The frequency band between 863 MHz and 870 MHz belongs to the ISM bands and can be used by anyone without a license. Only the product manufacturers have to observe some rules regarding the transmission power and the sub-bands.
The advantage of using sub- GHz frequencies over microwaves is the significantly lower free- space attenuation. The level advantage between 868 MHz and 2.4 GHz is about 9 dB. This means that it is possible to operate at a lower transmit power, cover longer distances at the same transmit power, or bridge long distances at low data rates.
Therefore, the 868 MHz band is used for many radio applications. For example, for low power WLAN( LPWAN) and short range devices( SRD), which is why it is also referred to as the SRD-860 frequency band. Other applications include radio-based alarm systems, wireless thermometers, cordless telephony, wireless mice, wireless sensors, wireless keyboards and RFID. In addition to short-range radio with point-to-point links, the 868 MHz frequencies are also used in network configurations such as ZigBee, sensor networks, IEEE 802.15.4 and low-power WLAN (LPWAN), and with remote keyless entry ( RKE) and passive keyless entry (PKE) also in automotive technology.
As far as SRD technology is concerned, for example, this technology divides the 868 MHz frequency band into seven subbands with different bandwidths. Between two subbands there is a frequency gap of 100 kHz, which is reserved for non-public applications. Five subbands are broadband, and two have a channel spacing - in one case of 25 kHz. As for the equivalent radiated power( ERP), it is usually 25 mW. This corresponds to 14 dBm. In two frequency bands 5 mW and 10 mW and in a narrow frequency band between 869.4 MHz and 869.65 MHz 500 mW. Another limitation is the duty cycle, which can be between 0.1 % and 10 %.