Transcript OFDM Technology -- Physical Layer, Standards, Advances

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
Instructional Workshop on Wireless Networks : Physical Layer Aspects
DRDO-IISc Program on Mathematical Engineering, Feb. 14, 2003
IEEE Symp./ IISc -2001
IIT Madras
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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
IEEE Symp./ IISc -2001
IIT Madras
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Basics of Radio Propagation
Power
Exponential
0.1 -1 m
(10-100 msecs)
Short-term Fading
Long-term Fading
10-100 m
(1-10 secs)
Distance
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Multi-path Propagation
r(t) = a0 s(t-t0) + a1 s(t-t1) + a2 s(t-t2) + a3 s(t-t3)
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Multi-path Propagation -- contd.
r(t) = a0 s(t-t0) + a1 s(t-t1) + a2 s(t-t2) + a3 s(t-t3)
a0
Impulse
Response h(t)
t3 - t0
a3
time
Input
(Tx signal)
Output
(Rx signal)
channel
Frequency
Response H(f)
freq.
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Frequency Selective Fading
Fading
2.0 secs
Time 3.0
2.5 secs
secs
Frequency Selective Fading Channels can provide
-- time diversity (can be exploited in DS-CDMA)
-- frequency diversity (can be exploited in OFDM)
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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
IEEE Symp./ IISc -2001
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TDMA, CDMA, and OFDM
Wireless Systems

Time Division Multiple Access (TDMA) is the most
prevalent wireless access system to date


Direct Sequence Code Division Multiple Access (DSCDMA) became commercial only in the mid 90’s


GSM, ANSI-136, EDGE, DECT, PHS, Tetra
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
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TDMA (with FDMA) Principle
Carriers
Power
Freq.
Time-slots
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Time
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Direct Sequence CDMA Principle
(with FDMA)
User Code
Waveforms
Power
Freq.
Time
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OFDM (with TDMA & FDMA) Principle
Tones
Carriers
Power
Freq.
Time-slots
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Time
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Other Multiple Access Techniques

Multi-Carrier TDMA


Frequency Hopped Spread Spectrum
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Bluetooth
CSMA/CA


DECT, PACS
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.
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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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Generic OFDM Transmitter
OFDM symbol
FEC
bits
Serial to
Parallel
IFFT
Pulse shaper
&
Linear
PA
DAC
add cyclic extension
fc
view this as a time to
frequency mapper
Complexity (cost) is transferred back from the digital to the analog domain!
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OFDM Transmitter -- contd.
d [n,0]
s[n,0]
Serial/
Add
s[n,1]
IFFT
d [n,1]
Parallel
s[ n, N ]
d[n, N ]
Parallel/
Cyclic
Serial
Prefix

S/P acts as Time/Frequency mapper

IFFT generates the required Time domain waveform
1
d [n, i] 
N

N 1
 s[n, k ]e
j 2i
k
N
k 0
Cyclic Prefix acts like guard interval and makes equalization easy
(FFT-cyclic convolution vs channel-linear convolution)
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OFDM Receiver
Remove
Serial/
d ' [n,0]
r[n,0]
d ' [n,1]
r[n,1] Parallel/
FFT
Cyclic
Parallel
Prefix
Serial
d ' [n, N ]
r[ n, N ]

Cyclic Prefix is discarded

FFT generates the required
Frequency Domain
signal
i
N 1

1
r[n, k ] 
N
 d '[n, i]e
 j 2k
N
i 0
P/S acts like a Frequency/Time Mapper
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Generic OFDM Receiver
Slot &
Timing
Sync.
AGC
Sampler
FFT
fc
VCO
fine offset
P/S and
Detection
Error
Recovery
gross offset
Freq. Offset
Estimation
(of all tones sent in one OFDM symbol)
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OFDM Basics

To maintain orthogonality



f
= sub-carrier spacing
Ts = symbol duration
1
 f
Ts
where
If N-point IDFT (or FFT) is used
 Total bandwidth (in Hz) = W  Nf

TS  TCP
IEEE Symp./ IISc -2001
= symbol duration after CP addition
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Condition for Orthogonality
Time
T
Base frequency = 1/T
T= symbol period
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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
r[n, k ]  H [n, k ]s[n, k ]  w[n, k ]



where
w[n, k ]
H [n, k ]
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is additive noise
is channel frequency response
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Tx Waveform over a OFDM Symbol
(magnitude values, for 802.11a)
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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:




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


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
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Frequency Offset

