Acknowledgments

We are grateful to G.David Forney,Jr. for suggesting some of the problems addressed in this work. We would also like to thank Frank Kschischang for sending us preprints of his papers and Alexander Vardy for motivating this work, providing important feedback, and suggesting Methods 1 and 2. Finally, we would like to thank the anonymous referees for their valuable suggestions for tightening the presentation of this paper.