5G will usher in a new era. Is the RF front end ready?

The rise of big data, driverless cars and the Internet of Things has made the industry chain's desire for 5G reach an unprecedented level. Considering the advantages of the new generation of mobile communications in speed, bandwidth and latency, this attention is not surprising.

From a technical perspective, 5G mobile networks are the next major stage in mobile telecommunications standards following the current deployment of 4G LTE. According to the ITU definition, 5G's peak rate can reach 20Gbit/S, the user experience rate can reach 100Mbit/s, and the latency is at the millimeter level.

5G performance defined by ITU (source: IUT)

Although the industry has high hopes for 5G, the 5G-related standards have not yet been implemented. According to the IHS report, a lot of 5G standard work is proceeding as planned.

3GPP is working on Release 15, which will be completed in 2018 and is expected to be the first specification for the new 5G radio air interface (5G NR) and the next-generation network architecture (5G NextGen). 5G development work will continue to 3GPP Release 16 and beyond, but Release 15 will provide global specifications for 5G commercial use starting in 2019. At the same time, the work currently being carried out by 3GPP will be submitted to the ITU before the official release of the IMT-2020 specifications, which will be completed in 2020. It is worth noting that while these specifications are being gradually completed, pre-standard 5G commercial deployments will start earlier.

5G standardization timeline (source: IHS)

It is foreseeable that in such a high-speed application environment, 5G will be a difficult challenge for upstream semiconductor suppliers, such as faster processors, suitable baseband and RF devices.

5G brings great challenges to RF front-end

Before talking about the challenges of 5G, let us first understand what the RF front end is.

By definition, RF FEM refers to the entire system from antenna to modem. This part includes filters, LNA, PA, switches, duplexers, etc.

Schematic diagram of the mobile communication system structure (source: Guosen Securities)

Looking back at the development process from 1G to 4G, the uplink and downlink speeds of wireless networks are constantly increasing, and the radio frequency of equipment is also gradually evolving. In the 4G era, due to the need for multi-mode and multi-frequency, the complexity of the radio frequency front end has increased significantly. Facing the upcoming 5G, this challenge is unprecedented. Because from 4G to 5G, the wireless transmission speed has increased by leaps and bounds.

According to the relevant theories of wireless communication, such speed increase can be achieved by increasing spectrum utilization or spectrum bandwidth. However, from the current status of wireless applications, the commonly used frequency bands below 5 GHz are already very crowded. In order to obtain spectrum resources, the industry has no choice but to turn its attention to higher frequency millimeter waves. After solving the frequency band problem, RF must also keep pace.

Definition of millimeter wave (source: Qualcomm)

Qorvo, a well-known RF supplier, said that in order to meet the needs of 5G, the RF front-end must cover all frequency bands; at the same time, it is necessary to make full use of carrier aggregation (CA) of fragmented frequency bands, higher modulation schemes and Massive MIMO, and it is also necessary to promote the implementation of 5G standards. Only in this way can the industry chain develop more effectively; of course, infrastructure support is necessary.

Schematic diagram of carrier aggregation technology (source: Qorvo)

We can understand the challenges facing RF in the 5G era by breaking down the above technical requirements.

First, let’s talk about carrier aggregation.

Since the ITU has set a maximum downlink rate standard of 50Gbps for 5G, compared with the maximum support of 5 20Mhz carrier aggregation in 4G, the number of carrier aggregations in the 5G era may be as high as 32 or 64. For RF manufacturers, it is necessary to solve the problem of crosstalk. This brings new requirements for filters. In addition to filters, this multi-carrier aggregation will also put forward higher requirements for the linearity of PA and switch devices.

Band 1 and Band 3 dual carrier aggregation frequency crosstalk diagram (source: Qorvo)

Secondly, high frequency is also a challenge for filter transformation.

Before 4G, SAW filters were able to meet the needs of equipment due to their relatively low frequencies. However, as we enter the high-frequency era of 5G, the limitations of SAW become more prominent. BAW filters that maintain high Q values ​​at high frequencies have become the new favorite in the industry.

Comparison between BAW and SAW (source: Qorvo)

Third, 5G leads to changes in PA.

In the 5G millimeter wave era, the high frequency band makes the LDMOS process of traditional PA stretched, but the inherent performance defects make it lose its advantages in future high-frequency applications. Base stations are in urgent need of new process products with high power density, high operating voltage, high frequency and high bandwidth. Therefore, gallium nitride with material performance advantages has become a new outbreak point pursued by the industry. The high-frequency characteristics of 5G make it easy for signals to be blocked, so the use of micro base stations for signal coverage has become a consensus in the industry. Gallium nitride PA happens to perfectly meet the needs of micro base stations.

