ADAPTIVE ANTENNA AND BEAMFORMING SYSTEMS IN WIMAX

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.

 
Figure 1: Beamforming technique.

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

Benefits of Beamforming | Advanced Antenna Technologies in WiMAX

System Design Aspects on BS and MS

All the complexity and intelligence of a beamforming system is inside the base station. In addition to the spatial signal processing unit, the BS includes as many transceivers as the number of antenna elements. Usually, wireless systems implementing beamforming are operating between two to eight antenna elements and transceivers.

On the MS side, the impact for the support of beamforming is minor. Basically, beamforming can be applied to any MS. However, in order to provide better performance, the standards (and in particular IEEE 802.16e) define additional messages/procedures between the BS and the MS. Nevertheless, there is no hardware impact on the terminal side; only some additional software is needed for an optimised beamforming operation.

The beamforming algorithms aim to combine coherently the signal transmitted/received from different antenna elements. In order to achieve this correlation, the antenna elements need to be closely separated. For an optimum performance of beamforming, a spacing of half the wavelength λ is preferred. Consequently, assuming a four-element antenna array at 2.5 GHz would result in an antenna of about 22 to 25 cm width. The array itself is thus of a relatively small size, which is very beneficial for visual impacts on the environment.

Besides, in order to maintain the signal coherence, mechanisms for calibrating the different transmit/receive paths are required, the algorithm for doing this being vendor-specific.

Benefits of Beamforming

The benefits of beamforming are manyfold: range increase and power saving at the MS side, interference mitigation and capacity increase.

First, beamforming improves the link budget for the data transmission for both the downlink and the uplink. Indeed, by concentrating the energy in one direction, the resulting antenna gain in one direction is significantly increased (see Figure 1). This additional gain is beneficial for improving the coverage of the BS (less sites needed for a deployment) and/or for reducing the power needed by the MS to transmit signals (power saving).

Figure 1: Range extension with beamforming

Theoretical gains, compared with a conventional antenna, for an N-element antenna arrays are of 10×log(N) for the uplink and 20×log(N) for the downlink. For example, with a four-element antenna arrays the gains are respectively of 6 dB (12 dB) for the uplink (respectively the downlink). The gain in the downlink is higher since, on top of the beamforming gain, the power from each transmitter coherently increases. The value of those gains has been validated in many experiments on the field and proves to be in line with the theory. Additional gains are measured in the uplink due to the additional spatial diversity gain.

Second, because the energy is focused in the direction of the user, there is a general interference reduction in a cellular system employing beamforming. Indeed, when beamforming is deployed on the BS of a given geographical area, the beams are oriented as a function of the repartition of the users served in a cell; at one moment, on a given radio resource, a single user is served. As a consequence, the interference created by the communication of this user is only in a restricted angle compared to sectorised antenna deployment (see Figure 2).

Figure 2: Interference reduction with beamforming

The angle spread of the main lobe is approximately the total angle of the sector divided by the number of antenna elements N. For example, with a four-element antenna array and a 90° antenna, the resulting main lobe width (at −3 dB) is around 220°. Therefore, since the users are randomly spread, the beam directions change according to the user locations, which create additional interference diversity gain. The interference reduction is further improved with the use of explicit interference cancellation algorithms.

Two direct consequences of the interference reduction are: a better signal quality and availability across the cell area and a better capacity in the cell for systems using link adaptation. Indeed, since the CINR values are better, the possibility of using a better modulation and coding scheme is higher.