Introduction to Sionna RT

In this notebook, you will

  • Discover the basic functionalities of Sionna’s ray tracing (RT) module

  • Learn how to compute coverage maps

  • Use ray-traced channels for link-level simulations instead of stochastic channel models

Background Information

Ray tracing is a technique to simulate environment-specific and physically accurate channel realizations for a given scene and user position. Please see the EM Primer for further details on the theoretical background of ray tracing of wireless channels.

Sionna RT is a ray tracing extension for radio propagation modeling which is built on top of Mitsuba 3 and TensorFlow. Like all of Sionna’s components, it is differentiable.

Mitsuba 3 is a rendering system for forward and inverse light-transport simulation that makes use of the differentiable just-in-time compiler Dr.Jit. Sionna RT relies on Mitsuba 3 for the rendering and handling of scenes, e.g., its XML-file format, as well as the computation of ray intersections with scene primitives, i.e., triangles forming a mesh. The transformations of the polarized field components at each point of interaction between a ray and a scene object, e.g., reflection, are computed in TensorFlow, which is also used to combine the retained paths into (optionally) time-varying channel impulse responses. Thanks to TensorFlow’s automatic gradient computation, channel impulse responses and functions thereof are differentiable with respect to most parameters of the ray tracing process, including material properties (conductivity, permittivity), antenna patterns, orientations, and positions.

Scene files for Mitsuba 3 can be created, edited, and exported with the popular open-source 3D creation suite Blender and the Mitsuba-Blender add-on. One can rapdily create scenes from almost any place in the world using OpenStreetMap and the Blender-OSM add-on. In Sionna, scenes and radio propagation paths can be either rendered through the lens of configurable cameras via ray tracing or displayed with an integrated 3D viewer. For more detail on scene creation and rendering, we refer to Sionna’s API documentation and the available video tutorial.

GPU Configuration and Imports

import os
gpu_num = 0 # Use "" to use the CPU
os.environ["CUDA_VISIBLE_DEVICES"] = f"{gpu_num}"
os.environ['TF_CPP_MIN_LOG_LEVEL'] = '3'

# Colab does currently not support the latest version of ipython.
# Thus, the preview does not work in Colab. However, whenever possible we
# strongly recommend to use the scene preview mode.
try: # detect if the notebook runs in Colab
    import google.colab
    colab_compat = True # deactivate preview
    colab_compat = False
resolution = [480,320] # increase for higher quality of renderings

# Allows to exit cell execution in Jupyter
class ExitCell(Exception):
    def _render_traceback_(self):

# Import Sionna
    import sionna
except ImportError as e:
    # Install Sionna if package is not already installed
    import os
    os.system("pip install sionna")
    import sionna

# Configure the notebook to use only a single GPU and allocate only as much memory as needed
# For more details, see
import tensorflow as tf
gpus = tf.config.list_physical_devices('GPU')
if gpus:
        tf.config.experimental.set_memory_growth(gpus[0], True)
    except RuntimeError as e:
# Avoid warnings from TensorFlow

tf.random.set_seed(1) # Set global random seed for reproducibility

%matplotlib inline
import matplotlib.pyplot as plt
import numpy as np
import time

# Import Sionna RT components
from sionna.rt import load_scene, Transmitter, Receiver, PlanarArray, Camera

# For link-level simulations
from import cir_to_ofdm_channel, subcarrier_frequencies, OFDMChannel, ApplyOFDMChannel, CIRDataset
from import PUSCHConfig, PUSCHTransmitter, PUSCHReceiver
from sionna.utils import compute_ber, ebnodb2no, PlotBER
from sionna.ofdm import KBestDetector, LinearDetector
from sionna.mimo import StreamManagement

Loading Scenes

The Sionna RT module can either load external scene files (in Mitsuba’s XML file format) or it can load one of the integrated scenes.

In this example, we load an example scene containing the area around the Frauenkirche in Munich, Germany.

# Load integrated scene
scene = load_scene(sionna.rt.scene.munich) # Try also sionna.rt.scene.etoile

To visualize the scene, we can use the preview function which opens an interactive preview of the scene. This only works in Jupyter notebooks.

You can use the following controls:

  • Mouse left: Rotate

  • Scroll wheel: Zoom

  • Mouse right: Move

Please note that the preview does not work in Colab and is therefore deactivated when colab_compat is set to True. Further, only one preview instance can be open at the same time.

 # Open 3D preview (only works in Jupyter notebook)
if colab_compat:
    scene.render(camera="scene-cam-0", num_samples=512);
    raise ExitCell

It is often convenient to choose a viewpoint in the 3D preview prior to rendering it as a high-quality image. The next cell uses the “preview” camera which corresponds to the viewpoint of the current preview image.

