Increase Coverage & Save on Cellular Infrastructure
VEGA (Very High Gain Antenna)
Guide and use recommendations with RF Repeaters
√ Increasing coverage without adding BTS’s
√ Covering rural targets with minimal CAPEX
√ Providing service to difficult to serve corridors
√ Inexpensive solution for covering remote railroads and highways
Introduction:
In many rural areas there are hundreds of highway kilometers wandering through open areas and small villages.
Due to lack of telecom infrastructure, these un-serviced or under serviced areas represent a gold mine for cellular service providers who now can provide service to these areas with minimum CAPEX by implementing unique, Very-High-Gain-(VEGA) antennas with BTS, repeaters, or both.
By doing so, they expand their network coverage at a fraction of capital expenditure needed for conventional “all BTS” systems.
Even mature cellular networks, which cover mostly urban areas, use combined BTS and RF repeaters solutions to expand coverage into rural areas, as well as to provide service to remote, low capacity residential areas.
Since its operational introduction in 2004, the VEGA antenna can be viewed as the only efficient, cost-effective way to provide profitable coverage to areas that were previously viewed as commercially questionable by the operator.
The most convincing demonstration of the cost saving possibilities inherent in the use of the VEGA antenna in combination with base stations and RF repeaters is to examine the case of the rural highway.
Traditional deployment dictates the use of base stations set at certain intervals along a highway with wide beam panel antennas. In such cases where the wider beam is not required, its implementation at such scenarios adds noise and interferences and shrinks possibly covered range.
By using VEGA antennas and repeaters, one can save the expense of significant numbers of base stations including the backhaul link, air-conditioning and other ancillary cost items needed for such installations.
This technical note specifies the instances where the cellular operator will benefit most from the use of VEGA antennas with repeater systems. (A separate note deals with use of VEGA antennas with base stations).
Adding RF Repeaters to a Cellular Network
The most popular basic configurations for integrating RF repeaters into a cellular infrastructure are:
1. A single repeater per BTS sector (either omni or directional)
2. Two cascaded repeaters per BTS sector
With a single repeater per sector, the trunk between the BTS donor sector and the repeater can be either by fiber, over-the-air F1/F1, or frequency translated (F1/F2), the last two using a narrow beam antenna on the repeater’s donor side.
The service antenna of such a repeater, depending on the situation, can be omni, wide-beam panel antenna or narrow-beam high gain antenna.
These are mounted in such a way that their coverage areas extend the BTS coverage area in distinct directions.
A single repeater per BTS sector scenarios, extending the range of the BTS, is shown in Figs.1 and 2.
Note that by using very high gain antennas for both the donor side and the service sides of the repeater, the range from the repeater to the BTS can be extended significantly while the repeater is illuminating from a distance the village or a highway section otherwise-not-covered.
Fig.1 A repeater with VEGA antennas can replace a BTS covering a remote village from the distance.
Fig.2 Cascaded repeaters with VEGA antennas illuminating an Highway section.
In cascaded configuration as shown on Fig 2 , the base station signal is passed down the line from one repeater to another repeater (up to a max of 2 repeaters in a given chain).Cascading repeaters is particularly useful for covering long highways where required capacity is low.
When calculating the link budget for an optimal repeater link, it is obvious that the highest gain antenna is needed for the repeater's donor antenna to handle the extended distances between the BTS and the repeater. The service side (towards the mobiles in the covered area) must have the highest appropriate gain as well.
The rationale behind that is as follows:
1.The higher the composite gain of the antennas is, the lower the repeater required gain on both the down-link and up-link.
Lower repeater gain means lower required isolation between the two antennas (donor and service) to prevent possible oscillations.
2.The repeater’s service antenna picks up the surrounding noise and interference, amplifies them and sends them (with the mobiles signals of course) to the BTS receiver. These are added to the BTS sector noise, desensitizing BTS receiver and limiting the BTS sensitivity (and thus its range). The higher the composite gain of the antennas (meaning narrower beam and lower side-lobes), the lower the chance of interference from other BTS sectors and repeaters in the vicinity.
When may repeaters benefit from VEGA antennas?
