Insufficiency of Linear-Feedback Schemes in Gaussian Broadcast Channels With Common Message

We consider the K >= 2-user memoryless Gaussian broadcast channel (BC) with feedback and common message only. We show that linear-feedback schemes with a message point, in the spirit of Schalkwijk and Kailath's scheme for point-to-point channels or Ozarow and Leung's scheme for BCs with private messages, are strictly suboptimal for this setup. Even with perfect feedback, the largest rate achieved by these schemes is strictly smaller than capacity C (which is the same with and without feedback). In the extreme case where the number of receivers K -> infinity, the largest rate achieved by linear-feedback schemes with a message point tends to 0. To contrast this negative result, we describe a scheme for rate-limited feedback that uses the feedback in an intermittent way, i.e., the receivers send feedback signals only in few channel uses. This scheme achieves all rates R up to capacity C with an Lth order exponential decay of the probability of error if the feedback rate R-fb is at least (L - 1)R for some positive integer L.