System object: phased.FreeSpace
Package: phased
Propagate signal from one location to another
Y = step(SFS,F,origin_pos,dest_pos,origin_vel,dest_vel)
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.
returns
the resulting signal Y
= step(SFS
,F
,origin_pos
,dest_pos
,origin_vel
,dest_vel
)Y
when the narrowband signal F
propagates
in free space from the position or positions specified in origin_pos
to
the position or positions specified in dest_pos
.
For nonpolarized signals, either the origin_pos
or dest_pos
arguments
can specify more than one point. Using both arguments to specify multiple
points is not allowed. The velocity of the signal origin is specified
in origin_vel
and the velocity of the signal
destination is specified in dest_vel
. The dimensions
of origin_vel
and dest_vel
must
agree with the dimensions of origin_pos
and dest_pos
,
respectively.
The object performs an initialization the first time the object is executed. This
initialization locks nontunable properties (MATLAB)
and input specifications, such as dimensions, complexity, and data type of the input data.
If you change a nontunable property or an input specification, the System
object issues an error. To change nontunable properties or inputs, you must first
call the release
method to unlock the object.

Propagated signal, returned as a Melement complexvalued column vector, MbyN complexvalued matrix or MATLAB structure containing complexvalued fields. If If The output 
When the origin and destination are stationary relative to each other, you can write the output signal of a freespace channel as Y(t) = x(tτ)/L_{fsp}. The quantity τ is the signal delay and L_{fsp} is the freespace path loss. The delay τ is given by R/c, where R is the propagation distance and c is the propagation speed. The freespace 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 freespace 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 oneway propagation and 2v/λ for twoway propagation. The quantity v is the relative speed of the destination with respect to the origin.
For further details, see [2].
[1] Proakis, J. Digital Communications. New York: McGrawHill, 2001.
[2] Skolnik, M. Introduction to Radar Systems, 3rd Ed. New York: McGrawHill, 2001.