Capacity of frame-asynchronous multiple-access channels with memory

Summary form only given, as follows. The author considers discrete two-user multiple-access channels with memory where the transmitters are not synchronized so that their codewords start simultaneously. Frame-asynchronous multiple-access channels with memory arise, for example, in the modeling of completely asynchronous channels where there is no synchronization at either the symbol or the codeword level. It is known that the absence of frame-synchronism in memoryless multiple-access channels results in the removal of the convex hull operation from the expression of the capacity region. In this study is is shown that when the channel has memory, frame-asynchronism rules out nonstationary inputs in order to achieve any point in the capacity region, thereby allowing only coding strategies that involve cooperation in the frequency domain, but not in the time domain. This restriction drastically reduces the capacity region of some multiple-access channels with memory, and in particular the maximum rate-sum (total capacity), which is invariant to frame-asynchronism for memoryless channels.