CSI RS in a Nutshell

CSI RS  in Detail

As in LTE CSI, NR CSI (Channel Status Information) is a mechanism that a UE measure various radio channel quality and report the result to Network(gNB).

There are pretty complicated factors involved in CSI operation, but this page would focus on CSI signal generation and Resource Mapping. Other CSI operation procedure will be explained in other page.

For most of LTE case, we don't need any special signal for CSI since we used Cell Specific Reference Signal. However, from LTE TM9 we stared using a special reference signal for CSI. So if you have some understandings on LTE CSI Reference signal in TM9 or higher, it would be relatively easier for you to understand this page even if NR CSI RS(Reference Signal) is constructed more complicated way comparing to LTE CSI RS.

• Sequence Generation and Resource Mapping
• Tables for Resource Element Mapping
• Tables for CDM Sequence Generation
• RRC Parameter : NZP-CSI-RS-Resource
• How UE figure out which CSI-RS the network is using ? (CSI-RS-ResourceMapping)
• CSI RS Resource Mapping Examples
• Example 01 : p1, row1 = 0001, density = three, noCDM, timedomainsymbol = 4
• Example 02 : p1, row1 = 0100, density = three, noCDM, timedomainsymbol = 4
• Example 03 : p1, row1 = 1000, density = three, noCDM, timedomainsymbol = 4
• Example 04 : p2, row2 = 010000000000, density = one, noCDM, timedomainsymbol = 13
• Example 05 : p3, row2 = 001000, density = one, FD-CDM2, timedomainsymbol = 13
• Example 06 : p8, row6 = 011110, density = one, FD-CDM2, timedomainsymbol = 3
• Example 07 : p8, row7 = 000110, density = one, FD-CDM2, timedomainsymbol = 3
• Example 08 : p8, row8 = 000110, density = one, CDM4(FD2,TD2), timedomainsymbol = 3
•   CSI-RS Transmission Timing
• Resource for Interference Measurement (CSI-IM)
• Tracking Reference Signal (TRS)
• Zero Power(zp) CSI-RS
• Why ZP CSI RS ?
• Chat with Experts
• Papers/Articles
•   RRC Parameters for CSI RS
•   RRC Examples
• How to Avoid Other Signals ?
• Get the Test Procedure and Log / Amarisoft TechAcademy

Sequence Generation and Resource Mapping

As you may notice in the following equation, NR CSI is based on Pseudo Random Sequence. Then this sequence is multipied by sepcially designed weighting sequence in both time domain and frequency domain and than scaled by power scaling factor. And then this sequence is mapped to a set of specific resource elements in Resource Grid. All of these process can be summarized as follows and so many factors are involved in this procedure.

If you are not the physical layer developer who need to implement this part, you don't need to go through this to the full details, but it will be beneficial to pay attention to various parameters (especially RRC parameters) involved in this process.

Tables for Resource Element Mapping

Following table contains everything about the physical resource allocation for CSI-RS. If you just take a first look, it would be almost like an encrypted document and no idea on what it mean. Follow through the explaination in this note and try to get clear undestandings on examples and you will gradually get familiar with this table.

< 38.211-Table 7.4.1.5.3-1: CSI-RS locations within a slot. >

The reference location of CSI-RS in time domain is determined by RRC layer as below.

= CSI-RS-ResourceMapping.firstOFDMSymbolInTimeDomain

= CSI-RS-ResourceMapping.firstOFDMSymbolInTimeDomain2

The reference location of CSI-RS in frequency domain (k1,k2,k3) is determined by RRC layer as below

f(i) is the bit number of the i th bit in the bitmap(CSI-RS-ResourceMapping.frequencyDomainAllocation) set to one, repeated across every 1/density.

 

Ports, Density, cdm-Type are specified by following RRC parameters

• Ports = CSI-RS-ResourceMapping.nrofPorts
• cdm-Type = CSI-RS-ResourceMapping.cdm-Type
• Density = CSI-RS-ResourceMapping.density

Tables for CDM Sequence Generation

CDM table is used for CSI-RS signal generation as shown below.

Each element values of CDM tables are specified in 38.211 as shown below.

< 38.211 - Table 7.4.1.5.3-2: The sequences wf(k) f and wt(l) for cdm-Type equal to 'no CDM'. >

< 38.211 - Table 7.4.1.5.3-3: The sequences wf(k) f and wt(l) for cdm-Type equal to 'FD-CDM2'. >

< 38.211 - Table 7.4.1.5.3-4: The sequences wf(k) f and wt(l) for cdm-Type equal to 'CDM4'. >

< 38.211 - Table 7.4.1.5.3-5: The sequences wf(k) f and wt(l) for cdm-Type equal to 'CDM8'. >

RRC Parameter : NZP-CSI-RS-Resource

Almost all of the contents in this page is about the IE(Information Element) resourceMapping. The IE resourceMapping is a part of the NZP-CSI-RS-Resource as shown below.

NZP-CSI-RS-Resource ::= SEQUENCE {
   nzp-CSI-RS-ResourceId      NZP-CSI-RS-ResourceId,
   resourceMapping            CSI-RS-ResourceMapping,
   powerControlOffset         INTEGER (-8..15),
   powerControlOffsetSS       ENUMERATED{db-3, db0, db3, db6} OPTIONAL, -- Need R
   scramblingID               ScramblingId,
   periodicityAndOffset       CSI-ResourcePeriodicityAndOffset OPTIONAL,-
   qcl-InfoPeriodicCSI-RS     TCI-StateId OPTIONAL, -- Cond Periodic
   ...
}
                                

resourceMapping : Refer to the section CSI-RS-ResourceMapping for the details.

powerControlOffset : Power offset of PDSCH RE to NZP CSI-RS RE. Value in dB

powerControlOffsetSS : Power offset of NZP CSI-RS RE to SSS RE. Value in dB

qcl-InfoPeriodicCSI-RS : For a target periodic CSI-RS, contains a reference to one TCI-State in TCI-States for providing the QCL source and QCL type. For periodic CSI-RS, the source can be SSB or another periodic-CSI-RS. Refers to the TCI-State which has this value for tci-StateId and is defined in tci-StatesToAddModList in the PDSCH-Config included in the BWPDownlink corresponding to the serving cell and to the DL BWP to which the resource belongs to.

In short, this indicates the tci-StateId to which this CSI-RS is QCLed.

How UE figure out which CSI-RS the network is using ? (CSI-RS-ResourceMapping)

When Network allocate the CSI-RS, it selects a specific row from 38.211-Table 7.4.1.5.3-1 and fill in a set of resource elements with the specific signal. This part would be straightforward.. the challenging part is how UE can figure out which CSI-RS the gNB(Network) is using.

The answer is that Network informs UE of all the details of CSI-RS via RRC message. Let's look into what kind of RRC parameters get involved in this process.

How to determine CSI-RS port and cdm (in Row number of 38.211 Table 7.4.1.5.3-1) ?  

:This is specified by nrofPorts and cdm-Type in CSI-RS-ResourceMapping as shown below.

CSI-RS-ResourceMapping ::= SEQUENCE {
   frequencyDomainAllocation CHOICE {
      row1     BIT STRING (SIZE (4)),
      row2     BIT STRING (SIZE (12)),
      row4     BIT STRING (SIZE (3)),
      other    BIT STRING (SIZE (6))
   },
   nrofPorts                       ENUMERATED {p1,p2,p4,p8,p12,p16,p24,p32},
   firstOFDMSymbolInTimeDomain     INTEGER (0..13),
   firstOFDMSymbolInTimeDomain2    INTEGER (2..12) OPTIONAL, -- Need R
   cdm-Type                        ENUMERATED {noCDM, fd-CDM2, cdm4-FD2-TD2, cdm8-FD2-TD4},
   density CHOICE {
     dot5      ENUMERATED {evenPRBs, oddPRBs},
     one       NULL,
     three     NULL,
     spare     NULL
   },
   freqBand    CSI-FrequencyOccupation,
   ...
}
                

How to determine REs(Resource Elements) for the selected CSI-RS ?

: This is configured by (k_bar, l_bar) column and frequencyDomainAllocation and firstOFDMSymbolInTimeDomain of CSI-RS-ResourceMapping as shown below.

