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Investigation into Indoor Signal Propagation

Ground_Floor_Plan_Cover

Explore the intricacies of wireless signal propagation in indoor environments through detailed measurements and insightful observations.

Oscar Appelman, oscar.appelman@aptab.nl

This article was first published in Microwave & RF magazine.

1.    Introduction

In a world where wireless devices have become an integral part of daily life, the guarantee of reliable wireless networks is more crucial than ever. Accurately predicting the propagation characteristics of wireless signals is highly desirable.

However, due to the complex nature of wireless signal propagation in volatile environments, simulating such scenarios is notoriously difficult and lacks accuracy [1]. As a result, characterizing wireless signal propagation is often neglected and wireless networks are set up based on intuition and simple measurements.

This article employs measurements to describe the impact of environmental factors on wireless signal propagation and strives to offer insights into the requirements for an optimal implementation of wireless networks.

2.    Measurements and equipment

The measurements in this article were conducted to examine the effect of distance and objects on the signal reception of a wireless device. At one specific location in a building, the Received Signal Strength Indicator (RSSI) was determined for six different transmitter locations.

A MegiQ VNA-0460e (see Fig. 1) was employed for signal transmission and a MegiQ RMS-0460 (see Fig. 2) for signal reception [2]. A comparison between RSSI values provides insights into the behavior of wireless signals in a typical indoor scenario.

3.    Antenna characterization

The antennas that were used in the measurement setup are linearly polarized wideband paddle antennas from Summit Electronics, provided by Top Electronics (see Fig. 3) [3]. A full characterization of the RF impedance of these antennas was conducted using the MegiQ VNA for frequencies between 400 MHz and 6 GHz (see Fig. 4).

MegiQ_VNA0460e.png

MegiQ_RMS0460.png

Fig. 1.   The MegiQ VNA-0460e, used to excite the transmitting antennas during the measurements in this article [2].

Fig. 2.   The MegiQ RMS-0460, used to receive the transmitted signals during the measurements in this article [2].

 

Wideband_antenna_ERF4123.png

VNA0460e_on_RMS_turntable.png

Fig. 3.   Wideband paddle antenna from Summit Electronics, provided by Top Electronics and used during the measurements in this article.

Fig. 4.   The MegiQ VNA-0460e with wideband paddle antennas connected to port 1 and port 2. This setup is used to characterize the RF impedance of the antennas and to transmit horizontal and vertical linearly polarized waves from port 1 and port 2 respectively.

The wireless signal propagation of these antennas was characterized using the MegiQ RMS. RMS measurements include, but are not limited to, the Field Strength, Antenna Gain, Effective Isotropic Radiated Power (EIRP) and Total Radiated Power (TRP) for frequencies between 370 MHz and 6 GHz.

To obtain 3D electromagnetic radiation patterns of the antennas, a full scan over three antenna rotations was performed in a matter of minutes for the entire frequency range of the MegiQ RMS. The resulting radiation patterns for a horizontally polarized transmitting antenna at a frequency of 2.4GHz are visualized in Fig. 5.

Radiation_Pattern_2400MHz.png

Fig. 5.   Radiation patterns of the horizontally polarized wideband paddle antenna as shown in Fig. 3, visualized for a frequency of 2.4 GHz. Next to the 3D radiation pattern, the figure represents the YZ, ZX and XY rotation separately. ‘H’, ‘V’ and ‘HV’ denote the received horizontally, vertically and combined polarized fields respectively.

 Fig. 6 represents the Return Loss (RL) and Total Isotropic Gain (TIG) of the antennas, measured by the MegiQ VNA and MegiQ RMS respectively. A comparison between the RL and TIG illustrates their inversely proportional behavior.

A change in RL indicates a change in power reflection and thus in impedance matching. It would therefore be expected that the TIG responds inversely to the RL, i.e., when one increases, the other tends to decrease.

Return_Loss_and_Total_Isotropic_Gain.png

 

Fig. 6.   Visual representation of the Return Loss (RL) and Total Isotropic Gain (TIG) of the wideband paddle antenna as shown in Fig. 3. The comparison between the RL and TIG illustrates their inversely proportional behavior.

