Carrier Modulation in
Digital Communication Systems
Xavier Fernando
Ryerson Communications Lab (RCL)
Why Carrier Modulation?
Until now we have been looking at
baseband communications
 The information is sampled, quantized
pulse coded and transmitted in baseband
 However, baseband transmission is not
suitable in many situations
 Carrier modulation is needed in these
cases
 Fe examples are listed in the next few
slides

Wireless Communications
Examples:
FM Radio: 88 – 108 MHz
WLAN – 2.4 or 5 GHz
Cellular Radio: 806-890 MHz
GPS: 1215 – 1240 MHz
The air-interface is shared by many different
users & services
 Each service has a certain allocated frequency
 Carrier modulation is needed to occupy only
the given spectrum

Digital Telephony/Cable Modem
Many of you may have Rogers Digital
Phone & Cable Modem
 The voice and internet data is modulated
on a carrier frequency (not overlapping
with TV Bands) and transmitted via cable
in addition TV Channels using QPSK or
16QAM modulation


TV Bands: 60-88 MHz, 180 – 216 MHz
and 476-890 MHz
Up Conversion
Carrier modulation
up converts the
signal to a suitable
band
Baseband 
Bandpass
Also note the
bandwidth
doubles
Frequency Division
Multiplexing (FDM)

Carrier Modulation enables sharing a common
channel by number of users/services