  Although the successive cancelation (SC) algorithm works well for very long
polar codes, its error performance for shorter polar codes is much worse.
Several SC based list decoding algorithms have been proposed to improve the
error performances of both long and short polar codes. A significant step of SC
based list decoding algorithms is the updating of partial sums for all decoding
paths. In this paper, we first proposed a lazy copy partial sum computation
algorithm for SC based list decoding algorithms. Instead of copying partial
sums directly, our lazy copy algorithm copies indices of partial sums. Based on
our lazy copy algorithm, we propose a hybrid partial sum computation unit
architecture, which employs both registers and memories so that the overall
area efficiency is improved. Compared with a recent partial sum computation
unit for list decoders, when the list size $L=4$, our partial sum computation
unit achieves an area saving of 23\% and 63\% for block length $2^{13}$ and
$2^{15}$, respectively.
