A New Approach to Circuit Depth
Communication Complexity describes a new intuitive model for studying circuit networks that captures the essence of circuit depth. Although the complexity of boolean functions has been studied for almost 4 decades, the main problems the inability to show a separation of any two classes, or to obtain nontrivial lower bounds remain unsolved. The communication complexity approach provides clues as to where to took for the heart of complexity and also sheds light on how to get around the difficulty of proving lower bounds. Karchmer's approach looks at a computation device as one that separates the words of a language from the non-words. It views computation in a top down fashion, making explicit the idea that flow of information is a crucial term for understanding computation. Within this new setting, Communication Complexity gives simpler proofs to old results and demonstrates the usefulness of the approach by presenting a depth lower bound for st-connectivity. Karchmer concludes by proposing open problems which point toward proving a general depth lower bound.
Communication Complexity received the 1988 ACM Doctoral Dissertation Award.