Communication, Signal System & Telecommunication
EEE Job Exam Q&A Series — Part 1 (Q1–Q15)
Q1. State Nyquist Sampling Theorem. Why is it necessary to follow Nyquist theorem during analog to digital conversion of a signal?
Nyquist Sampling Theorem:
A continuous analog signal can be fully reconstructed from its sampled version, provided the sampling frequency (fs) is at least twice the highest frequency component (fm) present in the signal.
Condition: fs ≥ 2fm
This minimum rate — 2fm — is known as the Nyquist Rate.
Why it matters in ADC:
When converting analog signals to digital, samples are taken at fixed intervals. Sample too slowly (fs < 2fm) and the reconstructed signal comes out distorted — a problem called Aliasing. Aliased signals have overlapping frequency components that can never be untangled, so the original signal is lost permanently. Nyquist theorem sets the boundary that keeps this from happening.
Practical Example:
Human voice reaches up to about 4 kHz. To capture it accurately:
fs = 2 × 4000 = 8000 samples/second (8 kHz)
This is exactly why standard telephone PCM systems sample at 8 kHz.
Diagram Reference: Nyquist Theorem & Aliasing (YouTube)
Q2. A message signal with a frequency of 5 kHz modulates a carrier of 88 MHz to produce an FM signal with a modulation index of 1. Sketch the spectrum, and find the bandwidth and spectral power density of the FM signal.
Given:
- Message frequency, fm = 5 kHz
- Carrier frequency, fc = 88 MHz
- Modulation index, β = 1
Bandwidth — Carson’s Rule:
BW = 2(β + 1) × fm
BW = 2(1 + 1) × 5 kHz
BW = 20 kHz
Significant Sidebands (Bessel function, β = 1):
For β = 1, significant spectral components appear at:
- fc ± fm → 88 MHz ± 5 kHz
- fc ± 2fm → 88 MHz ± 10 kHz
- fc ± 3fm → 88 MHz ± 15 kHz
Spectral Power Density:
Total FM signal power: Pt = Ac2/2
Unlike AM, this total power stays constant regardless of modulation index. The power is simply redistributed among the carrier and sideband components according to Bessel coefficients, spread across the 20 kHz bandwidth centered at 88 MHz.
Diagram Reference (FM Spectrum): FM Signal Spectrum Explained (YouTube)
Q3. What is noise? Explain the characteristics of various types of noise in a telecommunication system.
Noise:
Noise is any unwanted electrical signal that corrupts the original information signal during transmission or reception. It degrades signal quality and sets a hard limit on how well any communication system can perform.
Types of Noise:
1. Thermal Noise (Johnson Noise)
Comes from random electron movement in conductors due to heat. Present in every component above absolute zero. Power = kTB (k = Boltzmann’s constant, T = temperature in Kelvin, B = bandwidth). Spreads uniformly across all frequencies — which is why it is also called White Noise. Cooling the component reduces it but can never eliminate it entirely.
2. Shot Noise
Happens because electric charge is not continuous — it moves in discrete electron packets. Common in diodes, transistors, and photodetectors. Gets worse at higher frequencies.
3. Flicker Noise (1/f Noise)
Found in semiconductors and vacuum tubes. Power spectral density increases as frequency drops, so it dominates at low frequencies. Also called Pink Noise.
4. Intermodulation Noise
When two or more signals pass through a nonlinear device like an amplifier or mixer, they produce unwanted sum and difference frequencies. These extra components interfere with the original signals.
5. Crosstalk
Signal leakage from one channel into another. Typically happens in telephone cables or transmission lines running close together due to electromagnetic coupling.
6. Impulse Noise
Sudden high-amplitude spikes caused by lightning, switching transients, or power surges. Short in duration but highly destructive — especially in digital systems where a single spike can flip multiple bits.
Diagram Reference: Types of Noise in Communication (YouTube)
Q4. What is hand-off margin? How can proper hand-off be made to prevent call drop? Explain with necessary figures.
