Initialization

The following variables control both the Rayleigh and Rician channel objects. By default, the channel is modeled as four fading paths, each representing a cluster of multipath components received at around the same delay.

sampleRate500kHz = 500e3;% Sample rate of 500K HzsampleRate20kHz = 20e3;% Sample rate of 20K HzmaxDopplerShift = 200;% Maximum Doppler shift of diffuse components (Hz)delayVector = (0:5:15)*1e-6;% Discrete delays of four-path channel (s)gainVector = [0 -3 -6 -9];% Average path gains (dB)

The maximum Doppler shift is computed as v*f/c, where v is the mobile speed, f is the carrier frequency, and c is the speed of light. For example, a maximum Doppler shift of 200 Hz (as above) corresponds to a mobile speed of 65 mph (30 m/s) and a carrier frequency of 2 GHz.

By convention, the delay of the first path is typically set to zero. For subsequent paths, a 1 microsecond delay corresponds to a 300 m difference in path length. In some outdoor multipath environments, reflected paths can be up to several kilometers longer than the shortest path. With the path delays specified above, the last path is 4.5 km longer than the shortest path, and thus arrives 15 microseconds later.

Together, the path delays and path gains specify the average delay profile of the channel. Typically, the average path gains decay exponentially with delay (i.e., the dB values decay linearly), but the specific delay profile depends on the propagation environment. In the delay profile specified above, we assume a 3 dB decrease in average power for every 5 microseconds of path delay.

The following variables control the Rician channel System object. The Doppler shift of the specular component is typically smaller than the maximum Doppler shift (above) and depends on the direction of travel of the mobile relative to the direction of the specular component. The K-factor specifies the linear ratio of average received power from the specular component relative to that of the associated diffuse components.

KFactor = 10;% Linear ratio of specular power to diffuse powerspecDopplerShift = 100;% Doppler shift of specular component (Hz)

Creating Channel System Objects

With the parameters specified above, we can now create thecomm.RayleighChannelandcomm.RicianChannelSystem objects. We configure the objects to use their self-contained random stream with a specified seed for path gain generation.

% Configure a Rayleigh channel objectrayChan = comm.RayleighChannel(...'SampleRate'sampleRate500kHz,...'PathDelays',delayVector,...'AveragePathGains',gainVector,...'MaximumDopplerShift',maxDopplerShift,...'RandomStream','mt19937ar with seed',...'Seed',10,...'PathGainsOutputPort',true);% Configure a Rician channel objectricChan = comm.RicianChannel(...'SampleRate'sampleRate500kHz,...'PathDelays',delayVector,...'AveragePathGains',gainVector,...'KFactor',KFactor,...'DirectPathDopplerShift'specDopplerShift,...'MaximumDopplerShift',maxDopplerShift,...'RandomStream','mt19937ar with seed',...'Seed', 100,...'PathGainsOutputPort',true);

Modulation and Channel Filtering

Create acomm.QPSKModulator系统对象调节通道的数据,has been generated using therandifunction. In the code here a 'frame' refers to a vector of information bits. A phase offset of pi/4 is used for this example.

qpskMod = comm.QPSKModulator('BitInput',true,'PhaseOffset',pi/4);% Number of bits transmitted per frame is set to be 1000. For QPSK% modulation, this corresponds to 500 symbols per frame.bitsPerFrame = 1000; msg = randi([0 1],bitsPerFrame,1);% Modulate data for transmission over channelmodSignal = qpskMod(msg);% Apply Rayleigh or Rician channel object on the modulated datarayChan(modSignal); ricChan(modSignal);

Visualization

The fading channel System objects have built-in visualization to show the channel impulse response, frequency response, or Doppler spectrum when the object runs. To invoke it, set theVisualizationproperty to the desired value before calling the object. Release the Rayleigh and Rician channel System objects now so that to change their property values.

释放(rayChan);release(ricChan);

Wideband or Frequency-Selective Fading

Setting theVisualizationproperty to'Impulse response'shows the bandlimited impulse response (yellow circles). The visualization also shows the delays and magnitudes of the underlying fading path gains (pink stembars) clustered around the peak of the impulse response. Note that the path gains do not equal theAveragePathGainsproperty value because the Doppler effect causes the gains to fluctuate over time.

Similarly, setting theVisualizationproperty to'Frequency response'shows the frequency response (DFT transformation) of the impulses. You can also setVisualizationto'Impulse and frequency responses'to display both impulse and frequency responses side by side.

You can control the percentage of the input samples to be visualized by changing theSamplesToDisplayproperty. In general, the smaller the percentage, the faster the simulation runs. Once the visualization figure opens up, click thePlaybackbutton and turn off the "Reduce Updates to Improve Performance" or "Reduce Plot Rate to Improve Performance" option to further improve display accuracy. The option is on by default for faster simulation. To see the channel response for every input sample, uncheck this option and setSamplesToDisplayto'100%'.

rayChan。Visualization ='Impulse and frequency responses';rayChan。SamplesToDisplay ='100%';% Display impulse and frequency responses for 2 framesnumFrames = 2;fori = 1:numFrames% Create random datamsg = randi([0 1],bitsPerFrame,1);% Modulate datamodSignal = qpskMod(msg);% Filter data through channel and show channel responsesrayChan(modSignal);end

As you can see, the channel frequency response is not flat and may have deep fades over the 500K Hz bandwidth. Because the power level varies over the bandwidth of the signal, it is referred to as frequency-selective fading.

