[Technical Sharing] Wi-Fi HaLow Technology Overview

This article is translated from the Wi-Fi Alliance's May 2020 publication, 'Wi-Fi HaLow™ Technology Overview'.

Introduction

Wi-Fi® connectivity is accelerating the development of IoT applications, from sensors that help track the location of warehouse equipment to health monitors and climate control sensors. Wi-Fi Alliance® introduced Wi-Fi HaLow™ to meet the unique needs of the growing IoT market. Wi-Fi 4 is a mature technology based on the IEEE 802.11n standard and provides the data rates and range needed for email, online shopping, and smart TV streaming. Wi-Fi CERTIFIED 6™ is the latest generation of foundational Wi-Fi connectivity, providing improved device power savings, supporting IoT capabilities in certain environments, and also handling bandwidth-intensive applications. However, many IoT environments require longer-range connectivity, the ability to penetrate multiple walls, and devices that can run for months or years on a single battery charge.

Wi-Fi HaLow is based on the IEEE 802.11ah standard and provides the range, data rates, penetration, and low-power profile expected for IoT deployments. These include products for industrial, agricultural, smart city environments, and home and building automation. Wi-Fi HaLow operates in the sub-1GHz band, extending Wi-Fi ubiquity and security to more IoT environments. Its native IP support simplifies access to the internet and cloud-based applications without additional costly infrastructure such as hubs, repeaters, or gateways.

This technology overview document elaborates on the key points in the white paper 'Wi-Fi HaLow™: Wi-Fi® for IoT Applications' and provides additional background on why Wi-Fi HaLow is an excellent solution for the growing IoT market, including:

• Underlying wireless technology characteristics, such as sub-1GHz frequencies, network bandwidth, modulation and coding schemes

• Innovative new power-saving features defined by Wi-Fi HaLow, such as target wake time (TWT), restricted access window (RAW), and longer maximum idle periods

• Wi-Fi HaLow security approach

In addition, this overview compares Wi-Fi HaLow with other unlicensed low-power wide-area networks (LPWAN), licensed wide-area networks (WAN), wireless personal area networks (PAN), and local area networks (LAN) technologies on important characteristics such as battery life, coverage, data rate, and scalability. Readers will learn why Wi-Fi HaLow is considered the preferred choice for many different IoT environments.

Technology Overview

Sub-1GHz and Narrowband

The laws of electromagnetism and communication theory dictate the relative tradeoffs among power, distance, and radio frequency (RF) signal reliability. Generally, achieving longer distances at any given frequency requires higher signal transmission power or a lower data rate. For any given transmit amplification power, radio waves transmitted in the 2.4 GHz and 5 GHz bands do not propagate as far as lower-frequency radio waves such as 915 MHz. Therefore, Wi-Fi HaLow primarily operates in the sub-1GHz spectrum.

Another factor in predicting the success of long-range RF signal transmission is the channel width over which energy is concentrated. Narrower-band channels within a given frequency range can concentrate transmission and reach farther than wider channels at the same frequency. As shown in Figure 1, narrower channel modes operating at sub-1GHz frequencies provide greater coverage than wider channels at 2.4GHz frequencies.

To be precise, the portion of the sub-1GHz band used by Wi-Fi HaLow varies according to market regulatory requirements. The United States currently uses 902 MHz to 928 MHz. Australia and New Zealand use 915 MHz to 928 MHz. Europe has allocated 7 MHz of spectrum between the 800 MHz and 900 MHz bands. Device manufacturers should consult local regulators to confirm which part of the sub-1GHz band is approved for use. To support product development for global deployment, the Wi-Fi Alliance actively advocates globally harmonized spectrum access for Wi-Fi HaLow in the 915 MHz to 925 MHz range.

Lower frequencies and narrower channels improve the link budget

One way to determine which IoT radio technology best meets application requirements is to evaluate the available link budget. Basically, the link budget uses transmit power and transmission gains and losses (in decibels (dB)) to determine the received power of a transmission at a given distance. The IEEE 802.11ah standard specifies effective channel width options of 1, 2, 4, 8, and 16 MHz. Taking the United States as an example, in scenarios where Wi-Fi is considered as an IoT radio option, Wi-Fi HaLow leverages sub-1GHz operating frequencies and narrow channel widths to achieve an 8 to 12 dB advantage compared with Wi-Fi in the 2.4GHz band using 20MHz-wide channels.

