Bluetooth Low Energy – The technology

The main advantage of Bluetooth Low Energy as its name has it is low power consumption. This is due to features like sleep mode, less than 15mA peak and 1uA average power consumption, which in turn were achievable through very fast connection time and very short message lengths. Therefore, BT LE can be up to 10 times more energy efficient than Bluetooth. I keep looking at the big brother when analyzing the performance tests of BT LE not because this newer standard is supposed to replace the older one but because it gives assessment criteria.

The layered architecture can be seen in the picture to the right. BT LE uses the unlicensed 2.4 GHz ISM (Industrial Scientific Medical) band like other proprietary radios. The frequency planning is hence crucial for data integrity. Just like classic Bluetooth, BT LE uses frequency hoping with 40 2MHz channels instead of 79 1MHz ones, in order to reduce costs. Out of the 40 channels, 3 are used for discoverability, connect ability, broadcasting and to avoid know 802.11 frequencies, while the rest are used to send application data in a connection (see picture below). The simple state machine in a star topology can advertise, scan, initiate connection or be connected. The advertising function, repeated periodically, enables a unit to announce if, for example, a process value or an event has occurred. The active scanning is used to get more data out of the advertiser.

In order to achieve backwards compatibility with Classic Bluetooth and not to unnecessarily increase costs, the chipset is available in the following two versions:

• Dual-mode: Bluetooth low energy technology as well as Classic Bluetooth functionality
• Stand-alone: Bluetooth low energy technology only in order to optimize cost, power consumption, and size which are particularly useful for small battery powered devices.

An additional improvement of 3dB in the link budget is due to a modified modulation, which allows a BT LE unit to be up to 200-300 meters in line of site without the need of an additional power amplifier. Despite the fact that this number is rarely used in any application (usually up to 50 m is required), it is important to have a large reserve in order to bridge temporary obstacles and interference, so as to really ensure robustness and reliability.

The BT LE technology meets all the requirements of a wireless solution for sensors and actuators and was chosen as standard in healthcare by Continua Alliance. “Devices that support Bluetooth low energy Gateway functionality provide a transparent pipe from a device to an IP address. Middleware at the IP address can access the device directly as if it were a collector talking to it locally. The Gateway device plays no part other than in acting as a pipe.” – Joe Decuir, Standards Architect, CSR plc; IEEE Region 6 NW Area chair

Bluetooth Low Energy has the differentiation it takes to stick around for a long time:

• Simple star topology to reduce complexity
• Small silicon footprint to make it cost effective for stand-alone solutions
• Robustness inherited from Classic Bluetooth - the most robust industrial wireless solution with more than 10 years experience
• Long range
• Very fast connection
• Low latency
• Low power consumption