The Industrial, Scientific, and Medical (ISM) radio frequency spectrum was originally set aside to accommodate mechanical equipment which generated significant radio emissions.
At that time, this was limited to microwave ovens, RF heating elements, medical heat therapy (e.g., diathermy) devices and other industrial systems. The intent was to create a frequency spectrum where the radio output, which is essentially noise, of these devices would not interfere with actual communications systems.
The ISM frequency band was designated as an un-licensed spectrum – any equipment utilizing the 915 MHz to 5.800 GHz frequencies had no regulatory protections. However, in the US, the use of the ISM bands are governed under the FCC rules and regulations (Parts 15 and 18). Developers found that this frequency band was quite suitable for low power, short range communications. To be effective, any communication devices would need to comply with the ISM specifications and develop a method of coexistence for the noise produced by all the other devices operating in the same frequency.
For communications devices, the ITU-R 5.138, 5.150 and 5.280 call out the standards for designing ISM frequency based systems.
Common applications and their operating frequencies:
- Wireless Sensor Networks (WSN) – 915 MHz and 2.450 GHz
- Wireless LANs / Home Networks – 915 MHz and 2.450 GHz
- Cordless Phones – 915 MHz, 2.450 GHz and 5.800 GHz
- Bluetooth – 2.450 GHz
- WiFi (802.11) – 2.450 GHz and 5.800 GHz
- ZigBee (802.15.4) – 915 MHz and 2.450 GHz
In order for each device to operate properly, the developers need to employ one or more coexistence methods. These are generally divided into two types: “Collaborative” and “Non-Collaborative” .
Devices employing a collaborative scheme will generally attempt to work together to most efficiently share the available bandwidth and frequency spectrum. This may include a time domain sharing in which each active device takes a turn for its communications, and then waits for a period of time to allow other devices to communicate. More advanced methods will also provide for a prioritization of communications – for example voice packets over a Bluetooth connection will have a higher priority and usage than data packets using a wireless LAN (WLAN) home network. In this case, time sensitive data can be identified by a device type or through information encoded in the transmission.
A non-collaborative approach ignores the other devices and makes a best attempt at optimizing communications for itself. This includes methods such as adaptive frequency hopping to select a frequency channel which is least used, scheduling bulk data transmissions and other types of data traffic controls. To be effective, the device must be able to detect other devices in the vicinity, determine which frequencies they are operating on and calculate an error ratio or interference level for each frequency. The non-collaborative communications device must also be capable of quickly processing this information and either continuously or periodically acting on it to hop frequencies.
The options, as well as complexity, for ISM Band coexistence increase with the number of nodes in each network. For example, a Wireless Sensor Network (WSN) with multiple sensors in a mesh configuration communicating with each other and a common gateway. This implementation could be used in building automation, smart metering or precision agriculture.
Remote sensors can best select which frequencies to communicate on, whether to relay data from node-to-node or from node-to-gateway, what size data buffer to use for transmitting short or long bursts, and how to prioritize when nodes can communicate.
The next challenge for coexistence is just beginning in the area of vehicle networks. In this application, cars, buses and trucks will be able to communicate with the surrounding vehicles and also stationary nodes. This can be implemented with dedicated in-vehicle electronics or through smart phones. This mesh network is continuously, and often rapidly, changing as vehicles come into and out of range of each other.
(Image Credit – IEEE )
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