PUCCH

Physical Uplink Control Channel, PUCCH carries uplink control information(UCI).

Questions that arises can be listed as:

1. What all information is carried in UCI.
2. On which subframe is UCI transmitted.
3. UE procedure for determining PUCCH assignment.

We’ll find answer for each of these one by one.

What all information is carried in UCI:-

UCI basically carries CQI, SR and HARQ ACK/NACK. 

CQI refers to Channel Quality Indicator. CQI is an indicator telling how good the channel quality is. It ranges from 0 to 15, with 15 indicating the best quality. The reason for sending this is that if UE reports high channel quality then network will send a large transport block size. Basically eNb will use a higher modulation scheme and coding rate to increase efficiency. 20 bits per subframe are used to carry CQI.

Granularity of a CQI report can be classified into 3 levels in frequency domain:-

• Wideband CQI– 1 CQI value is reported for the entire downlink channel bandwidth.

• UE selected subband CQI- This is different in periodic and aperiodic reporting(explained below).

For aperiodic reporting, UE divides the bandwidth into a number of subbands, selects best M subbands and reports 1 CQI for the selected subbands.

For periodic CQI, channel bandwidth is divided into bandwidth parts, each of which includes a number of subbands. Ue considers a single bandwidth part each time a ue selected subband report is generated. Ue selects preferred subband from bandwidth part and generates CQI based upon subband.

• Higher layer configured subband CQI– UE reports subband CQI values for each band in a single report.

PUCCH transfers various combinations of SR, HARQ and CQI information based on it’s format.

Formats 1, 1a and 1b transfer HARQ acknowledgments and SR. Formats 2, 2a and 2b transfer HARQ and CSI. Format 3 carries HARQ for carrier aggregation and SR. Following is the summary:-

There are 2 types of reporting modes in time domain:-

• Periodic CQI–  UE reports CQI, PMI and RI. CQI is transmitted periodically with reporting period determined by higher layers. Minimum periodicity can be 2 msec. Carri ed by PUCCH or PUSCH. It supports wideband and ue-selected subband CQI. Rate at which UE sends report depends upon CQI-PMI configuration index given to UE in RRC connection setup/reconfiguration message.

 

UE sends different CSI components periodically, each combination is described as a mode according to following table:-

For each transmission mode, a CSI reporting mode is supported-

Transmission mode 1 : Modes 1-0, 2-0

Transmission mode 2 : Modes 1-0, 2-0

Transmission mode 3 : Modes 1-0, 2-0

Transmission mode 4 : Modes 1-1, 2-1

Transmission mode 5 : Modes 1-1, 2-1

Transmission mode 6 : Modes 1-1, 2-1

Transmission mode 7 : Modes 1-0, 2-0

Transmission mode 8 : Modes 1-1, 2-1 if the UE is configured with PMI/RI reporting; modes 1-0, 2-0 if the UE is

configured without PMI/RI reporting

Transmission mode 9 : Modes 1-1, 2-1 if the UE is configured with PMI/RI reporting and number of CSI-RS ports>1; modes 1-0, 2-0 if the UE is configured without PMI/RI reporting or number of CSI-RS ports=1.

Transmission mode 10 : Modes 1-1, 2-1 if the UE is configured with PMI/RI reporting and number of CSI-RS ports>1; modes 1-0, 2-0 if the UE is configured without PMI/RI reporting or number of CSI-RS ports=1.

To summarize, reporting modes and transmission modes:-

Also, different PUCCH types are defined for different CSI reports-

– Type 1 report supports CQI feedback for the UE selected sub-bands

– Type 1a report supports subband CQI and second PMI feedback

– Type 2, Type 2b, and Type 2c report

supports wideband CQI and PMI feedback

– Type 2a report supports wideband PMI feedback

– Type 3 report supports RI feedback

– Type 4 report supports wideband CQI

– Type 5 report supports RI and wideband PMI feedback

– Type 6 report supports RI and PTI feedback

 Summary of reporting types and reporting information:-

 

• Aperiodic CQI– UE provide detailed reports. It is based on request by eNb therefore, reporting period is given in DCI 0 /DCI 4 or RACH response. Carried by PUSCH. It is not applicable to 1.4MHz band. It supports wideband, ue-selected subband and higher layer configured subband CQI. Similar to periodic reporting, different reporting modes are defined for aperiodic reporting as follows:-

Different reporting modes are chosen according to different transmission modes.

RRC Connection setup/reconfiguration/re-establishment is used to configure the reporting mode.

RI (Rank Indicator) depicts the number of layers. Basically, it will indicate how well multiple antennas would work together i.e. signals from each antenna would interfere or not.

PMI(Precoding Matrix Indicator) denotes the direction to be given to signals i.e. how to distribute incomming data to antenna ports.

SR and HARQ are explained in different posts.

On which subframe is UCI transmitted:-

When only wideband reporting is configured reports are generated at subframes according to the following formula-

(10 * n_f + k – N_OFFSET,CQI) mod N_pd = 0                                        (1)

n_{f = SFN (0 to 1023)}

k = subframe no. (0 to 9)

N_OFFSET,CQI = reporting offset determined from CQI_PMI configuration index according to table Table 7.2.2-1A specs 36.213.

N_pd = reporting period from table.

 

From cqi-pmi-ConfigIndex, we determine Npd (periodicity)=10 and Noffset,CQI = 1 (with the help of table). Now, we can find the sfn and sf combination that will satisfy above equation.

Now if RI is also configured alongwith wideband CQI, the reporting instances for RI are subframes satisfying:-

(10 * n_f + k – N_{OFFSET,CQI- NOFFSET,RI}) mod (N_pd *M_RI)= 0                             (2)

In case of collision of RI and wideband CQI/PMI the wideband CQI/PMI is dropped.

