Accessible capacity of secondary users over the Gaussian interference channel

A new problem formulation is presented for the Gaussian interference channels (GIFC) with two pairs of users, which are distinguished as primary users and secondary users, respectively. The primary users employ a pair of encoder and decoder that were originally designed to satisfy a given error performance requirement (EPR) under the assumption that no interference exists. In the case when the secondary users attempt to access the same medium, we are interested in the maximum transmission rate (defined as accessible capacity) at which secondary users can communicate reliably without affecting the EPR under the constraint that the primary encoder (not the decoder) is kept unchanged. The relation of the accessible capacity to the capacity region of the GIFC is revealed. By modeling the primary encoder as a generalized trellis code (GTC), we are able to treat the secondary links as finite state channels. Then upper and lower bounds on the accessible capacity are derived and computed by using the BCJR algorithm. The numerical results show us either expected or interesting facts.