Lior Weiss
Lior Weiss
Published: June 15, 2022


Dual-band (2.4GHz and 5GHz) Wi-Fi Access Points (APs) and client devices are widely used in homes, businesses, and public spaces. While the client devices typically operate in either one of the two bands at any given time, the APs generally operate on both bands concurrently.

To add capacity and reduce congestion, a few years ago some AP vendors started offering 'Tri-Band' solutions. These APs operate in the same two bands as before but they use two separate channels in the 5GHz band in addition to the one in the 2.4GHz band. So effectively there are three independent Wi-Fi networks (one in 2.4GHz and two in 5GHz) operating with a single AP hence the reference to "Tri-Band". Still what may have been adequate in Wi-Fi 5 with 80MHz, is not practical in Wi-Fi 6 with 160MHz since the 5GHz band is limited in its amount of available bandwidth for the operation of two independent radios in maximum performance.

Tri-Band AP

Fast forward to Wi-Fi 6 era. Around the time of the introduction of the Wi-Fi 6, regulatory agencies around the world released new unlicensed spectrum in the 6GHz band which is more than double the spectrum available in the other two bands. Wi-Fi 6 operation in the 6GHz band is referred to as Wi-Fi 6E to distinguish it from Wi-Fi 6 operation in 2.4GHz and 5GHz bands.

6GHz Spectrum - Tri-Band AP

With this newly available 6GHz band Tri-Band will become the new entry-level for APs that can operate in 2.4GHz, 5GHz, and 6GHz concurrently. A Tri-Band solution with a huge spectrum in the 6GHz band can deliver a much-needed boost in Wi-Fi capacity and higher speeds while maintaining compatibility with older dual-band devices and alleviating congestion in all bands.

Different design and architectural choices lead to different Tri-Band AP solutions.

Tri-Band AP Solutions

In addition to the three bands, the APs need to offer MIMO capability for each band. Together the bands and MIMO combinations present a significant challenge for the chip designers. Operating multiple radios in different bands concurrently can cause significant mutual interference within an AP. For simplicity, some chip designers implement one frequency band per chip and three such chips are used in a single AP for a Tri-Band solution. The required number of interface ports with the host processor in an AP also increases when using separate chips for each band.

Renesas is the only company offering a single chip Wi-Fi 6/6E solution that supports concurrent dual-band operation. Further, it is the only company offering a two-chip Wi-Fi 6/6E solution that supports Tri-Band solutions.

Dynamic Frequency Selection

A challenge for an AP operating in the 5GHz band is that a majority of the spectrum is shared with other incumbent services (e.g., weather radar) that have priority over Wi-Fi operation in those channels. For an AP to be able to use any of these channels, it has to first perform Channel Availability Check (CAC), i.e., scan the spectrum and ensure with high reliability that a channel is not used for radar. The overall channel selection process is called Dynamic Frequency Selection (DFS).

Dynamic Frequency Selection

As per EU regulations, it is possible to have "Zero-Wait-DFS" through the use of "Off-Channel-CAC" which is a scan on a channel different from the channel on which the AP is actually active at that time.

In North America, since the FCC rules do not allow channel pre-clearance, an AP must perform CAC for 1 minute (or 10 minutes for some channels) before using a DFS channel. Further, if a channel being scanned is found to be used for radar, then the AP must switch to another channel and start over the entire CAC process.

Spending one minute, ten minutes, or even more time for CAC by an AP can have a significant negative impact on the entire network including all the devices connected to the AP in the 5GHz band, especially noticeable for latency-sensitive and real-time applications.

Introducing Tri-Band "Plus": Adding a Built-in Listening Radio

The Tri-Band Plus AP operates in the 2.4GHz, 5GHz, and 6GHz bands concurrently while in parallel monitors channels in 5GHz or 6GHz bands. For example, a Tri-Band Plus AP can operate on a non-DFS channel while monitoring a DFS channel in parallel using the built-in listening radio and clear it while meeting the specified reliability and CAC duration requirements. All this is done before switching to the DFS channel without any interruptions to the Wi-Fi operation on its current channels in any of the three bands serving a large number of devices and total throughput in multi-gigabits per second.

An example of such a Tri-Band Plus AP design using the CL8080 and CL8086 chips is illustrated below. In this architecture, the single CL8080 chip provides 2.4GHz and 5GHz concurrent dual-band Wi-Fi 6 operation with 4x4 MIMO in each band. In this example, the single CL8066 chip provides 3x3 Wi-Fi 6E operation in the 6GHz band while in parallel supports a dedicated "Listening Radio" for either 5GHz or 6GHz. Note that only two PCIe interfaces are needed for integration with the host CPU.

Tri-Band Plus AP Design

Renesas is the only company offering a two-chip Wi-Fi 6/6E solution that supports Tri-Band Plus (concurrent transmission over 3 bands plus a dedicated listening radio) solution.

Renesas chips also support a 'Quad-Band' AP solution that offers concurrent transmission over the 2.4GHz band, two independent channels in the 5GHz band, and in the 6GHz band. A Quad-Band AP can dedicate one of the 5GHz or 6GHz bands for the home wireless backbone between the Gateway and an extender without having to share that band with serving clients.

Quad-Band AP Design

Benefits of Dedicated Listening Radio

Beyond the obvious benefit of Zero-Wait-DFS and elimination of CAC interruptions to the normal Wi-Fi operation on the channel on which an AP operates, the Dedicated Listening Radio in a Tri-Band Plus AP offers other important advantages.

Spectrum Agility: Spectrum usage in today's dense networks can very rapidly degrade. Having the ability to refresh the AP's view of the entire spectrum usage faster with the Dedicated Listening Radio can improve the AP's performance enormously. It can yield proactive actions (e.g. transition to a higher power channel if available or to a less occupied channel) and not only reactive transitions (in case interference/blockage of the current channel becomes unbearable).

Interference Characterization: If an AP can monitor a channel for a longer duration, it can characterize the interference better. For instance, it can detect whether another Wi-Fi device is operating on that channel and it can even decode the Wi-Fi transmissions (e.g. the beacons). This can help determine whether the interference is coming from a neighboring AP that is within the same service management scope to enable better coordination. It can also allow the AP to identify non-Wi-Fi interferer types. It is especially important in dense neighboring deployments such as in MDUs and office buildings.

Technical Analysis: Dedicated Listening Radio can be used to monitor a channel and decode the ongoing Wi-Fi packet transmissions and flows over it. This is practically equivalent to a computer equipped with a sniffer radio and software tools that assist engineers onsite in debugging networks and connectivity issues. An AP equipped with a Dedicated Listening Radio can save technician physical presence in the network premise.

For more information, visit our Tri-Band-Plus: Wi-Fi 6/6E APs page.

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Tri-Band-Plus: Wi-Fi 6/6E APs Page Jun 15, 2022