OFDM Physical Layer -- Fundamentals, Standards,

1
OFDM Physical Layer -- Fundamentals, Standards,
Advances K. GiridharAssociate Professor of
Electrical EngineeringTelecom and Computer
Networks (TeNeT) GroupIIT Madras, Chennai
600036http//www.tenet.res.in
Instructional Workshop on Wireless Networks
Physical Layer Aspects DRDO-IISc Program on
Mathematical Engineering, Feb. 14, 2003
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

3
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
4
Multi-path Propagation
r(t) a0 s(t-t0) a1 s(t-t1) a2 s(t-t2) a3
s(t-t3)
5
Multi-path Propagation -- contd.
r(t) a0 s(t-t0) a1 s(t-t1) a2 s(t-t2) a3
s(t-t3)
a0
t3 - t0
Impulse Response h(t)
a3
time
channel
Output (Rx signal)
Input (Tx signal)
Frequency Response H(f)
freq.
6
Frequency Selective Fading
Frequency Selective Fading Channels can
provide -- time diversity (can be exploited in
DS-CDMA) -- frequency diversity (can be exploited
in OFDM)
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

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 90s
• 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

9
TDMA (with FDMA) Principle
Carriers
Power
Freq.
Time
Time-slots
10
Direct Sequence CDMA Principle(with FDMA)
User Code Waveforms
Power
Freq.
Time
11
OFDM (with TDMA FDMA) Principle
Tones
Carriers
Power
Freq.
Time
Time-slots
12
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-rates and high capacity (for data users) --
GPRS, DPRS, etc.
13
What is an OFDM System ?

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

14
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!
15
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)

16
OFDM Receiver

• Cyclic Prefix is discarded

• FFT generates the required Frequency Domain
  signal

• P/S acts like a Frequency/Time Mapper

17
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)
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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

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Condition for Orthogonality
Base frequency 1/T
T symbol period
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OFDM Basics -- contd.

• If the Cyclic Prefix gt 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

21
Tx Waveform over a OFDM Symbol(magnitude values,
for 802.11a)
22
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

24
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)

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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
• gt very robust to uncompensated frequency offsets
• If cyclic prefix is larger than the rms delay
  spread, range of (equally good) timing phases
  become available
• gt robust to estimation errors

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Slot and Timing Synchronization in OFDM
Example 4 tones per slot (OFDM symbol)
T
Traffic Slot
IFFT
PA
t
T secs
T/2
T
Preamble/Control Slot
IFFT
PA
t
T secs
self-symmetry can be exploited for non- coherent
timing recovery
zero tones
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Effect of Delay Spread

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

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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

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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

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 SNRgt poor
gross error rate performance
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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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OFDM Receiver Algorithms -- Recap
AGC
Error
P/S and Detection
Sampler
DFT
Recovery
Freq.
-- Gross Freq. Offset -- Channel Estimation
and Equalization
-- Fine Freq. Offset -- Timing Estimation
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Frequency Domain Equalisation -- Conventional OFDM
Add CP
Symbol Mapping S/P
Tx Mod.
Conventional OFDM
IDFT
Frequency Domain Equaliser
Remove CP
Rx Algos.
Detection P/S
DFT
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Frequency Domain Equalisation -- Single Carrier
FDE (SC-FDE)
Add CP (of symbols)
Tx Mod.
Symbol Mapping
Tx -- low-complexity, TDMA Rx -- implements
SC-FDE Linear Equaliser or DFE
to permit FDE
Frequency Domain Equaliser
Remove CP
Rx Algos.
IDFT
DFT
Detector
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Time Frequency Domain Equalisation -- for OFDM
in large delay spread channels
Add CP
Symbol Mapping S/P
Tx Mod.
TDE FDE for OFDM
IDFT
Frequency Domain Equaliser
Remove CP
Time- Domain Equaliser
Detection P/S
Rx Algos.
DFT
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Fading and Antenna Diversity

• Short-term fading exhibits spatial correlation
• Two antennas, spaced l/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

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

37
Channel Estimation in OFDM -- Example
Frame (say, 4 slots)
Control Training Slot
Control Training Slot
Traffic Slot 3
Traffic Slot 2
Traffic Slot 1
Training Tones (for channel identification)
Phase Correction Tones
MAC message (broadcast)

• 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

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)

39
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

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)

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)

