EEE Recruitment Exam: Communication & Telecommunication (Theory) (Part-1)
EEE Job Preparation: Communication & Telecommunication (Theory)
Preparing for EEE recruitment exams in Bangladesh requires a solid grasp of both mathematical problems and theoretical concepts. Among the various subjects, Communication and Telecommunication often carry significant weight in the written exams conducted by BUET, MIST, or the departmental boards of BPDB, PGCB, NESCO, and BREB.
In this Part 1 of our series, I have compiled and solved 18 essential theory questions that frequently appear in Assistant Engineer (AE) and Sub-Assistant Engineer (SAE) exams. From the fundamentals of the Nyquist Theorem to the complexities of 5G standards and Optical Fiber networks, these answers are structured to help you provide concise, high-quality responses that can secure full marks.
1. Nyquist Sampling Theorem
Question: State Nyquist sampling theorem. Why is it necessary to follow Nyquist theorem during analog to digital conversion of a signal? Explain with necessary figures?
Answer:
Statement: The Nyquist Sampling Theorem states that a continuous-time signal can be completely represented in its samples and recovered back if the sampling frequency (fs) is at least twice the highest frequency component (fm) present in the signal:
Necessity: In Analog to Digital Conversion (ADC), if we sample at a rate lower than 2fm, high-frequency components overlap with low-frequency ones. This distortion is called Aliasing. Following the theorem ensures that the original signal can be perfectly reconstructed without loss of data.
Question: State Nyquist sampling theorem. Why is it necessary to follow Nyquist theorem during analog to digital conversion of a signal? Explain with necessary figures?
Answer:
Statement: The Nyquist Sampling Theorem states that a continuous-time signal can be completely represented in its samples and recovered back if the sampling frequency (fs) is at least twice the highest frequency component (fm) present in the signal:
Necessity: In Analog to Digital Conversion (ADC), if we sample at a rate lower than 2fm, high-frequency components overlap with low-frequency ones. This distortion is called Aliasing. Following the theorem ensures that the original signal can be perfectly reconstructed without loss of data.
2. FM Signal Analysis (Math & Spectrum)
Question: A message signal with a frequency of 5 kHz modulates a carrier of 88 MHz to produce FM signal with a modulation index of 1. Sketch the spectrum, and find the bandwidth and spectral power density of the FM signal?
Answer:
Question: A message signal with a frequency of 5 kHz modulates a carrier of 88 MHz to produce FM signal with a modulation index of 1. Sketch the spectrum, and find the bandwidth and spectral power density of the FM signal?
Answer:
3. Noise in Telecommunication
Question: What is noise? Explain the characteristics of various types of noise created in telecommunication system?
Answer: Noise is any unwanted electrical signal that interferes with the transmitted information.
Thermal Noise: Caused by the random motion of electrons in a conductor; it exists at all temperatures above absolute zero.
Shot Noise: Arises due to the discrete nature of electron flow in electronic devices like diodes and transistors.
Crosstalk: Occurs when a signal from one transmission path leaks into another nearby path via electromagnetic coupling.
Impulse Noise: Sudden, high-amplitude bursts caused by lightning, industrial switching, or power lines.
Question: What is noise? Explain the characteristics of various types of noise created in telecommunication system?
Answer: Noise is any unwanted electrical signal that interferes with the transmitted information.
Thermal Noise: Caused by the random motion of electrons in a conductor; it exists at all temperatures above absolute zero.
Shot Noise: Arises due to the discrete nature of electron flow in electronic devices like diodes and transistors.
Crosstalk: Occurs when a signal from one transmission path leaks into another nearby path via electromagnetic coupling.
Impulse Noise: Sudden, high-amplitude bursts caused by lightning, industrial switching, or power lines.
4. Hand-off Margin & Call Drop Prevention
Question: What is hand-off margin? How proper hand-off can be made to prevent call drop? Explain with necessary figures?
