2.6 Advanced Antenna Systems for HSPA+ and LTE
Advanced Antenna System – Introduction
Traditionally we consider a radio channel to have one input and one output and we normally consider this as one ‘radio channel’ – only one connection between the mobile and the network, and one in reverse (separated in Frequency FDD, or in time TDD).
For macro deployment in mobile antenna systems, we sometimes add an extra antenna on the base station uplink, traditionally implemented via a cross polarized antenna – one antenna enclosure with two input/outputs. The base station connects the combined Tx/Rx to one port and the RxDiversity to the other port. The purpose of receive diversity is to provide better resistance against fading on the UL, and the Base Station uses both signals in micro-diversity processing. Simple algorithms such as maximal ratio combining can combine the two signals optimally to improve the overall signal quality. Selection diversity is typically used for macro diversity processing such as with soft handover in CDMA networks.
This Receive diversity serves the purpose of giving better ‘fading resistance’ and will typically improve the fading margin on the UL by 3–5 dB, dependant on the environment (best in high multipath environments), and antenna separation.
The motivation for the improvement of the UL is that the mobile network will in many cases be UL limited (for macro deployment).
The use of diversity makes little sense in indoor DAS for several reasons, one being that ...