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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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Ofdm

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


Ofdm

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


Ofdm

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 orthogonalitywhere

    • = 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


    Comparing tdma cdma and ofdm

    Comparing TDMA, CDMA, and OFDM


    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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