PHY layer (Modulation) Reference: 2.5.2 from Computer Networks by - PowerPoint Presentation

Slide1

PHY layer (Modulation)Reference: 2.5.2 from Computer Networks by Tenenbaum, Wetherall (uploaded on Canvas)

Slide2

CommunicationExchange of information from point A to point B

100001101010001011101

100001101010001011101

Transmit

Receive

Slide3

Wireless CommunicationExchange of information from point A to point B without a wire

100001101010001011101

100001101010001011101

Receive

Transmit

Slide4

Wireless 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

Slide5

ModulationConverting 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)

Slide6

Amplitude ModulationSuppose we have 4 voltage levels (symbols) to represent bits.

Each voltage level would represent a pair of bits

 

-

 

 

 

00

01

10

11

Slide7

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 symbols

This example shows how bits are converted into signals in amplitude modulation

Slide8

Modulated 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

Slide9

Received 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

Slide10

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

The received symbols are mapped to closest

voltage levels among possible transmitted symbols

 

Slide11

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

Slide12

Modulation 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

 

 

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

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

Slide15

That 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

Slide16

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)

 

Frequency

Bandwidth (B) of a typical WiFi channel is 20 MHz

Slide17

Upconversion: Transmission of modulated symbols on carrier

 

 

 

 

We will send this signal on a carrier – WiFi, 4G, 5G or any other choice

Slide18

Upconversion: Transmission of modulated symbols on carrier

A carrier wave is a sinusoidal function at a frequency. Frequency of WiFi is around 2.45GHz.

Slide19

Upconversion: 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

)

Slide20

Upconversion 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

 

Slide21

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

 

Slide22

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)

 

Slide23

Upconversion and Downconversion summary

x

m(t)

 

x

r(t)

 

 

Slide24

Upconversion and Downconversion summary

x

I(t)

 

x

r(t)

 

 

Slide25

Beyond amplitude modulationWe have learnt communication with amplitude modulationThere is a simple idea to double the data rate

called QAM (quadrature amplitude modulation)

Slide26

Quadrature 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)

Slide27

Demodulation: Recovering QAM messageGoal: Recover messages I and Q from the received signal on link ..

 

x

 

x

 

 

 

Slide28

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

 

 

Slide29

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

 

 

Slide30

Quadrature amplitude modulation: SummaryAchieves double data rate compared to amplitude modulation alone

I(t)

x

 

Q(t)

x

 

+

 

x

 

x

 

 

 

Slide31

Symbols with QAM

Slide32

Each QAM symbol uses two sets of voltages – I and QThus, we represent each symbol as a 2-dimensional element (I,Q)

Symbols with QAM

Slide33

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

1000

Slide34

64 QAM

Denser modulation can be used when symbol distortion is less in the channel

Slide35

BPSK (binary phase shift keying)

Coarser modulation can be used when symbol distortion is huge

Slide36

Amplitude Modulation

Slide37

Frequency ModulationEncode ‘0’s and ‘1’s by changing frequencies of transmitted signals.

0

1

0

1