Subarray delta-sigma beamforming for low-cost underwater acoustic imaging

In real-time digital underwater acoustic imaging, the size, power consumption and cost of the front-end hardware are critical design issues especially for applications like UUV. The number of active array channels and sampling scheme directly affect these specifications. For low-cost front-end, we present a beamforming architecture based on subarray processing with nonuniform oversampling delta-sigma (ΔΣ) data conversion. We use a small-sized active subarray multiplexed over a large transducer array to acquire low-resolution beam lines, where the RF echo signal received at each subarray channel is digitized using 1-bit ΔΣ modulator with nonuniform oversampling. In the nonuniform oversampling scheme, samples are taken in the time instants associated with the beamforming delays. The ΔΣ coded partial beam outputs from the subarrays are first demodulated and then coherently summed to form a high-resolution image frame. By the way, the limited number of acquired beam lines are upsampled using beam space interpolation, thus allowing real-time processing in acquiring beams and also maintaining acceptable image quality. Our experimental results show that the proposed beamforming scheme provides high-resolution acoustic imaging while simplifying the front-end circuitry.