1. Modulation of Digital Data
Digital-to-Analog Conversion
Amplitude Shift Keying (ASK)
Frequency Shift Keying (FSK)
Phase Shift Keying (PSK)
Quadrature Amplitude Modulation (QAM)
Bit/Baud Comparison

2. Digital-to-analog modulation
Types of digital-to-analog modulation

3. Aspects to digital-to Analog conversion
Bit Rate / Baud Rate
Bit rate is the number of bits per second. Baud rate is the number of signal units per second. Baud rate is less than or equal to the bit rate.
Bit rate is important in computer efficiency
Baud rate is important in data transmission.
Baud rate determines the bandwidth required to send signal
Baud rate = bit rate / # bits per signal unit
An analog signal carries 4 bits in each signal unit. If 1000 signal units are sent per second, find the baud rate and the bit rate
Baud rate = 1000 bauds per second (baud/s) Bit rate = 1000 x 4 = 4000 bps
The bit rate of a signal is If each signal unit carries 6 bits, what is the baud rate?
Baud rate = 3000/6 =500 bauds/sec

4. Amplitude Shift Keying (ASK)
On/Off keying
The strength of the carrier signal is varied to represent binary 1 and 0.
Frequency and phase remains the same.
Highly susceptible to noise interference.
Used up to 1200 bps on voice grade lines, and on optical fiber.

5. Relationship between baud rate and bandwidth in ASK
BW = (1 + d) * Nd
Find the minimum bandwidth for an ASK signal transmitting at 2000 bps. The transmission mode is half-duplex.
In ASK the baud rate and bit rate are the same. The baud rate is therefore An ASK signal requires a minimum bandwidth equal to its baud rate. Therefore, the minimum bandwidth is 2000 Hz.

6. Full duplex ASK
Given a bandwidth of 10,000 Hz (1000 to 11,000 Hz), if draw the full-duplex ASK diagram of the system. We can find the carriers and the bandwidths in each direction. Assume there is no gap between the bands in the two directions.
For full-duplex ASK, the bandwidth for each direction is
BW = / 2 = 5000 Hz
The carrier frequencies can be chosen at the middle of each band
fc (forward) = /2 = 3500 Hz
fc (backward) = – 5000/2 = 8500 Hz

7. Frequency Shift Keying
Frequency of the carrier is varied to represent digital data (binary 0/1)
Peak amplitude and phase remain constant.
Avoid noise interference by looking at frequencies (change of a signal) and ignoring amplitudes.
Limitations of FSK is the physical capabilities of the carrier.
f1 and f2 equally offset by equal opposite amounts to the carrier freq.
In MFSK more than 2 freq are used, each signal element represents more than one bit

8. Relationship between baud rate and bandwidth in FSK
FSK shifts between two carrier frequencies
FSK spectrum = combination of two ASK spectra centered on fc1 and fc0.
BW = fc1-fc0 + Nbaud

9. FSK Examples (cont.)
What is the Find the minimum bandwidth for an FSK signal transmitting at 2000 bps. Transmission is in half-duplex mode, and the carriers are separated by 3000 Hz.
Because For FSK
BW = baud rate + fc1 - fc0
BW = bit rate + fc1 - fc0 = = 5000 Hz
What is the maximum bit rates for an FSK signal if the bandwidth of the medium is 12,000 Hz and the difference between the two carriers is 2000 Hz. Transmission is in full-duplex mode.
Because the transmission is full duplex, only 6000 Hz is allocated for each direction.
BW = baud rate + fc1 - fc0
Baud rate = BW - (fc1 - fc0 ) = = 4000
But because the baud rate is the same as the bit rate, the bit rate is 4000 bps.

10. Phase Shift Keying
Phase of the carrier is varied to represent digital data (binary 0 or 1)
Amplitude and frequency remains constant.
If phase 0 deg to represent 0, 180 deg to represent 1. (2-PSK)
PSK is not susceptible to noise degradation that affects ASK or bandwidth limitations of FSK

11. 4-PSK (QPSK) method

12. 8-PSK
We can extend, by varying the the signal by shifts of 45 deg (instead of 90 deg in 4-PSK)
With 8 = 23 different phases, each phase can represents 3 bits (tribit).

13. Relationship between baud rate and bandwidth in PSK
Bandwith similar to ASK, but data rate can 2 or more times greater.
Given a bandwidth of 5000 Hz for an 8-PSK signal, what are the baud rate and bit rate?
For PSK the baud rate is the same as the bandwidth, which means the baud rate is But in 8-PSK the bit rate is 3 times the baud rate, so the bit rate is 15,000 bps.

14. Quadrature Amplitude Modulation
PSK is limited by the ability of the equipment to distinguish between small differences in phases.
Limits the potential data rate.
Quadrature amplitude modulation is a combination of ASK and PSK so that a maximum contrast between each signal unit (bit, dibit, tribit, and so on) is achieved.
We can have x variations in phase and y variations of amplitude
x • y possible variation (greater data rates)
Numerous variations. (4-QAM, 8-QAM)
# of phase shifts > # of amplitude shifts

15. 8-QAM and 16-QAM Second example, recommendation of OSI.
not all possibilities are used, to increase
readability of signal, measurable differences
between shifts are increased
First example handles noise best
Because of ratio of phases to amplitudes
ITU-T recommendation.

16. Bit Baud comparison
Modulation
Units
Bits/Baud
Baud rate
Bit Rate
ASK, FSK, 2-PSK
Bit 1 N
4-PSK, 4-QAM
Dibit 2 2N
8-PSK, 8-QAM
Tribit 3 3N
16-QAM
Quadbit 4 4N
32-QAM
Pentabit 5 5N
64-QAM
Hexabit 6 6N
128-QAM
Septabit 7 7N
256-QAM
Octabit 8 8N
Assuming a FSK signal over voice-grade phone line can send 1200 bps, it requires 1200 signal units to send 1200 bits (each frequency shift represents one bit, baud rate 1200)
Assuming 8-QAM, baud rate is only 400 to achieve same data rate.

17. Bit Baud comparison (examples)
A constellation diagram consists of eight equally spaced points on a circle. If the bit rate is 4800 bps, what is the baud rate?
The constellation indicates 8-PSK with the points 45 degrees apart. Since 23 = 8, 3 bits are transmitted with each signal unit. Therefore, the baud rate is 4800 / 3 = 1600 baud
What is the bit rate for a 1000-baud 16-QAM signal.
A 16-QAM signal has 4 bits per signal unit since log216 = 4. Thus, (1000)(4) = 4000 bps
Compute the baud rate for a 72,000-bps 64-QAM signal.
A 64-QAM signal has 6 bits per signal unit since log2 64 = 6.
Therefore, / 6 = 12,000 baud

18. Thanks..