How to Select the Right Short-Range RF Technology for Your Application

By Reuben Townsend, Field Applications Engineer, Future Electronics (UK)

READ THIS TO FIND OUT ABOUT The benefits of individual RF technologies for a given application How the fundamental differences between propriety and industry-standard solutions can have a significant effect on design time and bill of materials

The choice of technologies available to the design engineer for short-range RF communication or networking is bewildering. Certain technologies, such as Bluetooth, ZigBee™ and Wi-Fi™ (or 802.11), have gained widespread recognition. But just because they are well known does not mean that they are necessarily the right technical fit for every application. Other technologies from semiconductor vendors such as Micrel, Microchip and Cypress Semiconductor might lack the marketing clout of an industry Special Interest Group, but have characteristics that can make them preferable.

 

 

This article demystifies the choice of wireless communication products that use the unlicensed ISM band (see 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.

 

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 industry-standard 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 Wi-Fi’s. 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 such that if any of the nodes 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. Thus environments that are damp or that are exposed to rain and snow can suffer attenuation in the signal path, leading to severely reduced range.

Finally, it is important to remember that when you decide to adopt Bluetooth or ZigBee, you are also committing yourself to the cost of membership of the consortium, and 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. And indeed, if your 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 is a range of proven technologies to consider.

The license-free bands at the 433MHz and 868MHz frequencies (in Europe) are popular, as they avoid the interference problems that can affect the 2.4GHz band. On top of this, 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 are no consortiums 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 on, the designer can 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.

 

 

Micrel’s MicrelNet (see Figure 2), is an IEEE 15.247-compliant star-topology 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 that available from a ZigBee system. This 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 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 (typically) for ZigBee.

If your end product is to be marketed worldwide, however, then use of Europe’s lower ISM frequencies will not be suitable, and you are forced to adopt the worldwide standard 2.4GHz frequency. Even here, however, you have different options from 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 than a ZigBee system: 10kB compared to 32kB-64kB for ZigBee. 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 you need a simple implementation, 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.