1. Problem: Prolonged Deep Fades at the Indoor Wireless Receiver
Wireless communication receivers are often plagued by prolonged deep fades, which degrade and possibly drop communication between the transmitter and the intended receiver. As the transmitted electromagnetic wave undergoes multiple reflections and diffraction through a highly cluttered electromagnetic propagation environment, the transmitted signal reaches the receiver through diverse propagation paths. Deep fades may occur for either of two reasons. (1) These signals from multiple propagation paths (i.e., multipath signals; “multipath signals” are often called “multipaths” in communications slang) may. all arrive with powers too weak for the receiver to detect. Each such multipath signal's power depends on the polarization interaction among the transmitter, the scatterers in the propagation environment, and the receiving antenna. (2) The multipath signals cancel each other at the receiver, even if one or more of these multipath signals are individually strong enough for detection by the receiver. For indoor radio systems, the typical delay spread is of the order of several nanoseconds: very small compared to the typical information symbol's duration, which is on the order of microseconds. Thus, the indoor radio channel is largely non-frequency-selective (i.e., frequency-flat), and the arriving multipath signals' anival delays are largely synchronized among themselves at the receiver. Whether these multipath signals sum constructively or destructively depends mostly on their relative phases and their respective polarizations. If the receiver happens to be immobile for many symbol periods, such deep fades may lead to a disconnection of the link between the transmitter and the receiver.