Bluetooth Goes Ultra-Low-Power

By John Donovan

Contributed By Convergence Promotions LLC


There’s hardly a cell phone on the planet that doesn’t have a Bluetooth® transceiver for connecting to a wireless headset. Most new PCs now incorporate Bluetooth chips for the same purpose, letting you type while you talk or listen. Many, if not most new cars, have Bluetooth to let you talk hands free while driving. However, while that’s all well and good, there is a wide range of applications for which Bluetooth isn’t appropriate – or at least it wasn’t until now.

Bluetooth is a connection-oriented protocol designed to handle continuous streaming of data at relatively high speeds, making it well-suited to connecting wireless headsets to cell phones. While attempting to remain low power, most changes to the Bluetooth specification have concentrated on boosting the data rate. The basic rate (BR) enables synchronous and asynchronous connections at up to 720 kbps. Bluetooth Version 2.0 (2004) added an extended data rate (EDR) of 3 Mbps (in practice more like 2.1 Mbps). Bluetooth 3.0 (2009) added a high-speed (HS) data capability of up to 24 Mbps by using an alternative MAC/PHY (AMP) that communicates over a co-located 802.11 link. Despite some clever engineering, the quest for higher speed necessarily resulted in higher power consumption.

Bluetooth Low Energy, in contrast, was designed from the beginning to be an ultra-low-power (ULP) protocol to service short range wireless devices that may need to run for months or even years on a single coin cell battery. Introduced in Bluetooth Version 4.0 (2010), Bluetooth Low Energy uses a simple stack that enables asynchronous communication with low-power devices, such as wireless sensors that send low volumes of data at infrequent intervals. Connections can be established quickly and released as soon as the data exchange is complete, minimizing PA on time and thus power consumption.

The Bluetooth Core Specification Version 4.0 includes the complete specs for “Classic Bluetooth,” Bluetooth High Speed, and Bluetooth Low Energy. While Bluetooth Low Energy is entirely new, the designers realized the advantages of maintaining backward compatibility with over two billion Bluetooth devices already in circulation. To finesse this problem without compromising the design of Bluetooth Low Energy, Version 4.0 introduced two types of devices:
  • Dual-mode chips that can communicate with Classic Bluetooth devices as well as single-mode chips in ultra-low-power devices.
  • Single-mode chips running the compact Bluetooth Low Energy protocol stack. These can communicate with other single-mode chips or dual mode ones that are using the Bluetooth Low Energy part of their architecture.
Dual-mode chips – just now coming on the market – will provide Bluetooth functionality in all cell phones, PCs and cars going forward. They can communicate with legacy Bluetooth devices as well as new, low-power ones running Bluetooth Low Energy in a wide range of medical, industrial, and consumer applications. This added capability comes at very little extra silicon cost.

It pays to advertise

Bluetooth Low Energy radios operate in the 2.4 GHz ISM band, frequency hopping over 40 channels using GFSK modulation to achieve a data rate of up to 1 Mbps. Bluetooth Low Energy employs two data access schemes: frequency division multiple access (FDMA) and time division multiple access (TDMA).

In the FDMA scheme, LE radios hop between 40 physical channels separated by 2 MHz, three of which are used as advertising channels and 37 for data channels. In the TDMA scheme, one device transmits a packet at a predetermined time and the scanning device responds with another packet after a predetermined interval. Data is transmitted between Bluetooth Low Energy devices during two time units known as events: advertising events (see Figure 1) and connection events (see Figure 2).

The advertising channels enable the discovery of devices available in the vicinity. An advertiser sends out packets indicating that it has data to communicate. A scanning device may then request the advertising device to send another advertising packet – enabling broadcast communication on just that advertising channel – or more commonly it will request setting up a bi-directional communication link.

Advertising events

Figure 1: Advertising events (Courtesy Bluetooth SIG).

