UNIT-3
DIGITAL COMMUNICATION
DISCUSSION OUTLINE
• Advantages , Drawbacks and Applications of
ASK, FSK, PSK and QAM techniques.
• Comparison of all modulation techniques.
DIGITAL MODULATION
ASK
• Advantage - Simplicity in design, inexpensive.
- It offers high bandwidth efficiency.
- It has simple receiver design.
• Drawbacks - very susceptible to noise interference.
- It offers low power efficiency.
• Applications – used to transmit digital data over
optical fiber.
FSK
• Advantage – Higher immunity to noise.
- It provides high SNR.
- Low BER.
- Simple to design.
• Drawbacks – It requires more BW.
• Applications – Over voice lines, in high frequency
radio transmission.
PSK
• Advantage – Less susceptible to noise.
- Higher data rate transmission.
• Drawbacks – It has lower bandwidth efficiency.
• Applications – Used in WLAN IEEE 802.11b(1999)
BPSK in RFID
QPSK in satellite broadcasting, DVB,
in Zigbee IEEE 802.15.4, in HD and SD channels.
QAM
• Advantages- More efficient use of BW.
- Increase the efficiency of transmission.
• Drawbacks - More susceptible to noise.
-QAM receiver is more complex.
-Linear amplifier is needed which consumes
more power.
• Applications – Data communication, Wireless
communication ,DVB, Cable modem.
Waveform Comparison
CONSTELLATION DIAGRAM
Comparison of various Digital modulation
schemes
Comparison of various Digital
modulation schemes
Comparison of various Digital modulation schemes
DM
ASK
DAM
ASK-Output
ASK
FSK
FSK
FSK
BPSK
BPSK
BPSK
Applications
• The BPSK modulation is a very basic technique used
in various wireless standards such as CDMA,
WiMAX (16d, 16e), WLAN 11a, 11b, 11g, 11n,
Satellite, DVB, Cable modem etc.
• It is considered to be more robust among all the
modulation types due to difference of 180 degree
between two constellation points.
QPSK
QPSK
QPSK
QPSK
Advantages
• QPSK provide very good noise immunity
• It provides low error probability
• Bandwidth is twice efficient is compared to BPSK
modulation
• For the same BER, the bandwidth required by QPSK
is reduced to half as compared to BPSK
• It is more efficient utilization of the available
bandwidth of the transmission channel
• Carrier power almost remains constant because of
OQPSK amplitude is not much
• Disadvantages of QPSK:
• QPSK is not more power efficient modulation
technique compare to other modulation types as more
power is required to transmit two bits.
• QPSK is more complex compared to BPSK receiver
due to four states needed to recover binary data
information.
Applications
• QPSK allows the signal to carry twice as much
information as ordinary PSK using the same
bandwidth.
• QPSK is used for satellite transmission of MPEG2
video, cable modems, videoconferencing, cellular
phone systems, and other forms of digital
communication over an RF carrier.
• QPSK (Quadrature Phase Shift Keying) scheme is
used in various applications in modern digital
communication system; it provides high performance
on bandwidth efficiency and bit error rate. The areas
include wireless communication, mobile
communication and Satellite Communication.
QPSK
• Why preferred in Wireless Communication?
• Compared to modulation schemes that transmit one
bit per symbol, QPSK is advantageous in terms of
bandwidth efficiency.
• A QPSK system can use a baseband signal of the
same frequency, yet it transmits two bits during each
symbol period. Thus, its bandwidth efficiency is
(ideally) higher by a factor of two.