CSI-RS-ResourceMapping ::= SEQUENCE {
   frequencyDomainAllocation CHOICE {
      row1     BIT STRING (SIZE (4)),
      row2     BIT STRING (SIZE (12)),
      row4     BIT STRING (SIZE (3)),
      other    BIT STRING (SIZE (6))
   },
   nrofPorts                       ENUMERATED {p1,p2,p4,p8,p12,p16,p24,p32},
   firstOFDMSymbolInTimeDomain     INTEGER (0..13),
   firstOFDMSymbolInTimeDomain2    INTEGER (2..12) OPTIONAL, -- Need R
   cdm-Type                        ENUMERATED {noCDM, fd-CDM2, cdm4-FD2-TD2, cdm8-FD2-TD4},
   density CHOICE {
     dot5      ENUMERATED {evenPRBs, oddPRBs},
     one       NULL,
     three     NULL,
     spare     NULL
   },
   freqBand    CSI-FrequencyOccupation,
   ...
}

CSI-FrequencyOccupation ::= SEQUENCE {
   startingRB    INTEGER (0..maxNrofPhysicalResourceBlocks-1),
   nrofRBs       INTEGER (24..maxNrofPhysicalResourceBlocksPlus1),
   ...
}
                                

startingRB : PRB where this CSI resource starts in relation to common resource block #0 (CRB#0) on the common resource block grid. Only multiples of 4 are allowed (0, 4, ...)

nrofRBs : Number of PRBs across which this CSI resource spans. Only multiples of 4 are allowed. The smallest configurable number is the minimum of 24 and the width of the associated BWP. If the configured value is larger than the width of the corresponding BWP, the UE shall assume that the actual CSI-RS bandwidth is equal to the width of the BWP.

Combining thse two, we come come up with an example as shown below.

How to figure out what kind of (Logical) Antenna Configuration Network is using ?

: This is another complicated story and is explained in a separate page here.

CSI RS Resource Mapping Examples

This section is to show various examples of configuring following RRC Parameters.

CSI-RS-ResourceMapping ::= SEQUENCE {
   frequencyDomainAllocation CHOICE {
      row1     BIT STRING (SIZE (4)),
      row2     BIT STRING (SIZE (12)),
      row4     BIT STRING (SIZE (3)),
      other    BIT STRING (SIZE (6))
   },
   nrofPorts                       ENUMERATED {p1,p2,p4,p8,p12,p16,p24,p32},
   firstOFDMSymbolInTimeDomain     INTEGER (0..13),
   firstOFDMSymbolInTimeDomain2    INTEGER (2..12) OPTIONAL, -- Need R
   cdm-Type                        ENUMERATED {noCDM, fd-CDM2, cdm4-FD2-TD2, cdm8-FD2-TD4},
   density CHOICE {
     dot5      ENUMERATED {evenPRBs, oddPRBs},
     one       NULL,
     three     NULL,
     spare     NULL
   },
   freqBand    CSI-FrequencyOccupation,
   ...
}
                                

Followings are some of the examples showing the location (Resource Elements) within a PRB(physical resource block). Since these examples are showing the location only within one RB, n N^RB_sc term is removed in the equation determining k value.

Example 01 >

Given the following RRC parameters,

density = three

nrofPorts = p1

cdm-Type = noCDM

frequencyDomainAllocation.row1 = 0001

firstOFDMSymbolinTimeDomain = 4

According to 38.211-Table 7.4.1.5.3-1,

k prime = {0} ==> k prime[0] = 0

l prime = {0}  ==> l prime[0] = 0

According to the given, RRC parameters

k0 = 0 (based on frequencyDomainAllocation.row1 = 0001)

l0 = 4(based on firstOFDMSymbolinTimeDomain = 4)

Based on all these information and , we get 38.211-Table 7.4.1.5.3-1,

(k,l) = (k0+kprim[0],l0+lprim[0]) ,(k0+4+kprim[0],l0+lprim[0]) ,(k0+8+kprim[0],l0+lprim[0])

       = (0+0,4+0) ,(0+4+0,4+0) ,(0+8+0,4+0)

       = (0,4) ,(4,4) ,(8,4)

Example 02 >

Given the following RRC parameters,

density = three

nrofPorts = p1

cdm-Type = noCDM

frequencyDomainAllocation.row1 = 0100

firstOFDMSymbolinTimeDomain = 4

According to 38.211-Table 7.4.1.5.3-1,

k prime = {0} ==> k prime[0] = 0

l prime = {0}  ==> l prime[0] = 0

According to the given, RRC parameters

k0 = 2 (based on frequencyDomainAllocation.row1 = 0100)

l0 = 4(based on firstOFDMSymbolinTimeDomain = 4)

Based on all these information and , we get 38.211-Table 7.4.1.5.3-1,

(k,l) = (k0+kprim[0],l0+lprim[0]) ,(k0+4+kprim[0],l0+lprim[0]) ,(k0+8+kprim[0],l0+lprim[0])

       = (2+0,4+0) ,(2+4+0,4+0) ,(2+8+0,4+0)

       = (2,4) ,(6,4) ,(10,4)

Example 03 >

Given the following RRC parameters, (this example is based on TRS case in 38.508-1 Table 4.6.3-45: CSI-RS-ResourceMapping)

density = three

nrofPorts = p1

cdm-Type = noCDM

frequencyDomainAllocation.row1 = 1000 // From density,nrofPorts we can guess that this is for row1.

firstOFDMSymbolinTimeDomain = 4

According to 38.211-Table 7.4.1.5.3-1,

k prime = {0} ==> k prime[0] = 0

l prime = {0}  ==> l prime[0] = 0

According to the given, RRC parameters

k0 = 3 (based on frequencyDomainAllocation.row1 = 1000)

l0 = 4(based on firstOFDMSymbolinTimeDomain = 4)

Based on all these information and , we get 38.211-Table 7.4.1.5.3-1,

(k,l) = (k0+kprim[0],l0+lprim[0]) ,(k0+4+kprim[0],l0+lprim[0]) ,(k0+8+kprim[0],l0+lprim[0])

       = (3+0,4+0) ,(3+4+0,4+0) ,(3+8+0,4+0)

       = (3,4) ,(7,4) ,(11,4)

Example 04 >

Given the following RRC parameters,

density = one

nrofPorts = p1

cdm-Type = noCDM

frequencyDomainAllocation.row2 = 010000000000

firstOFDMSymbolinTimeDomain = 13

According to 38.211-Table 7.4.1.5.3-1,

k prime = {0} ==> k prime[0] = 0

l prime = {0}  ==> l prime[0] = 0

According to the given, RRC parameters

k0 = 10 (based on frequencyDomainAllocation.row2 = 010000000000)

l0 = 13(based on firstOFDMSymbolinTimeDomain = 13)

Based on all these information and , we get 38.211-Table 7.4.1.5.3-1,

(k,l) = (k0+kprim[0],l0+lprim[0])

       = (10+0,13+0)

       = (10,13)

Example 05 >

Given the following RRC parameters,

density = one

nrofPorts = p2

cdm-Type = FD-CDM2

frequencyDomainAllocation.other = 001000

firstOFDMSymbolinTimeDomain = 13

Assume that row = 3 (you need additional information to determine the row number other than 1,2,4. In this example, it is given)

According to 38.211-Table 7.4.1.5.3-1,

k prime = {0,1} ==> k prime[0] = 0, k prime[1] = 1

l prime = {0}  ==> l prime[0] = 0

According to the given, RRC parameters

k0 = 2 x 3(bit position) = 6 (based on frequencyDomainAllocation.other = 001000)

l0 = 13(based on firstOFDMSymbolinTimeDomain = 13)

Based on all these information and , we get 38.211-Table 7.4.1.5.3-1,

(k,l) = (k0+kprim[0],l0+lprim[0]),(k0+kprim[1],l0+lprim[0])

       = (6+0,13+0),(6+1,13+0)

       = (6,13),(7,13)

Example 06 >

Given the following RRC parameters, (this example is based on FR1 case in 38.508-1 Table 4.6.3-45: CSI-RS-ResourceMapping)

density = one

nrofPorts = p8

cdm-Type = fd-CDM2

frequencyDomainAllocation.other = 011110 //From density,nrofPorts,cdm-Tye it is assumed that this is for row6.

firstOFDMSymbolinTimeDomain = 3

According to 38.211-Table 7.4.1.5.3-1,

k prime = {0,1} ==> k prime[0] = 0, k prime[1] = 1

l prime = {0}  ==> l prime[0] = 0

According to the given, RRC parameters

k0 = 2 (based on frequencyDomainAllocation.other = 011110)

k1 = 4 (based on frequencyDomainAllocation.other = 011110)

k2 = 6 (based on frequencyDomainAllocation.other = 011110)

k3 = 8 (based on frequencyDomainAllocation.other = 011110)

l0 = 3(based on firstOFDMSymbolinTimeDomain = 3)

Based on all these information and , we get 38.211-Table 7.4.1.5.3-1,

(k,l) = (k0+kprim[0],l0+lprim[0]) ,(k1+kprim[0],l0+lprim[0]) ,(k2+kprim[0],l0+lprim[0]), (k3+kprim[0],l0+lprim[0])

         (k0+kprim[1],l0+lprim[0]) ,(k1+kprim[1],l0+lprim[0]) ,(k2+kprim[1],l0+lprim[0]), (k3+kprim[1],l0+lprim[0])

       = (2+0,3+0) ,(4+0,3+0) ,(6+0,3+0),(8+0,3+0),(2+1,3+0) ,(4+1,3+0) ,(6+1,3+0),(8+1,3+0)

       = (2,3) ,(4,3) ,(6,3) ,(8,3), (3,3) ,(5,3) ,(7,3) ,(9,3)

NOTE : The above resource mapping  is showing the CSI-RS for all antenna ports (8 ports in this case) superimposed in one resource grid. In real transmission, these resources are devided among the 8 CSI-RS antenna ports. How they are splitted among each CSI-RS antenna port is specified in 38.211 section 7.4.1.5.3. Applying the logic in the spec to this example, the antenna port distribution become as follows.

