By using the diversity made available by multiple-antenna communications, links can be improved [349, 109]. For example, the multiple degrees of freedom can be used to provide robustness through channel redundancy or increased data rates by exploiting multiple paths through the environment simultaneously. These advantages can even potentially be employed to reduce the probability of interception [1]. Knowledge of the channel can even be used to generate cryptographic keys [332].

The basic concept of a multiple-input multiple-output (MIMO) wireless communication link is that a single source distributes data across multiple transmit antennas, as seen in Figure 8.1. Potentially independent signals from the multiple transmit antennas propagate through the environment, typically encountering different channels. The multiple-antenna receiver then disentangles the signal from the multiple transmitters [99, 209, 308, 33, 116, 258, 84]. There is a wide range of approaches for distributing the data across the transmitters and in implementations for the receiver.

While MIMO communication can operate in line-of-sight environments (at the expense of some of the typical assumptions used in MIMO communications), the most common scenario for MIMO communications is to operate in an environment that is characterized by complicated multipath scattering. Consequently, most, if not all, of the energy observed at the receive array impinges upon the array from directions different from the direction to the source. Consequently, the line-of-sight environment assumption employed in Chapters 6 and 7 is not valid for most applications of MIMO communications.