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Introduction to WiMAX and its power amplifier test solution
WiMAX (Worldwide Interoperability for Microwave Access) is a wireless communication technology based on the IEEE 802.16 standard. Designed to provide high-speed, long-range broadband access, WiMAX was developed as an alternative to traditional wired connections like DSL and cable. It enables both fixed and mobile broadband access, using OFDM (Orthogonal Frequency Division Multiplexing) and OFDMA (Orthogonal Frequency Division Multiple Access) technologies. These techniques allow for efficient data transmission over non-line-of-sight (NLOS) environments, achieving data rates up to 75 Mbps. The IEEE 802.16-2004 standard defines two physical layer modes: OFDM and OFDMA. In OFDM mode, all carriers are used for data transmission, while in OFDMA, the available carriers are divided into subchannels to serve multiple users simultaneously.
The IEEE 802.16e standard extends WiMAX capabilities for mobile applications, supporting variable carrier numbers depending on the FFT size used (such as 128, 512, 1024, or 2048). WIBRO, a Korean variant of 802.16e, is specifically tailored for mobile broadband. Unlike WLAN, which typically uses fixed bandwidths, WiMAX signals can operate across a variable bandwidth range from 1.25 MHz to 28 MHz, offering greater flexibility in deployment.
When testing power amplifiers for WiMAX systems, several key parameters must be considered. For general-purpose amplifiers, important metrics include power gain, 1dB compression point, and maximum output power. These are usually measured using continuous wave (CW) signals, requiring a signal source with excellent RF performance and power scanning capabilities.
However, for WiMAX-specific power amplifiers, additional parameters are crucial. These include burst output power, frequency error, symbol clock error, EVM (Error Vector Magnitude), ACPR (Adjacent Channel Power Ratio), spectrum flatness, spectrum difference, and spectrum mask. Each of these factors plays a vital role in ensuring signal integrity and minimizing interference in multi-carrier systems.
Frequency error can cause inter-carrier interference (ICI) by shifting subcarrier positions relative to the receiver’s FFT bins. Similarly, symbol clock errors affect subcarrier spacing, leading to ICI and degrading EVM performance. EVM is a critical parameter that reflects the quality of the transmitted signal, measuring the deviation between the actual and ideal signal points.
ACPR measures the distortion introduced by the amplifier, indicating how much power leaks into adjacent channels. Spectrum flatness and spectrum difference assess the uniformity of power distribution across subcarriers, while the spectrum mask ensures that the transmitter adheres to regulatory limits and avoids excessive out-of-band emissions.
To accurately test these parameters, especially for complex WiMAX signals, a dedicated signal generator capable of producing real WiMAX waveforms is essential. While some standard signal sources can simulate WiMAX signals, they often lack the precision required for detailed measurements. This makes a specialized WiMAX signal source indispensable for comprehensive testing.
Rohde & Schwarz offers a complete and efficient solution for WiMAX power amplifier testing. Their system includes a signal source, a spectrum analyzer, and automated software for control and reporting. The signal source, such as the SMU200A or SMJ100A, can generate various WiMAX signals, including 802.16-2004 OFDM, 802.16e OFDMA, and WIBRO. The spectrum analyzer, like the FSQ series, supports accurate analysis of these signals with appropriate options.
The DemoMeas_WiMAX software streamlines the testing process, allowing users to select test parameters and automatically configure the system. It generates detailed reports, improving efficiency and reducing manual effort. The software also provides different setting files for OFDM, OFDMA, and WIBRO standards, enabling users to customize tests for uplink or downlink scenarios.
Overall, Rohde & Schwarz's WiMAX power amplifier test solution leverages advanced signal generation and analysis tools to deliver fast, accurate, and reliable results. By automating complex tasks, it enhances testing efficiency and ensures compliance with industry standards. This makes it an ideal choice for engineers and researchers working on next-generation wireless systems.