Answer:
Hand-off Margin: This is the difference in signal strength required between a new base station and the current one to trigger a hand-off.
Prevention of Call Drops:
Hysteresis: A margin is set so the mobile doesn't rapidly switch between two towers (Ping-pong effect).
Soft Hand-off: A "make-before-break" approach where the phone connects to a new cell before dropping the old one.
Guard Channels: Reserving a small portion of the total channels exclusively for hand-off calls.
Question: What is hand-off margin? How proper hand-off can be made to prevent call drop? Explain with necessary figures?
Answer:
Hand-off Margin: This is the difference in signal strength required between a new base station and the current one to trigger a hand-off.
Prevention of Call Drops:
Hysteresis: A margin is set so the mobile doesn't rapidly switch between two towers (Ping-pong effect).
Soft Hand-off: A "make-before-break" approach where the phone connects to a new cell before dropping the old one.
Guard Channels: Reserving a small portion of the total channels exclusively for hand-off calls.
5. Multiplexing and TDM
Question: What is multiplexing? Explain the principles of Time Division Multiplexing (TDM) with a sketch to show how the interleaving of channels takes place?
Answer:
Multiplexing: The process of combining multiple signals into one composite signal to share a single communication channel
- Principles of TDM
- Time Slot Allocation: The overall bandwidth of the shared channel is divided into recurring time slots, which are assigned to specific input signals.
- Interleaving: The multiplexer (MUX) gathers small segments (bits or bytes) from each channel sequentially to create a single high-speed composite signal.
- Synchronization: The demultiplexer (DEMUX) at the receiving end must be perfectly synchronized with the MUX to ensure each piece of data is routed to the correct destination.
- Framing: A single TDM frame contains one or more slots dedicated to each input source.
- Fixed Rotation: The multiplexer works like a fast-rotating switch (commutator), cycling through inputs in a constant order.
TDM principle
Question: What is multiplexing? Explain the principles of Time Division Multiplexing (TDM) with a sketch to show how the interleaving of channels takes place?
Answer:
Multiplexing: The process of combining multiple signals into one composite signal to share a single communication channel
- Principles of TDM
- Time Slot Allocation: The overall bandwidth of the shared channel is divided into recurring time slots, which are assigned to specific input signals.
- Interleaving: The multiplexer (MUX) gathers small segments (bits or bytes) from each channel sequentially to create a single high-speed composite signal.
- Synchronization: The demultiplexer (DEMUX) at the receiving end must be perfectly synchronized with the MUX to ensure each piece of data is routed to the correct destination.
- Framing: A single TDM frame contains one or more slots dedicated to each input source.
- Fixed Rotation: The multiplexer works like a fast-rotating switch (commutator), cycling through inputs in a constant order.
TDM principle
6. Mobile Radio vs. Cellular Systems
Question: What is meant by mobile communication? Distinguish between mobile radio and cellular mobile radio system.
Answer:
Mobile Communication: Communication where the sender or receiver can move while the link remains active.
Comparison:
Mobile Radio: Uses a single, high-power transmitter to cover a large area. It has limited capacity and no frequency reuse.
Cellular System: Divides the area into small "Cells," each with its own low-power transmitter. This allows for Frequency Reuse, massively increasing capacity.
Question: What is meant by mobile communication? Distinguish between mobile radio and cellular mobile radio system.
Answer:
Mobile Communication: Communication where the sender or receiver can move while the link remains active.
Comparison:
Mobile Radio: Uses a single, high-power transmitter to cover a large area. It has limited capacity and no frequency reuse.
Cellular System: Divides the area into small "Cells," each with its own low-power transmitter. This allows for Frequency Reuse, massively increasing capacity.
7. GSM Technology
Question: Draw mobile communication system diagram using GSM technology and explain its operation?
Answer:
GSM (Global System for Mobile Communications): A digital system using TDMA/FDMA.
Subsystems:
Mobile Station (MS): The handset and SIM.
