Beamforming & MIMO

TCI States —
Linking beams to channels

A Transmission Configuration Indicator (TCI) state associates a reference signal with a downlink channel — telling the UE which beam direction to use when receiving PDCCH or PDSCH. This section explains TCI states, how they are configured, how they are activated via MAC CE, and how they connect back to the SSB beam we detected.

TS 38.214 §5.1.5 TS 38.321 §6.1.3.14 TS 38.331 §6.3.2

What a TCI state is

When the gNB transmits PDSCH or PDCCH, it uses a specific beam. The UE must know which beam so it can apply the corresponding receive beamforming. A TCI state links a reference signal (SSB or CSI-RS) to a channel — effectively saying "this channel uses the same spatial filter as this reference signal."

The UE measures the reference signal, learns the spatial channel for that beam direction, and applies it when receiving the associated channel. This is quasi co-location (QCL): the UE assumes the channel and reference signal share certain propagation properties.

A TCI state provides a configuration for quasi-colocation relationships between a reference signal and another reference signal or physical channel. The UE uses the TCI state to determine the receive spatial filter for the downlink physical channel.

3GPP TS 38.214, Section 5.1.5

QCL types — four flavours

QCL types — what properties are sharedTS 38.214 §5.1.5, Table 5.1.5-1

QCL type	Shared property	Impact
QCL-TypeA	Doppler shift, Doppler spread, average delay, delay spread	Channel estimation accuracy
QCL-TypeB	Doppler shift, Doppler spread	Frequency offset tracking
QCL-TypeC	Doppler shift, average delay	Timing and frequency reference
QCL-TypeD	Spatial Rx parameter (beam direction)	Receive beamforming ← most important for 5G

TCI state configuration

TCI states are configured by RRC in tci-StatesToAddModList. Each TCI state has an ID and one or two reference signals. The gNB can configure up to 128 TCI states. Only a subset are active at any time.

TCI state configuration — our UE exampleTS 38.331 §6.3.2

// RRC configures 4 TCI states (simplified):
TCI-State ID=0:
  referenceSignal: SSB 0
  qcl-Type: TypeD
  ← UE uses SSB beam 0's spatial filter for PDSCH

TCI-State ID=1:
  referenceSignal: SSB 1
  qcl-Type: TypeD

TCI-State ID=2:
  referenceSignal: SSB 2
  qcl-Type: TypeD

TCI-State ID=3:
  referenceSignal: SSB 3    ← our best beam
  qcl-Type: TypeD
  ← UE points receive beam toward SSB 3 direction

// Additional for channel estimation quality:
TCI-State ID=3 also includes:
  referenceSignal2: CSI-RS resource 0
  qcl-Type: TypeA  ← for delay/Doppler estimation

TCI state activation via MAC CE

After RRC configures a set of TCI states, the gNB activates specific ones using a MAC CE. The TCI State Indication MAC CE is a 1-byte message containing two 3-bit TCI state IDs — one for each codeword (or for CORESET-0 and other CORESETs).

The UE shall, for each activated TCI state, apply the TCI state to the associated downlink physical channel or reference signal. The activation takes effect after a 3 ms delay from reception of the MAC CE.

3GPP TS 38.321, Section 6.1.3.14

TCI state activation — MAC CETS 38.321 §6.1.3.14

// TCI State Indication MAC CE (1 byte):
Bits [7:4]: Serving Cell ID = 0
Bits [3:0]: TCI state ID    = 3   ← activate TCI state 3 (SSB beam 3)

// Activation timing:
MAC CE received in slot n
TCI state becomes active: slot n + 3 ms = slot n + 6 slots (30 kHz)

// Effect:
// From slot n+6 onwards, UE uses SSB beam 3's receive spatial filter
// for all PDSCH on the configured CORESET
// The beam pointing matches the gNB's transmit beam for that TCI state

The complete beamforming loop — from SSB to data

We can now trace the complete beam management journey from initial cell search to steady-state data reception:

SSB beam sweep

UE scans 8 SSB beams. SSB index 3 has highest RSRP = −85 dBm. UE identifies best beam direction.

RACH on beam-linked PRACH

UE sends preamble on PRACH occasion linked to SSB 3. gNB receives on its beam 3 receive panel. Timing advance derived from arrival timing.

RRC configures TCI states

In RRCSetup (or RRCReconfiguration), gNB configures TCI states 0–7, each linked to one SSB beam. Also configures CSI-RS for finer channel estimation.

MAC CE activates TCI state 3

gNB sends TCI State Indication MAC CE selecting TCI=3. UE activates beam 3 spatial filter 3 ms later. PDSCH now received using optimal beam direction.

CSI-RS measurement → beam refinement

gNB periodically transmits narrow CSI-RS beams for refinement (P2). UE measures and reports PMI/CQI/RI. As UE moves, MAC CE updates the active TCI state to track the best beam.

TCI states — final summary

TCI state → links reference signal → channel beam direction

Our active TCI → TCI state 3 → SSB beam 3 → QCL-TypeD

Activation → MAC CE, takes effect after 3 ms

Refinement → CSI-RS P1/P2 procedures update TCI as UE moves

You can now derive every number in a 5G NR connection procedure from first principles: OFDM → cell search → PBCH/MIB → CORESET → SIB1 → RACH → RRC → scheduling → protocol stack → beamforming.

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