Multiple access Gaussian channels with feedback

We consider the problem of multiple access communication with feedback, and show that the well known schemes of Ozarow and Kramer may be represented as feedback systems with a marginally unstable decoder. Other approaches represent these schemes as feedback systems with a strictly unstable encoder. Both approaches achieve the same rate of convergence, however the latter has a Bode sensitivity function which can be very large, indicating the potential for poor robustness and disturbance response.