7.6 Updating a Passive DAS from 2G to 3G
In recent years RF designers have tried to prepare their passive DAS designs for UMTS to some extent. Often the planning of these passive DAS was done years prior to the launch of their UMTS service. Thus, it lacks a UMTS link budget, guidelines and knowledge as to how the NF caused by the attenuation of the passive system would impact the 3G uplink performance once implemented.
Many of these GSM systems were ‘future proofed’ to support UMTS, normally by using passive components that covered the 2100 MHz band, and sometimes a slightly heavier coaxcable.
For some of these projects it has become clear that, in some areas of the building, especially in the areas serviced by the most remote antennas, the antennas with the highest loss have problems with UL data speed on UMTS, and in particularly with HSUPA.
7.6.1 The 3G/HSPA Challenge
The reason for this is often the increased loss, the higher noise figure on the passive DAS when the frequencies get higher, but also the much stricter SNR requirements for the high data rate on 3G/HSPA. These requirements are covered in more detail in Chapter 8.
As we have just explored in the first sections of Chapter 7, we can use amplifiers to improve the performance in the part of the passive DAS that is having difficulties with the loss/noise figure.
Often antennas close to the base station and those with relatively low loss will perform acceptable. Typically it will be just the more distant part ...