A High Power Multi-carrier Power Amplifier
Power amplifiers are critical radio frequency components of wireless network systems, typically representing 10 to 40 percent of the cost of a basestation. There are a variety of wireless standards associated with 2G, 2.5G and 3G wireless systems, and power amplifiers are designed for each, with frequencies ranging from 800 MHz to 2.4 GHz. Typically, power amplifiers are specifically designed to match the air interface employed for the cellular basestation. However, it is becoming increasingly popular to deploy amplifiers that support multiple air interfaces simultaneously on a frequency band, be it cellular (800 to 900 MHz), PCS (1800 to 1900 MHz) or Universal Mobile Telecommunications System (UMTS) (2100 MHz).
As service carriers migrate to air interfaces that support high speed data, transceiver elements that support these new standards are added to basestation equipment. In order to save on equipment costs, service carriers are attempting to re-use as much of the basestation equipment as possible while running multiple standards. Antenna and feed-line sharing has been used in the past. Recently, the trend is to extend this notion to electronics within the basestation including filters, cables and power amplifiers. The Multi-carrier Power Amplifier (MCPA) specifically addresses this issue. It is designed to meet the stringent air interface requirements associated with 2G, 2.5G and 3G basestation wireless systems.
The MCPA is a unique feed-forward amplifier deploying the industry's most advanced adaptive digital pre-distortion correction technique. Figure 1 shows the MCPA's block diagram. The PA boasts a rated output power level of up to 135 W, as referenced to the RF output connector. The unit is designed for a composite RF, multi-carrier signal having a peak-to-average ratio of 10 dB. The supported instantaneous RF bandwidth is 20 MHz, located anywhere within the PCS transmit band from 1930 to 1990 MHz. In-band intermodulation products are typically < -65 dBc. The typical efficiency at rated power is nine percent.
Fig. 1 The high power MCPA design architecture.
Although the unit may be used in single-mode air interface applications, the MCPA was designed primarily for mixed-mode basestation operations. For example, a wireless carrier may have an embedded base of TDMA users. Because of the growth in wireless usage, the carrier may need to increase the capacity of the TDMA basestation (that is, add carriers) and at the same time introduce high speed data such as General Packetized Radio System (GPRS)/Enhanced Data Rate for GSM Evolution (EDGE) capability. Figure 2 shows the MCPA's output signal spectrum for a typical multi-carrier waveform.
The MCPA features a comprehensive list of built-in test capabilities that are continuously running in the background. The unit continuously monitors transistor currents for proper operation. The amplifier load SWR is monitored for excessive reflections. Input line voltage is continuously measured and processed. Over- and under-voltage conditions also trigger the processor to shut down the amplifier. In addition, the MCPA detects RF overdrive conditions and automatically levels the output power to safe operating conditions.
Fig.2 Output signal spectrum for a multi-carrier waveform.
The product also provides several visual alarms, which are used during site maintenance - overdrive protection, excessive temperature, excessive intermodulation distortion, fan failure, excessive load SWR, DC failure, amplifier gain failure and an enable light-emitting diode (LED). All of the visual indicators are also available as messages over the amplifier communications interface.
The MCPA is designed to interface with all major basestations' operations and maintenance systems. In addition, a field technician or operator can adjust and customize alarm thresholds by downloading a new processor program for the power amplifier.
Designed from the ground up to meet multi-carrier 3G applications, the MCPA achieves an extremely low noise figure, necessary for the high demands of these applications. Specifications for these air interfaces require extremely wide dynamic range transmitter sources, simultaneously having large signals and very low noise levels. The amplifier is designed so that gain adjustment is performed without degrading the overall noise figure of the amplifier system - a feature that is key to achieving the air interface noise requirements. The amplifier is designed to operate in temperature environments ranging from -5° to +50°C, and its integrated cooling is controlled by an internal processor. The fan speed is automatically adjusted in relation to the amplifier operating temperature, thereby reducing current draw and noise emissions in nominal temperature environments. Individual fan speeds are continuously monitored for failure conditions. The amplifier is designed and thermally modeled at the transistor level for safe operating junction temperatures. All active devices are designed to be within safe operating limits over the entire specified operating range. High quality components and proven design techniques ensure the product meets the most demanding aspects of wireless basestation reliability performance.
Manufacturing and Deployment
The unit is designed from the start for high volume manufacturing in terms of assembly and test. Hand-soldered connections are virtually eliminated within the assembly by careful consideration of system partitioning and interfacing. All boards within the assembly are designed for high speed automated functional test during the manufacturing cycle, requiring minimal hand tuning operations, resulting in very high reliability.
The new MCPA is highly scalable and can be deployed in flexible configurations of up to three amplifier modules per sub-rack and up to three sub-racks per 19" cabinet. The unit supports smart combiner technologies for optimized performance when amplifiers are set up in parallel. Each amplifier contains the detection and control functions, eliminating the need for an external sub-rack controller. Smart combiner technology enables systems to operate at maximum available power, regardless of how many amplifier modules are plugged in to an amplifier rack.
Additional information on the MCPA products may be obtained via e-mail at firstname.lastname@example.org. "Celiant" is a trademark of Celiant Corp.
Celiant Corp. Warren, NJ (908) 546-4800
Circle No. 302