Communication Systems, 5e

Chapter 7: Analog Communication Systems

A. Bruce CarlsonPaul B. Crilly

Pop Quiz

1) PAM is often used in single channel communication

systems.

a) TRUE b) FALSE

2) PAM has greater transmission bandwidth requirements

than either PWM or PPM.

a) TRUE b) FALSE

Pop Quiz

1) PAM is often used in single channel communication

systems.

a) TRUE b) FALSE

2) PAM has greater transmission bandwidth requirements

than either PWM or PPM.

a) TRUE b) FALSE

Chapter 7: Analog Communication Systems

Receivers for CW modulation

Multiplexing systems

Phase locked loops

Television systems

Multiplexing systems

Ordinarily a means to have several users share a channel (multiple access).

A means of diversity (message redundancy) to improve signal reliability.

Multiplexing Methods

Frequency division multiplexing (FDM)

Time division multiplexing (TDM)

Quadrature-carrier multiplexing or quadrature amplitude modulation (QAM)

Code division multiplexing (see Chap. 15)

Spatial multiplexing Antenna direction

Signal polarization

Time Division Multiplexing (TDM) and Time Division Multiple Access (TDMA)

Sample different waveforms and interleave them in

time so they appear to be sent simultaneously

Guard times to prevent intersymbol interference

(ISI)

Time diversity

TDM system (a) block diagram (b) waveforms

Guard time

TDM System with Electronic Commutation

TDM Equations

A particular signal is sampled at f S≥2W⇒T S≤1

2WWith TDM and M signals⇒channel data rate⇒ r=M f S and r=2MW

To reduce intersymbol interference (ISI), we haveguard time between message bits.

Each set of M message bits makes up a frame .

TDM synchronization markers

(a) TDM transmitter with baseband filtering (b) baseband waveform

Telemetry System with main and sub-multiplexers

Crosstalk

T g=guard time

A channel acting like a low-pass filter will distort pulsescausing pulse overlap or interchannel crosstalk.

Crosstalk

Defining the crosstalk reduction factor as

k ct=10 log Act / A2

then, with Act=Ae−2 BT g

k ct=−54.5BT g dB

TDM and FDM Comparison

Many systems such as wireless phones are a hybrid of FDM and

TDM or FDM and CDM

TDM lends itself to digital systems and low cost VLSI

implementation

With submultiplexers, TDM can more easily accommodate different

types of signals

Wideband fading ⇒ may only affect some TDM pulses, but may

affect all FDM channels. Narrowband fading has the opposite

affect.

In Class Exercises

Problem 7.2-11

Problem 7.2-15