Wi-Fi HaLow can perform longer-distance data transmission within the sub-1GHz band, effectively addressing the needs of massive IoT and industrial IoT applications while freeing capacity in other bands where Wi-Fi operates. This makes Wi-Fi HaLow an excellent complement to the Wi-Fi portfolio, enabling all forms of Wi-Fi to handle almost any use case, from low-bandwidth IoT network requirements to high-bandwidth-intensive applications.

Beacons are packets broadcast by an access point (AP) to synchronize the client devices it serves, setting the upper bound of network range. Although Wi-Fi HaLow operates in a 1MHz channel using Modulation and Coding Scheme 0 (MCS0) at 300 Kbps, Wi-Fi HaLow beacons have a 10 dB advantage over beacons sent at 6 Mbps in a 20MHz channel. The combination of these effects creates a link budget advantage of approximately 20 dB, which translates into roughly a 10x distance advantage compared with 2.4GHz Wi-Fi.

At the farthest distances (approximately 1 km), Wi-Fi HaLow operates in MCS 10 mode, with data replicated at 300 kbps in a 1MHz channel, effectively achieving a data rate of 150 kbps. This meaningful redundancy provides the receiving device with additional opportunities to correct data errors and offers another improvement to the Wi-Fi HaLow link budget. This combination of data rate and distance exceeds many alternative wireless IoT technology options.

Orthogonal Frequency Division Multiplexing (OFDM) Modulation and Forward Error Correction (FEC)

Wi-Fi HaLow uses OFDM modulation, which has been used in previous Wi-Fi versions since 802.11g. As shown in Figure 2, the data to be transmitted is encoded using a powerful forward error correction (FEC) code that adds valuable redundant check information, and the encoded stream can be sent simultaneously over 26 or more subcarriers distributed across the operating channel. Transmission across multiple subcarriers enables robust reception under channel distortion, while redundancy improves the link budget of the received signal in the presence of interference and noise. The combination of OFDM and strong FEC provides a more robust solution than simple frequency shift keying (FSK) radios. Using OFDM combined with phase shift keying (PSK) or quadrature amplitude modulation (QAM), Wi-Fi HaLow carries more data in a given period than other radio technologies. This spectral efficiency provided by Wi-Fi HaLow is especially important in countries/regions where the sub-1GHz industrial, scientific, and medical (ISM) bands are not widely available and where sensor device duty cycle is subject to certain limits. In many countries, using the ISM bands does not require a license. Within this spectrum, Wi-Fi HaLow provides data rates several orders of magnitude higher than competing IoT technologies such as Sigfox, LoRaWAN, Zigbee, Z-Wave, Bluetooth Low Energy, NB-IoT, and other dedicated frequency shift keying (FSK) radio systems, as shown in the table below.

Modulation and Coding Scheme (MCS)

The Wi-Fi HaLow MCS table is derived from the IEEE 802.11ah specification. The table describes the modulation type arrangement, channel width, inter-symbol guard interval (GI), and resulting data throughput. The access point (AP) and client devices automatically adjust between the two best supported MCS options to maximize data throughput under current channel conditions. Such MCS tables exist in Wi-Fi 4, Wi-Fi 5, and Wi-Fi 6. The data rates in the IEEE 802.11ah table are generally specified as one-tenth of the IEEE 802.11ac MCS table rates and are available for up to four spatial streams. Although initial Wi-Fi HaLow implementations may target single-stream devices, there will be a path to four-stream multiple-input multiple-output (4x4 MIMO) Wi-Fi HaLow solutions in the future.

MCS options take into account that the distance and RF conditions between two devices may change. Like other Wi-Fi technologies, Wi-Fi HaLow can adapt to changing conditions. The access point (AP) and client devices broadcast their capabilities when joining the network. They can automatically adjust to the best MCS as needed. For example, if an access point (AP) has an associated client running MCS 0 on a 1MHz channel at 300 kbps, and both devices determine that the RF channel conditions between them are suitable to support MCS 4 on a 4MHz channel, they can agree to change their MCS and channel bandwidth to transmit data at 9 Mbps.

