Navigating 5G Complexity: Understanding Power Class 2 in RF Front-End Design

In this discussion, we will only be addressing Power Classes in FR1. In FR1, four different power classes are defined, each tailored to specific device requirements and use cases within the 4G LTE and 5G NR spectrum (PC1, 1.5, 2, 3 and 4). All FR1 frequency bands support Power Class 3 by default, but the support for other classes varies by band.

• PC1—targets fixed-wireless and vehicle-mounted applications as well as high-power scenarios like public safety. FR1 PC1 addresses selected bands with a maximum power of around 31 dBm.
• PC1.5—Targets bands like n41, n77 and n79 address the needs of smartphones and FWA devices. These UE devices have a maximum output power of 29 dBm, which is realized in a 2Tx configuration.
• PC2—Addresses NR TDD intra-band CA and high-power UEs with a maximum transmit power of 26 dBm.
• PC3—This class is a default power class unless otherwise specified. Its maximum transmit power is 23 dBm across all bands.
  • PC3 is optimized for device power efficiency as it is used in mobile handsets.
  • If the power maximum rating is configured with a value of 23 dBm or lower, or the UE has not provided a maximum uplink power class rating, then the UE operates using PC3.

The table below provides the FR1 band and maximum power ratings combined as spelled out by the 3GPP standards organization. The table below is for reference only; please refer to the latest version of the specification using the 3GPP link below this table.

Table 2: FR1 Power Class Ratings from 3GPP as stated here. (Source: 3gpp.org)

A Deeper Look into Power Class 2

As covered in our previous article, PC2 was introduced as a new 4G LTE standard in December 2016 to support high-power user equipment (HPUE) and enhance 2.5 GHz LTE TDD coverage globally. In the article, we noted that higher frequency signals in FR1 have shorter travel distances due to greater propagation path loss compared to lower frequency signals at the same power level. PC2 was developed to overcome this path loss effect without adding expensive towers but still achieving the desired network coverage capabilities. There was also the need for these higher frequencies to penetrate buildings for internal communications.  Additionally, PC2 can improve the received signal strength at a base station under more ideal conditions, allowing the network to achieve better Signal-to-Noise Ratios (SNR) and enable higher order modulations for more data throughput.

Previously, Power Class 3, the standard before December 2016, limited Bands like B41 uplink to 23 dBm to maintain compatibility with older technologies. As shown below in Figure 1, Power Class 2 allowed for output power levels of 26 dBm — increasing the maximum coverage range previously defined by Power Class 3.

Figure 1: Power Class 2 & 3 output power levels in a network.

Despite the increased power, phones with HPUE must still comply with specific absorption rate (SAR) limits set by the FCC and other regulatory bodies to ensure safe radiation levels. (The SAR is a value that corresponds to the relative amount of RF energy absorbed in the head, hand, etc., of a user of wireless handsets.) With TDD systems, PC2 transmissions have been limited in duty cycle to reduce the average amount of energy absorbed. With the introduction of FDD PC2, there is no duplex duty cycle for transmit and receive operation. Therefore, time restrictions on PC2 transmission must be enforced to keep the average energy absorbed over time at an acceptable level.

From an RF front-end module perspective, analyses show that increasing TDD band power to 26 dBm can improve battery life with the associated transmit duty cycles, improve cell-edge coverage and improve throughput under more typical operating conditions. It is expected that the addition of FDD PC2 will further improve network operation and user battery life.

The PC2 RF Challenge

 There are several challenges in supporting PC2 power levels for RF front ends that support higher frequency. The first challenge is efficiency, which directly relates to battery life. The higher power levels require a low-loss post-PA lineup including higher performance acoustic filters. These filters must have low insertion loss while still providing sufficient out-of-band (OOB) rejections. Harmonic and other spurious signal generation in the transmit chain must also be addressed to meet phone-level requirements since these signal levels generally increase with a higher power. Ruggedness is similarly a concern with higher power levels as both filters and power amplifiers can be more easily damaged under high-power, high-VSWR conditions if not properly addressed in the design.

FDD PC2 systems must also address all the TDD PC2 challenges and more. With the increased power levels, receive sensitivity can be impacted if RF parameters like transmit leakage and transmit noise in the receive band are not managed. If a multiplexer required 60dB of transmit-to-receive isolation in a particular band to meet receive sensitivity targets under PC3 transmit power conditions, it is reasonable to assume that an isolation of 63dB may be required with 3dB higher transmit power. Higher efficiency and additional methods of thermal management are also critically important, as increased power levels are maintained for relatively long periods of time.

RF design engineers must minimize matching losses and optimize system link budgets to meet the performance requirements of PC2. Qorvo uses highly integrated modules, antennaplexers, premium switch technology and high-performance RF filters to achieve this goal. Integrated RF modules like Qorvo's RF Flex™ and RF Fusion™ bring together filters, switches and PAs, enabling direct matching and reducing matching loss in both Tx and Rx paths by up to 0.5 dB. This reduction helps achieve the 26 dBm output target without overloading the PA.

Figure 2: Qorvo RF Fusion module.

BAW technology offers superior performance with high Q, low spurs, steep skirts and excellent thermal properties, making it ideal for meeting 5G PC2 requirements and reducing system heat from higher power levels. As 5G devices become more complex, efficient designs are necessary to maintain performance.

Figure 3: Qorvo BAW filter benefits

Qorvo's BAW filters, both discrete and on-module, are essential for addressing coexistence issues with superior attenuation and low insertion loss, crucial for avoiding interference and optimizing B41 link budgets. Additionally, filters for 5G bands n77, n78 and n79 benefit from optimized coupling, which is critical for designing multiplexers in PC2 carrier aggregation (CA) applications.

With the growing complexity of RF front ends, engineers face challenges managing the requirements of multiple CA and regional modes while minimizing antennas. These challenges can be addressed using switches and filters integrated into antennaplexers along with high-performance antenna tuning solutions. These components simplify antenna design with minimal insertion losses, reducing complexity while maintaining optimal coexistence. With PC2 being introduced into more bands, addressing these challenges becomes even more important for next-generation designs.

A Final Word

In the evolving landscape of RF front-end design, the increasing number of bands, higher frequencies and the expansion of carrier aggregation continue to present significant challenges. Power Class 2, while challenging, can be effectively managed by RF engineers when partnered with knowledgeable RF suppliers. High-performance technologies such as Qorvo's BAW filters, along with integrated design approaches like RF Flex™ and RF Fusion™, are essential tools that help handset designers achieve efficiency and success more quickly. These advanced solutions not only reduce matching losses and optimize link budgets but also address complex issues, ensuring robust performance even at higher power levels required by PC2. Overall, Qorvo's innovations in RF module integration play a crucial role in meeting the stringent demands of 4G and 5G in increasingly complex devices.

Additionally, you can find more information on this subject by visiting our Qorvo Design Hub, which has a rich assortment of videos, technical articles, white papers, tools and more. For technical support, 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 wireless 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; Dennis Mahoney (Technical Director of Systems & Architectures), Phil Warder (Principal Design Engineer) and David Schnaufer (Corporate, Technical Marketing Manager) for their contributions to this blog post, ensuring our readers stay informed with expert knowledge and industry trends.