Frequency Selective Fading Channels can provide-- time diversity (can be exploited in DS-CDMA)-- frequency diversity (can be exploited in OFDM)

Page 7: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 7

Contents

Wireless Propagation -- Overview OFDM Fundamentals Comparing TDMA, CDMA, and OFDM OFDM Standards Case Study: IEEE 802.11a OFDM WLAN Key Advances in Wireless Technology Space-Time Processing for OFDM Summary

Page 8: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 8

TDMA, CDMA, and OFDM Wireless Systems

Time Division Multiple Access (TDMA) is the most prevalent wireless access system to date GSM, ANSI-136, EDGE, DECT, PHS, Tetra

Direct Sequence Code Division Multiple Access (DS-CDMA) became commercial only in the mid 90’s IS-95 (A,B, HDR,1x,3x,...), cdma-2000 (3GPP2), W-CDMA (3GPP)

Orthogonal Frequency Division Multiplexing (OFDM) is perhaps the least well known can be viewed as a spectrally efficient FDMA technique IEEE 802.11A, .11G, HiperLAN, IEEE 802.16 OFDM/OFDMA

options

Page 9: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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TDMA (with FDMA) Principle

Power

Time

Freq.

Time-slots

Carriers

Page 10: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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Direct Sequence CDMA Principle(with FDMA)

Power

Time

Freq.

User CodeWaveforms

Page 11: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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OFDM (with TDMA & FDMA) Principle

Power

Time

Freq.

Time-slots

Carriers

Tones

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Other Multiple Access Techniques

Multi-Carrier TDMA DECT, PACS

Frequency Hopped Spread Spectrum Bluetooth

CSMA/CA IEEE 802.11 (1 or 2 Mbps standard)

DS-CDMA with Time Slotting 3GPP W-CDMA TDD (Time Division Duplex)

Packet Switched Air Interface is vital for high bit-ratesand high capacity (for data users) -- GPRS, DPRS, etc.

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What is an OFDM System ?

Data is transmitted in parallel on multiple carriers that overlap in frequency

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IEEE Symp./ IISc -2001 IIT Madras 14

FEC IFFT

DAC

LinearPA

add cyclic extension

bits

fc

OFDM symbol

Pulse shaper &

view this as a time tofrequency mapper

Generic OFDM Transmitter

Complexity (cost) is transferred back from the digital to the analog domain!

Serial toParallel

Page 15: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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Add

Cyclic

Prefix

Serial/

Parallel

]0,[ns

]1,[ns

],[ Nns

Parallel/

SerialIFFT

]0,[nd

]1,[nd

],[ Nnd

OFDM Transmitter -- contd.

S/P acts as Time/Frequency mapper

IFFT generates the required Time domain waveform

Cyclic Prefix acts like guard interval and makes equalization easy (FFT-cyclic convolution vs channel-linear convolution)

1

0

2],[

1],[

N

k

N

kij

eknsN

ind

Page 16: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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OFDM Receiver

Cyclic Prefix is discarded

1

0

2],['

1],[

N

i

N

ikj

eindN

knr

FFT

]0,[nr

]1,[nr

],[ Nnr

Parallel/

Serial

Serial/

Parallel

Remove

Cyclic

Prefix

]0,[' nd

]1,[' nd

],[' Nnd

FFT generates the required Frequency Domain signal

P/S acts like a Frequency/Time Mapper

Page 17: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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AGC

fc

VCO

Sampler FFTError

gross offset

Slot &

fine offset

Freq. Offset

Estimation

TimingSync.

(of all tones sent in one OFDM symbol)

Generic OFDM Receiver

RecoveryP/S and

Detection

Page 18: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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OFDM Basics

To maintain orthogonalitywhere

= sub-carrier spacing = symbol duration

If N-point IDFT (or FFT) is used Total bandwidth (in Hz) =

= symbol duration after CP addition

fTs

1

fsT

fNW

CPS TT

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Condition for Orthogonality

Time

T

Base frequency = 1/T

T= symbol period

Page 20: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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OFDM Basics -- contd.