Carrier recovery and tracking critical for OFDM

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Residual freq. offset causes
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
Offsets can be comparable to sub-carrier spacing in OFDM
Non-coherent detectors possible with differential coding
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)
IEEE Symp./ IISc -2001
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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
introducing self-similarity


timing
recovery
techniques
by
=> 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
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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
T secs
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PA
t
self-symmetry can be
exploited for noncoherent 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
only 20-30% of symbol duration)


rms delay spread is
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


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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 SNR=> 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 MultiCarrier 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
Sampler
DFT
P/S and
Detection
Error
Recovery
Freq.
-- Gross Freq. Offset
-- Channel Estimation
and Equalization
-- Fine Freq. Offset
-- Timing Estimation
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Frequency Domain Equalisation
-- Conventional OFDM
Symbol
Mapping
& S/P
IDFT
Add
CP
Tx
Mod.
Conventional
OFDM
Rx
Algos.
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Remove
CP
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DFT
Frequency
Domain
Equaliser
Detection
& P/S
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Frequency Domain Equalisation
-- Single Carrier FDE (SC-FDE)
Symbol
Mapping
Add
CP
(of symbols)
Tx
Mod.
Tx -- low-complexity, TDMA
Rx -- implements SC-FDE;
Linear Equaliser or DFE
to permit FDE
Rx
Algos.
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Remove
CP
DFT
Frequency
Domain
Equaliser
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IDFT
Detector
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Time & Frequency Domain Equalisation
-- for OFDM in large delay spread channels
Symbol
Mapping
& S/P
IDFT
Add
CP
Rx
Algos.
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Tx
Mod.
TDE + FDE
for OFDM
TimeDomain
Equaliser
Remove
CP
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DFT
Frequency
Domain
Equaliser
Detection
& P/S
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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
IEEE Symp./ IISc -2001
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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
IEEE Symp./ IISc -2001
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Channel Estimation in OFDM -- Example
Frame (say, 4 slots)
Control +
Training Slot
Traffic Slot 1
Traffic Slot 2
Traffic Slot 3
Control +
Training Slot
Training
Tones
(for channel
identification)
MAC message
(broadcast)


Phase
Correction
Tones
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
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Fading Compensation in OFDM

OFDM using a FDE, observes only “flat” fading at the subcarrier 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”)
IEEE Symp./ IISc -2001
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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
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Phy Layer Issues in Macro-cell OFDM

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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)
IEEE Symp./ IISc -2001
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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)
IEEE Symp./ IISc -2001
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MAC Layer Issues in Macro-Cell OFDM
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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
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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
IEEE Symp./ IISc -2001
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DS-CDMA versus OFDM
DS-CDMA can exploit
time-diversity
a0
Impulse
Response h(t)
a3
time
Input
(Tx signal)
Output
(Rx signal)
channel
Frequency
Response H(f)
OFDM can exploit
freq. diversity
freq.
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Comparing Complexity of TDMA,
DS-CDMA, & OFDM Transceivers
TDMA
Easy, but requires
overhead (sync.) bits
Timing Sync.
Easy, but requires
overhead (sync.) bits
Freq. Sync.
Timing Tracking
Freq. Tracking
Channel
Equalisation
Analog Front-end
(AGC, PA, VCO, etc)
CDMA
OFDM
Difficult, and requires
sync. channel (code)
Very elegant, requiring
no extra overhead
More difficult than TDMA
Gross Sync. Easy
Fine Sync. is Difficult
Modest Complexity
Complexity is high in
Asynchronous W-CDMA
Usually not required
within a burst/packet
Easy, decision-directed
techniques can be used
Modest Complexity
(using dedicated correlator)
Requires CPE Tones
(additional overhead)
Modest to High Complexity RAKE Combining in CDMA
(depending on bit-rate and usually more complex than
extent of delay-spread)
equalisation in TDMA
Very simple
(especially for CPM signals)
IEEE Symp./ IISc -2001
Fairly Complex
(power control loop)
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Frequency Domain
Equalisation is very easy
Complexity or cost is
very high (PA back-off
is necessary)
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Comparing Performance of TDMA,
DS-CDMA, & OFDM Transceivers
TDMA
Fade Margin
(for mobile apps.)
Range
OFDM
Modest requirement
(RAKE gain vs powercontrol problems)
Required for mobile
applications
Range increase by reducing
allowed noise rise (capacity)
Difficult to support large
cells (PA , AGC limitations)
Modest (in TDMA) and
High in MC-TDMA
Modest
Re-use planning is
crucial here
FEC optional for voice
FEC is usually inherent (to
increase code decorrelation)
FEC is vital even for
fixed wireless access
Required for mobile
applications
Very easy to increase
cell sizes
Re-use & Capacity
FEC Requirements
Variable Bit-rate
Support
CDMA
Low to modest support
Very elegant methods
to support VBR & VAD
Modest
Poor to Low
Spectral Efficiency
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Powerful methods
to support VBR
(for fixed access)
Very High
(& Higher Peak Bit-rates)
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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
IEEE Symp./ IISc -2001
IIT Madras
47
Proprietary OFDM Flavours
Wireless Access (Macro-cellular)
Wideband-OFDM
(W-OFDM) of Wi-LAN
www.wi-lan.com
-- 2.4 GHz band
-- 30-45Mbps in 40MHz
-- large tone-width
(for mobility, overlay)
Flash OFDM
from Flarion
www.flarion.com
-- Freq. Hopping for
CCI reduction, reuse
-- 1.25 to 5.0MHz BW
-- mobility support
Vector OFDM
(V-OFDM) of Cisco, Iospan,etc.
www.iospan.com
-- 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)
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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
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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
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Preamble structure for 802.16a Wireless MAN

