4.7 Indoor DAS for MIMO Applications
In Section 2.6 we covered the basics of advanced antenna systems, utilizing MIMO so that we could optimize the data performance in the building. At this point in time it seems unrealistic from a practical viewpoint to implement more than 2 × 2 MIMO inside buildings, due to installation restrictions. As mentioned in Section 2.6 more advanced indoor antennas, utilizing cross polarized antennas, might ease the challenge of implementing MIMO in real life.
The key to good MIMO performance is a total separation of the MIMO links throughout the DAS, from the two antennas all the way to the base station, so it is not possible to use the same passive infrastructure at any point in the indoor DAS – that would destroy the isolation between the MIMO paths in the system.
4.7.1 Calculating the Ideal MIMO Antenna Distance Separation for Indoor DAS
Recent studies and calculations have provided the ‘ideal’ antenna separation of 3–7λ; this also applies when implementing indoor MIMO DAS solutions in order to benefit from a maximum de-correlation between the MIMO paths created by the scattering of the indoor environment (Figure 4.23).