If the Cyclic Prefix > Max. Delay Spread, then the received signal after FFT, at the nth tone for the kth OFDM block can be expressed as

where is additive noise is channel frequency response

],[],[],[],[ knwknsknHknr

],[ knw],[ knH

Page 21: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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Tx Waveform over a OFDM Symbol(magnitude values, for 802.11a)

Page 22: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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Sync Basis Functions(of equal height for single-ray channel)

Shape gets upset by(a) Fine Frequency Offset(b) Fading

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OFDM -- PHY layer tasks

Signals sent thro’ wireless channels encounter one or more of the following distortions:

additive white noise frequency and phase offset timing offset, slip delay spread fading (with or without LoS component) co-channel interference non-linear distortion, impulse noise, etc

OFDM is well suited for high-bit rate applications

Page 24: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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Frequency Offset

Carrier recovery and tracking critical for OFDM Offsets can be comparable to sub-carrier spacing in OFDM Non-coherent detectors possible with differential coding

Residual freq. offset causes constellation rotation in TDMA loss of correlation strength over integration window in CDMA

(thereby admitting more CCI or noise) increased inter-channel interference (ICI) in OFDM

OFDM can easily compensate for gross freq. offsets (offsets which are an integral multiple of sub-carrier width)

Page 25: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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Timing Synchronisation

Timing recovery (at symbol level) is easily achieved in OFDM systems Can easily overcome distortions from delay spread

Can employ non-coherent timing recovery techniques by introducing self-similarity => very robust to uncompensated frequency offsets

If cyclic prefix is larger than the rms delay spread, range of (equally good) timing phases become available

=> robust to estimation errors

Page 26: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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Slot and Timing Synchronization in OFDM

Example: 4 tones per slot (OFDM symbol)T

self-symmetry can be exploited for non-coherent timing recovery

zero tones

IFFT PA

T secs

t

IFFT PA

T secs

t

T/2 T

Traffic Slot

Preamble/Control Slot

Page 27: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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Effect of Delay Spread

Typical rms delay spread in macro-cells Urban : 1-4 secs, Sub-urban : 3-6 secs Rural (plain, open country) : 3-10 secs Hilly terrain : 5-15 secs

TDMA requires equalization (even if rms delay spread is only 20-30% of symbol duration)

higher bit-rates would imply more Inter-Symbol Interference (ISI)

therefore, equalization complexity increases with bit rate

Page 28: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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Effect of Delay Spread -- contd. 1

Effect of delay spread on DS-CDMA is multi-fold On the Uplink, the time diversity inherent in the delay

spread can be used to mitigate fading On the Downlink, multipath delay spread upsets

channelization (short) code orthogonality

Sectorisation vital in CDMA to reduce CCI on the Uplink

However, sectorisation reduces delay spread as well, thereby reducing the RAKE performance

Page 29: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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Effect of Delay Spread in OFDM

Delay spread easily compensated in OFDM using : Cyclic Prefix (CP) which is longer than the delay spread Thereby, converting linear convolution (with multipath

channel) to effectively a circular convolution enables simple one-tap equalisation at the tone level

However, the frequency selectiveness could lead to certain tones having very poor SNR=> poor gross error rate performance

Data Payload CP

3.2secs 0.8secs

Example: IEEE 802.11 A (and also in HiperLAN)

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Delay Spread Compensation in OFDM

Two basic ideas to combat freq. selectivity in OFDM

Feed-forward only techniques Temporal FEC and interleaving Transmit diversity and space-time coding

Feed-back based techniques (similar to approaches used in Multi-Carrier Modulation in the ADSL modems)

Water-pouring (bit-loading) Pre-equalisation or pre-distortion

Sectorisation in macro-cell OFDM can help reduce delay spread

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AGC

Sampler DFTError

-- Gross Freq. Offset-- Channel Estimation and Equalization

OFDM Receiver Algorithms -- Recap

RecoveryP/S and

Detection

Freq.

-- Fine Freq. Offset-- Timing Estimation

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Conventional OFDM

Frequency Domain Equalisation-- Conventional OFDM

DFTFrequency

DomainEqualiser

RemoveCP

RxAlgos.