When a connection is requested, these same advertising channels serve to connect the devices, which then proceed to use the data channels for communication. In making the connection, the initiating device defines the channel and the timing of the data interchange, having assessed the latency of communications with the advertising or slave device, which can subsequently request a change in these parameters in order to optimize power consumption.

Connection events

Figure 2: Connection events (Courtesy Bluetooth SIG).

Once a connection is established, the scanner becomes a master device and the advertiser the slave. Frequency hopping is initiated, with data packets being exchanged during so-called connection events on each frequency. The master initiates each connection event, though either the master or the slave can terminate communications at any time.

Saving energy

One of the ways Bluetooth Low Energy manages to minimize power consumption is to switch the radio on for only very brief periods of time. Bluetooth Low Energy radios only need to scan three advertising channels to search for other devices – which they can do in 0.6 to 1.2 ms – while Classic Bluetooth radios must constantly scan 32 channels, which require 22.5 ms each time. This trick alone enables low-energy devices to consume 10 to 20 times less power than Classic Bluetooth ones.

Like Classic Bluetooth, Bluetooth Low Energy utilizes adaptive frequency hopping to minimize interference with co-located radios. However, Bluetooth Low Energy uses three fixed advertising channels, which aren't protected from interference. However, these channels were chosen because in only one instance (2402 GHz, channel 37) is there an occasional conflict with Wi-Fi (channel 1), and of course by design there is no conflict with Bluetooth Low Energy data.

With a data bandwidth of 1 Mbps, a Bluetooth Low Energy transceiver will be on for one-eighth of the time that a ZigBee® node – operating on the same 2.4 GHz band – requires to transmit the same data, resulting in an eight times saving in battery life due to data speed alone.

Bluetooth Low Energy devices can scan for other devices, connect, send data, confirm valid reception, and terminate the link in 3 ms. Classic Bluetooth devices typically require hundreds of milliseconds to accomplish the same series of tasks, requiring a couple of orders of magnitude more energy in the process.

Bluetooth Low Energy packets are also much shorter than those used by Classic Bluetooth, resulting in less on time for the PA and thus longer battery life.

Finally, Bluetooth Low Energy’s modulation scheme also contributes to its low energy profile as well as greater robustness. Both Classic Bluetooth and Bluetooth Low Energy utilize Gaussian frequency shift keying (GFSK). However, Classic Bluetooth uses a modulation index of 0.35, while the modulation index for Bluetooth Low Energy is set at between 0.45 and 0.55, which is close to the level for Gaussian minimum shift keying (GMSK). This results in higher spectral efficiency and hence greater robustness than Classic Bluetooth, though at the risk of slightly increased inter-symbol interference (ISI). The slight increase in power required by the higher modulation index is more than offset by the other power saving techniques previously described. Table 1 summarizes the differences between Classic Bluetooth and Bluetooth Low Energy.

Technical Specification Classic Bluetooth Bluetooth low energy
Distance/Range 100 m (330 ft) 50 m (160 ft)
Over the air data rate 1-3 Mb/s 1 Mb/s
Application throughput 0.7-2.1 Mb/s 0.26 Mb/s
Active slaves 7 Not defined; implementation dependent
Security 64/128-bit and application layer user defined 128-bit AES with Counter Mode CBC-MAC and application layer user defined
Robustness Adaptive fast frequency hopping, FEC, fast ACK Adaptive frequency hopping, Lazy Acknowledgement, 24-bit CRC, 32-bit Message Integrity Check
Latency (from a non-connected state Typically 100 ms 6 ms
Total time to send data (det.battery life) 100 ms 6 ms
Voice capable Yes No
Network topology Scatternet Star-bus
Power consumption 1 as the reference 0.01 to 0.5 (depending on use case)
Peak current consumption <30 mA <20 mA (max 15 mA to run on coin cell battery)
Service discovery Yes Yes
Profile concept Yes Yes
Primary use cases Mobile phones, gaming, headsets, stereo audio streaming, automotive, PCs, security, proximity, healthcare, sports & fitness, etc. Mobile phones, gaming, PCs, watches, sports and fitness, healthcare, security & proximity, automotive, home electronics, automation, Industrial, etc.

