ts. The receiver senses which of the two antennas is able to provide the highest signal strength and switches to that antenna. Adaptive beams or beam-forming antennas consist of two or more antenna elements in an array and a so-called beam-forming algorithm, which assigns a specific gain and phase shift to the signal sent to or received from each antenna element. The result is an adjustable radiation Chapter Three Constructive interference in the desired direction Phase shift between two antenna signals Figure 3-18: Phase Shift Between Two Antennas Resulting in a Directed Beam pattern that can be used to steer the main lobe of the beam in the direction of the desired maximum gain (Figure 3-18). As well as focussing its beam pattern towards a particular node, the adaptive beam antenna can also place a "null" or zero gain point in the direction of a source of interference. Because the gain and phase shift applied to individual array elements is under real-time software control (Figure 3-19), the antenna can dynamically adjust its beam pattern to compensate for multipath and other sources of interference and noise. Summing Receiver amplifier Antenna array Amplitude and Adaptive elements phase control control algorithm Figure 3-19: Adaptive Beam Antenna Wireless Network Physical Architecture Like adaptive beam arrays, Multi-input Multi-output or MIMO radio, described in the Section "MIMO Radio, p. 124", also uses multiple antennas to increase network capacity. The key difference between these two techniques is that MIMO radio exploits multi-path propagation between a single transmitter and receiver, while adaptive beam arrays use multiple antennas to focus a single spatial channel. Some other differences are described in Table 3-6. Table 3-6: Adaptive Beam Arrays and MIMO Radio Compared Adaptive beam array MIMO radio Objective Focus the propagation pattern along Exploit multi-path propagation a single desired spatial direction, to increase data capacity by to allow multiple access, reduce multiplexing data streams ov