After a protracted conception, the Bluetooth Special Interest Group (SIG) announced late last year it adopted Bluetooth low-energy wireless technology as a “hallmark feature of the Bluetooth Core Specification Version 4.0.”
Since the spec’s adoption, the Bluetooth SIG prefers to use the umbrella term of “Bluetooth v4.0” rather than “Bluetooth low energy.” For the engineering community, however, that can be a little confusing as, for the first time, Bluetooth wireless technology now includes two distinctly different types of chip.
Bluetooth low-energy chips are ultra-low-power (ULP) devices, typically operating at microampere average currents. In contrast, Bluetooth v4.0 chips are similar to their v3.0 and v2.1 forerunners, except for the fact that they can communicate with Bluetooth low-energy devices. This vital distinction promises to dramatically extend the Bluetooth “ecosystem” to a range of products that had been excluded due to size, power, or cost constraints.
“Almost all existing Bluetooth-enabled phones are expected to migrate to the 4.0 standard,” says Peter Cooney, practice director, Semiconductors, ABI Research, “which will result in over a billion Bluetooth low-energy-capable hosts in the next few years in this market alone.”
A Tale Of Two Chips
The Bluetooth low-energy chip, which is brand new to the Bluetooth specification, is optimised for ULP operation. These devices can communicate with other Bluetooth low-energy chips and Bluetooth v4.0 chips when the latter use the Bluetooth low-energy technology part of their architecture to transmit and receive (Fig. 1).
Bluetooth low-energy chips can operate for long periods (months or even years, depending on the application’s duty cycle) from a coin cell battery such as a 3V, 220mAh CR2032. These compact, inexpensive batteries have limited energy capacity, though, typically in the range of 90 to 240mAh. A 220mAh CR2032 coin cell can sustain a maximum nominal current (or discharge rate) of just 25μA if it’s to last for at least a year.
Bluetooth v4.0 devices are capable of both conventional Bluetooth and Bluetooth low-energy communication, but require at least two AAA cells (which have 10 to 12 times the capacity of a coin cell and much higher peak current tolerance). Importantly, these chips will be able to communicate with all legacy Bluetooth devices already on the market, in addition to all future Bluetooth low-energy devices.
ULP Wireless Technology
Conventional Bluetooth technology is “connection-oriented” with a fixed connection interval that’s ideal for high-activity connections, such as mobile phones linking with wireless headsets. In contrast, Bluetooth low-energy technology employs a variable connection interval that can be set from a few milliseconds to several seconds, depending on the application. Furthermore, because it features a very rapid connection, Bluetooth low-energy technology typically is able to be in a “not connected” state (saving power). 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.
The operational mode of Bluetooth low-energy technology ideally suits transmission of data from compact wireless sensors (exchanging data every half second) or other peripherals like remote controls that can employ fully asynchronous communication. These devices send low volumes of data—i.e., a few bytes—infrequently (for example, a few times per second to once every minute or more seldom).
Switching the radio “on” for anything other than very brief periods will dramatically reduce battery life. Therefore, any transmitting or receiving must be done quickly. To minimise air time, Bluetooth low-energy technology employs only three “advertising” channels to search for other devices, or promote its own presence to devices that might be looking to make a connection. In comparison, conventional Bluetooth technology uses 32 channels.
This means Bluetooth low-energy technology has to switch “on” for just 0.6 to 1.2ms to scan for other devices, while conventional Bluetooth technology requires 22.5ms to scan its 32 channels. Consequently, Bluetooth low energy uses 10 to 20 times less power than conventional Bluetooth technology to locate other radios.
Note that using three advertising channels is a slight compromise. It’s a tradeoff between “on” time (and hence power) and robustness in what is a very crowded part of the spectrum (with fewer advertising channels, there’s a greater chance of another radio broadcasting on one of the chosen frequencies and corrupting the signal). The specification’s designers are confident they have balanced this compromise. They have, for example, chosen the advertising channels so that they don’t clash with Wi-Fi’s default channels (Fig. 2).