# The preview camera can be directly rendered as high-quality image
if not colab_compat:
    scene.render(camera="preview", num_samples=512);
    print("Function not available in Colab mode.")
Function not available in Colab mode.

One can also render the image to a file as shown below:

render_to_file = False # Set to True to render image to file

# Render scene to file from preview viewpoint
if render_to_file:
    scene.render_to_file(camera="scene-cam-0", # Also try camera="preview"

Instead of the preview camera, one can also specify dedicated cameras with different positions and look_at directions.

# Create new camera with different configuration
my_cam = Camera("my_cam", position=[-250,250,150], look_at=[-15,30,28])

# Render scene with new camera*
scene.render("my_cam", resolution=resolution, num_samples=512); # Increase num_samples to increase image quality

Note that each SceneObject (camera, transmitter,…) needs a unique name. Thus, running the cells above multiple times will lead to an error if the object name is not changed or the object is not removed from the scene.

Ray Tracing for Radio Propagation

We need to configure transmitters and receivers prior to computing propagation paths between them. All transmitters and all receivers are equipped with the same antenna arrays which are defined by the scene properties scene.tx_array and scene.rx_array, respectively. Antenna arrays are composed of multiple identical antennas. Antennas can have custom or pre-defined patterns and are either single- or dual-polarized. One can add multiple transmitters and receivers to a scene which need to have unique names, a position, and orientation which is defined by yaw, pitch, and roll angles.

# Configure antenna array for all transmitters
scene.tx_array = PlanarArray(num_rows=1,

# Configure antenna array for all receivers
scene.rx_array = PlanarArray(num_rows=1,

# Create transmitter
tx = Transmitter(name="tx",

# Add transmitter instance to scene

# Create a receiver
rx = Receiver(name="rx",

# Add receiver instance to scene

tx.look_at(rx) # Transmitter points towards receiver

Each SceneObject has an assigned RadioMaterial that describes the electromagnetic properties of the object whenever it interacts with a ray. This behavior can be frequency-dependent and the ray tracing is done for a specific frequency.

We now set the carrier frequency of the scene and implicitly update all RadioMaterials.

scene.frequency = 2.14e9 # in Hz; implicitly updates RadioMaterials

scene.synthetic_array = True # If set to False, ray tracing will be done per antenna element (slower for large arrays)

The default scenes have RadioMaterials assigned to each scene object. However, the RadioMaterial of a specific object can be modified and customized by the user.

# Select an example object from the scene
so = scene.get("Altes_Rathaus-itu_marble")

# Print name of assigned radio material for different frequenies
for f in [3.5e9, 2.14e9]: # Print for differrent frequencies
    scene.frequency = f
    print(f"\nRadioMaterial: {} @ {scene.frequency/1e9:.2f}GHz")
    print("Conductivity:", so.radio_material.conductivity.numpy())
    print("Relative permittivity:", so.radio_material.relative_permittivity.numpy())
    print("Complex relative permittivity:", so.radio_material.complex_relative_permittivity.numpy())
    print("Relative permeability:", so.radio_material.relative_permeability.numpy())
    print("Scattering coefficient:", so.radio_material.scattering_coefficient.numpy())
    print("XPD coefficient:", so.radio_material.xpd_coefficient.numpy())

RadioMaterial: itu_marble @ 3.50GHz
Conductivity: 0.017550057
Relative permittivity: 7.074
Complex relative permittivity: (7.074-0.090132594j)
Relative permeability: 1.0
Scattering coefficient: 0.0
XPD coefficient: 0.0

RadioMaterial: itu_marble @ 2.14GHz
Conductivity: 0.0111273555
Relative permittivity: 7.074
Complex relative permittivity: (7.074-0.09346512j)
Relative permeability: 1.0
Scattering coefficient: 0.0
XPD coefficient: 0.0

Let us run the ray tracing process and compute propagation paths between all transmitters and receivers. The parameter max_depth determines the maximum number of interactions between a ray and a scene objects. For example, with a max_depth of one, only LoS paths are considered. When the property scene.synthetic_array is set to False, antenna arrays are explicitly modeled by finding paths between any pair of transmitting and receiving antennas in the scene. Otherwise, arrays are represented by a single antenna located in the center of the array. Phase shifts related to the relative antenna positions will then be applied based on a plane-wave assumption when the channel impulse responses are computed.

