WN - 1.01 (Impairments and Mitigation Techniques)

Question 1

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

Answer 1

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

Question 2

Name all the possible mitigation techniques

Answer 2

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.

Question 3

Describe Path Loss

Answer 3

Path loss

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

pl = 1/d^2

Question 4

Describe shadowing

Answer 4

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

Question 5

Describe Multipath

Answer 5

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.

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

Question 6

Describe fading

Answer 6

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

Question 7

Describ noise

Answer 7

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.

Question 8

What metric is used to assess channel quality?

Answer 8

SNR

Question 9

Describe diversity

Answer 9

1. Combine independently receiver versions of the desired signal

Question 10

Name the different types of diversity

Answer 10

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

Question 11

Explain directional antennas

Answer 11

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

Question 12

Explain modulation and coding

Answer 12

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.

Question 13

Explain spread spectrum

Answer 13

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

Question 14

Explain frequency hopping

Answer 14

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.

Question 15

Explain direct sequence spread spectrum

Answer 15

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