Different components are used to split the signal power between the antennas in passive DAS. These components are passive, which means they do not need external power supply. With passive DAS, the signal is distributed between the antennas using the following elements:
- Coaxial cables
The number of antennas to use and the output power are important parameters that must be planned carefully and will be very dependent on the kind of environment. Both in single cell and multi-cell environments, DAS using an efficient power control increase capacity and reduce interference. It is interesting to compare the performance of DAS with a system using a single antenna to evaluate the interest of such an approach.
Active DAS is unlike passive systems which do not require use of electronic component active DAS use different active elements:
- The master unit
- Remote unit
The passive components are not needed any more, which is why the installation of an active system is easier. Indeed, the losses are automatically compensated by the master unit and the remote unit: there is no need to choose splitters and attenuators to adjust the losses, and no need to measure accurately the length of the cables.
Some other solutions combining passive DAS and active DAS have been installed. The idea is to connect the remote units via fiber optic, but to use passive coaxial cabling to link these remote units to the antennas. The combined method has the advantages of covering a long distance thanks to the fiber optic connection, and a cheaper price due to the passive components. Hybrid DAS can also combine different systems with different frequency bands. Due to the simplicity of radio over fiber installation, there are many possible solutions, such as combining the distributed antenna system with repeaters.
Passive systems are less expensive but the initial setup is costly due to the coaxial cabling. In fact active systems provide better performance and easier installation but more expansive. The advantage of passive systems is that there no electronic system has to be installed or power supplied. Hybrid DAS is a good compromise however it requires installation of coaxial cables. One good advantage of active systems is that it’s easier to manager because of the automatic diagnostics and alarms are integrated into the remote units, which makes the system failure problems easier to solve.
This technology is ideal for covering long narrow spaces. The main advantage of the radiating cable is that the energy is well distributed. For example, in a corridor, it provides homogeneous coverage all along the cable compared with the use of numerous base stations along the corridor, where the energy is distributed around the base stations. With a radiating cable, a single base station may be able to provide coverage over a large area, reducing the cost of system implementation. A disadvantage is the difficult and expensive installation. The installation is time consuming and it is not always easy to find the available space to install it. The cable must be aligned perfectly so that the slots can leak with minimum loss. Moreover the cable must not be installed directly against a wall but some space must be left, thanks to some special adapters Finally, especially inside tunnels (for example with trains making dust), the dirt degrades the performance of the cable. Therefore the cable must be regularly cleaned.
Self Organizing Network (SON) functionality is typically geared towards managing base stations (macro, pico or femto) in cellular networks. DAS coverage solution networks are often provided by vendors besides the vendors providing the RAN. These coverage solution networks are often managed by an EMS separate from that managing the RAN.
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