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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 - PowerPoint PPT Presentation
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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
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
IEEE Symp./ IISc -2001 IIT Madras 3
Basics of Radio Propagation
Distance
Powe
r
10-100 m(1-10 secs)
0.1 -1 m(10-100 msecs)
Exponential
Long-term Fading
Short-term Fading
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)
IEEE Symp./ IISc -2001 IIT Madras 5
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
time3
0
freq.
Frequency Response H(f)
IEEE Symp./ IISc -2001 IIT Madras 6
Frequency Selective Fading
Time2.0 secs 2.5 secs 3.0 secs
Delay Spreadrms =
5secs
Gain
(in vo
lts)
Fading
Frequency Selective Fading Channels can provide-- time diversity (can be exploited in DS-CDMA)-- frequency diversity (can be exploited in OFDM)
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
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
IEEE Symp./ IISc -2001 IIT Madras 9
TDMA (with FDMA) Principle
Power
Time
Freq.
Time-slots
Carriers
IEEE Symp./ IISc -2001 IIT Madras 10
Direct Sequence CDMA Principle(with FDMA)
Power
Time
Freq.
User CodeWaveforms
IEEE Symp./ IISc -2001 IIT Madras 11
OFDM (with TDMA & FDMA) Principle
Power
Time
Freq.
Time-slots
Carriers
Tones
IEEE Symp./ IISc -2001 IIT Madras 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-ratesand high capacity (for data users) -- GPRS, DPRS, etc.
IEEE Symp./ IISc -2001 IIT Madras 13
What is an OFDM System ? Data is transmitted in parallel on multiple
carriers that overlap in frequency
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
IEEE Symp./ IISc -2001 IIT Madras 15
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
Nkij
eknsN
ind
IEEE Symp./ IISc -2001 IIT Madras 16
OFDM Receiver
Cyclic Prefix is discarded
1
0
2],['1],[
N
i
Nikj
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
IEEE Symp./ IISc -2001 IIT Madras 17
AGC
fc
VCO
Sampler FFTError
gross offset
Slot &
fine offsetFreq. OffsetEstimation
TimingSync.
(of all tones sent in one OFDM symbol)
Generic OFDM Receiver
RecoveryP/S and
Detection
IEEE Symp./ IISc -2001 IIT Madras 18
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
fTs
1
fsT
fNW
CPS TT
IEEE Symp./ IISc -2001 IIT Madras 19
Condition for Orthogonality
Time
T
Base frequency = 1/T
T= symbol period
IEEE Symp./ IISc -2001 IIT Madras 20
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
IEEE Symp./ IISc -2001 IIT Madras 21
Tx Waveform over a OFDM Symbol(magnitude values, for 802.11a)
IEEE Symp./ IISc -2001 IIT Madras 22
Sync Basis Functions(of equal height for single-ray channel)
Shape gets upset by(a) Fine Frequency Offset(b) Fading
IEEE Symp./ IISc -2001 IIT Madras 23
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
IEEE Symp./ IISc -2001 IIT Madras 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)
IEEE Symp./ IISc -2001 IIT Madras 25
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
IEEE Symp./ IISc -2001 IIT Madras 26
Slot and Timing Synchronization in OFDMExample: 4 tones per slot (OFDM symbol) T
self-symmetry can be exploited for non-coherent timing recovery
zero tones
IFFT PA
T secst
IFFT PA
T secst
T/2 T
Traffic Slot
Preamble/Control Slot
IEEE Symp./ IISc -2001 IIT Madras 27
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
IEEE Symp./ IISc -2001 IIT Madras 28
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
IEEE Symp./ IISc -2001 IIT Madras 29
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)
IEEE Symp./ IISc -2001 IIT Madras 30
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
IEEE Symp./ IISc -2001 IIT Madras 31
AGC
Sampler DFTError
-- Gross Freq. Offset-- Channel Estimation and Equalization
OFDM Receiver Algorithms -- Recap
RecoveryP/S and
Detection
Freq.
-- Fine Freq. Offset-- Timing Estimation
IEEE Symp./ IISc -2001 IIT Madras 32
Conventional OFDM
Frequency Domain Equalisation-- Conventional OFDM
DFTFrequency
DomainEqualiser
RemoveCP
RxAlgos.
Detection& P/S
IDFT AddCP
TxMod.
SymbolMapping
& S/P
IEEE Symp./ IISc -2001 IIT Madras 33
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)Tx
Mod.SymbolMapping
to permit FDE
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
IDFT AddCP
TxMod.
SymbolMapping
& S/P
Time-Domain
Equaliser
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
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
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
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”)
IEEE Symp./ IISc -2001 IIT Madras 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
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)
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)
IEEE Symp./ IISc -2001 IIT Madras 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
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
IEEE Symp./ IISc -2001 IIT Madras 44
DS-CDMA versus OFDM
channelInput (Tx signal)
Output(Rx signal)
ImpulseResponse h(t)
time3
0
freq.
Frequency Response H(f)
DS-CDMA can exploit
time-diversity
OFDM can exploitfreq. diversity
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 OFDMVery elegant, requiring
no extra overhead
CDMAEasy, but requires
overhead (sync.) bitsDifficult, and requiressync. channel (code)
Easy, but requiresoverhead (sync.) bits More difficult than TDMA Gross 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)
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
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
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)
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
IEEE Symp./ IISc -2001 IIT Madras 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) • RS+Convolutional coding
• Block Turbo coding (optional)• Convolutional Turbo coding(optional)
• QPSK, 16QAM, 64QAM• Two different preambles for UL and DL
IEEE Symp./ IISc -2001 IIT Madras 51
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
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)
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)
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
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
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
IEEE Symp./ IISc -2001 IIT Madras 57
Rate Dependent ParametersCoded bits
persubcarrier
(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
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 estimationSignal detect, AGC, Timing
Recovery, 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
IEEE Symp./ IISc -2001 IIT Madras 59
PPDU Frame format
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
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
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
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
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
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
IEEE Symp./ IISc -2001 IIT Madras 66
MMSE Technique5 10 15 20 25
10-3
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
IEEE Symp./ IISc -2001 IIT Madras 67
64-QAM Without Pilot De-rotation64 QAM before pilot correction
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
-2 -1 0 1 2
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
IEEE Symp./ IISc -2001 IIT Madras 69
BER Curves for Different Channel Models For AWGN Channel
AWGN case
-5
-4
-3
-2
-1
00 5 10 15
Eb/n0 in db
BE
Rin
db QPSK1/2
12Mbps16QAM 1/224Mbps64QAM2/348MBPSBPSK1/26Mbps
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
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.
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
IEEE Symp./ IISc -2001 IIT Madras 73
ST Block Code Example
-d*(k+1), d(k)
d*(k), d(k+1)
TxRxr(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)
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
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!)
IEEE Symp./ IISc -2001 IIT Madras 76
Permutation Tx Diversity for OFDM
Courtesy:http://www.research.att.com/~justin/
IEEE Symp./ IISc -2001 IIT Madras 77
ST Coded Tx Diversity for OFDM
Courtesy:http://www.research.att.com/~justin/
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
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
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
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!