Radio Frequency (RF) Bands

Introduction

The electromagnetic spectrum includes waves of Extremely Low Frequency (ELF) of 1Hz through to visible light between 400 and 790 THz and further up to X-rays and Gamma rays in the order of exa-Hertz (EHz, 10 to the power of 18 Hertz). Radio Frequency (RF) is defined as the range of frequencies of the electromagnetic spectrum between 3kHz and 300GHz. The International Telecommunications Union (ITU) divides this band of radio frequencies into several smaller ranges (called bands) including, as examples, the Very High Frequency (VHF) band from 30 MHz to 300 MHz, and the Ultra High Frequency (UHF) band from 300MHz to 3GHz.

ISM Bands

ISM bands (Industrial Scientific and Medical), are RF frequency ranges which must be  considered by designers. An end user is free to transmit within these bands provided they have an appropriate Class License. ISM bands are used freely and liberally but are limited through regulation to low interference, low power devices (LIPD). End users are free to use these devices without permission from the ACMA. It is not necessary to buy spectrum. The user is allowed to transmit in an ISM band under this Class Licence as long as they meet power levels and certain transmitter and receiver requirements. Very importantly, the end user must not cause interference to other radio communication services. Spectrum is costly and therefore highly protected by its purchasers. ISM band users are not afforded regulatory protection from interference caused by other radio communications services. Designers and users should be aware and make allowance for fact that their signals may be subject to interference.

The main ISM bands used for IoT today are typically grouped around 433 MHz and 920 MHz. The 920MHz range is 915-928 MHz, and a power level of one watt is typical. There is also the 2.4 GHz band which is well known because it is the band used by consumer WIFI access points under a LIPD Class License.

In the 915-928 MHz band, users are regulated to a maximum of one watt effective isostropically radiated power (EIRP). Importantly, designers and end users must use modules that are type approved for Australia. Problems are common where modules are imported which do not meet the Australian requirements.

An example of a problem caused by using imported non-compliant modules is as follows. Overseas the 900 MHz band goes from 868 to 928MHz, compared to 915 to 928MHz in Australia. Vodafone is a mobile phone carrier which uses a band from 850 to 890MHz. If a module from overseas is transmitting 868 MHz, Vodafone will issue a disconnect warning to the offending user very quickly and the ACMA will as well. So, it is important to use modules which use the Australian, slightly more restricted bandwidth of 915 to 928MHz.

A advantage of the 920MHz band in comparison to the higher ISM frequency bands is that communication outdoors over longer distances is possible. The Australian Wool Industry and a firm called Digibale, which is a spin off from Australian Wool Industry uses this band for farm automation which is a typical use of this band.

Another example application of the 920MHz band is between the floor plates of a building. Readers will be familiar with WiFi which is ubiquitous in businesses and in homes. Wifi is effective for use on one floor however, when communicating between floors, the penetration of Wifi (which uses the 2.4 GHz band) between the floors is poor. That's an application where the 920 MHz band could be used, especially in WiFi systems which are replacing legacy cabling (such as old RS485) and which requires wireless communication covering an entire building.

Another example application of the 920MHz band is bridging Ethernet. Ethernet can be bridged at up to one megabit per second in this band with one watt, and has been tested with wireless links spanning over a hundred miles (over 160 km). However, to get a line of sight distance for a hundred miles is rare. It occurs, for example, where there are very high mountains on which communications equipment can be placed in order to get that line of sight distance.

The 2.4 GHz band is one which many are familiar with. The following picture shows a WiFi analyzer diagram from a phone.

It illustrates that this band gets congested. This scan was taken on the 88th floor of an office block by randomly scanning for WiFi channels in use. Note that there are eight WLANs centered on channel one  which are 20 MHz wide. Normally, peaks are expected at channel one, six, and eleven due to the fact that this how a lot of WiFi routers are set up by default. A question that is frequently asked in relation to Zigbee which also uses 2.4 GHz is, will it inter-operate with WiFi? The answer is yes because the channel spacing used by Zigbee is a lot smaller and can fit inbetween WiFi channels. There are instances where public WiFi networks are known to operate simultaneously with Zigbee networks for lighting control in cities utilizing the same channels.

Sources: Material on this page has primarily been sourced from the following:

• Presentation by Phillip Lark, Engineering Manager, Braetec titled Front End Integration: Connecting sensors to the cloud