LOS Channel

Narrowband line-of-sight propagation channel

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Libraries:
Phased Array System Toolbox / Environment and Target

Description

The LOS Channel block propagates signals from a point in space to another point in space by line-of-sight (LOS) channels. The block can also propagate signals from one point to multiple points or from multiple points back to one point. The block models propagation time, free-space propagation loss, Doppler shift, and atmospheric as well as weather loss. The block assumes that the propagation speed is much greater than the object speed in which case the stop-and-hop model is valid.

When propagating a signal in an LOS channel to an object and back, you can use a single block to compute two-way LOS channel propagation delays or two blocks to perform two one-way propagation delays in each direction. Because the LOS channel propagation delay is not necessarily an integer multiple of the sampling interval, it can turn out that the total round trip delay in samples when you use a two-way propagation block differs from the delay in samples when you use two one-way propagation blocks. For this reason, it is recommended that, when possible, you use a single two-way propagation block.

Ports

Input

X — Narrowband signal

M-element complex-valued column vector, M-by-N complex-valued matrix.

Narrowband signal, specified as an M-element complex-valued column vector or M-by-N complex-valued matrix. The quantity M is the number of sample values of the signal and N is the number of signals to propagate. When you specify N signals, you need to specify N signal origins or N signal destinations.

The size of the first dimension of the input matrix can vary to simulate a changing signal length. A size change can occur, for example, in the case of a pulse waveform with variable pulse repetition frequency.

Pos1 — Signal origin

3-by-1 real-valued column vector | 3-by-N real-valued matrix

Signal origin, specified as a 3-by-1 real-valued column vector or 3-by-N real-valued matrix. The quantity N is the number of propagated signals and equals the dimension specified in the signal input port X. If Pos1 is a column vector, it takes the form [x; y; z]. If Pos1 is a matrix, each column specifies a different signal origin and has the form [x; y; z]. Pos1 and Pos2 cannot both be specified as matrices — at least one must be a 3-by-1 column vector. Position units are meters.

Data Types: double

Pos2 — Signal destination

3-by-1 real-valued column vector | 3-by-N real-valued matrix

Signal destination, specified as a 3-by-1 real-valued column vector or 3-by-N real-valued matrix. The quantity N is the number of propagated signals and equals the dimension specified in the signal in the input port X. If Pos2 is a column vector, it takes the form [x; y; z]. If Pos2 is a matrix, each column specifies a different signal origin and has the form [x; y; z]. Pos2 and Pos1 cannot both be specified as matrices — at least one must be a 3-by-1 column vector. Position units are meters.

Data Types: double

Vel1 — Signal origin velocity

3-by-1 real-valued column vector | 3-by-N real-valued matrix

Signal origin velocity, specified as a 3-by-1 real-valued column vector or 3-by-N real-valued matrix. The quantity N is the number of propagated signals and equals the dimension specified in the signal into the port X. If Vel1 is a column vector, it takes the form [Vx; Vy; zV]. If Vel1 is a matrix, each column specifies a different signal origin and has the form [Vx; Vy; Vz]. The dimensions of Vel1 must agree with the dimensions of Pos1. Vel1 and Vel2 cannot both be specified as matrices — at least one must be a 3-by-1 column vector. Position units are meters.

Data Types: double

Vel2 — Signal destination velocity

3-by-1 real-valued column vector | 3-by-N real-valued matrix

Signal destination velocity, specified as a 3-by-1 real-valued column vector or 3-by-N real-valued matrix. The quantity N is the number of propagated signals and equals the dimension specified in the signal into the port X. If Vel2 is a column vector, it takes the form [Vx; Vy; zV]. If Vel2 is a matrix, each column specifies a different signal origin and has the form [Vx; Vy; Vz]. The dimensions of Vel2 must agree with the dimensions of Pos2. Vel2 and Vel1 cannot both be specified as matrices — at least one must be a 3-by-1 column vector. Position units are meters.

Data Types: double

Output

Port_1 — Propagated narrowband signal

M-element complex-valued column vector, M-by-N complex-valued matrix.

Propagated signal, returned as a M-element complex-valued column vector, M-by-N complex-valued matrix. Y will have the same dimensions as X.

