Beamforming, Multi-user MIMO and Massive MIMO
Recently, Verkotan has provided a beamforming antenna pattern measurement for the University of Oulu. One of our professionals, Chief Scientist Jani Kallankari has kindly shared his observations and conclusions with us.
Antenna Pattern measurement
Antenna may be considered to be the most important element in a receiving system, as its properties strongly influence the overall performance. In the antenna pattern measurement the magnitude and phase of an electromagnetic signal received or transmitted from the antenna under test (AUT) are measured.
Antenna array is simply an array of antenna elements. In this case we have used planar antenna array of 15 dual polarized antenna elements illustrated on Figure 1 on the right. Antenna is operating at 5GHz frequency range and has horizontal and vertical arrays of antennas. Each of the 30(=2×15) antennas amplitude and phases are measured separately. This enables us to study how beamforming and multi-user MIMO works in practice with base stations.
- Beamforming technology
Modern radio networks use beamforming technology in order to enable the efficient use of the spectrum. Beamforming technology in its turn, is a technology that projects the signal in a specific direction, while traditionally the signal is sent in all direction or a fixed direction. Beamforming is achieved by changing phases of multiple antenna elements. As a result you get a better Wi-Fi signal for your devices.
Beamforming can be achieved by adjusting phases of multiple antenna elements. The more antenna elements are used the greater gains in radio link can be achieved. In the figure below, we can see the animation of beamforming for angles ranging -60° to 60° degrees. Individual antenna element radiation patterns are coloured in grey. The red line in the Figure 2 presents combined antenna pattern with phases optimized to maximum gain to given angle.
On average about 22dB gain over single antenna element for sector -60° to 60° can be achieved with this antenna array. If we normalize total power of single antenna and antenna array to equal there is still about 11dB directivity gain for the array antenna. This greatly helps to improve the connection quality.
MIMO stands for multiple input and multiple output.It has a single principal claiming that a wireless network allows transferring of more than one data stream simultaneously over the same radio channel. MIMO provides fast and resource efficient of data transmission.
Multi-user MIMO can be considered as extension of beamforming. In comparison to standard MIMO base station has multiple data streams to different devices simultaneously. Idea is not only try to maximize the signal level to one direction, but also try to minimize signal levels to other user equipment (UE) directions. This can be successfully done if base station has enough antenna elements compared to UE’s. The illustration below shows how Multi-user MIMO works in case of two UEs.
In Multi-user MIMO it is important that all, but one antenna patterns have minimum to the UE direction. This enables simultaneous communication to multiple UEs with minimized interference from other UE’s signals.
In Multi-user MIMO user equipment complexity can be kept low and most of the it is in the base station (BS).Down side is that BS needs accurate channel estimation for all the UEs in the network.
- Massive MIMO
In comparison to multi-user MIMO networks, massive MIMO is using high number of antennas in the base station. It can be seen as a direct extension of Multi-user MIMO. With more antenna elements BS can serve more UEs simultaneously, therefore has better beamforming performance at the same time. This significantly improves capacity and range of the base station.
Using antenna arrays covering 360° one can avoid traditional antennas sector border problems. As massive MIMO base station can optimally adapt to movement of the EU to different directions. This works like the soft handover in WCDMA.
Antenna element can also be located in different base station locations. This enables to perform handovers from different antenna locations in optimal way.
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