5G is coming to your phone RF can support 5G

The accelerated development of the 5G standard enables mobile operators to advance their 5G deployment plans, some of which will be completed early next year. According to Strategy AnalyTIcs, once the deployment begins, 5G handsets may become the fastest growing segment of the smart phone industry in the next decade, with shipments increasing from 2 million units in 2019 to 1.5 billion units in 2025. A recent survey showed that almost 50% of consumers may choose 5G smart phones as their next mobile device, partly because the data rate is expected to increase.

However, the frenzy of 5G development has brought a huge RF challenge to mobile phone design. Due to the pressing timeline of the standard setting, the key details of the basic RF specifications remain uncertain, such as power back-off levels, regional band combinations, uplink MIMO, and supplemental uplink (SUL).

Since operators insist that timely inclusion of 5G content in mobile phones is required to complete their network deployment plans, smart phone manufacturers face pressure to formulate implementation strategies to meet challenging requirements, even as regulations continue to develop. 5G RF requirements. These requirements include an unprecedented bandwidth, peak-to-average power ratio, very high power amplifier (PA) linearity, and extensive carrier aggregation-driven frequency congestion.

5G will eventually support many applications. However, the initial implementation of mobile operators focused on enhanced mobile broadband (eMBB) and is expected to increase data rates by up to 20 times faster than today's 4G data rates.

Implementing true 5G technology requires new hardware in smart phones and 5G new radio (NR) infrastructure, rather than just increasing the 4G data rate and reshaping it to 5G, just as it did in the previous 3G to 4G technology transition.

The initial 5G NR specification set was delivered in December 2017 and defined in 5G Phase 1 (3GPP Release 15). These specifications focus on technologies that use non-independent (NSA) 5G NR technology for mobile broadband deployments, which can be used for most early 5G network deployments (Figure 1).

The NSA can be used to accelerate 5G deployments by leveraging LTE anchors for control and the 5G NR band to increase data rates. With this approach, operators can quickly implement 5G speeds by simply extending their existing LTE network without the need to build a new 5G core network. The 5G Independent (SA) specification eliminates the need for LTE anchors and will require the expansion of a full 5G network, which is currently scheduled for delivery one year later (December 2018).

Figure 1. Gradual transition from LTE to 5G deployment.

The Release 15 NSA specification incorporates the many 5G specifications required to begin designing 5G smartphones, including new frequency bands, carrier aggregation (CA) combinations, and key RF characteristics such as waveform, modulation, and subcarrier spacing.

As expected, the specification defines two broad spectrum ranges, namely sub-6 GHz (FR1) and millimeter wave (FR2) frequencies. They include the first set of new 5G FR1 frequency bands (n77, n78 and n79) and will be used in many global 5G deployments (Figure 2). In the long run, many LTE bands have been designated for redistribution into the 5G band, but only a small portion is expected to be used in the near future, including n41, n71, n28, and n66. The version 15 specification also includes more than 600 new CA combinations.

Figure 2. New area allocation for the 5G FR1 band (n77, n78, and n79).

The 5G specification defines two alternative waveforms: CP-OFDM and DFT-s-OFDM. CP-OFDM provides high spectrum encapsulation efficiency (up to 98%) in the resource module and provides good support for MIMO. Therefore, this waveform may be used when the operator prioritizes the highest possible network capacity (for example, in a dense urban environment). DFT-s-OFDM is the same waveform used for LTE uplinks, and its spectrum encapsulation efficiency is lower, but the range is wider (Table 1).

Table 1. Key 5G specifications.

The specification also confirms that although the data rate is improved, the time schedule of 5G mobile broadband is just like LTE, and it will not have any additional impact on the core RF implementation. However, 5G technology greatly reduces the delay, so the antenna switching and antenna tuning have less time available. This may lead to the need to use switch technology that is 10 times faster than 4G in some applications.

