in the uplink, the BS can directly estimate the channel from pilots, transmitted by the UE. From this estimated channel, the receiver (for example zero forcing) can be found. With this information, the transmission parameters (CQI, PMI and RI) are calculated. Then these values are transmitted back to the UE via a control channel, that is not explicitly modeled in our simulator. Now, the signalled CQI, PMI and RI values are used by the UE for the next transmission.
For details on how to optimize the feedback parameters from the estimated channel, please refer to the following works
S. Schwarz, C. Mehlführer, M. Rupp:
“Calculation of the Spatial Preprocessing and Link Adaption Feedback for 3GPP UMTS/LTE”
S. Schwarz, M. Wrulich, M. Rupp:
“Mutual Information based Calculation of the Precoding Matrix Indicator for 3GPP UMTS/LTE”