Communication, Signal System & Telecommunication
EEE Job Exam Q&A Series — Part 2 (Q16–Q30)
Q16. Differentiate among the 2G, 3G, 4G and 5G wireless standards.
Wireless communication has evolved through distinct generations, each solving the limitations of the previous one.
| Parameter | 2G | 3G | 4G (LTE) | 5G |
|---|---|---|---|---|
| Introduced | 1991 | 2001 | 2009 | 2019 |
| Technology | GSM, CDMA | WCDMA, CDMA2000 | LTE, WiMAX | NR (New Radio) |
| Data Speed | Up to 64 kbps | Up to 2 Mbps | Up to 1 Gbps | Up to 20 Gbps |
| Multiplexing | TDMA, CDMA | CDMA | OFDMA | OFDMA, NOMA |
| Frequency Band | 850–1900 MHz | 850–2100 MHz | 700 MHz–2.6 GHz | Sub-6 GHz & mmWave (up to 86 GHz) |
| Latency | ~300–1000 ms | ~100–500 ms | ~30–50 ms | <1 ms |
| Services | Voice, SMS | Voice, video call, mobile internet | HD video, VoIP, gaming | IoT, autonomous vehicles, AR/VR |
| Switching | Circuit switching | Packet & circuit switching | Full packet switching | Full packet switching |
Key Takeaway:
2G digitized voice. 3G brought mobile internet. 4G made broadband wireless practical. 5G is designed not just for faster phones but for connecting billions of machines and sensors with ultra-low latency.
Diagram Reference: 2G 3G 4G 5G Evolution Explained (YouTube)
Q17. 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.
Multiplexing vs Multiple Access:
| Parameter | Multiplexing | Multiple Access |
|---|---|---|
| Definition | Combining multiple signals from one source onto a single channel. | Allowing multiple independent users to share a common channel simultaneously. |
| Where Applied | Between fixed points (e.g., between two exchanges). | Wireless/cellular networks where many users share the same spectrum. |
| Control | Controlled by the network — users do not compete. | Users compete for or are assigned channel access. |
| Examples | TDM, FDM, WDM | TDMA, FDMA, CDMA, OFDMA |
| Application | Backbone links, optical fiber transmission, PCM telephony. | Mobile networks (GSM, LTE, 5G), Wi-Fi, satellite uplinks. |
In short — multiplexing is a hardware/link technique for efficient use of a transmission medium, while multiple access is a protocol/system technique for managing shared access among many users.
Optical Fiber Types — Refractive Index Profile and Ray Transmission:
1. Multimode Step Index Fiber:
The core has a uniform high refractive index (n1) throughout and the cladding has a uniform lower refractive index (n2). The boundary is abrupt — a sharp step. Light rays travel in a zigzag path, bouncing between the core-cladding boundaries by total internal reflection. Different rays travel different path lengths and arrive at different times, causing modal dispersion which limits bandwidth and distance.
- Core diameter: 50–100 μm
- Used for short-distance, low-bandwidth applications.
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2. Multimode Graded Index Fiber:
The core refractive index is highest at the center and decreases gradually toward the cladding in a parabolic profile. Light rays travel in smooth curved paths. Rays taking longer outer paths move faster (lower refractive index = higher speed), so all modes arrive at nearly the same time — greatly reducing modal dispersion.
- Core diameter: 50–62.5 μm
- Used for medium-distance, higher-bandwidth applications.
| Parameter | Step Index (Multimode) | Graded Index (Multimode) |
|---|---|---|
| Refractive Index Profile | Uniform core, sharp boundary | Parabolic, gradually decreasing |
| Ray Path | Zigzag (total internal reflection) | Smooth curved paths |
| Modal Dispersion | High | Much lower |
| Bandwidth | Low | Higher |
| Distance | Short range | Medium range |
Diagram Reference: Step Index vs Graded Index Fiber (YouTube)
Q18. Draw the block diagram of optical fiber communication and briefly explain each block.
An optical fiber communication system converts electrical signals into light, transmits them through fiber, and converts them back to electrical signals at the receiver.
