An upper bound on bit error rate for a single user of a two-user system as a function of the interfering user´s signal strength

Many communication systems are characterized by two or more users transmitting simultaneously while only one user´s message is of interest. To achieve optimum performance through such systems, we must understand the limitations imposed by the interfering users. We consider the problem of detecting a single user´s signal transmitted through a dispersive channel in the presence of a second user. Our goals are to determine the conditions under which the performance is optimum as well as bounds on performance in the presence of the unknown interferer. We begin by describing the binary sequence detection problem and present maximum likelihood sequence estimation, which can be efficiently implemented by the Viterbi algorithm, as an optimum solution. Next, we formulate the problem mathematically and define a metric used to characterize the system performance. We then derive a simple but robust upper bound on performance using a recursive search that minimizes the performance metric with respect to the strength of the interfering user´s signal