CHAPTER Information Theory of Underspread WSSUS Channels 1 Chalmers 2 University of Technology, Gothenburg, Sweden 2 ETH Zurich, Switzerland 3 Robert Bosch GmbH, Stuttgart, Germany 4 Technion ­ Israel Institute of Technology, Haifa, Israel ¨ Giuseppe Durisi 1 , Veniamin I. Morgenshtern 2 , Helmut B olcskei 2 , 3 , Shlomo Shamai (Shitz) 4 Ulrich G. Schuster 2.1 THE ROLE OF A SYSTEM MODEL 2.1.1 A Realistic Model In this chapter, we are interested in the ultimate limit on the rate of reliable communication through