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.png "Block Diagram of Optical Fiber communication")
Transmitter Side:
1. Input Signal: The original information — voice, data, or video — in electrical form.
2. Signal Conditioner / Encoder: Performs ADC, encoding, and multiplexing of multiple channels if needed.
3. Light Source Driver: Converts electrical signal into drive current for the light source.
4. Light Source (LED or Laser Diode): Converts electrical signal to light. LED is used for multimode fiber and short distances. Laser Diode (LD) is used for single-mode fiber, long distances, and high bandwidth.
5. Optical Fiber Cable: Carries the light signal using total internal reflection. May include EDFA (optical amplifiers) at intervals for very long distances.
Receiver Side:
6. Photodetector (PIN Diode or APD): Converts received optical signal back to electrical signal. PIN diode for shorter distances; APD for weak signals over longer distances.
7. Signal Amplifier: Boosts the weak converted electrical signal to a usable level.
8. Signal Conditioner / Decoder: Performs equalization, filtering, DAC, and demultiplexing to recover individual channels.
9. Output: The recovered original information delivered to the end user.
| Block | Function |
|---|---|
| Input Signal | Original message (voice/data/video) |
| Encoder/Conditioner | ADC, multiplexing, signal formatting |
| Light Source Driver | Drives LED/LD with modulated current |
| LED / Laser Diode | Converts electrical signal to light |
| Optical Fiber | Transmits light signal via TIR |
| Photodetector | Converts light back to electrical signal |
| Amplifier | Boosts weak received signal |
| Decoder/Conditioner | DAC, demultiplexing, signal recovery |
| Output | Recovered original information |
Diagram Reference: Optical Fiber Communication Block Diagram (YouTube)
Q19. Write short notes on: (a) GSM (Global System for Mobile) (b) BPSK and QPSK modulation.
(a) GSM (Global System for Mobile Communication):
GSM is a second-generation (2G) digital cellular standard developed in Europe and adopted globally. It replaced analog 1G systems and introduced digital voice, SMS, and basic data services.
GSM operates in frequency bands of 900 MHz and 1800 MHz. It uses TDMA (Time Division Multiple Access) combined with FDMA — the spectrum is divided into 200 kHz channels, and each channel is further divided into 8 time slots shared among 8 users.
Key features of GSM:
- SIM-based subscriber identity, allowing users to switch handsets easily.
- Supports voice calls, SMS, GPRS (2.5G data), and EDGE (2.75G data).
- International roaming capability.
- Encrypted communication between handset and BTS.
- Architecture: MS → BTS → BSC → MSC → PSTN/other networks.
(b) BPSK (Binary Phase Shift Keying):
BPSK is the simplest form of PSK. It uses two phase states to represent binary data:
- Binary 1 → 0° phase
- Binary 0 → 180° phase
BPSK signal: s(t) = Accos(2πfct + φ), where φ = 0° or 180°
Bits per symbol = 1. Very robust against noise but low spectral efficiency.
QPSK (Quadrature Phase Shift Keying):
QPSK uses four phase states, each representing 2 bits:
- 00 → 45°
- 01 → 135°
- 11 → 225°
- 10 → 315°
Bits per symbol = 2. Same bandwidth as BPSK but double the data rate. Used in satellite communication, cable modems, and LTE.
| Parameter | BPSK | QPSK |
|---|---|---|
| Phase States | 2 | 4 |
| Bits per Symbol | 1 | 2 |
| Bandwidth Efficiency | Low | Higher (2x BPSK) |
| Noise Immunity | Best | Good |
| Application | Deep space, satellite uplink | Satellite, LTE, cable modem |
Diagram Reference: BPSK and QPSK Explained (YouTube)
Q20. Describe the process of sampling and how the message signal is reconstructed from its samples. Also illustrate the effect of aliasing with neat sketch.
Sampling:
Sampling is the process of measuring the amplitude of a continuous analog signal at regular time intervals to produce a discrete-time sequence. The result is a series of impulses whose amplitudes match the original signal at those instants.
Types of Sampling:
- Ideal (Impulse) Sampling: Signal multiplied by a train of impulses. Theoretical model.
- Natural Sampling: Signal multiplied by a pulse train. Pulse tops follow the signal shape.