The benefit of multiple MCS options is that administrators can tailor data rates to a specific application, and mixed device types can automatically optimize to improve conditions. For example, a remote sensor that only needs 150 kbps can be served by the same access point (AP) as a nearby video camera that needs 10 Mbps. Devices close to the access point (AP) typically operate at faster MCS rates, transmit their data, and save energy.

The table below shows the MCS for a single-stream connection. Note that MCS 10 supports the lowest effective rate for long-distance connections, while MCS 0 through MCS 9 have progressively higher data rates.

Medium Access Control (MAC) Efficiency and Power Savings

One of the key design criteria for Wi-Fi HaLow technology is low power consumption, enabling battery-powered IoT devices to operate for years. New MAC features enable devices in Wi-Fi HaLow networks to save energy, reduce congestion, and increase capacity and device density. Transmitting radio signals typically consumes more power than receiving them. Any reduction in device transmissions generally saves energy. A key factor in low power consumption is ensuring that the radio can reliably remain asleep for long periods without being dropped or disconnected by the access point (AP). By allowing Wi-Fi devices to sleep for longer periods, the average energy consumption of such devices can be greatly reduced. Devices in sleep or passive listening states free up available spectrum for active client devices to transmit their data. The main Wi-Fi HaLow efficiency and power-saving features are listed below.

Non-Traffic Indication Map (TIM) mode option

In some wireless local area networks (WLANs), devices must wake frequently, multiple times per second, to monitor and respond to the Traffic Indication Map (TIM) sent by the access point (AP) in beacon frames. The TIM is used to indicate which client devices should expect inbound data. Wi-Fi HaLow devices can save power in the optional non-TIM mode without needing to remain running to actively monitor beacon frames. This feature eliminates the need for Wi-Fi client devices to periodically check beacon messages. Freed from TIM mode, Wi-Fi HaLow can conserve energy and thus compete with other IoT sensor network technologies.

Note that non-TIM mode is an option, depending on the capabilities required by the device and network. Wi-Fi HaLow also supports TIM mode. A Wi-Fi HaLow AP can simultaneously support a mix of both types of devices.

Target Wake Time (TWT)

Client devices that wish to sleep for extended periods can negotiate a TWT agreement with the access point (AP). The AP stores all traffic destined for the client until the agreed wake time is reached. When the client device wakes at the scheduled time, it listens for its beacon and enables the AP to receive and send any required data before returning to sleep. The wake interval, negotiated between the client and AP, can range from very short (microseconds) to very long (years).

Restricted Access Window (RAW)

RAW is another scheduling method for communication among a group of client devices. For systems with predictable activity cycles, the AP can grant a subset of clients with RAW privileges permission to transmit their data, while other clients can be forced to sleep, buffer non-urgent data, or both. This allows client devices to save power and preserves more network capacity for other time-critical traffic.

By combining TWT and RAW features, network designers can minimize channel contention and reduce power consumption across the entire system.

Extended Maximum Idle Time

The Extended Maximum Idle Time feature extends the time a client device is allowed to remain asleep before the AP disassociates it. This retained state prevents power-sensitive sensors from wasting energy on re-authentication that would be required if they dropped off the network. The feature provides a maximum idle time of more than five years. In practice, the idle time depends on implementation and application requirements.

Hierarchical Traffic Indication Map (TIM)

Hierarchical TIM is a more efficient method of encoding the TIM described above, reducing its broadcast time and accommodating more clients. Different encoding modes are defined to efficiently manage large numbers of client devices in sleep mode.

Short MAC Header

Removing unnecessary header fields at the beginning of a packet reduces the occupied time for transmission and reception as well as the associated power consumption. For example, an 8% reduction in overhead for a small 100-byte packet transmission reduces it from 40% to 32%.

Null Data PHY Protocol Data Unit (NDP Frame)

The Null Data PHY Protocol Data Unit (called an NDP frame) incorporates MAC-layer information into the PHY-layer signal field. Compared with legacy management and control frames, this reduces packet size and transmission time. For example, on a 1 MHz channel, the airtime for an NDP acknowledgment using MCS10 is about 0.56 milliseconds (ms), whereas a legacy acknowledgment takes 1.34 ms.

