Existence
In an ideal communication scenario, there is a line-of-sight path between the transmitter and receiver that represents clear spatial channel characteristics. In urban cellular systems, this is seldom the case as base stations are located on rooftops while many users are located either indoors or at streets far from base stations. Thus, there is a non-line-of-sight multipath propagation channel between base stations and users, describing how the signal is reflected at different obstacles on its way from the transmitter to the receiver. However, the received signal may still have a strong spatial signature in the sense that stronger average signal gains are received from certain spatial directions.
Spatial correlation means that there is a correlation between the received average signal gain and the angle of arrival of a signal.
Rich multipath propagation decreases the spatial correlation by spreading the signal such that multipath components are received from many different spatial directions. Short antenna separations increase the spatial correlation as adjacent antennas will receive similar signal components. The existence of spatial correlation has been experimentally validated.
Spatial correlation is often said to degrade the performance of multi antenna systems and put a limit on the number of antennas that can be effectively squeezed into a small device (as a mobile phone). This seems intuitive as spatial correlation decreases the number of independent channels that can be created by precoding, but is not true for all kinds of channel knowledge as described below.
Read more about this topic: Spatial Correlation
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“What is a novel if not a conviction of our fellow-men’s existence strong enough to take upon itself a form of imagined life clearer than reality and whose accumulated verisimilitude of selected episodes puts to shame the pride of documentary history?”
—Joseph Conrad (1857–1924)
“The Frenchman Jean-Paul ... Sartre I remember now was his last name had a dialectical mind good as a machine for cybernetics, immense in its way, he could peel a nuance like an onion, but he had no sense of evil, the anguish of God, and the possible existence of Satan.”
—Norman Mailer (b. 1923)
“To me the sole hope of human salvation lies in teaching Man to regard himself as an experiment in the realization of God, to regard his hands as God’s hand, his brain as God’s brain, his purpose as God’s purpose. He must regard God as a helpless Longing, which longed him into existence by its desperate need for an executive organ.”
—George Bernard Shaw (1856–1950)