- Flat-Top Sampling (Sample and Hold): Each pulse has a constant amplitude equal to the signal value at sampling instant. Used in practical ADC circuits.
Signal Reconstruction:
The sampled signal contains the original spectrum repeated at multiples of the sampling frequency fs. To reconstruct the original signal:
- Pass the sampled signal through an ideal low-pass filter (LPF) with cutoff frequency fm.
- The filter removes all repeated spectral copies above fm, leaving only the original baseband signal.
- This recovered signal is a perfect replica of the original, provided fs ≥ 2fm.
Aliasing Effect:
.png "Fig: Spectrum of the sampled signal for the case fs < 2fm")
When fs < 2fm, the repeated spectral copies overlap with the original spectrum. This overlap is called aliasing. The overlapping components mix and become inseparable — the LPF cannot isolate the original signal, and the reconstructed output is a distorted version at a false (alias) frequency.
Example: Sampling a 1 kHz signal at 1.5 kHz (instead of minimum 2 kHz) produces an alias at 0.5 kHz in the reconstructed output.
Prevention: Use an anti-aliasing filter (LPF) before the sampler to remove all frequency components above fs/2.
Diagram Reference: Sampling and Aliasing Illustrated (YouTube)
Q21. Describe PCM waveform and decoder with neat sketch and list the merits compared with analog coders.
PCM (Pulse Code Modulation) converts an analog signal into a digital bit stream through three stages: Sampling, Quantization, and Encoding.
PCM Encoder Process:
Step 1 — Sampling: Analog signal sampled at fs ≥ 2fm. For voice: fs = 8000 Hz.
Step 2 — Quantization: Each sample is rounded to the nearest of 2n discrete levels. For 8-bit PCM: 28 = 256 levels. The difference between the actual and rounded value is the quantization error (noise).
Step 3 — Encoding: Each quantized level is assigned a unique n-bit binary code word and transmitted serially.
Bit Rate = fs × n = 8000 × 8 = 64 kbps
PCM Decoder Process:
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- Regenerator: Cleans and reshapes the degraded binary pulses before decoding.
- Decoder (DAC): Converts each n-bit code word back to a quantized voltage level.
- Reconstruction LPF: Smooths the staircase waveform into the original continuous analog signal.
Merits of PCM over Analog Coders:
| Parameter | PCM (Digital) | Analog Coder |
|---|---|---|
| Noise Immunity | Excellent. Binary pulses regenerated perfectly. | Poor. Noise accumulates at each repeater. |
| Signal Quality | Consistent over any distance. | Degrades with distance. |
| Multiplexing | Easy TDM integration. | FDM required, more complex. |
| Security | Easy to encrypt digitally. | Difficult to secure. |
| Error Control | Error detection and correction possible. | Not possible. |
| Storage | Lossless digital storage. | Analog storage degrades over time. |
Diagram Reference: PCM Encoder Decoder Explained (YouTube)
Q22. Write short notes on: Delta Modulation, PCM, Mobile Assisted Handoffs.
Delta Modulation (DM):
Delta modulation is a simplified form of differential PCM where only 1 bit is transmitted per sample — indicating whether the signal has gone up or down since the last sample. The encoder compares the current sample with a staircase approximation and sends a 1 if the signal increased and a 0 if it decreased.
.jpeg "Delta Modulation Waveform")
Advantages: Very simple hardware, low bit rate.
Disadvantages: Two types of distortion occur — slope overload distortion (when signal rises too fast for the staircase to follow) and granular noise (when signal is nearly constant but the staircase oscillates around it).
PCM (Pulse Code Modulation):
PCM is the standard digital encoding technique for analog signals. It involves three steps: sampling the analog signal at Nyquist rate, quantizing each sample to the nearest discrete level, and encoding each level as a binary code word. For voice telephony, 8-bit PCM at 8 kHz sampling gives 64 kbps. PCM provides excellent noise immunity and forms the basis of modern digital telephony and audio.
Mobile Assisted Handoff (MAHO):
In MAHO, the mobile station (handset) actively participates in the handoff decision rather than leaving it entirely to the network. The mobile continuously measures the signal strength from neighboring base stations and reports these measurements to the serving BSC/MSC. The network uses this information to decide when and where to hand off the call.