Short Beacon Option

Beacons are typically transmitted at the lowest MCS rate to reach the farthest client devices in the coverage area, but their transmission time is long. Wi-Fi HaLow recognizes two types of beacons sent by an AP: full beacons sent less frequently, and short beacons that carry the minimum information needed to maintain synchronization, use less airtime, and are sent more frequently. Short beacons reduce power consumption for listening devices and free up valuable airtime in the spectrum.

Basic Service Set (BSS) Color

BSS coloring assigns a different “color” to each AP or to each BSS on an AP. This color coding is a simple indicator that enables client devices to pay attention to transmissions matching their BSS color and ignore transmissions from neighboring networks that are not intended for their BSS—that is, transmissions whose BSS color does not match the one with which they are associated. This is intended to reduce medium contention overhead and increase overall capacity, especially in dense IoT networks.

Security

Authentication and Encryption

Wi-Fi HaLow supports the latest and most advanced Wi-Fi security technologies: Wi-Fi CERTIFIED WPA3™ and Wi-Fi CERTIFIED Enhanced Open™, based on Opportunistic Wireless Encryption (OWE). For example, the latter provides privacy in public environments where devices need to access cloud servers. Over time, Wi-Fi HaLow will adopt future Wi-Fi Alliance security enhancements.

High-Speed Symmetric Data Throughput for Over-the-Air Firmware Updates

Wi-Fi HaLow’s lowest effective MCS 10 rate is 150 kbps, suitable for longer-range use. Nearby devices can use higher rates supporting tens of megabits per second. If a device requires new firmware to continue operating effectively and securely, Wi-Fi HaLow can deliver the information quickly and reduce downtime. This offers an advantage over PAN and wireless WAN, whose data rates are much lower—down to 100 bits per second. With such low data rates, these networks cannot provide the ability to respond quickly to threats that require security updates to be pushed over the air to thousands of devices in the field. Devices using these technologies may experience long downtime, or personnel may need to be sent to each device site to perform manual updates, resulting in higher maintenance costs. Wi-Fi HaLow has the security, capacity, coverage, and data rates to support over-the-air updates with minimal downtime.

Competitive Analysis of IoT Technologies

The Wi-Fi Alliance compared Wi-Fi HaLow with other IoT technology options, analyzing data rate, coverage, battery life, ease of IP network integration, energy efficiency, and scalability. These attributes are defined in the table below.

The technologies analyzed include:

• Unlicensed LPWAN: LoRaWAN, Sigfox, and Wi-SUN

• PAN and LAN: Zigbee, Bluetooth Low Energy, Z-Wave, and 2.4 GHz Wi-Fi

• Licensed WAN: NB-IoT, LTE-M

Before Wi-Fi HaLow, wireless IoT systems were typically designed around meeting a key operating parameter (such as long battery life) at the expense of other parameters (such as longer range, simplicity, or high data throughput). Low-power systems implemented using PAN technologies such as Bluetooth, Zigbee, Z-Wave, or proprietary radios sacrificed range, speed, or network simplicity.

The information below shows a comparison of Wi-Fi HaLow with other IoT technology options that use these attributes. For this analysis, the closer a line is to the center, the less effective that aspect of the technology is. The farther an attribute line is from the center, the better.

Low-power WAN systems

As shown in Figure 3, LPWAN systems such as LoRaWAN and Sigfox provide long-range connectivity but perform poorly in terms of data rate, scale, energy efficiency, ease of IP integration, or security.

In the analysis, LoRaWAN is represented by light blue lines; it is the only technology close to Wi-Fi HaLow in terms of range, battery life, and scalability. However, it still has many shortcomings in data rate and IP network integration. Similarly, Sigfox also performs very well in range, but compared with its competitors in this scenario, it performs poorly in all other respects.

Compared with other LPWAN technologies, Wi-Fi HaLow excels in five of six aspects. Historically, people have believed that Wi-Fi cannot be used for IoT because it is not energy-efficient. This is no longer true for Wi-Fi HaLow. Wi-Fi HaLow was developed specifically to improve device efficiency; it features the device power-saving capabilities described earlier, enabling device batteries (including coin-sized batteries) to last for years.

