12.4 Digital Distribution on DAS
The latest versions of DAS system utilize digital processing and transport which gives rise to some challenges and many advantages with regard to performance and application.
There are several ways of utilizing digital technology, and the following is a general description.
The latest versions of digital DAS will interface directly with the base station at the digital baseband level interfacing to the OBSAI (Open Base Station Architecture Initiative) and CIPRI (Common Public Radio Interface). This has several advantages and saves cost by having the DAS as a ‘Remote Radio Head’ saving the cost of the PA and power supply, etc. at the base station location.
In principle, digital DAS works like this (Figure 12.9):
1. If the signal from the base station is not already at the digital level, using CPRI or OBSAI, the Central Master Unit (MH), the MU will convert/digitize the RF from the base station.
2. Then via signal processing the Digital DAS will divide the signal into a timeslot/frame structure.
3. Then internal timeslots/frames are then converted into optical signals and distributed over the fiber. This can be to/from a single remote unit (RU) or the same signal/frame could be fed to/from several RUs in a simulcast scheme, as shown in Figure 12.8.
The same procedure works in reverse; where the analog RF on the UL from the ...