The output Y contains signal samples arriving at the signal destination within the current time frame. The current time frame is defined as the time spanned by the current input. Whenever it takes longer than the current time frame for the signal to propagate from the origin to the destination, the output contains no contribution from the input of the current time frame.

Parameters

Propagation speed (m/s) — Signal propagation speed

physconst('LightSpeed') (default) | positive scalar

Signal propagation speed, specified as a real-valued positive scalar. The default value of the speed of light is the value returned by physconst('LightSpeed').

Data Types: double

Signal carrier frequency (Hz) — Signal carrier frequency

3e8 (default) | positive scalar

Signal carrier frequency, specified as a positive scalar. Units are in Hz.

Data Types: double

Specify atmospheric parameters — Enable atmospheric attenuation model

off (default) | on

Select this parameter to enable to add signal attenuation caused by atmospheric gases, rain, fog, or clouds. When you select this parameter, the Temperature (degrees Celsius), Dry air pressure (Pa), Water vapour density (g/m^3), Liquid water density (g/m^3), and Rain rate (mm/hr) parameters appear in the dialog box.

Data Types: Boolean

Temperature (degrees Celsius) — Ambient temperature

15 (default) | real-valued scalar

Ambient temperature, specified as a real-valued scalar. Units are in degrees Celsius.

Dependencies

To enable this parameter, select the Specify atmospheric parameters checkbox.

Data Types: double

Dry air pressure (Pa) — Atmospheric dry air pressure

101.325e3 (default) | positive real-valued scalar

Atmospheric dry air pressure, specified as a positive real-valued scalar. Units are in pascals (Pa). The default value of this parameter corresponds to one standard atmosphere.

Dependencies

To enable this parameter, select the Specify atmospheric parameters checkbox.

Data Types: double

Water vapour density (g/m^3) — Atmospheric water vapor density

7.5 (default) | positive real-valued scalar

Atmospheric water vapor density, specified as a positive real-valued scalar. Units are in g/m³.

Dependencies

To enable this parameter, select the Specify atmospheric parameters checkbox.

Data Types: datetime

Liquid water density (g/m^3) — Liquid water density

0.0 (default) | nonnegative real-valued scalar

Liquid water density of fog or clouds, specified as a nonnegative real-valued scalar. Units are in g/m³. Typical values for liquid water density are 0.05 for medium fog and 0.5 for thick fog.

Dependencies

To enable this parameter, select the Specify atmospheric parameters checkbox.

Data Types: double

Rain rate (mm/hr) — Rainfall rate

0.0 (default) | non-negative real-valued scalar

Rainfall rate, specified as a nonnegative real-valued scalar. Units are in mm/hr.

Dependencies

To enable this parameter, select the Specify atmospheric parameters checkbox.

Data Types: double

Perform two-way propagation — Enable two-way propagation

off (default) | on

Select this check box to perform round-trip propagation between the origin and destination. Otherwise the block performs one-way propagation from the origin to the destination.

Inherit sample rate — Inherit sample rate from upstream blocks

on (default) | off

Select this parameter to inherit the sample rate from upstream blocks. Otherwise, specify the sample rate using the Sample rate (Hz) parameter.

Data Types: Boolean

Sample rate (Hz) — Sampling rate of signal

1e6 (default) | positive real-valued scalar

Specify the signal sampling rate as a positive scalar. Units are in Hz.

Dependencies

To enable this parameter, clear the Inherit sample rate check box.

Data Types: double

Maximum one-way propagation distance (m) — Maximum one-way propagation distance

10e3 (default) | positive real-valued scalar

Specify the maximum distance between the signal origin and the destination as a positive scalar. Units are in meters. Amplitudes of any signals that propagate beyond this distance will be set to zero.

Simulate using — Block simulation method

Interpreted Execution (default) | Code Generation

Block simulation, specified as Interpreted Execution or Code Generation. If you want your block to use the MATLAB^® interpreter, choose Interpreted Execution. If you want your block to run as compiled code, choose Code Generation. Compiled code requires time to compile but usually runs faster.