42
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

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

44
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.
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Comparing Complexity of TDMA, DS-CDMA, OFDM
Transceivers
TDMA
OFDM
CDMA
Very elegant, requiring no extra overhead
Easy, but requires overhead (sync.) bits
Difficult, and requires sync. channel (code)
Timing Sync.
Easy, but requires overhead (sync.) bits
Gross Sync. Easy Fine Sync. is Difficult
Freq. Sync.
More difficult than TDMA
Usually not required within a burst/packet
Complexity is high in Asynchronous W-CDMA
Timing Tracking
Modest Complexity
Freq. Tracking
Requires CPE Tones (additional overhead)
Modest Complexity (using dedicated correlator)
Easy, decision-directed techniques can be used
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
Complexity or cost is very high (PA back-off is
necessary)
Analog Front-end (AGC, PA, VCO, etc)
Very simple (especially for CPM signals)
Fairly Complex (power control loop)
46
Comparing Performance of TDMA, DS-CDMA, OFDM
Transceivers
TDMA
OFDM
CDMA
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
Difficult to support large cells (PA , AGC
limitations)
Very easy to increase cell sizes
Modest (in TDMA) and High in MC-TDMA
Re-use planning is crucial here
Re-use Capacity
Modest
FEC Requirements
FEC is vital even for fixed wireless access
FEC is usually inherent (to increase code
decorrelation)
FEC optional for voice
Variable Bit-rate Support
Powerful methods to support VBR (for fixed access)
Very elegant methods to support VBR VAD
Low to modest support
Very High ( Higher Peak Bit-rates)
Spectral Efficiency
Poor to Low
Modest
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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

48
Proprietary OFDM Flavours
Wireless Access (Macro-cellular)
Flash OFDM from Flarion www.flarion.com
Vector OFDM (V-OFDM) of Cisco, Iospan,etc. www.ios
pan.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, overlay)
-- 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
MAN Cisco leads the Broadand Wireless Internet
Forum (BWIF)
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

50
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)
• RSConvolutional coding
• Block Turbo coding (optional)
• Convolutional Turbo coding(optional)
• QPSK, 16QAM, 64QAM
• Two different preambles for UL and DL

51
Preamble structure for 802.16a Wireless MAN

• Two different preamble structures for DL and UL

52
Calculations for 802.16a -- Example 5MHz
53
Broadband Wireless Standards

• ETSI BRAN activity
• HiperLan gt HiperLink gt HiperAccess

Hiperaccess (PMP, 25Mbps, 40GHz) or ETSIs FWA
(2-11 GHz)
Hiperlink (155Mbps, 17GHz upto 150m)
HiperLan (1,2) (19 or 54Mbps, 5GHz)
2-5 miles, LoS(gt 11GHz) or non-LoS (lt11GHz)
54
Broadband Access Standards -- contd.

• IEEE LAN and MAN standards

IEEE 802.16 (10 to 66 GHz)
IEEE 802.16a,b (2 to 11 GHz)
1-3 miles, non-LoS
IEEE 802.11a or .11b, or .11g
2-5 miles, LoS(gt 11GHz)
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

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 gt 20 MHz

57
Rate Dependent Parameters

• Coded bits
• per
• subcarrier
• (NBPSC)

• Coded bits
• per OFDM
• symbol
• (NCBPS)

• Data bits
• per OFDM
• symbol
• (NDBPS)

58
802.11A -- Frame and Slot Structure
Number of Sub-carriers 64 (only 48452
are non-zero)
59
PPDU Frame format
60
Preamble Structure -- Implications


0
1
2
3
4
5
6
7
8
9

Only every 4th tone is non-zero. This implies 10
replicas (in time) within 44 8msecs
Even if delay spread in 0.2 msecs (for a 100m
cell), we can use 9 of 10 replicas to recover
timing use less than 9 for higher fade rates
61
Auto-correlation and Piece-wise Cross-correlation
for Slot Boundary Detection

• Auto-correlation for timing and freq. estimation

• Piece-wise Cross-correlation can also be used

62
Timing Recovery in 802.11A --Simulation Results
N0 represents start of 1st preamble length of
channel impulse response set to 8 samples
(0.4msecs)
Acceptable Range
63
Auto-correlation Result
64
Piece-wise cross-correlation Result
65
Fine Frequency Offset Estimation

• Quantity of interest is the Standard Deviation,
  ? f of the frequency estimate.
• It is given by ? f E (( fest - fo )2 )
  1/2

Approximate by using ensemble averaging of many
Monte-Carlo runs
66
Comparison of the Two Fine Frequency Estimation
Algorithms
MMSE Technique
Self-Correlation
67
64-QAM Without Pilot De-rotation
68
64-QAM After Pilot De-rotation
69
BER Curves for Different Channel Models

• For AWGN Channel

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

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.

72
Wireless Advances -- contd.
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
VAD, AMR, VBR
Fixed Beamforming
Power Control
Range
Multi-user Detection
Re-use Efficiency
DCS
73
ST Block Code Example
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)
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

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!)

76
Permutation Tx Diversity for OFDM
Courtesyhttp//www.research.att.com/justin/
77
ST Coded Tx Diversity for OFDM
Courtesyhttp//www.research.att.com/justin/
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

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-slot1
  OFDM symbol)
• Fine granularity from tones (blocks) inside a
  time-slot

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

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!