Bluetooth Low Energy Operation: how it works

Bluetooth Tutorial / Summary Includes:
Bluetooth technology basics     How Bluetooth works     Bluetooth Classic: how it works     Bluetooth Low Energy: how it works     File transfer     Bluetooth profiles     Pairing & networking     Security     Bluetooth 2 EDR     How to connect Bluetooth devices    

Bluetooth Low Energy or BLE is the term given to describe Versions 4 and 5 of Bluetooth which provide very much lower power operation for Bluetooth

The low power capability of BLE enables small and remote devices to operate over extended periods when compared to that which would be possible with Bluetooth Classic.

This means that not only remote devices and sensors, etc used for applications like the Internet of Things, IoT can run over extended periods, but also items like small wireless earbuds can operate satisfactorily.

In addition to this, Bluetooth Low Energy is used for many other products including wireless headphones, smart speakers, Bluetooth speakers, laptops, computers, tablets and a host of other electronic devices.

Bluetooth Low Energy features

Bluetooth Low Energy provided many improvements to the Bluetooth technology apart from the low power consumption. Each release has improved the performance in one aspect or another.

The low power consumption was brought about by the fact that BLE devices remain mostly inactive for around 99.9% of the time. This considerably saves on battery power.

In addition to this a much faster set-up or connection time was possible, making devices much easier to securely pair.

In terms of the other changes, Versions 4.1 and 4.2 provided some relatively minor extensions and improvements. However Version 5 introduced a number of more significant improvements, including a much higher data rate.

It should be remembered that Bluetooth Classic and Low Energy are similar but not compatible with one another. If a Bluetooth device must provide both modes, it must support dual mode operation, having controllers for both Bluetooth Classic and Bluetooth Low Energy. This type of operation is more suited to electronic items like laptops, computers, tablets where low power operation is not so crucial, but where it may need to connect to devices that might be of either type.

Devices like remote sensors and actuators, wireless earbuds, where power is more of an issue are unlikely to be dual mode.

BLE communications types

Bluetooth Low Energy provides the capability for several types of communication. A node or device connected in BLE can have up to 4 different functions:

• Broadcaster:   A broadcaster can act as a server and it regularly transmits data to a device, but it does not accept any incoming messages.

• Observer:   An observer is a device or node that listens to and interprets data sent by a broadcaster. In this situation, the object cannot send connections to the server.

• Central:   A central node or device is one that communicates in two ways: it can act in an advertising mode or in a connected mode. It forms the central device in any communications system and any exchange that occurs starts with it. This might be a smartphone or laptop, etc.

• Peripheral:   This type of device has connections and periodically sends data.

In order to enable devices to know what other devices are available for pairing, Bluetooth LE introduced the concept or "advertising." Communications initially take place via the special advertising channels, and the initial set-up data sent across them.

it is also possible to use the "Beacon" facility to locate devices: primary function of beacons is to permit positioning within a defined area along with related services. This can be used, for example within a shop to locate devices and then provide relevant advertising, etc. Beacons are typically only operate in transmit mode.

To save energy, devices can opt out of these specific communications during scanning; this is implemented using a white list filter.

In order to exchange data, a proper connection needs to be set up. This can be considered to be the piconet from Bluetooth Classic.

BLE frequency hopping

The frequency hopping figures for Bluetooth Low Energy are rather different to those used for the Classic version.

Originally hopping occurred over 79 channels, but for Bluetooth LE, it took places over only 40 channels, but each channel has a bandwidth of 2 MHz to provide the higher data capacity required.

The hopping range extends from 2402 MHz which is designated as RF channel 0; logical channel 37 to 2480 MHz which is designated as RF channel 39; logical channel 39.

Three channels: namely logical channels 37, 38 and 39 are referred to as advertising channels and logical channels 0 to 36 are the data channels.

The advertising channels were chosen so that so that they would not be not disturbed by the non-overlapping WLAN channels 1, 6 and 11 in the 2.4 GHz band.

Also hopping is only used when a connection exists between two devices and this means that advertising and the data exchanges that occur in connection wit this do not use frequency hopping.

In view of this Bluetooth Low Energy may be considered by some, including ETSI not to be a frequency hopping system.

However the new format has meant that connection between devices is much faster and more reliable.