Once connected, Bluetooth low-energy technology switches to one of its 37 data channels. During the short data-transmission period, the radio switches between channels in a pseudo-random pattern using adaptive frequency hopping (AFH), which was pioneered by conventional Bluetooth technology (although conventional Bluetooth uses 79 data channels).
Bluetooth low-energy technology also can minimise on-air time thanks to its raw data bandwidth of 1Mbps—greater bandwidth allows more information to be sent in less time. Alternative technologies with, say, 250kbps bandwidths must be “on” eight times as long (using more battery energy) to send the same amount of information.
Bluetooth low-energy technology can “complete” a connection (i.e., scan for other devices, link, send data, authenticate, and “gracefully” terminate) in just 3ms. With conventional Bluetooth, a similar connection cycle is measured in hundreds of milliseconds. Remember, more on-air time requires more energy from the battery.
There are two other ways Bluetooth low-energy technology can keep a lid on peak power—by employing more “relaxed” RF parameters than its big brother, and by sending very short packets. Both technologies utilise Gaussian frequency-shift-keying (GFSK) modulation. However, Bluetooth low-energy technology uses a modulation index of 0.5 compared to the 0.35 of conventional Bluetooth technology. An index of 0.5, which is close to a Gaussian minimum-shift-keying (GMSK) scheme, lowers the radio’s power requirements (the reasons for this are complex and beyond the scope of this article). Nonetheless, lower modulation index does generate a couple of beneficial side effects: increased range and enhanced robustness.
Conventional Bluetooth uses a long packet length. When transmitting these longer packets, the radio must remain in a relatively high power state for a longer duration, which heats the silicon and ultimately changes the material’s physical characteristics. This would alter the transmission frequency (breaking the link), unless the radio was constantly recalibrated. However, recalibration consumes power (and requires a closed-loop architecture, increasing complexity and cost).
In contrast, Bluetooth low-energy technology uses very short packets, which keeps the silicon cool. As such, a Bluetooth low-energy transceiver doesn’t require recalibration or a closed-loop architecture.
The First Of Many New Applications
Bluetooth low-energy and v4.0 chips are now hitting the market. For example, Nordic Semiconductor offers the nRF8001 μBlue Bluetooth low-energy solution, which is available for sampling. The company also released an nRF8001 development kit and software development kit.
The nRF8001 features sub-12.5mA peak currents and connected-mode average currents dropping to sub-12μA (for 1s connection intervals). The fully qualified, Bluetooth v4.0 low-energy chip combines radio, link layer and host into one end product listing (EPL), enabling designers to create new Bluetooth end products without any additional listing fees. In fact, Casio selected the chip for its Bluetooth Low Energy Watch (Fig. 3).
Similarly, Broadcom’s BCM4330 is claimed as the first wireless combination chip solution certified with the Bluetooth v4.0 standard. In early August, Nordic Semiconductor announced successful wireless communication tests between a prototype design for a small, low-cost Bluetooth low-energy proximity fob and Broadcom’s BCM4330.
The proximity fob (a combination of the nRF8001 and the recently released Bluetooth v4.0 Proximity Profile) prevents a device, such as a laptop, from being accessed in the owner’s absence. After “pairing” with the chip in the mobile device, the user carries the fob on their person. If the distance between the user and the mobile device exceeds a pre-set threshold (e.g., if the mobile device is left behind or stolen), the pairing is broken and the mobile device locks automatically.
The fob application is an inexpensive solution to the problem of mobile device security. It illustrates how Bluetooth low energy can be incorporated into compact, coin-cell-powered devices to extend the Bluetooth ecosystem.
Further applications will soon become possible as the Bluetooth SIG gradually introduces more “Profiles” to customise the software protocol to a particular function. Profiles in the works include personal user interface devices or PUIDs (such as watches), remote control, battery status, and heart rate. Other health and fitness monitoring profiles, such as blood-glucose and -pressure, cycle cadence, and cycle crank power, are expected to follow.
Commercialisation of Bluetooth low energy has taken a while. But with fully qualified silicon now reaching the market, a slew of Bluetooth low-energy products is anticipated. For example, market analyst IMS estimates that by 2013, one billion Bluetooth low-energy devices will be sold every year. That represents the fastest adoption—by a wide margin—of any wireless technology.