Rank Indication is one of the important input to eNB , in selection of the transmission layer in downlink data transmission. Even though the system is configured in transmission mode 3 (or open loop spatial multiplexing) for a particular UE and if the same UE report the Rank Indication value 1 to eNB, eNB will start sending the data in Tx diversity mode to UE . If UE report Rank Indication 2 , eNB will start sending the downlink data in MIMO mode (Transmission Mode 3).
I tried to explain here , please let me know if we can explore more .
http://lte-question.blogspot.in/2013/07/rank-indicaton-in-lte.html
basic idea is to choose the precoder that maximizes the mutual information for a specific subcarrier- (1 . . .K) and temporal-range (1 . . .N) of interest (which is at least a single RB and can be up to the full system bandwidth and subframe duration)
The UE is requested to send either periodic or a periodic reports concerning
• Rank indication (RI)
Reports are applicable for closed- and open-loop spatial multiplexing. In case of open-loop TRI=1 corresponds to transmit diversity and TRI>1 to large delay CDD.
• Precoding matrix indicator (PMI)
PMI reporting is relevant for spatial multiplexing (open- and closed-loop), MU-MIMO, closed-loop RANK=1 precoding. PMI and RI are confined to a subset of the codebook which is created by means of a codebook Subset Restriction parameter.
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