Bluetooth LE power levels

Power is obviously a key element of Bluetooth Low Energy, as such power control can be implemented in the devices. Also the classes for the power levels of the devices were defined. In Bluetooth Classic, there were power classes 1 to 3, but BLE introduces a class 1.5.

Bluetooth LE modulation

The modulation scheme for any wireless communications system is of key importance. The same is true for Bluetooth LE. Unlike its predecessor, Bluetooth Classic, Bluetooth Low Energy uses only one form of modulation.

The adopted form of modulation is Gaussian Frequency Shift Keying, GFSK with a nominal frequency deviation of ±250kHz for V4 and 5.

Using this scheme, it is possible to achieve a gross data rate of 1 Mbps using LE 1M and a code PHY.

For Bluetooth V 5, there is an additional option to use a frequency deviation level of ±500 kHz to enable a gross data rate of 2 Mbps to be achieved using LE 2M.

Another concept known as a stable modulation index can also be used, but it is optional. Using this, devices guarantee a modulation index of between 0.495 and 0.505. This enables the system to achieve a greater range because of the more stable defined modulation index.

Bluetooth LE packets

The packet structure for the packets within BLE all follow the same format, consisting of a preamble, access address, protocol data and the cyclic redundancy check data.

• Preamble:   The preamble is a fixed sequence of zeros and ones which is used to support the receiver functions like synchronisation and the automatic gain control.

• Access Address:   The access address is a random number that identifies the access to a physical channel and provides security.

• Protocol Data Unit, PDU:   The Protocol Data Unit, PDU is the block within the data packet that carries the actual message. It includes both the transmitter and receiver addresses as well as user data.The Bluetooth addresses are included directly in the PDU payload: they are 6 bytes i.e. 48 bits in length. The source address and destination address are normally included.

• Cyclic Redundancy Check, CRC:   This is used for error checking and data recovery.

The generalised format for the packet structure is used for the various forms of packet, but modified according to the actual requirements.

When looking at the different types of packet and data transmission, the various lengths and slight modifications to the basic format can be seen.

• Packet format for uncoded PHYs:   The general packet format applies to the uncoded physical layer or PHY. There are differences in the length of the preamble and in the duration of the packet, as LE 2M PHY has double the bit rate of the 1M. This is shown by the fact that in Bluetooth LE V4.1, the PDU was only 39 bytes, whereas from Bluetooth 4.2 onwards the length was 257 bytes in what was called the extended packet length. The LE 2M PHY preamble was 16 bits to ensure a consistent duration.

  With the variable length of the PDU, an LE 1M packet was between 80 bits which lasted for 80 µs and 2120 bits which lasted for 2120 µs. This enabled an LE 2M packet to be between 88 bits which was 44 µs in length and 2128 bits which was 1064 µs.

• Packet Format for Coded PHYs:   This packet formation is used for Bluetooth long range links. For coded packets, the actual preamble remains uncoded. However the access address, coding indicator, CI which occurs after the access address and termination field 1, T1 which follows C1 form FEC block 1. These are always transmitted with an S8 code.

  The PDU, CRC and termination field 2, T2 form FEC block 2, which can be coded with either S=8 or S=2 coding.

• PDU for Advertising:   The PDU used for advertising consists of a header with a number of elements but the whole length is 16 bits. 8 of these bits are used to define the length of the actual payload which can be between 8 and 2048 bits in length.

• PDU for data:   The PDU that is used for data transmission has a similar format to that of the advertising PDU. Again it consists of a header of 16 bits, and again 8 bits are used to define the length of the payload. A message integrity check can be added after the payload.

Each of the different types of packet has its own function, and although the packets are different in many ways, they basically conform to the generic format.

Summary of Bluetooth Low Energy, BLE Specifications

A summary of the highlight specifications and parameters for Bluetooth Low Energy, BLE, i.e. Versions 4 & 5 is given below in the table listing.

Bluetooth Low Energy, BLE provided a significant jump in performance over Bluetooth Classic in terms of the performance required for the the latest technology. Low power usage, high bandwidth, easy pairing, security, longer range, more reliable links and many other features meant that it was far more effective for a variety of applications from wireless earbuds, wireless headphones, wireless speakers as well as a host of other applications including sensors and actuators for the Internet of Things, IoT and very much more.

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