Detection& P/S

IDFTAddCP

TxMod.

SymbolMapping

& S/P

Page 33: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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Tx -- low-complexity, TDMARx -- implements SC-FDE; Linear Equaliser or DFE

Frequency Domain Equalisation-- Single Carrier FDE (SC-FDE)

DFTFrequency

DomainEqualiser

RemoveCP

RxAlgos.

DetectorIDFT

AddCP

(of symbols)

TxMod.

SymbolMapping

to permit FDE

Page 34: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 34

TDE + FDE for OFDM

Time & Frequency Domain Equalisation-- for OFDM in large delay spread channels

DFTFrequency

DomainEqualiser

RemoveCP

RxAlgos.

Detection& P/S

IDFTAddCP

TxMod.

SymbolMapping

& S/P

Time-Domain

Equaliser

Page 35: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 35

Fading and Antenna Diversity

Short-term fading exhibits spatial correlation Two antennas, spaced /4 meters or greater apart,

fade independently Spatial diversity combining can mitigate fading

Switch diversity (least complex, modest improvement) Selection diversity Equal gain combining Maximal ratio combining (most complex, optimal)

TDMA, CDMA, and OFDM systems will invariably require antenna diversity to overcome fading

Page 36: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 36

Fading and Channel Estimation

Use of midamble in GSM and EDGE to avoid channel tracking within the slot duration

Unlike in TDMA and OFDM, fading affects not only signal quality, but also system capacity in DS-CDMA

Fast closed-loop power control required which can track short-term fading

For RAKE combining, multipath delays and gains are required to be estimated and tracked

By using orthogonal signaling, IS-95 uplink does not need gain estimation, but requires delay estimation

In OFDM systems, the long symbol duration makes channel estimation and tracking very important

Page 37: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 37

Channel Estimation in OFDM -- Example

Traffic slots may contain a few equally spaced tones for phase correction (due to residual freq. offset, phase noise, fading)

Control slot may also contain MAC messages

Frame (say, 4 slots)

Control +Training Slot Traffic Slot 1 Traffic Slot 3Traffic Slot 2

PhaseCorrectionTones

TrainingTones (for channelidentification)

MAC message(broadcast)

Control +Training Slot

Page 38: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 38

Fading Compensation in OFDM

OFDM using a FDE, observes only “flat” fading at the sub-carrier level

Fading manifests as ICI terms in the Frequency Domain

In OFDM Phy Layer, two basic ways to reduce ICI Reduce OFDM symbol duration (increase sub-carrier width)

802.16 has FFT sizes ranging from 256 to 4096

Transmit pulse shaping can reduce ICI (by providing excess “time-width”)

Page 39: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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Other PHY Issues in OFDM

High peak-to-average ratio of the signal envelope Linear Power Amp., with 5-8dB back-off required

(costly)

To support mobility (fast fading) it will require More training tones per symbol and also in every slot Tx diversity and/or ST coding support Exploit time, frequency, and space diversity /

processing

Page 40: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 40

Phy Layer Issues in Macro-cell OFDM

Macrocells will require larger cyclic extensions / prefix Microcells may not be economical during initial deployment

GPS locked base stations required To control ACI from neighbor BS sites (at cell edge) CCI can be estimated / controlled only if it is tone-aligned

Strict power control required may be required on uplink To minimize cross-talk between tones of different users

sharing the same OFDM symbol (time slot) To avoid uplink power control

allocate only one user per uplink slot or, make uplink a pure TDMA (not OFDM)

Page 41: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 41

Phy Layer Issues in OFDMA

Strict power control required required on uplink (OFDMA)

To minimize cross-talk between tones of different users sharing the same OFDM symbol (time slot)

To avoid uplink power control allocate only one user per uplink slot (OFDM) or, make uplink a pure TDMA (single-carrier)

Page 42: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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MAC Layer Issues in Macro-Cell OFDM

Many proprietary broad-band FWA based on OFDM are configured as primarily data networks providing Bridging functionality (Ethernet packets on air) Routing functionality (IP packets on air)

Some of the key issues then are How many modes (scheduling options) should MAC

support? How is voice and other streaming data to be handled?

