How to select the right short-range RF technology for your application
by Reuben Townsend, Field Applications Engineer, Future Electronics (UK)
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READ THIS TO FIND OUT ABOUT:
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- The benefits of individual RF technologies for a given application.
- How the fundamental differences between proprietary and industry-standard solutions can have a significant effect on design time and bill of materials.
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Just because Bluetooth, ZigBee™ and Wi-Fi™, or 802.11, are
well known does not mean that they are necessarily the right
technical fit for every application. Other technologies might
lack the marketing clout of an industry Special Interest
Group, but can have characteristics that can make them
preferable. Reuben Townsend, Field Applications Engineer,
Future Electronics (UK) explains.
This article demystifies the choice of wireless communication
products that use the unlicensed ISM band shown in Figure 1. This
includes technologies operating at the 433MHz, 868MHz and 915MHz
frequencies as well as the standard 2.4GHz systems such as Bluetooth
and Zigbee.
Fig. 1: The EU Industrial, Scientific and Medical (ISM) bands.
Choosing between industry-standard and proprietary solutions
The golden rule for designers to remember is this: does your product
need to offer interoperability with products from an uncontrolled
group of other vendors? If so, you will need to consider an industrystandard
technology such as Wi-Fi, Bluetooth or ZigBee. If not, there
are a range of alternatives that can be cheaper and easier to
implement.
The reason for this is that Wi-Fi, Bluetooth and ZigBee are designed
by committee, and try to be all things to all people. This means that
they have more features than most users will require, which in turn
makes their protocols complex, and means that software stacks
occupy a large memory footprint. These characteristics slow design
time and increase the Bill Of Materials (BOM).
A typical 802.11 wireless LAN network, for instance, requires 1MB of
processor memory to run the application. The high power consumption and
the processing power required for an 802.11 solution are not a problem for
computing applications, but will be completely unsuitable for remote-monitoring
and industrial applications.
The same argument applies
to ZigBee and Bluetooth,
albeit that their memory and
processor requirements are less
extreme than those of Wi-Fi. But
even the ZigBee stack occupies
32kB-64kB. This large stack is
needed in part to support
ZigBee’s mesh network
topology. The mesh network
allows data to be passed from
node to node so that if any
node fails or goes out of range,
the data can still find a path to
its destination.
At the RF level, it also is
worth noting that the 2.4GHz
frequency band used by
Bluetooth, Wi-Fi and ZigBee is
extremely crowded, and many
users report problems with
interference at this frequency.
In addition, the modulus of
water is 2.4GHz, which is why
microwave ovens use this frequency. So at 2.4GHz, water acts as a shield to the
RF signal. Therefore environments that are damp, or exposed to rain and snow,
can suffer attenuation in the signal path, leading to severely reduced range.
Finally, it is important to remember that, deciding to adopt Bluetooth or
ZigBee, means committing to the cost of membership of the consortium, and
to the fees for technology licensing and certification testing.
How to choose an alternative to Bluetooth and ZigBee
In the experience of Future Electronics, the first technology that designers of
industrial and commercial wireless applications evaluate is ZigBee. Indeed, if an
application requires interoperability and low power consumption while
transmitting small amounts of data, then ZigBee is often the right choice.
However, for the many industrial applications that are single-vendor systems,
which do not require interoperability, there are a number of proven
technologies to consider.
In Europe, the licence-free bands at the 433MHz and 868MHz
frequencies are popular because they avoid the interference
problems that can affect the 2.4GHz band. In addition, technologies
such as MicrelNet™ from Micrel and WirelessUSB™ from Cypress are
‘closed’ systems. This means they are owned and promoted by a
single vendor, so there is no consortium to join, and no technology
licensing fees to pay. The only official requirement is to obey
government regulations covering usage of the frequency band.
When searching for alternatives to Wi-Fi, ZigBee and Bluetooth
network configuration, the key elements that dictate the suitability
of wireless technologies for each application are code size, system
power and range.
By making the right choice of system architecture early in the
design cycle, it is possible to make huge savings in BOM and design
time. For example, the fail-safe properties of a mesh network do not
justify the required processing and memory overhead, and a simpler
star, point-to-point or multipoint-to-point topology will be sufficient.
Fig. 2: Typical MicrelNet network configuration.
Micrel’s MicrelNet (see Figure 2) is an IEEE 15.247-compliant startopology
protocol running on its RadioWire family of FSK transceiver
chips and modules. It performs frequency-hopping spread-spectrum
modulation for interference immunity in a 250kHz bandwidth.
The MicrelNet stack can occupy less than 8KB in an 8-bit
microcontroller, while delivering data rates up to 200kbps at ranges
up to 10 times greater than the data rates available from a ZigBee
system. Also, code size is at least 75% smaller than that of a ZigBee
implementation. This small hardware overhead does not just produce
a lower BOM – it also helps to cut power consumption. By implementing
duty cycling of the nodes to transmit and receive only when required,
remote meter-reading designs can provide battery life of over 10
years, compared to one year which is typical for ZigBee.
If the end product is to be marketed worldwide, however, then use
of Europe’s lower ISM frequencies will not be suitable, and it will be
necessary to adopt the worldwide standard 2.4GHz frequency. Even
here, however, there are alternative options to the well-known
standards.
Microchip, for instance, provides a proprietary technology called
MiWi™. While it is similar in some ways to ZigBee, it offers a reduced
feature set that is more suitable for many remote-monitoring and
industrial-networking applications.
Crucially, MiWi attracts no technology licensing or certification
testing fees, and it has a much smaller hardware overhead of 10kB
compared to 32kB-64kB for a ZigBee system. It provides a way of
getting ZigBee-like functionality at the 2.4GHz frequency band at
lower cost and with less design complexity. Microchip provides the
MRF24J40 MiWi transceiver and a wide range of microcontrollers.
If a simple implementation is required, with a data transfer rate of
up to 1Mbps, and a point-to-point or multipoint-to-point topology,
Cypress Semiconductor’s proprietary WirelessUSB™ technology
should also be seriously considered. It implements a direct
sequence spread spectrum scheme that provides excellent
immunity to interference. The company’s PRoC™ device
provides a highly-integrated programmable controller and
2.4GHz transceiver, offering reduced board footprint, BOM and
design time.
Licence-free and well supported by silicon devices, development
kits, user guides and source code, the alternatives to Wi-Fi, Bluetooth
and ZigBee from manufacturers such as Micrel, Microchip and
Cypress have been proven to work in real applications. There is
also no doubt that the standards-based technologies such as
Bluetooth have helped to drive forward huge opportunities in
applications such as personal connectivity and multimedia
devices and wireless computer networking, but the proprietary
technologies, should not be disregarded. For the embedded
world, they can often be the most suitable option, and should
be seriously evaluated by the designer of industrial
applications.
Development kits from Micrel, Microchip and Cypress are
available to members of Future Electronics’ Board Club. Future
Electronics also supplies RF devices, as well as Wi-Fi, Bluetooth
and ZigBee components and modules.
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