Antenna and Advanced Antenna Systems
Antennas are mechanical elements that transform varying electrical signals into an electro-magnetic field that transfers the signal energy into the space surrounding the antenna, which propagates away from the antenna. Antennas also capture the energy from surrounding electro-magnetic fields and transform it into varying electrical signals. The former perform a signal transmit function and the latter a signal receive function.
The transmitted electro-magnetic field, also known as an RF (radio frequency) wave, propagates in many directions and is reflected, refracted and diffracted by obstacles in its path. This splits the original energy into many components that travel through different paths. A receive antenna receives several of those paths, and its sum creates a unique signal. The received signal is subject to fading as the different wave components either add constructively or cancel each other.
Two receive antennas receive different wave components and the resulting received signals are not exactly the same. This can be explored to enhance the extraction of the transmitted information, by analyzing the signals received from two or more antennas.
The similarity between two received signals is defined by its cross-correlation, which defines how different the signals are from one another. This cross-correlation is similar to the orthogonality test explained in Chapter 4, and can be calculated by a sliding dot ...