Tenenbaum Wetherall uploaded on Canvas Communication Exchange of information from point A to point B 100001101010001011101 100001101010001011101 Transmit Receive Wireless Communication ID: 780148
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Slide1
PHY layer (Modulation)Reference: 2.5.2 from Computer Networks by Tenenbaum, Wetherall (uploaded on Canvas)
Slide2CommunicationExchange of information from point A to point B
100001101010001011101
100001101010001011101
Transmit
Receive
Slide3Wireless CommunicationExchange of information from point A to point B without a wire
100001101010001011101
100001101010001011101
Receive
Transmit
Slide4Wireless CommunicationExchange of information from point A to point B:
Modulation and
Upconversion
Key steps at transmitterDownconversion
and Demondulation Key steps at receiver
100001101010001011101
100001101010001011101
Modulation
Upconvert
Downconvert
Demodulation
Slide5ModulationConverting bits to signalsThese signals are later sent over the air (wireless) or a cable (wired)
The receiver picks these signals and decodes transmitted data
100001101010001011101
Modulation
Signals (voltages)
Slide6Amplitude ModulationSuppose we have 4 voltage levels (symbols) to represent bits.
Each voltage level would represent a pair of bits
-
00
01
10
11
Slide7Amplitude 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 symbols
This example shows how bits are converted into signals in amplitude modulation
Slide8Modulated symbols ready 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
Frequency
Slide9Received symbols with distortions due to noise
1 0 0 0 0 1 1 0 1 0 1 0 0 0 1 0 1 1 1 0
F
-F
FFT
Frequency
Slide10Demodulation
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
The received symbols are mapped to closest
voltage levels among possible transmitted symbols
Coping up with demodulation errorsIf the noise is too high, there may be too many bit flipsSymbols 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 follows
Slide12Modulation with sparser symbols to reduce bit flips
1 0 0 0 0 1 1 0 1 0 1 0 0 0 1 0 1 1 1 0
0
1
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
Demodulation1 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
Slide15That eliminated all the bit flips, which is goodHowever, what is the disadvantage of choosing only two voltage levels?
Takes longer to transmit, hence bit rate is very low
Slide16Bit 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)
Frequency
Bandwidth (B) of a typical WiFi channel is 20 MHz
Slide17Upconversion: Transmission of modulated symbols on carrier
We will send this signal on a carrier – WiFi, 4G, 5G or any other choice
Slide18Upconversion: Transmission of modulated symbols on carrier
A carrier wave is a sinusoidal function at a frequency. Frequency of WiFi is around 2.45GHz.
Slide19Upconversion: Transmission of modulated symbols on carrier
Shown in Red is the “Amplitude modulated” message
Shown in Blue is the “Amplitude modulated carrier” sent over communication medium (
E.g
, WiFi, Ethernet, 4G
etc
)
Slide20Upconversion summary
x
m(t)
y(t)
m(t) is modulated message,
is the carrier signal
y(t) is the signal sent over communication link such as WiFi or Ethernet
Receiving: Down-conversion
Goal: Recover m(t) from y(t)
The receiver needs to perform an operation of down-conversion
The received signal is a high frequency signal (f can be multiple GHz)
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 bits
Down-conversion bringing signal back to baseband
Low pass filter used to eliminate the high frequency term above ---
This leaves us with m(t)
(after low pass filtering), the transmitted message is recovered from m(t)
Upconversion and Downconversion summary
x
m(t)
x
r(t)
Upconversion and Downconversion summary
x
I(t)
x
r(t)
Beyond amplitude modulationWe have learnt communication with amplitude modulationThere is a simple idea to double the data rate
called QAM (quadrature amplitude modulation)
Slide26Quadrature amplitude modulation (QAM)Achieves double data rate compared to amplitude modulation alone
I(t)
x
Q(t)
x
+
Modulated messages
Sin and Cosine carrier waves
Signal sent over communication link (
E.g
, WiFi, Ethernet)
Slide27Demodulation: Recovering QAM messageGoal: Recover messages I and Q from the received signal on link ..
x
x
Demodulation: Recovering I
Received signal on link is
Multiply
Low pass filter used to eliminate the high frequency term above ---
After this, we are left with
, now the modulated message on I(t) can be recovered
Demodulation: Recovering Q
Received signal on link is
Multiply
Low pass filter used to eliminate the high frequency term above ---
After this, we are left with
, now the modulated message on Q(t) can be recovered
Thus, we can recover messages modulated on both I and Q
Quadrature amplitude modulation: SummaryAchieves double data rate compared to amplitude modulation alone
I(t)
x
Q(t)
x
+
x
x
Symbols with QAM
Slide32Each QAM symbol uses two sets of voltages – I and QThus, we represent each symbol as a 2-dimensional element (I,Q)
Symbols with QAM
Slide33Symbols 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
1000
Slide3464 QAM
Denser modulation can be used when symbol distortion is less in the channel
Slide35BPSK (binary phase shift keying)
Coarser modulation can be used when symbol distortion is huge
Slide36Amplitude Modulation
Slide37Frequency ModulationEncode ‘0’s and ‘1’s by changing frequencies of transmitted signals.
0
1
0
1