Advantages of MAHO: Faster handoff decisions, reduced call drop probability, better use of network resources. Used in GSM and CDMA systems.
Diagram Reference: Delta Modulation Explained (YouTube)
Q23. Give the truth table of a 4×1 MUX. Implement this 4×1 MUX using only 2×1 MUXs.
4×1 Multiplexer:
A 4×1 MUX has 4 data inputs (I0, I1, I2, I3), 2 select lines (S1, S0), and 1 output (Y). The select lines choose which input is routed to the output.
Truth Table:
| S1 | S0 | Output (Y) |
|---|---|---|
| 0 | 0 | I0 |
| 0 | 1 | I1 |
| 1 | 0 | I2 |
| 1 | 1 | I3 |
Boolean Expression:
Y = S̅1S̅0I0 + S̅1S0I1 + S1S̅0I2 + S1S0I3
Implementation using 2×1 MUXs:
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Three 2×1 MUXs are needed to build a 4×1 MUX:
- MUX 1: Inputs = I0, I1. Select = S0. Output = Y1
- MUX 2: Inputs = I2, I3. Select = S0. Output = Y2
- MUX 3 (Final): Inputs = Y1, Y2. Select = S1. Output = Y (final output)
MUX 1 and MUX 2 handle the lower select line S0, while MUX 3 uses S1 to choose between the two intermediate outputs. The final output correctly replicates the 4×1 MUX function.
Diagram Reference: 4x1 MUX using 2x1 MUX (YouTube)
Q24. What is the difference between keying and modulation? Briefly explain ASK, FSK, and PSK.
| Parameter | Modulation | Keying |
|---|---|---|
| Message Signal | Analog (continuous) | Digital (binary) |
| Carrier Variation | Continuously varying | Switches between discrete states |
| Examples | AM, FM, PM | ASK, FSK, PSK |
| Application | Radio broadcasting, analog telephony | Digital data transmission, modems |
ASK (Amplitude Shift Keying):
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The amplitude of the carrier changes to represent binary data. Binary 1 is represented by a carrier with full amplitude and binary 0 by zero or reduced amplitude. Simple to implement but very sensitive to noise and amplitude variations in the channel. Used in optical fiber data links and RFID.
FSK (Frequency Shift Keying):
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Two different carrier frequencies represent binary 1 and binary 0. The amplitude stays constant — only frequency switches. Much better noise immunity than ASK. Used in caller ID transmission, early modems, and radio teletype systems.
PSK (Phase Shift Keying):
The phase of the carrier shifts to encode data. In BPSK (binary PSK), 0° represents binary 1 and 180° represents binary 0. Amplitude and frequency remain constant. PSK has the best noise performance among the three. Used in Wi-Fi, satellite communication, and LTE.
| Type | Parameter Changed | Noise Immunity | Complexity |
|---|---|---|---|
| ASK | Amplitude | Low | Simple |
| FSK | Frequency | Medium | Moderate |
| PSK | Phase | High | Higher |
Diagram Reference: ASK FSK PSK Explained (YouTube)
Q25. What is quadrature amplitude modulation (QAM)? Explain about 16-QAM and 64-QAM.
QAM (Quadrature Amplitude Modulation):
QAM combines both amplitude and phase variation to encode data. Two carrier signals of the same frequency but 90° out of phase — called the In-phase (I) and Quadrature (Q) components — are independently amplitude-modulated and then summed.
QAM signal: s(t) = I(t)cos(2πfct) − Q(t)sin(2πfct)
Each unique combination of amplitude and phase is called a symbol and is represented as a point on a constellation diagram. More points = more bits per symbol = higher data rate for the same bandwidth.
16-QAM:
Has 16 constellation points arranged in a 4×4 grid. Each symbol encodes 4 bits (since log216 = 4). Requires reasonably good SNR to distinguish all 16 points. Used in LTE, DVB-T, and cable modems.