Two different preamble structures for DL and UL
CP
128
128
Tg
Tb
Preamble structure of 802.16a Uplink
CP 64 64 64 64
Tg
Tb
CP
128
Tg
128
Tb
Preamble structure of 802.16a Downlink
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Calculations for 802.16a -- Example: 5MHz
Carrier frequency
Channel Bandwidth
Number of inputs to IFFT/FFT
Number of data subcarriers
Number of pilots
Subcarrier frequency spacing f
Period of IFFT/FFT Tb
Length of guard interval
Length of the preamble for Downlink
Length of the preamble for Uplink
Guard interval for Uplink preamble
OFDM symbol duration
IEEE Symp./ IISc -2001
2-11 GHz
5 MHz
256
192
8
19.53125 KHz (5 MHz/256)
51.2 ms (1 / f)
12.8 ms (Tb / 4)
128 ms (640 sub-carriers)
76.8ms (384/5 MHz)
25.6 ms (128/5 MHz)
64 ms (320/5 MHZ)
IIT Madras
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Broadband Wireless Standards

ETSI BRAN activity
 HiperLan > HiperLink > HiperAccess
Hiperaccess
(PMP, 25Mbps, 40GHz)
or
ETSI’s FWA (2-11 GHz)
Hiperlink
(155Mbps, 17GHz
upto 150m)
HiperLan (1,2)
(19 or 54Mbps, 5GHz)
2-5 miles, LoS(> 11GHz)
or non-LoS (<11GHz)
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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(> 11GHz)
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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
IEEE Symp./ IISc -2001
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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
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Rate Dependent Parameters
Coded bits
per
Data rate Modulation Coding rate
subcarrier
(Mbits/s)
(R)
(NBPSC)
6
BPSK
9
BPSK
12
Coded bits
per OFDM
symbol
(NCBPS)
Data bits
per OFDM
symbol
(NDBPS)
1/2
1
48
24
3/4
1
48
36
QPSK
1/2
2
96
48
18
QPSK
3/4
2
96
72
24
16 QAM
1/2
4
192
96
36
16 QAM
3/4
4
192
144
48
64 QAM
2/3
6
288
192
54
64 QAM
3/4
6
288
216
IEEE Symp./ IISc -2001
IIT Madras
57
802.11A -- Frame and Slot Structure
Details of the preamble field
10 short symbols (0.8*10 = 8ms)
0
1
2
3
4
5
6
7
8
2 long symbols (1.6 + 2*3.2 = 8ms)
9
8ms
P2
8ms
MAC
Header
T1
T2
Freq. Offset estimation
and channel estimation
Signal detect, AGC, Timing
Recovery, Freq. acquisition
P1
GI 2
Data Data
…….
4ms
Data Pream
ble2
Data …
4ms
Number of Sub-carriers = 64 (only 48+4=52 are non-zero)
IEEE Symp./ IISc -2001
IIT Madras
58
PPDU Frame format
IEEE Symp./ IISc -2001
IIT Madras
59
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 4+4 = 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
IEEE Symp./ IISc -2001
IIT Madras
60
Auto-correlation and Piece-wise Crosscorrelation for Slot Boundary Detection

