BehrTech Blog

Where Do My IoT Sensors Live?

An Overview of the Sub-GHz ISM Bands

We’ve heard the numbers; IoT devices will reach 50 billion by 2025 etc. Most of these devices will be wireless, but where will they reside on the radio spectrum? Within the radio frequency (RF) spectrum there are both licensed and unlicensed bands. While a few Low Power Wide Area Networking (LPWAN) solutions such as NB-IoT operate in the licensed portion of the spectrum, most other solutions such as MYTHINGS and LoRa operate in the sub-gigahertz (GHz), the one unlicensed portion of the spectrum reserved for industrial, scientific and medical (ISM) devices.

Operating in the unlicensed spectrum, sub-GHz ISM bands have some significant benefits for organizations such as reduced deployment cost as there is no need to pay for licensed bandwidth as well as the fact that the traffic resides in a completely different part of the spectrum from Wi-Fi and Bluetooth. However, there is still potential traffic from devices on the same LPWAN system, devices on different LPWAN networks, as well as traffic from other types of devices such as RFID tags and alarm systems. In this blog, we examine the history and characteristics of ISM bands.

Brief History of Radio Spectrum

Radio communication was invented towards the end of the 19th century. In its infancy, radio was mostly used for Morse communication largely for maritime and transoceanic communication. In those days, there was little-to-no regulation of radio traffic. The regulation of radio began in Europe in the early 20th century, however it wasn’t until the sinking of the Titanic, that the US adopted the Radio Act which legislated the requirement for radio station licenses.

The invention of AM radio and the ability to transmit voice led to the first commercial radio stations and with it, an explosion of amateur and commercial broadcasters. The invention of FM radio and its lower interference features made the radio spectrum much more dynamic. As the popularity of radio increased, the US government, in 1934 created the Federal Communications Commission (FCC) to regulate the radio spectrum in the United States.

Introduction of the ISM Band and Regulations

In addition to the broadcasting of voice and music, new applications and technologies began to use the radio spectrum. Examples include microwaves for cooking food, industrial induction heating, as well as medical applications such as diathermy machines using radio waves to apply deep heating to the body. The International Telecommunication Union set aside a portion of the radio spectrum and established the Industrial Scientific and Medical (ISM) bands in 1947 to provide dedicated spectrum for non-telecommunication devices.

Initially, the ISM bands were limited to Industrial Scientific and Medical devices, and telecommunication usage was forbidden. However, over time, the explosive growth of microelectronics and computing along with the attractiveness of an unlicensed spectrum, several factors brought about the pressure to use these unlicensed bands for wireless communication. In 1985, the FCC in the United States decided to allow communications on the ISM bands. However, soon after, rules were put in place to require pre-certification of all new products using unlicensed bands. To enforce these new regulations, the European Telecommunications Standard Institute (ETSI) was created in 1988, and in 1989, new regulations were introduced within the FCC.

There are actually several bands within the radio spectrum set aside for ISM equipment. Some such as the 2.4 GHz band (used by Wi-Fi and Bluetooth) is of worldwide standards. Others are regional with specific ranges being governed by individual countries or regions.

As mentioned, LPWAN solutions operate in both the unlicensed or licensed bands. The unlicensed ISM bands used by LPWAN solutions operate below the 1 GHz level. The following figure displays the sub-GHz radio spectrum bands and the amounts of spectrum set aside by various regions around the world.

sub-GHz ISM bands

Other Uses of the Sub-GHz ISM Bands

In addition to LPWAN wireless solutions, many other technologies and devices operate within the same sub-GHz ISM bands. These types of devices include radio frequency identification (RFID) devices, garage door openers, cordless telephones, wireless drones, wireless microphones, baby monitors and alarm systems.

Regulations to Keep Usage Under Control

Because anyone can use the sub-GHz ISM bands, it becomes difficult to limit the number of devices operating within them. As such, regional and national regulatory bodies have created rules and regulations to control usage within these bands and prevent them from becoming saturated.

One common regulation is to establish a limit on the maximum transmission (Tx) power of the transmitting device. For example, in the US, the limit on Tx power is 24 decibels per milliwatt (dBm) which translates to 250 milliwatts. In Europe, the limit is more often 14 dBm (25 milliwatts).

A limit on transmission power may not be enough to protect bandwidth usage. If devices are allowed to take a very long time to transmit, and especially if there are many devices on the network, additional devices might be prevented from using the channel. To address this issue, some regions implement a duty cycle. A duty cycle represents a percentage of time that a device can be actively transmitting in the band. For example, a duty cycle of 1 percent means that a device can only actively transmit 1 percent of the time. In a given hour, a device could transmit no more than 36 seconds. Alternative and complementary policies can be used as well including “frequency hopping” which forces the radio technology to use different sub-channels within a band to prevent one channel from being saturated.

Still, even with limits on transmission power and duty cycles, there is the potential for a very large number of devices transmitting in a campus environment, such as a factory or a building. As such, you need to ensure that your LPWAN solution offers superior robustness and can scale as needed. MYTHINGS by BehrTech with its patented Telegram Splitting technology offers superior power efficiency, high distance, robustness, scalability and supports mobility of 120 km/h and beyond.

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