Simplifying LTE Complexity: The First Reconfigurable RF Front End
By Dylan Kelly, VP of Engineering, Peregrine Semiconductor Corporation
The market for LTE devices is growing rapidly, and it is putting unprecedented demands on the performance of the RFFE (radio-frequency front end). ABI Research predicts there will be 375.2 million LTE subscriptions in 2014 and that will increase by more than 60 percent to 588.9 million in 2015. Its research brief “The Connected World of Tomorrow: Predictions for 2014 and 2015” also notes that enhanced capabilities delivered by LTE and other connected devices will spur “global, annual, 4G-mobile-network data traffic to more than double in 2014, reaching 12.4 exabytes.” An increasing number of frequency bands combined with carrier aggregation are being implemented to support this increased data demand, but this radically increases the complexity of the RFFE compared with prior mobile wireless generations. In upcoming RF front-end systems, there will conservatively be more than a 5000 fold increase in the number of possible states of the RFFE (see Figure 1). This 5000x level of RF front-end complexity occurs by multiplying the increasing number of frequency bands, different modulation schemes, power amplifier modes, antenna tuning states and downlink carrier aggregation. With this high level of complexity, a truly reconfigurable RFFE is now a requirement for the industry.
The Elusive Single, Global SKU
The RFFE is comprised of all the components between the transceiver output and the antenna. Traditionally, it has been a collection of products designed independently by a broad range of different vendors on a mix of disparate technologies. This was an acceptable solution before mobile-data demand drove the proliferation of bands and advanced technologies such as LTE and carrier aggregation. Today’s market demands more, but existing RFFE technology has limited OEMs’ ability to deliver a single reference design that works in all global regions — a single, global SKU.
Consider Apple’s recent iPhone 5S launch, which had five SKUs to accommodate different regions. Our research showed that the only obvious difference between the devices was the RFFE content. These findings were confirmed in discussions we had recently with leading industry analyst firms, such as Gartner, IHS iSuppli and Strategy Analytics. Had the technology been available for Apple to release an iPhone with a single, global SKU, imagine the cost savings in engineering, validation, manufacturing and supplier and inventory management.
Using the current, hybrid model of CMOS-based RF switches and antenna tuners combined with GaAs power amplifiers (PAs), it is not possible to increase die-level integration. Now that RFFE complexity is increasing exponentially, integration is critical. Some companies are attempting to solve this by developing complex multi-chip modules that provide incremental improvements but are reaching their technological limitation. Only a truly reconfigurable RFFE will enable a single, global SKU, and a reconfigurable RFFE is only possible if the entire system is based on CMOS.
The CMOS Advantage
For 25 years, Peregrine and its founders have been pioneering RF SOI with the vision of creating an integrated RFFE based on CMOS. The benefits of a CMOS design include the wide availability of CMOS foundries, tight process controls and the ability to monolithically integrate a wide range of functions, including tuning and control functions. Peregrine’s UltraCMOS® 10 technology platform, which launched in October 2013, is a 130 nm RF-SOI technology that delivers twice the RF applications performance due to half the Ron*Coff of competing solutions (see Figure 2).
Peregrine’s UltraCMOS® Global 1 System
Leveraging performance and design enhancements from the UltraCMOS 10 platform, Peregrine has created the first reconfigurable RFFE system that is capable of achieving a single, global SKU. Named UltraCMOS Global 1, it delivers the scalability to easily support higher band counts through low loss switching and tunability high isolation to solve interoperability issues, simple digitally-controlled adaptation across modes and bands and, most importantly, PA performance equivalent to GaAs.
Peregrine’s Global 1 system (see Figure 3) is comprised of:
• 3-path, multimode, multiband PA
• Post-PA switch
• Antenna switch
• Antenna tuner
• Support for envelope tracking
• Common RFFE MIPI interface
The Industry’s First CMOS PA to Meet GaAs Performance
Before now, no vendor has been able to deliver GaAs PA performance in a CMOS PA, which prevented CMOS PAs from competing in the LTE world, where any reduction in performance is deemed unacceptable. The Global 1 system integrates Peregrine’s established, best-in-class RF switches and tuners seamlessly with the first CMOS PA to meet the performance of GaAs PAs. This level of performance is reached without enhancements from envelope tracking or digital predistortion, which are often used when benchmarking CMOS PAs with GaAs PAs.
Figure 4 shows the standard industry comparison of a narrow band PA-performance benchmark of PAE (power-added efficiency) using a WCDMA (voice) waveform at an adjacent channel leakage ratio (ACLR) of -38 dBc. Under these conditions, the performance of the UltraCMOS Global 1 PA is approaching 50 percent PAE. This is on par with the leading GaAs products and exceeds the performance of existing CMOS PAs by 10 percentage points, which represents a 33 percent efficiency increase.
