Digital Modulation Techniques Lecture Notes
1. What do
you understand about Digital Modulation.
The mapping
of a series of input binary digits into a set of corresponding high-frequency
signal waveforms is referred to as digital modulation.
2. Write
down the different types of digital modulation techniques.
I. Based
on the method of detection, there are mainly 2 types of digital modulation
techniques:
1. Coherent
digital modulation
2.
Non-coherent digital modulation
II. Based
on the mapping techniques, the different techniques are:
|
Binary Scheme |
Quaternary Scheme |
M-ary Scheme |
Hybrid Scheme |
|
1. BASK |
1. QPSK |
1. M-ary ASK |
1. QAM |
|
2. BFSK |
2. MSK |
2. M-ary FSK |
2. APK |
|
3. BPSK |
|
3. M-ary PSK |
|
III. Based
on the performance of the modulation scheme and properties of the modulated
signal, the different techniques are:
1. Power
efficient scheme / Bandwidth efficient scheme.
2. Continuous
Phase (CP) Modulation / In phase – Quadrature Phase (IQ) Modulation.
3. Constant
Envelope Modulation / Non-constant Envelope Modulation.
4. Linear
Modulation / Non-linear Modulation.
5. Modulation
scheme with memory / Modulation scheme without memory.
3. Describe
the digital communication systems design objectives.
The objectives of digital communication systems design are:
1. Maximum
data rate
2. Minimum
possibility of symbol error.
3. Minimum
transmitted power.
4. Minimum
channel bandwidth.
5. Maximum
resistance to interfering signals.
6. Minimum
circuit complexity.
4. What do
you know about the coherent binary modulation technique?
There are
three basic types of binary modulation schemes. They are:
1. Binary
Amplitude Shift Keying (BASK)
2. Binary
Frequency Shift Keying (BFSK)
3. Binary
Phase Shift Keying (BFSK)
Coherent
binary modulation techniques are defined as binary modulation systems that use
coherent detection at the receiver.
The local
carrier generated at the receiver is phase synchronized with the carrier at the
transmitter incoherent detection.
5. Define
ASK, FSK and PSK
ASK: The
modulation process in Amplitude Shift Keying (ASK) includes switching or keying
the amplitude of the carrier signal following the incoming data.
FSK: The
modulation process in Frequency Shift Keying (FSK) involves switching or keying
the carrier signal's frequency following the incoming data.
PSK: The
modulation process in Phase Shift Keying (PSK) includes switching or keying the
phase of the carrier signal following the incoming data.
6. Mention
the merits and demerits of BPSK?
Merits
• BPSK
consumes less bandwidth than BFSK, and it has the lowest likelihood of a mistake.
As a result, when compared to BFSK and BASK systems, it delivers great
outcomes.
• It has a
significant level of noise immunity.
Demerits
• Since the
information in PSK is maintained in the phase, it cannot be detected
non-coherently.
7. What are the advantages and disadvantages of BFSK?
Advantages :
• It is simple to implement.
• It has more noise resistant
than ASK.
Disadvantages:
• When compared to BPSK and BASK,
BFSK takes a large amount of bandwidth.
8. What are the advantages and disadvantages of BASK?
Advantage
• BASK is simple to produce and
detect.
Disadvantage:
• It is highly sensitive to noise.
9. What is meant by the Non-coherent binary modulation
technique?
Non-coherent modulation refers to a modulation technique in
which the detection process does not need the receiver carrier to be phase synchronized
with the transmitter carrier. Non-coherent binary modulation techniques are:
1. Differential Phase Shift Keying (DPSK)
2. Binary Amplitude Shift Keying (BASK)
3. Binary Frequency Shift Keying (BFSK)
10. Define DPSK?
Differential Phase Shift Keying (DPSK) is a "Pseudo
PSK" technology that is essentially a non-coherent version of BPSK. By
merging two fundamental procedures at the transmitter, it eliminates the need
for a coherent reference signal at the receiver. They are namely:
1. Differential encoding of the input binary wave
2. Phase shift keying
11. List down the merits and
demerits of DPSK?
Advantages
• At the receiver end, the DPSK
system does not require a carrier. As a result, it has simplified the system.
• The necessary bandwidth is lower
than for BPSK.
Disadvantages
In comparison to BPSK, it has a
larger likelihood of a mistake.
• There's a lot more noise.
• Previous bit detection is utilized
in DPSK to identify the following bit. As a result, mistakes in pairings are
possible.
12. What do you know about the coherent quadrature modulation
technique?
Quadrature modulation is an M-ary modulation technique with
m=4. The coherent quadrature modulation approach refers to a quadrature
modulation system in which coherent detection is used at the receiver. Two
quadrature modulation methods that save bandwidth are available. They are
1. QuadriPhase Shift Keying (QPSK)
2. Minimum Shift Keying (MSK)
13. Define QPSK
In QPSK, as in BPSK, the phase contains the information
delivered by the transmitted signal. The bits per symbol in QPSK is k = 2,
hence m = 2k = 22 = 4. As a result, two quadrature
carriers are modulated by two consecutive bits (dibit) in the data sequence.
14. What do
you understand about signal constellation?