Since this example is using FD-CDM2, it is assumed that 's' is determined by the index value of the following table and 'L' becomes 2. N is given in this example (38.211-Table 7.4.1.5.3-1) is 8.

< 38.211 - Table 7.4.1.5.3-3: The sequences wf(k) f and wt(l) for cdm-Type equal to 'FD-CDM2'. >

Based on this information, j and s can be calculated as follows.

s = 0,1 (index of Table 7.4.1.5.3-3)

j = 0,1,...,N/L-1 = 0,1,...,(8/2-1) = 0,1,2,3

Now we can calculate the antenna port as follows.

p = 3000 + s + j L , where s = {0,1}, j = {0,1,2,3}

   = 3000 + {0,1} + {0,1,2,3} 2

   = {3000, 3002, 3004, 3006, 3001, 3003, 3005, 3007}

NOTE : I posted Matlab simulation for this case showing the resource mapping for each antenna port on this note.

Example 07 >

Given the following RRC parameters, (this example is based on FR1 case in 38.508-1 Table 4.6.3-45: CSI-RS-ResourceMapping)

density = one

nrofPorts = p8

cdm-Type = fd-CDM2

frequencyDomainAllocation.other = 000110 //From density,nrofPorts,cdm-Tye it is assumed that this is for row7.

firstOFDMSymbolinTimeDomain = 3

According to 38.211-Table 7.4.1.5.3-1,

k prime = {0,1} ==> k prime[0] = 0, k prime[1] = 1

l prime = {0}  ==> l prime[0] = 0

According to the given, RRC parameters

k0 = 2 (based on frequencyDomainAllocation.other = 000110)

k1 = 4 (based on frequencyDomainAllocation.other = 000110)

l0 = 3(based on firstOFDMSymbolinTimeDomain = 3)

Based on all these information and , we get 38.211-Table 7.4.1.5.3-1,

(k,l) = (k0+kprim[0],l0+lprim[0]) ,(k1+kprim[0],l0+lprim[0]) ,

         (k0+kprim[0],l0+lprim[0]+1), (k1+kprim[0],l0+lprim[0]+1)

         (k0+kprim[1],l0+lprim[0]) ,(k1+kprim[1],l0+lprim[0]) ,

         (k0+kprim[1],l0+lprim[0]+1), (k1+kprim[1],l0+lprim[0]+1)

       = (2+0,3+0) ,(4+0,3+0) ,(2+0,3+0+1) ,(4+0,3+0+1),(2+1,3+0) ,(4+1,3+0) ,(2+1,3+0+1) ,(4+1,3+0+1)

       = (2,3) ,(4,3) ,(2,4),(4,4),(3,3),(5,3),(3,4),(5,4)

NOTE : The above resource mapping  is showing the CSI-RS for all antenna ports (8 ports in this case) superimposed in one resource grid. In real transmission, these resources are devided among the 8 CSI-RS antenna ports. How they are splitted among each CSI-RS antenna port is specified in 38.211 section 7.4.1.5.3. Applying the logic in the spec to this example, the antenna port distribution become as follows.

Since this example is using FD-CDM2, it is assumed that 's' is determined by the index value of the following table and 'L' becomes 2. N is given in this example (38.211-Table 7.4.1.5.3-1) is 8.

< 38.211 - Table 7.4.1.5.3-3: The sequences wf(k) f and wt(l) for cdm-Type equal to 'FD-CDM2'. >

Based on this information, j and s can be calculated as follows.

s = 0,1 (index of Table 7.4.1.5.3-3)

j = 0,1,...,N/L-1 = 0,1,...,(8/2-1) = 0,1,2,3

Now we can calculate the antenna port as follows.

p = 3000 + s + j L , where s = {0,1}, j = {0,1,2,3}

   = 3000 + {0,1} + {0,1,2,3} 2

   = {3000, 3002, 3004, 3006, 3001, 3003, 3005, 3007}

NOTE : I posted Matlab simulation for this case showing the resource mapping for each antenna port on this note.

Example 08 >

Given the following RRC parameters, (this example is based on FR1 case in 38.508-1 Table 4.6.3-45: CSI-RS-ResourceMapping)

density = one

nrofPorts = p8

cdm-Type = CDM4(FD2,TD2)

frequencyDomainAllocation.other = 000110 //From density,nrofPorts,cdm-Tye it is assumed that this is for row8.

firstOFDMSymbolinTimeDomain = 3

According to 38.211-Table 7.4.1.5.3-1,

k prime = {0,1} ==> k prime[0] = 0, k prime[1] = 1

l prime = {0,1}  ==> l prime[0] = 0, l prime[1] = 1

According to the given, RRC parameters

k0 = 2 (based on frequencyDomainAllocation.other = 000110)

k1 = 4 (based on frequencyDomainAllocation.other = 000110)

l0 = 3(based on firstOFDMSymbolinTimeDomain = 3)

Based on all these information and , we get 38.211-Table 7.4.1.5.3-1,

(k,l) = (k0+kprim[0],l0+lprim[0]) ,(k1+kprim[0],l0+lprim[0]),

         (k0+kprim[1],l0+lprim[0]) ,(k1+kprim[1],l0+lprim[0]),

         (k0+kprim[0],l0+lprim[1]) ,(k1+kprim[0],l0+lprim[1]),

         (k0+kprim[1],l0+lprim[1]) ,(k1+kprim[1],l0+lprim[1]),

       = (2+0,3+0) ,(4+0,3+0) ,(2+1,3+0) ,(4+1,3+0),(2+0,3+1) ,(4+0,3+1) ,(2+1,3+1) ,(4+1,3+1)

       = (2,3) ,(4,3) ,(3,3),(5,3),(2,4),(4,4),(3,4),(5,4)

NOTE : wft0 = wf(0) wt(0), wft1 = wf(1) wt(1)

NOTE : The above resource mapping  is showing the CSI-RS for all antenna ports (8 ports in this case) superimposed in one resource grid. In real transmission, these resources are devided among the 8 CSI-RS antenna ports. How they are splitted among each CSI-RS antenna port is specified in 38.211 section 7.4.1.5.3. Applying the logic in the spec to this example, the antenna port distribution become as follows.

Since this example is using FD-CDM2, it is assumed that 's' is determined by the index value of the following table and 'L' becomes 4. N is given in this example (38.211-Table 7.4.1.5.3-1) is 8.

< 38.211 - Table 7.4.1.5.3-4: The sequences wf(k) f and wt(l) for cdm-Type equal to 'CDM4'. >

Based on this information, j and s can be calculated as follows.

s = 0,1,2,3 (index of Table 7.4.1.5.3-4)

j = 0,1,...,N/L-1 = 0,1,...,(8/4-1) = 0,1

Now we can calculate the antenna port as follows.

p = 3000 + s + j L , where s = {0,1}, j = {0,1,2,3}

   = 3000 + {0,1,2,3} + {0,1} 4

   = {3000, 3004, 3004, 3006, 3001, 3003, 3005, 3007}

NOTE : I posted Matlab simulation for this case showing the resource mapping for each antenna port on this note.

CSI RS Transmission Timing

CSI Transmission Timing in slot is determined by RRC parameter CSI-ResourcePeriodicityAndOffset based on following equation.

