Femtocells are being developed to improve indoor wireless coverage using a broadband connection, such as digital subscriber line (DSL), coaxial cable, and fiber-optic cable. In the future, these home access points (APs) may be part of a combined cell-phone base station and WiFi home-networking router. According to analysts, initial femtocell deployments will be standalone before they are embedded with other technologies. For dual-purpose use, future femtocells must provide suitable power and linearity with complex modulated waveforms. The key to satisfying these demands lies in a robust, efficient power-amplifier (PA) module. The PA module and the femtocell design in general also must be highly integrated so that the femtocell can support multiple wireless standards and frequencies at a low manufacturing cost.
Many PA manufacturers are already finding ways to deliver such integration and performance. Using its indium-gallium-phosphide (InGaP) -Plus heterojunction-bipolar-transistor (HBT) technology and engineering expertise, for example, ANADIGICS has launched an amplifier module series that is optimized for femtocell applications. The first product in this line is a 4.5-V isolated, fully matched, multi-chip PA module dubbed the AWB7220. It operates from 2.3 to 2.7 GHz with 30 dB gain (Fig. 1). The AWB7220 offers up to +28 dBm output power with an orthogonal-frequency-division-multiple-access (OFDMA) waveform. According to Joe Cozzarelli, the firm's Director of Wireless Infrastructure, that is more than enough output power to cover a home- or small-office space. This three-stage, Class AB PA has a balanced configuration in a 7-x-7-x-1-mm surface-mount package. It incorporates matching networks that are optimized for output power, efficiency, and linearity in a 50- Ω system.
Because OFDMA waveforms are geared toward high-data-rate transmissions, they require lower bit error rate (BER) and error-vector- magnitude (EVM) performance than for voice alone. The AWB7220 was designed to meet these goals while providing extremely linear performance. It can handle high peak-to-average-ratio (PAR) signals like LTE and WiMAX. The module's high efficiency also lends itself to low junction temperatures for higher reliability. According to Cozzarelli, ANADIGICS will be releasing additional products in the AWB line for different frequency bands in the coming months. The next version will include functions like a power detector and step attenuator.
Another analog and mixed-signal semiconductor manufacturer readying PA modules for femtocell base stations is Skyworks Solutions, Inc. Leveraging its InGaP HBT process, Skyworks has crafted several PA modules to cover bands I, II, V, and VII for wideband-code-division-multipleaccess (WCDMA) femtocell base stations. For example, the model SKY65120 is optimized for 2110 to 2170 MHz. This fully matched, two-stage, Class AB module comes in a 20-pin, surface-mount package (Fig. 2). For optimum efficiency and linearity, all input and output matching circuits as well as the active bias for both input and output stages are incorporated in a single galliumarsenide (GaAs), monolithic microwave integrated circuit (MMIC). Key characteristics of the SKY65120 include +25 dBm linear output power, 24.6 dB gain, and 42-percent power-added efficiency (PAE). The module's output third-order intercept point (IP3) is rated at +48 dBm. It delivers +33.5 dBm output power at 1-dB compression over the temperature range of -40 to +85C.
Additional PAs for this application include the SKY65124 and SKY65126. The SKY65124 is optimized for band II femtocell applications, and the SKY65126 is tailored for band V. Both PA modules are pin-to-pin compatible. Operating in the 1930-to-1990-MHz frequency band, the 6-x-6-mm SKY65124 features 24 dB gain, 35-percent PAE, and an output IP3 of more than +45 dBm. It delivers greater than +33 dBm output power at 1-dB compression. The adjacent-channel leakage ratio (ACLR) is rated at -45 dBc for output-power levels above +25 dBm. In contrast, the 6-x-6- mm SKY65126 PA module covers 800 to 900 MHz. It boasts 31.5 dB gain, 33 percent PAE, more than +52 dBm output IP3, and -45 dBc ACLR at output-power levels above +25 dBm. Skyworks also is developing a band VII solution, slated for release this quarter.
For its femtocell base stations, Analog Devices, Inc. has taken a radio-solutions approach. The manufacturer has architected a two-chip CMOS transceiver covering the UMTS bands. It comprises a mixed-signal data-converter chip, the AD9863, which works with the ADF4602-1 front-end RF transceiver. The 0.35-m, CMOS-based AD9863 offers dual 12-b analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) on a single chip. In contrast, the 0.18-m, CMOSderived ADF4602-1 is a direct-conversion, multiband, third-generation (3G) transceiver device. As a result, the AD9863 samples ADF4602-1 receive signals and provides transmit baseband signal to the ADF4602-1. For twostage power amplification, ADI recommends a combination of the ADL5542 gain block and ADL5320 driver.
Unlike the transceiver, the power amplifiers are implemented in an InGaP HBT process. Covering a broad band of 50 MHz to 6 GHz, the ADL5542 offers 20 dB gain with +20.6 dBm output power at 1-dB compression and an output IP3 of +46 dBm at 500 MHz. The input and output are internally matched to 50 Ohms and bias control is on chip. For its part, the ADL5320 covers 400 to 2700 MHz with +25.4 dBm output power at 1-dB compression at 880 MHz. The module is housed in a low-cost SOT-89 package.
To evaluate the performance of these components in femtocell basestation applications, ADI has readied an evaluation board called the EVALADF4602EB1Z (Fig. 3). This board is designed to interface with picoChip's development platform, the PC7202. According to Dale Wilson, ADI's Senior Marketing Manager for the RF Group, output power is +12.5 dBm at the antenna output and meets the femtocell ACLR requirements as per 3GPP WCDMA standard TS24.104.
Semiconductor supplier MAXIM Integrated Products also is offering a radio solution for femtocell base stations in the form of a two-chip transceiver. When the integrated PAa complete transmitter is combined with the accompanying receiver (data converter) chip, a complete radio-transceiver solution is achieved. In compliance with the TS25.104 standard, the MAX2599 is a monolithic direct-conversion intermodulator- quadruature (I/Q) transmitter for WCDMA/HSPA femtocell base stations in the 2110-to-2170-MHz band. Key specifications include +15 dBm power output at the antenna port, less than 6 percent error vector magnitude (EVM), and adjacent-channel power ratio (ACPR) below -50 dBc. The MAX2599 is tailored for European and Japanese markets while the MAX2597 is the North American version.
With increased activity on the femtocell front and trials under way, base-stations sales are expected to ramp up this year. According to market-research firm ABI Research , they will increase to double-digit millions in volume by 2010. Another analyst, In-Stat, projects worldwide femtocell subscriptions to grow to 40 million by 2011, representing a market opportunity of over $4 billion.