 

Forward_Loss_and_Total_Isotropic_Gain.png

Fig. 7.   Visual representation of the Forward Loss (FL) and Total Isotropic Gain (TIG) of the wideband paddle antenna as shown in Fig. 3. FL is defined as the amount of power that is accepted by the antenna. The comparison between the FL and TIG illustrates their directly proportional behavior.

To determine the correlation between these two properties, the MegiQ VNA software can translate the RL to the Forward Loss (FL), representing the amount of power that is accepted by the antenna. Fig. 7 represents both the FL and TIG.

In an ideal situation, the FL and TIG should be equal if all accepted power is transmitted by the antenna. The deviation between them therefore reveals the importance of wireless signal propagation measurements. The TIG includes all antenna losses and environmental effects, which are not detected with the VNA impedance measurements.

 4.    RSSI measurement setup

During all RSSI measurements, the paddle antennas were connected to the MegiQ VNA to excite them at an accurately controlled power level. The MegiQ RMS then measured the RSSI using the same paddle antennas, as this represents a realistic scenario in which two similar devices are communicating.

The antennas were oriented such that both a horizontal and a vertical linearly polarized electromagnetic field were transmitted and received. A visualization of the measurement setup is provided in Fig. 8.

 

RSSI_Measurement.png

 

 

Fig. 8.   The adjusted MegiQ RMS measurement setup for the RSSI measurements conducted in this article. The RMS receiver is shown on the left and the VNA is shown on the right.

 

To measure the RSSI for different transmitter locations, the turntable of the MegiQ RMS had to be positioned at these locations as well. Due to the versatility of the MegiQ RMS, this was achieved by controlling the turntable wirelessly using a Silex DS-600 Network USB Server. After connecting the turntable to the Silex DS-600, the RMS software was able to control it without the need for a wired connection between the RMS and the turntable.

5.    RSSI variability due to frequency and transmitter location

Fig. 9 and Fig. 10 illustrate the measured RSSI for different transmitter locations on a floorplan, with values averaged over the transmitted horizontal and vertical linearly polarized electromagnetic fields. This consideration is significant as many wireless devices are often used in varying orientations, randomizing the direction of polarization.

Fig. 9 and Fig. 10 represent a selection of six frequencies.

The receiver antenna is positioned on the ground floor and indicated with a cross. The locations of transmitter antennas are indicated with black dots.

The bar charts visualize the measured RSSI for the different transmitter locations, where the grey part represents the noise, the light colored part represents the minimum RSSI and the dark colored part the average RSSI.

  • 450 MHz due to its various applications, like Amateur Television (ATV) and smart meters;
  • 868 MHz is used for LoRa, Zigbee, Thread and RFID;
  • 1900 MHz is a commonly used frequency for 3G and 4G wireless communication;
  • 2400 MHz is popular for WiFi, Bluetooth and Thread;
  • 3400 MHz is commonly used for 5G communication;
  • 5800 MHz is a popular WiFi frequency as well, but is also used for Dedicated Short-Range Communications (DSRC).
Ground_Floor_Plan.png

Fig. 9.   Floorplan of the ground floor illustrating the averaged RSSI for different transmitter locations, taken over the transmitted horizontal and vertical linear electromagnetic polarization. The transmitter locations are indicated with black dots. The receiver is indicated with a cross. Grey is noise floor, light color is minimum RSSI, dark color is average RSSI.

 

First_Floor_Plan.png

Fig. 10.   Floorplan of the first floor illustrating the averaged RSSI for different transmitter locations, taken over the transmitted horizontal and vertical linear electromagnetic polarization. The transmitter locations are indicated with black dots. The receiver is located on the ground floor (see Fig. 9). Grey is noise floor, light color is minimum RSSI, dark color is average RSSI.

As expected, the bar charts indicate that the RSSI decreases as the distance between the transmitter and receiver increases. Furthermore, they show that this rate of decrease is larger for higher frequencies, which can be explained by Friis transmission equation for free space, defined by [4]:

Friis_Transmission_Equation.png

where

  • Pr = Received power
  • Pt = Transmitted power
  • Gt = Transmitter antenna gain
  • Gr = Receiver antenna gain
  • c = Speed of light
  • R = Distance between transmitter and receiver
  • f = Frequency

For higher frequencies ( ), an increase in distance between the transmitter and receiver ( ) results in a lower received power ( ) than for lower frequencies.

