A cell decides almost everything, but it cannot decide well without knowing what the UE is experiencing. Six separate channels of information run upward for that reason, and each one produces a different command coming back down.

This page is an index to those six. Each section says in a few lines what the feedback is for, and then links to the page that covers it properly.

• UE feedback and Scheduling / Control
• (1) TPC : Power Control
• (2) Timing Control : Timing Advance
• (3) SR (Scheduling Request)
• (4) BSR (Buffer Status Report)
• (5) CQI (Channel Quality Indicator)
• (6) PMI(Precoding Matrix Information)/RI (Rank Indicator)
• Reference

UE feedback and Scheduling / Control

In LTE (actually in most of cellular communication), Network (eNB) and UE are interacting with each other in a kind of Master and Slave. It am not talking about such a clearly defined Master and Slave in IP communication. I am using the term Master / Slave in the sense that eNB (Network) decide on almost everything and UE just follows what the network tells them to do. However, the Master (eNB) in LTE is not such a heartless dictator who doesn't listen to others at all. The master's eary is always open to the slaves and they are constantly observing the slave to maintain the communication in best possible status with maximum efficiency of resource allocation.

Then you would ask what eNB is listening from UE and what eNB is observing ? Followings are a list of those things poping up in my mind. The number shown here does not have any special meaning ..  They are just the numbers for my explanation. You can assume that all of these things are running at the same time.  

The six exchanges, numbered as the sections below are. Blue runs toward the eNB and red runs back, and the three yellow boxes are what consumes each one.

• Every row has the same shape. A blue arrow labelled Input points at the eNB, and a red arrow labelled Output points back at the UE.
• Three yellow boxes sit on the eNB side. Power Controller takes row 1, Timing Controller takes row 2, and one tall Scheduler box takes rows 3 to 6.
• Rows 1 and 2 read measured by eNB, and rows 3 to 6 read from UE. The first two are things the network observes for itself, and the last four are things the UE chooses to send.
• Rows 3 and 4 produce the same output, UL Grant. SR and BSR differ in what they ask for rather than in what comes back.
• Rows 5 and 6 both act on PDSCH. CQI produces MCS Change (Code Rate Change) for PDSCH and PMI / RI produces PMI Index setting for PDSCH.

It would take a couple of separate pages for each topics. So I would not put down any detailed explanation here about these procedure. I will write down a few lines of comments here and give you the link that will lead you to the full details.

The same six rows as the drawing, in the order the sections below take them.

Reading the last column downward is the quickest way to see how the six divide. Two of them are handled outside the scheduler entirely. The other four are the scheduler deciding what the UE may send, and how the network should send back.

• Two of the six are not reports at all : TPC and Timing Advance start from something the eNB measures, so the UE sends nothing to trigger them.
• The same output can have two causes : SR and BSR both end in a UL Grant, and which one to use depends on how much data is waiting.
• CQI and PMI / RI both aim at PDSCH : one changes how robustly the data is coded and the other changes how it is spread across antennas.
• All six run at once : the numbering is for explanation, as the paragraph above says, and no order is implied by it.

(1) TPC : Power Control

This is the process by which the open loop power control performs. Basic mechanism is that eNB measures the uplink power (UE Tx power) and transmit the appropriate TPC commands until it gets the desired UE power.  See Power Control page for the details.

(2) Timing Control : Timing Advance

This is the mechanism by which eNB adjust UE transmission timing by observing the arrival time of PRACH signal or PUSCH and sending proper adjustment command. In case where eNB observes PRACH signal, it sends the adjustment command via RAR(RACH response) and in case where eNB observes PUSCH, it sends the adjusment command vis Timing Advanced MAC CE. See following pages for further details.

• RACH Procedure during Initial Registration - RACH Procedure Summary
• Timing Advance

(3) SR (Scheduling Request)

UE cannot transmit PUSCH (UL Data) on its own. It always should get approval (UL Grant) from eNB. What if eNB does not provide UL Grant when UE need to get ? In this case, UE can request eNB to provide UL Grant by sending SR (Scheduling Request). See Scheduling Request page for full details.

(4) BSR (Buffer Status Report)

SR is a physical layer message to request one time UL Grant. However, if UE needs large amount of UL Grants to send big chunk of data, it can send a MAC layer command called BSR and eNB would send multiple UL Grants in sequence enough for UE to transmit the data. See BSR page for the details.

(5) CQI (Channel Quality Indicator)

When radio channel quality gets poor and UE frequently fails to decode the downlink data (PDSCH), UE can notify the eNB to send data in more robust format (i.e, lower code rate). CQI is a kind of command from UE to eNB to adjust the code rate of the downlink data, then eNB can adjust the code rate by changing MCS.  See CQI page for the details.

(6) PMI(Precoding Matrix Information)/RI (Rank Indicator)

These two report (indicators) are used only for MIMO configuration. You can think of these of an indicator representing the quality of MIMO channel configuration. When these report are received by eNB, eNB changes MIMO channel configuration (Precoding Matrix) dynamically to optimize the formation of downlink data so that UE can easily decode it. See Closed Loop MIMO for the details.

Reference

• TS 36.321 v19.3.0 (Release 19) - E-UTRA Medium Access Control (MAC) protocol specification. Clause 5.4.4 Scheduling Request, clause 5.4.5 Buffer Status Reporting, and clause 6.1.3.5 Timing Advance Command MAC Control Element.
• TS 36.213 v19.4.0 (Release 19) - E-UTRA Physical layer procedures. Clause 4.2.3 Transmission timing adjustments, clause 5.1 Uplink power control, and clause 7.2 UE procedure for reporting Channel State Information (CSI).