DECT and DECT ULE

DECT ULE is a low-energy extension to DECT. It was developed by the Dutch firm SiTel Semiconductor and RTX Telecom of Denmark as a special solution for wireless sensor devices that need to run for years powered by a single battery. This makes DECT ULE a powerful extension on the proven standard, and brings all the benefits of DECT to the sensor/actuator domain. DECT and DECT ULE use the same API, so the DECT OSGi Module can work with both.

Current Consumption

The typical standby consumption of a synchronous DECT ULE device is 20-40 microamps. This means that it can run for many years on two AAA batteries.

DECT ULE's low current consumption is achieved in the same way as in other low-power systems – by allowing the IC to operate in a sleep mode for most of the time. This sleep state is available as a service within CAT-iq.

Frequency Range

DECT operates in the 1880 to 1900 MHz frequency band (outside Europe the 1910-1930 MHz and 1900- 1920 MHz bands are also used). This 20 MHz of radio spectrum is split into ten channels with a spacing of 1.728 MHz. In addition to this division by frequency (Frequency Division Multiple Access or FDMA), DECT also splits up the available space into time slots (Time Division Multiple Access or TDMA). In total, there are 24 time slots per frequency band (12 "down" and 12 "up"). And 100 frames per second can be sent over each of the ten frequency bands.

To transmit data, DECT equipment first scans the entire DECT band and selects a channel by choosing a specific time slot combination on a specific frequency. For instance, after scanning it may decide that downlink slot 2 and uplink slot 14 in frequency band 2 (1888.248 MHz) are available. With twelve possible time slot combinations and ten frequency bands, there are a total of 120 radio channels available. The total capacity can be extended further by adding more base stations. As long as the base stations are far enough from one another so as not to occur interference, each base station can manage its own sector. In this way, hundreds of thousands of users in a single office environment can be supported. Unlike many other low-power technologies, there is no need for frequency planning because the system dynamically selects the best channel to use.

Interference

Interference between radio signals reduces the chance of reaching their desired destinations. Many proposed home networking technologies operate in the popular 2.4 GHz Industry Science Medicine (ISM) band. This is available worldwide but is very crowded with Wi-Fi, Bluetooth and microwave radiation. For sensor networks with multiple devices, this is a big obstacle. By contrast, DECT operates in the 1.9 GHz band. Uniquely, this band is licensed (reducing interference issues) but is free from royalties. Moreover, DECT is designed at protocol level to prevent interference. The dynamic channel selection system ensures that the best available band is used. In addition, a 1 Mbit/s raw data rate and high link budget make high voice quality and reliable data transmission possible in DECT. Some baby monitors and door phones even send video over DECT.

Benefits for Wireless Sensors

In a DECT network, the Base Station emits a timing signal every ten milliseconds to synchronize all the devices. However, if a device has no connection, to save its battery, it goes into standby mode and only listens for the timing signal every 640 milliseconds. This so-called low scan paging mode enables a balance between battery usage and system reaction time. Sensors do nothing most of the time so they can go into sleep mode, making much lower current consumption possible. By stretching the low scan paging mode interval from 640 milliseconds to 20 seconds and reducing the current during this period to a few microamps, it is possible to create a device that can run on a single set of batteries for years. The 20 second sleep time is a practical trade-off between power consumption and response requirements.