Band filters between the 5GHz and 6GHz bands may become a prohibiting factor for massive adoption and timely Wi-Fi 6+6E infrastructure designs
As we covered earlier in Use-cases and Derived AP Architectures, Wi-Fi 6+6E infrastructure creates a new type of Tri-Band (2.4+5+6GHz). Such Tri-Band is not an esoteric configuration, it is expected to be the mainstream configuration for any AP to take advantage of the new 6GHz spectrum while continuing to provide service on the existing 2.4GHz and 5GHz bands. Sounds like an easy task, right? We had 2.4GHz + 5GHz concurrent dual bands before, we just need to add another radio for the 6GHz. Unfortunately, there are some system-level technical challenges that need to be addressed beforehand. Band filters are one critical aspect that may easily turn out as a technical and/or commercial prohibitor for such Tri-Band AP designs. This blog covers the topic of band filters for Wi-Fi 6E in detail.
Blocking and non-blocking interferers
Interference can occur between adjacent multiple devices as well as within a single device. The severity of such interference is very different in various scenarios (and thus mitigation of these interferers). For the sake of this blog I would like to outline two distinct cases:
- Non-blocking interferer - This type of interferer can impact the quality of transmission/reception on the channel, e.g. a neighbor AP operating on the same channel as the AP, a neighbor AP operating on the adjacent channel or adjacent band to the interfered AP, an adjacent room wireless audio system, microwave in the kitchen
- Such interferers are characterized by some level of spectral pollution injected into the AP's operational channel, depending on the strength of the interference and its duration and duty cycle, this would in-turn cause the AP to back off (when it detects that the channel is busy), and may cause rate drops or link-errors and data retransmissions
- This is referred to as "spectral coexistence" and Wi-Fi has excelled for many years in coexisting with other Wi-Fi and non-Wi-Fi radios
- Blocking interferer - This refers to an interference that does not enable proper Wi-Fi transmission/reception on the channel.
- This occurs when the wireless radio and interferer are physically co-located, i.e. an AP with both radios operating simultaneously
- On top of the spectral coexistence topics discussed above, a more severe phenomenon occurs: When the interferer transmits, it completely deafens the adjacent radio as its RF receive path (specifically the LNA in the receive path) is saturated. Simply explained – imagine trying to hear someone talking quietly from across the room while a kid is screaming his heart out in your ear... this is a blocker scenario, whereas if the kid screaming is two floors away it may disturb but not prevent you completely from hearing the other person.
Illustrated in the figure below, is a 6GHz Wi-Fi 6E radio that operates on a neighbor's AP B as illustrated in image 1.A below would be a non-blocking interferer to AP A as it is sufficiently far enough from AP B, while a Wi-Fi 6E (in 6GHz) radio operating concurrently with a Wi-Fi 6 (5GHz) radio as illustrated in the Tri-Band AP in image 1.B below would be a blocking interferer.
Let us explain why this is happening, and why is it so dramatic.
What happens when we have two radio systems operating adjacent to one another?
In Figure 2, we have an example of two radios, each operating on a different band - Band A and Band B. Each radio transmits an RF signal through a Power Amplifier (PA) and receives RF signals through a Low Noise Amplifier (LNA). The transmit and receive paths are joined through a Switch (SW) that directs the transmit (Tx) and receive (Rx) into the antenna (this is a scheme of any half-duplex system such as Wi-Fi). When Band A transmits a radio signal through its transmit path, it also propagates into the antenna of the second radio as illustrated by the red dotted line. This interference is of such high power that it completely saturates the Rx LNA of Band B preventing it from any attempt at proper reception even though they operate on different channels and different bands – the LNA input is of such high energy that it is compressed and its output is too distorted to be processed.
Mitigation of this is completed with band filters implemented conventionally on the receive path of each radio as illustrated in Figure 3. Now, the Band A transmit signal is filtered out by the yellow filter, allowing Band B radio to receive a proper signal from an associated device as illustrated by the yellow dotted line.
The above is true for the case of 2.4GHz and 5GHz, 5-Low and 5-High (of traditional Tri-Band APs), as well as for the discussed case of 5GHz and 6GHz. So why is it that the latter case is so complicated? To address this question, we need to take a closer look at the Wi-Fi spectrum:
Figure 4 is an in-scale, proportional view of the Wi-Fi spectrum. From the left, we see the 2.4GHz band, the 5GHz band where IEEE 802.11ac existed, and then the 5GHz UNII4, which had just recently been repurposed by the FCC in a recent Report & Order. Originally the 75MHz UNII4 band was allocated mainly for vehicular applications and Dedicated Short Range Communications (DSRC), in the recent repurposing the lower 45MHz would be used for the expansion of unlicensed mid-band spectrum operations (e.g. for WISP operations based on Wi-Fi technology) while the upper 30MHz kept for vital intelligent transportation system (ITS) operations.
The 2.4GHz band is very narrow with less than 100MHz available spectrum. The 5GHz band is not continuous. It has an isolation band shown in light yellow in Figure 4 (between UNII2 and UNII2 Ext).
