11 September 2012
How the 458MHz ISM band suits short range, low data rate applications
The radio spectrum is a crowded place – and some parts of the spectrum are more crowded than others. So it's no surprise that licensing authorities around the world control the use of certain frequencies in order to avoid overcrowding.
Some obvious licensed areas are those used for mobile phone communications – depending on where you are in the world, mobile phones operate at 850, 900, 1800, 1900 and 2100MHz. But there are many other parts of the spectrum which are unlicensed: effectively, there's a communications 'free for all', subject for the need for products to conform to some basic requirements.
In general, these bands are known as the industrial, scientific and medical bands – or ISM – and are licence exempt. According to Ofcom, 6.9% of the spectrum available in the UK was allocated to licence exempt communications in 2010.
The Radiocommunications Agency – the predecessor to Ofcom – said licence exemption can offer significant advantages for users. "[It] offers obvious advantages in relation to mass produced items for domestic use, such as remote controls or garage door openers, for which individual licensing would not be feasible, and for private users or small businesses who wish to use radio equipment in a domestic or office setting."
A range of ISM bands is available to UK users and these differ in some instances from ISM allocations in other parts of the world. At one end of the spectrum are frequencies as low as 9kHz. At the other is the 76GHz band, allocated for vehicle radar systems. In between are at least 10 other bands for various applications.
Perhaps the best known of the ISM bands is the one at 2.4GHz, which supports Wi-Fi and Bluetooth communications – and it's also the frequency at which your microwave oven operates. But those developing industrial applications will also be familiar with three particular centre frequencies – 433, 458 and 868MHz.
Asked about the attraction of the two lower frequency bands, John Sharples, managing director of Low Power Radio Solutions (LPRS), said: "Apart from the fact that they are licence free, the bands are less crowded than other areas of the spectrum."
John Jackson is telemetry product manager with Radiometrix, a leading designer and manufacturer of ISM based wireless connectivity products. He said the 458MHz band had been in use for about 20 years. "It gives significant advantages to UK users," he noted, "because it offers them the possibility of a 100% duty cycle. The specification supports a maximum output power of 500mW, give provides users with good range." He added that duty cycle is an important consideration when developing wireless links. "While you can send a lot of data over the link, most systems working in this band will only be transferring a few bytes a few times a day. That means users don't have to worry about what transmissions they are making."
Unlike Sharples, Jackson believes the 458MHz band is crowded. "But because the transmissions can only use a maximum power of 500mW, there's room for a lot of systems."
One of the benefits of the 458MHz band is transmission range, which can be in excess of 1km in free space. Sharples said: "In many areas, the use of lower frequencies will offer you better signal propagation and less degradation than you might experience at, say, 2.4GHz. However – and this may have to do with the kind of customers we target – we don't come across applications which need to cover 1km."
While there are some applications which need long range communications, the 458MHz band comes into its own within buildings. Sharples said: "There's a large block of LPRS' clients who are building warehouse type applications or systems that need to work in larger buildings. Designing these systems can be complicated because substantial buildings have constraints."
Jackson agreed. "The 458MHz band is ideal for use within buildings. For example, it's good for data terminals within warehouses." So did Ian Siddall, applications group manager with design in distributor Solid State Supplies. "One of the big problems for wireless communications is coverage and designers have to determine how the various options they have will operate within buildings. Lower frequencies certainly have better in building coverage."
Jackson said 458MHz modules are useful for applications such as traffic light control. "But 458MHz is also useful for location services. If you have, for example, 100 vehicles, you could track them using only one channel; all the vehicles need to do is send their gps coordinates."
Siddall is seeing growing interest in wireless, rather than wired, communications. "It's an obvious solution for remote monitoring," he offered. "We are talking with a customer involved with architectural lighting about how rf communications can be used to monitor, configure and control their systems. But we're also seeing interest from security system developers, for example."
The 458MHz band is reserved for the UK only, so if you're building a system to be used across Europe, for example, then you'll need to look at other frequencies. There are two main choices: 433MHz and 868MHz. According to Jackson, while 433MHz is a useful option, it has a power limit of 10mW, which reduces range. "If you're looking at short distance comms, then 433MHz is best from the unit cost point of view," he said. "Using 458MHz modules will cost more because you're transmitting at higher power."
All three believe it's important for designers to think about their applications and not go for a 'hot technology' such as ZigBee.
Sharples said: "Design engineers have a set of targets and start from a particular point – which might be 'I've got to use ZigBee' – without understanding the problem they're dealing with. We spend a lot of time back tracking them, but engineers can often be blinded by hype. We ask them to tell us what they want to achieve; what's the power, data rate and distance?"
Siddall said engineers have to ask themselves some basic questions. "What are you trying to configure, monitor or control? How many bytes are you transmitting and how often? And what's the throughput?"
Jackson said: "Because broadband speeds have gone well into the Mbit/s range, everyone believes speed is important. If you're looking to send video data, then it's a problem, but if you're only looking to send a few bytes from a temperature sensor, then it doesn't matter. The challenge is getting engineers to understand what it's all about. Pushing 19.2kbit/s over a dual channel or 9.6kbit/s over a 12.5kHz channel at 458MHz will be hard."
What other considerations are there? "Where do you put the antenna and how do you enclose the product," Jackson concluded. "Make sure there's no interference from digital circuits and don't use a directional antenna, because you'll be breaking the regulations; a quarter wave antenna will be just fine."
Long range, high reliability
Radiometrix' WFX2 radio transceiver module is said to be suited to industrial applications requiring long range and high reliability. The 256 channel module has an rx to tx switching time of 5ms, a receive sensitivity of -118dBm and a usable range of 5km over open ground.
According to Radiometrix, the WFX2 is suited to industrial applications including on site paging, telecommand, machine control, fast, long range data modems and robotic/mechatronic control.