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Digital Communications
ECP 601
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Professional Masters Program
Wireless Channels
Impairments and Models
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What affects Communication Quality
Effects due to • Receiver Design• Transmitter Design• Environment (Channel)• Other users (Interference)
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Receiver Design
• Receiver algorithms• MIMO decoder• Equalize design• Decoder (Viterbi, Turbo ,… )• others
• RF effects• IQ imbalance• Quantization• VGA performance
• Noise• Phase noise
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Noise
• Noise is (additive)• Noise is (random)• Noise can (not) be subtracted• Noise is always there!• Noise and capacity?
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Noise
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Thermal noiseCaused by random motion of electrons
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Modeling thermal noise
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Properties of Thermal Noise
• Exist in any conductor• Independent of DC current flow• Result of a large number of random,
superimposed collisions of electrons• Gaussian amplitude distortion• Can (not) be predicted• Its power can be predicted
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Equivalent noise model
• Equivalent noise temperature of the receiver
N0 = KTPn = N0B
• k: Boltzman constant• T: Equivalent noise temperature at the
receiver
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Modeling of receiver noise
• Additive : noise is added to the signal
• White: noise power spectral density is flat
• Gaussian: noise at any time is a Gaussian RV
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Modeling of receiver noise
Phase noise
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Transmitter Design
Power amplifier non-linearity
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Channel
• Free space loss• Small scale fading• Multipath effects• Slow versus fast fading
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Wireless Channel Impairments
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Wireless Channel Impairments
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Path loss
Why there is a competition on the TV band?• Between 400MHz-800MHz
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Path loss
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More models!
Two ray model
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More models!
10 ray model
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Empirical path-loss models
• Previous models are too theoretical• Why we need channel models?• Hence, field tests were made and more
models were developed
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Other attenuations (e.g. penetration)
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Penetration Loss
Penetration Loss
Source: “The mobile radio propagation channel”, J.D.Parson25
General path-loss model
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Okumura - Hata Model
• Distances: 1-100km• Frequency range: 150-1500 MHz• Based on extensive measurements
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Okumura - Hata Model
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Okumura - Hata Model
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Okumura - Hata Model
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Okumura - Hata Model
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Okumura – Hata Model
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COST 231 – Hata Model
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COST 231–Walfish–Ikegami
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Shadowing
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Fading
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Small scale fading
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Statistical multipath models
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Small scale fading
• The effect of the channel in small scale fading is complex Gaussian.
• Its amplitude follows Rayleigh distribution. • Its phase is uniform.
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WidebandFrequency Selective
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Power delay profile
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Power delay profile
•Mean Excess delay
•RMS delay spread
•Coherence BW
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Frequency selectivity
•Condition for frequency “flat” fading
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Effect of mobility
Doppler power spectrum
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Other channel models
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Other channel models
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Other channel models