Another major change in the 4G to 5G transition is that the handset must support an unprecedented bandwidth. Increasing bandwidth is the basic tenet of 5G: It is the key to achieving higher data rates targeting the new 5G band. The single-carrier bandwidth can be up to 100 MHz, which is 5 times the maximum bandwidth of 20 MHz for LTE (Figure 3), and in the FR1 frequency range, there can be 2 uplinks and 4 downlink carriers to achieve 200 MHz respectively. The total bandwidth of 400 MHz. The challenges of managing this bandwidth are expected to affect the entire RF subsystem so that even the most innovative RF companies will raise their standards.

The challenge for smart phone manufacturers is how to quickly add 5G support to handsets that already have a lot of 4G LTE capabilities, and how to achieve this without delaying the product release cycle or without affecting the goal of achieving global shipments. increase.

Although 5G NSA is key to speeding up 5G deployment, it also greatly increases RF complexity because it requires simultaneous 4G LTE and 5G connectivity.

In many cases, operators are expected to integrate the 4G FDD-LTE band and the 5G band. The NSA specification allows the handset to transmit data in one or more LTE bands while receiving data in a 5G band. This greatly increases the possibility that the transmission frequency harmonics degrade the sensitivity of the receiver.

For example, LTE bands 1, 3, 7, and 20 are integrated with the 5G band n78. Band n78 has a higher frequency range than any LTE band and is extremely wide (3.3-3.8 GHz). Therefore, there is a greater threat that the harmonic frequencies generated by transmitting data on one of the LTE anchor bands will fall into the n78 frequency range, and if the frequency attenuation is insufficient, the receiver sensitivity may be degraded. However, implementing the necessary filtering for CA attenuation may result in increased RFFE insertion loss, which increases the PA output power requirement and reduces overall system efficiency.

In addition, dual connectivity brings other challenges. For example, accommodating two main mobile phone antennas in a mobile phone would be a desirable solution. Simultaneous data transmission in the LTE and 5G bands also creates power management issues and requires the use of an additional DC converter that occupies more space, leaving no room for further expansion of the antenna capacity.

Figure 4 shows this trend, that if the number of key RF function groups increases, the available antenna capacity and number of antennas in a typical flagship smartphone will decrease. As shown in the figure, even with the larger form factor of some 18:9 screen aspect ratio smartphones, the available antenna capacity will be reduced until the ability to add more antennas is limited.

Figure 4. As mobile phone RF content increases, the ability to add antennas will be limited.

"Mandatory 4X4 MIMO has more impact."

The 5G MIMO requirement makes this problem even worse. Unlike 4G LTE, 5G handsets must support 4x4 MIMO in the downlink above 1 GHz when MIMO is optional. This applies not only to new bands (such as n77) but also to the reassigned LTE bands. For example, if Band 3 is re-assigned to 5G NR to become n3, then the handset must now comply with the 5G NR specification. Therefore, the LTE receive diversity requirements (ie, two receive paths) are immediately required for the four receive paths.

For some handset designs that already support optional 4x4 LTE MIMO, this change is not obvious. However, for many other handsets, this change will require a substantial increase in RF content, signal routing complexity, and antenna bandwidth. In general, this means that more content is crowded into the congested space that has been allocated to the RF front end because it requires 4 antennas and 4 separate RF channels. All of these do not even consider the impact of 2x2 uplink MIMO, as specified for n77, n78, n79, and n41.

This architectural change will have many effects. One of the most obvious and crucial effects is that antenna tuning and antenna switching will become more important. Today's smart phones have to rely on antenna tuning to improve radiation efficiency, but antenna tuning will play a greater role in the transition to 5G while helping smartphones by allowing each antenna to efficiently support a wider frequency range. Manufacturers keep the number of antennas within an acceptable range.

According to related notes, duplex signals are common today (for example, low-band and mid-range/high-band signals), but 5G has taken signal routing complexity to a whole new level. Given that the maximum number of antennas starts to stabilize (as shown in Figure 4), ultra-high frequency and dual connectivity uplink requirements will require substantial changes to the way the signal is routed to the antenna. High-performance antenna switches can maximize the number of signal connections while meeting stringent CA suppression requirements while maintaining low insertion loss, and will quickly replace simple duals.

