Telecom operators provide IBS to overcome Coverage Issues, Quality Issues and to Increase Capacity. To elaborate this more refer the following Figure: Issues at different heights of a building. Figure shows us a building which consists of over 40 floors.
As mentioned in the figure, at different heights we have different issues. When we provide an IBS we need to clearly identify the issue and then needed to provide a suitable solution for that. There are two types of IBS solutions:
· Distributed Antenna System (DAS) – mainly provided solution out of the two
· Nano BTS (2G) or Femto Cell (3G)
Distributed Antenna System (DAS)
In a DAS solution we lay a network of antennas as required inside the building using splitters. Used equipment in a DAS solution:
Splitters
· Symmetric (2way, 3 way, 4 way)
· Asymmetric (2way tapper)
Using symmetric splitters we can divide a link and get similar output powers from the output (i.e. when the input power= 30 dBm, 27 dBm output at both outputs of a 2 way splitter.). And if we are using an asymmetric splitter we can divide output power asymmetrically (i.e. input power= 30 dBm, output power1= 29 dBm, output power2= 23 dBm).
Antennas
· Omnidirectional Antenna
This kind of antennas can be used to cover a room or a seminar room.
· Panel Antenna
Panel antennas can be used cover a place like corridor.
Design Steps of a DAS
· Existing Coverage
We can get to know the existing coverage by doing a Drive Test.
· Requirement
Based on the results obtained in the drive test we need to identify the issue prevailing in that floor. It may be coverage, quality or capacity issue. Requirement may differ from floor to floor.
· Antenna Locations
Then we need to plan the antenna types and their locations in each floor. Proposed antenna locations for the Ground floor and for the 1st floor are depicted in the Figure: Proposed Antenna Locations.
· Antenna Tree (EIRP Table)
We need to calculate the output power of each antenna considering BTS transmit power, splitter losses, jumper losses, connector loses, antenna gain and cable length along the path from BTS to antenna. We have to keep EIRP values of all the antennas in the range of ±2 dBm. Why? To distribute the Rx power of the base evenly, If we want to adjust power of any antenna without effecting other and all of these antennas should be work as a one antenna (because they act as a one antenna covering the building). To keep these values in the same range we may even use attenuators. But it is not recommended.
After implementing the planned solution we do another drive test to verify the results. If a building is already covered using a DAS few other service providers can share the resources to provide their services. They only have to provide a BTS and they can use the existing antenna network.
Difficulties in DAS:
Rent to the building
High cost
Installation Difficulty
DAS should be installed when the building construction takes place. Otherwise it is difficult to set up it after the completion of the building construction.
Nano BTS (2G) or Femto Cell (3G)
Now the trend is towards IP based systems. Using a Nano BTS or a Femto cell we can easily solve issues inside a building. This solution is becoming popular in these days. Nano BTS is for 2G and Femto cell is to 3G. No need to do any construction to setup this service. These cells only require internet connectivity.
Nano BTS
It is just plug and play. There is an option called auto configuration which will configure the BTS when it is placed in the desired location. Or we can manually configure it. A Nano BSC can handle up to 100 Nano BTSs at the current licence. They even support power over Ethernet (using a POE adapter) so with minimum number of external connections we can setup a 2G solution. Nano BSC will authenticate its BTSs using their MAC address and IP address.
Femto Cell
For 3G requirement we can use Femto cells. It has a throughput of 7.2 Mbps. And it is functioning in a similar manner to the Nano BTS. Through internet we connect it to the MGW. Using a Femto cell we can provide various services. We can address users that are connected to that Femto Cell separately. Also we can assign a certain user for that cell.