Interpreted execution is useful when you are developing and tuning a model. The block runs the underlying System object™ in MATLAB. You can change and execute your model quickly. When you are satisfied with your results, you can then run the block using Code Generation. Long simulations run faster with generated code than in interpreted execution. You can run repeated executions without recompiling, but if you change any block parameters, then the block automatically recompiles before execution.

This table shows how the Simulate using parameter affects the overall simulation behavior.

When the Simulink^® model is in Accelerator mode, the block mode specified using Simulate using overrides the simulation mode.

Acceleration Modes

Block Simulation	Simulation Behavior
	Normal	Accelerator	Rapid Accelerator
Interpreted Execution	The block executes using the MATLAB interpreter.	The block executes using the MATLAB interpreter.	Creates a standalone executable from the model.
Code Generation	The block is compiled.	All blocks in the model are compiled.

For more information, see Choosing a Simulation Mode (Simulink).

Programmatic Use

Block Parameter:SimulateUsing

Type:enum

Values:Interpreted Execution, Code Generation

Default:Interpreted Execution

More About

Attenuation and Loss Factors

Attenuation or path loss in the Wideband LOS channel consists of four components. L = L_fspL_gL_cL_r, where

• L_fsp is the free-space path attenuation

• L_g is the atmospheric path attenuation

• L_c is the fog and cloud path attenuation

• L_r is the rain path attenuation

Each component is in magnitude units, not in dB.

Propagation Delay, Doppler, and Free-Space Path Loss

When the origin and destination are stationary relative to each other, you can write the output signal of a free-space channel as Y(t) = x(t-τ)/L_fsp. The quantity τ is the signal delay and L_fsp is the free-space path loss. The delay τ is given by R/c, where R is the propagation distance and c is the propagation speed. The free-space path loss is given by

$$L_{fsp}=\frac{{(4\pi R)}^2}{{\lambda }^2},$$

where λ is the signal wavelength.

This formula assumes that the target is in the far field of the transmitting element or array. In the near field, the free-space path loss formula is not valid and can result in a loss smaller than one, equivalent to a signal gain. Therefore, the loss is set to unity for range values, R ≤ λ/4π.

When the origin and destination have relative motion, the processing also introduces a Doppler frequency shift. The frequency shift is v/λ for one-way propagation and 2v/λ for two-way propagation. The quantity v is the relative speed of the destination with respect to the origin.

Atmospheric Gas Attenuation Model

This model calculates the attenuation of signals that propagate through atmospheric gases.

Electromagnetic signals attenuate when they propagate through the atmosphere. This effect is due primarily to the absorption resonance lines of oxygen and water vapor, with smaller contributions coming from nitrogen gas. The model also includes a continuous absorption spectrum below 10 GHz. The ITU model Recommendation ITU-R P.676-10: Attenuation by atmospheric gases is used. The model computes the specific attenuation (attenuation per kilometer) as a function of temperature, pressure, water vapor density, and signal frequency. The atmospheric gas model is valid for frequencies from 1–1000 GHz and applies to polarized and nonpolarized fields.

The formula for specific attenuation at each frequency is

$$\gamma ={\gamma }_o(f)+{\gamma }_w(f)=0.1820f{N}^{″}(f).$$

The quantity N"() is the imaginary part of the complex atmospheric refractivity and consists of a spectral line component and a continuous component:

$$N^{″}(f)={\displaystyle \sum _i{S}_i{F}_i+{N^{″}}_D^{}(f)}$$

The spectral component consists of a sum of discrete spectrum terms composed of a localized frequency bandwidth function, F(f)_i, multiplied by a spectral line strength, S_i. For atmospheric oxygen, each spectral line strength is

$$S_i=a_1\times {10}^{-7}{\left(\frac{300}{T}\right)}^3\exp \left[a_2(1-\left(\frac{300}{T}\right)\right]P.$$

For atmospheric water vapor, each spectral line strength is

$$S_i=b_1\times {10}^{-1}{\left(\frac{300}{T}\right)}^{3.5}\exp \left[b_2(1-\left(\frac{300}{T}\right)\right]W.$$

P is the dry air pressure, W is the water vapor partial pressure, and T is the ambient temperature. Pressure units are in hectoPascals (hPa) and temperature is in degrees Kelvin. The water vapor partial pressure, W, is related to the water vapor density, ρ, by

$$W=\frac{\rho T}{216.7}.$$

The total atmospheric pressure is P + W.

