Remark on “On the capacity of a multiple-access vector adder channel” by A. A. Frolov and V. V. Zyablov

For vector adder channel, we present an upper bound on the capacity, which coincides with the lower bound given in [1]. Thus, we prove optimality of the uniform probability distribution of symbols for $\gamma\in(0,\gamma^*]$ and of the twisted distribution for $\gamma\in(\gamma^*,\infty)$, where $\gamma$ is the ratio of the number of users to the number of subchannels and $\gamma^*=1.3382$.