Digital Modulation Techniques Lecture Notes

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. M-ary ASK

1. QAM


2. MSK

2. M-ary FSK

2. APK



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?


• 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.


• 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.


• When compared to BPSK and BASK, BFSK takes a large amount of bandwidth.

8. What are the advantages and disadvantages of BASK?


• BASK is simple to produce and detect.


• 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?


• 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.


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?


•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.


• 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) = 1/2๐‘‡๐‘ = ๐‘…๐‘/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?


• 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. 


• 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.

Sreejith Hrishikesan

Sreejith Hrishikesan is a ME post graduate and has been worked as an Assistant Professor in Electronics Department in KMP College of Engineering, Ernakulam. For Assignments and Projects, Whatsapp on 8289838099.

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