TCI (Transmission Configuration Indicator) States & QCL (Quasi Co-Location) of 5G NR Ref Signals.

Below article is based on my interpretation of the TCI states & QCL concepts mentioned in 3GPP TS 38.213 & 38.214, I may not be 100% accurate with the interpretation, Experts please feel free to share your views.

What is QCL (Quasi Co Location): As per 3GPP TS 38.214 two Ref Signals can have a QCL relationship, QCL is " Two antenna ports are said to be quasi co-located if properties of the channel over which a symbol on one antenna port is conveyed can be inferred from the channel over which a symbol on the other antenna port is conveyed"

• For example two Ref signals X & Y are transmitted by the same same TRP(Antenna Array) applying same spatial filter would go through similar channel conditions & have similar channel properties.
• Since Ref signals X & Y have similar channel properties, On the receiver side UE will detect Ref signal X and it can apply the channel properties of X and detect Ref Signal Y, This is called Ref Signal X & Y are Quasi-Co Located.

What are the channel properties to make two signals Quasi-Co Located : There could be many factors that determine a channel condition, however as per 3GPP TS 38.214 5.1.5 ( Antenna ports quasi co-location) defines few channel properties as listed below.

• Doppler Shift : Doppler shift is a shift in the wavelength of the radio wave relative to motion of the receiver, example, eNB transmits a radio wave of a wavelength "x" how ever the receiver (UE) may be traveling away from the eNB so that same radio wave due to the distance of the UE by the time it reaches the UE the Wavelength is changed to "Y" , this effect is called Doppler shift, another simple example is the sound of a speeding car that comes close and speeds away, this is called Doppler shift of sound wave.
• Doppler Spread: Doppler Spread is also known as fading rate, Difference between the Signal Frequency at the Tx & Tx with respect to time is called Doppler spread, example: taking the speeding car example the rate at which the speeding car sound changes over time is called Doppler spread.
• Average Delay : In multi path scenario the average time taken to receive all the Multi path components at the receiver.
• Delay Spread : Difference between the time of arrival of the earliest significant multi path component (typically the line of sight component) and the time of arrival of the last multi path component.
• Spatial Rx Parameter: for Beam Forming

QCL Types based on channel properties : As per 3GPP TS 38.214 5.1.5, Based on above channel properties i have put together below table correlating QCL Types and their purpose.

QCL Relationship between NR Ref Signals : Again as per 38.214 5.1.5,Based on TRS info & Repetition in nzp-CSI-RS-ResourceSet & CSI Resource Type (periodic & a periodic) in csi-ResourceConfig, I have put together below table out lining possible QCL Relationship and their QCL Type's between NR ref signals for both sub 6 and mmwave, (In this article, Iam not going into details of the conditions on how the QCL Relationship's & Types are derived, 38.214 5.1.5 has all the details). I think i made it simpler by populating below table. "This was the most complex part to interpret in the spec" :) .

What is TCI (Transmission Configuration Indicator) State:

• A Transmission Configuration Indicator (TCI) states are dynamically sent over in a DCI message which includes configurations such as QCL-relationships between the DL RSs in one CSI-RS set and the PDSCH DMRS ports.
• UE can be configured with a list of up to "M" TCI-State configurations within the higher layer (RRC Re Config) parameter PDSCH-Config to decode PDSCH according to a detected PDCCH with DCI intended for the UE and the given serving cell.
• where M depends on the UE capability maxNumberActiveTCI-PerBWP
• Each TCI-State contains parameters for configuring a quasi co-location relationship between one or two downlink reference signals and the DM-RS ports of the PDSCH, the DM-RS port of PDCCH or the CSI-RS port(s) of a CSI-RS resource. 
• The quasi co-location (QCL) relationship is configured by the higher layer (RRC Reconfig) parameter qcl-Type1 for the first DL RS, and qcl-Type2 for the second DL RS , Maximum two qcl-types per TCI state can be configured.
• For the case of two DL RSs, the QCL types shall not be the same, regardless of whether the references are to the same DL RS or different DL RSs.

Keeping all the above pointers together, Below i have put together an example of how CSI-RS & SSB Ref Signals are QCL'ed for Beam Management, This example is for mmwave band n260 with SCS 120Khz .

 Beam Management Condition: SSB Beam switching or Wide Beam Switching due to serving beam RSRP is getting weaker, Based on the above table that i have populated below is the condition.Below example shows QCL relationship between CSI-RS & SSB

Current serving SSB Beam:

Beam Index of current serving SSB is 6 and BRSRP is -80, as the Beam RSRP is getting bad new TCI state id has been triggered by MAC DCI for SSB beam switch to a better beam.

New TCI State Id = 1 has been triggered , now by looking up the TCI Sate ID =1 from the TCI list provided the upper layer (RRC Reconfig) respective csi-rs and QCL Types are derived.

From above message TCI State id 1 has CSI-RS Ref Signal: 2 with two qcl-Types , TypeA (for channel estimation) & Type D (for Beam Forming)

Now to determine the Quasi co located SSB Index , Look up the csi-rs:2 against the corresponding nzp-CSI-RS-Resource from the list provided by upper later (RRC Re config) message.

From the above message nzp-CSI-RS-Resource Id 2 carries the qcl-InfoPeriodicCSI-RS 13 which needs to be looked up in the SSB TCI State's list provided by upper layer (RRC Re config message)

From above message the new SSB Index that the UE need to switch is SSB#3 and it has qcl-Types C & D, to perform RSRP measurements and do beam forming.

After the beam switching is completed SSB beam 3, will become the new Serving SSB Beam index as shown in below snap.

At this point based on the Quasi Co Location between CSI-RS and SSB, SSB Beam Switching has been successfully completed from SSB Beam 6 to SSB Beam 3

Like wise as per below table that i put together QCL relationship between PDCCH,PDSCH,SSB & CSI-RS can be determined.

Below is an example of PDCCH TCI State ID#0 is QCLed with  referenceSignal ssb : 2