Indeed, mixing of voice and data not good for statistical multiplexing CDMA example – the new cdma2000 / HDR standard, where

distinct voice-only and data-only base stations are proposed

Page 43: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 43

Contents

Wireless Propagation -- Overview OFDM Fundamentals Comparing TDMA, CDMA, and OFDM OFDM Standards Case Study: IEEE 802.11a OFDM WLAN Key Advances in Wireless Technology Space-Time Processing for OFDM Summary

Page 44: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 44

DS-CDMA versus OFDM

channelInput (Tx signal)

Output(Rx signal)

ImpulseResponse h(t)

time

3

0

freq.

Frequency Response H(f)

DS-CDMA can exploit

time-diversity

OFDM can exploitfreq. diversity

Page 45: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 45

Comparing Complexity of TDMA, DS-CDMA, & OFDM Transceivers

Timing Sync.

Freq. Sync.

Timing Tracking

Freq. Tracking

ChannelEqualisation

Analog Front-end(AGC, PA, VCO, etc)

TDMA OFDM

Very elegant, requiringno extra overhead

CDMA

Easy, but requiresoverhead (sync.) bits

Difficult, and requiressync. channel (code)

Easy, but requiresoverhead (sync.) bits

More difficult than TDMAGross Sync. Easy

Fine Sync. is Difficult

Modest Complexity Usually not requiredwithin a burst/packet

Requires CPE Tones(additional overhead)

RAKE Combining in CDMA usually more complex than

equalisation in TDMA

Modest Complexity(using dedicated correlator)

Easy, decision-directedtechniques can be used

Frequency DomainEqualisation is very easy

Complexity or cost is very high (PA back-off

is necessary)

Very simple(especially for CPM signals)

Complexity is high inAsynchronous W-

CDMA

Modest to High Complexity(depending on bit-rate and

extent of delay-spread)

Fairly Complex(power control loop)

Page 46: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 46

Comparing Performance of TDMA, DS-CDMA, & OFDM Transceivers

Fade Margin(for mobile apps.)

Range

Re-use & Capacity

FEC Requirements

Variable Bit-rateSupport

Spectral Efficiency

TDMA OFDM

Required for mobileapplications

CDMA

Required for mobileapplications

Modest requirement(RAKE gain vs power-

control problems)

Range increase by reducing allowed noise rise (capacity)

Difficult to support large cells (PA , AGC limitations)

Modest (in TDMA) andHigh in MC-TDMA

Re-use planning iscrucial here

FEC is vital even forfixed wireless access

FEC is usually inherent (to increase code decorrelation)

FEC optional for voice

Powerful methodsto support VBR

(for fixed access)

Very High(& Higher Peak Bit-rates)

Modest

Modest

Low to modest support

Poor to Low

Very elegant methodsto support VBR & VAD

Very easy to increasecell sizes

Page 47: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 47

Contents

Wireless Propagation -- Overview OFDM Fundamentals Comparing TDMA, CDMA, and OFDM OFDM Standards Case Study: IEEE 802.11a OFDM WLAN Key Advances in Wireless Technology Space-Time Processing for OFDM Summary

Page 48: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 48

Proprietary OFDM Flavours

Wideband-OFDM(W-OFDM) of Wi-LAN

www.wi-lan.com

Flash OFDMfrom Flarion

www.flarion.com

Vector OFDM(V-OFDM) of Cisco, Iospan,etc.

www.iospan.com

Wireless Access (Macro-cellular)

-- 2.4 GHz band-- 30-45Mbps in 40MHz-- large tone-width (for mobility, overlay)

-- Freq. Hopping for CCI reduction, reuse-- 1.25 to 5.0MHz BW -- mobility support

-- MIMO Technology-- non-LoS coverage, mainly for fixed access-- upto 20 Mbps in MMDS

Wi-LAN leads the OFDM Forum -- many proposals submitted to IEEE 802.16 Wireless MANCisco leads the Broadand Wireless Internet Forum (BWIF)

Page 49: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 49

OFDM based Standards

Wireless LAN standards using OFDM are HiperLAN-2 in Europe IEEE 802.11a, .11g

OFDM based Broadband Access Standards are getting defined for MAN and WAN applications