Hand-off (Handover):
As a mobile user moves from one cell to another, their active call gets transferred from the current base station (BTS) to a new one. This process — done automatically without dropping the call — is called hand-off.
Hand-off Margin:
Hand-off margin (ΔH) is the minimum signal strength difference between the serving BTS and the target BTS required before a hand-off is triggered. Typically set between 6–10 dB. It acts as a buffer — the system does not wait until the signal fully collapses before switching.
Condition: Hand-off triggers when —
RSL of current BTS ≤ Threshold Level (Lth) + ΔH
How proper hand-off prevents call drop:
- The MSC (Mobile Switching Center) continuously monitors signal strength from all nearby base stations.
- When the current BTS signal approaches the threshold, MSC identifies a stronger neighboring BTS.
- Hand-off is initiated before signal quality becomes unusable — the call is transferred while the link is still active.
- A hysteresis margin prevents the Ping-Pong effect — unnecessary repeated switching between two BTS of similar strength.
- In CDMA, soft hand-off lets the mobile communicate with two BTS simultaneously during the transition, making the switch even smoother.
Types of Hand-off:
| Type | Mechanism | Used In |
|---|---|---|
| Hard Hand-off | Break-before-make. Old link drops before new one forms. | GSM |
| Soft Hand-off | Make-before-break. Mobile links to new BTS before releasing old one. | CDMA |
Diagram Reference: Hand-off in Cellular Networks (YouTube)
Q5. What is multiplexing? Explain the principles of Time Division Multiplexing (TDM) with a sketch to show how the interleaving of channels takes place.
Multiplexing:
Multiplexing is the process of combining multiple independent signals and sending them over a single shared communication channel. A demultiplexer at the other end separates them back. It maximizes channel utilization and cuts transmission costs significantly.
Time Division Multiplexing (TDM):
TDM divides the channel’s total time into repeating frames, and each frame is split into fixed time slots. Each input signal gets one slot per frame. The signals do not actually transmit simultaneously — they take turns so fast it appears simultaneous.
Working Principle:
- A TDM frame with N slots carries N channels.
- Each channel is sampled once per frame and its sample placed in its designated slot.
- The multiplexer switches between channels at high speed using a commutator mechanism.
- The demultiplexer at the receiver must be perfectly synchronized to route each slot to the correct output channel.
Channel Interleaving Example:
For three channels A, B, C:
| Frame | Slot 1 | Slot 2 | Slot 3 |
|---|---|---|---|
| Frame 1 | A1 | B1 | C1 |
| Frame 2 | A2 | B2 | C2 |
| Frame 3 | A3 | B3 | C3 |
This sequential slot-by-slot arrangement is called interleaving. If individual bits are interleaved it is called bit interleaving; if complete samples are interleaved it is word interleaving.
Applications: PCM telephony (T1/E1 systems), GSM (uses TDMA), satellite communication.
Diagram Reference: TDM Explained with Diagram (YouTube)
Q6. What is meant by mobile communication? Distinguish between mobile radio and cellular mobile radio system.
Mobile Communication:
Mobile communication is wireless transmission of voice, data, and multimedia between users who are moving or at non-fixed locations. No physical connection is needed between sender and receiver — radio frequency links handle the channel.
| Parameter | Mobile Radio System | Cellular Mobile Radio System |
|---|---|---|
| Coverage | One high-power transmitter covers a single large area. | Total area divided into small cells, each with its own low-power BTS. |
| Frequency Reuse | No reuse. Each frequency is assigned once across the whole area. | Same frequencies reused in non-adjacent cells, multiplying system capacity. |
| User Capacity | Limited. Bottlenecked by available frequencies. | High capacity due to frequency reuse and cell splitting. |
| Transmitter Power | High power needed to cover large distances. | Low power per cell is sufficient. |
| Handover | Not needed. | Essential as users cross cell boundaries. |
| Interference | Co-channel interference is a persistent problem. | Controlled through frequency planning, cell design, and power management. |
| Examples | Police radio, taxi dispatch, walkie-talkie networks. | GSM, CDMA, LTE, 5G. |
Diagram Reference: Cellular Mobile Communication System (YouTube)
Q7. Draw mobile communication system diagram using GSM technology and explain its operation.
