Siddarth Sundar - Senior Product Manager, Wi-Fi Solutions
Over the last two decades, Wi-Fi has transformed connectivity with more than 15 billion installed devices in the world.
Such a broad and pervasive installed base, along with its throughput capabilities and native IP capabilities, makes Wi-Fi an attractive protocol for IoT wireless applications. However, a key limitation of most Wi-Fi based IoT systems is high power consumption. As Wi-Fi was primarily designed to optimize bandwidth, using traditional Wi-Fi in power constrained battery powered applications can be a significant challenge. This whitepaper discusses the factors contributing to Wi- Fi power consumption, different power saving modes, as well as design and hardware considerations for optimizing power consumption and extending battery life in IoT devices.
There are multiple flavors of Wi-Fi based on different IEEE standards. These typically operate on the 2.4 and 5 GHz bands, with a multitude of modulation schemes. The maximum data rates range from single digit Mbps to hundreds of Mbps, depending on the antenna configuration and the modulation scheme employed. Before developers can understand best practices for optimizing power consumption and system efficiency, they must first understand the different Wi-Fi technologies and why certain ones have advantages for power-constrained devices.
The following table lists the most common flavors of Wi-Fi:
|Band||Technology||Max Speed (Mbps)||Comments|
|2.4 GHz||802.11 b/g||11-54 Mbps||Historical standards. Superseded by 802.11n|
|2.4 & 5 GHz||802.11 n||72-600 Mbps||Faster speeds, MIMO support|
|5 GHz||802.11 a||54 Mbps||Original 5 GHz standard|
|5 GHz||802.11 ac||Gigabit||Flagship 5 GHz high speed standard|
|2.4 & 5 GHz||802.11 ax||Gigabit||Next-gen Wi-Fi standard|
|Sub-GHz||802.11 ah||< 40 Mbps||New standard designed for Sub-GHz Wi-Fi operation|
These are some of the key aspects to consider while selecting the Wi-Fi standard for IoT applications:
Power consumption in Wi-Fi varies dramatically across various modes of operation starting from few hundred mA in transmit mode to few μA in sleep mode. It’s important to understand the various modes of Wi-Fi operation and optimize those modes from a system and application level to reduce the overall power consumption. From a power consumption perspective, we can classify the Wi-Fi operations in to the following modes:
|Mode||Current Consumption*||Memory Retained||Time to Wake Up and Associate*|
|Snooze||500 μA||Yes||1 ms|
|Associated Sleep||30 μA||Yes||~3-5 ms|
|Deep Sleep||2 μA||No||100s of ms|
One strategy to reduce the overall power consumption is to stay in the lowest power mode as much as possible. When data needs to be transmitted or received, do it as quickly as possible then go back to lowest power state possible. As shown in the above table, wake up time increases as you enter deeper sleep states. This means higher latency. Also, certain sleep modes cannot retain memory, which means applications need to re-establish the connection with the AP and cloud. This can lead to additional power consumption.
There are several best practices for optimizing these modes to ensure low power consumption: optimizing Transmit and Receive modes, optimizing various sleep modes, WMM and power save modes, and standby and shutdown modes.
As mentioned previously, Wi-Fi devices typically have significant power consumption in Transmit and Receive modes (Transmit power consumption is typically in the 200-400 mA range, while Receive power consumption is typically in the 50-100 mA range). Therefore, choosing devices with the lowest possible Transmit and Receive power consumption is important to reduce overall power consumption.
For most IoT devices, receive power is more significant as you spend most of your time listening for data. In cases where the device wakes up periodically and transmits a chunk of data (e.g. IP camera triggered on motion), power consumption may be dominated by the Tx mode.
Most Wi-Fi devices are optimized for range and throughput, which means they attempt to transmit at close to peak power all the time. A power-optimized Wi-Fi device may have a lower power TX or Receive mode that allows you to trade off power consumption with range for your system.
Figure 1. Downlink/Uplink Power Consumption
Downlink Power Consumption (High Throughput)
Uplink Power Consumption (High Throughput)
Beyond average power consumption, the Transmit (Tx) power consumption typically sets the peak current requirement for the whole system, which means the entire power subsystem must be able to deliver that amount of peak power. This can set limits on device size, cost, and battery technologies. Therefore, it is important to look at peak Transmit (Tx) current consumption, even if that is not a dominant factor in overall power consumption. In addition, there may also be thermal considerations in products like lightbulbs and switches, which could be a further limitation on peak current consumption.
