Accessible capacity of high-mobility secondary users

In this paper, we are concerned with an asymmetric multiple-access channel (MAC) model with two users, which are distinguished as primary user and secondary user, respectively, according to their mobility. The low-mobility user (primary user) has a time-invariant uplink and employs a single-user code that was originally designed to satisfy a given error performance requirement under the assumption that no high-mobility user exists. In contrast, the high-mobility user (secondary user) has a rapidly time-varying uplink and is allowed to design a code by considering the signaling structure of the primary user. While the high-mobility user is passing through the same cell, we are interested in the maximum transmission rate (defined as accessible capacity) at which the secondary user can reliably send data to the base station without affecting the error performance requirement by the primary user under the constraint that the primary encoder is kept unchanged. By modeling the primary encoder as a generalized trellis code (GTC), we are then able to treat the secondary uplink as a finite state channel (FSC) with fast fading. Based on this, upper and lower bounds on the accessible capacity are derived. For some special cases, these bounds can be computed numerically using the Bahl-Cocke-Jelinek-Raviv (BCJR) algorithm.