Wi-Fi today is a ubiquitous connectivity technology and is one of the pillars one which Internet of Things (IoT) connectivity rests. The problem, however, is that Wi-Fi has traditionally been a power hungry technology. Imagination’s Ensigma Whisper low-power Wi-Fi IP has been architected from the ground-up to provide an ultra-low power solution that’s ideal for IoT and other battery operated devices. In this post, we share some actual power measurements performed on a customer chip that uses the Whisper Wi-Fi core. Measuring the power consumption enables us to determine the battery life of the end product.
Test setup description
Our measurements were carried out using a Techtronix TDS 784D Oscilloscope and Techtronix TCP202 current probe. For low-current measurements, we used a digital multimeter (Metrix MX56). The current measurements for connected standby mode were conducted by connecting the Device under Test (DUT) to a TP-Link AC750 router. All current measurements were carried out on Channel 1 for channel bandwidth of 20MHz in Mixed Mode (802.11 b/g/n) with no encryption enabled. The set-up measures both active (transmit and receive) and inactive (sleep mode) power consumption.
Figure 1: Test setup and measuring instruments
The Whisper Radio Processing Unit (RPU) has multiple power domains for each of the functional blocks. It also has a sleep controller, which provides power to each power domain at the latest possible time and cuts off power at the earliest possible time. The power domains are comprised of the following power rails: 3.3V for the power amplifier (PA), 1.6V for the RF and 1.1V digital blocks. We carried out the power measurements for each of the power domains.
Figure 2: Power domains in Whisper Radio Processing Unit (RPU)
Figure 3: Sleep controller in Whisper Radio Processing Unit (RPU)
Power Amplifier (PA) power consumption at 3.3V
The power consumption of the PA is measured both at the highest output power (lowest data rate) and at lowest output power (higher data rate) as depicted in the below photos. As you can see, the current measured is 98mA at a transmit output power of 10.5dBm. Correspondingly, the current measured at 17.5dBm is 136mA. The PA is the predominant source of power consumption in the transmit path, and the Whisper architecture focuses on power reduction for each component in the transmit chain including the PA.
Figure 4: Transmission of an 802.11n MCS7 signal @ 10.5 dBm
Figure 5: Transmission of an 802.11b 1 Mbps signal @ 17.5 dBm
RF Power Consumption at 1.8V
The RF power consumption is measured for both the transmit and receive paths and is depicted in the below photos. As you can see, the current measured is 29mA in the receive path and 59mA in the transmit path. The receive power consumption is a key parameter for the connected standby power consumption, and the Whisper core’s power consumption in the receive path has been specifically optimized for this use case.
Figure 6: Reception of an 802.11n MCS7 signal
Figure 7: Transmission of an 802.11n MCS7 signal
Power consumption at 1.1V
The digital power consumption is measured for both the transmit and receive path and is depicted in the below photos. In this mode, the receive current is 16mA and the transmit current is 13mA. The digital power consumption is optimised through a combination of innovative algorithms and power gating mechanisms in the baseband.
Figure 8: Reception and transmission of an 802.11n MCS7 signal
Summary of current consumption
A key power consumption metric for IoT devices is the receive power consumption. As can be seen in the table below, the Whisper core’s receive power consumption in active mode is 64mW. Based on the extensive data mining conducted on similarly featured devices, it is fair to state that the power consumption of Whisper’s receive power is far better – by 50% in most cases.
Application energy consumption calculation example:
Consider an intermittently connected device such as the smoke alarm sensor. The devices awaken at regular intervals to communicate with the server (access point). There are two predominant states: a static power consumption state (sleep state) and an active power consumption state (during which the various sub-blocks of the RF transceiver are powered and the device receives data).
This connected standby state of this scenario is captured in the measurement:
power consumption = 29mA*33ms + 0.5mA*(304-33)ms = 1.1mC
During a one-hour interval, if the device wakes up at two-minute intervals (30 times in one hour), the total power consumption = 1.1 *30 = 33mAs.
For a battery capacity of 1000mAH, the lifetime = (1000 mAh/33(mAs))/24 hours = 12.45 years.
As our tests prove, Ensigma Whisper low-power Wi-Fi IP consumes roughly half the power consumption of similarly featured devices, offering a compelling solution for IoT applications where low cost, low power consumption and robust performance are key drivers.
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