A recently published article by Alan Aronoff provided an excellent introduction into the world of IoT. If you are interested in designing chips for IoT, make sure you keep following our blog because we plan to touch upon two important topics in the upcoming months:

  • Power requirements and power management
  • Processing and connectivity requirements

Inside the industry leading Ensigma Whisper low-power Wi-Fi

Imagination’s Ensigma Whisper radio processing units (RPUs) support low power wireless standards such as Wi-Fi, Bluetooth Smart, IEEE 802.15.4 and others. However, in this article I intend to delve mainly into the Ensigma Whisper low-power Wi-Fi family and how it achieves industry-leading power consumption for Wi-Fi 802.11n (roughly half of what competitor devices achieve!).

Ensigma Whisper RPU for low power connectivity

Before going into the specific details of how Whisper RPUs achieve such low power consumption, here is a brief presentation of the generic methodology for lowering Wi-Fi power consumption.

Some of the common techniques include:

  • Sending data at the highest possible rate to maximize sleep time
  • Using a single antenna as opposed to multiple antennas (SISO vs. MIMO systems)
  • Optimizing the power consumption during packet reception

The IEEE 802.11 a/b/g/n standards also provide a number of features to drive lower power in Wi-Fi devices including:

  1. IEEE Power Save Mode: In this mode radio activity is suspended after a period of inactivity to save power. The device wakes up periodically to check if the access point has data queued for it to ensure no valid packets are lost.
  2. Wireless Multimedia (WMM) Power Save: In this mode the client can request for queued traffic any time rather than waiting for the next beacon frame. The asynchronous mode is called Unscheduled Automatic Power Save Delivery (UAPSD) and the Synchronous Mode is called Scheduled Automatic Power Save Delivery (SAPSD). These techniques are suitable for lighter traffic loads such as voice.

The Ensigma Whisper RPU architecture uses a combination of techniques discussed above, as well as several PHY, MAC and RF optimizations to attain the lowest power consumption possible.

Whisper RPUs also go further with specific optimizations for IoT. In general, IoT devices are asleep most of the time; they wake up only once in a while to determine if a packet needs to be decoded or transmitted. Given these considerations, Whisper RPUs have been designed to optimize the sleep mode power consumption.

For the technical folks among you, the next section discusses Whisper power optimizations in a fair bit of detail.

PHY level optimizations

  • Clocking the PHY at lower frequency compared to traditional designs enables reduction in dynamic power consumption.
  • Reduced bit-widths in the data path enable reduction in both dynamic and leakage power consumption.
  • Low false synchronization probabilities combined with early aborts during false syncs helps optimize listen mode power.
  • Rapid Automatic Gain Control (AGC) algorithms allow quicker preamble detection to optimize sleep/wake cycles. Sleep/wake cycles are critical to connected standby mode power consumption.
  • Apriori estimation of Carrier Frequency Offset, decoding fewer bits than needed, and other techniques ensure minimal beacon mode power consumption.

MAC level optimizations

The MAC optimizations ensure that the entire stack (from the layer 2 to application-level logic) can run on the embedded processor (see the diagram above), thus eliminating the need for a paired general purpose CPU.

RF level optimizations

The RF power savings are achieved through several optimizations, including in areas such as analog filtering or PLL/VCO/LNA design.

The PHY/MAC/RF optimizations are combined with sophisticated power management systems including DC-DC converters, LDO regulators, 32 kHz real time clock, timers and control sequencers etc. These ensure that the power management software can turn on power at the latest possible time while at the same time ensuring that the power can be turned off at the earliest time, ensuring lowest energy consumption. Additionally, Whisper RPUs provide mechanisms to control the frequency of the MAC clock, thus allowing high clock frequency during initialization periods or power down periods enabling the processor to sleep longer.

The picture below showcases the multiple power states in Whisper and compares the same with a direct competitor. As is evident from the figure, the receive power consumption is roughly half that of the competition. The various power optimizations ensure that Whisper-powered devices can wake up quickly, consuming the lowest power in the process.

Ensigma Whisper RPUs - power consumption

These power-sipping characteristics make Whisper RPUs highly suitable for IoT applications running on coin cell batteries.

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About the author: Narayanan Raman

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