Refinement of the Sphere-Packing Bound: Asymmetric Channels
  We provide a refinement of the sphere-packing bound for constant composition codes over asymmetric discrete memoryless channels that improves the pre-factor in front of the exponential term. The order of our pre-factor is $\Omega(N^{-1/2(1+\epsilon + \rho_{R}^{\ast})})$ for any $\epsilon >0$, where $\rho_{R}^{\ast}$ is the maximum absolute-value subdifferential of the sphere-packing exponent at rate $R$ and $N$ is the blocklength. 
Yucel Altug and Aaron B. Wagner
cs.IT math.IT