Bluetooth Low Energy Features and coin cells

Bluetooth low energy (BLE) technology is the key feature of the Bluetooth Core Specification 4.0 (Bluetooth v4.0) and it is one of the latest technology to incorporate a very low power, relatively short range (50m) features [1] while using for communication a coin cell battery that can last almost two years. It has inherited several technical features from Classic Bluetooth technology, since it is a further evolution of it, thus provides power efficiency, excellent resistance to interference, small size and scalability. The key features of Bluetooth Low Energy can be summarized as follows:

• Ultra-low power consumption

• Ability to operate for long periods (months or even years) from a coin-cell batteries

• Low cost due to the already existing Bluetooth ecosystem

• Interoperability

• Robustness and Reliability

Bluetooth Low Energy has features similar to those of the Classic Bluetooth technology but in some ways, BLE technology is a very innovative wireless technology including new features such as proximity sensing and a variable connection interval up to few millisecond to transmit small amounts of data instead of the Classic Bluetooth fixed connection interval for periodic high activity connections. Similarly to the Classic Bluetooth, BLE uses Adaptive Frequency Hopping (AFH) spread spectrum for accessing to the shared channel, in order to have a robust transmission against the possible noise in RF environments such as medical environments or generic buildings. AFH also decrease interference probability from Bluetooth technology to other wireless technologies in the same radio space, since Bluetooth technology, IEEE 802.15.4/ZigBee ,Wireless LAN, use the license-free 2.4GHz Industrial Scientific Medical (ISM) band. However, since the priority of the BLE technology is to reduce energy consumption, the number of hops was reduced to 40 and the channel width is 2MHZ [2] as opposed to the Classic Bluetooth technology where the number of hops was 79 and the channel width was 1 MHz.

Bluetooth  low energy channels

Bluetooth Low Energy can operate either in a master or slave role, but never both; there is no scatternet topology for Bluetooth low energy technology. BLE has a star topology; a master can command multiple simultaneous connections with a number of slave devices controlling also how often the slaves are allowed to communicate, but a slave can only be connected to a single master communicating by request from the master. A relevant feature that can be seen when comparing the BLE with the Classic Bluetooth technology is the “advertising” functionality. In a BLE discovery is done so that a device acting as a slave advertises on one or several advertisement channels that it has something to transmit to the master. Then the devices acting as masters scan these channels in order to find possible slaves. Once there is a discovery, transmission messages can also include connection events in which the master and the slave wake up in synchrony for exchanging frames.
The ultra low power consumption and coin cell battery operations use also has it own limitations. The ultra Low power BLE technology has an average current consumption of less than 1 mA; this assures there is no overlap since a ultra low power standard can still work in this range if the data throughput is low, since transfer rates for BLE technology are below 100 kbps. This means that some applications works better with the Bluetooth Low Energy connection than others. The simple lithium coin cells batteries, also used for watches for example, have been selected for the BLE wireless applications, but in order to be able to function with a good battery lifetime some important improvements are necessary.The battery lifetime used in a typical ultra low power standard [3], is dependent on the battery capacity and the average current consumption. Therefore while working with ultra low power consumption it is not only important to focus on the average current consumption that should obviously be as low as possible, but it is also necessary to consider that unlike traditional coin cell applications, wireless applications battery capacity is not fixed and is dependent on the various usage pattern.When considering coin cell battery for ultra low power wireless operations to maximize the battery lifetime  it is important to select devices with the lowest peak current for minimizing the current drain; the low peak current is the main  reason why the coin cell battery is very suited to Bluetooth Low Energy applications. 

When talking about BLE technology, is it almost obvious form the definition and from the name itself that it consumes very little energy, but going through a more technological overview the truth is that, indeed this technology is low powered but there are some applications for which the BLE technology is not sufficiently low. In order to have low power consumption different solutions can be applied. For instance, a good approach can be to modify the way the radio operates. If the radio is switched “on” only when it has data to send for a very short period and then after sending data returns to its sleep mode, then the power consumption will be absolutely reduced. On the other side, with this type of solution, also transmitting and receiving times are quick and the connection is rapid and a “not connected” state is enabled. When the “not connected” state is used then the devices at the two ends are aware of each other but only connect when needed and for a very short time. 
In this case, the two ends of a link are aware of each other, but only link up when absolutely necessary and, even then, it’s for as short a time as possible.
There are other ways Bluetooth low-energy technology can control the energy efficiency. A good solution is to allow more packets to be sent within a connection event and by implementing AFH to avoid interference. In addition it is possible to reduce the low power consumption by sending very short packets; Bluetooth low-energy technology uses a modulation index of 0.5 for the Gaussian frequency-shift-keying (GFSK) modulation which is close to a Gaussian minimum-shift-keying (GMSK) scheme thus, lowers the radio’s power requirements. Moreover, lower modulation index does generate a couple of beneficial side effects: increased range and enhanced robustness.
When considering BLE technology, it is interesting to understand how it is implemented in tablets, mobile phones or health care centres. The Bluetooth devices are connected to a web based application where it can share its radio and can be considered thus, as a conventional Bluetooth Health Device Profile (HDP) and a BLE controller with the advantage of having very little additional circuital parts required in the chips, making this technology ideal for low power home and hospital medical applications.



References:
[1] C. Sorrel, “Casio Bluetooth low energy watch has two year battery life: http://www.wired.com/gadgetlab/2011/03/casio-bluetooth-low-energy-watch-has-two-year-battery-life/,” Wired magazine, Mar. 2011.
[2] http://www.connectblue.com/press/articles/bluetooth-low-energy-technology-and-healthcare/
[3] http://www.eetimes.com/ContentEETimes/Documents/Schweber/C0924/C0924post.pdf