Design Article

How to use optional wireless power-save protocols to dramatically reduce power consumption

Bill McFarland, CTO, Atheros Communications

5/27/2008 4:20 PM EDT

While a variety of methods can be used to reduce the power consumption of wireless LAN (WLAN) and Bluetooth applications, most of these methods must be implemented at the chip level, and their effectiveness is not always obvious in simple metrics for transmit and receive power levels. Understanding how these methods work is thus crucial for minimizing power consumption at the system level.

This two-part series of articles provides an overview of today's power-saving methods, ranging from power-reduction protocols and system optimization to low-power circuit design and process technology. Since protocol and system solutions have the greatest influence on power, the articles focus on how these methods can minimize power consumption while ensuring high performance of the wireless system.

Considering just the 802.11 wireless portion of a VoIP handset, the battery would last for more than 100 hours in continual VoIP mode on a standard 3.7V, 800mAh phone battery (as demonstrated by Atheros next week at COMPUTEX TAIPEI 2008).

Similarly, considering just the power consumption of the Wi-Fi system, a portable media player can download over 200 gigabytes of data on the same battery charge. This level of power efficiency is possible without requiring users to switch the Wi-Fi functionality on and off, because the techniques described in these articles automatically manage the wireless power modes.

Part One

This first article in the series begins with a brief look at protocol modes and then provides an overview of standard power-reduction protocols for wireless systems. While WLAN and Bluetooth standards define a number of power-reduction protocols, many of these protocols are optional. Even among chips that implement the protocols, the way the protocols are implemented can make a big difference in power consumption from one chip to another. An understanding of these modes is thus vital for system developers who want to minimize the power consumption of WLAN and Bluetooth products.

Protocol modes
As wireless devices operate, their physical layers (PHYs) can be considered to be in one of five states: (ul>

  • Off—The only power consumption is leakage current, but coming out of the off state can take a long time (many milliseconds).
  • Sleep/Standby—The device may be consuming as little as 175 μW and can wake quickly unless the main crystal is turned off.
  • Listen—The device is listening for a packet to arrive, so most of the radio receiver must be on. State-of-the-art power numbers for WLAN and Bluetooth devices in this mode are 110 and 46 mW, respectively.
  • Active Rx—Similar to the listen state, but use of additional circuitry may push power consumption for WLAN (802.11g) and Bluetooth devices to 140 and 52 mW, respectively.
  • Active Tx—In the transmit state, the device's active components include the RF power amplifier, which often dominates in high-power transmit systems. State-of-the-art power consumption for an 802.11g WLAN device is 450 mW at 15 dBm Tx power. For Bluetooth, values are 78 mW at 3 dBm Tx power and 55 mW at "18 dBm.

    • These wireless physical states are used in combination to create wireless protocol modes:

    • Searching for a network
    • Connected but idle
    • Media traffic flow
    • Max-throughput traffic flow

    To illustrate the contrast between one protocol mode and another, Figure 1 shows the amount of time spent in various states when searching for a network, while Figure 2 shows the states for media traffic flow. When searching for a network, the device spends almost all its time in sleep mode.

    Surprisingly, the device still spends most of its time in sleep mode when sending and receiving media traffic. The required data rate is lower than the radio capacity, and the device can be in this state continuously for hours.

    Click here for Figure 1.
    Figure 1: Wireless power mode: Searching for a network.

    Click here for Figure 2.
    Figure 2: Wireless power mode: Media traffic.

    In the max-throughput state, a wireless device is trying to move a large amount of data as quickly as possible (e.g., a file transfer). Devices are usually in this mode for a short burst. The higher the data rate, the sooner the device can go back to idle.


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    kruimer

    6/22/2008 11:32 AM EDT

    don't forget to take into account power consumed by the connection of the chip to a computer. A very low power wireless chip doing nothing may still eat up 10% of a laptop's battery life just by the USB hardware accessing computer memory using DMA

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