Bluetooth power reduction
Bluetooth technology is fundamentally a TDMA system. Operating according to the basic modes of Bluetooth standards achieves most of the power-save advantages available from protocols, but there are some additional methods chip designers can use to further reduce power consumption.

Sniff Subrating
One of these methods is to use the capabilities of sniff subrating, which was standardized in the Bluetooth 2.1 specification. Sniff subrating allows the period between listen and transmit periods to be adjusted dynamically. Similar to the beacons and sleep periods in WLANs, Bluetooth devices have standby periods. After these periods, a device wakes up and sniffs, and the sniff timing can be negotiated.

Sniff subrating is particularly beneficial for bursty traffic from devices such as mice and keyboards. When a mouse is moving, it needs to be quite reactive, but when the mouse is just sitting still, it should go into a very low-power mode. Using sniff subrating, a Bluetooth device can constantly renegotiate how long it goes to sleep and how often it wakes up and transmits data.

Simultaneous Sniff and HV3 Operation
A Bluetooth device operating in headset mode (HV3) is active about one-third of the time. It is possible for such a device to transfer data in two-thirds the time that the headset voice data is not consuming. Originally these devices had to listen at the start of each time slot to see if such data were present.

However, starting with the Bluetooth 2.0 specification, the device can use the sniff mode while HV3 is running to remain ready for data transmissions while dissipating much less power. In this way, the device operating in HV3 mode can sleep for most of the time that it is not transmitting or receiving voice data, waking only occasionally to see if there is additional data that needs to be received in other slots. Unfortunately, some manufacturers have not implemented this feature.

Transmit Power Control
Bluetooth technology includes a sophisticated transmit power control mechanism. When two Bluetooth devices are close to each other, they can successfully communicate with lower transmit power. Reducing the transmit power reduces the draw on the battery from the RF power amplifier.

The Bluetooth power control system is a closed-loop power control system. The receiver notifies the transmitter if the power level can safely be reduced. The same procedure can operate simultaneously in the other direction. The mechanism is quite reliable because the receiver is able to specify the exact power level it needs to receive adequately.

Getting results from power-reduction protocols
Wireless communication standards offer many ways to reduce the power consumption of wireless chips. The operating modes combine with the power-reduction protocols described in this article to provide many choices for managing power consumption.

Many of these choices are optional, and results depend greatly on the way the protocols are implemented by chip designers. The next article in this series will show how implementation details—such as the speed of transitioning from one power mode to another—can have a profound affect on overall power consumption.

Part 2 of this article is available at Power-reduction details determine a chip's overall power consumption (Part 2 of 2).

About the author
Bill McFarland is Chief Technology Officer for Atheros Communications, a semiconductor company specializing in the development of high-performance wireless systems solutions.