Approximate Hybrid High Radix Encoding for Energy-Efficient Inexact Multipliers

Approximate computing forms a design alternative that exploits the intrinsic error resilience of various applications and produces energy-efficient circuits with small accuracy loss. In this paper, we propose an approximate hybrid high radix encoding for generating the partial products in signed multiplications that encodes the most significant bits with the accurate radix-4 encoding and the least significant bits with an approximate higher radix encoding. The approximations are performed by rounding the high radix values to their nearest power of two. The proposed technique can be configured to achieve the desired energy-accuracy tradeoffs. Compared with the accurate radix-4 multiplier, the proposed multipliers deliver up to 56% energy and 55% area savings, when operating at the same frequency, while the imposed error is bounded by a Gaussian distribution with near-zero average. Moreover, the proposed multipliers are compared with state-of-the-art inexact multipliers, outperforming them by up to 40% in energy consumption, for similar error values. Finally, we demonstrate the scalability of our technique.