Base Station Subsystem (BSS): Consists of BTS (Antenna) and BSC (Controller).
Network Switching Subsystem (NSS): Contains the MSC (Switch), HLR (Home database), and VLR (Visitor database).
Question: Draw mobile communication system diagram using GSM technology and explain its operation?
Answer:
GSM (Global System for Mobile Communications): A digital system using TDMA/FDMA.
Subsystems:
Mobile Station (MS): The handset and SIM.
Base Station Subsystem (BSS): Consists of BTS (Antenna) and BSC (Controller).
Network Switching Subsystem (NSS): Contains the MSC (Switch), HLR (Home database), and VLR (Visitor database).
8. Satellite Communication
Question: Draw satellite communication system diagram and explain its working principle.
Answer:
Working Principle: A satellite acts as a microwave repeater. It receives an Uplink signal from Earth, its Transponder amplifies and shifts the frequency, and then it retransmits the Downlink signal back to Earth.
Question: Draw satellite communication system diagram and explain its working principle.
Answer:
Working Principle: A satellite acts as a microwave repeater. It receives an Uplink signal from Earth, its Transponder amplifies and shifts the frequency, and then it retransmits the Downlink signal back to Earth.
9. CCTV System Operation
Question: Draw and explain the operation of a CCTV system.
Answer:
Operation: The camera lens focuses light onto a CCD/CMOS sensor $\rightarrow$ the sensor converts light to electrical signals $\rightarrow$ signals are transmitted via cables/Wi-Fi to a DVR/NVR $\rightarrow$ the recorder compresses the video and displays it on a monitor.
Question: Draw and explain the operation of a CCTV system.
Answer:
Operation: The camera lens focuses light onto a CCD/CMOS sensor $\rightarrow$ the sensor converts light to electrical signals $\rightarrow$ signals are transmitted via cables/Wi-Fi to a DVR/NVR $\rightarrow$ the recorder compresses the video and displays it on a monitor.
10. Optical Fiber Communication (OFC)
Question: Explain the features and advantages of the optical fiber communication system?
Answer:
Features: Uses light to transmit data through glass/plastic fiber via Total Internal Reflection (TIR).
Advantages:
High Bandwidth: Can carry terabits of data.
EMI Immunity: It is not affected by lightning or electrical noise.
Low Loss: Requires fewer repeaters than copper.
Security: Hard to tap without signal loss detection.
Question: Explain the features and advantages of the optical fiber communication system?
Answer:
Features: Uses light to transmit data through glass/plastic fiber via Total Internal Reflection (TIR).
Advantages:
High Bandwidth: Can carry terabits of data.
EMI Immunity: It is not affected by lightning or electrical noise.
Low Loss: Requires fewer repeaters than copper.
Security: Hard to tap without signal loss detection.
11. Modulation & Demodulation Circuits
Question: Explain with necessary circuit diagram modulation and demodulation used in signal transmission.
Answer:
Modulation: A Square Law Modulator (using a diode and Band Pass Filter) is commonly used for AM.
Demodulation: An Envelope Detector (Diode + RC Low Pass Filter) is used to recover the message by tracing the peaks of the AM wave.
Question: Explain with necessary circuit diagram modulation and demodulation used in signal transmission.
Answer:
Modulation: A Square Law Modulator (using a diode and Band Pass Filter) is commonly used for AM.
Demodulation: An Envelope Detector (Diode + RC Low Pass Filter) is used to recover the message by tracing the peaks of the AM wave.
12. Drone and Counter-Drone Systems
Question: Briefly describe drone and counter drone system?
Answer:
Drone (UAV): An unmanned aircraft used for surveillance or delivery, controlled via Radio Frequency (RF) or GPS.
Counter-Drone: Technology to detect (Radar/RF) and neutralize (Jamming/Spoofing) unauthorized drones.
Question: Briefly describe drone and counter drone system?