802.16 Working Group of IEEE 802.16 -- single carrier, 10-66GHz band 802.16a, b -- 2-11GHz, MAN standard

Page 50: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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Key Parameters of 802.16a Wireless MAN

• Operates in 2-11 GHz• SC-mode, OFDM, OFDMA, and Mesh support• Bandwidth can be either 1.25/ 2.5/ 5/ 10/ 20 MHz• FFT size is 256 = (192 data carriers+ 8 pilots +56 Nulls) • RS+Convolutional coding

• Block Turbo coding (optional)• Convolutional Turbo coding(optional)

• QPSK, 16QAM, 64QAM• Two different preambles for UL and DL

Page 51: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

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Preamble structure for 802.16a Wireless MAN

TbTg

CP 128 128

Preamble structure of 802.16a Uplink

Two different preamble structures for DL and UL

TgTg Tb Tb

CP 64 64 64 64 CP 128 128

Preamble structure of 802.16a Downlink

Page 52: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 52

Calculations for 802.16a -- Example: 5MHz

Carrier frequency 2-11 GHz Channel Bandwidth 5 MHz Number of inputs to IFFT/FFT 256 Number of data subcarriers 192 Number of pilots 8 Subcarrier frequency spacing f 19.53125 KHz (5 MHz/256) Period of IFFT/FFT Tb 51.2 s (1 / f)Length of guard interval 12.8 s (Tb / 4)Length of the preamble for Downlink 128 s (640 sub-carriers)Length of the preamble for Uplink 76.8s (384/5 MHz)Guard interval for Uplink preamble 25.6 s (128/5 MHz)OFDM symbol duration 64 s (320/5 MHZ)

Page 53: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 53

Hiperaccess(PMP, 25Mbps, 40GHz)

orETSI’s FWA (2-11 GHz)

Broadband Wireless Standards

ETSI BRAN activity HiperLan > HiperLink > HiperAccess

HiperLan (1,2)(19 or 54Mbps, 5GHz)

Hiperlink(155Mbps, 17GHz

upto 150m)

2-5 miles, LoS(> 11GHz) or non-LoS (<11GHz)

Page 54: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 54

IEEE 802.16(10 to 66 GHz)

Broadband Access Standards -- contd.

IEEE LAN and MAN standards

IEEE 802.11a or.11b, or .11g

IEEE 802.16a,b(2 to 11 GHz)

2-5 miles, LoS(> 11GHz)

1-3 miles, non-LoS

Page 55: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 55

Contents

Wireless Propagation -- Overview OFDM Fundamentals Comparing TDMA, CDMA, and OFDM OFDM Standards Case Study: IEEE 802.11a OFDM WLAN Key Advances in Wireless Technology Space-Time Processing for OFDM Summary

Page 56: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 56

IEEE 802.11a Overview

Carrier frequency= 5 GHz Total allotted bandwidth= 20 MHz x 10 =

200MHz Size of the FFT= 64 Number of data subcarriers= 48 Number of Pilot subcarriers= 4 FFT period= 3.2 µs Channel bandwidth used= 64/3.2 µs => 20

MHz

Page 57: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 57

Rate Dependent Parameters

Coded bitsper

subcarrier(NBPSC)

Coded bitsper OFDM

symbol(NCBPS)

Data bitsper OFDM

symbol(NDBPS)

Data rate(Mbits/

s)

ModulationCoding rate

(R)

6

9

12

18

24

36

48

54

BPSK

BPSK

QPSK

QPSK

16 QAM

16 QAM

64 QAM

64 QAM

1/2

1/2

3/4

3/4

1/2

3/4

2/3

1

1

2

2

4

4

6

6

288

48

96

96

192

192

48

288

24

36

48

72

96

144

192

2163/4

Page 58: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 58

802.11A -- Frame and Slot

Structure Details of the preamble field

10 short symbols (0.8*10 = 8s) 2 long symbols (1.6 + 2*3.2 = 8s)

