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Internet of things & SigFox Protocol

Updated: Jul 28, 2022

Testing requirements for wireless systems evolve faster and more dynamically, in these we explore the term IoT and the challenges of building validation systems to ensure their correct functioning and approval.


The term IoT, an acronym for Internet of Things has become increasingly popular in recent years. The term refers nothing more or less to a network of physical objects, such as vehicles, buildings and even more everyday objects, such as your refrigerator and the internet.

Within this universe, more and more companies are developing solutions for this field and one of them is Sigfox, a French global network operator founded in 2010 that builds wireless networks to connect objects.


"Sigfox, a French global network operator founded in 2010 that builds wireless networks to connect objects. One of the great challenges addressed by this company was to reconcile objects that need to be continuously activated and emitting with a system of low energy use."

One of the great challenges addressed by this company was to reconcile objects that need to be continuously activated and emitting with a system of low energy use. This ensures that the requirements of most IoT applications are met: low cost, low energy usage, high communication range and high network reliability.


SigFox Technology and Development Challenges


Sigfox technology is based, like all wireless technology, on basic operating principles of telecommunications systems. During the course of this text I will address each of these points.


Technology Principles

As a basis for operation, Sigfox uses the Ultra Narrow Band technique for data transmission. This technique consists of a receiver that can reject noise and interference that can enter the receiver outside its bandwidth¹, thus ensuring an extremely advantageous signal-to-noise ratio, allowing relatively weak and low signals to be received and interpreted. As a result, transmitter power levels may be lower and the effective transmission range may be greater than when using other technologies that do not use this signal-to-noise selectivity approach.


Main Features


The technology is supported by three pillars: low energy consumption, low implementation cost and information reliability.


Operating Principles

Imagine the following hypothetical situation: consider a device monitoring the temperature of a certain environment and it is connected to a Sigfox device, its temperature data will be converted to fit the communication protocol, which are basically the characteristics necessary for you to communicate within from the sigfox network. Just like the languages ​​where for a conversation to be established both participants must speak the same language, the communication protocols worked in the same way, that is, for you to send the signal, some characteristics must be taken into account.

One of these characteristics is the central frequency that will establish which road I will travel on, and the other is the size of that road (number of lanes where cars can pass) and we will call this bandwidth. So my center frequency will be my middle lane and my bandwidth will delimit the amount of lanes on the left and right on my highway. As shown in the drawing below:


Within my transmission bandwidth my SigFox device will assemble your message, or Payload, which will contain the information I want to send, my device information and its cryptographic data.

Unlike cellular protocols, the Sigfox device is not directly connected to a signal station, it will only send messages in the following way: it will send 3 messages around the Fc occupying 100Hz of my bandwidth, the station will receive the message and the will pass to Cloud Sigfox. The communication from my device to the station is called Uplink and the response will be called Downlink, where each of these communication sides has a specific transmission speed, by analogy it would be how fast the cars travel and in which direction and it is called Uplink Data Rate and Downlink Data Rate measured in bits per second.


Technical Characteristics of the Protocol


There are currently 7 Sigfox operating zones in the world as we can see in the chart below:


Source: https://build.sigfox.com/sigfox-radio-configurations-rc

There are currently 7 Sigfox operating zones in the world as we can see in the chart below:

Source: https://build.sigfox.com/sigfox-radio-configurations-rc

Focusing on the last two lines of the table some concepts to be explained, let's start with the EIRP (Effective Isotropic Radiated Power) which is basically the maximum power of my center frequency.

In the last line we have some particularities regarding the sending of messages, they are: Duty Cycle, Frequency hopping and Listen Before Talk.


Duty Cicle: duty cycle or duty cycle corresponds to the amount of time a system is operating at high level and low level, as shown in the graph below:


As described in the table, this device operating mode in the RC1 and RC7 regions uses this mechanism with a duty cycle of 1%, that is, at 1% it stays in a high level state per hour (36 seconds) for a payload of 8 to 12 bytes.


