While “MIMO” has been a buzz word in the mobile communications industry for some time, it is only now gaining real traction and will be a key enabler as networks migrate from 4G to 5G. Base Station Antennas are a critical component in MIMO architectures, and there is a science to proper spacing in order to achieve the highest Quality of Service (QoS) and Quality of Experience (QoE) while minimizing interference and PIM.
Let’s take a quick look at what how MIMO works, what it brings to mobile networks, and how proper antenna spacing is a key to maximizing throughput.
A Quick Tour of MIMO
MIMO, or Multiple Input Multiple Output, utilizes multiple antennas at both the transmitter and the receiver (smartphone) to increase link reliability and spectral efficiency. Spatial Multiplexing makes it possible to transmit separate data streams from multiple antennas on the same frequencies. Signal processing hardware splits the data into multiple streams and transmits these streams using multiple antennas. The receiver then reverses this process, recreating the original data stream inside the phone. Obviously, propagation conditions between the transmitter and the receiver must be good for MIMO to work effectively.
What’s the result of all this? MIMO increases the capacity of a cell without using more bandwidth. With 2×2 MIMO (two transmit and two receive) it is theoretically possible to double the throughput, while 4×4 MIMO can quadruple throughput. In an LTE network, the peak throughput using SISO (single input, single output) is about 100 Mbps. Utilizing 2×2 MIMO and 4×4 MIMO, throughput can ideally reach 173 Mbps and 326 Mbps, respectively.
Mobile operators have implemented 2×2 MIMO in their LTE 4G networks for a number of years and are now beginning to deploy 4×4 MIMO to meet increased data demands. Just last fall, Samsung’s Galaxy S7 became the world’s first 4×4 MIMO capable smartphone. The challenge with placing four antennas so close together in a phone (along with Wi-Fi, GPS and Bluetooth antennas) is that it can cause the transmission paths to couple, limiting MIMO performance and increasing signal interference.
While handset manufacturers were busy developing smartphones with 4×4 MIMO capable antennas, antenna manufacturers, like Kathrein, were developing 4×4 MIMO ready antennas for the cell sites.
Optimum Macro Antenna Spacing for 4×4 MIMO
A lot of research that has been done on the proper placement of 4×4 ready MIMO antennas on cell towers. When determining optimum spacing between horizontal antenna columns for 4×4 MIMO, a balance must be found between improving gain while reducing inter-sector interference (I-SI).
Mounting antennas with proper spacing helps operators achieve maximum MIMO performance by keeping the antenna pattern in the “desired” area of the sector, with minimum energy in the “undesired” area where there is higher inter-sector interference.
Measurement studies performed by Kathrein engineers at the low band shows the optimal spacing between columns to be 0.8λ (wavelengths). For perspective, at 780 MHz, one wavelength is about 15 inches. It was determined that at mid band (1.7-2.7 GHz) gain became more important than I-SI to improve 4×4 MIMO performance. Therefore, the preferred spacing between columns for the low band has been set near 0.8λ to ensure I-SI is minimized; however, the spacing at high-band in a shared aperture is chosen for improved gain, near the 1.3λ or 1.8λ spacing, (based on bands under the radome) in an attempt to minimize I-SI as well.
Kathrein Makes Antenna Spacing Easy
Kathrein recently released new 4, 8 and 12 port macro antennas that are 508 mm in width. These wider antennas support 4×4 MIMO under one radome (click to view the datasheets: 80010901, 80010964, 80010991). Operators can mount these antennas and be 4×4 MIMO ready without worrying about spacing two separate antennas. For 378 mm antennas, Kathrein offers 2x Panel Mounting Kits (85010103/850108) that provide pre-configured optimum spacing for 4×4 MIMO applications.
Moving Forward with MIMO
MIMO is already offering huge dividends by increasing network throughput and capacity. Moving forward, we will see more 4×4 MIMO implementations, as well as 8×8 MIMO, and eventually 64×64 Massive MIMO as operators move into 5G and beyond.
Learn More by visiting us at MWC Americas in San Francisco, CA, September 12-14: Booth S.1042.