Resource for Interference Measurement (CSI-IM)

CSI IM resource is a set of specific resource elements reserved for Interference Measurement. This resources are configurable by RRC message. The frequency and time domain location is defined in 38.214 - 5.2.2.4 as illustrated below.

CSI-IM-Resource ::= SEQUENCE {
   csi-IM-ResourceId                   CSI-IM-ResourceId,
   csi-IM-ResourceElementPattern       CHOICE {
      pattern0 SEQUENCE {
         subcarrierLocation-p0         ENUMERATED { s0, s2, s4, s6, s8, s10 },
         symbolLocation-p0             INTEGER (0..12)
      },
      pattern1 SEQUENCE {
         subcarrierLocation-p1         ENUMERATED { s0, s4, s8 },
         symbolLocation-p1             INTEGER (0..13)
      }
   } OPTIONAL, -- Need M
   freqBand                            CSI-FrequencyOccupation OPTIONAL, 
   periodicityAndOffset                CSI-ResourcePeriodicityAndOffset OPTIONAL, 
   PeriodicOrSemiPersistent
...
}
                                

pattern 0 / pattern1 : illustrated above.

CSI-FrequencyOccupation ::= SEQUENCE {

   startingRB    INTEGER (0..maxNrofPhysicalResourceBlocks-1),

   nrofRBs       INTEGER (24..maxNrofPhysicalResourceBlocksPlus1),

   ...

}

startingRB : PRB where this CSI resource starts in relation to common resource block #0 (CRB#0) on the common resource block grid. Only multiples of 4 are allowed (0, 4, ...)

nrofRBs : Number of PRBs across which this CSI resource spans. Only multiples of 4 are allowed. The smallest configurable number is the minimum of 24 and the width of the associated BWP. If the configured value is larger than the width of the corresponding BWP, the UE shall assume that the actual CSI-RS bandwidth is equal to the width of the BWP.

Tracking Reference Signal (TRS)

5G TRS, or Tracking Reference Signal, is a key component in 5G wireless technology, primarily used for beam management. TRS in 5G plays a crucial role in maintaining time and frequency synchronization, particularly in high mobility scenarios, distinguishing it from CSI-RS, which is more aligned with beam management and channel quality assessment

NZP-CSI-RS-ResourceSet ::= SEQUENCE {
   nzp-CSI-ResourceSetId     NZP-CSI-RS-ResourceSetId,
   nzp-CSI-RS-Resources      SEQUENCE (SIZE (1..maxNrofNZP-CSI-RS-ResourcesPerSet)) 
                                  OF NZP-CSI-RS-ResourceId,
   repetition                ENUMERATED { on, off } OPTIONAL, 
   aperiodicTriggeringOffset INTEGER(0..4) OPTIONAL, 
   trs-Info                  ENUMERATED {true} OPTIONAL, 
   ...
}
                

38.331 defines trs-Info as follows.

trs-Info : Indicates that the antenna port for all NZP-CSI-RS resources in the CSI-RS resource set is same. If the field is absent or released the UE applies the value false

38.214-5.2.2.3.1 provides additional information as follows :

trs-Info in NZP-CSI-RS-ResourceSet is associated with a CSI-RS resource set and for which the UE can assume that the antenna port with the same port index of the configured NZP CSI-RS resources in the NZP-CSI-RSResourceSet is the same as described in Subclause 5.1.6.1.1 and can be configured when reporting setting is not configured or when the higher layer parameter reportQuantity associated with all the reporting settings linked with the CSI-RS resource set is set to 'none'.

38.214-5.1.6.1.1 specifies the condition about how multiple csi-rs is grouped into a trs as stated below.

For a NZP-CSI-RS-ResourceSet configured with the higher layer parameter trs-Info, the UE shall assume the antenna

port with the same port index of the configured NZP CSI-RS resources in the NZP-CSI-RS-ResourceSet is the same. For frequency range 1(FR1), the UE may be configured with one or more NZP CSI-RS set(s), where a NZP-CSI-RS-ResourceSet consists of four periodic NZP CSI-RS resources in two consecutive slots with two periodic NZP CSI-RS resources in each slot.

For frequency range 2(FR2) the UE may be configured with one or more NZP CSI-RS set(s), where a NZP-CSI-RSResourceSet consists of two periodic CSI-RS resources in one slot or with a NZP-CSI-RS-ResourceSet of four periodic NZP CSI-RS resourcesin two consecutive slots with two periodic NZP CSI-RS resources in each slot.

NOTE : trs-info setting affects the applicable QCL type which is described in this note.

Zero Power CSI-RS

As shown in the configuration below, in terms of resource element configuration and resource set definition it is same as NZP CSI-RS. But there are two big differences between NZP CSI-RS and ZP CSI RS as below

• ZP CSI-RS is a configuration within PDSCH-Config. It means the location of ZP CSI RS would be tightly associated with the area scheduled for PDSCH.
• ZP-CSI-RS does not transmit any signal. As the term 'ZP(zero power)', this is like a 'blanked resource element' that does not transmit any signal/data.

PDSCH-Config ::=                        SEQUENCE {
    ...
    zp-CSI-RS-ResourceToAddModList          SEQUENCE (SIZE (1..maxNrofZP-CSI-RS-Resources))
                                                OF ZP-CSI-RS-Resource   OPTIONAL,   -- Need N
    zp-CSI-RS-ResourceToReleaseList         SEQUENCE (SIZE (1..maxNrofZP-CSI-RS-Resources))
                                                OF ZP-CSI-RS-ResourceId OPTIONAL,   -- Need M
    aperiodic-ZP-CSI-RS-ResourceSetsToAddModList    SEQUENCE (SIZE (1..maxNrofZP-CSI-RS-Sets))
                                                OF ZP-CSI-RS-ResourceSet   OPTIONAL, -- Need N
    aperiodic-ZP-CSI-RS-ResourceSetsToReleaseList   SEQUENCE (SIZE (1..maxNrofZP-CSI-RS-Sets))
                                                OF ZP-CSI-RS-ResourceSetId   OPTIONAL, -- Need N
    sp-ZP-CSI-RS-ResourceSetsToAddModList   SEQUENCE (SIZE (1..maxNrofZP-CSI-RS-Sets))
                                                OF ZP-CSI-RS-ResourceSet     OPTIONAL, -- Need N
    sp-ZP-CSI-RS-ResourceSetsToReleaseList  SEQUENCE (SIZE (1..maxNrofZP-CSI-RS-Sets))
                                                OF ZP-CSI-RS-ResourceSetId   OPTIONAL, -- Need N

    ...
}

ZP-CSI-RS-Resource ::= SEQUENCE {
   zp-CSI-RS-ResourceId                 ZP-CSI-RS-ResourceId,
   resourceMapping                         CSI-RS-ResourceMapping,
   periodicityAndOffset                    CSI-ResourcePeriodicityAndOffset OPTIONAL, --Cond PeriodicOrSemiPersistent
   ...
}

ZP-CSI-RS-ResourceSet ::= SEQUENCE {
   zp-CSI-RS-ResourceSetId             ZP-CSI-RS-ResourceSetId,
   zp-CSI-RS-ResourceIdList             SEQUENCE (SIZE(1..maxNrofZP-CSI-RS-ResourcesPerSet)) 
                                                            OF ZP-CSI-RS-ResourceId,
   ...
}

ZP-CSI-RS-ResourceSetId ::= INTEGER (0..maxNrofZP-CSI-RS-ResourceSets-1)
                

Why ZP CSI RS ?

In terms of configuration of ZP CSI RS, it looks clear since it is same as NZP CSI RS, but it does not look clear (at least to me) on why we need ZP CSI RS ? Why this concept is introduced ?

To be honest, this is still open question to me. So I am trying to complile various opinions in this section. If you have any different opinions, ping me via email or linkedIn.

< Chat with Experts >

First I had chance to chat with Sakshama Ghoslya. He is working as L1/L2 stack developer with better knowledge than I. So the chat went as I raised question and he answered. The chat was pretty long back-and-forth but I tried to compacted as much as possible and I am sharing this with the approval of Sakshama.

[Sharetechnote] Why do we need ZP CSI-RS ? What is it designed for ?  Vaguely I think it would act in a similar role as CSI-IM which can be a reference power measurement for Noise Floor(Serving cell only environment) or interference power from neighbour cells(Serving cell + neighbour cell environment).

