WN - 1.01 (Impairments and Mitigation Techniques) Flashcards
Name all the impairments mentioned in the lecture (6 impairments)
- Path loss
- Shadowing
- Fading
- Multipath
- Noise
- Interference
Name all the possible mitigation techniques
- Diversity
- Directional antennas
- Coding and modulation
- Spread system; transporting data through a wide range of frequencies
- System-level mitigation: frequency and code planning, multiple access methods etc.
Describe Path Loss
Path loss
- Attenuation undergone by an electromagnetic wave in transit between a tx and rx.
pl = 1/d^2
Describe shadowing
- Models attenuation from obstacles
- Random event, random number of number and types of obstacles
- Typically modelled with a log-normal distribution
Describe Multipath
- Due to reflections and scattering, multiple versions of transmitted signal may arrive at the receiver.
Severe case -> out-of-phase versions cancel each other.
- Delay spread: original signal is spread due to different delays of parts of the signal -> leads to variability in frequency domain
Describe fading
- Quick variations in received power due to time-varying channel characteristics
- Various stochastic models (Rayleigh, Rician) for indoor and outdoor environments
- Variability in time domain
Describ noise
- Unwanted signals added to the message signal
- Unwanted natural (lightning) or man-made sources (tx and rx in passing cars).
- Often modelled as white noise; random signal in which power is distributed uniformly.
What metric is used to assess channel quality?
SNR
Describe diversity
- Combine independently receiver versions of the desired signal
Name the different types of diversity
- Spatial diversity; multiple antennas
- Frequency diversity; multiply frequencies
- Temporal diversity; transmit the same data or parts of it at different moments in time.
- Polarisation diveristy; different polarisations
Explain directional antennas
- Antenna array elements arranged in a certain geometry
- Maximise the gain in one direction while minimising it in other directions.
- Reduce interference
Explain modulation and coding
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.
Explain spread spectrum
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
Explain frequency hopping
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.
Explain direct sequence spread spectrum
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