RF Power Amplifiers Must Meet Demands of Mobile Wireless Broadband Services
By Glenn Eswein, Director of Product Marketing, Broadband Business, ANADIGICS, Inc.

Voice calls and text messaging may be the most-used services in the wireless world, but they are no longer the new glamour applications. Today’s mobile device users are eagerly adopting a generation of technology and services that include music downloads, e-mail access, data communications, photo transmission, multimedia, Internet browsing, even video capture and transmission. To take full advantage of these services, users demand anywhere, anytime, broadband connectivity.

Carriers are responding with higher-order modulation schemes that deliver the bandwidth to provide these new services. Higher-order modulation presents greater challenges to mobile device designers. For example, it requires lower signal distortion, and uses higher peak-to-average signal power ratios; both are characteristics which place additional performance demands on the transceiver.

Because such modulation schemes can reduce signal coverage, mobile transceivers require higher outputs to achieve the desired reach. Meanwhile, longer transmit times are required to handle applications with large file uploads, such as e-mail and 2-way video. This combination of factors increases battery drain, putting a premium on power efficiency.

The net effect of all of these trends is to place more stringent demands on the performance and efficiency of RF power amplifiers (PAs). Since PAs are the mobile device’s gateway to the network, and can consume a significant portion of its power supply budget, the choice of PA is critical to the success of any mobile broadband product. The device designer must choose a PA that meets the performance requirements of these new mobile wireless broadband systems while at the same time providing a power-efficient solution that extends battery life.

Fortunately, today’s PA technology is quite capable of providing robust performance that meets or exceeds the requirements for the mobile wireless broadband services currently being established. In fact, features such as high linearity, low distortion and high efficiency can already be found in the handful of PA modules now on the market.

One example is the AWM6423 WiMAX RF Power Amplifier module manufactured by ANADIGICS, Inc. (see Figure 1). The AWM6423 is optimized for the 2.5 GHz to 2.7 GHz frequency band used predominantly by the Sprint/Clearwire venture and other service providers in the US, as well as by providers in Japan and Taiwan. Powered by a +3.3V supply, the AWM6423 conforms to WiMAX spectrum mask limits at +23.5 dBm output power, and under these conditions boasts a typical power-added efficiency of 20%. (With a +4.2V supply, the device also meets mask limits at +25 dBm.)

A companion product is available for the 2.3 GHz to 2.4 GHz frequency band used in Korea. The AWM6422 has the same functional integration as the AWM6423, and provides essentially identical electrical performance. Using a +3.3 V supply, the device conforms to WiMAX spectrum mask limits at +23.5 dBm output power and boasts a typical power-added efficiency of 20%. (With a +4.2V supply, the AWM6422 device also meets mask limits at +25 dBm.)

The packaging, layout footprint, electrical connections and application circuit for the AWM6422 are exactly the same as for the AWM6423. This makes it easy to develop mobile devices that support either the US/Japan/Taiwan frequency band or the Korean band, depending on which PA module is selected.

Form factors for today’s mobile broadband wireless transceivers include PC cards and USB dongles for computers, and embedded modules for notebook PCs, PDAs and “smart phones.” Important for all these applications is a compact solution with a small thermal footprint. The ANADIGICS products include low-loss RF impedance matching, which maximizes power efficiency and minimizes demands on thermal design. They also save space by using a patented Bi-FET process called InGaP-Plus™ to integrate several discrete RF functions into a single device. Attenuators and output power voltage detectors included in these power amplifier modules enable small form factor solutions for most mobile applications.

But, in the wireless world progress never stops, and the need for ever-higher efficiency, output power, and levels of integration will undoubtedly spur a continuing evolution of packaging and semiconductor process technology in the years to come. While today’s PA modules meet the needs of today’s mobile broadband wireless networks, a look at the commercial services rolling out this year, and at enhanced applications expected in years to come, will help point the direction in which the technology needs to go.

WiMAX currently is the leading technology for mobile broadband connectivity, and the one for which ANADIGICS developed the AWM6423 and AWM6422. Experts tell us that WiMAX offers higher throughput than today’s 3G data services. In early trials, Clearwire has reported seeing speeds of 5 to 6 Mbps on the downlink and 2 to 3 Mbps on the uplink while going down the freeway. An expected advantage of WiMAX for progressive commercial deployments is that it can be scaled efficiently to support a wide range of subscriber densities.

Based on IEEE 802.16 standards, commercial fixed-point WiMAX services are already available around the world. In Korea, mobile WiMAX services have been offered for the past few years under the trade name “WiBro.” Sprint/Clearwire is rolling out its “Xohm” service, based on WiMAX technology, in the United States this year. And other service providers are establishing WiMAX-based services in Taiwan, Japan, the US, and parts of Europe, Africa, and South America.

Competing technologies, such as Long-Term Evolution (LTE), claim benefits similar to those of WiMAX, but are earlier in the development cycle and thus are not yet commercially available.

From an RF power amplifier point of view, however, the needs of WiMAX, LTE, or any other next-generation technology are similar: higher linearity, lower distortion, better efficiency, smaller size. Today’s power amplifier technology, and solutions such as the two PA modules mentioned above, will lay the groundwork for future progress in mobile broadband wireless services. The obvious question is, “What products and technologies will we need in the future?”

The history of mobile devices presents an unbroken record of escalating capabilities. This translates into more power drain, but less space for circuitry and batteries. With so many new applications being developed, broadband service only compounds the problem. The next generation of PAs must display a trend of increased power efficiency. Add in the size constraints of ever-shrinking form factors, and packaging and functional integration become even more important for future applications.

For improvements in power efficiency, we may see a combination of new process technologies, new signal modulation approaches and increased system-level control of the PA. Advances in semiconductor device technology and the use of new materials (e.g. GaN) may lead to core amplifier devices with inherently better linearity, which in turn relaxes output power requirements, thus reducing power consumption. Modulation schemes that can achieve higher data rates with lower peak-to-average power ratios, or system solutions that minimize the need for the PA to support high peak-to-average power ratios, can also provide improvements in overall system power efficiency.

Although the transmit power requirements of mobile broadband wireless devices will likely limit size reductions for the core devices in power amplifiers, even higher levels of functional integration can provide some benefits. These benefits may be obtained by combining the PA along with additional functional blocks, such as LNAs, switches and filters, into a front-end module (FEM) or front-end IC (FEIC). In a FEM, some of the passive RF components (both discrete components and filters) can be integrated into an organic or ceramic substrate, thus reducing footprint area and perhaps profile height. A FEIC approach provides further integration and size reduction by moving not only passive but also active components onto the semiconductor die alongside the PA. While advanced process technologies are required to support the FEIC approach, it provides a path to very small footprint and very low profile solutions.

For future mobile wireless broadband products to demonstrate increased power efficiencies and levels of integration, new design approaches must view the power amplifier as an integral part of the overall system solution, rather than as a separate stand-alone component. Systems that make the best use of the PA, taking advantage of available integration approaches and the characteristics inherent in specific process technologies, will result in the best possible solutions for tomorrow’s generation of mobile wireless broadband services.

ANADIGICS, Inc.
www.anadigics.com
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