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In the orthogonal frequency-division multiple-access (OFDMA) communication system, resource allocation plays an important role in improving the transmission performance. The maximum spectral efficiency can be achieved through channel quality measurement, resource allocation, adaptive modulation, and so on. In this brief, we design a partial cached fast Fourier transform (FFT) processor that accounts for the distribution of allocated resources to the users of the OFDMA system. We also design a constellation- and power-aware twiddle-factor multiplier for the variable FFT length and modulation order. We implement a 128- to 1024-point mixed pipelined/cached-FFT processor using a 0.18-??m 1P6M CMOS technology. The chip measurement results show that its energy dissipation ranges from 0.09 to 1.90 nJ per FFT point and scales to the allocated resources in the OFDMA system.