Listen Before Talk and Automatic Frequency Agility

This device implements Listen Before Talk (LBT) and Automatic Frequency Agility (AFA). The advantage of LBT with AFA is that the device bypasses the Duty Cycle requirement imposed by European standards. LBT+AFA requires that you use at least two frequencies for transmission.

This feature provides a level of fairness to the devices in a given area. Before this device transmits, it senses a channel to determine if there is activity by taking an RSSI measurement for 5 ms. If the measurement is below the threshold, the device transmits on that channel. If there is activity, that channel is not used, and the device listens for at least 5 ms to allow transmissions to be received.

After the device transmits on a channel, it will not transmit on that channel again until the minimum TX off time has been met, which is greater than 100 ms. It is useful to have many channels in your channel mask, so transmissions are less likely to be delayed.

European requirements also state that only 100 seconds of transmission may occur over the period of an hour on 200 kHz of spectrum. This method simplifies and optimizes the calculations of spectrum use over the period of one hour. The standard states that the more channels you have, the more transmission time you have in a one hour period. Calculate the effective duty cycle based on the number of available channels enabled as follows:

Effective Duty Cycle = (number of channels * 100)/3600.

For example, if you enabled two channels you would have an effective duty cycle of 5.6%.

The XBee 868LP RF Module uses a sliding bucket algorithm to calculate usage over the period of 1 hour for each channel. Each bucket accumulates for 6 minutes.

This device has a maximum of 30 AFA channels to choose from, and channels can be excluded by setting the channel mask (CM) to reduce them. Since not all countries allow for all of these channels, the set may be dramatically smaller for some countries. For a complete list, refer to www.digi.com.