We consider several illustrative situations to show that the details of the physical layer have a strong impact on the optimal protocols for a wireless network, and thus on the capacity of the network. These examples will also serve to highlight some of my current research.

The first example considers a wireless ad hoc network, with a 'Ultrawideband-like' physical layer. Specifically, the assumption is that each link has low spectral efficiency, with finite power and infinite bandwidth, which is typical of UWB, or sensor networks. We show that, contrary to the intuition in [GuptaKumar00], the capacity of such a network **increases** with node density, on the order of $n^{(\alpha-1)/2}$, where $n$ is the node density and $\alpha$ is the distance-loss exponent. This also results in a different set of optimal protocols.

The second example considers the problem of joint optimization of an ad hoc network, with network level metrics. We show that the problem decomposes neatly into 'layers' of networking, where we are able to quantify the concept of 'network layering'. We solve the problem by designing optimal algorithms. An application of the algorithms to a UWB ad hoc network verifies the dramatic information theoretic result shown in the first example.

Finally, if time permits, I will show a third example in which we have designed a 'queued channel code' for operation over a time-varying wireless channel. A queued channel code is an information theoretic channel code, which incorporates ideas from network queuing theory. We show that this code achieves better performance than either pure coding or pure queuing. This is along the lines of our previous work, where we defined an 'effective capacity' notion for a time-varying wireless channel.

Rohit Negi received the B.Tech. degree in Electrical Engineering from the Indian Institute of Technology, Bombay, India in 1995. He received the M.S. and Ph.D. degrees from Stanford University, CA, USA, in 1996 and 2000 respectively, both in Electrical Engineering. He has received IIT Bombay's prestigious President of India Gold medal in 1995.

Since September 2000, he has been with the Electrical and Computer Engineering department at Carnegie Mellon University, Pittsburgh, PA, USA, where he is currently an Associate Professor. His research interests include communications systems, information theory, and networking with a cross-layer viewpoint.