3D Beamforming

3D beamforming is being considered as one of the enabling technologies for fifth generation (5G) wireless networks alongside mm-waves with higher bandwidth, massive MIMO, etc. As the current cellular networks deployed so far, it is seen that the antenna elements at the base station (BS) or the access point are arranged along the horizontal axis to cover beam pattern radiation in the horizontal plane. This arrangement is known as 2D beamforming (2DBF). With the advancement of this technology, when the radiation beam pattern is directed in both the elevation and azimuth plane (Fig. 1), to provide more degrees of freedom, it adds one more dimension to the beamforming. This is called 3D beamforming.

The 3DBF has many advantages such as higher user capacity, less intercell and inter sector interference, higher energy efficiency, improved coverage and increased spectral efficiency. In 2DBF, the beam pattern is focused only in the horizontal plane [1]. The main idea is to adapt the vertical dimension of the antenna radiation pattern and to steer the transmitted energy in a useful direction, as in beamforming or precoding in the horizontal plane. Beamforming and MIMO schemes are extensively used to produce the horizontal beam pattern of mobile wireless base stations (eNodeB) to maximize performance. Several limitations need to be accounted for to adjust the 3D antenna geometry. 3DBF using an Active Antenna System (AAS) works in both ways, regulating the antenna weights of the horizontal antenna terminals and also of each antenna element in the vertical direction. Thus, it exploits channel’s elevation degree of freedom. The transmit power can be more concentrated on the target UEs and the interference can also be reduced. The adjustment range of the vertical beam pattern is much less than in the horizontal direction. Further, the vertical beam usually is much narrower than the horizontal beam and the half-power beamwidth (HPBW) is typically in the 5° to 10° range. So, the vertical beam requires a higher accuracy of the downtilt adjustment, to improve vertical beam separation. The way to get a narrow vertical beam is to arrange a large number of transmitting elements vertically and feeding the same signal with proper phase shift between the transmitting elements.

Downtilt adjustment can be realized by selectable passive feeding networks. While this approach is suitable for adapting the downtilt of the full signal bandwidth but for the 3D beamforming approach dynamic beam pattern adaptation per assigned resource block is required i.e., per UE. Therefore, each transmitting element must be intended to transmit each frequency sub-band with a specific phase shift [2].

References:

[1] S. Mohammad Razavizadeh, MinkiAhn, and Inkyu Lee, “A new enabling technology for 5G wireless networks Three-Dimensional Beamforming”, IEEE Signal Processing Magazine, Vol. 31, Issue 6, pp. 94 – 101, Nov. 2014.

[2] Johannes Koppenborg, Hardy Halbauer, Stephan Saur and Cornelis Hoek, “3D beamforming trials with an active antenna array”, 2012 International ITG Workshop on Smart Antennas (WSA), pp. 110-114, April 2012.