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In this paper, we present a low power and variable-length design of fast Fourier transform (FFT) processor for flexible MIMO-OFDM applications. In this work, mixed-radix-2/4/8 algorithm and new continuous-flow method are applied to achieve variable-length of 1K/2K/4K/8K points and in-order output. Furthermore, ping-pong cache memory architecture and optimized data scaling strategy are also applied to reduce main memory accesses up to 50% and achieve higher SQNR requirements of 16/64-QAM signals for multi-standards. A test chip of the proposed FFT processor has been designed and fabricated using CMOS 0.18 mum 1P6M process with the core area of 4.96 mm2. Our proposed processor can perform four independent data sequences of 2048-point FFT within 205.2 mus at operating clock rate of 20 MHz. The power consumption of calculating single 8192-point FFT sequence is only 20.88 mW at operating clock rate of 10 MHz.