Physical layer concepts

Distorsion

Effect of attenuation, distortion, and noise on transmitted signal.

Point-to-point communication

Mikrofon Högtalare

Source coding Source decodingDigitalizatingcompression

0110 0110

Error management Error control.

0100010

Bit error

0110010

Flow control Flow control

Modulation Demodulation

0110010NACKACK

Layer6

Layer2

Layer1

Layer7

Digital modulation methods

Binary signal

ASK = Amplitude Shift Keying (AM)

FSK = Frequency Shift Keying (FM)

PSK = Phase Shift Keying (PSK)

0 0.005 0.01-2

0

2000

0 0.005 0.01-2

0

2001

0 0.005 0.01-2

0

2011

0 0.005 0.01-2

0

2010

0 0.005 0.01-2

0

2100

0 0.005 0.01-2

0

2101

0 0.005 0.01-2

0

2111

0 0.005 0.01-2

0

2110

8QAM example:Below you find eight symbols used for a so called 8QAM modem (QAM=Quadrature Amplitude Modulation). The symbols in the first row represent the messages 000, 001, 011 and 010 respectively (from left to right). The second row representents 100, 101, 111 and 110.

a) The signal below is transmitted from the modulator. What bit sequency is transmitted?

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04-2

0

2

Tid [sekunder]

Sp

än

nin

g [V

olt]

Modulatorns utsignal

b) The time axis is graded in seconds. What is the symbol rate in baud or symbols/s?

c) What is the bit rate in bit/s?

Example 2 cont.

Bit rate vs baud rate

Bit rate in bit/s:

Where M is the number of symbols and fs is the symbol rate in baud or symbols/s.

2logb Sf f M

Bit and baud rate comparison

ModulationModulation UnitsUnits BitsBits/symbol/symbol Baud rateBaud rate Bit Rate

ASK, FSK, 2-PSKASK, FSK, 2-PSK Bit 1 N N

4-PSK, 4-QAM4-PSK, 4-QAM Dibit 2 N 2N

8-PSK, 8-QAM8-PSK, 8-QAM Tribit 3 N 3N

16-QAM16-QAM Quadbit 4 N 4N

32-QAM32-QAM Pentabit 5 N 5N

64-QAM64-QAM Hexabit 6 N 6N

128-QAM128-QAM Septabit 7 N 7N

256-QAM256-QAM Octabit 8 N 8N

Sine wave example

I

5 Volt

л/2 radians = 90º

Complex representation

Figure 5.14 The 4-QAM and 8-QAM constellations

Q (Quadrature phase)

I (Inphase)

Q (Quadrature phase)

I (Inphase)

Figure 5.11 The 4-PSK characteristics

Figure 5.12 The 8-PSK characteristics

Figure 5.16 16-QAM constellations

Figure 5.20 The V.32 constellation and bandwidth

Spectrum of ASK, PSK and QAM signal

Figure 9.3 ADSL modem

Figure 9.4 DSLAM

Other DSL Technologies

• SDSL (Symmetric DSL) divides frequencies evenly

• HDSL (High-rate DSL) provides DS1 bit rate in both directions Short distances

Four wires

• VDSL (Very high bit rate DSL) provides up to 52 Mbps Very short distance

Requires Optical Network Unit (ONU) as a relay