Compared with other low-power wide-area networks, Wi-Fi HaLow excels in most measured attributes

PAN and LAN networks

Many types of PAN networks are designed for short-range connectivity. They attempt to achieve longer range through mesh architectures. Mesh architectures create multiple communication paths between devices. Although mesh architectures can simplify initial installation for users, they increase complexity, latency, and cost in IoT environments. Zigbee devices that relay traffic for other nodes within a mesh consume more battery energy and are limited to 250 kbps data throughput. The bottlenecks and latency of mesh architectures can prevent these systems from achieving the simplicity of Wi-Fi. Z-Wave mesh networks are limited to a maximum of 232 devices. Wi-Fi HaLow allows 8,191 devices per access point (AP) and does not require a proprietary gateway to access IP networks.

As shown in Figure 4, although each PAN/LAN technology performs well in terms of installation and operational efficiency, the only technology among the other categories that competes with Wi-Fi HaLow is 2.4 GHz Wi-Fi. This is where managers need to evaluate the primary goals of the IoT environment. When the most important factors are longer range and longer battery life, wall penetration, ease of installation, and scalability to more client devices, Wi-Fi HaLow is the clear choice. Ease of IP network integration and data throughput within an access point (AP) range favor 2.4 GHz Wi-Fi, because the technology is mature and already exists in many access points (APs) today. Adding Wi-Fi HaLow to a Wi-Fi 4, Wi-Fi 5, or Wi-Fi 6 network enables users to meet virtually any requirement on the same IT infrastructure without interference.

Compared with other IoT technology options, Wi-Fi HaLow meets key requirements

Licensed WAN technologies

WAN technologies such as NB-IoT or LTE-M use licensed spectrum from carriers or mobile telephony service providers. Despite the promise of ubiquitous network coverage, these systems add a recurring cost burden to IoT networks by requiring data plan subscriptions to use mobile cellular infrastructure.

Compared with LTE-M, Wi-Fi HaLow achieves significantly higher data rates for clients within an access point (AP) range, reaching a maximum downlink data rate of 4 Mbps using 5 MHz of bandwidth; compared with NB-IoT, it reaches a data rate of 127 kbps using 180 kHz of bandwidth. In this analysis, Wi-Fi HaLow outperforms the other technologies in all respects except coverage (see Figure 5). With a coverage range of 1 km, it is possible for Wi-Fi HaLow to meet the requirements of many IoT use cases within enterprises.

Wi-Fi HaLow provides a low-cost solution for applications in which devices are within an access point (AP) range, and it offers the advantages of longer battery life, easy IP network integration, and higher data rates. Wi-Fi HaLow is a good choice for aggregating traffic in dense deployments of low-power client devices, which can then be routed to the Internet. If a carrier network is unavailable, the Wi-Fi HaLow network can continue to operate as a LAN.

Wi-Fi HaLow's cost-effectiveness, scalability, and native IP support make it an excellent alternative to expensive licensed or proprietary IoT technologies

Comparison of IoT device energy efficiency

The IMEC research team conducted a study comparing the energy consumption of sub-1 GHz wireless technologies. The team conducted two different experiments. One experiment compared the long-range technologies NB-IoT, LoRaWAN, and Sigfox; the other compared Wi-SUN (based on IEEE 802.15.4 Zigbee technology) and Wi-Fi HaLow.

To provide a fair and objective comparison of these five solutions, the study assumed that a 12-byte data packet was sent every 10 minutes. Power consumption was measured during operation of each system. Note that because Sigfox was included in the comparison, this study chose a 12-byte packet, which is very small for Wi-Fi HaLow. In practical applications, Wi-Fi HaLow transmits packets up to 1,500 bytes at a rate 15 times that of Sigfox.

IMEC study assumptions

The following table lists the assumptions used to calculate battery life shown in the IMEC simulation results. They are based on energy consumption for a 10-minute transmission interval: one message every 600 seconds, using an example radio + microcontroller (MCU) semiconductor solution. Note that other Wi-Fi HaLow technology vendors may show different results.

Energy efficiency (bits/joule)

This study defines energy efficiency as how many bits of data can be sent or received using 1 joule of energy. As shown in Figure 6, when comparing key IoT technology options and assuming a 10-minute transmission interval, Wi-Fi HaLow is much more energy-efficient in terms of bits per joule because its transmission time is relatively short. In this study, the other technologies take much longer to transmit a packet of the same length. For example, Wi-Fi HaLow sends 22.4 kb per joule, while NB-IoT sends 3.7 kb per joule. This sixfold advantage for Wi-Fi HaLow translates into longer device battery life.

