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OFDM. Lecture 9. Basics of Radio Propagation. Exponential. Power. 0.1 -1 m (10-100 msecs). Short-term Fading. Long-term Fading. 10-100 m (1-10 secs). Distance. Gain (in volts). Fading. Delay Spread rms = 5 m secs. Time. 3.0 secs. 2.0 secs. 2.5 secs. Frequency Selective Fading.

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Presentation Transcript
slide1

OFDM

Lecture 9

basics of radio propagation
Basics of Radio Propagation

Exponential

Power

0.1 -1 m

(10-100 msecs)

Short-term Fading

Long-term Fading

10-100 m

(1-10 secs)

Distance

frequency selective fading
Gain (in volts)

Fading

Delay Spread

rms = 5msecs

Time

3.0 secs

2.0 secs

2.5 secs

Frequency Selective Fading

Frequency Selective Fading Channels can provide

-- time diversity (can be exploited in DS-CDMA)

-- frequency diversity (can be exploited in OFDM)

tdma cdma and ofdm wireless systems
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
tdma with fdma principle
TDMA (with FDMA) Principle

Carriers

Power

Freq.

Time

Time-slots

direct sequence cdma principle with fdma
Direct Sequence CDMA Principle (with FDMA)

User Code

Waveforms

Power

Freq.

Time

ofdm with tdma fdma principle
OFDM (with TDMA & FDMA) Principle

Tones

Carriers

Power

Freq.

Time-slots

Time

what is an ofdm system
What is an OFDM System ?
  • Data is transmitted in parallel on multiple carriers that overlap in frequency

IIT Madras

slide9
Generic OFDM Transmitter

OFDM symbol

bits

Serial to

Parallel

Pulse shaper

FEC

LinearPA

IFFT

&

DAC

fc

add cyclic extension

view this as a time to

frequency mapper

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

IIT Madras

ofdm transmitter contd
Add

Cyclic

Prefix

Parallel/

Serial

Serial/

Parallel

IFFT

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)

IIT Madras

ofdm receiver
Remove

Cyclic

Prefix

Parallel/

Serial

Serial/

Parallel

FFT

OFDM Receiver
  • Cyclic Prefix is discarded
  • FFT generates the required Frequency Domain signal
  • P/S acts like a Frequency/Time Mapper

IIT Madras

slide15
Generic OFDM Receiver

Slot &

Timing

AGC

Sync.

Error

P/S and

Detection

Sampler

FFT

Recovery

fc

gross offset

VCO

Freq. Offset

Estimation

fine offset

(of all tones sent in one OFDM symbol)

IIT Madras

ofdm basics
OFDM Basics
  • To maintain orthogonality where
      • = sub-carrier spacing
      • = symbol duration
  • If N-point IDFT (or FFT) is used
    • Total bandwidth (in Hz) =
    • = symbol duration after CP addition

IIT Madras

condition for orthogonality
Time

T

Condition for Orthogonality

Base frequency = 1/T

T= symbol period

IIT Madras

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

Shape gets upset by

(a) Fine Frequency Offset

(b) Fading

IIT Madras

ofdm phy layer tasks
OFDM -- PHY layer tasks
  • Signals sent throh 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

IIT Madras

effect of delay spread in ofdm
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

Example: IEEE 802.11 A (and also in HiperLAN)

Data Payload

CP

3.2msecs

0.8msecs

However, the frequency selectiveness could lead to certain tones

having very poor SNR=> poor gross error rate performance

IIT Madras

ds cdma versus ofdm
Output

(Rx signal)

Input

(Tx signal)

channel

DS-CDMA versus OFDM

DS-CDMA can exploit

time-diversity

a0

Impulse

Response h(t)

a3

time

Frequency

Response H(f)

OFDM can exploit

freq. diversity

freq.

IIT Madras

comparing complexity of tdma ds cdma ofdm transceivers
Comparing Complexity of TDMA, DS-CDMA, & OFDM Transceivers

TDMA

CDMA

OFDM

Difficult, and requires

sync. channel (code)

Very elegant, requiring

no extra overhead

Easy, but requires

overhead (sync.) bits

Timing Sync.

Easy, but requires

overhead (sync.) bits

Gross Sync. Easy

Fine Sync. is Difficult

Freq. Sync.

More difficult than TDMA

Complexity is high in

Asynchronous W-CDMA

Usually not required

within a burst/packet

Timing Tracking

Modest Complexity

Freq. Tracking

Easy, decision-directed

techniques can be used

Modest Complexity

(using dedicated correlator)

Requires CPE Tones

(additional overhead)

Channel

Equalisation

Modest to High Complexity

(depending on bit-rate and

extent of delay-spread)

RAKE Combining in CDMA

usually more complex than

equalisation in TDMA

Frequency Domain

Equalisation is very easy

Analog Front-end

(AGC, PA, VCO, etc)

Complexity or cost is

very high

Fairly Complex

(power control loop)

Very simple

comparing performance of tdma ds cdma ofdm transceivers
Comparing Performance of TDMA, DS-CDMA, & OFDM Transceivers

TDMA

CDMA

OFDM

Fade Margin

(for mobile apps.)

Modest requirement

(RAKE gain vs power-

control problems)

Required for mobile

applications

Required for mobile

applications

Range increase by reducing

allowed noise rise (capacity)

Range

Very easy to increase

cell sizes

Difficult to support large

cells (PA , AGC limitations)

Modest (in TDMA) and

High in MC-TDMA

Re-use planning is

crucial here

Re-use & Capacity

Modest

FEC Requirements

FEC is usually inherent (to

increase code decorrelation)

FEC is vital even for

fixed wireless access

FEC optional for voice

Variable Bit-rate

Support

Powerful methods

to support VBR

(for fixed access)

Very elegant methods

to support VBR

Low to modest support

Very High

(& Higher Peak Bit-rates)

Spectral Efficiency

Poor to Low

Modest

proprietary ofdm flavours
Proprietary OFDM Flavours

Wireless Access (Macro-cellular)

Flash OFDM

from Flarion

www.flarion.com

Vector OFDM

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

www.iospan.com

Wideband-OFDM

(W-OFDM) of Wi-LAN

www.wi-lan.com

-- Freq. Hopping for

CCI reduction, reuse

-- 1.25 to 5.0MHz BW

-- mobility support

-- 2.4 GHz band

-- 30-45Mbps in 40MHz

-- large tone-width

(for mobility)

-- MIMO Technology

-- non-LoS coverage,

mainly for fixed access

-- upto 20 Mbps.

Wi-LAN leads the OFDM Forum -- many proposals submitted to

IEEE 802.16 Wireless MAN

Cisco leads the Broadand Wireless Internet Forum (BWIF)

wireless advances
Wireless Advances

Spatial Multiplexing

Transmit Diversity

Spectral

Efficiency

OFDM

Turbo Coding

Link

Adaptation

Sectorisation

Space-Time Coding

CCI Suppression

Transmit Diversity

Freq. Hopping

Smart Antennas

Receive Diversity

Fixed Beamforming

Power Control

Range

Multi-user Detection

Re-use

Efficiency

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