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9. Multi Carrier Modulation and OFDM
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Transmission of Data Through Frequency Selective Time Varying Channels
We have seen a wireless channel is characterized by time spreadand frequencyspread.
Time Spread
Frequency Spread
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ifsymbol duration >> time spreadthen there is almost no Inter Symbol Interference
(ISI).
1 0 time
channel
1 0
phase still recognizable
Problem with this: Low Data Rate!!!
Single Carrier Modulation in Flat Fading Channels
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this corresponds to Flat Fading
Frequency Frequency
channel
Flat Freq. Response
Frequency
in the Frequency Domain
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ifsymbol duration ~ time spreadthen there is considerable Inter Symbol Interference
(ISI).
1 0
time
channel
? ?
phase not recognizable
Single Carrier Modulation in Frequency Selective Channels
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One Solution: we need equalization
channel equalizer
1 0
time
1 0
time
Channel andEqualizer
Problems with equalization: it might require training data (thus loss of bandwidth)
if blind, it can be expensive in terms computational effort
always a problem when the channel is time varying
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let symbol duration >> time spreadso there is almost no Inter Symbol Interference(ISI);
send a block of data using a number of carriers (Multi Carrier)
1 0
time channel
time
time
0
0
1
1
symbol symbol
The Multi Carrier Approach
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Compare Single Carrier and Multi Carrier Modulation
Frequency Frequency
channel
0 1 0 1 1 1
Block ofsymbols
subcarriers
Each subcarrier seesa Flat Fading
Channel: EasyDemod
MC
Frequency
1
One symbol
Frequency
Flat Fading Channel:Easy Demod
SC
101 1
0 1 0 1 1 1
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In MC modulation each MC symbol is defined on a time interval and it contains ablock of data
data intervalguard interval
OFDM Symbol
data datadatadata
data
MAX channel time spreadwith
Structure of Multi Carrier Modulation
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the guard time is long enough, so themultipath in one block does not affect
the next block
Data BlockData Block
TX RX
We leave a guard time betweenblocks to allow multipath
Guard Time
data+guard
Guard Time
TX
RX
NO Inter Block Interference!
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Baseband Complex Signal:
MC Signal
Transmitted Signal:
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Orthogonal Subcarriers and OFDM
data intervalguard interval
Choose:
Orthogonality:
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still orthogonal at the receiver!!!
Orthogonality at the Receiver
transientresponse
Transmittedsubcarrier Channel
(LTI)
Receivedsubcarrier
steady stateresponse
OFDM b l i di t ti
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Let
be the sampling frequency;
be the number of data samples in each symbol; the subcarriers spacing
Then:
with the guard time.
OFDM symbols in discrete time
S OFDM S b l
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Summary OFDM Symbol
Sampling Interval
guard data
TIME:
Freq spacing
FREQUENCY:
# samples# subcarriers
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OFDM Symbol and FFT
Where:
positive subcarriers
negative subcarriers
unused subcarriers
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Guard Time with Cyclic Prefix (CP)
CP from the periodicity
IFFT{X}CP
OFDM Demod lator
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OFDM Demodulator
with
See each block:
No Inter Block Interference
O ll St t f OFDM C S t
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Overall Structure of OFDM Comms System
IFFT +CP P/S
FFT -CP S/P
Simple One Gain Equalization
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To recover the transmitted signal you need a very simple one gain equalization:
received transm. noise
channel
Use simple Wiener Filter:
Simple One Gain Equalization
OFDM as Parallel Flat Fading Channels
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OFDM as Parallel Flat Fading Channels
Significance: a Freq. Selective Channel becomes NFlat Fading Channels
OFDMMod
OFDMDemod
FrequencySelectivechannel
NFlatFading
Channels
OFDM Parameters
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OFDM Parameters
Summarize basic OFDM Parameters:
sampling rate in Hz
N length of Data Field in number of samples
L length of Cyclic Prefix in number of samples
total number of Data Subcarriers
data
time
data
frequency
guard guard guard
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IEEE 802.11a:
Frequency Bands: 5.150-5.350 GHz and 5.725-5.825 GHz (12 channels)
Modulation OFDM
Range: 100m
IEEE 802.11g
Frequency Bands: 2.412-2.472GHz
Modulation: OFDM
Range: 300m
Channel Parameters: FCC
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Channel Parameters: FCC
Example: the Unlicensed Band 5GHz U-NII (Unlicensed National Information Infrastructure)
4 channels in the range 5.725-5.825GHz
8 channels in the range 5.15-5.35GHz
Channel Parameters: Example IEEE802 11
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Channel Parameters: Example IEEE802.11
In terms of a Transmitter Spectrum Mask (Sec. 17.3.9.2 in IEEE Std 802.11a-1999)
Typical SignalSpectrum
Typical BW~16 MHz
In either case:
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In either case:
Sampling frequency
FFT size
Cyclic Prefix
DATACP
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Sub-carriers: (48 data + 4 pilots) + (12 nulls) = 64
Pilots at: -21, -7, 7, 21
Frequency TimeIFFT
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DATA
Frequencies:
Subcarriers index
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Time Block:
Overall Implementation (IEEE 802.11a with 16QAM).