Answer:
Drone (UAV): An unmanned aircraft used for surveillance or delivery, controlled via Radio Frequency (RF) or GPS.
Counter-Drone: Technology to detect (Radar/RF) and neutralize (Jamming/Spoofing) unauthorized drones.
13. DSB-SC Modulation
Question: Explain the double-sideband suppressed-carrier (DSB-SC) modulation with necessary diagram?
Answer:
Definition: A type of AM where the carrier is suppressed and only the Upper and Lower Sidebands are transmitted.
Merit: Since the carrier carries no information but consumes 66.7% power, suppressing it makes the system much more power-efficient.
Question: Explain the double-sideband suppressed-carrier (DSB-SC) modulation with necessary diagram?
Answer:
Definition: A type of AM where the carrier is suppressed and only the Upper and Lower Sidebands are transmitted.
Merit: Since the carrier carries no information but consumes 66.7% power, suppressing it makes the system much more power-efficient.
14. Pulse Code Modulation (PCM)
Question: Describe PCM waveform coder and decoder with a neat sketch and list the merits compared with analog coders?
Answer:
Process: Sampling $\rightarrow$ Quantization $\rightarrow$ Encoding.
Merits: High noise immunity, easy data encryption, and perfect signal regeneration using digital repeaters.
Question: Describe PCM waveform coder and decoder with a neat sketch and list the merits compared with analog coders?
Answer:
Process: Sampling $\rightarrow$ Quantization $\rightarrow$ Encoding.
Merits: High noise immunity, easy data encryption, and perfect signal regeneration using digital repeaters.
15. Keying vs. Modulation
Question: What is the difference between keying and modulation? Briefly Explain FDM and QAM?
Answer:
Keying vs. Modulation: Modulation uses analog inputs; Keying (ASK, FSK, PSK) uses digital (binary) inputs.
FDM: Sharing a channel by dividing the frequency range.
QAM: Combines both Amplitude and Phase changes to increase data transmission speed.
Question: What is the difference between keying and modulation? Briefly Explain FDM and QAM?
Answer:
Keying vs. Modulation: Modulation uses analog inputs; Keying (ASK, FSK, PSK) uses digital (binary) inputs.
FDM: Sharing a channel by dividing the frequency range.
QAM: Combines both Amplitude and Phase changes to increase data transmission speed.
16. 2G to 5G Wireless Standards
Question: Differentiate among the 2G, 3G, 4G AND 5G wireless standards?
Answer:
2G: Digital Voice/SMS.
3G: Mobile Data (CDMA).
4G: High-speed video/LTE.
5G: Ultra-low latency (<1ms), high speed, and Massive Internet of Things (IoT) support.
Question: Differentiate among the 2G, 3G, 4G AND 5G wireless standards?
Answer:
2G: Digital Voice/SMS.
3G: Mobile Data (CDMA).
4G: High-speed video/LTE.
5G: Ultra-low latency (<1ms), high speed, and Massive Internet of Things (IoT) support.
17. Multiplexing vs. Multiple Access & Fiber Profiles
Question: Differentiate between multiplexing and multiple access technology. Draw the refractive index profile and ray transmission for multimode (1) step index fibre, and (2) graded index fiber?
Answer:
Multiplexing: Point-to-point sharing of a link.
Multiple Access: Point-to-multipoint sharing of a medium (e.g., TDMA).
Fiber Profiles: Step-Index (sharp change, zig-zag rays) vs. Graded-Index (gradual change, curved rays, lower dispersion).
Question: Differentiate between multiplexing and multiple access technology. Draw the refractive index profile and ray transmission for multimode (1) step index fibre, and (2) graded index fiber?
Answer:
Multiplexing: Point-to-point sharing of a link.
Multiple Access: Point-to-multipoint sharing of a medium (e.g., TDMA).
Fiber Profiles: Step-Index (sharp change, zig-zag rays) vs. Graded-Index (gradual change, curved rays, lower dispersion).
