How to Improve Wi-Fi Performance Using BAW Filter Technology

In an increasingly connected world, the demand for robust and reliable wireless communication has grown exponentially. Wi-Fi Customer Premises Equipment (CPE) access points play a crucial role in maintaining seamless connectivity across various devices. However, the dense occupancy of the Wi-Fi Tri-band RF spectrum and the inherent challenges in multi-band operations necessitate the use of specialized filters in CPE access points (APs). These filters ensure that Wi-Fi signals remain strong and stable, even in the presence of potential sources of interference. This article explores the critical need for filters in CPE APs, delving into the challenges posed by the RF spectrum, self-generated interference, and the technical requirements of these filters.

Figure 1: The many devices inside a home that can cause Wi-Fi AP congestion and interference challenges.

As shown in Figure 2, tri-band Wi-Fi 6 and 7 offer increased bandwidth, relieving congestion and enhancing performance, along with offering greater capacity to provide consumers with a larger data pipeline. However, this also creates more opportunities for interference, which can degrade performance across various applications.

Figure 2: Tri-band AP Wi-Fi spectrum.

Using RF filters in these more advanced tri-band AP applications not only mitigates signal interference but also plays a vital role in extending coverage, enhancing frequency performance, and increasing network capacity. They also address key design challenges faced by engineers developing Wi-Fi routers for congested RF environments.

Self-Generated Interference in Multi-Band Routers

While this tri-band capability (i.e., 2.4 GHz, 5 GHz, and 6) enhances network flexibility and performance, it also introduces a significant challenge: self-generated interference. This self-generated interference arises whenever a multi-band AP transmits. Since the multi-band transmitters are located in the same product as the receivers in the AP, mitigation of cross-band interference must be a major design consideration.

Figure 3: Access point showing many areas where self-generated inference can occur.

For example, as shown in above Figure 3, when an AP transmits signals across multiple bands simultaneously, strong RF signals from one band can interfere with others. This is especially problematic because Wi-Fi receivers are highly sensitive to detecting weak signals, making them more susceptible to interference from both external sources and other bands within the AP. This phenomenon can cause receiver desensitization.

RF Receiver Desensitization: A Critical Concern

Intra-device coexistence issues arise when multiple radios in a system interfere with each other.  This interference, combined with external AP transmit signals, increases the noise power at the affected receiver, degrading the signal-to-noise ratio and leading to reduced receiver sensitivity, or "desensitization." This results in dropped or interrupted wireless connections.

While desensitization has long been an issue, it's especially problematic today for devices like smartphones, Wi-Fi APs, IoT and Bluetooth systems. There are two effective ways to prevent this: providing sufficient isolation between transmit and receive signals and using RF filters. While coexistence filters to reduce desense are common in smartphones and client devices, their use in Wi-Fi APs is becoming increasingly important. 

Most RF chain antenna designs provide 20-30 dB of isolation between interfering and intended signals to mitigate desense. However, to maintain good throughput, interfering signals should not exceed -70 to -90 dBc, meaning designers need an additional 40-60 dB of isolation in the Wi-Fi front end. Filters play a critical role in achieving this.

Technical Requirements for RF Filters

To ensure that a Wi-Fi AP operates efficiently and reliably, it must be equipped with filters that meet specific technical criteria. An adequate filter for a CPE AP should possess the following characteristics:

• Low Insertion Loss: Insertion loss refers to the reduction in signal strength as it passes through the filter. A low insertion loss is crucial for both the transmit and receive paths. On the transmit side, minimizing insertion loss reduces heat generation and power consumption, which are critical for maintaining the AP's efficiency. On the receiving side, low insertion loss is essential for preserving receiver sensitivity, which directly impacts the router's coverage area and performance.
• Steep Filter Skirts: The filter skirts define how rapidly the filter's response transitions from low insertion loss to high rejection in the frequency domain. Steeper filter skirts are desirable because they allow for better coexistence with neighboring frequency bands, both within the Wi-Fi spectrum and with external bands. This characteristic is particularly important in densely populated RF environments, where precise filtering is necessary to prevent interference.
• High Rejection Levels: Rejection refers to the filter's ability to attenuate unwanted signals and prevent them from interfering with the desired signal. Adequate rejection is necessary to minimize receiver desensitization, especially in the presence of out-of-band interference. A filter with high rejection levels ensures that the Wi-Fi receiver remains sensitive and reliable, even in challenging RF environments.
• Small Size: In addition to their technical performance, filters used in CPE APs must be small and cost-effective. As APs become more compact and manufacturers strive to reduce costs, finding filters that meet these criteria while still delivering high performance is a significant challenge.

Figure 4: Typical RF filter response.

