Communication is the transmission of information from a source to one or more destinations. To enable this, communications protocols or communications models are required. The most widely used communications protocol is the transmission control protocol/internet protocol (TCP/IP) model, which has four layers stacked from the bottom to the top as the network interface layer, the internet layer, the transport layer, and the application layer. Another frequently used model is the open systems interconnection (OSI) model, which has seven layers stacked from the bottom to the top as the physical layer, the link layer, the network layer, the transport layer, the session layer, the presentation layer, and the application layer. These two models have certain equivalence in terms of functions. In wireless communications, it is often the physical layer and the link layer in the OSI model, equivalent to the network interface layer in the TCP/IP model, that are of interest. The physical layer performs the transmission and reception of the physical bits, while the link layer provides control functions to enable the efficient operations of transmission and reception. Figure 4.1 shows diagrams of these models. One sees that the physical layer lies at the bottom of the whole system. Any issue in this layer will affect the whole system. Thus, this layer is the most important layer in any communications systems. It will be studied first.