[Sakshama] ZP-CSI-RS can be used for the following purpose:

1. Beam Mobility: gNB can configure multiple beams with identical physical layer settings and the UE finds out which beam is the best based on the power levels. For an example consider 2 adjacent beams B0 and B1. at a particular time T0, gNB can configure B0 as ZP-CSI-RS & B1 as NZP-CSI-RS and at time T1, it can configure B0 as NZP-CSI-RS & B1 as ZP-CSI-RS. This helps UE to measure CSI-RSRP individually. This helps gNB to decide (based on CQI reports) which beam to use for the UE (B0 or B1). Whether to switch or not.

Image Source : Zero Power CSI-RS (ZP-CSI-RS) in 5G NR

2. Interference Awareness/Measurement: In this case ZP-CSI-RS is configured as CSI-IM. It allows the UE to obtain awareness of the interference. In this case gNB makes sure the ZP-CSI-RS resources are configured such that they collide with PDSCH resources of the neighbouring cells.

[Sharetechnote] Are there any other use cases you can think of ?

[Sakshama] Yes, ZP-CSI-RS can also be used for PDSCH rate matching. When a transport block is not equal to the PDSCH scheduled resources, the rate has to match. gNB can use ZP-CSI-RS resource elements to match the data rate. Configuration of ZP-CSI-RS is the same as CSI-IM, just the purpose is different.

Image Source : Zero Power CSI-RS (ZP-CSI-RS) in 5G NR

< Papers/Articles >

I want to quote statements from some papers/aritcles that I have read.

VI-C of this paper would state as follows:

Zero-power CSI-RS can be used as a masking tool to protect certain REs by making them unavailable for PDSCH mapping. This masking supports transmission of UE specific CSI-RS, but the design is also a tool for allowing introduction of new features to NR with retained backward compatibility

It is worth noting the statements from this paper :

• It should be clear that CSI-IM and ZP CSIRS have different functions, where CSI-IM defines the set of resource elements from which the interference is measured, and ZP CSI-RS defines a set of resource elements where physical downlink shared channel (PDSCH) is not mapped and UE can not make any assumptions of the content of these resources (Section III-B)-2))
• In CSI-IM method, the NZP CSI-RSs are protected from inter-cell-interference by allocating either CSI-IM or ZP CSI-RS to the REs overlapping with NZP CSI-RSs from other gNBs. This allows the UE to obtain higher quality channel measurement due to reduced interference on top of the NZP CSI-RS (Section III-B)-2))

NOTE : I think you need to pay close attention to reading this paper and try to clearly understand the meaning of each figures and the test result. For me, two figures in the paper got my attention Fig 2 and Fig 3 as shown below.

First let look into the details of resource allocations in Fig 2 and try to understand the implications of each resource allocations.

For all cells, the symbol for DMRS are same, but further details of DMRS is not described in the paper.

[a] only NZP-CSI RS are configured for every cells (Serving gNB, gNB1, gNB2) in PDSCH regions. And the RE position of NZP CSI-RS for every cells are same. This implies that NZP CSI-RS from gNB1, gNB2 would interfere with NZP CSI RS of serving cell. In other words, NZP CSI-RS of Serving gNB is interfered by NZP CSI RS of gNB1 and gNB2. ==> This is labeld as [NZP CSI-RS non precoded IM]

[b] only NZP-CSI RS are configured for every cells (Serving gNB, gNB1, gNB2) in PDSCH regions. And the RE position of NZP CSI-RS for every cells are different in such a way that the position does not overlap with CSI-RS of any other cells. In this case, NZP CSI-RS of Serving gNB is interfered by PDSCH data RE of gNB1 and gNB2. ==> This is labeld as [NZP CSI-RS precoded IM]

[c] NZP-CSI RS and CSI-IM are configured for every cells (Serving gNB, gNB1, gNB2) in PDSCH regions. And the RE position of NZP CSI-RS for every cells are different in such a way that the position does not overlap with CSI-RS of any other cells. In this case, NZP CSI-RS of Serving gNB is NOT interfered (theoretically) by PDSCH data RE of gNB1/gNB2 nor NZP CSI-RS RE of gNB1/gNB2. ==> This is labeld as [CSI-IM non precoded IM]]

[d] NZP-CSI RS, CSI-IM and ZP CSI-RS are configured for every cells (Serving gNB, gNB1, gNB2) in PDSCH regions. And the RE position of NZP CSI-RS for every cells are different in such a way that the position does not overlap with CSI-RS of any other cells. In this configuation, the RE position of ZP CSI-RS is same as CSI-IM position of [c]. In this case, NZP CSI-RS of Serving gNB is NOT interfered (theoretically) by PDSCH data RE of gNB1/gNB2 nor NZP CSI-RS RE of gNB1/gNB2. ==> This is labeld as [CSI-IM precoded IM]]

BLER and Throughput measurement for each of the configuration are shown in Fig 3. In short, it is shown that when SINR is very good (e.g, greater than 25), you wouldn't see any differences in terms of performance regardless of which configuration you use. When SINR is poor (e.g, around cell boundary), non-precoded IM shows better performance comparing to precoded IM.

Image Source : Interference Measurement Methods in 5G NR: Principles and Performance - Fig 2

Image Source : Interference Measurement Methods in 5G NR: Principles and Performance - Fig 3

RRC Parameters for CSI RS

The hierarchy to configure CSI-RS is pretty complicated. Overall hierachy (procedure) to configure CSI-RS can be described as follows.

i) Configure CRS-RS resource element within single RB via CSI-RS-ResourceMapping

ii) Configure the RB locations(Start RB and N_RB)  in frequency domain via CSI-RS-ResourceMapping.freqBand

iii) Configure the periodicity and offset of CSI-RS in time domain via ZP-CSI-RS-Resource and/or NZP-CSI-RS-Resource

iv) Apply the whole setting(i, ii, iii) via PDSCH-Config or/and CSI-MeasConfig  

PDSCH-Config ::=                        SEQUENCE {
    dataScramblingIdentityPDSCH             INTEGER (0..1007)    OPTIONAL,
    dmrs-DownlinkForPDSCH-MappingTypeA      SetupRelease { DMRS-DownlinkConfig }  OPTIONAL,   
    dmrs-DownlinkForPDSCH-MappingTypeB      SetupRelease { DMRS-DownlinkConfig }  OPTIONAL,  
    tci-StatesToAddModList                  SEQUENCE (SIZE(1..maxNrofTCI-States)) 
                                                OF TCI-State    OPTIONAL,   -- Need N
    tci-StatesToReleaseList                 SEQUENCE (SIZE(1..maxNrofTCI-States)) 
                                                OF TCI-StateId  OPTIONAL,   -- Need N
    vrb-ToPRB-Interleaver                   ENUMERATED {n2, n4},
    resourceAllocation                      ENUMERATED { resourceAllocationType0,
                                                         resourceAllocationType1, 
                                                         dynamicSwitch},
    pdsch-AllocationList                    SEQUENCE (SIZE(1..maxNrofDL-Allocations)) 
                                                OF PDSCH-TimeDomainResourceAllocation ,
    pdsch-AggregationFactor                 ENUMERATED { n2, n4, n8 } OPTIONAL, 
    rateMatchPatternToAddModList            SEQUENCE (SIZE (1..maxNrofRateMatchPatterns)) 
                                                 OF RateMatchPattern    OPTIONAL, -- Need N
    rateMatchPatternToReleaseList           SEQUENCE (SIZE (1..maxNrofRateMatchPatterns)) 
                                                 OF RateMatchPatternId     OPTIONAL, -- Need N
    rateMatchPatternGroup1                  SEQUENCE (SIZE (1..maxNrofRateMatchPatterns)) 
                                                 OF RateMatchPatternId     OPTIONAL, -- Need R
    rateMatchPatternGroup2                  SEQUENCE (SIZE (1..maxNrofRateMatchPatterns)) 
                                                 OF RateMatchPatternId     OPTIONAL, -- Need R
    rbg-Size                                ENUMERATED {config1, config2},
    mcs-Table                               ENUMERATED {qam64, qam256},
    maxNrofCodeWordsScheduledByDCI          ENUMERATED {n1, n2} OPTIONAL,   -- Need R
    prb-BundlingType                    CHOICE {
        static                                  SEQUENCE {
            bundleSize                              ENUMERATED { n4, wideband }  OPTIONAL  
        },
        dynamic                                 SEQUENCE {
            bundleSizeSet1                          ENUMERATED { n4, 
                                                                 wideband,
                                                                 n2-wideband,
                                                                 n4-wideband
                                                                }  OPTIONAL,   -- Need S
            bundleSizeSet2                          ENUMERATED { n4, 
                                                                 wideband
                                                                } OPTIONAL    -- Need S
        }
    },
    zp-CSI-RS-ResourceToAddModList          SEQUENCE (SIZE (1..maxNrofZP-CSI-RS-Resources)) 
                                                OF ZP-CSI-RS-Resource   OPTIONAL,   -- Need N
    zp-CSI-RS-ResourceToReleaseList         SEQUENCE (SIZE (1..maxNrofZP-CSI-RS-Resources)) 
                                                OF ZP-CSI-RS-ResourceId OPTIONAL,   -- Need M
    aperiodic-ZP-CSI-RS-ResourceSetsToAddModList    SEQUENCE (SIZE (1..maxNrofZP-CSI-RS-Sets)) 
                                                OF ZP-CSI-RS-ResourceSet   OPTIONAL, -- Need N
    aperiodic-ZP-CSI-RS-ResourceSetsToReleaseList   SEQUENCE (SIZE (1..maxNrofZP-CSI-RS-Sets)) 
                                                OF ZP-CSI-RS-ResourceSetId   OPTIONAL, -- Need N
    sp-ZP-CSI-RS-ResourceSetsToAddModList   SEQUENCE (SIZE (1..maxNrofZP-CSI-RS-Sets)) 
                                                OF ZP-CSI-RS-ResourceSet     OPTIONAL, -- Need N
    sp-ZP-CSI-RS-ResourceSetsToReleaseList  SEQUENCE (SIZE (1..maxNrofZP-CSI-RS-Sets)) 
                                                OF ZP-CSI-RS-ResourceSetId   OPTIONAL, -- Need N

