Feedback Communication Using Orthogonal Signals

We consider a feedback communication system in which the forward and feedback channels are disturbed by additive noise and constrained in average power. Two block coding schemes are proposed in which the signals are orthogonal waveforms. A finite number of forward and feedback transmissions per message is made. Information received over the feedback channel is used to reduce the expected value of forward signal energy on all iterations after the first. Similarly, the expected value of feedback signal energy is reduced on all iterations after the first. These schemes, which are modifications of a feedback coding scheme due to Kramer, obtain improved error performance over one-way coding at all rates up to the forward channel capacity, provided only that the feedback channel capacity is greater than the forward channel capacity. They require less feedback power than existing feedback coding schemes to achieve a given error performance. © 1969 IEEE