Frequency hopping: The device will send 3 messages on 3 different frequencies with a maximum time of 400ms second per channel and will not allow reissue of messages within 20 seconds.


Listen Before Talk: The device will verify that the SigFox channel within its 200kHz bandwidth is free of any other signal with power greater than -80dBm before transmitting.


Modulation


In telecommunications, even IoT systems not escaping the Sigfox protocol from this rule, before being transmitted it has to go through a process called modulation. Everything we've talked about so far about sigfox messages has been about baseband, that is, before it was modulated to be transmitted over the air. Both uplink and downlink devices have to modulate and/or demodulate a signal before analyzing it. But what is the modulation process anyway? Basically it's the process in which my base signal, baseband, goes through a process of adding other electromagnetic waves that will allow it to be interpreted from the other side. Remembering that digital communications are based on binary signals (0 or 1) the modulation process will mark where these high and low level transitions exist. For example: amplitude modulation will change the amplitude of my signal so that my demodulator device understands when I encounter 0 or 1, for example: I can modulate a signal so that 24dBm represents 1 and 0 dBm represents zero. In this case, the other wave characteristics such as frequency and phase remain unchanged. Other types of modulation are: frequency modulation (FM) and phase modulation (PM).


Uplink Modulation – Sigfox


For Uplink communication, the Sigfox device will use 2-BPSK Binary Phase Shifting Key modulation, or binary modulation by phase shifting in Portuguese. Which basically means that the device will represent state changes 0 and 1 through a change in the phase of the wave, that is, if the wave keeps the phase constant I have a state, when the change of phase means that there has been a change of state. As can be seen in the demo below:

Source: https://www.disk91.com/2017/technology/sigfox/the-sigfox-radio-protocol/

Downlink Modulation – Sigfox


Unlike Uplink communication, Downlink uses 2-GFSK, 2-Level Gaussian Frequency Shift Key modulation, which is basically a modulation by frequency variation with an order 2 Gaussian filter applied to it. Below we have a demonstration of an FSK-type modulation without the filter applied to facilitate understanding:

Source: https://sites.google.com/site/nearcommunications/gfsk-modulation

Formato da Mensagem


As mentioned above, each communication protocol, as well as a language, has its standard formats for how to assemble a sentence, and with Sigfox it is no different. To explain the message format, let's first remember that each message transmission is sent 3 signals, we will call each of them a frame and we will divide each of these frames into 8 parts, they are: F.Sync, F.Type, Flag, SeqID , Device ID, Payload, HMAC and CRC16.

  • F.SYNC: It is the part of the frame that deals with synchronization, it is a standard sequence of zeros and ones that will allow the receiver to correctly synchronize the rest of the message.

  • F.Type: it will determine two things the size of the payload and indicates which repetition is a frame, remembering that sigfox sends three equal frames to compose a message.

  • Flag: Indicates the number of bytes added for the frame to fit the standard size. For example, the payload has a maximum size of 12 bytes, if you add an information of 8 bytes, the flag will indicate that you need 4 empty bytes to complete the default size. In addition, the flag will indicate whether or not the message has made a downlink request.

  • SEQ.ID: It is the message counter and indicates the message number, for example, the first message of the day is 1, the SEQ.ID will identify this to the SigFox API. This number is incremented for each message sent.

  • DEVICE.ID: This number is your Sigfox device's CPF an identifier that is used by the SigFox API.

  • Payload: It is the portion of the frame that contains the information that will be sent in the message.

  • HMAC: The HMAC is a unique hash that contains the encrypted communication protocol data, containing the error correction code (ECC) and the AES (Advanced Encrypt Standard) code.

  • CRC (Cyclic Redundancy Check): Represents the frame error correction method by detecting accidental changes in the data chain.

