Real-Time Digital Signal Processing Lab - Channel Impairments Overview
Explore the impact of channel impairments in communication systems, covering topics like additive thermal noise, LTI effects, and wireless propagation issues. Learn about signal processing techniques to address these challenges in the context of real-time digital signal processing lab.
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ECE 445S Real-Time Digital Signal Processing Lab Spring 2025 Channel Impairments Prof. Brian L. Evans Dept. of Electrical and Computer Engineering The University of Texas at Austin Lecture 12 http://www.ece.utexas.edu/~bevans/courses/realtime
Review Communication System Structure Information sources Voice, music, images, video, and data (baseband signals) Transmitter Signal processing block lowpass filters message signal Carrier circuits block upconverts baseband signal and bandpass filters to enforce transmission band baseband baseband bandpass bandpass baseband baseband Message Received Baseband Processing Analog/RF Front End Transmission Medium Analog/RF Front End Baseband Processing Message s(t) r(t) TRANSMITTER CHANNEL RECEIVER 12 - 2
Review Communication Channel Transmission medium Wireline (twisted pair, coaxial, fiber optics) Wireless (indoor/air, outdoor/air, underwater, space) Propagating signals degrade over distance Repeaters can strengthen signal and reduce noise Simple repeater is a bandpass filter and power amplifier Light painting Wi-Fi demo Light painting Wi-Fi demo baseband baseband bandpass bandpass baseband baseband Message Received Baseband Processing Analog/RF Front End Transmission Medium Analog/RF Front End Baseband Processing Message s(t) r(t) TRANSMITTER CHANNEL RECEIVER 12 - 3
Impairment: Additive Thermal Noise 12 - 4 Models thermal noise in the analog/RF front end of a receiver
Impairment: LTI Effects Wired linear time-invariant (LTI) effects Resistor-inductor-capacitor (RLC) model of wired channel LTI system must be at rest initial conditions must be zero (initialcurrent across inductors and voltage across capacitors) Second-order case Linear constant-coefficient differential equation (with zero initial conditions) Impulse response is a damped sinusoid h(t) + + t R L y(t) x(t) C t = 0 : 0.001 : 1; h = exp(-5*t) .* cos(2*pi*t); plot(t, h); xlabel('t'); ylabel('h(t)'); Impulse Response Derivation
Impairment: LTI Effects Wireless LTI effects due to wave propagation Reflection, absorption, and scattering Two Bounces Line of Sight 12 - 6
Impairment: LTI Effects Linear time-invariant effects Distortion in frequency: due to channel frequency response Spreading in time: due to channel impulse response ( ) y 0t ( ) x 0t input output b h+ b h Communication Channel t t x(t) y(t) -A Th -A Model channel as LTI system; then approximate the infinite impulse response as finite; further approximate using rect. pulse for timing analysis ( ) 1t x ( ) 1t y (t ) h A A Th 1 t b t h h h+ b t Bit of 0 or 1 12 - 7 Assume Th < Tb
Impairment: LTI Effects Spreading Effect Wired/wireless physical medium is IIR Model as FIR by truncating impulse response h(t) t
Impairment: Phase Jitter Linear time-varying effects: Phase jitter Sinusoid at same fixed frequency experiences different phase shifts when passing through channel Visualize phase jitter in periodic waveform by plotting it over one period, superimposing second period on the first, etc. fc = 1; Tc = 1 / fc; fs = 100*fc; Ts = 1/fs; t = Ts : Ts : Tc; numPeriods = 10; figure; hold on; for i = 1 : numPeriods phase = 0.05*randn(1, length(t)); plot(t, cos(2*pi*fc*t + phase)); pause(1); end hold off; 12 - 9
Impairment: Phase Jitter Linear time-varying effects: Phase jitter Transmission of two-level sinusoidal amplitude modulation Bit of value 1 becomes an amplitude of +1 Bit of value 0 becomes an amplitude of -1 Visualize by superimposing each period on first (eye diagram) fc = 1; Tc = 1 / fc; fs = 100*fc; Ts = 1/fs; t = Ts : Ts : Tc; numPeriods = 10; figure; hold on; for i = 1 : numPeriods symAmp = round(2*(randn() > 0) - 1); phase = 0.05*randn(1, length(t)); plot(t, symAmp*cos(2*pi*fc*t + phase)); pause(1); end hold off; 12 - 10
Impairment: Additive Interference Nonlinear effects Harmonics: due to quantization, voltage rectifiers, squaring devices, power amplifiers, etc. Additive noise: arises from many sources in transmitter, channel, and receiver (e.g. thermal noise) Additive interference: arises from other systems operating in transmission band Spectrogram of emissions from a microwave oven in unlicensed 2.4 GHz band sweeping across IEEE 802.11g channels 1, 6, 11 [Nassar, Lin & Evans, 2011] Keysight Wi-Fi demo Keysight Wi-Fi demo
Impairment: Additive Interference Power Spectral Density Estimate -75 -80 Power/frequency (dB/Hz) -85 -90 -95 -100 -105 -110 -115 -120 -125 0 10 20 30 40 50 60 70 80 90 Frequency (kHz) Measurement taken on a wall power plug in an apartment in Austin, Texas, on March 20, 2011 Home Powerline 12 - 12
Impairment: Additive Interference Power Spectral Density Estimate -75 -80 Power/frequency (dB/Hz) -85 -90 -95 -100 -105 -110 -115 Spectrally-Shaped Background Noise -120 -125 0 10 20 30 40 50 60 70 80 90 Frequency (kHz) Measurement taken on a wall power plug in an apartment in Austin, Texas, on March 20, 2011 Home Powerline 12 - 13
Impairment: Additive Interference Power Spectral Density Estimate -75 Narrowband Interference -80 Power/frequency (dB/Hz) -85 -90 -95 -100 -105 -110 -115 Spectrally-Shaped Background Noise -120 -125 0 10 20 30 40 50 60 70 80 90 Frequency (kHz) Measurement taken on a wall power plug in an apartment in Austin, Texas, on March 20, 2011 Home Powerline 12 - 14
Impairment: Additive Interference Power Spectral Density Estimate Periodic and Asynchronous Interference -75 Narrowband Interference -80 Power/frequency (dB/Hz) -85 -90 -95 -100 -105 -110 -115 Spectrally-Shaped Background Noise -120 -125 0 10 20 30 40 50 60 70 80 90 Frequency (kHz) Measurement taken on a wall power plug in an apartment in Austin, Texas, on March 20, 2011 Home Powerline 12 - 15
Impairment: Fading Same as wireline channel impairments plus others Fading: multiplicative noise Talking on a mobile phone and reception fades in and out Represented as time-varying gain that follows a particular probability distribution Model #4: fading, LTI effects additive noise, and additive interference FIR + + 0 a fading interference noise 12 - 16
