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AlwaysON DFS: Use the intelligent mechanism of DFS channel

Wi-Fi is the main technology for accessing the Internet in homes and businesses. More and more applications such as video and games use Wi-Fi as the main communication medium. In view of the ever-increasing number of high-speed and low-latency applications using Wi-Fi, there is an urgent need for efficient channel management solutions. ON semiconductor has developed SmartScan technology to solve this demand. AlwaysON DFS is just one of the representatives of SmartScan, which incorporates multiple mechanisms to effectively utilize the available spectrum in the dynamic frequency selection (DFS) channel.

Author: Hiten Dalal

Preface

Wi-Fi is the main technology for accessing the Internet in homes and businesses. More and more applications such as video and games use Wi-Fi as the main communication medium. In view of the ever-increasing number of high-speed and low-latency applications using Wi-Fi, there is an urgent need for efficient channel management solutions. ON Semiconductor has developed SmartScan technology to solve this demand. AlwaysON DFS is just one of the representatives of SmartScan, which incorporates multiple mechanisms to effectively utilize the available spectrum in the dynamic frequency selection (DFS) channel.

Introduction to DFS Channel

The DFS channel occupies a portion of the 5 GHz spectrum where existing radars are deployed. These include airport radars, military radars, and weather radars, listed as Terminal Doppler Weather Radar (TDWR) in Figure 1 below.

As shown in Figure 1, there are only two 160 Mhz bandwidth channels available for Wi-Fi in the 5GHz spectrum, and both of these channels are partially or completely composed of DFS channels. In many cases, due to the complexity of acquiring the channel, the DFS channel is the cleanest channel with the least number of devices. In addition, in some regions, such as the European Union, DFS channels support higher transmission power. Therefore, operating in these channels has inherent advantages.

In order to operate in these channels, each Wi-Fi access point (AP) must follow a protocol called Dynamic Frequency Selection (DFS). Table 1 below illustrates the requirements for operation in the DFS channel.

AlwaysON DFS: Use the intelligent mechanism of DFS channel

Figure 1: IEEE 802.11 5 GHz radio frequency band

Table 1: Requirements for operation in DFS channels

Features

Require

Channel acquisition

A channel availability check (CAC) must be performed. This involves listening to the radar pulse of the desired channel without transmission.

The time required for the weather channel to clear the channel can be 1 minute or 10 minutes (TDWR).

In-channel monitoring

Once the channel is cleared and used, the device must continue to monitor the radar. If a radar pulse is detected, the AP must notify all clients of the event and immediately stop all further transmissions and vacate the channel.

Re-enter the DFS channel

After the AP vacates the DFS channel, it must wait for the specified non-occupied time, then execute CAC again, and then re-enter the channel.

AlwaysON DFS: Use the intelligent mechanism of DFS channel

Challenges of operating in DFS channels

Today, the two main challenges facing Wi-Fi devices are to clear the DFS channel without interrupting traffic, and to stay in the cleared DFS channel for as long as possible.

Nowadays, most solutions stop all traffic during the channel acquisition phase of occupying the DFS channel. This is a huge disruption to the service, and in most cases, it is only performed when the AP is idle at night.

Another challenge is that it is impossible to determine on which subband the radar pulse is detected. This is especially important if the device clears channel 106 + channel 122 DFS channels to 160 Mhz. Even if a DFS event is detected, today’s equipment is forced to evacuate the entire 160 Mhz channel, even if the event occurs on channel 106. All work to clear the stricter weather channel (CH122) was wasted because the event occurred on the lower 80 MHz channel.

ON Semiconductor’s AlwaysON DFS features to solve

ON Semiconductor has solved all these problems in its current Wi-Fi 6 solution. Both QSR10GU-AX and QSR5GU-AX Plus chipsets use AlwaysON DFS to solve this problem. Table 2 below summarizes each mechanism included in AlwaysON DFS.

Table 2: Overview of the components of AlwaysON DFS

characteristic

solved problem

instruction

ZeroWait DFS

Channel acquisition

Ability to move one or more antennas away from the channel to perform CAC without interrupting traffic.

Broadband CAC (WCAC)

Channel acquisition

Ability to clear and adjacent 80 MHz DFS channels without interrupting traffic

Subband DFS (S-DFS)

Maximize the occupancy of cleared DFS channels

Able to identify on which sub-channel the DFS event is detected, and compress to the unaffected 80 Mhz channel.