Below we can see the frame format more clearly:


Fonte: https://www.disk91.com/2017/technology/sigfox/the-sigfox-radio-protocol/

Monarch Technology


Within the Sigfox technology is the SigFox Monarch certificate that consists of devices capable of connecting to the Sigfox network automatically anywhere in the world within the 7 operating zones. This works in a very simple way: the device will send a beacon signal that will help the device identify which zone it is in and thus adapt its characteristics to it. This is a unique feature of the SigFox network.


Applications


The Sigfox protocol is perfect for monitoring where there is no need for large volumes of data and/or very high monitoring speed, making it perfect for use with small data that needs to be transmitted sporadically. For example:

  • Temperature monitoring

  • Tracking in the agribusiness industry, monitoring of livestock

  • Monitoring of security breaches


Challenges in Building Test Systems and its Solutions


In the process of certifying a Sigfox product, the test stage is essential to ensure the success of the application, so Blue Eyes Systems opted for National Instruments technologies to ensure the rapid prototyping of the communication API using the LabVIEW language. And to manage the test supervisory, TestStand was used. The hardware used for the acquisition and generation of signals was as follows:

  • Chassis PXIe-1095, is the case where the modular components below were fitted.

  • PXIe-8880 controller with Windows 10, the controller is the brain of the system is the CPU that will control all the peripherals involved.

  • PXIe-5644R board – VST, Vector Signal Transceiver, board responsible for the generation and acquisition of radio frequency signals

  • PXIe-4145 SMU Board – Source Measurement Unit, software controlled voltage source


Left to right: PXIe-8880, PXIe-4145 and PXIe-5644R



Test Plan Configuration


Sigfox makes the complete test plan documents available and in this text we will only cover the high-level configurations and the primary requirements for each of them. The tests fundamentally connected the Sigfox device to be tested with the following arrangement:


Test Glossary

  • 2GFSK: 2-Level Gaussian Frequency Shift Key (Downlink Modulation)

  • DBPSK: Differential Binary Phase Shift Key (Uplink Modulation)

  • DUT: Device Under Test

  • Test Equipment: PXIe Chassis + PXIe Controller + VST + SMU

  • Test Mode Activation: Serial Command via PXIe Controller for the DUT to enter test mode


Modultion Quality Test Setup


Source: SigFox RF & Protocol Test Plan for RC3a-UDL-ENC, Versão 3.8.1 – 11 de Outubro, 2018

The PXI controller in this configuration fulfilled the role of sending the Test Mode Activation commands to the DUT, the SMU board fed it and the VST board was responsible for receiving the signals. In this test mode the DUT generates only baseband signals, so that the VST could analyze the signal components. Breaking down into high-level requirements like:

  • Frequency analysis to verify that channels do not exceed the 100Hz threshold, phase shift verification, frequency deviation stability and static frequency tolerance analysis

  • Modulation Analysis: When receiving the signal in baseband, the PXI must modulate the signal and check: the baud rate of the received information, the duration time of the extra bits added to the frame as guard band and the signal is correctly performing the changes of phase

  • Power analysis: Verification of power distribution within bandwidth and power levels by bandwidth division as seen in the two images below



Source: SigFox RF & Protocol Test Plan for RC3a-UDL-ENC, Versão 3.8.1 – 11 de Outubro, 2018

In the case of the image above, the test verifies that 99% of the power is distributed in up to 36kHz of the bandwidth.



Source: SigFox RF & Protocol Test Plan for RC3a-UDL-ENC, Versão 3.8.1 – 11 de Outubro, 2018

In the image above, each of the steps is analyzed and a mask is created over the signal in order to validate if the power distribution format is within the specifications.


Demodulated Information Analysis Test Setup


Source: SigFox RF & Protocol Test Plan for RC3a-UDL-ENC, Versão 3.8.1 – 11 de outubro, 2018

As in the previous configuration, the PXI controller would send the operation commands to the DUT via the serial channel, the SMU board would feed it and the VST would analyze the signals. In this configuration in question, the DUT sends a modulated message and the VST must be able to: parse the ID and encryption key as well as be able to record the modulation constellation graph and parse them for signal deviation verification, as well as checking the number of frames of a message, which must be equal to 3 and the time between them.


