【正文】 eric noise,manmade noise, and thermal noise from electronic ponents at the receiver are dominant disturbances for signal transmission in the MF band. Skywave propagation, as illustrated in Fig. 124 results from transmitted signals being reflected (bent or refracted) from the ionosphere, which consists of several layers of charged particles ranging in altitude from 50 to 400 km above the surface of the earth. During the daytime hours, the heating of the lower atmosphere by the sun causes the formation of the lower layers at altitudes below 120 km. These lower layers, especially the Dlayer, serve to absorb frequencies below 2 MHz, thus severely limiting skywave propagation of AM radio broadcast. However, during the nighttime hours, the electron density in the lower layers of the ionosphere drops sharply and the frequency absorption that occurs during the daytime is significantly reduced. As a consequence, powerful AM radio broadcast stations can propagate over large distances via sky wave over the Flayer of the ionosphere, which ranges from 140 to 400 km above the surface of the earth. A frequently occurring problem with electromagic wave propagation via sky wave in the HF frequency range is signal multipath. Signal multipath occurs when the transmitted signal arrives at the receiver via multiple propagation paths at different delays, tt generally results in intersymbol interference in a digital munication system. Moreover, the signal ponents arriving via different propagation paths may add destructively, resulting in a phenomenon called signal fading, which most people have experienced when listening to a distant radio station at night when sky wave is the dominant propagation mode. Additive noise at HF is a bination of atmospheric noise and thermal noise. Skywave ionospheric propagation ceases to exist at frequencies above approximately 30 MHz, which is the end of the HF band. However, it is possible to have ionospheric scatter propagation at frequencies in the range 3060 MHz, resulting from signal scattering from the lower ionosphere. It is also possible to municate over distances of several hundred miles by use of tropospheric scattering at frequencies in the range 40300 MHz. Troposcatter results from signal scattering due to particles in the atmosphere at altitudes of 10 miles or less. Generally, ionospheric scatter and tropospheric scatter involve large signal propagation losses and require a large amount of transmitter power and relatively large antennas. Frequencies above 30 MHz propagate through the ionosphere with relatively little loss and make satellite and extraterrestrial munications possible. Hence, at frequencies in the VHF band and higher, the dominant mode of electromagic propagation is lincofsight (LOS) propagation. For terrestrial munication systems, this means that the transmitter and receiver antennas must be in direct LOS with relatively little or no obstruction. For this reason, television stations transmitting in the VHF and UHF frequency bands mount their antennas on high towers to achieve a broad coverage area. In general, the coverage area for LOS propagation is limited by the curvature of the earth. If the transmitting antenna is mounted at a height h m above the surface of the earth, the distance to the radio horizon, assuming no physical obstructions such as mountains, is approximately d VlSh km. For example, a TV antenna mounted on a tower of 300 m in height provides a coverage of approximately 67 km. As another example, microwave radio relay systems used extensively for telephone and video transmission at frequencies above I GHz have antennas mounted on tall towers or on the top of tall buildings. The dominant noise limiting the performance of a munication system in VHF and UHF frequency ranges is thermal noise generated in the receiver front end and cosmic noise picked up by the antenna. At frequencies in the SHF band above 10 GHz, atmospheric conditions play a major role in signal propagation. For example, at 10 GHz, the attenuation ranges from about dB/km in light rain to about dB/km in heavy rain. At 100 GHz, the attenuation ranges from about dB/km in light rain to about 6 dB/km in heavy rain. Hence, in this frequency range, heavy rain introduces extremely high propagation losses that can result in service outages (total breakdown in the munication system). At frequencies above the EHF (extremely high frequency) band, we have the infrared and visible light regions of the electromagic spectrum, which can be used to provide LOS optical munication in free space. To date,these frequency bands have been used in experimental munication systems, such as satellitetosatellite links. Underwater Acoustic Channels Over the past few decades, ocean exploration activity has been steadily increasing. Coupled with this increase is the need to transmit data, collected by sensors placed under water, to the surface of the ocean. From there, it is possible to relay the data via a satellite to a data collection center. Electromagic waves do not propagate over long distances under water except at extremely low frequencies. However, the transmission of signals at such low frequencies is prohibitively expensive because of the large and powerful transmitters required. The attenuation of electromagic waves in water can be expressed in terms of the skin depth, which is the distance a signal is attenuated by 1/r. For sea water, the skin depth 250/v7, where f is expressed in Hz and 8 is in m. For example, at 10 kHz. the skin depth is contrast, acoustic signals propagate over distances of tens and even hundreds of kilometers. An underwater acoustic channel is characterized as a multipath channel due to signal reflections from the surface and the bottom of the sea. Because of wave motion, the signal multipath