Figure 5 shows that as part of the Global 1 system, Peregrine’s PA performance is not limited just to competitive WCDMA performance but maintains GaAs-equivalent PAE for LTE waveforms. Within the LTE standard, different resource-block (RB) allocations are used based on the channel bandwidth assigned to a user. A 5MHz channel equates to 25RBs, and a 20MHz channel equates to a 100RBs. Again, this data is without the use of digital predistortion techniques or envelope tracking.
Performance Enhancements with Third-Party Envelope Tracking
While the UltraCMOS Global 1 PA reaches GaAs-competitive performance levels without the use of envelope tracking, Global 1 will natively support envelope tracking (ET) and has been designed to support all of the major solutions currently on the market. The PAE at saturated power (PSAT) provides a good indication of what PAE is possible using an ET modulator, however, the efficiency enhancements that ET brings are very band specific. With an envelope tracker, Global 1’s PAE increases by up to 20 percentage points.
One of the biggest benefits of having the entire RFFE on a CMOS platform is the flexibility RF engineers get from its high level of reconfigurability. There are varying levels of reconfigurability — from simple bias control to full RF tuning. Peregrine leverages its expertise in RF antenna-tuning products to design this capability into the Global 1 RFFE. The reconfigurable system enables the performance to remain consistent across frequency. In narrow band solutions, this is rarely a major issue however, for broadband systems, significant roll-off can occur across frequency, which is further compounded by process tolerances, voltage and temperature variation.
The plot in Figure 6 shows the UltraCMOS Global 1 PA being tuned over three different tuning states. This means the tuning state will be selected depending on the frequency of operation in order to enable the best performance. For example, at 790MHz, tuning state 1 would be selected; however, at 860MHz you would use tuning state 2. This becomes increasingly important as a single PA is required to support multiple frequency bands as efficiently as possible. The performance of a typical GaAs broadband PA is shown to provide a benchmark for typical roll-off across the operating frequency range.
Global 1 offers several reconfiguration options in each of the components of the integrated RFFE:
• RF tuning: The PA can be optimized based on the frequency of operation, modulation scheme or power level used. The tuning enables each path of Global 1 to be optimized based on the frequency band, minimizing variations in PAE and linearity across the entire frequency range (see Figure 6).
• Band-specific interface optimization: In many single-band PA-and-duplexer (PAD) modules, the impedance between the PA and duplexer is optimized to optimize PAE. This is not possible in a fixed multiband amplifier, as it needs to support multiple duplexers. With a tunable PA, it is possible to optimize the impedances at this interface for each band, which improves the RFFE’s overall system level performance.
• Biasing on a per-band, per-mode basis: There are significant differences in how PAs need to be biased based on the band and mode of operation. This can simply be controlled through the flexible biasing made possible with a CMOS process.
• Make-tolerance correction: Global 1’s reconfigurable system enables RF engineers to eliminate the make tolerance due to processing at final test. This provides significant improvements in the system performance by removing a majority of the RFFE variation, which allows the system to be designed to a tighter specification.
• CMOS exclusives: Implementing this level of RF tuning and biasing flexibility requires a significant number of control bits and dense interconnect. Global 1 includes a MIPI RFFE control interface, more than 100 control lines, analog drivers and other support circuitry. This is all referenced off the same bias generator, making sure everything tracks with process, voltage and temperature. This level of control is not feasible in a multi-die solution, making it impossible for GaAs to support this level of functionality.
Timing and Availability
After shipping more than two billion switches and tuners, Peregrine has unveiled the industry’s only CMOS PA to challenge GaAs performance. The UltraCMOS Global 1 PA joins Peregrine’s family of the industry’s highest-performing RF antenna switches and tuners, making it the final component required to complete a fully reconfigurable RFFE that will enable a single, global SKU. The PA performance of Global 1 will be demonstrated at Mobile World Congress 2014, platform integration will be completed in 2014 and volume production will ramp up in 2015.
Peregrine’s Global 1 is designed to solve one of the LTE device sector’s biggest challenges — a single, global SKU. It is the first fully reconfigurable RFFE because it is designed on an advanced CMOS platform built with Peregrine’s 25 years of RF expertise. The system is complete today after the introduction of the industry’s first CMOS PA to meet the performance levels of GaAs-based alternatives, and the CMOS platform delivers reconfigurability and performance that make a single, global SKU possible.
RF design engineers will benefit most from Global 1 because it can dramatically reduce the engineering and validation time required, a major benefit in the mobile wireless industry known for incredibly short development cycles. In addition to the advantages RF design engineers can gain from Global 1, its benefits permeate the wireless ecosystem:
• Platform providers can develop a single reference platform, reducing reference design development costs and validation time.
• OEMs can design a single, global SKU, cutting R&D costs, accelerating time to market, streamlining supply chains and improved inventory management.
• Consumers can enjoy longer battery life, better reception, faster data rates and wider roaming range.
The UltraCMOS Global 1 system provides greater flexibility and choice to meet the next wave of global, mobile-device innovation.
For more information, visit our Website.
Peregrine Semiconductor Corporation
this article to a friend!