The signal
space diagram is used to analyze any modulation scheme. A map of probable
message points is used in the signal space technique. A signal constellation
refers to a collection of probable message points.
15. Mention
the merits and demerits of QPSK?
Advantage:
•QPSK provides a high level of noise immunity.
• It makes better use of the transmission channel's available bandwidth
and has a reduced mistake probability.
Drawback:
• The creation and detection of QPSK are both difficult.
16. Justify
Minimum Shift Keying (MSK)?
MSK (Minimum
Shift Keying) is a type of binary CPFSK signal. MSK refers to a Continuous
Phase Frequency Shift Keying (CPFSK) signal with a deviation ratio of h = 1/2.
17. Define
Deviation Ratio.
The deviation
ratio is defined as the difference between the frequencies f1 and f2, normalized
about the bit rate 1/Tb. The deviation ratio h = 1/2 for MSK.
18. Explain Minimum Shift Keying (MSK)
The deviation
ratio is h = Tb (f1 – f2)
For MSK, we
have h = 1/2. Therefore,
1/2 = Tb
(f1 – f2)
=> (f1
– f2) = 1/2𝑇𝑏 = 𝑅𝑏/2
Where Rb
=> bit rate
Hence, the
bit rate is half the frequency deviation (f1 – f2). This
is the smallest frequency spacing that permits the two FSK signals representing
symbols 1 and 0 to be coherently orthogonal in the sense of not interfering
with one another during detection. Minimum Shift Keying is the name given to a
CPFSK signal with a deviation ratio of half (MSK).
19. Discuss the merits and demerits of MSK?
Advantages
• The phase of the MSK signal is
constant.
• There is no need for sidelobe
suppression filters that generate interchannel interference.
• There is no need for sidelobe
suppression filters, which produce interchannel interference.
Disadvantages
• MSK signal production and detection
are more difficult.
• The neighboring channel
interference in a wireless communication system utilizing MSK is insufficient
to meet the practical needs of a multiuser communications environment.
20. Mention the two basic steps in a digital receiver?
Explain
1) Demodulation and
2) Detection is the two basic phases of a digital receiver.
1. Demodulator:
The frequency down-conversion block is the demodulator. The
signal demodulator's purpose is to transform the received waveform X(t) into an
N-dimensional vector X = [X1, X2,..... XN], where N is the sent signal
waveforms' dimension.
2. Detector
The detector's task was to find out which of the M possible
signal waveforms was sent depending on Vector X. The optimum detector minimizes
the probability of error.
21. Discuss coherent and non-coherent detection in a digital
receiver?
Coherent Detection
The local carrier generated at the receiver is phase
synchronized with the carrier generated at the transmitter in coherent
detection. The receiver is aware of the exact moments when the modulation
changes state. The synchronous detection scheme is coherent detection.
Non-Coherent detection
The detection process in non-coherent detection does not need
the receiver carrier to be phase synchronized with the transmitter carrier.
Between the transmitter and receiver, there is no time and phase synchronization.
As a result, non-coherent detection is an asynchronous scheme.
22. Illustrate about sampled matched filter
The impulse response of a matched filter is a delayed version
of the input signal waveform's mirror image (rotated on the t = 0 axis). As a
result, the impulse response h(t) of a filter that is matched to s(t) is
A sampled matched filter uses digital techniques and sampled waveforms to implement a matched filter.
23. What do you know about Correlator.
A correlator's primary function is to integrate the received
noisy signal with each of the reference carrier signals. The received signal is
divided into N-dimensional vectors (X1, X2, .... XN).
24. Define Time Division Multiplexing (TDM)
TDM involves sampling a collection of signals sequentially in
time at a common sampling rate and multiplexing them for transmission over a
single channel. TDM is a digital management information system. This allows us
to combine many digital signals into a single data stream with a higher bit
rate, such as computer outputs, digitized voice signals, digitized facsimile
signals, and television signals.
25. List the different types of TDM?
TDM is classified into two types. They are
1. Synchronous TDM
2) Asynchronous TDM
1. Synchronous or Deterministic TDM:
Every individual communication channel has a consistent
latency and bandwidth. Time slots are employed in a synchronous periodical way
and have a fixed length (capacity). ISDN, PDH, and SDH are all approaches that
utilize it.
2. Asynchronous or statistical TDM:
Each individual communication channel has a changeable
latency and bandwidth. Time slots come in a variety of lengths and are utilized
as needed. It is used in protocols such as X25, Frame Relay, ATM, and IP.
26. What is ASCII Framing?
In the ASCII frame for TDM, the American Standard Code for
Information Interchange (ASCII) codes in hexadecimal notation are used.
Character-oriented protocols are used to organize ASCII packets.
27. Write short notes on EI framing.
• The EI carrier-based framing is a European hierarchy.
• There are a total of 30 PCM encoded speech data channels.
• The data rate on the transmission line is 2.048 Mbps.
• It operates over coaxial cable and uses the A-law algorithm
for companding.
28. Write short notes on TI framing
• A North American hierarchy of TI framing is based on TI
carriers.
• There are a total of 24 PCM encoded voice data channels.
• It employs the -law algorithm for companding and operates
over shielded twisted pair wires with a data rate of 1.544 Mbps.