– Bo Jonsson, Senior RF expert at CellMax Technologies, says:
We have had antennas since the days of Guglielmo Marconi, the Italian scientist who invented the radio a little over 100 years ago. So by now we should all know how to use antennas. But do we? With data replacing voice as the ‘killer application’ in the networks, antenna tilt – the angle in which the antennas are directed – becomes a serious issue and an area where many base stations today are clearly suboptimized.
The long narrow form of the typical array antennas gives them a fan-shaped radiation pattern, wide in the horizontal direction and relatively narrow in the vertical direction. There is usually a downward beam tilt, or downtilt, so that the base station can more effectively cover its immediate area and not cause radio frequency interference to distant cells. For good coverage and call quality, the signal must be strong in the desired radiation area, but drop of sharply where it is not needed or where it interferes with signals from other base stations.
You can compare with a car’s headlights: you want to see everything as clearly as possible in the direction you are travelling, but don’t want to waste energy by illuminating something irrelevant.
The most common antenna in a three-sector base station is the 18 dBi antenna with 65° of horizontal beamwidth and around 6.5° of vertical beam width. The 15 dBi antennas are still quite common, especially on the lower frequencies, with a vertical beam width of around 14°. Most of the planning experience and rollout methods for mobile networks are based on these two antenna types. They are built on the assumption that there are interfering signals not in the adjacent cell, but further away. But in 3G and 4G systems, there is the interference mainly coming from the next cell; there are no longer any “transition zone” between service area and “disturbance area”.
So what does that mean in practice? Well, with data surpassing voice in new 3G and 4G mobile networks, interference is different and so must the antennas be to stay effective. The efficiency of a cellular network depends on its correct configuration and adjustment of radiant systems: their transmit and receive antennas. One of the more important system optimizations task is based on correctly adjusted tilts, or the inclination of the antenna in relation to its axis. When the antenna is tilted down, we call it ‘downtilt’, which is the most common use. The tilt is used when operators want to reduce interference and/or coverage in some specific areas, having each cell to meet only its designed area.
With data being the networks’ new “killer application” instead of voice, a high carrier-to-interference ratio (C/I) is the key parameter for efficiency, data rate and general success. Carrier-to-Interference ratio (C/I) is the ratio of desired signal power in an RF carrier to the unwanted interference power in the channel. In voice, it was a waste to have very high C/I. But with data replacing voice in the networks we want to have high data rate all the way to the cell border. Basically, we would like to have a coverage that provides a constant signal level all the way to the cell border and there, suddenly, magically, drop to zero. This is of course not possible, but antennas with sharp roll-off can help us to get a lot closer to that ideal situation.
In most sites a sharper upper roll-off will provide higher C/I and less soft handover load. Both of which will increase performance and release capacity, often by over 10 dB in C/I improvement can be seen if the tilt is properly optimized with an antenna having a sharper roll-off curve and high efficiency.
This sounds too good to be true! Can you really improve network efficiency by just swapping the antennas and tilting them differently? Yes, you can. But it comes at a price. It requires both a very accurate tilt setting and a better understanding of how to use the very sharp cell border that these antennas give. Basically, it means that setting tilt after the scale on the tilt bracket is history. Half a degree makes a lot of difference. So, use a good digital leveler.
The conclusion is that most sectors would benefit significantly from an antenna with higher gain and a sharper upper roll-off curve than the standard 18 dBi can offer. Almost every site can perform better if the tilt is optimized better and more often. That is an easy and inexpensive way to improve the networks’ efficiency.