Low Power WiFi Devices

Written by : Deepaknath Tandur, March 2017

The proliferation of IoT requires many of the end devices to work solely on battery power, sometimes for months or even years. The deployed wireless technology should support such long operations and need not drain the battery life of these devices. Various wireless options are available that operate on unlicensed bands, for example for outdoor low power long range communication, networks based on technologies such as LoRaWAN, Ingenu, Sigfox can be utilized. These technologies provide long battery life for end devices for months or even years. However, the supported data rate is typically in bits per second. Similarly, personal area network (PAN) technologies such as Zigbee and 6LowPAN support low power operations of end devices with data rates of up to 250 kbps. Technologies offering medium to higher data rates, such as in megabits per second (mbps) along with long battery life are very limited. Bluetooth, provides data rate of 1 mbps, however it is typically used in the context of point-to-point communication. WiFi is a popular broadband technology with supported data rates in several mbps, however it is a power hungry solution and it is not expected to support battery powered end devices for weeks or months. The table below compares the different unlicensed wireless technologies.

Recently several WiFi vendors (TI, Gainspan, Silex Technology) have come up with mainstream WiFi (802.11b/g/n) based chipsets for end devices that consume very low power. The main advantage here is that WiFi networks are ubiquitous and provide native IP support. Thus, an end device with WiFi connectivity can be directly hooked on to an existing WiFi network in a factory or a plant. These devices are then directly accessible via the internet.

Traditional Wi-Fi requires several megabytes of memory on a host system to run a driver/supplicant. However, the low power WiFi devices have minimal memory requirements, which can be as low as 128 KByte for code and 16 KByte for data. These devices allow the usage of a much smaller and lower power host CPU or even no host CPU at all, creating an overall smaller power draw for the system. These devices have the capability to switch off different parts of the board in order to reduce the current consumption. They quickly go to sleep and wake-up states based on the application requirements. Thus based on the components and parts of the board that are switched off, the device can achieve different low power states. Some of the lowest current consumption modes may disable the real time clock (RTC), and in these cases the device can be brought up only by an external input or an event. Such states are useful if the application does not require the device to remain switched on all the time and data transmission happens only when there is an external event. A device may have varying current consumption levels depending on whether it is in one of the active states such as transmission state, receiving state, wakeup state, or one of the non-active states such as sleep, deep sleep, hibernation, etc. The absolute current consumption levels among the vendors are implementation specific, however the different device states and their current consumption levels are generally comparable. The key is to operate the device in non-active states as much as possible in order to maximize the longevity of the battery.

In addition to small compact sizes and implementation technology, additional Medium Access Control (MAC) layer mechanisms are also utilized for power saving. The access point (AP) from the network has to ensure that it maintains connectivity or association with the end devices for as long as possible. AP also buffers the respective data when the devices are in the non-active states. The extent of support may vary from one AP to another.