The 6GHz band begins right after the 5GHz band at about a span of 1.2GHz in FCC regions; in ETSI regions, only UNII5 is available for the 6GHz band.
Figure 5 below illustrates schematically the different AP configurations and their required band filter profiles.
For previous Tri-Band designs implementing radios on the 2.4 + 5.a + 5.b GHz bands, the two 5GHz radios required more sophisticated filtering. The isolation of the 2.4GHz was implemented in the same way as the dual-band case, while each of the 5GHz bands required a band-pass filter using the 100MHz gap between the UNII-2 and UNII-2-Ext used as the transition band for such filters, this is illustrated in Figure 4 below in AP Type B.
For the future Wi-Fi 6+6E (2.4+5+6GHz) Tri-Band implementation, the required band filters are illustrated in AP Type C below. You may notice that the transition band between the 5GHz and 6GHz bands is much narrower, requiring sharper filters while the pass band required for the 6GHz is quite wide.
Wi-Fi 6 Dual-Band Filter Architecture
A dual-band solution is depicted in AP Type A in Figure 5 above. Having 2.4GHz and 5GHz in one system, the filters have moderate specification requirements since they are so far apart in the spectrum. Furthermore, as illustrated in Figure 5 the upper band filter is not required to isolate and block the 6GHz lower band which further relaxes its design requirements, having the filter cutoff bands very relaxed.
Implementing band filters to isolate between these two bands is a well-known art and is implemented conventionally with an inexpensive diplexer.
Wi-Fi 6 Traditional Tri-band Filter Architecture
A "traditional" Tri-Band filter design is depicted in AP Type B in Figure 5 above. Having 2.4GHz radio, 5GHz "Low-band" (depicted as 5.a above) and 5GHz "High-band" (depicted as 5.b above) operating concurrently in one system. Instead of having one radio that operates in the full 5GHz band, the frequency is separated into two sub-bands and has one operating in UNII1 and UNII2 and the other operating in UNII2Ext and UNII3. Those two radios are hard-wired or limited to serve the lower part of the band and the upper part, respectively. The 5GHz filters for such a configuration are more complex and more expensive than for the case of the dual-band concurrent as they must meet a more challenging specification – there is only the 100MHz restricted band between UNII-2 and UNII2-Ext for the transition. Another factor that affects filter complexity, size and cost is that the higher the frequency, the harder it is to implement sharper filters.
Wi-Fi 6+6E Tri-Band Filter Architecture
A new Tri-Band Wi-Fi 6+6E design is depicted in AP Type C in Figure 5 above. Having 2.4GHz, 5GHz and 6GHz radios operating concurrently. There is no big difference in the 2.4GHz radio filter as was the case in AP Type A or AP Type B above. Implementation of band filters between the 5GHz and 6GHz however is a completely different challenge:
- The 5GHz band filter needs to cover (pass-band) the entire 5GHz band (~700MHz) while blocking the 6GHz near interferer with a very short transition band
- The 6GHz band filter needs to cover (pass-band) the entire 6GHz band (1.2GHz!!) while blocking the 5GHz near interferer with a very short transition band
Additional points to consider:
- As noted, filter design complexity grows as a function of the frequency, it is becoming harder to ensure a smoother pass-band, low insertion loss (attenuation of the desired signal in the pass-band), and sharp transition band in the higher frequencies.
- The filters induce some insertion loss on the desired signal and not only reject the unwanted signal (out-of-band). Initial indications are that the high-frequency filters induce higher attenuation than diplexers or 5GHz sub-banding filters.
- Some initial samples indicate that a cost-sensible filter solution may be at the expense of the first (lowest) 160MHz channel of the 6GHz band – this can be an acceptable limitation on FCC-regulated designs (which have quite a few additional 160MHz channels) but a likely "no-go" for EU like regulations that have only 3 non-overlapping 160MHz channels – not utilizing a third of the available spectrum is a very severe system deficit.
- Another factor that we did not detail yet is the regulations EMI and specifically the out-of-band restrictions that filter designs need to cope with.
- UNII-4 recent FCC Report & Order allows unlicensed WLAN operation in the lower 45MHz – a Tri-Band system aiming to use this spectrum as well would require an even greater challenge as the frequency gap between the 5GHz and the 6GHz spectrum is as little as 30MHz.
The industry is not standing still despite these challenges and new filter technologies such as the Bulk Acoustic Wave (BAW) are providing positive indications – only time will tell if the industry will be able to deliver filters that would not impose technical limitations (insertion loss, channel loss due to transition band), to severe form-factor and design constraints of the boards affecting the size of such designs, and mostly the system bill of materials cost – to enable Tri-Band 6+6E to become the mainstream configuration of Wi-Fi APs.
Learn more about overall AP design considerations for upgrading from Wi-Fi 5 and older generation products to Wi-Fi 6/6E, as well as other engineering topics of Tri-Band Wi-Fi 6+6E AP design considerations in our coming blog.