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RF interference and the role of filters in Wi-Fi 6E designs

Widespread availability of Wi-Fi functionality on smart devices, the potential for an overall improved quality of service (QoS), and the ability to stream video to smartphones in dense environments has driven customer retention.

Wi-Fi is a brand name, not an acronym, and the IEEE 802.11 standard defines the protocols that enable communications with current Wi-Fi-enabled wireless devices, including wireless routers and wireless access points (APs). The standards operate on varying frequencies, deliver different bandwidths, and support different numbers of channels. Moreover, the naming convention of Wi-Fi includes a number to add specificity, such as Wi-Fi 6 and Wi-Fi 6E.

With Wi-Fi 5, the effect of multiple devices requiring high-speed wireless connectivity has an immediate impact on the network, resulting in lag time and a drop off in data speeds, negatively impacting user experience. On the other hand, Wi-Fi 6 will retain the same data speed when multiple devices are in use. This increased ability to handle more devices results from two technologies incorporated into Wi-Fi 6: multi-user, multiple input, multiple output (MU-MIMO) and orthogonal frequency division multiple access (OFDMA). Other benefits of Wi-Fi 6 include lower latency because of the better “packing” of data within the signal, improved battery life, and a significant security protocol upgrade in WPA3, the first in almost a decade.

Using the same encoding and channel widths as Wi-Fi 6, Wi-Fi 6E refers to Wi-Fi in support of the new unlicensed, 6 GHz frequency band. The “E” represents “extended” and refers to the 1,200 MHz of unlicensed spectrum in the 6 GHz range opened by the FCC in April 2020 after Wi-Fi 6 was first announced. Significant growth in Wi-Fi 6E is anticipated because of the combination of such large bandwidth and the upgraded technologies of Wi-Fi 6.

Figure 1 The unit shipments highlight the projected growth of Wi-Fi 6E. Source: IDC

Wi-Fi and interference

The increase in unlicensed spectrum available for use by Wi-Fi is an extremely important development for the global proliferation of Wi-Fi. The unlicensed spectrum allocated in the United States will be essential for cellular offload to preserve a positive consumer experience, and many other countries are looking to follow suit. Within a particular set of frequencies allocated for unlicensed use, there is a set of “channels” of specific bandwidth: 20 MHz, 40 MHz, 80 MHz, or 160 MHz. The larger the channel bandwidth, the higher the data speed. Given the total frequencies available at 2.4 GHz, 5 GHz and 6 GHz, the non-overlapping options are limited at 2.4 GHz but dramatically increase at the higher frequencies (Figure 2).

 

Figure 2 Non-overlapping channel packing is crucial in Wi-Fi 6 spectrums. Source: Resonant

The interference challenge for Wi-Fi has two distinct elements—interfering signals from incumbents within the unlicensed band and interfering signals outside of the unlicensed band—which cause harmonics within the band. Dynamic frequency selection (DFS), transmit power control (TPC), low power indoor (LPI), and automated frequency coordination (AFC) define how in-band interference is managed in Wi-Fi.

However, none of these techniques address potential interfering signals outside of the unlicensed frequency band that can produce in-band harmonic “noise,” and worse case, cause saturation of the low-noise amplifier (LNA) and “blocking” of the required signal. This is the function of filtering—to attenuate potential interferers—to a point that they do not generate in-band noise.

Role of filters in Wi-Fi 6E

The proliferation of 4G LTE networks, the deployment of new 5G networks, and the pervasive nature of Wi-Fi are driving a dramatic increase in the number of radio frequency (RF) bands that wireless devices must support. Each band needs to be isolated using filters to keep the signal in the right “lane.” As the volume of traffic increases, the requirements to allow essential signals to pass efficiently will grow, preventing battery drainage and increasing data speeds.

Filters are critical for wide bandwidth and high frequency functions, the most challenging being the new Wi-Fi 6E with a bandwidth of 1,200 MHz and maximum frequency of 7.125 GHz. As more traffic takes advantage of 5G and Wi-Fi in the 3 GHz – 7 GHz frequency range, interference between the bands will jeopardize the coexistence of these advanced wireless technologies and limit their performance. Thus, higher performance filters will be required to maintain the integrity of each band. Furthermore, the limited number of antennas available in both mobile devices and APs will transform architectures to increased use of antenna sharing, which will further escalate filter performance requirements.

Filters must evolve to meet the requirements of new Wi-Fi 6 and Wi-Fi 6E, as well as 5G operation. Filter technologies—such as surface acoustic wave (SAW), temperature compensated surface acoustic wave (TC-SAW), solidly mounted resonators-bulk acoustic wave (SMR-BAW) and film bulk acoustic resonator (FBAR)—from previous wireless applications can be extended in bandwidth and to higher frequency. But this comes at the expense of other critical parameters such as loss and power durability. Alternatively, multiple filters can cover the wide bandwidths, either as a hybrid with non-acoustic filters or in multiple sections.

With newer high-performance filtering, the result will be the higher data speeds, lower delay, and more robust coverage. In the pandemic’s remote work environment, everyone has encountered the experience of stalled video on a Zoom call, lag in video games, and lost connectivity at the house perimeter. New Wi-Fi technology, combined with new wide bandwidth frequencies, protected with advanced filtering will provide the solution moving forward.

These filters facilitate the required wide bandwidth, high frequency operation, low loss, and high-power capability. For instance, XBAR is based on the bulk acoustic wave (BAW) resonator technology. These resonators comprise a single crystal, piezoelectric layer, and a metal interdigital transducer (IDT) on the top surface.

Figure 3 The comparison shows measured performances of XBAR-based filters and hybrid Wi-Fi 6E filter. Source: Resonant

In the Figure 3 comparison, the hybrid integrated passive device (IPD)/FBAR Wi-Fi 6E filter provides interference protection only against signals in the 5 GHz unlicensed band, and not to 5G sub-6 GHz or UWB channels, while the XBAR Wi-Fi 6E filter protects the Wi-Fi 6E band against all of the potential interference issues.

RF filters for Wi-Fi 7

Wi-Fi complements cellular in meeting the demand for capacity and data speeds. Wi-Fi 6 and the massive addition of spectrum makes Wi-Fi even more attractive. However, the coexistence of Wi-Fi and 5G will require filters to combat potential interference issues. And these filters will need to provide wide bandwidth, high frequency operation, low loss and high-power capability.

With the certification of Wi-Fi 7 devices expected in early 2024, the need for filters targeting even more demanding requirements will only be exacerbated. Additionally, the shift in lifestyle and workspace following the pandemic mean there will only be more new device types and data hungry applications.

Mike Eddy is VP of product marketing at Resonant Inc.

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