64-QAM:
Has 64 constellation points arranged in an 8×8 grid. Each symbol encodes 6 bits (since log264 = 6). Higher data throughput but needs significantly better signal quality to avoid errors — points are closer together on the constellation, making them harder to distinguish under noise. Used in LTE-Advanced, Wi-Fi (802.11n/ac), and cable TV.
| QAM Type | Constellation Points | Bits per Symbol | SNR Requirement | Application |
|---|---|---|---|---|
| 4-QAM (QPSK) | 4 | 2 | Low | Satellite, deep space |
| 16-QAM | 16 | 4 | Medium | LTE, DVB-T |
| 64-QAM | 64 | 6 | High | LTE-A, Wi-Fi, cable TV |
| 256-QAM | 256 | 8 | Very High | 5G, DOCSIS 3.1 |
Diagram Reference: QAM 16-QAM 64-QAM Constellation (YouTube)
Q26. Mention the SNR importance in telecommunications engineering.
SNR (Signal-to-Noise Ratio):
SNR is the ratio of the desired signal power to the background noise power in a communication system.
SNR (dB) = 10 log10(Psignal / Pnoise)
A higher SNR means the signal is much stronger than the noise — better quality. A low SNR means the signal is buried in noise — poor quality or communication failure.
Importance of SNR in Telecommunications:
1. Channel Capacity (Shannon’s Theorem):
C = B × log2(1 + SNR)
Shannon’s theorem directly links SNR to the maximum achievable data rate. Higher SNR allows higher channel capacity — more data transmitted per second.
2. Modulation Scheme Selection:
Higher-order modulation (64-QAM, 256-QAM) requires high SNR to distinguish closely spaced constellation points. Systems adaptively select modulation based on measured SNR — this is called Adaptive Modulation and Coding (AMC).
3. Receiver Design:
SNR determines the sensitivity requirements of the receiver. Low SNR demands more sophisticated receivers with better low-noise amplifiers (LNA) and error correction.
4. Link Budget Planning:
Engineers calculate the expected SNR at the receiver during system design to ensure acceptable performance margins for the required coverage distance and data rate.
5. Audio and Video Quality:
In analog systems, SNR directly maps to perceived audio quality (higher SNR = clearer sound) and video quality (higher SNR = sharper image with less grain).
Diagram Reference: SNR in Communication Systems (YouTube)
Q27. Digital signal requires much more bandwidth than original analog signal. Why?
When an analog signal is converted to digital through PCM, the resulting bit stream requires significantly more bandwidth. Here is why:
1. Sampling Increases Data Volume:
A voice signal of maximum 4 kHz must be sampled at 8000 samples/second (Nyquist). Each sample generates a data point that did not exist in the original continuous analog waveform.
2. Quantization Adds Bits per Sample:
Each sample is encoded as an n-bit binary word. For 8-bit PCM, each of the 8000 samples becomes 8 bits. Total bit rate = 8000 × 8 = 64 kbps for a signal whose analog bandwidth was only 4 kHz.
3. Bandwidth of Digital Signal:
The minimum bandwidth required to transmit a digital signal at bit rate Rb is:
Bmin = Rb/2 = 64,000/2 = 32 kHz
This is 8 times the original 4 kHz analog bandwidth.
4. Why It Is Worth the Trade-off:
Despite the higher bandwidth requirement, digital systems are preferred because:
- Binary signals can be perfectly regenerated at repeaters — noise does not accumulate.
- Error detection and correction can be applied.
- Digital signals can be encrypted, compressed, and multiplexed easily.
- Modern compression techniques (MP3, AAC, video codecs) significantly reduce the bandwidth overhead.
Q28. What are the noise contributing factors in an earth station receiving channel?
In satellite communication, the earth station receiver faces noise from multiple sources. Understanding these is critical for designing reliable links.
1. Thermal Noise from Receiver Electronics:
All electronic components in the receiving chain (LNA, cables, mixers, amplifiers) generate thermal noise. The total receiver noise is characterized by the system noise temperature (Tsys) in Kelvin.
2. Antenna Noise Temperature:
The receiving antenna picks up noise from the sky, ground, and nearby warm objects. At low elevation angles, the antenna points toward warmer ground — increasing antenna noise temperature significantly.