5G Mobile Communication Base Station Technology Evolution Roadmap

Traditional giants have strong layout

According to a report by Mobile Experts LLC, the market for RF front-ends for mobile devices will reach $19 billion by 2020. Considering the explosion of micro base stations brought about by 5G, this is a new growth driver for RF front-end manufacturers. Traditional giants have already made arrangements in related fields, hoping to get a piece of the pie.

Growth of RF front-ends for mobile devices between 2016 and 2020 (source: Mobile Experts LLC)

Let’s first talk about the layout of traditional giants.

In the past few years, the RF front-end market has experienced a series of acquisitions and mergers. After Avago acquired Broadcom to form New Broadcom, and RFMD and Triquint merged to become the new Qorvo, the giant pattern of the RF front-end has basically been determined.

Among these manufacturers, there are SAW filter manufacturers represented by Murata, Taiyo Yuden and TDK. For the BAW filters that can meet the high-frequency characteristics required by future 5G, there are currently Qorvo and Broadcom, and the two have almost divided up all the current BAW market. In the PA market, Qorvo, Broadcom and Skyworks share the world. Let's focus on filters and PAs:

Source: Guosen Securities

In a RF circuit, the function of the filter is to allow signals of a specific frequency to pass through, and then suppress interference from signals of other frequencies as much as possible. Usually, SAW is used for frequencies below 1900Mhz, and BAW is used for higher frequency bands.

SAW is a surface acoustic wave filter. At the input end, the piezoelectric effect converts wireless signals into acoustic signals that propagate on the surface of the medium. At the output end, the inverse piezoelectric effect converts acoustic signals into wireless signals. SAW filters combine low insertion loss and good suppression performance. Not only can they achieve wide bandwidth, but their volume is also much smaller than traditional cavity or even ceramic filters. However, SAW filters have limitations. When the frequency is higher than 1Ghz, its selectivity decreases. At 2.5Ghz, it can only meet applications with low performance requirements. At this time, BAW is needed.

SAW Filters

BAW is the abbreviation of bulk acoustic wave. The sound waves in this filter propagate vertically and have excellent high-frequency characteristics. It is reported that BAW can solve complex filtering problems due to its favorable performance and steep suppression curve, such as enabling RF spectrum in adjacent frequency bands at the same time without interference. In the face of multi-carrier and spectrum-intensive future 5G high-frequency communications, BAW filters have become an inevitable choice. Qorvo is one of the two major representatives of BAW filters.

BAW Filters

At the "Evison Annual China ICT Forum and 2017 Industry and Technology Outlook Seminar" held in early 2017, Jiang Xiong, Qorvo China Mobile Product Sales Director, said that Qorvo's highly integrated Baw filter can support the Power Class 2 standard, which can not only increase the transmission range by 30%, but also increase the power of mobile phone RF by 3db.

Filter requirements in different regions (source: Qorvo)

Qorvo总裁Bob Bruggeworth在接受微波杂志采访的时候提到，Qorvo独有的的LowDrift™ BAW滤波器技术相比其他声学滤波器技术具有本质上的性能优势，可以帮助下一代智能手机用户显著改进移动数据服务。高性能BAW滤波器能满足严苛的技术要求，如高度的频率选择性、收发频段的狭窄缝隙，确保温度性能一致性等。

Qorvo’s Baw solution (source: Qorvo)

We can also see that Qorvo's advanced filter solutions not only increase performance, but also significantly reduce their size, giving developers much more flexibility when integrating them with PAs, switches and other RF front ends.

Another key component in the RF circuit is the PA (Power Amplifier), which is the power amplifier. Its main function is to amplify the signal. From the current application point of view, the power amplifier is mainly composed of gallium arsenide power amplifier (GaAs PA) and complementary metal oxide semiconductor power amplifier (CMOS PA), among which GaAs PA is the mainstream. However, with the advent of 5G, gallium arsenide devices will not be able to maintain integration at such a high frequency, so GaN has become the next hot spot.

Qorvo predicts that GaAs will remain the mainstream below 8GHz, and GaN will replace GaN above 8GHz. As a wide-bandgap semiconductor, GaAs can withstand higher operating voltages, which means that its power density and operating temperature are higher. Therefore, it has the characteristics of high power density, low energy consumption, suitable for high frequency, and wide bandwidth. Several industry pioneers, including Qorvo, have invested huge amounts of money in GaN research.

