4.5 Hybrid Active DAS Solutions
It is important to distinguish between the pure active DAS that we covered in the previous section and hybrid DAS solutions. As the name suggests, a ‘hybrid’ DAS is a mix of an active DAS and a passive DAS.
The passive part of the hybrid DAS will, as for the pure passive DAS, limit the installation possibilities, impact data performance on 3G/HSPA and to some extent dictate the design, due to installation limitations.
4.5.1 Data Performance on the Uplink
The basic of RF design is to limit any loss prior to the first amplifier in the receive chain. The fact is that the passive portion of the hybrid solution between the antenna and the hybrid remote unit (HRU) will degrade the UL data performance. This is explained in more detail in Chapter 7. The impact of the passive portion of the hybrid DAS becomes a concern for UMTS and HSUPA performance.
4.5.2 DL Antenna Power
Even though the typical hybrid DAS produces medium to high power levels out of the HRU, the power at the antenna points will typically be significantly lower. The reason for this is that the power is attenuated by the passive DAS between the HRU and the antenna, but if you only service a few antennas and keep the losses low, you can obtain relatively high radiated power from the antennas.
4.5.3 Antenna Supervision
The small passive DAS after the HRU will give a high return loss back to the HRU; therefore it is often a problem for the HRU to be able to detect any VSWR problems due to the ...