WN - 1.01 (Impairments and Mitigation Techniques) Flashcards

1
Q

Name all the impairments mentioned in the lecture (6 impairments)

A
  1. Path loss
  2. Shadowing
  3. Fading
  4. Multipath
  5. Noise
  6. Interference
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2
Q

Name all the possible mitigation techniques

A
  1. Diversity
  2. Directional antennas
  3. Coding and modulation
  4. Spread system; transporting data through a wide range of frequencies
  5. System-level mitigation: frequency and code planning, multiple access methods etc.
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3
Q

Describe Path Loss

A

Path loss

  • Attenuation undergone by an electromagnetic wave in transit between a tx and rx.

pl = 1/d^2

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4
Q

Describe shadowing

A
  1. Models attenuation from obstacles
  2. Random event, random number of number and types of obstacles
  3. Typically modelled with a log-normal distribution
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5
Q

Describe Multipath

A
  1. Due to reflections and scattering, multiple versions of transmitted signal may arrive at the receiver.

Severe case -> out-of-phase versions cancel each other.

  1. Delay spread: original signal is spread due to different delays of parts of the signal -> leads to variability in frequency domain
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6
Q

Describe fading

A
  1. Quick variations in received power due to time-varying channel characteristics
  2. Various stochastic models (Rayleigh, Rician) for indoor and outdoor environments
  3. Variability in time domain
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7
Q

Describ noise

A
  1. Unwanted signals added to the message signal
  2. Unwanted natural (lightning) or man-made sources (tx and rx in passing cars).
  3. Often modelled as white noise; random signal in which power is distributed uniformly.
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8
Q

What metric is used to assess channel quality?

A

SNR

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9
Q

Describe diversity

A
  1. Combine independently receiver versions of the desired signal
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10
Q

Name the different types of diversity

A
  1. Spatial diversity; multiple antennas
  2. Frequency diversity; multiply frequencies
  3. Temporal diversity; transmit the same data or parts of it at different moments in time.
  4. Polarisation diveristy; different polarisations
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11
Q

Explain directional antennas

A
  1. Antenna array elements arranged in a certain geometry
  2. Maximise the gain in one direction while minimising it in other directions.
  3. Reduce interference
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12
Q

Explain modulation and coding

A

Digital modulation; QPSK, QAM

Multi-carrier modulation; splits the bit stream into several lower bit rate streams, each sent using an independent carrier frequency. e.g. OFDM

  • Frequency selective fading only affects some subcarriers, not the entire signal.

Forward error correction mitigates the effect of channel errors.

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13
Q

Explain spread spectrum

A

A signal is transmitted in a bandwidth considerably greater than the frequency content of the original information.

Wideband signals are noise-like -> difficult to detect or interfere with (jamming, interception)

Frequency hopping and direct sequence spread spectrum variants

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14
Q

Explain frequency hopping

A

A method of transmitting radio signals by rapidly switching the carrier between different frequency channels.

Hopping between different frequencies should reduce the interference of the signal.

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15
Q

Explain direct sequence spread spectrum

A

A modulation technique used in telecommunications, particularly in wireless communication systems, to transmit data by spreading it across a broader bandwidth than the original signa

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