Table 1: Classic Bluetooth versus Bluetooth Low Energy (Source: Wikipedia).

Stack Architecture

A single-mode Bluetooth Low Energy transceiver, surprisingly, when running flat out isn’t much lower power than a Classic Bluetooth one (20 mA versus 30 mA). The energy savings is enabled largely by the stack architecture, which enables a range of battery-saving techniques.

The Bluetooth Low Energy protocol stack (see Figure 3) consists of a controller and a host. The various parts of the stack consist of:
  • The Physical Layer (PHY) transmits and receives packets over the physical channel—in this case GFSK packets at 1 Mbps on the 2.4 GHz ISM band.
  • The Link Layer (LL) controls the RF state of the transceiver, determining whether it’s advertising, scanning, initiating, connected, or standing by.
  • The Host-Controller Interface (HCI) processes all communications between the host and controller, usually by SPI, USB, or a UART.
  • The Logical Link Control and Adaptation Protocol (L2CAP) provides data encapsulation services for the upper layers. It provides traffic management, controlling the order of submission of protocol data unit to the baseband and ensuring QoS access to the physical channel.
  • The Attribute Protocol (ATT) enables a device to reveal certain of its attributes to another device. The ATT block sets up peer-to-peer communication between an attribute server and a client to be able to exchange this information over a dedicated L2CAP channel.
  • The Security Manager (SM) generates, manages, and stores encryption and identity keys to enable two devices to communicate securely over a dedicated L2CAP channel. Bluetooth Low Energy uses 128-bit AES encryption with counter mode CBC-MAC and a user-defined application layer.
  • The Generic Attribute Profile (GATT) defines the sub-procedures for using ATT and specifies the structure of Bluetooth profiles. All communications between Bluetooth devices are handled through GATT sub-procedures.
  • The Generic Access Profile (GAP) block provides the interface between the application and Bluetooth profiles and handles device discovery, connection, and services, including security procedures.

BLE protocol stack

Figure 3: BLE protocol stack (Courtesy Texas Instruments).

Bluetooth applications and devices communicate with each other through a highly structured set of procedures called profiles. Profiles can be quite low level – such as profiles for connecting to IrDA devices or setting up secure links – or they can be grouped together to serve high level applications. For example, there are Bluetooth Low Energy protocols to monitor a patient’s blood pressure (BLS), heart rate (HRS), and temperature (HTP); together they form a group of healthcare profiles. The Bluetooth SIG is in the process of defining profiles for consumer, healthcare, and industrial applications, where it predicts rapid proliferation of Bluetooth Low Energy devices.

Bluetooth Low Energy chips

As of late 2011, Bluetooth Low Energy chips are only now starting to appear on the market. Texas Instruments is one of the first to market with its CC2540 Bluetooth System-on-Chip. The CC2540 is a single-mode chip that integrates an 8051 MCU, 8 KB of RAM, 128-256 KB of flash, and RF transceiver, a single-mode protocol stack, embedded profile software, and a full range of peripherals. The CC2540 can operate in either master or slave mode and consumes as little as 15.8 mA at 3V in receive mode and 18.6 mA in transmit (-6 dBm).

For developers TI offers the CC2540DK-MINI development kit for the CC2540. The kit contains a CC2540 USB dongle, a CC2540 keyfob board and plastic case, a CC debugger with cables, a mini USB cable, a CR2032 battery, and kit documentation. Using the supplied BTool application program and the Simple Keys GATT profile you can form a connection between the dongle and the keyfob and then experiment with different security and other low-level features. TI supplies sample source code for a blood pressure sensor, heart rate sensor, and health thermometer that you can compile; download to the CC2540 using SmartRF Flash Programmer and the CC debugger; and then run it on the dongle.

CC2540DK-MINI development kit

Figure 4: CC2540DK-MINI development kit (Courtesy Texas Instruments).

