Energy and information in antenna synthesis

The development of MIMO antennas, has pushed the attention toward the antenna synthesis in complex environments. Broadly speaking, MIMO antennas follow an "informational goal", e.g. maximization of the bit rate, while in classic antenna synthesis method we have an "energetic goal", based on the distribution of the radiated power density in the space. Consequently, classic and MIMO antennas follow a different synthesis philosophy, "energy driven" the former, "information driven" the latter. A starting point to develop a more general theory on antenna synthesis including MIMO antennas is to clarify the differences of these two philosophies from an electromagnetic point of view. The NDF of the field represents the amount of ldquomoneyrdquo available to a radiating system. The radiating system can choose to spend its NDF budget in two different way, to obtain a given density power distribution in the space, or to send statistically independent information. Classic antennas and MIMO antennas differ basically due to the different use the available NDF. Maximization of the energy on the receiving antenna does not assure maximization of the information throughput of the communication system. This is due to the fact that it is more advantageous to use the available degrees of freedom to transmit statistically independent information than to maximize the received energy. Only if the NDF of the field on the observation manifold covered by the receiving antenna is one, the ldquoenergeticrdquo and ldquoinformationalrdquo point of view give the same result.