SMUL-FFT: A Streaming Multiplierless Fast Fourier Transform

Beamspace processing is an emerging paradigm to reduce hardware complexity in all-digital millimeter-wave (mmWave) massive multiple-input multiple-output (MIMO) basestations. This approach exploits sparsity of mmWave channels but requires spatial discrete Fourier transforms (DFTs) across the antenna array, which must be performed at the baseband sampling rate. To mitigate the resulting DFT hardware implementation bottleneck, we propose a fully-unrolled Streaming MUltiplierLess (SMUL) fast Fourier Transform (FFT) engine that performs one transform per clock cycle. The proposed SMUL-FFT architecture avoids hardware multipliers by restricting the twiddle factors to a sum-of-powers-of-two, resulting in substantial power and area savings. Compared to state-of-the-art FFTs, our SMUL-FFT ASIC designs in 65nm CMOS demonstrate more than 45% and 17% improvements in energy-efficiency and area-efficiency, respectively, without noticeably increasing the error-rate in mmWave massive MIMO systems.