Low power reconfigurable FP-FFT core with an array of folded DA butterflies

Abstract

A variable length (32 ~ 2,048), low power, floating point fast Fourier transform (FP-FFT) processor is designed and implemented using energy-efficient butterfly elements. The butterfly elements are implemented using distributed arithmetic (DA) algorithm that eliminates the power-consuming complex multipliers. The FFT computations are scheduled in a quasi-parallel mode with an array of 16 butterflies. The nodes of the data flow graph (DFG) of the FFT are folded to these 16 butterflies for any value of N by the control unit. Register minimization is also applied after folding to decrease the number of scratch pad registers to (log2N − 1) × 16. The real and imaginary parts of the samples are represented by 32-bit single-precision floating point notation to achieve high precision in the results. Thus, each sample is represented using 64 bits. Twiddle factor ROM size is reduced by 25% using the symmetry of the twiddle factors. Reconfigurability based on the sample size is achieved by the control unit. This distributed floating point arithmetic (DFPA)-based design of FFT processor implemented in 45-nm process occupies an area of 0.973 mm2 and dissipates a power of 68 mW at an operating frequency of 100 MHz. When compared with FFT processor designed in the same technology with multiplier-based butterflies, this design shows 33% less area and 38% less power. The throughput for 2,048-point FFT is 222 KS/s and the energy spent per FFT is 7.4 to 14 nJ for 64 to 2,048 points being one among the most energy-efficient FFT processors.