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.
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).
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).
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.
