Insight into wireless technology
Despite its everyday usage, few people have much appreciation of the differences between the various RF (radio frequency) technologies in use today. Chris Irwin of WEMS takes us on a trip through the radio spectrum.
Wireless technology is becoming increasingly important in our lives. We have all become completely comfortable with long-range wireless communications in the form of mobile phones, and many of us will already be living with a variety of short-range wireless technologies in our own homes.
Advantages and disadvantages
In Europe there are several ‘license-free’ frequency ranges (bands) which are utilised by various applications. Each band has its own advantages and disadvantages. At 2.4 GHz and 5 GHz there are more ‘channels’ available, so devices using this frequency can be set to avoid channels being used by other devices; the bandwidth is also greater, so more data can be passed faster than at lower frequencies.
Unfortunately the higher the frequency the shorter the range, and 2.4 GHz is not great at travelling through walls and floors, so the effective range can be quite limited, as must of us appreciate as we sometimes struggle to get an adequate Wi-Fi signal even though we may be within 10 m or so of the nearest Wi-Fi access point.
Lower frequencies inherently travel further and penetrate walls and floors better, so 868 MHz is better than 2.4 GHz. 433 MHz is better still, giving a range many times superior to 2.4 GHz for a given RF transmission power.
However, this discussion oversimplifies the issue as each frequency band has different rules applied as to the maximum permitted transmission power level. 868 MHz transmissions are allowed at a higher level than 433 MHz, but this is not of help if the device is battery powered since the higher power required to provide equivalent range to 433 MHz will drain the device's battery faster.
Sources of interference
There are different sources of interference in each of the different frequency bands; the 2.4 GHz spectrum has become very crowded with a multiplicity of devices using this band, and items such as video senders and microwaves causing havoc to transmissions. This is partly why 5 GHz is increasingly used for Wi-Fi as well as its ability to carry far more data faster, but its range is even worse than 2.4 GHz.
Lower frequencies for increased range
Whilst Wi-Fi networking requires large amounts of data to be sent fast, monitoring and control networks require far less data to be transmitted, and the speed of communication (MB per second) is also less important. Lower frequencies can therefore be used, with the opportunity for increased range — which matters a great deal when installing such systems in large houses and commercial buildings where the distances between devices are greater. Several standard protocols have become quite widely deployed; these include ZigBee (2.4 GHz), Z-wave (868 MHz) and EnOcean (868 MHz).
From my earlier comments you can guess that of these protocols, Zigbee has an inherently shorter range, so it uses a technique called ‘mesh’ to extend its effective range. A mesh network has the ability to re-transmit the message from one device through another one, ‘hopping’ several times between devices (nodes) before reaching its intended destination. Such meshing can operate dynamically so that messages auto-route through different nodes depending on the interference experienced at any one time. Z-wave also offers a mesh network capability.
Such networks are effective if there are (a) lots of evenly distributed nodes to facilitate the hopping and (b) if the devices are locally powered — since battery operated nodes cannot be used for hopping as the need to stay ‘awake’ in order to listen and re-transmit drains batteries much too quickly.
EnOcean does not offer meshing as its forte is that it employs ‘energy harvesting’ to avoid the need for batteries in its sensors. However, the focus on low power also means the range is not great, and powered repeaters must be used quite extensively to achieve the necessary ranges.
WEMS RF protocol
In contrast, the WEMS RF protocol, which has now been successfully deployed in over 6000 commercial sites across the UK and elsewhere, uses the 433 MHz band since this offers much longer range potential, without the use of repeaters. The WEMS protocol also uses narrow-band technology, so all the allowed power can be utilised over a very narrow range, further boosting the effective range.
The latest version of WEMS wireless devices also has the ability to amplify the RF signal when necessary to achieve the required range and overcome local interference issues. The protocol is optimised for wireless comms with a lower message 'overhead' than more verbose protocols such as ZigBee, so that the risk of interference compromising transmission us minimised. The WEMS wireless network can be configured as a mesh with hopping, but the range is so good anyway that hopping is frequently unnecessary. In free air, a range of over 2 km is possible; in buildings ranges of over 100 m can also be achieved.
The great success achieved by WEMS in commercial applications can also be attributed to the overall system design; the whole system is designed to be wireless, not just the sensors. The wireless I/O modules, which are switching electrical loads, are locally powered (since they are always next to a power source), so unnecessary use of batteries is avoided and higher transmission power can be used. 868 MHz modules are also supported, which can transmit up to 5 km due to the higher output power allowed at this level. In the North American market WEMS also supports the 915 MHz band which can be used there.
Chris Irwin is commercial director at WEMS.