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The performance assessment of worldwide interoperability for microwave access (WiMAX) systems is dealt with. A fully digital signal processing approach for modulation quality measurement is proposed, which is particularly addressed to transmitters based on orthogonal frequency-division multiplexing (OFDM) modulation. WiMAX technology deployment is rapidly increasing. To aid researchers, manufactures, and technicians in designing, realizing, and installing devices and apparatuses, some measurement solutions are already available, and new ones are being released on the market. All of them are arranged to complement an ad hoc digital signal processing software with an existing specialized measurement instrument such as a real-time spectrum analyzer or a vector signal analyzer. Furthermore, they strictly rely on a preliminary analog downconversion of the radio-frequency input signal, which is a basic front-end function provided by the cited instruments, to suitably digitize and digitally process the acquired samples. In the same way as the aforementioned solutions, the proposed approach takes advantage of existing instruments, but different from them, it provides for a direct digitization of the radio-frequency input signal. No downconversion is needed, and the use of general-purpose measurement hardware such as digital scopes or data acquisition systems is thus possible. A proper digital signal processing algorithm, which was designed and implemented by the authors, then demodulates the digitized signal, extracts the desired measurement information from its baseband components, and assesses its modulation quality. The results of several experiments conducted on laboratory WiMAX signals show the effectiveness and reliability of the approach with respect to the major competitive solutions; its superior performance in special physical-layer conditions is also highlighted.