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In reality, the capacity of a MIMO channel is limited by the number and strength of independent paths. The number of independent paths, in turn, is largely determined by the spatial structure of the physical channel. MIMO systems have the added benefit that they can strike a trade-off between multiplexing and diversity gain. By changing the way in which information is coded at the transmitter, we can either send multiple copies of the same message on different antennas (diversity gain), send different messages on different antennas (multiplexing gain), or strike a balance somewhere in between.

Each path is associated with its own complex gain, labeled h1 to h4. Each channel coefficient is a complex-Gaussian random variable. If the correlation between channels E{h1 hÃ2 } is close to 1, then we say that the fading between paths 1 and 2 is strongly correlated. Equivalently, we say that the two scatterers s1 and s2 are strongly correlated. In a similar fashion, let E{h3 hÃ4 } ¼ 1. Let there be no other significant correlation. In this example, we consider fs1, s2g and fs3, s4g to be correlated scatterers.

The SIMO channel is illustrated in Fig. 6. Depending on the composition of the physical channel, the signal at a given receive-antenna can be independent of all others, or it can correlate in some way. 2, correlation between signals at different antennas allows us to deduce the location of scatterers in the physical channel. 6 Block diagram of the SIMO channel. 6 31 The Narrowband MIMO Channel Consider the case where both the transmitter and receiver are equipped with multiple antennas. This introduces the ability to resolve the spatial structure at both link-ends, and thus gives us an added degree of freedom.