GSM (Global System for Mobile Communication) is a digital cellular network standard. The system is built around four major subsystems that work together to handle calls and data.
Major Components of GSM Architecture:
1. Mobile Station (MS)
The user’s handset along with the SIM card. The SIM stores subscriber identity (IMSI) and authentication keys. The MS communicates with the network over the radio interface.
2. Base Station Subsystem (BSS)
Consists of two parts:
- BTS (Base Transceiver Station): Handles the radio link with the MS. Each cell has one BTS.
- BSC (Base Station Controller): Controls multiple BTS units. Manages radio channel assignment, handover decisions, and power control.
3. Network Switching Subsystem (NSS)
- MSC (Mobile Switching Center): Core of the GSM network. Routes calls, manages handovers between BSCs, and interfaces with PSTN.
- HLR (Home Location Register): Permanent database storing subscriber profile and current location.
- VLR (Visitor Location Register): Temporary database for subscribers currently active in the MSC area.
- AUC (Authentication Center): Handles security and authentication of subscribers.
- EIR (Equipment Identity Register): Tracks valid, stolen, or faulty handsets by IMEI.
4. Operation and Support Subsystem (OSS)
Monitors and manages the entire network — fault management, performance monitoring, and configuration.
Operation Flow:
When a call is made, the MS sends a request to the BTS over the radio channel. The BTS passes it to the BSC, which forwards it to the MSC. The MSC checks the HLR for subscriber data, authenticates via AUC, and routes the call to the destination — either within the GSM network or out through the PSTN. Throughout the call, BSC monitors signal strength and initiates handover if the user moves to another cell.
Diagram Reference: GSM Architecture Explained (YouTube)
Q8. Draw satellite communication system diagram and explain its working principle.
Satellite Communication:
Satellite communication uses an artificial satellite orbiting the Earth as a relay station to receive signals from one ground station and retransmit them to another, enabling long-distance communication across countries and oceans.
Main Components:
1. Space Segment (Satellite)
The satellite carries a transponder — a device that receives the uplink signal, amplifies it, shifts its frequency, and retransmits it as the downlink signal. It also has solar panels for power and attitude control thrusters for maintaining orbital position.
2. Ground Segment (Earth Stations)
Earth stations have large parabolic dish antennas to transmit and receive signals. They include high-power amplifiers (HPA) for uplink and low-noise amplifiers (LNA) for downlink, along with modulation and demodulation equipment.
3. User Segment
End-user terminals such as VSAT dishes, satellite phones, or broadcast receivers.
Working Principle:
- The transmitting earth station sends a signal toward the satellite on the uplink frequency (typically 6 GHz for C-band, 14 GHz for Ku-band).
- The satellite transponder receives this signal, amplifies it, and converts it to the downlink frequency (4 GHz for C-band, 12 GHz for Ku-band) to avoid interference between uplink and downlink.
- The retransmitted signal is received by the destination earth station and demodulated to recover the original information.
Types of Satellite Orbits:
| Orbit | Altitude | Application |
|---|---|---|
| GEO (Geostationary) | 35,786 km | TV broadcast, weather, VSAT |
| MEO (Medium Earth Orbit) | 2,000–35,786 km | GPS, navigation |
| LEO (Low Earth Orbit) | 160–2,000 km | Satellite phones, Starlink |
Diagram Reference: Satellite Communication System Diagram (YouTube)
Q9. Draw and explain the operation of a CCTV system.
CCTV (Closed-Circuit Television) is a video surveillance system where signals are transmitted to a specific, limited set of monitors — unlike broadcast television which is open to anyone. It is widely used for security monitoring in banks, offices, roads, and public places.
