Understand wireless short-range devices for global license-free systems

Designers of SRD wireless systems must use great care in choosing the communication frequency. In most cases, the choice is limited to those portions of the spectrum that allow license-free operation: 13.56 MHz, 40 MHz, 433 MHz, 2.4 GHz, and 5.8 GHz globally; 868 MHz and 915 MHz in Europe, U.S, Canada, Australia, and New Zealand.

The 2.4-GHz band is widely used by designers who want to build systems that can operate worldwide. In fact, it has become the frequency band of choice for such standards as Bluetooth, WLAN, and ZigBee. The 5.8-GHz band has also attracted some attention in cordless phones and 802.11a WLANs.

For systems that require both wider range and lower power, however, the sub 1 GHz bands remain compelling. Their reduced coexistence problems and greater transmission range affect power consumption, making them important considerations in battery-powered applications.

The improvement in propagation range for lower-frequency radiators can be shown by a simplified version of the Friis transmission equation, which relates the power available in a receiving antenna, P_r, to the power delivered to the transmitting antenna, P_t:

This equation assumes that both antennas have unity gain, and shows that for a fixed transmit power, P_t, the received power will decrease with the square of the distance, d, and the square of the frequency, f. If the received power goes below the minimum power needed to demodulate the signal correctly (the sensitivity point), the link will break down.

Worldwide Frequency Allocations Below 1 GHz
The 433-MHz band is one option for global usage, with a slight frequency modification required for Japan (easily handled by modern frequency-flexible transceivers, such as that shown in Figure 1). Unfortunately, less than 2 MHz of bandwidth is available, and applications such as voice, video, audio, and continuous data transmission are typically not allowed, thus restricting its use. As a result, it is more commonly used for keyless entry systems and basic telecontrol.

The bands around 868 MHz (Europe) and 902 MHz to 928 MHz (U.S.) are more useful; they do not restrict applications, and they allow more compact antenna implementations. Other regions, such as Australia and Canada, have adopted versions of these specifications, thus making the band multiregional, albeit not quite fully global.

Prior to the latest EN 300 220 specification, U.S. and European bodies took vastly different regulatory approaches. The U.S. adopted a frequency-hopping approach, while Europe applied duty-cycle limits in each of the sub-bands as described in ERC REC-70. Both implementations are useful in minimizing interference, but manufacturers designing systems for both regions needed to completely rewrite the media-access control (MAC) layer of the system's communication protocol. Fortunately, the latest European EN 300 220 regulations have extended the frequency bands to allow for frequency-hopping spread-spectrum (FHSS) or direct-sequence spread-spectrum (DSSS). This makes the MAC implementations more similar to those designed for the U.S, but some fine-tuning will still be required.

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Frequency-Hopping Systems
Frequency-hopping spread-spectrum (FHSS) transmission technology spreads energy in the time domain by dividing the spectrum into a number of channels, switching between them in a pseudorandom sequence (hopping code) known by both the receiver and transmitter.

To welcome new nodes joining the network, the controller node periodically sends out a beacon signal to which the new node can synchronize. The synchronization time depends on the beacon interval and the number of hopping channels. U.S. and European standards both specify a similar number of hopping channels, and a maximum dwell time (the time spent at a particular frequency during any single hop) of 400 ms.

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