Unlicensed Band LPWAN a Cost Effective IoT Solution

Low-Power Wide Area Networks that transmit and receive on the unlicensed ISM (Industrial, Scientific, Medical) band are up against their licensed, cellular counterparts. Both have advantages, but the unlicensed option may be best for budget conscious organizations that would benefit from IoT solutions.

The Cost

The ISM band is unlicensed, so there is no cost associated with using the band. While the alternative, cellular, offers some technical and infrastructure advantages, it comes with a monthly cost per device. Each sensor in a cellular network would require a data subscription to the user's carrier of choice. The advantage here is that the cellular provider takes care of the network infrastructure and may offer (at additional cost) some options for the maintenance of end devices. The unlicensed option, operating on bands typically used for non-communication applications, only requires the acceptance of the fact that your device will be subject to any spurious emissions from other transmitters operating in the band. And, in addition, you would own the end devices, and the network infrastructure.

So why take on ownership of this network? ISM allows you subscription-free high-performance wireless communication when cellular costs are not practical - either due to the size of the organization implementing it (too small), or the size of the application (too large). This makes ISM band LPWAN solutions well suited for governments with tight budgets and large domains that want to implement smart city or smart building applications to further optimize the use of their already limited assets. It also allows small to medium size businesses easier access to sophisticated technologies that will help them make critical (money-saving) decisions with data that they, otherwise, could not afford to collect.

The Performance

The ISM band, being reserved by the FCC for applications ranging from microwave heating to military radar, is open to any device for communication purposes. In fact, it's use is quite common. The band covers technologies such as cordless phones, Bluetooth, Wi-Fi, near field communications, and more. The caveat with operating on ISM, is that the device must tolerate any potential interference from others using the band. Fortunately, many modulation techniques have been developed that obviate the need for concern. For a classic example of this - think about that "spread spectrum" sticker on your early 2000s cordless phone.

IoT applications use much more sophisticated techniques. One such method is called chirp spread spectrum (CSS). With CSS, information is encoded using a frequency "chirp" - Each bit of information, 1 or 0, is represented by a quick, continuous, and predictable up or down-tick in frequency. This rate of frequency change is known thus allowing the receiver to decode it without additional information in the signal.

CSS uses all allocated bandwidth making it very robust to noise. It resists multi-path fading (the result of transmitted signal being reflected from multiple objects before arriving at the receiver) and negative effects from the Doppler effect - experienced when one or more of the transceivers are in motion. The combination of these advantages makes encoding schemes like CSS well suited for deployments that require low-power, transmission in and around buildings, and applications like asset tracking where at least one of the devices will be moving.

If you are concerned signal range will suffer without the use of cell towers, many ISM technologies in IoT make use of sub-GHz frequencies which are less easily absorbed by water and oxygen in the atmosphere and less subject to reflection from conductive materials. Coupled with signal transmission techniques such as CSS that significantly increase the ability of a device to decode noisy, reflecting signals, these features make for excellent signal coverage both in and outdoors. And, that brings added range which means the user will need fewer gateways - typically the costliest part of the wireless network. Although the gateway is not needed for cellular networks, it becomes an advantage when you wish to deploy in a region where you may not have a nearby cell tower – say in agriculture deployments such as vineyards, cattle ranches, and orchards.

Conclusion

LPWAN systems using the ISM band play an important role in the IoT ecosystem. There’s no subscription needed which allows a lower cost of entry for organizations with limited budgets and large deployment needs, they use sophisticated modulation techniques that make them ideal for urban and asset tracking applications, and they have long-range capability coupled with great urban and indoor signal penetration which leads to fewer needed gateways. ISM band systems offer a solution where cellular coverage is too expensive or too spotty.