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comm.ThermalNoise System object

Package: comm

Add thermal noise to signal

Description

The ThermalNoise object simulates the effects of thermal noise on a complex, baseband signal.

To add thermal noise to a complex, baseband signal:

  1. Define and set up your thermal noise object. See Construction.

  2. Call step to add thermal noise according to the properties of comm.ThermalNoise.

    Note:   Starting in R2016b, instead of using the step method to perform the operation defined by the System object™, you can call the object with arguments, as if it were a function. For example, y = step(obj,x) and y = obj(x) perform equivalent operations.

Construction

tn = comm.ThermalNoise creates a receiver thermal noise System object, H. This object adds thermal noise to the complex, baseband input signal.

tn = comm.ThermalNoise(Name,Value) creates a receiver thermal noise object, H, with each specified property set to the specified value. You can specify additional name-value pair arguments in any order as (Name1,Value1,...,NameN,ValueN).

Properties

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Method used to set the noise power, specified as 'Noise temperature', 'Noise figure', or 'Noise factor'.

Receiver noise temperature, specified in degrees K as a nonnegative real scalar. This property is available when NoiseMethod is equal to 'Noise temperature'. Noise temperature is typically used to characterize satellite receivers because the input noise temperature can vary and is often less than 290 K. Tunable.

Noise figure, specified in dB as a nonnegative real scalar. This property is available when NoiseMethod is equal to 'Noise figure'. Noise figure describes the performance of a receiver and does not include the effect of the antenna. It is defined only for an input noise temperature of 290 K. The noise figure is the dB equivalent of the noise factor. Tunable.

Noise factor, specified as a real scalar greater than or equal to 1. This property is available when NoiseMethod is equal to 'Noise factor'. Noise factor describes the performance of a receiver and does not include the effect of the antenna. It is defined only for an input noise temperature of 290 K. The noise factor is the linear equivalent of the noise figure. Tunable.

Sample rate, specified as in Hz as a positive real scalar. The object computes the variance of the noise added to the input signal as kT×SampleRate. The value k is Boltzmann's constant and T is the noise temperature specified explicitly or implicitly via one of the noise methods.

Add 290 K antenna noise to the input signal, specified as a logical scalar. To add 290 K antenna noise, set this property to true. This property is available when NoiseMethod is equal to 'Noise factor' or 'Noise figure'.

The total noise applied to the input signal is the sum of the circuit noise and the antenna noise.

Methods

cloneCreate thermal noise object with same property values
getNumInputsNumber of expected inputs to step method
getNumOutputsNumber of outputs from step method
isLockedLocked status for input attributes and nontunable properties
releaseAllow property value and input characteristics changes
stepAdd receiver thermal noise

Examples

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Create a thermal noise object having a noise temperature of 290 K and a sample rate of 5 MHz.

thNoise = comm.ThermalNoise('NoiseTemperature',290,'SampleRate',5e6);

Generate QPSK-modulated data having an output power of 20 dBm.

data = randi([0 3],1000,1);
modData = 0.3162*pskmod(data,4,pi/4);

Attenuate the signal by the free space path loss assuming a 1000 m link distance and a carrier frequency of 2 GHz.

fsl = (4*pi*1000*2e9/3e8)^2;
rxData = modData/sqrt(fsl);

Add thermal noise to the signal. Plot the noisy constellation.

noisyData = thNoise(rxData);
scatterplot(noisyData)

Create a thermal noise object having a 5 dB noise figure and a 10 MHz sample rate. Specify that the 290 K antenna noise be included.

thermalNoise = comm.ThermalNoise('NoiseMethod','Noise figure', ...
    'NoiseFigure',5, ...
    'SampleRate',10e6, ...
    'Add290KAntennaNoise',true);

Generate QPSK-modulated data having a 1 W output power.

data = randi([0 15],1000,1);
modSig = qammod(data,16,'UnitAveragePower',true);

Attenuate the signal by the free space path loss assumgin a 1 km link distance and a 5 GHz carrier frequency.

fsl = (4*pi*1000*5e9/3e8)^2;
rxSig = modSig/sqrt(fsl);

Add thermal noise to the signal and plot its constellation.

noisySig = thermalNoise(rxSig);
scatterplot(noisySig)

Estimate the SNR.

mer = comm.MER;
snrEst1 = mer(rxSig,noisySig)
snrEst1 =

   22.6611

Decrease the noise figure to 0 dB, and plot the resultant received signal. Because antenna noise is included, the signal is not completely noiseless.

thermalNoise.NoiseFigure = 0;
noisySig = thermalNoise(rxSig);
scatterplot(noisySig)

Estimate the SNR. The SNR is 5 dB higher than in the first case, which is expected given the 5 dB decrese in the noise figure.

snrEst2 = mer(rxSig,noisySig)
snrEst2 - snrEst1
snrEst2 =

   27.8658


ans =

    5.2047

Algorithms

Wireless receiver performance is often expressed as a noise factor or figure. The noise factor is defined as the ratio of the input signal-to-noise ratio, Si/Ni to the output signal-to-noise ratio, So/No, such that

F=Si/NiSo/No.

Given receiver gain G and receiver noise power Nckt, the noise factor can be expressed as

F=Si/NiGSi/(Nckt+GNi)=Nckt+GNiGNi.

The IEEE defines the noise factor assuming that noise temperature at the input is T0, where T0 = 290 K. The noise factor is then

F=Nckt+GNiGNi=GkBTckt+GkBT0GkBT0=Tckt+T0T0.

Tckt is the equivalent input noise temperature of the receiver and is expressed as

Tckt=T0(F1).

The overall noise temperature of an antenna and receiver, Tsys, is

Tsys=Tant+Tckt,

where Tant is the antenna noise temperature.

The noise figure, NF, is the dB equivalent of the noise factor and can be expressed as

NF=10log10(F).

Introduced in R2012a

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