ZeroWait DFS

ON Semiconductor’s Wi-Fi 6 products can bring one or more antennas to non-adjacent channels to perform the channel availability check (CAC) required to occupy the DFS channel. This will not interrupt user traffic. This saves the device from having to wait until late at night to clear the DFS channel. Therefore, in some cases, users can be transferred to cleaner, higher-power channels faster, resulting in an overall better user experience.

An example use case is shown in Figure 2 below. The AP starts on channel 42 (non-DFS channel), and then uses ZeroWait DFS to clear channel 106 without interrupting the traffic on channel 42.

AlwaysON DFS: Use the intelligent mechanism of DFS channel

AP selects non-DFS channel on Boot-up: AP selects non-DFS channel on Boot-up

Operating Channel: Operating Channel

Clears CH106 using ZeroWait: Clear CH106 with ZeroWait

Figure 2: Zero-Wait DFS can clear non-adjacent DFS channels without interrupting traffic

Broadband CAC

Wideband CAC (WCAC) is a unique feature that can seamlessly clear adjacent 80 MHz DFS channels without interrupting traffic. Based on the ZeroWait DFS scenario shown above, once CH106 is cleared, AP can use WCAC to CH122. Please note that CH122 is a weather radar channel and requires 10 minutes of CAC to clear it. The ability to clear CH122 is essential for 160 MHz operation. Figure 3 below shows how to use WCAC to support 160 MHz operation, or just move to CH122 and operate in 80 MHz mode.

AlwaysON DFS: Use the intelligent mechanism of DFS channel

AP Operates in Lower 80 MHz DFS Channel: AP operates on the lower 80 MHz DFS channel

Operating Channel: Operating Channel

AP Clears Weather Radar Channels: AP Clears Weather Radar Channels

AP Expands to 160 MHz Channel: AP expands to 160 MHz channel

AP Switches to Weather Channel: AP switches to weather channel

Still Use DFS Channel: Still Use DFS Channel

Opportunistically use Weather Radar Channel: Use Weather Radar Channel when appropriate

Cleaner Spectrum Less Interference: Cleaner spectrum and less interference

Better Performance: Better performance

Figure 3: Wideband CAC (WCAS) can seamlessly clear adjacent 80 MHz DFS channels

Subband DFS (S-DFS)

Subband DFS is another advanced technology used by ON Semiconductor to determine the 80 Mhz channel where DFS events are detected. This is essential for staying longer in the DFS channel. Similarly, based on the WCAC example above, the AP cleared CH106 and CH122 and now operates on the 160 MHz channel Ch114. The radar pulse is an extremely narrow-band pulse and does not occupy the entire 80 Mhz channel. If there is no S-DFS, the entire 160 Mhz spectrum must be cleared for radar pulses to appear in the Ch106 frequency or the CH122 frequency. Advantageously, using our S-DFS feature, we can even determine in which subband the radar occurs. If it happens in CH122, we only need to compress the bandwidth to 80 MHz and run in CH106 completely. In this way, we can make the most of the time it takes to clear the DFS channel. Figure 4 below visually shows this feature.

AlwaysON DFS: Use the intelligent mechanism of DFS channel

Radar is detected: Radar is detected

AP Operates in 160 MHz DFS Channel: AP operates on 160 MHz DFS Channel

Desired Channel: Desired channel

Operating Channel: Operating Channel

Radar: radar

AP Operates in lower 80 MHz DFS Channel: AP operates on the lower 80 MHz DFS channel

Figure 4: Subband DFS (S-DFS) can identify in which subband the DFS event occurs

Once the non-occupied time of Ch122 expires, we can use WCAC and seamlessly clear CH122 again and resume 160 MHz operation.

Summarize

ON Semiconductor continues to bring innovative features to the Wi-Fi field, resulting in a better overall user experience. Our AlwaysON DFS feature set is the most effective way to clear DFS channels and maximize the use of these channels. The main advantages of this technology are:

・ Supports seamless clearing of DFS channels without interrupting traffic.This brings huge benefits to end users as they can clear the DFS channel at any time of the day, not just at night with low traffic

・ Achieve efficient operation of 160 MHz on the DFS channel, including the use of broadband CAC (WCAC) weather channels. The weather channel is one of the cleanest and most utilized channels available for Wi-Fi.This provides end users with a much better experience when streaming real-time video or playing online games

・ Extends the occupancy rate of the cleaner DFS channel, and can detect on which subband the radar is detected

As the demand for real-time video streaming including 4K and real-time online games continues to grow, ON Semiconductor’s current and future Wi-Fi 6 products provide innovative solutions, through AlwaysOn DFS, to use these channels to get the best Wi-Fi user experience.

The Links:   CM200DY-34A PK110FG160