0 1 2 3 4 5 6 7 8 9 GI 2 T1 T2

Freq. Offset estimation and channel estimation

Signal detect, AGC, TimingRecovery, Freq. acquisition

Number of Sub-carriers = 64 (only 48+4=52 are non-zero)

P1 P2 MACHeader

Data Data ……. Data Preamble2

Data …

8 s 8 s 4 s 4 s

Page 59: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 59

PPDU Frame format

Page 60: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 60

Preamble Structure --

Implications

0 1 2 3 4 5 6 7 8 9

Only every 4th tone is non-zero. Thisimplies 10 replicas (in time) within 4+4 = 8secs

Even if delay spread in 0.2 secs (for a 100m cell), we can use 9 of 10 replicas to recover timing; use less than 9 for higher fade rates

Page 61: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 61

Auto-correlation and Piece-wise Cross-correlation for Slot Boundary Detection

79k

0k

* 159to0nfor16) k(nk)yy(n z(n) ||

Auto-correlation for timing and freq. estimation

Piece-wise Cross-correlation can also be used

Page 62: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 62

Timing Recovery in 802.11A --Simulation Results

N=0 represents start of 1st preamble; length of channel impulse response set to 8 samples (0.4secs)

Probability of the corresponding n being detected as the startof the frame at different SNRs

Value of index n inthe transmitteddata s(n) 5 db 10 db 15 db 20 db No noise

n<7 (outside theacceptable range)

0.062 0.008 0 0 0

N=7 0.032 0.009 0.002 0 0

N=8 0.057 0.048 0.022 0.013 0.013

N=9 0.096 0.091 0.081 0.080 0.083

N=10 0.144 0.195 0.226 0.236 0.231

N=11 0.204 0.276 0.322 0.327 0.313

N=12 0.148 0.216 0.205 0.208 0.228

N=13 0.118 0.109 0.113 0.106 0.103

N=14 0.070 0.036 0.027 0.027 0.026

N=15 0.033 0.036 0.002 0.003 0.003

N=16 0.019 0.008 0 0 0 n>16 (outside theacceptable range

0.017 0.003 0 0 0

Performance of timing recovery algorithm using 1st preamble

AcceptableRange

Page 63: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 63

Auto-correlation Result

autocorrelation result

0

0.2

0.4

0.6

0.8

1

1.2

1 11 21 31 41 51 61 71 81 91 101 111 121 131 141 151 161

Page 64: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 64

Piece-wise cross-correlation Result

Cross correlation Result

0

0.5

1

1.5

2

2.5

3

3.5

4

1

10 19 28 37 46 55 64 73 82 91

100

109

118

127

136

145

154

Page 65: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 65

• Quantity of interest is the Standard Deviation, f of the frequency estimate.

• It is given by: f = [E (( fest - fo )2 )] 1/2

Fine Frequency Offset Estimation

Approximate by using ensembleaveraging of many Monte-Carlo runs

Page 66: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 66

MMSE Technique

5 10 15 20 2510

-3

10-2

10-1

snr(db)

S.D

300 Hz

30 Hz

5 10 15 20 2510

-3

10-2

10-1

snr(db)

S.D

30 Hz

300 Hz

Self-Correlation

Comparison of the Two Fine Frequency Estimation Algorithms

Page 67: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 67

64-QAM Without Pilot De-rotation

64 QAM before pilot correction

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

-2 -1 0 1 2

Page 68: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 68

64-QAM After Pilot De-rotation

64 QAM after pilot rotation

-1.5

-1

-0.5

0

0.5

1

1.5

-1.5 -1 -0.5 0 0.5 1 1.5

Page 69: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 69

BER Curves for Different Channel Models

For AWGN Channel

AWGN case

-5

-4

-3

-2

-1

0

0 5 10 15

Eb/n0 in db

BE

Rin

db QPSK1/2

12Mbps16QAM 1/224Mbps

64QAM2/348MBPSBPSK1/26Mbps

Page 70: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 70

Contents

Wireless Propagation -- Overview OFDM Fundamentals Comparing TDMA, CDMA, and OFDM OFDM Standards Case Study: IEEE 802.11a OFDM WLAN Key Advances in Wireless Technology Space-Time Processing for OFDM Summary

Page 71: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 71

Motivation for Advances

Increase Erlang Capacity (Re-use Efficiency) – more users per square area

Increase Range and/or Reliability Increase Channel Capacity (Spectral

Efficiency) -- higher average bit rate or lower Tx power

Increase Coverage -- must for fixed wireless Support for asymmetric and bursty traffic

-- high peak to average bit rate traffic like Internet

Support for mobility, inter-operability etc.

