Two-Way AWGN Channel Error Exponents at Zero Rate

Achievable error exponent regions of a two-way additive white Gaussian noise (AWGN) channel, where two terminals exchange a fixed number of messages M, are derived. In particular, error exponent regions for M = 2 messages under expected power and M = 3 messages under almost sure power constraints are considered. For M = 2 messages the use of active feedback is shown to lead to an error exponent gain over that when feedback / interaction is ignored. For M = 3 messages and asymmetric channels, it is shown that the error exponent of the weaker channel may be improved through active feedback, at the expense of a decreased error exponent of the stronger direction. This may, for sufficiently asymmetric channel gains, outperform the error exponent region achieved by having both terminals operate independently of one another (ignoring the possibility of sending feedback for the other).