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Overall Implementation (IEEE 802.11a with 16QAM).
1. Map encoded data into blocks of192 bits and 48 symbols:
data
EncodeInterleave
010011010101
Buffer(192 bits)
111001111000
1101
4x48=192 bits
Map to16QAM
+1+j3
-1+j
+3-j3
+1-j
Overall Implementation (IEEE 802.11a with 16QAM).
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Overall Implementation (IEEE 802.11a with 16QAM).
2. Map each block of48 symbols into 64 samples
+1+j3
-3-j
+3-j3
+1-j
IFFT
time domainfrequency domain
null
null
24 data2 pilots
24 data2 pilots
Channel Parameters: Physical
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Constraints on OFDM Symbol Duration:
to minimize CP overhead
roughly!!!
Frequency Spread
Time Spread
for channel Time Invariant
C y
S f OFDM d Ch l P t
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Summary of OFDM and Channel Parameters
Channel:
1. Max Time Spread sec
2. Doppler Spread Hz
3. Bandwidth Hz
4. Channel Spacing Hz
OFDM (design parameters):
1. Sampling Frequency
2. Cyclic Prefix
3. FFT size (power of 2)
4. Number of Carriers
Example: IEEE802.11a
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Channel:
1. Max Time Spread
2. Doppler Spread
3. Bandwidth
4. Channel Spacing
OFDM (design parameters):
1. Sampling Frequency
2. Cyclic Prefix
3. FFT size (power of 2)4. Number of Carriers
p
Applications: various Area Networks
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According to the applications, we define three Area Networks:
Personal Area Network (PAN), for communications within a few meters. This is the typical
Bluetooth or Zigbee application between between personal devices such as your cell phone,desktop, earpiece and so on;Local Area Network (LAN), for communications up 300 meters. Access points at theairport, coffee shops, wireless networking at home. Typical standard is IEEE802.11 (WiFi) orHyperLan in Europe. It is implemented by access points, but it does not support mobility;Wide Area Network (WAN), for cellular communications, implemented by towers. Mobilityis fully supported, so you can move from one cell to the next without interruption. Currently itis implemented by Spread Spectrum Technology via CDMA, CDMA-2000, TD-SCDMA,EDGE and so on. The current technology, 3G, supports voice and data on separate networks.For (not so) future developments, 4G technology will be supporting both data and voice on thesame network and the standard IEEE802.16 (WiMax) seems to be very likely
pp
More Applications
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pp
1. WLAN (Wireless Local Area Network) standards and WiFi. In particular:
IEEE 802.11a in Europe and North America
HiperLAN /2 (High Performance LAN type 2) in Europe and North America MMAC (Mobile Multimedia Access Communication) in Japan
2. WMAN (Wireless Metropolitan Network) and WiMax
IEEE 802.16
3. Digital Broadcasting Digital Audio and Video Broadcasting (DAB, DVB) in Europe
4. Ultra Wide Band (UWB) Modulation
a very large bandwidth for a very short time.
5. Proposed for IEEE 802.20 (to come) for high mobility communications (cars,trains )