Another effect of all this new RF content is that although the functionality is increasing, the area available for RF implementation is not. Therefore, this trend may accelerate the deployment of integrated RF front-end modules. Highly integrated modules that integrate PAs, switches, filters, and LNAs (such as Qorvo's RF FusionTM) require less space while reducing losses and supporting carrier aggregation.

The unprecedented bandwidth and new waveforms used to achieve high 5G data rates present great challenges for RF power output, power management, and linearity.

Today's flagship LTE handsets typically use envelope tracking (ET) and PA to minimize power consumption. ET optimizes efficiency by continuously adjusting the PA supply voltage to track the RF envelope. However, the envelope tracker is expected to support only up to 60 MHz of bandwidth during 5G deployments, while the new 5G bands (such as the n77 and n79) will support single-carrier transmissions up to 100 MHz bandwidth. Therefore, the PA will need to operate in the average power tracking (APT) fixed voltage mode to achieve broadband 5G transmission while reducing efficiency.

The new 5G waveform highlighted in Table 1 adds to the challenge. Combining higher CP-OFDM peak-to-average power ratio (PAR) and mass channel bandwidth requirements

The PA fallback value is added in 5G to avoid exceeding the specified limit and maintain the linearity required for high quality data links. As a result, transmission chain efficiency may be degraded and PA designs may need to meet challenging high linear power requirements.

This does not seem to be that complicated, and the RF front-end (RFFE) may also need to support LTE in order to achieve backwards compatibility in areas where FR1 frequencies have been used for LTE. To maximize battery life, handset manufacturers want to use ET as much as possible, which means using ET to implement LTE transmissions and 5G signals at frequencies up to 60 MHz. Therefore, the PA must achieve high saturation efficiency in the ET mode and high linearity efficiency in the APT mode. Balancing PA operations between high-bandwidth APT mode and low-bandwidth ET mode presents additional complexity challenges to RFFE vendors. In addition, we need advanced power management to switch between ET and APT modes.

Redistributing the LTE frequency band to meet the 5G NR specification creates additional complexity. In the next few years, many existing 3G/4G spectrum allocations will be gradually re-allocated into 5G NR bands. Prior to completing this transition in each market, smart phone PAs will need to be able to efficiently support 4G and 5G transmissions in each of the bands. It is expected that the full transition to 5G NR across all frequency bands may take a decade or more. The need to support both LTE and 5G deployments in this frequency range adds additional complexity to RFFE.

For example, Band 41 is one of a set of bands that was initially reassigned (reallocated as n41). When used as an LTE band, the theoretical maximum bandwidth is 60 MHz (implemented by aggregating 3 20 MHz carriers), and ET can be used to save power. When used as a 5G band, the single-carrier bandwidth can be up to 100 MHz, and the PA is required to operate in the APT mode; the increase in signal bandwidth also affects the RF filter design.

In addition, in some cases, the number of resource blocks (RBs) allocated per channel bandwidth should be reviewed as part of the 4G to 5G reallocation transition. Many RB limits were established when the LTE specification was first created several years ago; since then, technology and knowledge have evolved to the extent that there is still room for improvement. Mobile operators are very interested in these improvements because they can improve spectrum efficiency. For handset OEMs and RF front-end vendors, this raises the complexity to another level because the RF chain may need to operate in a way that is not included in its original design.

"As in the previous technology transition, solving complex 5G challenges will require innovative RF solutions."

Due to the accelerated development of standards and radical deployment plans, 5G has grown faster than originally expected, thus increasing the pressure on smartphone makers to quickly adjust their handsets to support 5G. The new standard brings unprecedented RF challenges, and while these specifications continue to evolve, existing system knowledge and expertise must still be used to estimate the impact on RF design. In addition, the mobile industry faces a series of unprecedented challenges due to the limitations of the smart phone's external dimensions. As in previous technology transitions, solving complex 5G challenges requires innovative RF solutions. RF vendors must increase standards in key areas such as PA design, RFFE module integration, antenna tuning, and antenna switching. These core 5G capabilities will play a vital role in helping mobile phone OEMs release on time data-centric mobile devices that are essential to consumer life.

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