In the fast-evolving world of telecommunications, new technologies are continuously emerging. Among them, the 802.16 standard—commonly known as WiMAX—is gaining momentum and is expected to become a major force in future technology development. WiMAX, or Worldwide Interoperability for Microwave Access, has quickly made an impact across the industry, drawing attention from small rural operators, large service providers, and original equipment manufacturers (OEMs). Its ability to deliver high-speed wireless connectivity has made it a compelling option for both fixed and mobile applications.
Originally designed for fixed broadband communication, WiMAX is now expanding into more dynamic areas such as point-to-multipoint connectivity, cellular backhaul, and enterprise or university-based high-speed LANs. While earlier technologies could offer similar capabilities, WiMAX stands out due to its strong focus on interoperability. This ensures that systems complying with the 802.16 standard can work seamlessly together, giving vendors greater flexibility. Small companies can now enter the market without worrying about compatibility issues, while service providers have more choice in selecting components from various suppliers.
WiMAX operates on several frequency bands, including 3.5 GHz, 5.6 GHz, and 2.5 GHz, which are license-free. Additionally, newer bands like 4.9 GHz and 700 MHz are also being used for WiMAX deployments. Despite the standardization, there are still many variables in system design that influence RF performance. Companies are constantly seeking innovative ways to enhance performance while ensuring compliance with WiMAX standards. The RF chipset must be flexible enough to support multiple implementations and powerful enough to meet all technical requirements.
WiMAX Transmitter
The key performance metric for a WiMAX transmitter is its Error Vector Magnitude (EVM) at a given power level. EVM measures the quality of the digital constellation after transmission. The main contributors to EVM degradation are phase noise from the local oscillator (LO) and distortion from the power amplifier. Since the power amplifier plays a significant role, we evaluate EVM performance at specific output power levels. The reference EVM is set at 2.7%, equivalent to -31.4 dB. This is much stricter than the EVM requirements in cellular or 802.11 systems, where values are typically measured in dB for higher precision.
For Client Equipment (CPE), the outdoor link between the base station and the user is usually clear, so the rated power is around 20 dBm. However, in indoor environments with multipath interference, the power may need to increase to 24–27 dBm. If the base station transmits at 4W to 20W, even higher power ratings may be necessary depending on the distance and system design.
When increasing the system's output power, the power amplifier becomes the primary component to adjust. A larger, more robust amplifier is often required to maintain EVM at -31.4 dB or better. However, this alone isn't sufficient. The standard also sets a strict limit on spurious emissions, requiring them to be no higher than -40 dBm, regardless of the output power.
As output power increases, if the input power from the baseband processor remains constant, the transmitter gain must also increase. This affects not only the desired signal but also unwanted spurious signals. With the parasitic output limit fixed at -40 dBm, an increase in gain reduces the tolerance for spurious emissions. Therefore, maintaining compliance at higher power levels becomes more challenging. To address this, the RF chipset should provide good EVM performance of around -37 dB before the power amplifier and allow for 7–10 dB of tolerance in spurious emissions. This gives designers more flexibility in choosing the right power amplifier while meeting all performance and regulatory requirements.
WiMAX Receiver
The receiver’s key performance parameter is its sensitivity. The standard specifies a minimum bit error rate (BER) of 1E-6, which is essential for reliable communication. In practice, measuring BER directly during RF testing can be difficult, so it is often converted to an EVM value for evaluation. For a 64-QAM signal, a sensitivity of -21.5 dB corresponds to the required BER. To account for real-world variations, a 2 dB margin is typically added, resulting in a target of -23.5 dB for the receiver’s EVM performance.
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