ed cell (REC). In contrast to that, ad hoc and peer-to-peer multihop networks [31] have to work without any infrastructure; because they do not have a planned structure, they are not well suited for commercial infrastructure-based cellular networks. The goal of the described multihop-augmented, infrastructure- based networks is the almost-ubiquitous provision of coverage and throughput with very high data rates for any UT within the cell. Figures 8.2 and 8.3 show the two Figure 8.2 Left: single-hop cellular hexagonal layout (frequency reuse 3). Right: multihop geometry to increase the capacity of the cell. © 2009 by Taylor & Francis Group, LLC Multihop Extensions to Cellular Networks Figure 8.3 Multihop cellular geometry to increase the coverage area of a cell. With three relays, the covered area is increased +200%. ways from a conventional cellular layout to multihop-augmented cells for both goals. A BS is located in the center of the cell and RNs are within the cell (to increase the capacity; Figure 8.2) or a normal cell radius apart (to increase the COV- erage of a single cell; Figure 8.3). The following are some facts about cellular multihop networking: Relays are low-cost, low-transmit power devices (compared to BSs). They are not connected to a wired backhaul. Data received from the BS to the UT and vice versa are stored and for- warded later on the next transmit opportunity. Relays can reduce the propagation losses between BS and UT. Networks using fixed infrastructure relays compared to ad hoc networks do not need complicated distributed routing algorithms. The topology is a tree. Simultaneous transmissions of BS and RN, or only among RN, are possible with spatial multiplexing, SO additional capacity gains are achieved. 8.2 Fundamentals 8.2.1 Protocol Stack In the ISO-OSI reference model terminology, the physical layer, or layer 1, is responsible for the transport of bit streams over the channel, modulation and channel coding, equalization, synchronization, orthogonal frequency division © 2009 by Taylor &