Finally, while MIMO architectures and newer standards like 802.11ac/ax can offer higher data rates, this often comes at the expense of Transmit (Tx) and Receive power consumption. For IoT systems with limited bandwidth requirements, 802.11 b/g/n can offer the right compromise of throughput, power consumption, and cost.
For many IoT applications, most of the time is spent in the idle mode, not transmitting data. The Wi-Fi standards have two mechanisms to reduce power consumption when the device is not sending or receiving data:
Wi-Fi Multimedia (WMM) Power Save: this mechanism allows the access point (AP) to buffer downlink frames (based on the QoS parameters defined in WMM), allowing the client device to doze between packets to save power. WMM doesn’t need to send specific PS-Poll requests because it can be triggered with any data frame. Also, for integration with 802.11e QoS, different application types can use different queues so services that can tolerate higher latency will let the device stay asleep longer.
Delivery Traffic Indication Message (DTIM) Intervals: Wi-Fi access points send out a beacon every 100 ms. The standard allows the AP to only send broadcast or multicast data every ‘n’ beacons (defined as the DTIM interval), allowing the client to sleep for multiple beacons. This is valuable for power saving, since listening to each beacon consumes a lot of Receive current, and this Receive current typically dominates average current consumption. The longer the DTIM interval, the more the power savings. However, this comes at the cost of higher latency.
By using these two power-saving mechanisms, the device can sleep for most of the time it is not sending or receiving data, reducing power consumption dramatically. Keep in mind that the device must retain connection information (SSID, keys, IP addresses, etc.) even when asleep. Therefore, this type of sleep mode (often referred to as “associated sleep”) consumes a bit more power than a true shutoff mode (as memory must be retained). However, this current is far smaller than the Receive or TX current. For example, an average sleep current number may be < 100 μA, compared to 10’s to 100’s of mA in TX and Receive modes. This can enable average power consumption for sleepy applications to drop well below 1 mA in many cases.
Figure 2. Power Save Power Consumption
Wi-Fi association and disassociation involves several message exchanges between the device and the access point. This means transmittingandreceivingseveralpacketsandthepowerconsumptionassociatedwithit. To save the maximum amount o fpower, the station can completely dissociate from the access point and go in shutoff mode (where the device can sometimes consume < 1 μA of current). However, this means that the client has to re-associate with the AP every time it wants to send data (which can take up to a few seconds).
This association process adds significant latency to any information that needs to be communicated and results in significant power consumption during this period. In addition, the device cannot receive any information when it is dissociated, which means remote wakeup via Wi-Fi is not possible.
However, this can be a very useful strategy to save a lot of power. Devices like ordering buttons (push button to order a specific item to be delivered) are only used sporadically, perhaps a few times a month. Additionally, latency of a few seconds in such a device is acceptable. Staying dissociated from the Wi-Fi access point, and only re-associating on a button push to transmit information, allows such devices to have multi-year lifetimes on a small battery.
Unlike many wireless protocols, Wi-Fi power consumption is significantly impacted by RF performance and network conditions:
Figure 3. Channel Utilization with Adjacent Channel Interference (5 Mbps Target Throughput)
Goal–maintain throughput during interference
Figure 4. Data Throughput with Adjacent Channel Interference
Goal–minimize re-tries during interference
Here are ways to optimize power consumption:
Wi-Fi datasheets are complicated, and there are several tricks to watch out for:
Power consumption is highly dependent on the application and use case. In general, we can classify the applications into three major categories:
These are some of the application level factors that needs to be considered while selecting the Wi-Fi technology:
While Wi-Fi is not optimized from the ground up for low power operations, there are several techniques that can be used to significantly reduce power consumption. These range from features built into the specification, to RF performance, to system level optimizations on connectivity. Developers must understand all the contributing factors to overall energy consumption in IoT devices. They must understand both system level factors, and deep application factors to achieve low energy consumption in their applications. The right mix of power-saving Wi-Fi modes and the right hardware are the keys to dramatically reducing power consumption. Leveraging hardware and software designed specifically for IoT devices can reduce long term costs, overcome development challenges, extend battery life, and potentially enhance the life of their products and customer satisfaction.
Learn more about Silicon Labs low power IoT solutions at www.siliconlabs.com/wi-fi
About the Author:
Siddharth Sundar is a Senior Product Manager at Silicon Labs based out of Austin, TX. He has more than 10 years of experience in the semiconductor industry in engineering and marketing roles. He holds a B.S and a Master of Engineering degree from the Massachusetts Institute of Technology.