Page 72: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 72

Wireless Advances -- contd.

Transmit Diversity

Smart Antennas

Sectorisation

CCI Suppression

Freq. Hopping

Multi-user Detection

Power Control

VAD, AMR, VBRReceive Diversity

Fixed Beamforming

Transmit Diversity

Spatial Multiplexing

Space-Time CodingLink

Adaptation

Re-use Re-use EfficiencyEfficiency

RangeRange

Spectral Spectral EfficiencyEfficiency

DCS

Turbo Coding OFDM

Page 73: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 73

ST Block Code Example

-d*(k+1), d(k)

d*(k), d(k+1)

TxRx

r(k+1), r(k)

Recall Example – Permutation Tx Diversity Scheme

Alamouti and other Tx diversity / coding schemes are suitable only for frequency-flat channels

OFDM converts frequency selective channel to parallel flat channels (one for every sub-carrier)

Page 74: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 74

Contents

Wireless Propagation -- Overview OFDM Fundamentals Comparing TDMA, CDMA, and OFDM OFDM Standards Case Study: IEEE 802.11a OFDM WLAN Key Advances in Wireless Technology Space-Time Processing for OFDM Summary

Page 75: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 75

MIMO OFDM

In addition to time and space, OFDM systems can exploit frequency diversity

If feedback channels are available, Space-Time-Frequency “water pouring” possible!

OFDM can convert delay-spread diversity into space diversity (diversity conversion!)

Page 76: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 76

Permutation Tx Diversity for OFDM

Courtesy:http://www.research.att.com/~justin/

Page 77: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 77

ST Coded Tx Diversity for OFDM

Courtesy:http://www.research.att.com/~justin/

Page 78: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 78

Contents

Wireless Propagation -- Overview OFDM Fundamentals Comparing TDMA, CDMA, and OFDM OFDM Standards Case Study: IEEE 802.11a OFDM WLAN Key Advances in Wireless Technology Space-Time Processing for OFDM Summary

Page 79: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 79

Why OFDM for Broadband Access?

Why not CDMA ? DS-CDMA cannot support high bit rates efficiently

Advantages of OFDM Fundamentally, well suited for high bit rate applications Simple frequency domain equalisation

lower complexity than RAKE or TDMA equalization Timing recovery is very straight forward Timing jitter easier to handle (due to long symbol duration) Good support for highly variable bit rate applications

Coarse granularity from time-slots(1 time-slot=1 OFDM symbol)

Fine granularity from tones (blocks) inside a time-slot

Page 80: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 80

Summary -- contd. 1

OFDM is emerging as popular solution for wireless LAN, and also for fixed broad-band access

The questions that remain to be answered are Will OFDM be good when there is vehicular mobility?

Pulse-shaping or large tone-widths reduce throughput

What about macro-cellular, non-LoS coverage issues?

What about OFDM deployment in unlicensed bands?

Will OFDM be cost-effective? If not right now, when? Analog (linear PA) with dynamic PAR control

Page 81: IEEE Symp./ IISc -2001 IIT Madras 1 OFDM Physical Layer -- Fundamentals, Standards, & Advances K. Giridhar Associate Professor of Electrical Engineering.

IEEE Symp./ IISc -2001 IIT Madras 81

Summary -- contd. 2 Space-Time processing for OFDM is a very hot area of

current research

The cost-effectiveness of many of these space-time techniques is not clear at present Multiple RF/IF chains versus faster base-band (MIPS) costs

Will 4G see a combination of OFDM, DS-CDMA & TDMA ?

Key Question is: Where are those high-bit rate, high usage applications ? -- at low cost ?

Thank You!