Channel combining and splitting for cutoff rate improvement

The cutoff rate R₀(W) of a discrete memoryless channel (DMC) W is often used as a figure of merit, alongside the channel capacity C(W). Given a channel W consisting of two possibly correlated subchannels W₁, W₂, the capacity function always satisfies C(W₁) + C(W₂) les C(W), while there are examples for which R₀(W₁) + R₀(W ₂) > R₀(W). This fact that the cutoff rate can be "created" by channel splitting was noticed by Massey in his study of an optical modulation system modeled as a M\´ary erasure channel. This paper demonstrates that similar gains in cutoff rate can be achieved for general DMC\´s by methods of channel combining and splitting. Relation of the proposed method to Pinsker\´s early work on cutoff rate improvement and to Imai-Hirakawa multi-level coding are also discussed