communication systems, 5e - university of evansvillerichardson/courses/elcomm/ee471... ·...
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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 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 .
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.