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Although many power-reduction methods have been standardized for WLANs and Bluetooth, the way a wireless chip implements these techniques has a profound effect on power consumption in real-world applications.
For example, shortening the time needed to go into standby mode achieves better results than reducing the amount of power consumed in standby mode. Increasing data rates can also achieve significant power reductions.
Power relationships such as these are explored in this article—the second in a two-part series on power-reduction methods for wireless devices. The first article in the series provided an overview of device operating modes and the power-saving protocols available in today's wireless standards.
To read Part 1:
How to use optional wireless power-save protocols to dramatically reduce power consumption.
System behavior and host interaction
Beyond protocol-based power savings, several additional methods can save power equally well for Bluetooth or WLAN systems:
- Data rates
- Quick wakeup and return to sleep
- Scan optimization
- System and host interaction
These methods look at the entire host system and involve interactions between a wireless chip and its host.
As Table 1 shows, high data rates can save power. This idea is somewhat counterintuitive because higher data rates usually increase a device's instantaneous power consumption while actively transmitting and receiving.
Click here for Table 1.
Table 1: Effect of Data Rate on Power Consumption.
The truth is that radios have a large amount of power-consumption overhead that is independent of the data rate. The synthesizer, low-noise amplifier (LNA), mixers, filters, and other components have to be on for transmitting and receiving independent of the data rate. That power consumption is a fixed overhead.
The increase in the power of the ADC and digital processing to support higher data rates is actually fairly modest. For most wireless systems, scaling the data rate up and down does not significantly change the average power consumption while in active transmit-and-receive mode.
However, the protocols allow the radio to enter an order-of-magnitude, lower-power sleep mode as soon as data has been transferred. So when moving a file or sending data for VoIP or video, the system can spend more of its time in sleep if the data rate is high.
The important metric here is the energy-per-bit transferred, which is the average power consumption divided by the average data rate. This energy can be measured in nanojoules (nJ) per bit transferred, and is the metric to determine how long a battery will last while doing tasks such as VoIP, video transmissions, or file transfers.
For example, Table 1 shows that for 802.11g the data rate is 22 Mbps and the corresponding receive power-consumption average is around 140 mW. While actively receiving, the energy consumed in receiving each bit is about 6.4 nJ. On the transmit side, the energy is about 20.4 nJ per bit.
Looking at these same cases for 802.11n, the data rate has gone up by almost a factor of 10, while power consumption has gone up by only a factor of 5, or in the transmit case, not even a factor of 3.
Thus, the energy efficiency in terms of nJ per bit is greater for 802.11n.
The Bluetooth numbers in Table 1 are for the traditional Bluetooth mechanism (1 Mbps) and Bluetooth EDR (enhanced data rate, 3 Mbps), supported by Atheros. The latter version came into existence with the Bluetooth 2.0 standard, so it is relatively recent and is more power-efficient than the original version.
Perhaps surprisingly, both Bluetooth modes are less efficient than the WLAN modes. Designers need to consider this fact carefully when thinking about what kind of wireless system to put in products. When using multiple wireless systems, designers need to consider which physical layer to use for moving data.
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