The VEGA antenna is unique in its extremely high gain, narrow beam and low side lobes and dual polarizations features. These features will significantly improve repeater performance within the cellular network for the following reasons:
VEGA antennas higher gain – usually repeaters use high gain vertically polarized only Yagi antennas of 13 to 17 dB for the donor side antenna.
When used as a donor antenna, the VEGA antenna can provide a higher gain of 6 to 13 dB. The more focused beam of the VEGA high gain antenna enables the signal to span distances between the towers with higher signal level at the repeaters ports.
When used as the service side antenna, the VEGA will provide 6 to 9 dB higher gain in comparison to a common panel antenna and thus extend the service range for narrow corridor coverage by up to 50%.
VEGA antennas narrow beam & low side lobes – When a repeater uses the VEGA antenna as its service antenna, the narrow beam & low side lobes of the VEGA will cause less unwanted spatial interferences to other areas on the down link as well and will significantly reduce accumulated received interferences from the other BTSs and other noise sources in the vicinity resulting in lower interference being sent back to the BTS on the up link.
VEGA antennas dual polarization feature - VEGA is the only very high gain, very narrow beam RAN antenna on the market featuring dual polarization.
This feature enables all types of repeaters to take full advantage of the Donor antennas slant polarized signal and by this gaining an extra 3dB signal strength on both uplink and downlink compared to a vertically polarized Yagi.
VEGA is the perfect antenna choice for both the donor side & the service side for those repeaters which maintain polarization diversity.
The following figures (3-5) explain the VEGA dual-polarization benefits.
Fig.1 : Extended range of the VEGA over another antenna
A common single channel or single band repeater, as shown on Fig. 3, has just a single donor antenna port and a single service antenna port. Usually both these antennas are vertically polarized. With this configuration, only the extra high gain and the narrow beam and low side lobes features of the VEGA can be implemented with this type of repeater.
Fig. 4 Dual channel channelized repeater with vertical polarization antennas
Fig. 4 presents a dual channel channelized repeater with single donor antenna port and a single service antenna port. As with a single channel repeater, usually both the repeater’s antennas are vertically polarized. As with the single channel repeater, only the extra high gain and the narrow beam and low side lobes features of the VEGA can be implemented with this kind of repeater.
The main drawback in using the repeater as in Fig. 4 is the need to combine both channelizers by power divider/combiners both at the donor side and the service side of the repeater. These devices have 3dB inherent losses minimum, per side (6dB total loss).
Fig. 5 Dual Channel repeater implementing Dual Polarization VEGA antennas
Fig. 5 shows how this problem can be overcome by using the VEGA with dual polarization feeds.
By using the VEGA on the donor side the divider/combiner losses are saved, lowering by 3 dB the repeater noise figure on the down link to the mobile and getting 3dB more power from the repeater on the up-link to the BTS.
For the same reasoning, when the VEGA is used on the service side, the repeater uplink noise figure is lower by 3dB and the down link output power of the repeater can be higher by 3dB for the same power amplifier.
All these performance improvements are extremely significant for repeater performance.
Another extremely important benefit in using dual-pol VEGA is when the donor BTS transmits the selected two channels onto two different polarizations of the BTS sector antenna.
By using the VEGA for the repeater donor antenna the donor BTS signals will be received at 3dB higher level each compared to the signal level received with a vertical donor antenna of the same gain.
The same reasoning makes the VEGA antenna the perfect choice for “ True Diversity” repeaters whereas the diversity gain is kept through the use of Diversity Repeater.
Concluding Recommendations
a. The first choice for repeater antennas should be the highest possible gain antennas like the VEGA for the donor side, service side or both antennas of the repeater.
b. Use lowest repeater gain possible to keep the signal levels at appropriate level. Using high gain antennas as the VEGA, enables using lower gain within the repeater.
c. In applicable situations, use a narrow beam and low side-lobes service antenna such as the VEGA, so that minimum interferences are spatially emitted and received.
This way the BTS receiver is less de-sensitized by the repeater.
d. Use dual-polarization donor antenna like the VEGA for dual-channel repeaters when the two channels are transmitted form the donor BTS sector antenna on separate +/- 45˚ polarizations.
e. Use dual-polarization service antenna like the VEGA for dual channel repeaters to combine the two channels in the air and save 3dB of power of the combiner/divider unit.