    ...
}

CSI-MeasConfig ::= SEQUENCE {
   nzp-CSI-RS-ResourceToAddModList          SEQUENCE (SIZE (1..maxNrofNZP-CSI-RS-Resources)) 
                                                OF NZP-CSI-RS-Resource OPTIONAL, 
   nzp-CSI-RS-ResourceToReleaseList         SEQUENCE (SIZE (1..maxNrofNZP-CSI-RS-Resources)) 
                                                OF NZP-CSI-RS-ResourceId OPTIONAL, 
   nzp-CSI-RS-ResourceSetToAddModList       SEQUENCE (SIZE (1..maxNrofNZP-CSI-RS-ResourceSets)) 
                                                OF NZP-CSI-RS-ResourceSet OPTIONAL, 
   nzp-CSI-RS-ResourceSetToReleaseList      SEQUENCE (SIZE (1..maxNrofNZP-CSI-RS-ResourceSets)) 
                                                OF NZP-CSI-RS-ResourceSetId OPTIONAL, 
   csi-IM-ResourceToAddModList              SEQUENCE (SIZE (1..maxNrofCSI-IM-Resources)) 
                                                OF CSI-IM-Resource OPTIONAL, 
   csi-IM-ResourceToReleaseList             SEQUENCE (SIZE (1..maxNrofCSI-IM-Resources)) 
                                                OF CSI-IM-ResourceId OPTIONAL, 
   csi-IM-ResourceSetToAddModList           SEQUENCE (SIZE (1..maxNrofCSI-IM-ResourceSets)) 
                                                OF CSI-IM-ResourceSet OPTIONAL,
   csi-IM-ResourceSetToReleaseList          SEQUENCE (SIZE (1..maxNrofCSI-IM-ResourceSets)) 
                                                OF CSI-IM-ResourceSetId OPTIONAL, 
   csi-SSB-ResourceSetToAddModList          SEQUENCE (SIZE (1..maxNrofCSI-SSB-ResourceSets)) 
                                                OF CSI-SSB-ResourceSet OPTIONAL, 
   csi-SSB-ResourceSetToAddReleaseList      SEQUENCE (SIZE (1..maxNrofCSI-SSB-ResourceSets)) 
                                                OF CSI-SSB-ResourceSetId OPTIONAL, 
   csi-ResourceConfigToAddModList           SEQUENCE (SIZE (1..maxNrofCSI-ResourceConfigurations))
                                                OF CSI-ResourceConfig OPTIONAL,
   csi-ResourceConfigToReleaseList          SEQUENCE (SIZE (1..maxNrofCSI-ResourceConfigurations))
                                                OF CSI-ResourceConfigId OPTIONAL,
   csi-ReportConfigToAddModList             SEQUENCE (SIZE (1..maxNrofCSI-ReportConfigurations)) 
                                                OF CSI-ReportConfig OPTIONAL,
   csi-ReportConfigToReleaseList            SEQUENCE (SIZE (1..maxNrofCSI-ReportConfigurations)) 
                                                OF CSI-ReportConfigId OPTIONAL,
   reportTriggerSize                        INTEGER (0..6) OPTIONAL,
   aperiodicTriggerStateList                SetupRelease { CSI-AperiodicTriggerStateList },
   semiPersistentOnPUSCH-TriggerStateList   SetupRelease { 
                                                CSI-SemiPersistentOnPUSCH-TriggerStateList 
                                            } OPTIONAL,
...
}

ZP-CSI-RS-Resource ::= SEQUENCE {
   zp-CSI-RS-ResourceId        ZP-CSI-RS-ResourceId,
   resourceMapping             CSI-RS-ResourceMapping,
   periodicityAndOffset        CSI-ResourcePeriodicityAndOffset OPTIONAL,
   ...
}

ZP-CSI-RS-ResourceSet ::= SEQUENCE {
   zp-CSI-RS-ResourceSetId    ZP-CSI-RS-ResourceSetId,
   zp-CSI-RS-ResourceIdList   SEQUENCE (SIZE(1..maxNrofZP-CSI-RS-ResourcesPerSet)) 
                                OF ZP-CSI-RS-ResourceId,
...
}

NZP-CSI-RS-Resource ::= SEQUENCE {
   nzp-CSI-RS-ResourceId      NZP-CSI-RS-ResourceId,
   resourceMapping            CSI-RS-ResourceMapping,
   powerControlOffset         INTEGER (-8..15),
   powerControlOffsetSS       ENUMERATED{db-3, db0, db3, db6} OPTIONAL, -- Need R
   scramblingID               ScramblingId,
   periodicityAndOffset       CSI-ResourcePeriodicityAndOffset OPTIONAL,-
   qcl-InfoPeriodicCSI-RS     TCI-StateId OPTIONAL, -- Cond Periodic
   ...
}

NZP-CSI-RS-ResourceSet ::= SEQUENCE {
   nzp-CSI-ResourceSetId     NZP-CSI-RS-ResourceSetId,
   nzp-CSI-RS-Resources      SEQUENCE (SIZE (1..maxNrofNZP-CSI-RS-ResourcesPerSet)) 
                                  OF NZP-CSI-RS-ResourceId,
   repetition                ENUMERATED { on, off } OPTIONAL, 
   aperiodicTriggeringOffset INTEGER(0..4) OPTIONAL, 
   trs-Info                  ENUMERATED {true} OPTIONAL, 
   ...
}

CSI-RS-ResourceMapping ::= SEQUENCE {
   frequencyDomainAllocation CHOICE {
      row1     BIT STRING (SIZE (4)),
      row2     BIT STRING (SIZE (12)),
      row4     BIT STRING (SIZE (3)),
      other    BIT STRING (SIZE (6))
   },
   nrofPorts                       ENUMERATED {p1,p2,p4,p8,p12,p16,p24,p32},
   firstOFDMSymbolInTimeDomain     INTEGER (0..13),
   firstOFDMSymbolInTimeDomain2    INTEGER (2..12) OPTIONAL, -- Need R
   cdm-Type                        ENUMERATED {noCDM, fd-CDM2, cdm4-FD2-TD2, cdm8-FD2-TD4},
   density CHOICE {
     dot5      ENUMERATED {evenPRBs, oddPRBs},
     one       NULL,
     three     NULL,
     spare     NULL
   },
   freqBand    CSI-FrequencyOccupation,
   ...
}

CSI-ResourcePeriodicityAndOffset ::= CHOICE {
   slots4       INTEGER (0..3),
   slots5       INTEGER (0..4),
   slots8       INTEGER (0..7),
   slots10      INTEGER (0..9),
   slots16      INTEGER (0..15),
   slots20      INTEGER (0..19),
   slots32      INTEGER (0..31),
   slots40      INTEGER (0..39),
   slots64      INTEGER (0..63),
   slots80      INTEGER (0..79),
   slots160     INTEGER (0..159),
   slots320     INTEGER (0..319),
   slots640     INTEGER (0..639)
}

CSI-FrequencyOccupation ::= SEQUENCE {
   startingRB    INTEGER (0..maxNrofPhysicalResourceBlocks-1),
   nrofRBs       INTEGER (24..maxNrofPhysicalResourceBlocksPlus1),
   ...
}

