Decoding of Polar Codes Using Quadratic Unconstrained Binary Optimization

Polar codes encounter challenges in decoder complexity while preserving good error-correction properties. Instead of conventional decoders, a quantum annealer (QA) decoder has been proposed to explore untapped possibilities. For future QA applications, a crucial prerequisite is transforming the optimization problem into quadratic unconstrained binary optimization (QUBO) form. However, existing QUBO forms for polar decoding result in suboptimal frame error rate (FER) performance for codes exceeding 8 bits. This paper redesigns the QUBO form for polar decoding. We first introduce a novel receiver constraint modeled by the binary cross-entropy (BCE) function. Utilizing a simulated annealing (SA) solver with the proposed QUBO form with BCE (QUBO-BCE) achieves maximum-likelihood (ML) performance for a code length of 32 bits. Next, to reduce the number of variables, we remove the frozen variables and introduce a simplified QUBO-BCE form (SQUBO-BCE). Additionally, CRC polynomials are modelled into constraints in QUBO form, resulting in a CRC-aided SQUBO-BCE (CA-SQUBO-BCE) form for polar decoding to further enhance the FER. Numerical results demonstrate that SQUBO-BCE achieves ML performance and reduces up to 61.5% of variables compared to QUBO-BCE. Furthermore, the proposed CA-SQUBO-BCE achieves near CRC-aided ML performance. The proposed SQUBO-BCE requires the lowest number of SA processes to reach a specific FER.