n Phones the left to the receiver in the center. Other aspects of the Local Area Network Commercial Cellular environment are shown to highlight the challenges in- Phones herent in wireless communications. These challenges are the focus of much research aimed at improving RF communications. Fiber Optics First, even in the best of situations, we have free space attenuation, where the signal loses strength at a rate Satellite proportional to the square of the distance traveled. This Communications limits the signal propagation distance. The electromag- netic radio waves are further attenuated due to blockage Telephone Modem Walkie-Talkie by objects (including atmospheric particles) in their propagation paths. Both types of attenuation limit the ability of the receiver to capture and interpret the Transistor Commercial TV transmitted signal. Additionally, radio signals are subject to reflection, which leads to multipath fading, another form of signal One-Way Land Mobile Voice Radio loss. A reflected signal, since it travels a longer distance between transmitter and receiver, arrives at the receiver FCC Established with a time delay relative to a signal following a direct (or shorter reflected) path. Two similar signals, offset in time, may cancel each other out. Transcontinental Telephone Broadcast Radio Another difficulty facing the receiver operating in the unprotected wireless environment is the possibility of other similar signals sharing the same medium and ar- riving at the receiver simultaneously with the desired signal, thus causing interference. Finally, the unprotected medium also allows the possibility of eavesdropping, or Telephone Wireless Telegraph interception, where a third party captures the trans- mitted signal without permission, potentially compro- mising the privacy of the system's users. Each of the challenges illustrated in Figure 1.0-4 identifies an area where wireless communications is at Figure 1.0-1 The graph indicates general telecommunications advances on the left and wireless-specific ad