18. OFC Block Diagram
Question: Draw the block diagram of optical fiber communication and briefly explain each block?
Answer: An optical fiber communication system converts electrical signals into light, transmits them via fiber optic cables, and converts them back to electrical signals, consisting primarily of a transmitter (LED/Laser), an optical fiber channel, and a receiver (photodetector) to achieve high-speed data transmission.
- Information Source: Generates the input signal, which can be analog (voice) or digital (computer data).
- Transmitter / Modulator: Converts the input electrical signal into a suitable form for the optical source, often modulating light intensity to encode information.
- Light Source (LED or Laser Diode): Converts electrical energy into optical energy (light). Lasers are used for long distances/high speeds, while LEDs are for short distances.
- Channel Coupler: Efficiently launches the light generated by the source into the optical fiber, minimizing power loss.
- Optical Fiber Cable: The transmission medium (channel) that guides the light signal to the destination with very low loss.
- Optical Amplifier / Repeater: Used in long-haul systems to amplify weak light signals or regenerate them when they become too faint.
- Receiver / Photodetector: Receives the attenuated light signal and converts it back into an electrical signal. Common detectors are photodiodes (PIN or APD).
- Signal Processor / Demodulator: Amplifies the weak electrical signal from the receiver and filters it to remove noise, restoring the original input signal.
- Destination: The final recipient of the message, such as a computer, phone, or monitor.
Mastering these 18 topics will give you a strong foundation for the communication portion of any technical recruitment test. In competitive exams, the clarity of your diagrams and the precision of your technical definitions are what set you apart from the crowd.
This concludes Part 1 of our communication theory guide. I hope this compilation helps you in your preparation journey. Remember to practice drawing the block diagrams for GSM and Optical Fiber, as they are almost always part of the written segment.
Stay tuned for Part 2, where I will be diving into the Mathematical Solutions for these communication topics, including PCM bit rate calculations, FM bandwidth problems, and signal-to-noise ratio (SNR) exercises.
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Question: Draw the block diagram of optical fiber communication and briefly explain each block?
Answer: An optical fiber communication system converts electrical signals into light, transmits them via fiber optic cables, and converts them back to electrical signals, consisting primarily of a transmitter (LED/Laser), an optical fiber channel, and a receiver (photodetector) to achieve high-speed data transmission.
- Information Source: Generates the input signal, which can be analog (voice) or digital (computer data).
- Transmitter / Modulator: Converts the input electrical signal into a suitable form for the optical source, often modulating light intensity to encode information.
- Light Source (LED or Laser Diode): Converts electrical energy into optical energy (light). Lasers are used for long distances/high speeds, while LEDs are for short distances.
- Channel Coupler: Efficiently launches the light generated by the source into the optical fiber, minimizing power loss.
- Optical Fiber Cable: The transmission medium (channel) that guides the light signal to the destination with very low loss.
- Optical Amplifier / Repeater: Used in long-haul systems to amplify weak light signals or regenerate them when they become too faint.
- Receiver / Photodetector: Receives the attenuated light signal and converts it back into an electrical signal. Common detectors are photodiodes (PIN or APD).
- Signal Processor / Demodulator: Amplifies the weak electrical signal from the receiver and filters it to remove noise, restoring the original input signal.
- Destination: The final recipient of the message, such as a computer, phone, or monitor.
Mastering these 18 topics will give you a strong foundation for the communication portion of any technical recruitment test. In competitive exams, the clarity of your diagrams and the precision of your technical definitions are what set you apart from the crowd.
This concludes Part 1 of our communication theory guide. I hope this compilation helps you in your preparation journey. Remember to practice drawing the block diagrams for GSM and Optical Fiber, as they are almost always part of the written segment.
Stay tuned for Part 2, where I will be diving into the Mathematical Solutions for these communication topics, including PCM bit rate calculations, FM bandwidth problems, and signal-to-noise ratio (SNR) exercises.
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