For the same channel specification, we now display the Doppler spectrum for its first discrete path, which is a statistical characterization of the fading process. The System object makes periodic measurements of the Doppler spectrum (blue stars). Over time with more samples processed by the System object, the average of this measurement better approximates the theoretical Doppler spectrum (yellow curve).

释放(rayChan);rayChan。Visualization ='Doppler spectrum';% Display Doppler spectrum from 5000 frame transmissionnumFrames = 5000;fori = 1:numFrames msg = randi([0 1],bitsPerFrame,1); modSignal = qpskMod(msg); rayChan(modSignal);end

Narrowband or Frequency-Flat Fading

When the bandwidth is too small for the signal to resolve the individual components, the frequency response is approximately flat because of the minimal time dispersion caused by the multipath channel. This kind of multipath fading is often referred to as narrowband fading, or frequency-flat fading.

We now reduce the signal bandwidth from 500 kb/s (250 ksym/s) to 20 kb/s (10 ksym/s), so the delay span (15 microseconds) of the channel is much smaller than the QPSK symbol period (100 microseconds). The resultant impulse response has very small intersymbol interference (ISI) and the frequency response is approximately flat.

释放(rayChan);rayChan。Visualization ='Impulse and frequency responses';rayChan。SampleRate = sampleRate20kHz; rayChan.SamplesToDisplay ='25%';% Display one of every four samples% Display impulse and frequency responses for 2 framesnumFrames = 2;fori = 1:numFrames msg = randi([0 1],bitsPerFrame,1); modSignal = qpskMod(msg); rayChan(modSignal);end

To simplify and speed up modeling, narrowband fading channels are typically modeled as a single-path fading channel. That is, a multipath fading model overspecifies a narrowband fading channel. The following settings correspond to a narrowband fading channel. Notice that the shape of the bandlimited impulse response is flat.

释放(rayChan);rayChan。PathDelays = 0;% Single fading path with zero delayrayChan。AveragePathGains = 0;% Average path gain of 1 (0 dB)fori = 1:numFrames% Display impulse and frequency responses for 2 framesmsg = randi([0 1],bitsPerFrame,1); modSignal = qpskMod(msg); rayChan(modSignal);end

The Rician fading channel System object models line-of-sight propagation in addition to diffuse multipath scattering. This results in a smaller variation in the magnitude of path gains. To compare the variation between Rayleigh and Rician channels, we make use of atimescopeobject to view their path gains over time. Note that the magnitude fluctuates over approximately a 10 dB range for the Rician fading channel (blue curve), compared with 30-40 dB for the Rayleigh fading channel (yellow curve). For the Rician fading channel, this variation would be further reduced by increasing the K-factor (currently set to 10).

释放(rayChan);rayChan。Visualization ='Off';% Turn off Rayliegh object visualizationricChan.Visualization ='Off';% Turn off Rician object visualization% Same sample rate and delay profile for the Rayleigh and Rician objectsricChan.SampleRate = rayChan.SampleRate; ricChan.PathDelays = rayChan.PathDelays; ricChan.AveragePathGains = rayChan.AveragePathGains;% Configure a Time Scope System object to show path gain magnitudegainScope = timescope(...'SampleRate',rayChan.SampleRate,...'TimeSpanSource','Property',...'TimeSpan',bitsPerFrame/2/rayChan.SampleRate,...% One frame span'Name','Multipath Gain',...'ShowGrid',true,...'YLimits',[-40 10],...'YLabel','Gain (dB)');% Compare the path gain outputs from both objects for one framemsg = randi([0 1],bitsPerFrame,1); modSignal = qpskMod(msg); [~,rayPathGain] = rayChan(modSignal); [~,ricPathGain] = ricChan(modSignal);% Form the path gains as a two-channel input to the time scopegainScope(10*log10(abs([rayPathGain,ricPathGain]).^2));

Fading Channel Impact on Signal Constellation

We now return to our original four-path Rayleigh fading channel. We use acomm.ConstellationDiagramSystem object to show the impact of narrowband fading on the signal constellation. To slow down the channel dynamics for visualization purposes, we reduce the maximum Doppler shift to 5 Hz. Compared with the QPSK channel input signal, you can observe signal attenuation and rotation at the channel output, as well as some signal distortion due to the small amount of ISI in the received signal.

clearhRicChanhMultipathGain;释放(rayChan);rayChan。PathDelays = delayVector; rayChan.AveragePathGains = gainVector; rayChan.MaximumDopplerShift = 5;% Configure a Constellation Diagram System object to show received signalconstDiag = comm.ConstellationDiagram(...'Name','Received Signal After Rayleigh Fading'); numFrames = 16;forn = 1:numFrames msg = randi([0 1],bitsPerFrame,1); modSignal = qpskMod(msg); rayChanOut = rayChan(modSignal);% Display constellation diagram for Rayleigh channel outputconstDiag(rayChanOut);end

When we increase the signal bandwidth to 500 kb/s (250 ksym/s), we see much greater distortion in the signal constellation. This distortion is the ISI that comes from time dispersion of the wideband signal. The delay span (15 microseconds) of the channel is now larger than the QPSK symbol period (4 microseconds), so the resultant bandlimited impulse response is no longer approximately flat.

释放(rayChan);release(constDiag); rayChan.SampleRate = sampleRate500kHz;forn = 1:numFrames msg = randi([0 1],bitsPerFrame,1); modSignal = qpskMod(msg); rayChanOut = rayChan(modSignal); constDiag(rayChanOut);end