Anadigics attacks the femtocell market with BiFET power amplifiers

Smartphones such as the Samsung Epic 4G and the Apple iPhone are spawning a new generation of gadget lovers. Armed with such a device, it is possible to dip in and out of a vast array of applications while socially networking any time, anywhere.

Thanks to these alluring attributes, smartphone sales are rocketing, and now account for one-fifth of all handset purchases. The owners of these cutting-edge devices are exchanging more and more data as time goes by, so it is not surprising that mobile operator networks are
creaking under increasing strain.

This May Anadigics introduced its first two power amplifier modules for the femtocell market. These single-ended amplifiers feature InGaPbased HBTs and produce an average power of 24.5 dBm

Today’s networks are built around a traditional infrastructure model, which involves blanket deployment of macrocells to a geographic area. Any gaps that the macrocells cannot cover are filled with microcells. Regardless of the local cell technology, calls tend to be routed to their destinations through either terrestrial T1 lines or microwave backhaul, which are typically leased by the network operator.

However, even with this extensive and costly infrastructure in place, many people have either poor or no coverage in their homes and offices. Small cell solutions offer significant relief to this problem.

Complementing today’s networks with small cells, and femtocells in particular, is the best way forward from both an economic and quality-of-service perspective. By placing coverage exactly where it’s needed, the mobile operators will be able to keep pace with the growing customer demand and redirect backhaul traffic to the user’s internet. Imagine a network where the user’s sessions are almost always open. These cells are an attractive option for both upgrading the existing 3G network infrastructure and for deploying 4G. By adding hundreds of thousands of small local cell sites, users will enjoy great coverage in their homes, shops or businesses.

The femtocell market is expected to expand to approximately 49 million access points by 2014, according to the industry organization Femto Forum. By then 114 million users across the globe will be accessing mobile networks via femtocells. Considering that these small cells contain all the functional elements of a traditional base station, the importance of the RF power amplifier (PA) module becomes apparent.

To enjoy significant commercial success in this growing market, the femtocell design must balance features, functionality and pricing. For the PA, these requirements translate to characteristics that include exceptional RF and DC performance, multi mode support, multi-standard support and reliability. The biggest factor that influences all of these characteristics is the semiconductor process used to manufacture the PA itself.

Going with GaAs
GaAs-based chips are employed in the vast majority of mobile handsets, as well as many components for low- tomid-power infrastructure. By building devices around thismaterial it is possible to create amplifiers delivering greatperformance at competitive prices, and there is everyreason to believe that this material will be widely used tobuild the amplifiers deployed during the build-out of 3Gand 4G networks.

For the last ten years or so most PAs have been built from GaAs-based HBTs. Initially these transistors combined GaAs with AlGaAs, but more recently alternatives employing the pairing of GaAs and InGaP have been introduced that offer superior performance.

At Anadigics, which is based in Warren, NJ, we have built upon the huge success of the InGaP/GaAs HBT. Our InGaP-Plus process combines bipolar and FET devices on the same GaAs die, a move that allows features usually residing off of the chip to be integrated into conveniently sized, surface mount parts. The upshot is that switches, step attenuators, power detectors, and voltage regulators are commonly found in our PAs.

Pioneering RF Performance
We are currently designing a family of balanced and single-ended power amplifier modules for use in femtocells, picocells and in-home customer premises equipment. Each of our modules is specifically designed to deliver optimal performance in one or more of the several popular frequency bands used by wireless carriers. While specific features vary from module to module and are based on the target application, the design approach for each is similar.

The availability of a family of devices offers significant advantages to design teams, which may be tasked with providing femtocell products that implement different standards and operate over different frequency bands.

PA module designers want amplifiers that combine linearity with adequate RF power for good coverage and the capability of handling high capacity waveforms with high peak-to-average ratios (PAR). The PAs that we produce excel on all these fronts, uniting high power with outstanding linearity, plus good thermal performance for high reliability. As expected, these products draw on the many years of experience that we have in developing Pas for mobile handsets and broadband infrastructure products.

One of our primary goals is to create modules that combine extremely linear performance with a full complement of functional integration. To realize this ambition, we exploit the native efficiency and broadband capabilities of GaAs devices, and turn to state-of-the-art RF circuit simulation and thermal analysis tools to design the circuitry.

Since small cell products are available in several transmit powers, we developed single-ended and balanced PA modules with common features for each of the popular wireless bands. Earlier this year we released our first Pas for the small cell market, the AWB7123 and AWB7127, a pair of singled-ended parts with average powers of +24.5 dBm.