The study also shows that for simple networks, Wi-Fi HaLow is four times more energy-efficient than LoRaWAN and Wi-SUN. As more nodes join the mesh network, Wi-SUN's energy efficiency decreases slightly, while Wi-Fi HaLow's energy efficiency remains relatively unchanged.

The study demonstrates that Wi-Fi HaLow is more energy-efficient than the other technology options.

Wi-Fi HaLow is at least four times more energy-efficient than several other IoT technology options

Battery life

Based on battery capacity

By understanding the energy consumption of each technology, battery life can be predicted for different battery capacities for each IoT technology. The energy stored in a battery in joules is related to its rated capacity, usually expressed in milliampere-hours (mAh).

The left-hand chart in Figure 7 shows Wi-Fi HaLow battery life for different battery capacities. Many common 3V coin cells have a capacity of 250 mAh. A typical AA 1.5V alkaline battery has an average rated capacity of 2,000 mAh and can be used in larger devices. The results show that in this challenging IoT use case, Wi-Fi HaLow can operate for more than a year and a half on a coin cell while transmitting every 10 minutes. Regardless of battery size, the study shows that Wi-Fi HaLow can support small IoT devices powered by coin cells and outperforms other technologies in many applications.

Wi-Fi HaLow battery life based on battery capacity (left) and transmission frequency (right)

Based on Transmission Frequency

The study focused on comparing technologies using a 10-minute transmission interval. Technologies that require more frequent communication between data transmissions will be affected by reduced battery life. Wi-Fi HaLow has been shown to extend sleep time to longer periods. Features such as TWT and BSS Extended Max Idle enable Wi-Fi HaLow devices to sleep for hours, days, months, or even years between wake-up operations.

Study Conclusions

When comparing bandwidth, data rate, and topology, Wi-Fi HaLow demonstrates clear advantages over Sigfox, NB-IoT, LoRaWAN, and Wi-SUN. In similar use cases, Wi-Fi HaLow is the most energy-efficient compared with these other technologies. The IMEC research team's assessment determined that Wi-Fi HaLow is not only more than four times more energy-efficient than other widely recognized long-range technologies, but also provides higher data throughput than other IoT technology options. The team also determined that Wi-Fi HaLow devices using typical alkaline AA batteries can operate for more than 10 years, longer than the typical battery shelf life.

Summary

Wi-Fi HaLow clearly meets, and in many cases exceeds, the key requirements for IoT connectivity and applications. The ability to accommodate a wide range of IoT use cases in terms of range, data rate, and energy efficiency is critical to the success of IoT technology.

Wi-Fi HaLow's sub-1 GHz operating frequency and narrow channels enable longer range (approximately 1 km) and improved penetration through walls and obstacles. A unique suite of energy-saving features reduces device power consumption, improves energy efficiency, supports multi-year battery operation, and enables coin-cell battery support. It supports a variety of data rates, accommodating use cases ranging from low-data sensor networks to high-data-rate video systems. New Wi-Fi HaLow PHY and MAC features enable each access point (AP) to support thousands of devices while improving radio spectrum utilization.

Because Wi-Fi HaLow is part of the IEEE 802.11 and Wi-Fi portfolio, it is an open standard that delivers more efficient installation and operation without the need for proprietary controllers, hubs, or gateways. Wi-Fi HaLow networks can be deployed in the presence of existing Wi-Fi 4, Wi-Fi 5, and Wi-Fi 6 networks without affecting their RF performance. Native IP support and advanced WPA3™ security make accessing cloud-based applications and over-the-air updates simpler and more secure.

Related comparative studies show that Wi-Fi HaLow is several times more energy-efficient than other wireless technologies. More data can be transmitted per joule of energy, ensuring longer device battery life. Its star-oriented architecture provides long-range connectivity and eliminates the data bottlenecks and latency associated with mesh networks. As part of the complete Wi-Fi portfolio, Wi-Fi HaLow offers a more comprehensive connectivity approach and extends current Wi-Fi coverage to hard-to-reach places such as garages, basements, attics, warehouses, factories, and large outdoor areas. Wi-Fi HaLow enables network designers to deploy IoT networks using a single, standards-based, IP-ready, and reliable architecture without sacrificing simplicity and efficiency.

This article is reposted from Xinzhihui.