ered to focus energy from the antenna in a given direction, and the steering can even be dynamic to allow for some degree of track- ing of a mobile client. Phased-array antennas achieve directional selectivity by elec- tronically imposing minute delays on the signals, referred to as a phase shift, moving from (or to) different parts of the array. Arrays can be programmed to send a grid of electronically formed radio beams to track moving targets or, alternatively, to receive signals from only certain directions. Chapter 13 Wireless Communications Basics Amplifiers Input Phase Shifters Figure 13.2 Phased-array antennas Beam steering and intelligent antenna techniques increase the signal directed toward an intended receiver and reduce the reception of stray signals intended for other targets, which appear at the receiver as noise. Phased-array antennas enable 100-fold spectrum reuse (i.e., a capacity increase by a factor of 100) and may enable us to reach 1,000-fold reuse. Phased arrays have the additional advantage of being flat, which makes them more suitable than other antennas for many applica- tions, particularly mobile installations. Magnetic Polarization Antennas Magnetic polarization antennas can be used to increase bandwidth. They work by reusing the same frequency at different polarizations. Waves vibrating in perpen- dicular directions cannot interfere with each other, SO the same frequency can be used twice in the same place. Polarization is also applied to the magnetic waves, which are transmitted alongside their electric counterparts and have very similar properties. Different combinations of these polarizations make a sixfold capacity increase possible, and this can be achieved by simply upgrading the base stations to fit the extra antennas. MIMO Antennas A recent advance in antenna design is the multiple-input multiple-output (MIMO) system, which is one of the key building blocks for next-generation wireless access. Antennas MIMO is an advanced antenna technology that can carry several times more da