# Compute propagation paths
paths = scene.compute_paths(max_depth=5,
                            num_samples=1e6)  # Number of rays shot into directions defined
                                              # by a Fibonacci sphere , too few rays can
                                              # lead to missing paths

# Visualize paths in the 3D preview
if colab_compat:
    scene.render("my_cam", paths=paths, show_devices=True, show_paths=True, resolution=resolution);
    raise ExitCell
scene.preview(paths, show_devices=True, show_paths=True) # Use the mouse to focus on the visualized paths

Remark: only one preview instance can be opened at the same time. Please check the previous preview if no output appears.

The Paths object contains all paths that have been found between transmitters and receivers. In principle, the existence of each path is determininistic for a given position and environment. Please note that due to the stochastic nature of the shoot-and-bounce algorithm, different runs of the compute_paths function can lead to different paths that are found. Most importantly, diffusely reflected or scattered paths are obtained through random sampling of directions after each interaction with a scene object. You can seet TensorFlow’s random seed to a specific value before executing compute_paths to ensure reproducibility.

The Paths object contains detailed information about every found path and allows us to generated channel impulse responses and apply Doppler shifts for the simulation of time evolution.

Let us now inspect some of the available properties:

# Show the coordinates of the starting points of all rays.
# These coincide with the location of the transmitters.
print("Source coordinates: ", paths.sources.numpy())
print("Transmitter coordinates: ", list(scene.transmitters.values())[0].position.numpy())

# Show the coordinates of the endpoints of all rays.
# These coincide with the location of the receivers.
print("Target coordinates: ",paths.targets.numpy())
print("Receiver coordinates: ",list(scene.receivers.values())[0].position.numpy())

# Show the types of all paths:
# 0 - LoS, 1 - Reflected, 2 - Diffracted, 3 - Scattered
# Note that Diffraction and scattering are turned off by default.
print("Path types: ", paths.types.numpy())
Source coordinates:  [[ 8.5 21.  27. ]]
Transmitter coordinates:  [ 8.5 21.  27. ]
Target coordinates:  [[45.  90.   1.5]]
Receiver coordinates:  [45.  90.   1.5]
Path types:  [[0 1 1 1 1 1 1 1 1 1 1 1 1]]

We can see from the list of path types, that there are 14 paths in total. One LoS and 13 reflected paths.

# We can now access for every path the channel coefficient, the propagation delay,
# as well as the angles of departure and arrival, respectively (zenith and azimuth).

# Let us inspect a specific path in detail
path_idx = 4 # Try out other values in the range [0, 13]

# For a detailed overview of the dimensions of all properties, have a look at the API documentation
print(f"\n--- Detailed results for path {path_idx} ---")
print(f"Channel coefficient: {paths.a[0,0,0,0,0,path_idx, 0].numpy()}")
print(f"Propagation delay: {paths.tau[0,0,0,path_idx].numpy()*1e6:.5f} us")
print(f"Zenith angle of departure: {paths.theta_t[0,0,0,path_idx]:.4f} rad")
print(f"Azimuth angle of departure: {paths.phi_t[0,0,0,path_idx]:.4f} rad")
print(f"Zenith angle of arrival: {paths.theta_r[0,0,0,path_idx]:.4f} rad")
print(f"Azimuth angle of arrival: {paths.phi_r[0,0,0,path_idx]:.4f} rad")

--- Detailed results for path 4 ---
Channel coefficient: (-1.1461009307822678e-05-3.821776886070438e-07j)
Propagation delay: 0.46001 us
Zenith angle of departure: 1.7007 rad
Azimuth angle of departure: 0.5157 rad
Zenith angle of arrival: 1.7007 rad
Azimuth angle of arrival: 0.5368 rad

From Paths to Channel Impulse Responses

Once paths are computed, they can be transformed into channel impulse responses (CIRs). The class method apply_doppler can simulate time evolution of the CIR based on arbitrary velocity vectors of all transmitters and receivers for a desired sampling frequency and number of time steps. The class method cir generates the channel impulse responses which can be used by other components for link-level simulations in either time or frequency domains. The method also allows you to only consider certain types of paths, e.g., line-of-sight, reflections, etc.