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Main Components of a CCTV System:
1. Camera
Captures video footage of the monitored area. Types include fixed cameras, PTZ (Pan-Tilt-Zoom) cameras, dome cameras, and IP cameras. Each camera has a lens, image sensor (CCD or CMOS), and IR LEDs for night vision.
2. Transmission Medium
The video signal travels from camera to recorder via coaxial cable (analog systems), twisted pair (with balun), or network cable/Wi-Fi (IP-based systems).
3. DVR / NVR (Recording Unit)
DVR (Digital Video Recorder) is used with analog cameras. It converts analog video to digital, compresses it (H.264/H.265), and stores it on a hard drive.
NVR (Network Video Recorder) is used with IP cameras. It receives already-digitized video over the network and stores it.
4. Monitor
Displays live or recorded footage. Can show single or multiple camera feeds simultaneously using a split-screen view.
5. Power Supply
Cameras are powered via separate adapters or through PoE (Power over Ethernet) in IP-based systems.
Operation:
The camera captures video and sends it through the transmission medium to the DVR/NVR. The recorder digitizes (if analog), compresses, and stores the footage with timestamps. Live feed is simultaneously displayed on the monitor. Remote viewing is possible through internet connection to the NVR/DVR via smartphone or PC.
Diagram Reference: CCTV System Diagram and Working (YouTube)
Q10. Explain the features and advantages of the optical fiber communication system.
Optical Fiber Communication:
Optical fiber communication transmits data as light pulses through thin strands of glass or plastic fiber. Light travels through the fiber by the principle of Total Internal Reflection (TIR) — when light hits the core-cladding boundary at an angle greater than the critical angle, it reflects back into the core and propagates forward without escaping.
Structure of Optical Fiber:
- Core: Central glass strand where light travels. Higher refractive index.
- Cladding: Outer glass layer surrounding the core. Lower refractive index — causes TIR.
- Buffer Coating: Protective plastic layer around the cladding.
Features:
- Operates at wavelengths of 850 nm, 1310 nm, and 1550 nm (infrared range).
- Two types: Single-mode fiber (SMF) for long distance, Multi-mode fiber (MMF) for shorter distances.
- Uses WDM (Wavelength Division Multiplexing) to carry multiple channels on one fiber.
Advantages over Copper Cable:
| Parameter | Optical Fiber | Copper Cable |
|---|---|---|
| Bandwidth | Extremely high (Tbps range) | Limited (Gbps range) |
| Attenuation | Very low (0.2 dB/km) | High |
| EMI Immunity | Complete immunity | Susceptible to EMI |
| Security | Very difficult to tap | Easier to intercept |
| Weight | Light and flexible | Heavy |
| Distance | Up to 100+ km without repeater | Needs repeaters frequently |
Diagram Reference: Optical Fiber Communication System (YouTube)
Q11. Explain with necessary circuit diagram modulation and demodulation used in signal transmission.
Modulation:
Modulation is the process of varying a high-frequency carrier signal’s properties (amplitude, frequency, or phase) in accordance with the low-frequency message signal, so it can be efficiently transmitted over long distances.
Why Modulation is Needed:
- Baseband signals (voice, audio) have low frequency and cannot travel long distances efficiently.
- Modulation shifts the signal to higher frequencies suitable for antenna transmission.
- Allows multiple signals to share the same medium (multiplexing).
- Reduces noise and interference effects.
Types of Modulation:
| Type | Parameter Varied | Example |
|---|---|---|
| AM (Amplitude Modulation) | Amplitude of carrier | AM radio broadcast |
| FM (Frequency Modulation) | Frequency of carrier | FM radio, audio broadcast |
| PM (Phase Modulation) | Phase of carrier | Digital communication |
AM Modulation Expression:
s(t) = Ac[1 + m·cos(2πfmt)]cos(2πfct)
where m = modulation index = Am/Ac
Demodulation:
Demodulation is the reverse process — extracting the original message signal from the modulated carrier at the receiver.