Source: SigFox RF & Protocol Test Plan for RC3a-UDL-ENC, Versão 3.8.1 – 11 de Outubro, 2018

In this test configuration, the VST board is responsible for sending a standard frame to the DUT and it must return the power at which the signal was received via serial, this parameter serves as the basis for the correct calibration of the DUT information reception system.


Configuration for Downlink Validation


Source: SigFox RF & Protocol Test Plan for RC3a-UDL-ENC, Versão 3.8.1 – 11 de Outubro, 2018

In this configuration, the VST must send and receive signals from the DUT, that is, it receives a message with a downlink request and must be able to detect which bandwidth channel the message was sent on, and respond to it and wait for an acknowledgment. of the DUT that the message exchange was successful, we call this procedure Bi-Directional. This configuration validates the temporal specifications of message exchange, in addition to checking the times between the 3 frames sent within the message, following the specifications below:


Source: Sigfox connected Objects: Radio specifications – Fev 2019

In table 4-1 we have the times and intervals between an Uplink message in the first line, in the second we have the Downlink times for response and in the last two the minimum and maximum times for the confirmation message.


Listen Before Talk Test Setup


Source: SigFox RF & Protocol Test Plan for RC3a-UDL-ENC, Versão 3.8.1 – 11 de Outubro, 2018

In this test configuration, used only for zone RC3 and RC5, the test system must be able to receive the signal from the DUT and at the same time send a blocking signal. The Listen Before Talk (LBT) operation consists of the DUT analyzing if the band is free of other signals before sending a frame, so in this procedure the VST will send a blocking signal concomitantly with the DUT trying to send 6 frames, to validate the behavior of the LBT the DUT must send only 2, the first and the last as shown in the image below:

Source: SigFox RF & Protocol Test Plan for RC3a-UDL-ENC, Versão 3.8.1 – 11 de Outubro, 2018

In red we have the DUT signal trying to be transmitted, in light blue we have the blocker and in green the frames that were not influenced by the blocker, which are the signals actually sent to the VST.








References:

[01] Sigfox connected Objects: Radio Specification – Fevereiro 2019

[02] Sigfox RF & Protocol Test Plan for RC3a-UDL-ENC – Versão 3.8.1 – 11 Outubro de 2018

[03] Sigfox RF & Protocol Test Plan for RC2-UDL-ENC – Versão 3.8.1 – 11 Outubro de 2018

[04] MEDEIROS, Julio Cesar de Oliveira. Princípios de Telecomunicações – Teoria e Prática. 5. Ed. São Paulo: Saraiva, 2009

[05] National Instruments Corporation, Managing Software Engineering in LabVIEWᵀᴹ Course Manual, Austin, Texas, 2009.

[06] National Instruments Corporation, LabVIEWᵀᴹ Core 3 Course Manual, Austin, Texas, 2014.

[07] National Instruments Corporation, Advanced Architectures for LabVIEWᵀᴹ, Austin, Texas, 2011.

[08] T. Janssen, M. Aernouts, R. Berkvens and M. Weyn, "Outdoor Fingerprinting Localization Using Sigfox," 2018 International Conference on Indoor Positioning and Indoor Navigation (IPIN), Nantes, 2018, pp. 1-6, doi: 10.1109/IPIN.2018.8533826.

[09] A. Lavric, A. I. Petrariu and V. Popa, "Long Range SigFox Communication Protocol Scalability Analysis Under Large-Scale, High-Density Conditions," in IEEE Access, vol. 7, pp. 35816-35825, 2019, doi: 10.1109/ACCESS.2019.2903157.

[10] Pozar, David M. Microwave and RF Design of Wireless Sysstems, John Wiley & Sons, Inc. Ed. 1.


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