Qorvo's Best-in-Class Filters

Qorvo, a leading provider of RF solutions, has developed filters that excel in meeting the demanding requirements of modern CPE APs. One of the key factors contributing to the superior performance of Qorvo's filters is their use of Bulk Acoustic Wave (BAW) technology. BAW filters are known for their high-quality factor (Q), which is a measure of the filter's efficiency in terms of insertion loss and the steepness of its skirts.

Figure 5: RF filter Q factor resonator response.

Qorvo creates compact, cost-effective filter designs that fit all Wi-Fi applications. Figure 6 shows the placement of two such filters. Qorvo's BAW filters have the highest Q in the industry, allowing them to achieve low insertion losses, steep filter skirts, and high rejection levels. These characteristics make them ideal for use in CPE APs, where the need for precise filtering is paramount to ensure reliable and high-performance Wi-Fi connectivity.

Figure 6: RF Filtering used in a typical Wi-Fi frequency AP plan.

Qorvo’s filters are designed to address the increasingly stringent requirements for coexistence between different Wi-Fi bands, sub-bands, and external systems like cellular networks. In support of this coexistence and compliance with out-of-band restricted emissions regulations, Qorvo's filters ensure that Wi-Fi APs operate efficiently without compromising the performance of neighboring bands.

This is particularly critical when transmitting over wider Wi-Fi channels, such as a 320 MHz channel in the UNII-5 band, commonly used in the 6 GHz spectrum. In this scenario, an AP broadcasts the lowest channel, channel 31, which spans from 5945 MHz to 6265 MHz, as shown in Figure 7 below. Without a filter, as seen in the light blue trace of the graph, significant spectral regrowth occurs both above and below the 320 MHz waveform. This regrowth represents noise that spills into adjacent frequency bands, such as UNII-2c and UNII-3 in the 5 GHz band. Such noise would desensitize the 5 GHz receivers, rendering them ineffective.

Figure 7: Wi-Fi channel 31 response using analyzer showing with filter and without filter.

Introducing a bandpass filter, shown in Figure 7, results in a much cleaner signal with significant noise reduction in the 5 GHz bands. The filter's high rejection characteristics, particularly in the lower frequencies of the 320 MHz waveform, allow the AP to maintain signal clarity and minimize interference with the adjacent Wi-Fi bands. A similar situation can be demonstrated with a 5 GHz Bandpass Filter with a Ch 155 transmission, as shown in below Figure 8. The ensuing noise level is much lower when a filter is present, ensuring stronger signal integrity.

Figure 8: Wi-Fi channel 155 response using analyzer showing with filter and without filter.

However, not all filters are suitable for this type of application. A filter with a very high Q-factor is necessary to achieve minimal insertion loss while ensuring steep skirts, meaning the transition from low insertion loss to high rejection happens within a narrow frequency range. This steep rejection is essential not only for preventing interference with other Wi-Fi bands but also for adhering to regulatory standards, such as the out-of-band restricted emissions imposed by the FCC in the United States.

RF Filters - A Critical Component in Future CPE Applications

As the RF spectrum becomes increasingly crowded and the demand for high-performance Wi-Fi continues to rise, the need for effective filters in CPE routers has never been more critical. These filters play a vital role in mitigating both external and self-generated interference, ensuring that Wi-Fi signals remain strong, stable, and reliable. By incorporating high-quality filters, such as those offered by Qorvo, CPE access points can achieve optimal performance, providing users with seamless and uninterrupted connectivity in even the most challenging RF environments.

For more on this topic and solutions, we encourage you to view these collateral pieces – Wi-Fi 7 & Matter Ratification: What You Need to Know, 4 Ways to Address the Most Common RF Filtering Challenges for Modern Applications, Exploring Automated Frequency Coordination (AFC) in the Wi-Fi 6 GHz Realm, or read our RF Filter Technology For Dummies book. Additionally, you can find more interesting collateral on this subject by visiting our Qorvo Design Hub for a rich assortment of videos, technical articles, white papers, tools and more.

For more information on this and other Qorvo 5G and 6G base station design solutions, please visit Qorvo.com or reach out to Technical Support.

About the Authors

Our authors bring a wealth of technical expertise in developing and optimizing Wi-Fi solutions. With a deep understanding of customer needs and industry trends, they collaborate closely with our design teams to drive innovation and deliver cutting-edge solutions that support industry-leading products

Thank you to our main contributors of this article; Chris Levesque (Principal Systems Architect), Jeremy Foland (Sr. Marketing Manager), Kathy DaSilva (Sr. Marketing Manager), Kevin Gallagher (Sr. Product Line Manager) and David Schnaufer (Corporate Technical Marketing Manager).