A Licence-Free Wireless Primer
Wireless spectrum that can be used with the need for a licence is a fundamental enabler for building the Internet of Things. Here in Part 1 of a series we provide an introduction to licence-free wireless and cover some of the key considerations to take into account when selecting technologies.
Since spectrum is a finite resource and demand outstrips supply it must be very carefully managed. This is typically done by telecoms regulators operating at the country level and most of the spectrum they manage is licensed to one or a small number of users. But certain frequency allocations or “bands” are given over for unlicensed use subject to certain conditions being met.
The spectrum available and conditions attached can vary from country to country, and it's important to ensure that equipment meets the regulatory requirements of the country in which it will be used.
One of the best known and most used licence-free spectrum allocations is the 2.4GHz ISM band employed by wireless LANs, Bluetooth and a whole host of simpler systems including wireless keyboards and CCTV. Originally intended for industrial, scientific and medical (ISM) purposes other than communications, the ISM bands have increasingly found use where there is a requirement for low power, short-range communications.
Other ISM bands include 433MHz (Europe, Middle East and Africa) and 915MHz (Americas, Greenland and Pacific Islands), both of which are used by consumer electronics such as remote car locks and wireless thermostats. But it's important to note that not all licence-free spectrum is an ISM band and one example is the 868MHz Short Range Device (SRD) allocation used in Europe.
When it comes to selecting a wireless solution some thought should be given to the band in which it operates, as this will not only dictate where the equipment can be used but may also have an impact on the range and data throughput. As a general rule of thumb and all other things being equal, the higher the frequency the shorter the range, as signals at higher frequencies become more line-of-sight and less able to penetrate walls etc. But benefits as you go up in frequency include that antennas get smaller and its common for more bandwidth to be made available.
Transmit power and other constraints
Even where a frequency allocation is common to different parts of the world the conditions for use may vary. An example of this is the 2.4GHz ISM band where the power limit is set at 100mW in the UK, whereas in the USA this can be 4 watts or even more for fixed point point-to-point links.
The figures quoted above are for a rating known as effective isotropically radiated power (EIRP), and while this sounds complicated all it means is the power level taking into account the gain of the antenna. It's important to not confuse transmitter power and EIRP when reviewing wireless regulations, and there may be times when it is necessary to calculate the EIRP for a given system. However, in practice turnkey systems are typically sold as being suitable for use in a particular region and so it's probably not very often that you would have to calculate EIRP.
Other constraints associated with licence-free wireless technology may include bandwidth, type of modulation and use of interference avoidance mechanisms. But again, it should generally be stipulated whether a device meets the regulatory requirements for use in a particular region.
The most basic wireless systems simply provide a means of conveying data between two or more points and may offer no guarantee as to its integrity, or provide addressing or any of the other features that are typically associated with networks. And for many applications this will be sufficient. E.g. where sensors continually broadcast and it doesn't matter if data is occasionally lost. But in cases where a reliable data service is required, or there is perhaps the need to scale to a large number of nodes or create a mesh network, it may be advisable to look to more complex systems.
Which wireless system is best suited to a particular application will depend upon various factors, including the desired range and bandwidth, number of nodes, power supply available and budget.
When designing for the wireless-enabled Internet of Things consideration should be given to all the countries in which a solution will be used. Off-the-shelf wireless systems should clearly state where they are suitable for use, but familiarity with equipment specifications and regional regulatory requirements may lead to opportunities for performance improvement. Thought should also be given to application requirements and whether simpler wireless systems just ends up relocating the complexity elsewhere, or conversely whether a more advanced system would prove to be an unnecessary overkill.
In the next part of this introductory series we will take a look at a range of hardware and software combinations currently available for use including the low cost and simple HopeRF RFM12B, all the way to more advanced options like Xbee, Redbee, and the RFDesign RFD900 designed for use in much longer range projects.
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