Auto-correlation for timing and freq. estimation
z(n)

k 79
|  y(n  k)y*(n  k  16) | for n  0 to 159
k 0

Piece-wise Cross-correlation can also be used
IEEE Symp./ IISc -2001
IIT Madras
61
Timing Recovery in 802.11A -Simulation Results
N=0 represents start of 1st preamble; length of channel impulse
response set to 8 samples (0.4msecs)
Value of index n in Probability of the corresponding n being detected as the start
of the frame at different SNRs
the transmitted
data s(n)
No noise
20 db
15 db
10 db
5 db
0.062
0.008
0
0
0
N=7
0.032
0.009
0.002
0
0
N=8
N=9
0.057
0.096
0.048
0.091
0.022
0.081
0.013
0.080
0.013
0.083
N=10
N=11
0.144
0.204
0.195
0.276
0.226
0.322
0.236
0.327
0.231
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
0.019
0.017
0.008
0.003
n<7 (outside the
acceptable range)
N=16
n>16 (outside the
acceptable range
0
0
0
0
Acceptable
Range
0
0
Performance of timing recovery algorithm using 1st preamble
IEEE Symp./ IISc -2001
IIT Madras
62
Auto-correlation Result
autocorrelation result
1.2
1
0.8
0.6
0.4
0.2
0
1
11 21 31 41 51 61 71 81 91 101 111 121 131 141 151 161
IEEE Symp./ IISc -2001
IIT Madras
63
Piece-wise cross-correlation Result
Cross correlation Result
4
3.5
3
2.5
2
1.5
1
0.5
IEEE Symp./ IISc -2001
IIT Madras
154
145
136
127
118
109
100
91
82
73
64
55
46
37
28
19
10
1
0
64
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
IEEE Symp./ IISc -2001
IIT Madras
65
Comparison of the Two Fine Frequency
Estimation Algorithms
-1
-1
10
-2
S.D
S.D
10
10
-2
10
300 Hz
300 Hz
30 Hz
30 Hz
-3
10
-3
5
10
15
snr(db)
20
25
10
5
MMSE Technique
IEEE Symp./ IISc -2001
10
15
snr(db)
20
25
Self-Correlation
IIT Madras
66
64-QAM Without Pilot De-rotation
64 QAM before p ilot correction
2
1.5
1
0.5
0
-2
-1
0
1
2
-0.5
-1
-1.5
-2
IEEE Symp./ IISc -2001
IIT Madras
67
64-QAM After Pilot De-rotation
64 QAM after p ilot rotation
1.5
1
0.5
0
-1.5
-1
-0.5
0
0.5
1
1.5
-0.5
-1
-1.5
IEEE Symp./ IISc -2001
IIT Madras
68
BER Curves for Different Channel Models

For AWGN Channel
AWGN case
0
0
5
10
15
-1
BERin db
-2
-3
-4
-5
Eb/n0 in db
IEEE Symp./ IISc -2001
IIT Madras
QPSK1/2
12Mbps
16QAM 1/2
24Mbps
64QAM2/3
48MBPS
BPSK1/2
6Mbps
69
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
IEEE Symp./ IISc -2001
IIT Madras
70
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.
IEEE Symp./ IISc -2001
IIT Madras
71
Wireless Advances -- contd.
Spatial Multiplexing
Turbo Coding
OFDM
Spectral
Efficiency
Link
Space-Time Coding
Adaptation
Transmit Diversity
Sectorisation
CCI Suppression
Transmit Diversity
Freq. Hopping
Smart Antennas
Receive Diversity
VAD, AMR, VBR
Power Control
Fixed Beamforming
Range
Multi-user Detection
Re-use
Efficiency
IEEE Symp./ IISc -2001
DCS
IIT Madras
72
ST Block Code Example
Recall Example – Permutation Tx Diversity Scheme
-d*(k+1), d(k)
Tx
a
b
r(k+1), r(k)
Rx
d*(k), d(k+1)
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)
IEEE Symp./ IISc -2001
IIT Madras
73
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
IEEE Symp./ IISc -2001
IIT Madras
74
MIMO OFDM



In addition to time and space, OFDM systems
can exploit frequency diversity
If feedback channels are available, SpaceTime-Frequency “water pouring” possible!
OFDM can convert delay-spread diversity into
space diversity (diversity conversion!)
IEEE Symp./ IISc -2001
IIT Madras
75
Permutation Tx Diversity for OFDM
Courtesy:http://www.research.att.com/~justin/
IEEE Symp./ IISc -2001
IIT Madras
76
ST Coded Tx Diversity for OFDM
Courtesy:http://www.research.att.com/~justin/
IEEE Symp./ IISc -2001
IIT Madras
77
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
IEEE Symp./ IISc -2001
IIT Madras
78
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
IEEE Symp./ IISc -2001
IIT Madras
79
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
IEEE Symp./ IISc -2001
IIT Madras
80
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!
IEEE Symp./ IISc -2001
IIT Madras
81