Area comparison of GaAs, Si-LDMOS, and GaN solutions (source: Qorvo)

Qorvo said that GaN is favored by markets such as power amplifiers and wireless infrastructure due to its high power density, wideband performance, high power handling, stable input power, and reduced part size and quantity.

According to tests, GaN can emit high power in a tiny area, and the heat received per unit area is more than ten times that of GaAsDE, so it is very suitable for the millimeter wave frequency band that 5G is pursuing.

We need to be clear that GaN devices are not new. They have been used in military radars and cable TV facilities for a long time. However, due to cost issues, they have not been promoted to the civilian field in the past. However, after the efforts of companies such as Qorvo and Macom, the cost of GaN materials and manufacturing costs have begun to decline.

For example, Qorvo announced earlier that it would shift its focus to 6-inch GaN on SiC, which effectively improved its cost competitiveness. However, we should also see that the power consumption problem it brings also needs to be solved by manufacturers.

Sumit Tomar, general manager of Qorvo's wireless infrastructure products division, believes that LDMOS devices have reached their physical limits, which is why gallium nitride devices have entered the market. Base station applications require higher peak power, wider bandwidth, and higher frequency, all of which have contributed to the acceptance of gallium nitride devices in base stations. However, GaN has encountered some obstacles in its entry into mobile phones. Qorvo said that the key factors restricting the use of GaN devices in mobile phones are that gallium nitride devices work in a low-voltage environment, must design new packaging forms to meet heat dissipation requirements, and are too expensive.

In addition to products, we also need to focus on one issue, that is, standards. Because no matter what device you make, you need to strictly refer to the standards set by 3GPP to develop high-quality products that meet the needs. For many companies, if they can participate in the formulation of standards, they can not only promote their research results to the standard setters, but also benefit themselves from a certain perspective. Many manufacturers have also participated in this, and Qorvo has also started early in this regard.

As a leading RF supplier, Qorvo believes that this expansion of network capacity can be categorized into two categories, based on the characteristics of spectrum and mobile or fixed wireless access. In the short term, the upgrade of mobile base stations (4.5G) operating between 2.6 GHz and 6 GHz will improve the customer experience through additional channel bandwidth. In the long term, true 5G mmWave fixed wireless access will drive and significantly increase bandwidth for fixed users. Both applications bring huge RF challenges to the industry.

As a full voting member of 3GPP, Qorvo can provide advice to the standards body on 4.5 and 5G RF solutions. As 5G standards evolve and global spectrum is allocated, this puts them in an advantageous position to provide a wide range of 5G connectivity solutions. Their current strategy is to support 4.5G deployments as well as the evolving 5G ecosystem.

Although 5G millimeter wave is still in the early stage of technical verification, Qorvo is also actively working with global network providers to conduct 5G field trials to prepare for the initial network deployment in 2020. So far, Qorvo has participated in more than 20 customer field trials. For example, in February this year, Qorvo joined China Mobile's 5G Joint Innovation Center, becoming the first RF front-end company to be absorbed by China Mobile. This will be of great help to the productization and commercialization of 5G RF in the future.

Chinese manufacturers have a long way to go

In the RF front-end market, it is no doubt that the high-end field is basically monopolized by foreign manufacturers, but the progress of Chinese manufacturers in recent years is also obvious to all.

According to a report by Guosen Securities, the manufacturing cost and difficulty of SAW device preparation are high. Therefore, there is a high entry barrier in this industry. At present, most domestic SAW filter manufacturers are still producing products in the public frequency band (lower frequency, below 1GHz). For higher RF operating frequencies (such as the frequency band above 2GHz covered by the current LTE band), there are higher technical requirements for the process, and China cannot catch up for the time being. In the field of BAW, China is almost blank, so in the future 5G era, it is inevitable that it will remain dependent on foreign countries for a long time.

Domestic acoustic filter (mainly SAW process) research and production units (source: Guosen Securities)

In the PA industry, the progress is relatively smooth. A number of manufacturers have emerged, including Hantianxia, ​​Guomin Feixiang, Zhongke Hantianxia, ​​Weige Chuangxin and Suzhou Yique, but they face the same problem as filters, that is, high-end is still a pain in the neck.

In the field of radio frequency, China still has a long way to go.