New outer bounds to capacity regions of two-way channels

Shannon´s two-way channel problem has attracted the attention of information theorists for many years. In a classic paper Shannon gave both an outer and an inner bound to the capacity region of the two-way channel. Schalkwijk recently obtained an improvement to the inner bound for the Blackwell multiplying channel (BMC). We present the first improvements on Shannon´s outer bound. Calculation shows that our results are close to optimum when applied to the BMC.