Android Bluetooth Low Energy (LBE) Technology Introduction

Bluetooth Low Energy (BLE) technology is a low-cost, short-distance, interoperable, robust wireless technology that works in a license-free 2.4GHz ISM RF band. It was designed to be ultra-low power (ULP) wireless technology from the start. It uses many intelligent means to minimize power consumption.

Bluetooth low power technology uses variable connection intervals, which can be set to a few milliseconds to a few seconds depending on the application. In addition, because BLE technology uses a very fast connection, it can usually be in a "non-connected" state (energy saving), at this time the link between the two sides only know each other, only when necessary to open the link, and then in the shortest possible time to close the link.

The operating mode of the BLE technology is ideal for transmitting data from micro wireless sensors (exchanging data once every half-second) or with other peripherals such as remote controls with fully asynchronous communication. These devices send a very small amount of data (usually several bytes) and are sent infrequently (for example, several times per second to a minute or less).

Ultra-low power wireless technology

The three main features of Bluetooth low energy technology are ULP performance, which is maximized standby time, fast connection, and low peak transmit/receive power.

When the wireless "on" time is not very short, the battery life is drastically reduced, so any required send or receive tasks need to be completed quickly. The first technique used by Bluetooth low-energy technology to minimize the time of wireless switching is to search for other devices with only 3 "ad" channels, or to announce that they exist for devices seeking to establish a connection. By contrast, standard Bluetooth technology uses 32 channels.

This means that Bluetooth low-power technology scans other devices just "on" for 0.6 to 1.2ms of time, while standard Bluetooth technology requires 22.5ms of time to scan its 32 channels. Results Bluetooth Low energy technology is 10 to 20 times times less power required to locate other wireless devices than standard Bluetooth technology.

It is worth noting that using 3 ad channels is a compromise of some degree: This is a tradeoff between the "on" time (which corresponds to power consumption) and robustness in the very congested part of the spectrum (the fewer ad channels, the more chances that another wireless device will broadcast on the chosen frequency, the more likely it will be to cause a signal conflict). But the spec's designers are quite confident in balancing this compromise-for example, the ad channel they choose doesn't conflict with the Wi-Fi default channel (see Figure 1)

Figure 1: The Bluetooth low energy technology advertising channel is carefully chosen to avoid conflicts with Wi-Fi

Once the connection is successful, Bluetooth low power technology switches to one of 37 data channels. During a brief transfer of data, the wireless signal will switch between channels in a pseudo-random manner using adaptive frequency hopping (AFH) technology advocated by standard Bluetooth technology (although standard Bluetooth technology uses 79 data channels).

The other reason for requiring Bluetooth low-power technology to have a minimum of wireless open time is that it has 1Mbps of raw data bandwidth--greater bandwidth allows for more information to be sent in a shorter period of time. Another wireless technology with 250kbps bandwidth, for example, needs to be turned on for 8 times times longer to send the same information (consumes more battery power).

Bluetooth Low energy Technology "complete" a single connection (that is, scanning other devices, establishing links, sending data, authenticating and ending appropriately) is only 3ms. Standard Bluetooth technology requires hundreds of milliseconds to complete the same connection cycle. Again, the longer the wireless is on, the more energy the battery consumes.

Bluetooth low-Power technology can also limit peak power consumption in two other ways: using more "loose" RF parameters and sending very short packets. Both technologies use high Spenn shift keying (GFSK) modulation, but Bluetooth low energy technology uses a modulation index of 0.5, while standard Bluetooth technology is 0.35. The 0.5 index approaches the Gaussian minimum frequency shift keying (GMSK) scheme, which can reduce the power consumption requirements of wireless devices (the reasons for this are more complicated, this article does not repeat). The lower modulation index also has two benefits, namely improved coverage and enhanced robustness.

Standard Bluetooth technology uses a longer packet length. When these longer packets are sent, the wireless device must remain in a relatively high power state for a longer period of time, making the silicon wafer more susceptible to heat. This heat will change the physical properties of the material and thus change the transmission frequency (interrupt link) unless the wireless device is frequently re-calibrated. Calibrating again will consume more power (and require a closed-loop architecture that makes the wireless device more complex, which pushes up the price of the device).

Conversely, Bluetooth low-energy technology uses very short packets-which keeps the silicon at a low temperature. As a result, Bluetooth low power transceivers do not require a more energy-efficient recalibration and closed-loop architecture.

Two kinds of chip architectures for BLE

Bluetooth low-Power architecture consists of two chips: single-mode and dual-mode. Bluetooth single-mode devices are a newly emerging Bluetooth specification chip that only supports Bluetooth low-energy technology-part of a technology optimized for ULP operations. Bluetooth single-mode chips can communicate with other single-mode chips and dual-mode chips, which require the use of Bluetooth low-energy technology in their own architecture to send and receive data (see Figure 2). Dual-mode chips can also communicate with standard Bluetooth technology and other dual-mode chips using traditional Bluetooth architectures.

The dual-mode chip can be used in any situation where the standard Bluetooth chip is currently used. This allows mobile phones, PCs, personal navigation devices (PND) or other applications with dual-mode chips to communicate with all the traditional standard Bluetooth devices already in use and all future Bluetooth low power devices. However, because these devices require standard Bluetooth and Bluetooth low-power tasks, the dual-mode chip is less optimized for ULP operations than a single-mode chip.

Single-mode chips can be used for a long period of time (months or even years) with one-button batteries (e.g. 3V, 220mAh CR2032). In contrast, standard Bluetooth technology (and Bluetooth low-power dual-mode devices) typically requires at least two AAA batteries (10 to 12 times times the power of the coin cell, which can tolerate much higher peak currents) and, in many cases, only a few days or weeks (depending on the application). Note that there are also some highly specialized standard Bluetooth devices that can work with batteries that are lower in capacity than AAA batteries.

Figure 2: The dual-mode chip communicates with the single-mode device using the Bluetooth low energy portion of its architecture.

Android Bluetooth Low Energy (LBE) Technology Introduction