Exchange of information from point A to point B 100001101010001011101 100001101010001011101 Transmit Receive Wireless Communication Exchange of information from point A to point B 100001101010001011101 ID: 673466
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Slide1
Modulation and OFDMSlide2
Communication
Exchange of information from point A to point B
100001101010001011101
100001101010001011101
Transmit
ReceiveSlide3
Wireless Communication
Exchange of information from point A to point B
100001101010001011101
100001101010001011101
Receive
TransmitSlide4
Wireless Communication
Exchange of information from point A to point B
100001101010001011101
100001101010001011101
Modulation
Upconvert
Downconvert
DemodulationSlide5
Modulation
Converting bits to signals
These signals are later sent over the air
The receiver picks these signals and decodes transmitted data
100001101010001011101
Modulation
Signals (voltages)Slide6
Amplitude Modulation
Suppose we have 4 voltage levels (analog) to represent bits.
-
00
01
10
11Slide7
Amplitude Modulation
1 0 0 0 0 1 1 0 1 0 1 0 0 0 1 0 1 1 1 0
-
00
01
10
11
-
Individual voltage levels are called as symbolsSlide8
Modulated symbols for transmission
1 0 0 0 0 1 1 0 1 0 1 0 0 0 1 0 1 1 1 0
F
-F
FFT
FrequencySlide9
Received symbols with distortions
1 0 0 0 0 1 1 0 1 0 1 0 0 0 1 0 1 1 1 0
F
-F
FFT
FrequencySlide10
Demodulation
1 0 0 0 0 1 1 0 1 0 1 0 0 0 1 0 1 1 1 0
-
00
01
10
11
-
1 0
Tx
bits
Rx bits decoded
0 0
0 0
1 0
1 0 1 0 0 0 1 1 1 1 1 1 Slide11
Coping up with demodulation errors
If the noise is too high, there may be too many bit flips
Symbols for modulation to be chosen as a function of this noise
For example, if we want to eliminate bit flips completely, we can choose voltage levels as followsSlide12
Modulation with sparser symbols
1 0 0 0 0 1 1 0 1 0 1 0 0 0 1 0 1 1 1 0
0
1
Slide13
Received symbols with distortion
1 0 0 0 0 1 1 0 1 0 1 0 0 0 1 0 1 1 1 0
0
1
Slide14
Demodulation
1 0 0 0 0 1 1 0 1 0 1 0 0 0 1 0 1 1 1 0
0
1
1 0 0 0 0 1 1 0 1 0 1 0 0 0 1 0 1 1 1 0 Slide15
That eliminated all the bit flips, which is good
However, what is the disadvantage of choosing only two voltage levels?
Takes longer to transmit, hence bit rate is very lowSlide16
Bit rates
1 0 0 0 0 1 1 0 1 0 1 0 0 0 1 0 1 1 1 0
-
Symbol duration
Bit per symbol
Symbol rate (Baud rate)
Bit rate
Symbol rate (Baud rate)
(bandwidth)
Bit rate
F
-F
FFT
(bandwidth)
FrequencySlide17
Transmission of modulated symbols
The modulated message has zero center frequency (baseband)
Impractical to have antennas at that frequencies
Causes interference if everyone wants to use baseband ..
(bandwidth)
(bandwidth)
UpconversionSlide18
Upconversion
shifting center frequency
(bandwidth)
(bandwidth)
Slide19
Down-conversion
bringing signal back to baseband
The receiver needs to perform an operation of down-conversion
The received signal is a high frequency signal in RF
Processing the data at these frequencies needs high clock digital circuits, which is impracticalWe need to convert the data back to baseband and process the low frequency signals for decoding bitsSlide20
Down-conversion
bringing signal back to baseband
(bandwidth)
(bandwidth)
Low pass filter eliminates this
Then, we recover baseband signalSlide21
Upconversion
and
Downconversion
summary
x
m(t)
x
r(t)
Slide22
Upconversion
and
Downconversion
summary
x
I(t)
x
r(t)
Slide23
Beyond amplitude modulation
We have learnt communication with amplitude modulation
There is a simple idea to double the data rate using QAM (quadrature amplitude modulation)Slide24
Quadrature amplitude modulation
Achieves double data rate compared to amplitude modulation alone
I(t)
x
Q(t)
x
+
x
x
Slide25
Symbols with QAM
-
This scheme uses 16 symbols (4 bits per symbol), hence called 16 QAM
0010
0011
0001
0000
0110
0111
0110
0100
1110
1111
1101
1100
1010
1011
1001
1000Slide26
64 QAM
Denser modulation can be used when symbol distortion is less in the channelSlide27
BPSK (binary phase shift keying)
Coarser modulation can be used when symbol distortion is hugeSlide28
Channel multipath
Tx
Rx
time
Amplitude
Channel Impulse Response
Channel Frequency Response
Slide29
Received data with channel multipath
Slide30
Deep channel fading
Slide31
Revisit the transmitted spectrum
F
-FSlide32
Increase the symbol duration
F
-F
Decreases the bandwidth of the signal
Symbol rate
(bandwidth)
F
-FSlide33
Modified signal passed through the channel
Data is unaffected since the fading frequencies do not overlap with data frequencies
Slide34
Coping with fading
Decrease bandwidth so that data frequencies do not overlap with fading frequencies
This helped eliminate the effect of fading
Disadvantage:
This would waste a lot of available bandwidthCan we do better to achieve throughput proportional to the channel quality, without wasting any bandwidthSlide35
F
-F
F
-F
F
-F
F
-F
F
-F
Coping with fading
Slide36
Coping with fading
x
x
x
x
F
-F
F
-F
F
-F
Slide37
F
-F
+
+
+
x
x
x
x
Coping with fading
Slide38
F
-F
x
x
x
x
+
+
+
Slide39
F
-F
+
+
Slide40
OFDM (Orthogonal frequency division multiplexing) transmission
IFFT
x
x
+
I = Real part
Q =
Imag
partSlide41
OFDM performance under deep fading
Slide42
x
x
OFDM reception
Slide43
OFDM vs Conventional
Robust to deep fading
Very efficient, achieves capacity limits, used widely in LTE/
WiFi
Robust to synchronization errorsRequires FFT/IFFT power intensiveHigh variation in signal amplitude – needs better h/w
Complete loss of performance under deep fading
Cannot reach maximum capacity
High synchronization overhead
Suitable for low power/battery-less communication
Low variation in signal amplitude