1 Beamforming
Beamforming takes advantage of interference to change the directionality of an antenna array system. A beamformer controls the amplitude and phase of the signal at each transmitting antenna element, to create a pattern of constructive interference (beamspots) and destructive interference (null) in the wavefront. To create a beamspot, the beamformer uses an array of closely spaced antennas, often enclosed in a single enclosure as illustrated in Figure 1. λ/2 antenna spacing between the antenna elements is commonly used (where λ is the wavelength of the transmitted signals, given by λ = c/f; f is the frequency of the transmitted signals, c is the speed of light). By varying the amplitude and phase of each antenna element, the beamformer is able to focus electromagnetic energy (beam) in the desired directions. The beams are directed to intended users, while nulls are focused on other unintended users reducing interference to the unintended users while increasing received SNR for the intended user. This provides a stronger link to the intended user and improves reach and capacity.
2 Adaptive Antenna System
AAS is one of the advanced antenna technologies specified in the WiMAX standard to improve performance and coverage. In the AAS system, the transmitter (base station, BS) adaptively tracks a mobile receiver as it moves around the coverage area of the transmitter (BS), and steers the focus of the beam (beam spot) on the receiver unit as it moves. The beam steering method can either be mechanical or electronic. Thus AAS creates narrow beams to communicate with desired user device, which helps to reduce interferences to unintended user devices and improves carrier-to-interference (C/I) and frequency reuse, giving rise to high spectral efficiency. Thus, through the use of adaptive processing (beam steering), AAS improves performance and coverage of the system significantly. In WiMAX networks, AAS will find wide applications both in the point-to-multipoint (PMP) as well as mesh network deployments. In the PMP mode, AAS operates in similar way as in the current 3G cellular system, and is used for enhancing coverage and performance. In the mesh mode, AAS is used to form physical or directed mesh links. Physical or directed mesh is a form of mesh where substantially directional antennas are used to create physical links between neighboring devices. Mesh nodes adaptively steer antennas towards other nodes in their neighborhood and direct the focus of the electromagnetic radiations accordingly, to create the physical link with the intended neighboring device. One of the main drawback of AAS however is the high complexity involved in designing antenna systems capable of adaptively switching (steering) antenna directionality toward users who may be highly mobile. The use of AAS technology in mobile network deployment is therefore very challenging from a complexity perspective. Another drawback of AAS technology is that in an urban environment, with rich scatterer, the beams get blurred at the receiver and are not focused as expected, due to the reflections of waves as it propagates from the transmitter to the receiver. This effect is known as angle spread and it impacts significantly the performance of AAS in urban areas with cluttered structures. The gains achieved using AAS in such places thus reduce considerably from the theoretical expectations. For example, an AAS system using an eight-column array would have an ideal gain of 6.9 dB but angle spread would reduce this to only 3.2 dB in an urban environment and 4.7 dB in a suburban environment. There are some techniques however to mitigate the effects of angle spread. Active research works are ongoing in this area
