Efficient digital implementation of a multirate-based variable fractional delay filter for wideband beamforming

In this brief, we propose a FPGA implementation of a novel Variable Fractional Delay (VFD) filter architecture for digital wideband (hundred of MHz or more) beamforming applications. The proposed architecture is based on a multirate structure that allows reaching high performance in terms of amplitude and group delay with a significant reduction in design complexity. Usually, the antenna’s beams are modified by introducing precise delays that can be multiples or fractions of the sampling period. The most used structures for realizing this operation are Farrow filters but, for wideband applications, the classical Farrow structure may require a huge amount of resources. A common solution to simplify the Farrow filter architecture is interpolation. However, this solution requires an increase of the clock frequency which is not always possible. In this context, our multirate approach mitigates the limitations in terms of computational complexity avoiding the requirement to increase of the clock frequency, while still exhibiting great performance. A comparison between a reference design based on the proposed approach and the literature is also provided. The experimental results confirm that the proposed architecture, characterized by a computational complexity similar to other works, exhibits better performance without the need to increase the clock frequency.