Urban Propagation Problems:
Shadowing: Dead spots in communications caused by urban structures that block or absorb signals. Shadowing increases with an increase in frequency. Shadowing may cause communication loss of 10 to 30 decibels.
Multi-path Effects: Fading of communications, caused by the reflection of the signal off urban terrain. The signal and one or more of its reflected versions meet at the antenna causing constructive or destructive interference. The severity of fading increases with frequency. For most VHF radios fading is usually less than 10 decibels. Higher frequency radios may experience fades as severe as 20 to 30 decibels.
Electrical: Manmade electrical noise, concentrated at low frequencies. Declines rapidly as frequency increases but still 15 to 20 decibels higher than a rural or suburban environment.
Mechanical: Caused by heavy concentration of manmade noise associated with heavy industry or manufacturing. The more audible the noise level, the higher the volume must be adjusted on communications equipment.
Power: Increase in power or antenna gain may increase communication capability. Trade-off is decreased battery life, increased load for Marine, and increase in susceptibility to Electronic Warfare.
Frequency: HF is susceptible to noise but best to reduce shadowing and multi-path effects. High UHF/SHF (above 500 megahertz) is very susceptible to shadowing and multi-path but best to reduce noise interference. VHF to low UHF range offers best trade off in offsetting various problems associated with operating in urban terrain.
Diversity: Can eliminate multi-path effects. Provided by using multiple receive antennas or multiple frequency transmissions. Frequency hopping or direct sequence spread spectrum can also provide diversity.
Relays: Shortens link distance and can overcome noise problems. Relays used on higher urban terrain, unmanned aerial vehicles or other air platforms will reduce shadowing.
Technology: Commercial systems developed for robotic use in manufacturing may have military application. These systems use spread spectrum modulation (direct sequence or frequency hopping) to overcome fading and interference.
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