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
Famous quotes containing the word existence:
“Every moment of one’s existence one is growing into more or retreating into less. One is always living a little more or dying a little bit.”
—Norman Mailer (b. 1923)
“The very existence of society depends on the fact that every member of it tacitly admits he is not the exclusive possessor of himself, and that he admits the claim of the polity of which he forms a part, to act, to some extent, as his master.”
—Thomas Henry Huxley (1825–95)
“The duty of the State toward the citizen is the duty of the servant to its master.... One of the duties of the State is that of caring for those of its citizens who find themselves the victims of such adverse circumstances as makes them unable to obtain even the necessities for mere existence without the aid of others.... To these unfortunate citizens aid must be extended by government—not as a matter of charity but as a matter of social duty.”
—Franklin D. Roosevelt (1882–1945)