Minimising Memory Access Conflicts for FFT on a DSP

Abstract: The FFT support in an Ericsson's proprietary DSP is to be improved in order to achieve high performance without disrupting the current DSP architecture too much. The FFT:s and inverse FFT:s in question should support FFT sizes ranging from 12-2048, where the size is a multiple of prime factors 2, 3 and 5. Especially memory access conflicts could cause low performance in terms of speed compared with existing hardware accelerator. The problem addressed in this thesis is how to minimise these memory access conflicts. The studied FFT is a mixed-radix DIT FFT where the butterfly results are written back to addresses of a certain order. Furthermore, different buffer structures and sizes are studied, as well as different order in which to perform the operations within each FFT butterfly stage, and different orders in which to shuffle the samples in the initial stage. The study shows that for both studied buffer structures there are buffer sizes giving good performance for the majority of the FFT sizes, without largely changing the current architecture. By using certain orders for performing the operations and shuffling within the FFT stages for remaining FFT sizes, it is possible to reach good performance also for these cases.