  A decoding algorithm for polar (sub)codes with binary $2^t\times 2^t$
polarization kernels is presented. It is based on the window processing (WP)
method, which exploits the linear relationship of the polarization kernels and
the Arikan matrix. This relationship enables one to compute the kernel input
symbols probabilities by computing the probabilities of several paths in Arikan
successive cancellation (SC) decoder.
  In this paper we propose an improved version of WP, which has significantly
lower arithmetic complexity and operates in log-likelihood ratios (LLRs)
domain. The algorithm identifies and reuses common subexpressions arising in
computation of Arikan SC path scores.
  The proposed algorithm is applied to kernels of size 16 and 32 with improved
polarization properties. It enables polar (sub)codes with the considered
kernels to simultaneously provide better performance and lower decoding
complexity compared with polar (sub)codes with Arikan kernel.
