|Issue:||Europe I 2014|
|Topic:||Antennas can make or break any network|
How to Choose the Optimal Base Station Antennas
|Author:||Torbjörn Kämpe & Bo Jonsson|
|Title:||CEO & Senior RF Expert|
Torbjörn Kämpe is co-founder and CEO of CellMax Technologies, a developer of ultra high-efficiency antennas for the global telecom market. Mr. Kämpe has decade-long experience in managing telecom-related companies and has been with CellMax Technologies since 2003. He Holds a BSBA in Marketing & Finance from University of Southern Europe.
Bo Jonsson is the senior radio frequency expert at CellMax Technologies, a developer of ultra high-efficiency antennas for the global telecom market. Mr. Jonsson has more than 30 years of working experience with radio systems and more than 15 years in R&D designing fast hopping synthesizers, transmitters, receivers, filters etc. Bo Jonsson got his MSCE by 1987 and has held various titles over the years such as: RF-group manager, R&D manager, CTO, Systems expert, project manager and many others.
The choice of antennas can make or break network coverage. For some years, the 18dBi antenna has been installed without a second thought, but the 21dBi antenna may do a better job. The choice of the best antenna is greatly influenced by its location. Antennas situated in rural, suburban and urban locations have different requirements, which are satisfied by high gain, tilt, efficiency and configuration. Analysis shows that surprisingly many situations call for the 21 dBi, not 18 dBi antennas, despite the more complex feeding networks.
The choice of antennas for base stations rarely receives any attention. Antennas are regarded as a cheap commodity that will ‘do the job’ regardless of which antenna you choose. Nothing could be further from the truth. Antennas have a tremendous impact on coverage, performance, capacity and efficiency, so choosing the right ones can make or break a mobile operator’s ability to cope with the rapidly increasing demand for data.
Looking at the installed base, one could get the impression that the obvious, or even optimal, choice for almost all sites would be the traditional 18 dBi antenna. This antenna has 65° of horizontal beamwidth and around 6.5° of vertical beam width, as do about 80 per cent of all installations on 1,700 to 2,100 MHz. The 15 dBi antenna is still quite common, especially on the lower frequencies, with a vertical beam width of around 14°.
There are also high gain 21 dBi antennas and new so-called ultra high-efficiency antennas using air as dielectric and virtually eliminating power losses. These antennas improve base stations’ transmission capacity, resulting in higher signal strength, an increase in geographical area coverage, improved indoor penetration, increased traffic, improved data throughput and reduced production costs per call.
18 dBi antenna is chosen for historical reasons, not with forethought
How much forethought is behind the choice of 18 dBi antennas? The decision to use this antenna comes down to historical reasons. The 18 dBi antenna was the highest gain available from antenna vendors who were able to deliver antennas in mass quantities. It was such an obvious choice that it became almost a de facto standard, and was installed almost without a second thought. This was of course not without reasons – it was a good compromise and has served us all very well.
However, the 18 dBi antenna does not deliver the highest gain anymore – high gain ultra high-efficiency antennas do. The 18 dBi antenna was always somewhat deficient, both for its narrow vertical beam width and for its horizontal beam width of ‘only’ 65°.
With Data traffic surpassing Voice in new 3G and 4G mobile networks, interference is different and the antennas must stay effective. 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.
The choice of antenna depends on the individual situation
So what is the optimum antenna for all installations? The answer to this question is: “there is no such thing as an optimum antenna for all installations!” It always depends on the situation. If there is a tall building nearby, a small antenna with lower gain like 15 dBi would do a good job. At a rural site with maximum coverage as the most important objective, a 21 dBi antenna is clearly the best option.
At suburban sites the focus shifts towards capacity, suppression of interference and less on coverage. In can be seen that a high gain antenna will provide a 3.5 dB stronger signal level in the center of the cell, and suppress interference from next cell better than the 18 dBi antenna. The 3.5 dB extra will improve in-building coverage and data transfer speed. Also, the lower interference will improve carrier-to-interference ratio (C/I) and further improve performance. A high C/I is the key parameter for efficiency, data rate and general success. So a high gain high efficiency antenna is again the best choice.
What about dense urban sites? After all, that is where most antennas are installed and the requirement is here to contain the signal within the cell while at the same time provide a very strong signal for good in-building penetration and high C/I for high speed data transfer. Simulations and real-life tests show that there is a very sharp ‘cut off’ at 650 meters from the high gain antenna at 6° tilt. This will provide a significant reduction of interference and a magnificent control of soft handover load. There is a remarkable difference between 5.5° and 6° which proves that antenna tilt is crucial to performance, and that antenna tilt setting is a precision job where a scale on a tilt bracket is far from accurate enough. The high gain antenna still outperforms the 18 dBi antenna in dense urban sites and the sharper upper roll off is the main asset, actually even more important than the stronger signal level.
When going further in tilt, it can be seen that the high gain antenna provides a stunning 5.5 dB stronger signal and even more in interference rejection from outside 450 meters. That does wonders for soft handover overhead and data transfer speed. So again a clear win for the 21 dBi high gain antenna with very sharp upper roll off.
High gain, high efficiency the way to go
In summary, if we generalize a bit, we can see that the 21 dBi antenna is a better choice in surprisingly many situations, for much the same reason as for the 18 dBi antenna replacing earlier antennas with lower gain. Very often, they offer significant improvements over the traditional 18 dBi antennas. However, they have a significantly more complex feeding network, so efficiency becomes a very important parameter. After all, to extend and improve coverage requires more radiated radio frequency power, not less. So look not only for a high gain antenna, but also for a high efficiency antenna!