# Default parameters in the PUSCHConfig
subcarrier_spacing = 15e3
fft_size = 48
# Print shape of channel coefficients before the application of Doppler shifts
# The last dimension corresponds to the number of time steps which defaults to one
# as there is no mobility
print("Shape of `a` before applying Doppler shifts: ", paths.a.shape)

# Apply Doppler shifts
paths.apply_doppler(sampling_frequency=subcarrier_spacing, # Set to 15e3 Hz
                    num_time_steps=14, # Number of OFDM symbols
                    tx_velocities=[3.,0,0], # We can set additional tx speeds
                    rx_velocities=[0,7.,0]) # Or rx speeds

print("Shape of `a` after applying Doppler shifts: ", paths.a.shape)

a, tau = paths.cir()
print("Shape of tau: ", tau.shape)
Shape of `a` before applying Doppler shifts:  (1, 1, 2, 1, 1, 13, 1)
Shape of `a` after applying Doppler shifts:  (1, 1, 2, 1, 1, 13, 14)
Shape of tau:  (1, 1, 1, 13)

Let us have a look at the channel impulse response for the 14 incoming paths from the simulation above.

t = tau[0,0,0,:]/1e-9 # Scale to ns
a_abs = np.abs(a)[0,0,0,0,0,:,0]
a_max = np.max(a_abs)
# Add dummy entry at start/end for nicer figure
t = np.concatenate([(0.,), t, (np.max(t)*1.1,)])
a_abs = np.concatenate([(np.nan,), a_abs, (np.nan,)])

# And plot the CIR
plt.title("Channel impulse response realization")

plt.stem(t, a_abs)
plt.xlim([0, np.max(t)])
plt.ylim([-2e-6, a_max*1.1])
plt.xlabel(r"$\tau$ [ns]")

Note that the delay of the first arriving path is normalized to zero. This behavior can be changed using the Paths’ call property normalize_delays. For link-level simulations, it is recommended to work with normalized delays, unless perfect synchronization is explicitly desired.

# Disable normalization of delays
paths.normalize_delays = False

# Get only the LoS path
_, tau = paths.cir(los=True, reflection=False)
print("Delay of first path without normalization: ", np.squeeze(tau))

paths.normalize_delays = True
_, tau = paths.cir(los=True, reflection=False)
print("Delay of first path with normalization: ", np.squeeze(tau))
Delay of first path without normalization:  2.739189e-07
Delay of first path with normalization:  0.0

The CIRs can now be loaded either in the time-domain or frequency-domain channel models, respectively. Please see cir_to_ofdm_channel and cir_to_time_channel for further details.

# Compute frequencies of subcarriers and center around carrier frequency
frequencies = subcarrier_frequencies(fft_size, subcarrier_spacing)

# Compute the frequency response of the channel at frequencies.
h_freq = cir_to_ofdm_channel(frequencies,
                             normalize=True) # Non-normalized includes path-loss

# Verify that the channel power is normalized
h_avg_power = tf.reduce_mean(tf.abs(h_freq)**2).numpy()

print("Shape of h_freq: ", h_freq.shape)
print("Average power h_freq: ", h_avg_power) # Channel is normalized
Shape of h_freq:  (1, 1, 2, 1, 1, 14, 48)
Average power h_freq:  1.0

The frequency responses h_freq are now ready to be processed by the ApplyOFDMChannel Layer.

# Placeholder for tx signal of shape
# [batch size, num_tx, num_tx_ant, num_ofdm_symbols, fft_size]
x = tf.zeros([h_freq.shape.as_list()[i] for i in [0,3,4,5,6]], tf.complex64)

no = 0.1 # noise variance

# Init channel layer
channel = ApplyOFDMChannel(add_awgn=True)

# Apply channel
y = channel([x, h_freq, no])

# [batch size, num_rx, num_rx_ant, num_ofdm_symbols, fft_size]
(1, 1, 2, 14, 48)

BER Evaluation

We now initialize a transmitter and receiver from the 5G NR PUSCH Tutorial notebook. These components could be replaced by your own transceiver implementations. Then we simulate PUSCH transmissions over the ray-traced CIRs that we generated in the previous cells.

# Init pusch_transmitter
pusch_config = PUSCHConfig()

# Instantiate a PUSCHTransmitter from the PUSCHConfig
pusch_transmitter = PUSCHTransmitter(pusch_config)

# Create a PUSCHReceiver using the PUSCHTransmitter
pusch_receiver = PUSCHReceiver(pusch_transmitter)
# Simulate transmissions over the
batch_size = 100 # h_freq is broadcast, i.e., same CIR for all samples but different AWGN realizations
ebno_db = 2. # SNR in dB

no = ebnodb2no(ebno_db,

x, b = pusch_transmitter(batch_size) # Generate transmit signal and info bits

y = channel([x, h_freq, no]) # Simulate channel output

b_hat = pusch_receiver([y, no]) # Recover the info bits

# Compute BER
print(f"BER: {compute_ber(b, b_hat).numpy():.5f}")
BER: 0.07575

Remark Contrary to other Sionna components, ray tracing does not have a dedicated batch dimension. However, multiple transmitter and receivers can be simulated in parallel, which effectively equals a batch-dimension.