AM Demodulation (Envelope Detector):
The AM signal passes through a diode (rectifier) which removes the negative half. A low-pass RC filter then smooths out the carrier ripple, leaving only the envelope — which is the original message signal.
FM Demodulation (PLL-based):
A Phase Locked Loop (PLL) tracks the instantaneous frequency of the FM signal and generates a voltage proportional to the frequency deviation — recovering the original message.
Diagram Reference: Modulation and Demodulation Circuit (YouTube)
Q12. Briefly describe drone and counter-drone system.
Drone (UAV — Unmanned Aerial Vehicle):
A drone is an aircraft that operates without an onboard human pilot. It is controlled remotely by an operator or flies autonomously using pre-programmed flight paths and onboard sensors.
Key Components of a Drone:
- Frame: Structural body, usually made of lightweight carbon fiber or plastic.
- Motors and Propellers: Generate lift and directional thrust.
- Flight Controller (FC): The brain of the drone. Processes sensor data and controls motor speeds to maintain stability.
- ESC (Electronic Speed Controller): Regulates power to each motor.
- GPS Module: Enables position hold, waypoint navigation, and return-to-home functions.
- Radio Receiver: Receives commands from the remote controller.
- Battery: Li-Po batteries provide power.
- Payload: Camera, sensors, or delivery package depending on application.
Applications: Aerial photography, agriculture, surveillance, delivery, military reconnaissance, search and rescue.
Counter-Drone System (C-UAS):
Counter-drone systems are designed to detect, track, identify, and neutralize unauthorized or hostile drones. They are critical for protecting airports, military bases, prisons, and public events.
Counter-Drone Methods:
| Method | How it Works |
|---|---|
| RF Jamming | Broadcasts strong radio signals to disrupt communication between drone and controller, forcing it to land or return home. |
| GPS Spoofing | Sends fake GPS signals to confuse the drone’s navigation system. |
| Laser System | High-energy laser beam destroys drone components or disables electronics. |
| Net Gun / Drone Capture | Fires a net to physically entangle and capture the target drone. |
| Radar Detection | Tracks drone position and flight path using radar for monitoring and targeting. |
Diagram Reference: Counter-Drone Systems Explained (YouTube)
Q13. Explain the double-sideband suppressed-carrier (DSB-SC) modulation with necessary diagram.
DSB-SC Modulation:
In standard AM, the carrier is transmitted along with both sidebands. The carrier itself carries no information — it just wastes power. DSB-SC removes the carrier entirely and transmits only the two sidebands, making it significantly more power-efficient.
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Mathematical Expression:
For a message signal m(t) = Amcos(2πfmt) and carrier c(t) = Accos(2πfct):
DSB-SC signal: s(t) = AcAmcos(2πfct)cos(2πfmt)
Expanding using trigonometric identity:
s(t) = (AcAm/2)[cos(2π(fc−fm)t) + cos(2π(fc+fm)t)]
This shows two sidebands at (fc − fm) and (fc + fm) — with no carrier component.
Generation (Balanced Modulator / Ring Modulator):
A balanced modulator (using diodes or a multiplier circuit) multiplies the message and carrier signals directly. The carrier components cancel each other out due to the balanced arrangement, leaving only the sidebands.
Demodulation (Coherent Detection):
DSB-SC cannot be demodulated with a simple envelope detector. A locally generated carrier of the same frequency and phase must be multiplied with the received signal, then passed through a low-pass filter to recover the message. This is called coherent or synchronous detection.
Comparison with Standard AM:
| Parameter | AM | DSB-SC |
|---|---|---|
| Carrier Transmitted | Yes | No |
| Power Efficiency | Low (carrier wastes power) | High (all power in sidebands) |
| Bandwidth | 2fm | 2fm |
| Demodulation | Simple envelope detector | Coherent detector required |
Diagram Reference: DSB-SC Modulation Explained (YouTube)
Q14. Describe PCM waveform coder and decoder with a neat sketch and list the merits compared with analog coders.
