Why using the 868 MHz frequency
For transmitting temperature values wirelessly to a central point for storing, monitoring and presentation, different radio standards are available. This section describes shortly the reasoning why 868 MHz has been chosen as the optimal standard for the Wisensys system.
In selecting the applied radio standard the following aspects are important:
- The distance between sensor and base station (range) should be as large as possible.
- The battery life time has to be as long as possible (low energy consumption)
- the amount of data to transmit per second is limited
- the transmission has to be reliable
In the license free bands there are various possibilities namely:
- the 2.4 GHz band using WiFi, Bluetooth or Zigbee
- the 868 MHz band with optional use of Zigbee
- the 434 MHz band
- the RFID frequencies (13.56 MHz, 125 kHz, etc)
WiFi (wireless LAN) is frequently used in an office environment, designed for transmitting large amount of data over a relative short distance (100 meters free line of sight, 10 meters typical in buildings). Energy consumption is relatively high with consequently low battery life time. Due to the extensive use congestion in the frequency band may happen. The advantage is the world wide license free band.
Bluetooth has similar characteristics as WiFi. Energy consumption is lower but still substantial. The Bluetooth architecture offers good possibilities for data security through the use of frequency hopping techniques.
Zigbee is a new standard used in the 2.4 GHz band especially designed for transmitting small amounts of data. Energy consumption is low and through the available network functionality a reliable transmission of measured values to the base station can be guaranteed. The standard can be used world wide.
The 868 MHz standard is used for Wisensys sensors because it offers superior characteristics with respect to range (free line of sight is 2 to 3 times more than the abovementioned standards in the 2.4 GHz band, typical in buildings 2 times). The implementation is relatively at low cost. The energy consumption is low, leading to battery lifetimes from 3 to 5 years (dependent on transmission intervals)
In the Wisensys system Zigbee is used in case large distances have to be bridged and repeaters have to be placed. The connection between sensor and repeater is done with 868 MHz while the connection between repeater and base station is done with Zigbee The build-in networking functions in Zigbee are used for a reliable transmission of measured data.
The 868 MHz version can not be used world wide. The US requires the 915 MHz band. Hardware for both standards however is identical. Different versions of software are required.
The 434 MHz band is slightly better with respect to range however reliable data transmission is not possible. Through the use of the large amount of devices in this band, especially remote controls, disturbance of transmission occurs frequently. Reliable transmission of data can not be assured
RFID frequencies can not be used for temperature measurements. The range is too limited. These frequencies are generally used for passive tags and the range is therefore limited to less than one meter.