Carrier modulation is needed for wireless communications, digital telephony/cable, and frequency division multiplexing (FDM) because: 1) It allows signals to be transmitted at radio frequencies suitable for their intended medium by upconverting baseband signals to a carrier frequency band. 2) In wireless communications, each service is allocated a certain frequency band to avoid interference through the use of carrier modulation. 3) Digital cable and telephone uses carrier modulation to transmit voice and data signals over cable infrastructure within allocated frequency bands without overlapping television channels.

1. Carrier Modulation in
Digital Communication Systems
Xavier Fernando
Ryerson Communications Lab (RCL)

2. 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

3. 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

4. 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

5. Up Conversion
Carrier modulation
up converts the
signal to a suitable
band
Baseband 
Bandpass
Also note the
bandwidth
doubles

6. Frequency Division
Multiplexing (FDM)
 Carrier Modulation enables sharing a common
channel by number of users/services