Part II. Physical Layer and Media

Chapter 5. Analog Transmission

COMP 3270 Computer NetworksComputing Science

Thompson Rivers University

Learning Objectives Given a bandwith and the number of signal elements to carry a bit,

determine the bit rate, for various types of ASK, FSK, PSK and QAM. Given a bit rate and a bandwith, design a QAM (or other conversion

type) scheme to meet the given requirements. List the advantages and disadvantages of AM and FM respectively.

1. DIGITAL-TO-ANALOG CONVERSION

Digital-to-analog conversion is the process of changing one of the characteristics of an analog signal based on the information in digital data.

• Aspects of Digital-to-Analog Conversion• Amplitude Shift Keying (ASK)• Frequency Shift Keying (FSK)• Phase Shift Keying (PSK)• Quadrature Amplitude Modulation (QAM)

Topics discussed in this section:

Aspects of DtA Conversion

Types of digital-to-analog conversion

Bit rate is the number of bits per second. Baud rate is the number of signal elements

per second.

In the analog transmission of digital data, the baud rate is generally less than or equal

to the bit rate.

Example:An analog signal carries 4 bits per signal element. If 1000 signal elements are sent per second, find the bit rate.

Solution:

Example:An analog signal has a bit rate of 8000 bps and a baud rate of 1000 baud. How many bits are carried by each signal element? How many different signal elements do we need?

Solution:

Amplitude Shift Keying (ASK)

Binary amplitude shift keying (BASK)

The minimum required bandwidth is equal to the baud rate: B = S.

☺ ☺ Advantages? - Simple☺ ☺ Disadvantages? - Susceptible to noise - ☺ ☺ Idle state?

Example:We have an available bandwidth of 100 kHz which spans from 200 to 300 kHz. What are the carrier frequency and the bit rate if we modulated our data by using ASK with d = 0?

Solution:

Example:In data communications, we normally use full-duplex links with communication in both directions. We need to divide the bandwidth into two with two carrier frequencies. The below figure shows the positions of two carrier frequencies and the bandwidths. The available bandwidth for each direction is now 50 kHz, which leaves us with a data rate of 50 kbps in each direction.

Multilevel ASK

More signal levels to modulate 2, 3, 4, or more bits at a time.

Frequency Shift Keying (FSK)

Binary frequency shift keying (BFSK)

☺ ☺ Advantages? - Strong to noise☺ ☺ Disadvantages? - More bandwidth is required

The minimum required bandwidth is equal to 2 times S.

B = 2S.

Example:We have an available bandwidth of 100 kHz which spans from 200 to 300 kHz. What should be the carrier frequency and the bit rate if we modulated our data by using FSK with d = 0?

Solution:

Binary phase shift keying (BPSK)

☺ ☺ Advantages? - Strong to noise - Smaller bandwidth than FSK☺ ☺ Disadvantages? - Difficult to manipulate

180 degree phase

Phase Shift Keying (PSK)

The minimum required bandwidth is equal to the baud rate: B = S.

Example:Find the bandwidth for a signal transmitting at 12 Mbps for QPSK (Quadrature PSK).

Solution:

2-PSK, 4-PSK, 8-PSK, …

Quadrature amplitude modulation (QAM) is a combination of ASK and PSK.

Quadrature Amplitude Modulation (QAM)

2-QAM, 4-QAM, 8-QAM, 16-QAM, …

The minimum bandwidth required for QAM is the same as that required for ASK and

PSK. QAM has the same advantages as PSK over

ASK.

Most analog transmissions use QAM.

• The idea of D-A• n-QAM

o The minimum required bandwidtho Bit rateo Baud rate

• Shannon capacity formula

Intermediate Summary

2. ANALOG TO ANALOG

Analog-to-analog conversion is the representation of analog information by an analog signal. One may ask why we need to modulate an analog signal; it is already analog. Modulation is needed if the medium is bandpass in nature or if only a bandpass channel is available to us.

• Amplitude Modulation (AM)• Frequency Modulation (FM)

Topics discussed in this section:

Types of analog-to-analog modulation

Amplitude Modulation (AM)

☺ ☺ Advantage?☺ ☺ Disadvantage?

Frequency Modulation

)

☺ ☺ Advantage?☺ ☺ Disadvantage?