Confirmed Exam Flashcards
List Advantages and Disadvantage of geo stationary and sub-synchronous orbiting satellites
Carsons BW equations?
BW = 2(∆f + fm)
∆f = (fmax - fmin) / 2
Fm = 1 / Tm
If an FM system has a modulating baseband with a period of 250ms and the maximum and minimum frequency outputs are 100 MHz and 99.85 MHz. What is the carson’s BW?
158 kHz
WRT Spread spectrum Techniques briefly explain operation of a Direct Sequence System
The process takes baseband signal and then spreads this power over a much wider bandwidth.
This ‘spread’ is then used to Phase Shift Key a sub-carrier.
The sub-carrier would be then up-converted to a higher frequency before being amplified and transmitted.
WRT Spread spectrum techniques briefly explain the operation of a frequency hopping system
Signal is rapidly switched or ‘hopped’ among many frequency channels in a predetermined sequence.
Each hop typically occurs within a frequency of a second, spreading signal over a wider bandwidth.
This technique reduces interference and increases security as an unintended receiver would need to know the hopping pattern to successfully intercept the signal.
What are the advantages of using spread spectrum techniques in communications systems?
Low Probability of Intercept and Exploitation (LPI/LPE): The wide bandwidth makes it harder to detect and intercept the signal.
• Resistance to Jamming: Spread spectrum signals are less susceptible to interference and jamming, as the wide bandwidth allows the system to filter out narrowband interference.
• Security: The use of pseudo-random codes for spreading and de-spreading adds a layer of security, making it difficult to eavesdrop without knowing the code.
• Multiple Access: Multiple users can transmit simultaneously on the same frequency range using different spreading codes, enabling efficient multiple access.
Explain ARQ and FEC
ARQ (Automatic repeat request) - Detects errors and then asks for re-transmission if errors are found (2 ways commas)
FEC (Forward Error Correction) - Adds extra bits to correct errors on its own, suitable for one-way broadcasts where re-transmission is not possible.
WTAD Explain which techniques overcome Burst Errors
Interleaving rearranges the data before transmission so that any burst errors are spread out over time when de-interleaved,making it easier for error correction codes to handle them effectively.
What systems is interleaving used in?
- Link 11 / 16
- Most digital transmission systems (including mobiles and CDs/DVDs)
What are the frequency bands and what propagation methods do they use?
How does a Sky wave Propagate?
- Transmission: A radio wave is transmitted from an antenna on Earth.
- Ionosphere Interaction: The wave travels upwards and reaches the ionosphere, a layer of the atmosphere containing ionized particles.
- Refraction: The ionosphere bends (refracts) the wave back towards the Earth.
- Return to Earth: The refracted wave returns to the Earth at a distant point from the transmitter, allowing long-distance communication.
- Skip Distance: The distance between the transmitter and the point where the refracted wave returns to Earth, creating a zone where the signal cannot be received directly.
- Multiple Hops: For even longer distances, the wave can reflect off the Earth’s surface and be refracted again by the ionosphere, creating multiple hops.
List and explain abnormal and regular variations in ionosphere
Regular -
Diurnal Change- Height and density of layers change throughout day. (At noon range/skip distance is extended due to increased ionisation)
Seasonal Change- All layers experience higher ionisation during summer due to more intense sunlight (Increased range/skip distance)
Sunspot cycle- 11 Year cycle of solar activity mainly affecting F-Layer (Extending Range/Skip Distance)
Abnormal-
Sporadic-E- Localised clouds in E Layers (Caused by meteors or electrical discharges)
Dellinger Fade out- Intense UV Radiation from Solar flares cause increased ionisation in D layer (Short term comms black outs/reduced range)
Magnetic Storms- Solar flares emit charges particles that cause rapid ionisation in all layers (Causes fading/reduced range/skip distance for days)
Atmospheric Disturbances- Volcanic eruptions or nuclear explosions altering ionospheric equilibrium (Causes unpredictable changes)
What is Sky wave frequency planning?
- Definition: Choosing specific frequencies for best communication performance.
- MUF: Highest usable frequency without signal loss.
- LUF: Lowest effective frequency without too much loss.
- FOT: Best reliable frequency, usually 85% of MUF.
- Daily Changes: Adjust frequencies based on time of day.
- Seasonal Changes: Adjust frequencies for different seasons.
- Sunspot Cycle: Change frequencies based on solar activity.
- Real-Time Monitoring: Measure conditions and adjust frequencies as needed.
- ALE: Automated systems pick best frequencies in real-time.
- Avoid Interference: Choose frequencies to avoid other signals and noise.
How does a ground wave propagate?
- Transmission: Sent from an antenna near the ground.
- Follows Earth’s Surface: Travels along the curvature of the Earth.
- Diffraction: Bends around obstacles and terrain.
- Penetration: Partially penetrates the ground or water.
- Attenuation: Loses strength over distance due to ground absorption.
- Best Over Water: Travels farther over water than land.
How does a space wave propagate
- Transmission: Sent from an elevated antenna.
- Line of Sight: Travels directly from transmitter to receiver.
- Limited Range: Limited to the horizon due to Earth’s curvature.
- Little Refraction: Minimal bending in the atmosphere.
- Direct and Reflected Paths: Can include both direct and ground-reflected signals.
- Higher Frequencies: Commonly used for VHF, UHF, and higher frequencies.
Draw and explain diode detector
1) Transformer (L) and Variable Capacitor (C1) act as filter to remove all but the required AM Signal.
2) Diode (D) removes negative half cycles of modulated carrier.
3) (R) and (C2) act as filter removing high frequency components (Charges/Slowly discharges).
4) (C3) acts as a DC blocker and removes bias voltage.
5) Signal may be smoothed by further filtering.
Draw and explain product detector
- Received AM signal (A) is mixed with locally produced replica of carrier (B) in a mixer.
- Output of mixer are A+B, A-B and harmonics of each (C)
- Difference frequency forms the baseband signal that was input at the transmitter. (D)
Draw and explain formation of dipole
- formed from an open circuit transmission line with conductors bent so currents run in same direction
- resonant length is λ/2 (half wavelength)
- requires 2 conductors with fields forming a donut shaped radiation pattern
Draw and explain Monopole
- formed by replacing one conductor of a dipole with an earth plane or reflective surface.
- Resonant length is λ/4 (quarter wavelength)
- Uses the ground or a conductive surface as the second conductor, which effectively reduces the length by half compared to a dipole
Explain PCM
- Definition: PCM is a method used to digitally represent analog signals.
- Sampling: The analog signal is sampled at regular intervals.
- Quantization: Each sample is then quantized to the nearest value within a range of digital steps.
- Encoding: The quantized values are encoded into a binary form.
- Transmission: The binary data is then transmitted as a series of pulses.
- Reconstruction: At the receiver, the binary data is decoded back into an analog signal.
Draw and Explain PRBS generator
- Initialization: Starts with an initial seed value.
- Shift Register: Uses a shift register where bits are shifted in each clock cycle.
- Feedback: Specific bits from the register are fed back through a predefined logic function (usually XOR).
- Sequence Generation: The feedback modifies the sequence, producing a long, repeating pseudo-random sequence.
- Repetition: The sequence eventually repeats after a maximum length determined by the register size and feedback taps.
- Applications: Used in communication systems for testing, synchronization, and encryption due to its predictable yet random-like output.