Also based on the CC2540, BlueRadios, Inc. offers the BR-EVAL-LE4.0-S2A evaluation kit. The kit provides a ready-to-use BLE device with simple AT modem-like commands and embedded Bluetooth stack and profiles (GAP, GATT, ATT, SMP, L2CAP, and BATT). This device can be configured, commanded, and controlled through simple ASCII strings over the RF Link or through the hardware serial connection via UART, SPI or USB.

Panasonic’s ENW-89820A1KF is a single-mode Bluetooth Low Energy module targeting mobile phones, PDAs, and laptops. It includes an 8051 MCU, up to 256 kbits of flash, 8 kbits of RAM, an AES security coprocessor, IR generation circuitry, a USB 2.0 interface, and a complete Bluetooth Low Energy software stack. The PAN1720-Module is manufactured in a very small 15.6 mm x 8.7 mm x 1.9 mm SMD package with a shielded case.

Panasonic’s ENW-89820A1KF

Figure 5: Panasonic’s ENW-89820A1KF single-mode Bluetooth Low Energy module (Courtesy Panasonic).

CSR has a number of Bluetooth Low Energy chips in production, including the CSR1000A04-IQQM-R. The CSR µEnergy™ platform provides everything required to create a Bluetooth low energy product with RF, baseband, microcontroller, qualified Bluetooth v4.0 stack, and customer application running on a single chip. The CSR µEnergy platform is available in two chip variants. The CSR1001 chip is optimized for keyboards and remote controls with dedicated hardware for keypad scanning. The CSR1000 offers a smaller package for fitness products, watches, mice and other sensors.

Is that all there is?

The short-range wireless market is both crowded and fragmented, since there is no “one size fits all” solution to the wide range of requirements out there. Most of these protocols are built on top of the IEEE 802.15.4 standard, which only defines the PHY and MAC layers; each protocol then goes on to define the network layer differently according to their needs. ZigBee is probably the most widely used 802.15.4 variant, but 6LoWPAN and WirelessHART™ also have their place, as do proprietary protocols such as ANT™, Z-Wave®, MiWi™, and SimpliciTI™. Where – and how well – does Bluetooth Low Energy fit in?

Each of the other standardized protocols just mentioned have found a niche where they make the most sense: 6LoWPAN supplying Internet connectivity and WirelessHART for self-healing mesh networks; these aren’t target markets for Bluetooth Low Energy. On the other hand, ZigBee’s RF4CE™ protocol targets many of the same applications as Bluetooth Low Energy, but being a more complex protocol, it’s better at networking on the one hand – Bluetooth Low Energy can only operate in a star configuration – but being considerably higher power, it’s not nearly as well-suited to ultra-low-power applications such as medical sensors and industrial and military applications that require as much battery longevity as possible.

In the consumer space, ANT is widely used in heart rate monitors for joggers and some other body area network (BAN) applications. Like Bluetooth Low Energy, ANT is also an ultra-low energy protocol targeting low data rate applications. ANT would be a serious competitor, but Bluetooth Low Energy has one huge advantage – compatibility with over two billion installed devices and a high level of acceptance in the consumer community. The fact that the major ANT chip vendors – Texas Instruments and Nordic Semiconductor – are so strongly backing Bluetooth Low Energy should remove any doubt that it will soon replace ANT and proprietary solutions to become the dominant protocol found in ultra-low-power wireless devices.

References:
  1. Bluetooth technology web site
    http://www.bluetooth.com/Pages/Bluetooth-Home.aspx
  2. Bluetooth Low Energy – Texas Instruments
    http://www.ti.com/ww/en/analog/bluetooth/index.htm?DCMP=BluetoothLowEnergy&HQS=NotApplicable+OT+bluetoothlowenergy
  3. Bluetooth Low Energy – Digi-Key
    http://search.digikey.com/scripts/DkSearch/dksus.dll?x=12&y=16⟨=en&site=us&KeyWords=bluetooth+low+energy
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