Abstract
A new generation of Wi-Fi chips and modules are emerging that can offer significantly lower power and simpler integration into a design. With Wi-Fi Direct and Wi-Fi HaLow also emerging for low power connections, there are many more options for low power wireless connectivity. This article looks at the usage models of these Wi-Fi devices and modules in typical applications to get the best battery life
Introduction
There is an increasing demand to connect all kinds of equipment to the Internet. This focus on the Internet of Things (IoT) brings many benefits, from low costs of management and upgrading to a dramatic improvement in the flexibility in building large networks.
There are many ways of providing a wireless link to equipment, but recent developments are making Wi-Fi (the IEEE802.11 standard) an increasingly popular option. This is being driven by the ubiquitous use of Wi-Fi networks in the home and industry. Silicon devices for the standards are being manufactured in the tens of millions, bringing down the cost. Frequency bands at 900MHz, 2.4GHz and 5GHz are unregulated and available worldwide, unlike cellular solutions such as GSM and LTE, and there is a broad and growing ecosystem of software that uses the direct connection to Internet through an access point.
However, using Wi-Fi for wireless sensor networks and the IoT has up until now had its challenges, mostly around the complexity of the implementation and the power consumption, as well as interference issues that also hit the power consumption. This is all changing with modern devices.
Wi-Fi was designed as a desktop technology for streaming data to an access point and so is not architected for low power. However modern silicon and different software architectures can dramatically reduce the power consumption to make it viable and cost effective even for battery powered devices. The higher performance of access point chips and new standards such as 802.11ac and 802.11ah are allowing many more devices to be attached to the network, and that same increase in performance is being used to reduce the power.
Reducing power
There are two ways to reduce the power in Wi-Fi for IoT. The first is to optimise the duty cycle. Unlike streaming applications, sensor and control data in the IoT is bursty. This means a Wi-Fi chip only needs to operate for less than 5% of the time to send data and, unlike Bluetooth, does not need to set up a link. As the latest chips from developers such as Broadcom, Microchip and Atmel are also designed with ultra low power ‘listening’ sleep modes, the chip can be quiescent for 95% or even 99% of the time, only waking up if there is data to send or control instructions coming in. This dramatically reduces the average power consumption and extends the battery life.
System architects can also make more use of the higher performance links. For battery systems what really matters is the energy consumed, so using a higher speed link (100Mbit/s for 11n or even 433Mbit/s for an 11ac channel) means the chip can send its data in a shorter time and shut down more quickly, using significantly less energy as a result.
The new standards such as 11ac also tackle the interference issues. One of the problems with Wi-Fi in the past has been interference, both from other networks and from other equipment. There are a limited number of channels available at 2.4GHz for 11b, 11g and 11n, and with a large network with many thousands of devices, there can be interference. This increases the power consumption as data has to be re-transmitted, increasing the probability of more interference. Many schemes increase the transmit power to overcome the interference, further increasing the power consumption.
This is mitigated by reducing the duty cycle, as any one device is only trying to transmit for a short period of time and therefore less likely to interfere with another device nearby. Similarly, reducing the time spent transmitting not only reduces the energy consumption but the risk of interference.
There are other networks that operate in the 2.4GHz such as Zigbee and Bluetooth, and, as it is unregulated, there can be other proprietary networks operating. However, modern design techniques have improved the interference protection, and newer coding schemes also reduce the interference. This is also important with other equipment, such as microwave ovens that radiate at the same 2.4GHz frequency, creating more interference. 802.11ac operating at 5GHz with 80MHz channels avoids many of these issues, providing higher data rates (headline figures up to 1Gbit/s) and minimising the effects of other networks
Radio front end
However, this brings its own challenges. Designing a radio front end at 2.4GHz is not trivial, and it is significantly more difficult at 5GHz, and this is also vitally important for the network implementation and the power consumption of the individual nodes.
The design of the front end antenna is vital to the power consumption. A good antenna design with minimal switching losses means a good link budget, which allows the node to transmit at a lower power. This can also allow the node to operate effectively at a longer range, reducing the number of nodes that are needed and reducing the cost and complexity of the network.
While 5GHz networks can provide lower power from higher data rates, the penetration is lower than 2.4GHz, so the links are shorter for the same power, or more devices are needed. The design of antenna systems for 5GHz 11ac networks is also different from the 2.4GHz versions. The opposite is true for networks operating at lower frequencies, with greater penetration and/or lower power possible at 900MHz.
All these issues can be addressed with a new generation of low power modules. These integrate the single chip transceiver with the optimised antenna design and the matched components to provide the best possible link budget. These modules can be easily added to existing equipment designs to connect them up to access points and then onto the Internet of Things. These modules, from suppliers such as Murata using the Broadcom chip, Microchip or Gainspan, allow designers to easily implement a link with a range of 50 to 70m indoors or 300 to 1000m outdoors with the built in antenna, and longer range is possible with an external power amplifier and antenna, although this comes with higher power consumption. As the modules are low power to start with, this gives the designer more options for the network implementation.
New variants
There are also new versions of Wi-Fi emerging. The next version of 11ac uses channels as wide as 160MHz, giving more opportunity for spread spectrum implementations that are more robust against interference.
There is also a point-to-point version of Wi-Fi called Wi-Fi Direct. This will allow nodes to be interrogated or linked directly without having to go across the network, potentially simplifying the network architecture and software by reducing the number of access points and reducing the power. This may be especially helpful in situations that are simple cable replacement applications and in diagnostic and test situations. Chip designers such as Atmel are including Wi-Fi Direct in its transceivers for industrial networking applications.
The latest standard introduced by the Wi-Fi Alliance, but awaiting certification, is Wi-Fi HaLow (802.11ah). HaLow extends Wi-Fi into the 900 MHz band, doubling the reach of the signal and arguably giving Wi-Fi the advantage over Bluetooth in the Internet of Things market. In addition to this there are also the benefits that HaLow will enable connection through commonplace Internet routers, and will have the same level of security as standard Wi-Fi. However, there are limitations to consider, one being the volume of data will be lower. Wi-Fi HaLow has been designed to transmit sporadic low-level communications, such as triggering a light to turn on in a Smart Home.
Conclusion
Wi-Fi is already a ubiquitous wireless network technology, and recent developments are driving down power consumption. This is opening up the opportunity to design Wi-Fi into a wider range of equipment, including portable, battery power devices. Cost effective modules that integrate and optimise the RF front end can be quickly and easily integrated into existing and new designs. This is helping open up a wide range of exciting opportunities for the Internet of Things and wireless sensor networks, connecting more and more devices to the Internet.