CSI-IM-Resource ::= SEQUENCE {
   csi-IM-ResourceId                   CSI-IM-ResourceId,
   csi-IM-ResourceElementPattern       CHOICE {
      pattern0 SEQUENCE {
         subcarrierLocation-p0         ENUMERATED { s0, s2, s4, s6, s8, s10 },
         symbolLocation-p0             INTEGER (0..12)
      },
      pattern1 SEQUENCE {
         subcarrierLocation-p1         ENUMERATED { s0, s4, s8 },
         symbolLocation-p1             INTEGER (0..13)
      }
   } OPTIONAL, -- Need M
   freqBand                            CSI-FrequencyOccupation OPTIONAL, 
   periodicityAndOffset                CSI-ResourcePeriodicityAndOffset OPTIONAL, 
   PeriodicOrSemiPersistent
...
}

CSI-IM-ResourceSet ::= SEQUENCE {
   csi-IM-ResourceSetId              CSI-IM-ResourceSetId,
   csi-IM-Resources                  SEQUENCE (SIZE(1..maxNrofCSI-IM-ResourcesPerSet)) 
                                         OF CSI-IM-ResourceId,
   ...
}

CSI-SSB-ResourceSet ::= SEQUENCE {
   csi-SSB-ResourceSetId            CSI-SSB-ResourceSetId,
   csi-SSB-ResourceList             SEQUENCE (SIZE(1..maxNrofCSI-SSB-ResourcePerSet)) 
                                        OF SSB-Index,
   ...
}

CSI-ResourceConfig ::= SEQUENCE {
   csi-ResourceConfigId           CSI-ResourceConfigId,
   csi-RS-ResourceSetList         CHOICE {
      nzp-CSI-RS-SSB                 SEQUENCE {
         nzp-CSI-RS-ResourceSetList   SEQUENCE (SIZE (1..maxNrofNZP-CSI-RS-ResourceSetsPerConfig))
                                              OF NZP-CSI-RS-ResourceSetId OPTIONAL,
         csi-SSB-ResourceSetList      SEQUENCE (SIZE (1..maxNrofCSI-SSB-ResourceSetsPerConfig))
                                              OF CSI-SSB-ResourceSetId OPTIONAL 
      },
      csi-IM-ResourceSetList          SEQUENCE (SIZE (1..maxNrofCSI-IM-ResourceSetsPerConfig)) 
                                             OF CSI-IM-ResourceSetId
   },
   bwp-Id                         BWP-Id,
   resourceType                   ENUMERATED { aperiodic, semiPersistent, periodic },
   ...
}

CSI-ReportConfig ::= SEQUENCE {
   reportConfigId                         CSI-ReportConfigId,
   carrier                                ServCellIndex OPTIONAL, 
   resourcesForChannelMeasurement         CSI-ResourceConfigId,
   csi-IM-ResourcesForInterference        CSI-ResourceConfigId OPTIONAL, 
   nzp-CSI-RS-ResourcesForInterference    CSI-ResourceConfigId OPTIONAL, 
   reportConfigType                       CHOICE {
      periodic SEQUENCE {
         reportSlotConfig                 CSI-ReportPeriodicityAndOffset,
         pucch-CSI-ResourceList           SEQUENCE (SIZE (1..maxNrofBWPs)) OF PUCCH-CSI-Resource
      },
      semiPersistentOnPUCCH SEQUENCE {
         reportSlotConfig                 CSI-ReportPeriodicityAndOffset,
         pucch-CSI-ResourceList           SEQUENCE (SIZE (1..maxNrofBWPs)) OF PUCCH-CSI-Resource
      },
      semiPersistentOnPUSCH SEQUENCE {
         reportSlotConfig                 ENUMERATED {sl5, sl10, sl20, sl40, sl80, sl160, sl320},
         reportSlotOffsetList             SEQUENCE (SIZE (1.. maxNrofUL-Allocations)) 
                                               OF INTEGER(0..32),
      p0alpha                             P0-PUSCH-AlphaSetId
      },
      aperiodic SEQUENCE {
         reportSlotOffsetList             SEQUENCE (SIZE (1..maxNrofUL-Allocations)) 
                                               OF INTEGER(0..32)
      }
   },
   reportQuantity CHOICE {
      none NULL,
      cri-RI-PMI-CQI                      NULL,
      cri-RI-i1                           NULL,
      cri-RI-i1-CQI                       SEQUENCE {
         pdsch-BundleSizeForCSI                ENUMERATED {n2, n4} OPTIONAL
      },
      cri-RI-CQI                          NULL,
      cri-RSRP                            NULL,
      ssb-Index-RSRP                      NULL,
      cri-RI-LI-PMI-CQI                   NULL
   },
   reportFreqConfiguration SEQUENCE {
      cqi-FormatIndicator                 ENUMERATED { widebandCQI, subbandCQI } OPTIONAL, 
      pmi-FormatIndicator                 ENUMERATED { widebandPMI, subbandPMI } OPTIONAL, 
      csi-ReportingBand CHOICE {
         subbands3                        BIT STRING(SIZE(3)),
         subbands4                        BIT STRING(SIZE(4)),
         subbands5                        BIT STRING(SIZE(5)),
         subbands6                        BIT STRING(SIZE(6)),
         subbands7                        BIT STRING(SIZE(7)),
         subbands8                        BIT STRING(SIZE(8)),
         subbands9                        BIT STRING(SIZE(9)),
         subbands10                       BIT STRING(SIZE(10)),
         subbands11                       BIT STRING(SIZE(11)),
         subbands12                       BIT STRING(SIZE(12)),
         subbands13                       BIT STRING(SIZE(13)),
         subbands14                       BIT STRING(SIZE(14)),
         subbands15                       BIT STRING(SIZE(15)),
         subbands16                       BIT STRING(SIZE(16)),
         subbands17                       BIT STRING(SIZE(17)),
         subbands18                       BIT STRING(SIZE(18)),
         ...,
         subbands19-v1530                 BIT STRING(SIZE(19))
      } OPTIONAL 
   } OPTIONAL, 
   timeRestrictionForChannelMeasurements          ENUMERATED {configured, notConfigured},
   timeRestrictionForInterferenceMeasurements     ENUMERATED {configured, notConfigured},
   codebookConfig                                 CodebookConfig OPTIONAL, 
   nrofCQIsPerReport                              ENUMERATED {n1, n2} OPTIONAL, 
   groupBasedBeamReporting                CHOICE {
      enabled                                 NULL,
      disabled                            SEQUENCE {
         nrofReportedRS                          ENUMERATED {n1, n2, n3, n4} OPTIONAL 
      }
   },
   cqi-Table                              ENUMERATED {table1, table2, table3, spare1} OPTIONAL, 
   subbandSize                            ENUMERATED {value1, value2},
   non-PMI-PortIndication                 SEQUENCE (SIZE (1..maxNrofNZP-CSI-RS-ResourcesPerConfig)) 
                                                OF PortIndexFor8Ranks OPTIONAL,
   ...,
   [[
   semiPersistentOnPUSCH-v1530 SEQUENCE {
      reportSlotConfig-v1530             ENUMERATED {sl4, sl8, sl16}
   } OPTIONAL
   ]]
}

CSI-ReportPeriodicityAndOffset ::= CHOICE {
   slots4          INTEGER(0..3),
   slots5          INTEGER(0..4),
   slots8          INTEGER(0..7),
   slots10         INTEGER(0..9),
   slots16         INTEGER(0..15),
   slots20         INTEGER(0..19),
   slots40         INTEGER(0..39),
   slots80         INTEGER(0..79),
   slots160        INTEGER(0..159),
   slots320        INTEGER(0..319)
}

PUCCH-CSI-Resource ::= SEQUENCE {
   uplinkBandwidthPartId         BWP-Id,
   pucch-Resource                PUCCH-ResourceId
}

