PHY layer Modulation Reference 2 5 2 from

PHY layer (Modulation) Reference: 2. 5. 2 from Computer Networks by Tenenbaum, Wetherall (uploaded on Canvas)

Communication Exchange of information from point A to point B 100001101010001011101 Transmit Receive 100001101010001011101

Wireless Communication Exchange of information from point A to point B without a wire 100001101010001011101 Transmit Receive 100001101010001011101

Wireless Communication Exchange of information from point A to point B: Modulation and Upconversion Key steps at transmitter Downconversion and Demondulation Key steps at receiver 100001101010001011101 Modulation Upconvert Downconvert Demodulation 100001101010001011101

Modulation • Converting bits to signals • These 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)

Amplitude Modulation • Suppose we have 4 voltage levels (symbols) to represent bits. 00 01 10 11 • Each voltage level would represent a pair of bits

Amplitude Modulation 1 0 0 1 1 0 1 0 0 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

Modulated symbols ready for transmission 1 0 0 1 1 0 1 0 0 0 1 1 1 0 FFT -F Frequency F

Received symbols with distortions due to noise 1 0 0 1 1 0 1 0 0 0 1 1 1 0 FFT -F Frequency F

Demodulation Tx bits 00 1 0 0 1 1 0 1 0 0 0 1 1 1 0 01 11 Rx bits decoded 10 1 0 0 0 1 1 1 1

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 follows

Modulation with sparser symbols to reduce bit flips 0 1 1 0 1 0 0 0 1 1 1 0

Received symbols with distortion 0 1 1 0 1 0 0 0 1 1 1 0

Demodulation 0 1 1 0 0 1 1 0 1 0 0 0 1 1 1 0

• 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 low

Bit rates 1 0 0 1 1 0 1 0 0 0 1 1 1 0 -F FFT Bandwidth (B) of a typical Wi. Fi channel is 20 MHz Frequency F

Upconversion: Transmission of modulated symbols on carrier We will send this signal on a carrier – Wi. Fi, 4 G, 5 G or any other choice

Upconversion: Transmission of modulated symbols on carrier A carrier wave is a sinusoidal function at a frequency. Frequency of Wi. Fi is around 2. 45 GHz.

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, Wi. Fi, Ethernet, 4 G etc)

Upconversion summary y(t) m(t) x

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 impractical • We need to convert the data back to baseband process the low frequency signals for decoding bits

Down-conversion bringing signal back to baseband This leaves us with m(t)

Upconversion and Downconversion summary m(t) x r(t) x

Upconversion and Downconversion summary I(t) x r(t) x

Beyond amplitude modulation • We have learnt communication with amplitude modulation • There is a simple idea to double the data rate • called QAM (quadrature amplitude modulation)

Quadrature amplitude modulation (QAM) • Achieves double data rate compared to amplitude modulation alone I(t) Sin and Cosine carrier waves x + Modulated messages Q(t) x Signal sent over communication link (E. g, Wi. Fi, Ethernet)

Demodulation: Recovering QAM message • Goal: Recover messages I and Q from the received signal on link. . x x

Demodulation: Recovering I •

Demodulation: Recovering Q •

Quadrature amplitude modulation: Summary • Achieves double data rate compared to amplitude modulation alone I(t) x x + Q(t) x x

Symbols with QAM

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

Symbols with QAM 0010 0011 0000 0111 0110 0100 1111 1100 1011 1000 This scheme uses 16 symbols (4 bits per symbol), hence called 16 QAM

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

BPSK (binary phase shift keying) Coarser modulation can be used when symbol distortion is huge

Amplitude Modulation

Frequency Modulation • Encode ‘ 0’s and ‘ 1’s by changing frequencies of transmitted signals. 0 1 0 1