Enhancing Polar Codes Efficiency on 3D Flash Memory by Exploiting Multiple Error Variations

3D flash memory is becoming the mainstream of Solid-State Drives (SSDs) because of its large storage capacity achieved by vertically stacking planar flash into multiple layers. This special vertical structure introduces two additional error sources: the intra-wordline error variation between upper pages and lower pages inside the same wordlines, and the inter-layer error variation across different layers inside one flash block. Recent works have studied polar code in flash memory, which is the first Error Checking and Correcting (ECC) that is proven to reach the Shannon's channel capacity. However, due to the special error characteristic mentioned above, polar codes cannot display effective error correction capabilities in 3D flash.

This paper initially analyzes the challenges of polar codes and proposes an effective polar code design for 3D flash memories, named as Error Variation Aware Polar Code (EvaPC). By exploiting layer-induced error variations and characteristics of polar code encoding/decoding, we implement two schemes: intra-wordline error aware data placement and layer-adaptive polar code. The former places important bits of codewords into lower pages with lower error rates, while the latter applies varied polar code rates across different layers to improve space efficiency by providing equivalent ECC capability. A series of experimental results show that EvaPC can enable polar codes on 3D flash memories with efficient error correction capability and space utilization efficiency.