From the earliest days of the feature phone market where devices were used mostly for talk and text to the smartphones capable of download speeds faster than many home Internet connections, there has been one constant, an underappreciation for the radio frequency (RF) front-end. Most smartphone users today aren’t even aware of what the RF front-end is, but it has remained one of the most critical aspects of mobile handset design since the product’s inception. The RF front-end (RFFE) is the functional area of a mobile handset between the RF transceiver and the antenna, comprised mostly of components like power amplifiers (PAs), low noise amplifiers (LNAs), switches, duplexers, filters, and other passive devices. Without an adequate RFFE, a device simply wouldn’t be able to connect to mobile networks and would be essentially useless to today’s mobile users. A properly designed RFFE is critical to the recent innovation occurring in regards to a phone’s performance, features, and industrial design.
As the smartphone market continues to mature, many of the innovative design changes have taken place in the premium market segment. Early premium smartphone variants had smaller screens, shorter battery lives, and broadband speeds which may not have been adequate enough to stream high definition video or download large files over the mobile network. Users were lucky to get a full workday out of their phones if they were doing much more than talk or text, especially with the first LTE devices. Over the past several years, smartphone user demands have evolved, driven partly by the growing popularity of social media applications like YouTube, Facebook, and Twitter. These applications have increased the production and consumption of user generated content; driving the need for faster and more consistent download and upload speeds. Even though the complexity of the RFFE has increased substantially since the first LTE devices, evolutions of other device features have received a lot more credit in improving the overall user experience, but these improvements have led to an even more challenging RFFE design environment.
Today the experience with activities such as video consumption has improved substantially, so much so that it is one of the more common activities of smartphone users. Consequently, large screen form factors have been growing rapidly as a percentage of smartphone shipments. Smartphones with screens measuring 5 inches and larger comprised about 73% of shipments in 2016, up from only 53% a year earlier. Big screens are typically a drag on battery life which has led to larger batteries as well. The combination of these and other design evolutions have led to less physical space for critical RFFE components. At the same time, it is now more important than ever for the RFFE to be as power efficient as possible because of the screen’s effect on battery life.
Gigabit LTE and the RFFE: It's Complicated
The complexity of the RFFE has increased with every generation of wireless wide area network (WWAN) technology. However, with the latest generation of flagships, there has been a step function in RF content and complexity compared to any previous generation. The move from LTE-Advanced to LTE Advanced Pro may be the most drastic increase in RFFE complexity to date.
One measure of increasing complexity in the RFFE is the number of transmit and receive paths present in a device. This generally correlates with the number of antennae used in RFFE designs and the number of supported spatial data streams. As seen in the images of the S6 Edge + above and the S7 Edge below, the antenna architecture remained relatively unchanged between Cat 6 and Cat 9/12 devices. However, Cat 16 devices will see a clear increase in the number of antennae.
With its expanded carrier aggregation abilities, higher orders of modulation, more complex antenna architectures, increasing number of spatial streams, and LTE-U capabilities, the RFFE of new premium smartphones like the Galaxy S8 and S8+ are arguably the most complex smartphone RF designs deployed at the time of their release.
The Galaxy S8 and S8+ are the first high volume smartphones capable of Category (Cat) 16 LTE which delivers downlink speeds of around one gigabit per second (1Gbps). This is a marked improvement over the previous generation flagship modems which were capable of Cat12 LTE, or downlink speeds of about 600 Megabits per second (Mbps). The faster download speed not only benefits the end-user but also benefits the mobile network operator and other devices on the network. Faster data rates brought on by Cat16 LTE minimize the on-duty cycle of the mobile devices, extending their battery life, but also freeing up network resources via more efficient network interaction. Additionally, operators will be able to leverage unlicensed spectrum via technologies such as LTE-U.
Despite the dramatic increase in complexity of the RFFE, the amount of PCB space allotted to the function has declined over time. In just the past few years premium smartphones have moved from supporting a handful of RF bands to supporting up to 34 bands with a single smartphone SKU in products such as the OnePlus 5. To help accommodate expanded band support in as little space as possible, the RFFE is becoming increasingly modular with more PA, filter, duplexer, switch, and LNA content being incorporated within front-end modules than ever before. As components are combined into modules and the component density of the printed circuit board (PCB) allocated for the RFFE rises, interference between components becomes more of an issue and the challenges of each RF component achieving adequate isolation intensifies.
Notwithstanding the aforementioned consolidation among components in the RFFE, the sophistication required to support an increasing number of frequency bands and faster mobile broadband speeds has resulted in more components and added cost from the RFFE, demonstrating the increased value it provides.
Design techniques and technologies such as 4x4 multiple-input-multiple-output (MIMO) antenna architectures and carrier aggregation are essential in order to achieve the steep step-up in performance brought by Cat16 LTE but also add to its cost. Carrier aggregation is the combining of separate blocks (component carriers) of frequencies to obtain higher bandwidth and throughput. With Cat16 LTE up to 4 component carriers can be aggregated (4xCA) for a total bandwidth of 80MHz and download speeds up to 1Gbps. Before Cat16, only three component carriers (3xCA) were combined for a total bandwidth of 60MHz. The increased bandwidth and higher downlink speed of Cat16 with 4x4 MIMO increase the complexity of the already complex RFFE. One of the biggest impacts is to RF components in the receive chain, especially through increasing filtering and switching content which may be combined into modules with other content such as LNAs.