Note that simulating multiple receivers in the ray tracer comes at little additional overhead. However, the complexity increases significantly for multiple transmitters as an individual ray tracing step is required for each transmitter. As the total number of rays is fixed, an increased number of transmitter requires also an increased number of rays in the compute_paths step for the same overall precision.

Runtime vs Depth

We will now investigate the complexity of the ray tracing algorithm for different values of max_depth, i.e., for a different number of bounces of the rays.

max_depths = 10 # evaluate performance up to 10 reflections
depths = range(1,max_depths+1)
ts = []
pl_avg = []
for d in depths:
    # save start time
    t = time.time()
    # run the ray tracer
    paths = scene.compute_paths(max_depth=d)
    # and measure the required time interval
# and plot results
plt.plot(depths, ts, color="b");
plt.xlabel("Max. depth")
plt.ylabel("Runtime (s)", color="b")
plt.xlim([1, max_depths]);

As can be seen, the computational complexity increases significantly with the number of ray interactions. Note that the code above does not account for scattering or diffraction. Adding these phenomea adds additional complexity as can be seen below:

t = time.time()
paths = scene.compute_paths(max_depth=3, diffraction=False)
print("Time without diffraction and scattering:" , time.time()-t)

t = time.time()
paths = scene.compute_paths(max_depth=3, diffraction=True)
print("Time with diffraction:" , time.time()-t)

t = time.time()
paths = scene.compute_paths(max_depth=3, scattering=True)
print("Time with scattering:" , time.time()-t)
Time without diffraction and scattering: 2.956139087677002
Time with diffraction: 3.0558841228485107
Time with scattering: 3.830286741256714

Although we have simulated scattering in the last example above, the scattered paths do not carry any energy as none of the materials in the scene has a positive scattering coefficient. You can learn more about scattering and diffraction in the dedicated tutorial notebooks.

Coverage Map

Sionna RT can be also used to simulate coverage maps for a given environment. We now put a new transmitter on top of the Frauenkirche and simulate a large-scale coverage map.

# Remove old transmitter and add new one

tx = Transmitter(name="tx",
                 position=[-210,73,105], # top of Frauenkirche

# We could have alternatively modified the properties position and orientation of the existing transmitter
#scene.get("tx").position = [-210,73,105]
#scene.get("tx").orientation = [0,0,0]

Let’s have a look at the new setup. The receiver can be ignored for the coverage map simulation.

 # Open 3D preview (only works in Jupyter notebook)
if colab_compat:
    scene.render(camera="scene-cam-0", num_samples=512, resolution=resolution);
    raise ExitCell
cm = scene.coverage_map(max_depth=5,
                        diffraction=True, # Disable to see the effects of diffraction
                        cm_cell_size=(5., 5.), # Grid size of coverage map cells in m
                        num_samples=int(20e6)) # Reduce if your hardware does not have enough memory

Once simulated, the coverage map object can be directly visualized with the preview or render function.

# Create new camera
tx_pos = scene.transmitters["tx"].position.numpy()
bird_pos = tx_pos.copy()
bird_pos[-1] = 1000 # Set height of coverage map to 1000m above tx
bird_pos[-2]-= 0.01 # Slightly move the camera for correct orientation

# Create new camera
bird_cam = Camera("birds_view", position=bird_pos, look_at=tx_pos)


if colab_compat:
    scene.render(camera="birds_view", coverage_map=cm, num_samples=512, resolution=resolution);
    raise ExitCell
# Open 3D preview (only works in Jupyter notebook)

Alternatively, a 2D visualization of the coverage map can be shown.

[29]:; # If multiple transmitters exist, tx selects for which transmitter the cm is shown

Note that it can happen in rare cases that diffracted rays arrive inside or behind buildings through paths which should not exists. This is not a bug in Sionna’s ray tracing algorithm but rather an artefact of the way how scenes are created which can lead to the false detection of diffraction edges.

Conclusion and Outlook

In this notebook, you have learned how to use the Sionna RT module. We have seen how paths can be found in complex environments such as in the Munich scene. In a second step, we have calculated the effective CIRs from the paths and used them for link-level simulations.

There is one key feature of Sionna RT that was not discussed in this notebook: Automatic gradient computation. Like most components of Sionna, the ray tracer is differentiable with respect to most system parameters, such as radio materials, transmitter and receiver orientations, array geometries, positions, etc. This enables a whole new line of research which is discussed in the Sionna RT paper and the related notebooks.

Hopefully you have enjoyed this tutorial on Sionna’s RT module!

Please have a look at the API documentation of the various components or the other available tutorials to learn more.