Wideband LOS Channel

Wideband line-of-sight propagation channel

Library

Environment and Target

phasedenvlib

Description

The Wideband LOS Channel block propagates signals from one point in space to multiple points or from multiple points back to one point via line-of-sight (LOS) channels. 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's 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 have the choice of either using a single block to compute two-way LOS channel propagation delay or two blocks to perform one-way propagation delays in each direction. Because the LOS channel propagation delay is not necessarily an integer multiple of the sampling interval, it may 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.

Parameters

Signal Propagation speed (m/s)

Specify the propagation speed of the signal, in meters per second, as a positive scalar. You can use the function physconst to specify the speed of light.

Signal carrier frequency (Hz)

Specify the carrier frequency of the signal in hertz of the narrowband signal as a positive scalar.

Number of subbands

The number of subbands used for subband processing, specified as a positive integer.

Specify atmospheric parameters

Select this check box to enable atmospheric attenuation modeling.

Temperature (degrees Celsius)

Ambient atmospheric temperature, specified as a real-valued scalar. Units are degrees Celsius. This parameter appears when you select the Specify atmospheric parameters check box. Units are degrees Celsius.

Dry air pressure (Pa)

Atmospheric dry air pressure, specified as a positive real-valued scalar. Units are Pascals (Pa). The value 101325 for this property corresponds to one standard atmosphere. This parameter appears when you select the Specify atmospheric parameters check box.

Water vapour density (g/m^3)

Atmospheric water vapor density, specified as a positive real-valued scalar. Units are gm/m³. This parameter appears when you select the Specify atmospheric parameters check box.

Liquid water density (g/m^3)

Liquid water density of fog or clouds, specified as a non-negative real-valued scalar. Units are gm/m³. Typical values for liquid water density are 0.05 for medium fog and 0.5 for thick fog. This parameter appears when you select the Specify atmospheric parameters check box.

Rain rate (mm/hr)

Rainfall rate, specified as a non-negative real-valued scalar. Units are in mm/hour. This parameter appears when you select the Specify atmospheric parameters check box.

Perform two-way propagation

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

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

Sample rate (Hz)

Specify the signal sampling rate (in hertz) as a positive scalar. This parameter appears only when the Inherit sample rate parameter is not selected.

Maximum one-way propagation distance (m)

The maximum distance between the signal origin and the destination, specified 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, 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 than they would in interpreted execution. You can run repeated executions without recompiling. However, if you change any block parameters, then the block automatically recompiles before execution.

When setting this parameter, you must take into account the overall model simulation mode. The table shows how the Simulate using parameter interacts with the overall simulation mode.

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
Block Simulation	`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.	Creates a standalone executable from the model.

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

Ports

Note

The block input and output ports correspond to the input and output parameters described in the step method of the underlying System object. See link at the bottom of this page.

Port	Description	Supported Data Types
`X`	Input signal.	Double-precision floating point
`Pos1`	Signal source position.	Double-precision floating point
`Pos2`	Signal destination position.	Double-precision floating point
`Vel1`	Signal source velocity.	Double-precision floating point
`Vel2`	Signal destination velocity.	Double-precision floating point
`Out`	Propagated signal.	Double-precision floating point

More About

expand all

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

$L_{f s p} = \frac{{(4 π R)}^{2}}{λ^{2}},$

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

$γ = γ_{o} (f) + γ_{w} (f) = 0.1820 f N^{″} (f) .$

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

$N^{″} (f) = \sum_{i} S_{i} F_{i} + {N^{″}}_{D}^{} (f)$

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

$S_{i} = a_{1} \times 10^{- 7} {(\frac{300}{T})}^{3} \exp [a_{2} (1 - (\frac{300}{T})] P .$

For atmospheric water vapor, each spectral line strength is

$S_{i} = b_{1} \times 10^{- 1} {(\frac{300}{T})}^{3.5} \exp [b_{2} (1 - (\frac{300}{T})] W .$

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

$W = \frac{ρ T}{216.7} .$

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.

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).

You can 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 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

$γ_{c} = K_{l} (f) M,$

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 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

$γ_{R} = k R^{α},$

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

$r = \frac{1}{0.477 d^{0.633} R_{0.01}^{0.073 α} f^{0.123} - 10.579 (1 - \exp (- 0.024 d))}$

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 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.

Subband Frequency Processing

Subband processing decomposes a wideband signal into multiple subbands and applies narrowband processing to the signal in each subband. The signals for all subbands are summed to form the output signal.

When using wideband frequency System objects or blocks, you specify the number of subbands, N_B, in which to decompose the wideband signal. Subband center frequencies and widths are automatically computed from the total bandwidth and number of subbands. The total frequency band is centered on the carrier or operating frequency, f_c. The overall bandwidth is given by the sample rate, f_s. Frequency subband widths are Δf = f _s/N_B. The center frequencies of the subbands are

$f_{m} = {\begin{matrix} f_{c} - \frac{f_{s}}{2} + (m - 1) Δ f, N_{B} even \\ f_{c} - \frac{(N_{B} - 1) f_{s}}{2 N_{B}} + (m - 1) Δ f, N_{B} odd \end{matrix}, m = 1, \dots, N_{B}$

Some System objects let you obtain the subband center frequencies as output when you run the object. The returned subband frequencies are ordered consistently with the ordering of the discrete Fourier transform. Frequencies above the carrier appear first, followed by frequencies below the carrier.

Version History

Introduced in R2016a