PCM (Pulse Code Modulation):
PCM is the standard method for converting analog signals to digital form. It involves three steps: Sampling, Quantization, and Encoding.
PCM Encoder (Transmitter Side):
Step 1 — Sampling:
The analog signal is sampled at regular intervals following the Nyquist criterion (fs ≥ 2fm). For voice, fs = 8000 samples/second.
Step 2 — Quantization:
Each sample amplitude is rounded to the nearest level in a finite set of discrete levels. For an n-bit PCM system, the number of quantization levels = 2n. For 8-bit PCM: 28 = 256 levels.
Step 3 — Encoding:
Each quantized value is converted to a binary code word of n bits. These binary bits are then serialized and transmitted.
Bit Rate of PCM:
Bit Rate = fs × n = 8000 × 8 = 64 kbps (for standard voice PCM)
PCM Decoder (Receiver Side):
- Regenerator: Reconstructs the binary pulses degraded by channel noise.
- Decoder: Converts each n-bit binary code word back to a quantized sample value (DAC).
- Reconstruction Filter (Low-Pass Filter): Smooths the staircase waveform of quantized samples back into the original continuous analog signal.
Merits of PCM over Analog Coders:
| Parameter | PCM (Digital) | Analog Coder |
|---|---|---|
| Noise Immunity | Excellent. Binary signal easily regenerated. | Poor. Noise accumulates with distance. |
| Signal Quality | Consistent over long distances. | Degrades with distance and repeaters. |
| Multiplexing | Easy TDM with other digital signals. | FDM required, more complex. |
| Security | Easy to encrypt. | Difficult to secure. |
| Error Detection | Error detection and correction possible. | Not possible. |
| Storage | Easy digital storage. | Analog storage degrades over time. |
Diagram Reference: PCM Encoder and Decoder (YouTube)
Q15. What is the difference between keying and modulation? Briefly explain FDM and QAM.
Keying vs Modulation:
| Parameter | Modulation | Keying |
|---|---|---|
| Signal Type | Used with analog message signals. | Used with digital (binary) message signals. |
| Output | Continuously varying modulated wave. | Discrete state transitions in carrier. |
| Examples | AM, FM, PM | ASK, FSK, PSK, QAM |
| Application | Radio broadcasting, analog telephony. | Digital communication, data transmission. |
In short — modulation is the analog counterpart; keying is what happens when the message is digital and the carrier switches between defined states instead of varying continuously.
FDM (Frequency Division Multiplexing):
FDM divides the total available bandwidth of a channel into several non-overlapping sub-bands. Each user or signal is assigned one sub-band and uses it continuously. All signals are transmitted simultaneously, just at different frequencies.
- Each sub-channel is separated by a guard band to prevent interference.
- At the receiver, bandpass filters separate each sub-channel.
- Used in: AM/FM radio broadcasting, cable TV, ADSL broadband, first-generation (1G) cellular systems.
QAM (Quadrature Amplitude Modulation):
QAM is a digital modulation technique that combines both amplitude and phase variations to encode data. Two carriers — same frequency but 90° out of phase (in-phase I and quadrature Q) — are independently amplitude-modulated and then combined.
QAM signal: s(t) = I(t)cos(2πfct) − Q(t)sin(2πfct)
Higher-order QAM encodes more bits per symbol:
| QAM Type | Constellation Points | Bits per Symbol |
|---|---|---|
| 4-QAM (QPSK) | 4 | 2 |
| 16-QAM | 16 | 4 |
| 64-QAM | 64 | 6 |
| 256-QAM | 256 | 8 |
Higher-order QAM achieves higher data rates but requires better signal quality (higher SNR). Used in: cable modems, LTE, Wi-Fi (802.11), DVB-C.
Diagram Reference: FDM and QAM Explained (YouTube)
Part 1 Complete — Q1 to Q15
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