Antenna performance is critical to the capacity and data throughput gains sought by the industry’s deployment of LTE Releases 10 and beyond, Ray Butler, vice president active wireless products engineering, Commscope, told AGL Bulletin at PCIA’s Wireless Infrastructure Show last month in Hollywood, Fla. He spoke on the “LTE Evolution” panel.
“We have done some analysis on antenna performance and what we are seeing is how well the pattern is shaped is very important for LTE in the macro-layer of the network and it is also true for the small cells,” he said.
Butler said there was a tendency in early deployments to use whip antennas in outdoor metro small cell sites, with a plan to place them on every street corner of the coverage area without performing the RF optimization. But this approach leads to system performance issues, because the radio issues that are present in the macro-cellular networks, – for example, carrier-to-interference ratio, containing the radio signal to the desired coverage area– also apply in small cells, according to his analysis.
“As operators get into the deployment, we are seeing a more practical approach being used, where a 2-foot to 4-foot antenna with electrical tilt capability is being specified instead of a whip antenna. You are better off using a high-performance antenna. It makes a huge difference in the network,” Butler said.
Small cells should be deployed with the same care and precision as a macrocell, according to Butler. “Many small cells will operate on the same frequencies as the umbrella macrocell, so you need to contain the radio signal and minimize the interference between the layers,” he said.
While the self-optimizing networks’ algorithms are powerful, Butler said that RF engineering is critical to unleashing maximum system throughput and capacity gains.
“You are never going to achieve full throughput and meet your capacity goals if there is too much overlap of coverage in each sector. If the signals overlap too much, you are going to have trouble with interference.”
In the area of high performance antennas, there are many considerations: MIMO versus beam forming and single-user MIMO versus multi-user MIMO for optimizing the antenna and the antenna configuration for the different LTE transmission modes.
“If you have a high mobility user, transmission diversity works the best. There is less dependence upon feedback,” Butler said. “If you have pedestrian traffic close to the site with a scattering environment where there are a lot of radio signal reflections, single- or multi-user MIMO is the best, because the traffic speed is slower and there is better signal-to-interference ratio.”
A user moving slowly close in to the antenna utilizes closed-loop feedback to maximize performance, so the antenna can be switched back and forth between beam forming and MIMO as needed to optimize the radio link, which creates the throughput gains.
The question arises, according to Butler, as to how antennas can be configured to maximize the throughput—either side by side or stacked vertically one on top of the other, for example. “The answer is rather intuitive: side by side, if users are spread horizontally and vertically for coverage down a corridor,” he said.
Butler discussed the various ways that high-performance antennas can increase the spectral efficiency of a wireless network.
“With single-user MIMO, the bit stream can be split into two paths for 2-way MIMO, which roughly doubles the spectral efficiency,” he said. “Multi-user MIMO sends multiple bit streams using the same spectrum to geographically separated users. Beam forming, on the other hand, sends a bit stream that focuses on or follows the user as he travels through the sector, which decreases interference and increases the signal-to-interference ratio.”