3. Sky Noise (Atmospheric Noise):
The atmosphere absorbs and re-emits radio energy as thermal noise. Rain, clouds, and atmospheric gases (especially oxygen and water vapor) contribute to sky noise. Severe at frequencies above 10 GHz.
4. Galactic and Cosmic Noise:
Background radiation from the galaxy and cosmos contributes noise, particularly at frequencies below 1 GHz.
5. Man-made Interference:
Interference from nearby terrestrial transmitters, industrial equipment, power lines, and other satellite systems operating in adjacent frequency bands.
6. Intermodulation Noise:
When multiple carriers pass through nonlinear amplifiers in the receiving chain, intermodulation products are generated that fall within the signal band.
Figure of Merit (G/T):
Earth station quality is measured by G/T ratio (antenna gain / system noise temperature). Higher G/T means better ability to receive weak satellite signals in the presence of noise.
Diagram Reference: Satellite Communication Link Budget and Noise (YouTube)
Q29. What is meant by frequency reuse?
Frequency Reuse:
Frequency reuse is the technique of using the same set of radio frequencies in different cells of a cellular network that are geographically separated by a sufficient distance, so that interference between them remains below an acceptable level.
Why It Is Needed:
The radio frequency spectrum is limited. Without frequency reuse, each frequency could serve only one user across the entire coverage area. Frequency reuse allows the same frequencies to serve multiple users simultaneously in different cells, dramatically increasing total system capacity.
Frequency Reuse Factor (N):
The reuse factor N defines how many cells form a cluster before frequencies repeat. Common values: N = 4, 7, 12.
- Small N (e.g., N = 4): Frequencies reused more aggressively → higher capacity but more co-channel interference.
- Large N (e.g., N = 12): Greater separation between same-frequency cells → less interference but lower capacity per area.
Co-Channel Interference:
Cells using the same frequency are called co-channel cells. The ratio of distance between co-channel cells (D) to cell radius (R) determines interference level:
D/R = √(3N)
Applications: GSM, CDMA, LTE all rely on frequency reuse to serve millions of users with limited spectrum.
Diagram Reference: Frequency Reuse in Cellular Networks (YouTube)
Q30. Write short notes on: Harmonics, Generation Scheduling, Swing Equation.
Harmonics:
Harmonics are sinusoidal components of a periodic waveform with frequencies that are integer multiples of the fundamental frequency (f0). The 2nd harmonic is at 2f0, 3rd at 3f0, and so on. In power systems, non-linear loads (inverters, rectifiers, variable speed drives) generate harmonics that distort the ideal sinusoidal waveform. Total Harmonic Distortion (THD) quantifies this distortion. Harmonics cause overheating in transformers, increased losses, and malfunction of sensitive equipment. Filters and power factor correction equipment are used to suppress them. In communication systems, harmonics from oscillators can cause unwanted interference on adjacent frequency bands.
Generation Scheduling:
Generation scheduling is the process of planning and allocating electrical power output among available generating units (power plants) to meet the forecasted load demand at minimum cost while satisfying all operational constraints. Also called Economic Load Dispatch (ELD). The scheduler determines how much power each plant should generate at each time interval, considering fuel costs, plant efficiency curves, transmission limits, spinning reserve requirements, and start-up/shut-down costs. Modern energy management systems use optimization algorithms for this purpose.
Swing Equation:
The swing equation describes the dynamic behavior of a synchronous generator’s rotor angle when subjected to a disturbance (such as a sudden load change or fault). It governs transient stability analysis in power systems.
The swing equation is:
(2H / ωs) × (d2δ/dt2) = Pm − Pe
Where:
- H = inertia constant of the machine (MJ/MVA)
- ωs = synchronous angular frequency (rad/s)
- δ = rotor angle (radians)
- Pm = mechanical power input
- Pe = electrical power output
If Pm > Pe, the rotor accelerates. If Pm < Pe, it decelerates. The swing equation is used to study whether a generator will remain in synchronism after a disturbance — this is the fundamental question of transient stability.
Diagram Reference: Swing Equation Power System Stability (YouTube)
Part 2 Complete — Q16 to Q30
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