Before the year is out we will launch balanced equivalents of these modules. These two additions - the AWB7223 and the AWB7227 - operate at frequencies centered on 1.9 GHz and 2.1 GHz, respectively. They deliver a linear output of +27 dBm, which is more than adequate to cover a home or small office space. The modules operate at 4.5 V, and can handle a high peak-to-average ratio (PAR) waveform, making them ideal for networks employing CDMA, WCDMA or LTE technology. All of these products take advantage of the capabilities of our patented InGaP-Plus technology.

The remainder of this article will focus on the higherfrequency, balanced PA module: the AWB7227. Measurements show that this device can deliver a high level of performance when driven with a WCDMA signal (see Figure 1). Even when this module is driven with the most demanding conditions, such as a ‘Test Mode 1’, 64- channel waveform with a PAR of 10.5 dB, there is headroom to the standards requirements. Figure 1 shows there is performance margin to the adjacent channel power (ACP) requirement, so there is no need to back-off the PA from its rated power to meet the ACP requirement at the antenna. Additionally, the module has integrated matching networks so it’s extremely easy to use.

Figure 1. Anadigics has recently developed the AWB7227 power amplifier module with a rated power of 27dBm. This balanced design that is slated for release later this year can be used to amplify the waveforms of the most demanding air interfaces such as CDMA, WCDMA, and LTE. When driven with a WCDMA waveform – test mode 1, 64 channel waveform, and a 10.5 dB peak-to-average ratio – this high-performance amplifier has margin to the ACOP requirement

The AWB7227 is also capable of supporting multi-carrier operation (see Figure 2). This provides deployment flexibility to mobile operators by providing handoff options. This feature will become even more important as the number of femtocells increases.

Figure 2. The AWB7227 amplifier is capable of supporting multiple carriers, a feature that can be useful in certain deployment scenarios. Each carrier represents a group of users operating at a particular frequency. Here, the device has been subjected to 2 WCDMA Test Mode 1, 64 DPCH carriers at maximum separation
 

Future proof technology
As air interfaces evolve, the associated technology must keep pace. The PA module is certainly affected by thesechanges. To prevent the AWB modules from becomingineffective as the latest standards are deployed, we havedesigned them withthe future in mind. Thanks to thisapproach, we have enabled the creation of femtocells that can adapt to standards and support migration and growth strategies.

One of the technologies that is on the horizon is frequency division duplex LTE, which is compatible with our AWB7227. Figure 3 shows the AWB7227 performance with an LTE waveform. Again, there is a healthy margin to the performance required by the standard. When building an infrastructure, particularly one based on femtocells, it is critical to deploy robust components that are capable of reliable operation for many years. This means that power amplifiers must have a long mean time-to-failure (MTTF). The MTTF is primarily dictated by the semiconductor material itself and the junction temperature.

Figure 3. When designing the AWB series, the engineers at Anadigics had one eye on the future. As mobile carrier technology evolves, it is likely that the frequency division duplex LTE protocol will be used widely. The AWB7227 is capable of making this transition, as shown by this measurement with a 10 MHz, fully filled 64 QAM (50RB) test model

Our PA modules are designed for long operating life. They employ HBTs featuring the ternary material InGaP, which have a very strong track record in creating PAs that combine high reliability with extremely high efficiency (see Figure 4). This pairing of attributes has made this type of module a very popular choice for many years in applications demanding similar power levels to those used in femtocell networks.

Figure 4. The power-added efficiency of the AWB7227 is best in class, thanks in part to the inclusion InGaP

Figure 5 shows the thermal scans of the AWB7227 when operated at its rated voltage and driven to its rated power with a CW signal. The measured die junction temperature is just 115 oC, which translates to an extremely high MTTF. The AWB series has been designed to do an excellent job of converting DC input power to usable RF transmit power. Considering that the PA remains one of the key consumers of power in any base station, efficiency is an extremely important parameter. Thanks to the high efficiency of the AWB7227, the overall power consumption of the femtocell will be more manageable, supporting ‘green’ initiatives and enabling other key femtocell features such as battery back-up.

Figure 5. A thermal scan of the AWB7227 power amplifier module reveals a junction temperature of just 115 oC when the device was operated at its rated voltage and driven to its rated power with a CW signal

By carefully selecting materials and applying sound design principles, we are continuing to build a growing portfolio of high-performance PA modules that should lead to the construction of higher-performance femtocells. As the small cell market for both service providers and consumers grow, our AWB series of PA modules will provide a catalyst for ubiquitous coverage from 3G and 4G wireless networks while delivering a high quality of service.