PortIndexFor8Ranks ::= CHOICE {
   portIndex8    SEQUENCE{
      rank1-8         PortIndex8 OPTIONAL, -- Need R
      rank2-8         SEQUENCE(SIZE(2)) OF PortIndex8 OPTIONAL, -- Need R
      rank3-8         SEQUENCE(SIZE(3)) OF PortIndex8 OPTIONAL, -- Need R
      rank4-8         SEQUENCE(SIZE(4)) OF PortIndex8 OPTIONAL, -- Need R
      rank5-8         SEQUENCE(SIZE(5)) OF PortIndex8 OPTIONAL, -- Need R
      rank6-8         SEQUENCE(SIZE(6)) OF PortIndex8 OPTIONAL, -- Need R
      rank7-8         SEQUENCE(SIZE(7)) OF PortIndex8 OPTIONAL, -- Need R
      rank8-8         SEQUENCE(SIZE(8)) OF PortIndex8 OPTIONAL -- Need R
   },
   portIndex4    SEQUENCE{
      rank1-4         PortIndex4 OPTIONAL, -- Need R
      rank2-4         SEQUENCE(SIZE(2)) OF PortIndex4 OPTIONAL, -- Need R
      rank3-4         SEQUENCE(SIZE(3)) OF PortIndex4 OPTIONAL, -- Need R
      rank4-4         SEQUENCE(SIZE(4)) OF PortIndex4 OPTIONAL -- Need R
   },
   portIndex2    SEQUENCE{
      rank1-2         PortIndex2 OPTIONAL, -- Need R
      rank2-2         SEQUENCE(SIZE(2)) OF PortIndex2 OPTIONAL -- Need R
   },
   portIndex1         NULL
}

CodebookConfig ::= SEQUENCE {
   codebookType CHOICE {
      type1 SEQUENCE {
         subType CHOICE {
            typeI-SinglePanel SEQUENCE {
                  nrOfAntennaPorts CHOICE {
                     two SEQUENCE {
                        twoTX-CodebookSubsetRestriction BIT STRING (SIZE (6))
                     },
                     moreThanTwo SEQUENCE {
                        n1-n2 CHOICE {
                           two-one-TypeI-SinglePanel-Restriction        BIT STRING (SIZE (8)),
                           two-two-TypeI-SinglePanel-Restriction        BIT STRING (SIZE (64)),
                           four-one-TypeI-SinglePanel-Restriction       BIT STRING (SIZE (16)),
                           three-two-TypeI-SinglePanel-Restriction      BIT STRING (SIZE (96)),
                           six-one-TypeI-SinglePanel-Restriction        BIT STRING (SIZE (24)),
                           four-two-TypeI-SinglePanel-Restriction       BIT STRING (SIZE (128)),
                           eight-one-TypeI-SinglePanel-Restriction      BIT STRING (SIZE (32)),
                           four-three-TypeI-SinglePanel-Restriction     BIT STRING (SIZE (192)),
                           six-two-TypeI-SinglePanel-Restriction        BIT STRING (SIZE (192)),
                           twelve-one-TypeI-SinglePanel-Restriction     BIT STRING (SIZE (48)),
                           four-four-TypeI-SinglePanel-Restriction      BIT STRING (SIZE (256)),
                           eight-two-TypeI-SinglePanel-Restriction      BIT STRING (SIZE (256)),
                           sixteen-one-TypeI-SinglePanel-Restriction    BIT STRING (SIZE (64))
                     },
                     typeI-SinglePanel-codebookSubsetRestriction-i2     BIT STRING (SIZE (16))
                  }
               },
               typeI-SinglePanel-ri-Restriction BIT STRING (SIZE (8))
            },
            typeI-MultiPanel SEQUENCE {
                  ng-n1-n2 CHOICE {
                     two-two-one-TypeI-MultiPanel-Restriction          BIT STRING (SIZE (8)),
                     two-four-one-TypeI-MultiPanel-Restriction         BIT STRING (SIZE (16)),
                     four-two-one-TypeI-MultiPanel-Restriction         BIT STRING (SIZE (8)),
                     two-two-two-TypeI-MultiPanel-Restriction          BIT STRING (SIZE (64)),
                     two-eight-one-TypeI-MultiPanel-Restriction        BIT STRING (SIZE (32)),
                     four-four-one-TypeI-MultiPanel-Restriction        BIT STRING (SIZE (16)),
                     two-four-two-TypeI-MultiPanel-Restriction         BIT STRING (SIZE (128)),
                     four-two-two-TypeI-MultiPanel-Restriction         BIT STRING (SIZE (64))
                  },
                  ri-Restriction BIT STRING (SIZE (4))
               }
         },
         codebookMode INTEGER (1..2)
      },
      type2 SEQUENCE {
         subType CHOICE {
            typeII SEQUENCE {
               n1-n2-codebookSubsetRestriction CHOICE {
                  two-one               BIT STRING (SIZE (16)),
                  two-two               BIT STRING (SIZE (43)),
                  four-one              BIT STRING (SIZE (32)),
                  three-two             BIT STRING (SIZE (59)),
                  six-one               BIT STRING (SIZE (48)),
                  four-two              BIT STRING (SIZE (75)),
                  eight-one             BIT STRING (SIZE (64)),
                  four-three            BIT STRING (SIZE (107)),
                  six-two               BIT STRING (SIZE (107)),
                  twelve-one            BIT STRING (SIZE (96)),
                  four-four             BIT STRING (SIZE (139)),
                  eight-two             BIT STRING (SIZE (139)),
                  sixteen-one           BIT STRING (SIZE (128))
               },
               typeII-RI-Restriction    BIT STRING (SIZE (2))
            },
            typeII-PortSelection SEQUENCE {
               portSelectionSamplingSize              ENUMERATED {n1, n2, n3, n4} OPTIONAL, 
               typeII-PortSelectionRI-Restriction     BIT STRING (SIZE (2))
            }
         },
         phaseAlphabetSize                            ENUMERATED {n4, n8},
         subbandAmplitude                             BOOLEAN,
         numberOfBeams                                ENUMERATED {two, three, four}
      }
   }
}
                

RRC Examples

Following is a simple example of CSI-RS allocation from 38.508-1.  You would have more details configurations from the same specification that I summarized in this note.

< Based on 38.508-1 Table 4.6.3-45: CSI-RS-ResourceMapping >

< Based on 38.508-1 Table 4.6.3-33: CSI-FrequencyOccupation >

How to Avoid Collision with Other Signals ?

According to what is explained above, it would be possible to allocate the CSI-RS at any symbol and in any slot, but in real situation where various other physical channels and signals running we cannot enjoy such a full degree of freedom for allocating CSI-RS.  In this aspect, I think I can list some tips that may helps.

• Avoid Symbols where SSB is transmitted
• Avoid slots and symbols that are configured for Uplink in TDD UL-DL configuration
• Avoid symbols where CORESET is configured
• Avoid symbols where PDSCH DMRS is configured

NOTE : If you want to see the contents of full log with Amarisoft Log viewer, go to LogAnalysis section and click on 'Sample Log' in this tutorial of Amarisoft TechAcademy.

Example 1 >

As an example, let's assume a case as follows.

SIB 1 :

 

      ssb-PositionsInBurst {

        inOneGroup 'FF'H

      },

      ssb-PeriodicityServingCell ms20,

      tdd-UL-DL-ConfigurationCommon {

        referenceSubcarrierSpacing kHz30,

        pattern1 {

          dl-UL-TransmissionPeriodicity ms5,

          nrofDownlinkSlots 7,

          nrofDownlinkSymbols 6,

          nrofUplinkSlots 2,

          nrofUplinkSymbols 4

        }

      },

First thing I want to suggest you to do is to draw a diagram and mark the configuration of ssb bitmap and tdd-UL-DL config and some additional symbols where CSI-RS is not allowed to be assigned as below.

Now you have a bunck of white spaces where you can allocate CSI-RS. Not so many white spaces, right ? Actually if you want to put the CSI-RS to another 5ms period where SSB is not transmitted, it would be much easier to configure, but in this example I intentionaly picked up tough situation. There can be so many different ways of configuring the CSI-RS, but you can determine any valid configuration as you want and mark it as shown below (this is just one example) and then populate the configuration into RRC message. It would be very error prone if you try configuring RRC without this kind of drawing beforehand.

NOTE : I have tested this configuration with Amarisoft Callbox (gNB+Core) and two different UE (Amarisoft UE simulator and a commericial UE).

Case 1 : I configured CSI for TRS at the different symbol number in slot 4 and 5. With this configuration, only Amarisoft UE simulator passed and commercial UE didn't pass (Radio Link broken right after RRC Setup).

Case 2 : I configured CSI for TRS at the same symbol number in slot 4 and 5 then both DUT (Amarisoft UEsimulator and Commerical UE) passed. I think both configuration is compliant to 3GPP and I personally think it is due to UE implementation of the commercial UE that failed at case 1).

Reference

• Interference Measurement Methods in 5G NR:Principles and Performance - ResearchGate
• 5G New Radio: Unveiling the Essentials of the Next Generation Wireless Access Technology - arxiv
• An Introduction to CSI-RS - NR LTE Blog