For each oxygen line, S_i depends on two parameters, a₁ and a₂. Similarly, each water vapor line depends on two parameters, b₁ and b₂. The ITU documentation cited at the end of this section contains tabulations of these parameters as functions of frequency.

The localized frequency bandwidth functions F_i(f) are complicated functions of frequency described in the ITU references cited below. The functions depend on empirical model parameters that are also tabulated in the reference.

This model applies to both narrowband and wideband atmospheric attenuation. To compute the total attenuation for narrowband signals along a path, the function multiplies the specific attenuation by the path length, R. Then, the total attenuation is L_g= R(γ_o + γ_w). To apply the attenuation model to wideband signals, first, divide the wideband signal into frequency subbands, and apply attenuation to each subband. Then, sum all attenuated subband signals into the total attenuated signal.

Fog and Cloud Attenuation Model

This model calculates the attenuation of signals that propagate through fog or clouds.

Fog and cloud attenuation are due to the same atmospheric phenomenon. The ITU model, Recommendation ITU-R P.840-6: Attenuation due to clouds and fog is used. The model computes the specific attenuation (attenuation per kilometer), of a signal as a function of liquid water density, signal frequency, and temperature. The model applies to polarized and nonpolarized fields. The formula for specific attenuation at each frequency is

$${\gamma }_c=K_l\left(f\right)M,$$

where M is the liquid water density in gm/m³. The quantity K_l(f) is the specific attenuation coefficient and depends on frequency. The cloud and fog attenuation model is valid for frequencies 10–1000 GHz. Units for the specific attenuation coefficient are (dB/km)/(g/m³).

To compute the total attenuation for narrowband signals along a path, the function multiplies the specific attenuation by the path length R. Total attenuation is L_c = Rγ_c. You can also apply the attenuation model to wideband signals. First, divide the wideband signal into frequency subbands, and apply narrowband attenuation to each subband. Then, sum all attenuated subband signals into the total attenuated signal.

Rainfall Attenuation Model

This model calculates the attenuation of signals that propagate through regions of rainfall. Rain attenuation is a dominant fading mechanism and can vary from location-to-location and from year-to-year.

Electromagnetic signals are attenuated when propagating through a region of rainfall. Rainfall attenuation is computed according to the ITU rainfall model Recommendation ITU-R P.838-3: Specific attenuation model for rain for use in prediction methods. The model computes the specific attenuation (attenuation per kilometer) of a signal as a function of rainfall rate, signal frequency, polarization, and path elevation angle. The specific attenuation, ɣ_R, is modeled as a power law with respect to rain rate

$${\gamma }_R=k{R}^{\alpha },$$

where R is rain rate. Units are in mm/hr. The parameter k and exponent α depend on the frequency, the polarization state, and the elevation angle of the signal path. The specific attenuation model is valid for frequencies from 1–1000 GHz.

To compute the total attenuation for narrowband signals along a path, the function multiplies the specific attenuation by the an effective propagation distance, d_eff. Then, the total attenuation is L = d_effγ_R.

The effective distance is the geometric distance, d, multiplied by a scale factor

$$r=\frac{1}{0.477d^{0.633}{R}_{0.01}^{0.073\alpha }{f}^{0.123}-10.579\left(1-\exp \left(-0.024d\right)\right)}$$

where f is the frequency. The article Recommendation ITU-R P.530-17 (12/2017): Propagation data and prediction methods required for the design of terrestrial line-of-sight systems presents a complete discussion for computing attenuation.

The rain rate, R, used in these computations is the long-term statistical rain rate, R_0.01. This is the rain rate that is exceeded 0.01% of the time. The calculation of the statistical rain rate is discussed in Recommendation ITU-R P.837-7 (06/2017): Characteristics of precipitation for propagation modelling. This article also explains how to compute the attenuation for other percentages from the 0.01% value.

You can also apply the attenuation model to wideband signals. First, divide the wideband signal into frequency subbands and apply attenuation to each subband. Then, sum all attenuated subband signals into the total attenuated signal.

Version History

Introduced in R2016a