Additionally, this effect is amplified by the objects in the environment. Due to the smaller wavelength of signals with a higher frequency, these signals tend to observe more details in objects. As a result, they are more likely to interact with, for example, the metal grid in reinforced concrete, metal components in furniture and even the coatings in insulated windows.

Despite their advantages, higher frequency signals clearly also have their disadvantages. To mitigate these and ensure reliable wireless networks, it is crucial to consider wireless signal propagation not only during the design of wireless products, but also during the construction of buildings and setup of wireless networks.

6.    Wireless signal disturbances

Moving the antennas to different locations tends to change the measured radiation pattern. As Fig. 11 indicates with RSSI measurements at a frequency of 915 MHz, the orientation, shape and distribution of electromagnetic polarization differ across the radiation patterns obtained from various transmitter locations.

These changes can be attributed to various phenomena, like multipath propagation, fading and polarization alteration [5]. Some of the most significant phenomena are mentioned below.

Polarization_change_at_915MHz.png

Fig. 11.   Polar radiation patterns at 915 MHz for a transmitted horizontally and vertically polarized signal, measured for three different transmitter locations. The RSSI is measured using a vertically polarized wideband paddle antenna and is represented in dBm. The locations correspond to the locations in Fig. 9 and Fig. 10.

Whenever a signal interacts with objects, it can be absorbed, reflected, transmitted, scattered and diffracted [5]. All these different interactions can lead to changes in the signal, like its intensity, frequency, phase, polarization and direction. It furthermore leads to different paths in which the signal travels to the receiver, called multipath propagation.

The detected RSSI at the receiver is highly affected by all these interactions due to the different properties of the multipath components and the receiver’s sensitivity to certain properties of the signal. Additionally, interactions between the individual multipath components also impact the RSSI. This is caused by constructive and destructive interference, where the combination of signals leads to an amplification or attenuation of the signal.

Depending on the composition and electrical properties of objects, interactions with objects potentially alter the polarization of a signal [5]. This, in turn, affects the interaction between multipath components on the receiver side and the shape of the detected radiation patterns.

Fig. 11 indicates the effect of multipath propagation and polarization alteration on the measured radiation patterns. Due to the multipath propagation, a change in orientation of the radiation patterns is visible. Furthermore, Fig. 11 shows a variation in the relative distribution of horizontally and vertically polarized signals between the different locations, indicating that the polarization of these signals have changed.

The polarization alteration is especially visible at approximately 150° at location 4, where the vertically polarized signal decreases in power and the horizontally polarized signal increases correspondingly. These figures are a prime example of why antenna diversity, i.e. integrating multiple antennas with different polarization in a device, can significantly enhance the received wireless signal strength of the device.

7.    Conclusion

The reception of wireless signals propagating in an indoor environment depends strongly on the characteristics of the signal, distance between antennas and layout of the environment. This was proven with a comparison between radiation patterns for different transmitter locations, which revealed how the RSSI and polarization can change by physically moving a wireless device within the same network.

Furthermore, the measurements showed that higher frequency signals suffer from more signal degradation as the distance between transmitter and receiver increases. These signals face a stronger interaction with the environment, making their wireless propagation increasingly unpredictable and prone to distortion.

To draw clear and concise conclusions about the precise effects of objects on wireless signal propagation, a more comprehensive analysis and measurement approach is needed. Nevertheless, the research conducted in this article demonstrates that the discussed phenomena significantly affect the reception of transmitted wireless signals.

It emphasizes the necessity of accounting for wireless signal propagation to ensure reliable wireless networks, not only during the design of new wireless products but also throughout the construction of environments housing wireless devices, as well as during the setup of wireless networks.

The affordability, high accuracy and versatility of the MegiQ VNA and MegiQ RMS rendered them the ideal equipment for the measurements conducted in this article.

8.    References

[1] O. Appelman, Ray Tracing Simulation Accuracy for Predicting Indoor Wireless Signal Reception, pp. 1-12, 2023. [Online]. Available: https://www.aptab.nl/articles/wireless_propagation_rt_simulation.pdf

[2] MegiQ BV., https://www.megiq.com/

[3] Top Electronics, https://www.top-electronics.com/

[4] D. M. Pozar, Microwave Engineering, 4th ed., 2012.

[5] A. F. Molisch, Wireless Communications (2nd edition), 2011.

 

 

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