More than Cat 16 LTE: Slimmer Bezels, Bigger Screens, Better Battery Life
OEMs are under increasing pressure to differentiate their products from one another in order to achieve a competitive advantage in the maturing market. There is a substantial effort from RF suppliers not only to enable Cat 16 LTE but also to prevent the RFFE from becoming an obstacle to innovative design elsewhere in the handset. Components such as modem-assisted antenna tuning solutions have become much more prominent in premium tier smartphones over the past few years and are now commonplace in flagship designs from the leading global OEMs. Antenna tuning has become an important element of the RF front-end and enables smartphones to mitigate environmental and design factors that can interfere with and degrade the smartphone’s RF signal and ability to achieve higher data rates as well as achieve better power efficiency. Without antenna tuning the act of simply holding the smartphone could deteriorate the quality of the RF signal and force the network to downgrade the download speeds to the device. Complicating the matter further are design choices by smartphone OEMs which can have a significant impact on the RF performance of a device. Smartphone designs with a (near) bezel-less display similar to the Galaxy S8 require the antennas to be placed beneath the display. This can interfere with the RF signal and create a challenging RF environment for the antenna. The IHS Markit teardown of a Galaxy S8+ found that the smartphone uses both impedance and aperture antenna tuning solutions from Qualcomm, the QAT3550 and the QAT3514, respectively to increase antenna performance sufficiently to allow the placement of antennas beneath the display. By deploying tuning solutions that use modem intelligence, such as these, OEMs can reduce the size of the antennas, increase overall power efficiency and signal consistency along with the highest possible data speeds.
Power efficiency has been a concern for smartphone designers since the advent of the product category. Along with the screen, one of the biggest drains on the battery of a device is the RF front-end. Implementing the most efficient power amplifiers possible has become more important over time and has led to the widespread deployment of technologies such as envelope tracking. Envelope tracking ICs dynamically adjust the power supply of the power amplifiers in order to achieve maximum PA efficiency. Companies such as Qualcomm and Qorvo supply ET ICs as part of their RF front-end portfolios, and other companies such as Samsung have adopted the technology as well.
In the past a limitation of ET is that it would only work on bandwidths up to 20MHz, however with its newest generation product, the QET4100, Qualcomm has been able to support bandwidths up to 40MHz an important feature for phones capable of 2xCA in the uplink. By doubling the uplink bandwidth users can upload their captured media, such as 360 VR video, faster in high demand venues such as stadiums. Uplink carrier aggregation will lead to better user experience as user generated content becomes increasingly common.
Average power tracking is another technology that is used to increase PA efficiency but is typically less effective at higher frequencies where many existing and new LTE bands are located. Over the past five years, LTE has moved from an environment where most devices operated on frequency bands of 1.9GHz or lower to one where essentially all premium smartphones support frequencies of 2.1GHz and higher. Some of the best opportunities for mobile network operators lie at these higher frequencies; for instance, Sprint has 160MHz of spectrum at 2.5GHz (band 41) in the United States. However, these higher frequencies don’t travel as far and can’t penetrate a building as easily which is why High Performance User Equipment (HPUE) is being deployed. HPUE devices are able to transmit at higher power levels in order to increase a device’s usable range and this scenario is where envelope tracking technology becomes critical.
Enabling the move from 4G to 5G
The progression to 4G+ and 5G new radio (NR) technologies in mobile handsets will not be possible without several technological advancements in the RFFE. The evolution of carrier aggregation to include support for 5xCA with Cat 18 LTE is making it easier for operators around the globe to enable higher aggregated bandwidths leveraging licensed and unlicensed spectrum with technologies like Licensed Assisted Access (LAA) and antenna sharing between LTE and Wi-Fi. In addition, more advanced modulation such as 256 QAM on the downlink and 64 QAM on the uplink is enabling mobile devices to interact with networks more efficiently and achieve the fast download and upload speeds required for an optimal mobile internet experience.
The frequency range of 4G+ will expand to support lower frequency bands at 600MHz and higher bands at 3.5GHz. Some component suppliers are already capable of supporting these new bands through hardware which can then be enabled via a future software update. In general, 4G+ will pose more of a challenge for the receive side of the RF front-end as downstream data rates climb above 1Gbps; however, the additional bands in the wider frequency range will require support from componentry on the transmit side as well, such as power amplifiers.
IHS Markit expects 5G devices to be commercially available by the end of 2019 and the move to supporting 5G NR will have a further strain on the RFFE. Component suppliers will have to add support for new waveforms and a wider range of frequency bands (relating to mobile broadband) from 400MHz to 6GHz, as well as an additional set of encoding. Like the other core smartphone ICs such as the baseband, the RFFE will need to provide backward compatibility to 4G/3G/2G operating modes. Without true system level expertise, the current and forthcoming RFFE requirements will make it even more difficult for component suppliers to prevent the RFFE from becoming a bottleneck in the mobile broadband performance of a device. Suppliers must offer a complete portfolio of components providing OEMs different levels of performance and flexibility to satiate end user demand.