When both wideband and sub-band reporting are configured–

Wideband CQI/PMI reports are sent on subframes satisfying:-

The reporting instances for wideband CQI/PMI and subband CQI are subframes satisfying:-

(10 * n_f + k – N_OFFSET,CQI) mod (N_pd) = 0                                            (3)

Now, wideband CQI/PMI report has period H*N_pd and is reported on subframes satisfying:-

(10 * n_f + k – N_OFFSET,CQI) mod (H*N_pd) = 0                                    (4)

where,

H = (J*K) +1

J = Number of sub-band parts associated with channel bandwidth (These are predefined in specifications)

K = signalled by eNB from 1 to 4.

N_{OFFSET,CQI is determined according to table Table 7.2.2-1A specs 36.213.}

Sub-band CQI reports are sent during subframes that satisfy eq (1) if the subframe is not already occupied with a wideband report. i.e. between every two consecutive wideband CQI/PMI reports, the remaining J*K ⋅ reporting instances are used in sequence for subband CQI report

 

RI are reported at different intervals. A table is defined in specifications (Table 7.2.2-1B) for RI configuration index and reporting period.

When RI is configured, reporting interval of RI is-

(10 * n_f + k – N_{OFFSET,CQI- NOFFSET,RI}) mod (H*N_pd *M_RI)= 0                                       (5)

 

UE procedure for determining PUCCH assignment:-

 

The number of RB’s that can be used for PUCCH transmission within a cell is determined by N ^PUCCH_RB  (pusch-HoppingOffset). It is broadcasted in SIB.

A single PUCCH transmission occupies 2 RB’s in different time domain. These 2 RB’s are located at the edge of channel bandwidth. i.e. each PUCCH transmission in a subframe comprises of a RB at one end of system bandwidth and an RB at the opposite end of bandwidth. The 2 RB’s are known as PUCCH region. Allocating PUCCH resources at the edges has several advantages:-

• Central RB’s can be used for PUSCH transmission for a single UE. This increases ul data rate. If PUCCH were allocated at the center, PUSCH would be assigned at either end of the control signal to maintain single carrier nature of ul.
• Frequency diversity achieved through frequency hopping is maximized by allowing hopping from 1 edge to another so that if channel conditions are bad at one edge, packets at the other edge can be decoded.
• Out Of Band emissions are reduced, if UE is transmitting on a slot compared to multiple slots.

PUCCH is never transmitted simultaneously (i.e. in the same TTI) with PUSCH from the same UE.

 

Some terms needs to be understood for PUCCH-

• n⁽¹⁾ _RB,
• n⁽²⁾ _RB,
• Cyclic Shift (n⁽¹⁾ _CS),
• Orthogonal Sequence(n_OC),
• Delta PUCCH Shift (Δ ^PUCCH _SHIFT).

 

 

If only HARQ is transmitted-

n⁽¹⁾ _PUCCH = n_CCE + N⁽¹⁾ _PUCCH  ; ranges from 0 to 2047, in FDD

For transmitting SR,

n⁽¹⁾ _PUCCH,SR is sent in RRC connection reconfiguration information from eNB. a single ue is given multiple PUCCH resources- a standard PUCCH resource and an SR PUCCH resource. Standard PUCCH resource is used when HARQ is transmitted without SR and SR PUCCH resource is used when sending SR with/without HARQ.

N⁽¹⁾ _PUCCH is common for all Ue’s , broadcasted in SIB1. It represents number of statically reserved PUCCH resources i.e. some of the first few PUCCH regions that are reserved for format 2/2a/2b or SPS.

n_CCE is the number of first CCE used for transmission of corresponding DCI.

For PUCCH format 2, 2a, 2b;  n⁽²⁾ _PUCCH  determines the PUCCH resource. This is sent by eNB in the RRC connection reconfiguration message.

Cyclic Shift (n⁽¹⁾ _CS)-

Multiple UE’s can be multiplexed on 1 PUCCH RB i.e. UE’s can share the PUCCH RB’s but the combination of orthogonal codes and cyclic shift is unique for each UE. This leads to giving different resources to different UE’s on 1 RB.

 

The number of PUCCH format 1/1a/1b resources per resource block, can be calculated as follows:

N^RB_{PUCCH Format 1  = (N^PUCCHRS} *12)/ Δ ^PUCCH _SHIFT

where

N^PUCCH_RS  is the number of reference signals on PUCCH format 1/1a/1b (= 3 with normal CP and 2 with extended CP).

Δ ^PUCCH _SHIFT  is the cyclic shift difference between two adjacent SR pucch resources using the same orthogonal cover sequence

For normal cyclic prefix,

If Δ ^PUCCH _SHIFT =1, 1 RB can contain 36 resources i.e. 36 UE’s can be multiplexed on 1 PUCCH RB.

If Δ ^PUCCH _SHIFT =2, 1 RB can contain 18 resources i.e. 18 UE’s can be multiplexed on 1 PUCCH RB

If Δ ^PUCCH _SHIFT =3, 1 RB can contain 12 resources i.e. 12 UE’s can be multiplexed on 1 PUCCH RB.

n⁽¹⁾ _RB or n⁽¹⁾ _PUCCH  represents resource used by a specific UE for  transmission of PUCCH format 1, 1a 1b.

Following is a UE log snippet for RRC Connection Reconfiguration message-

message c1 : rrcConnectionReconfiguration : {
rrc-TransactionIdentifier 2,
criticalExtensions c1 : rrcConnectionReconfiguration-r8 : {
radioResourceConfigDedicated {
physicalConfigDedicated {
cqi-ReportConfig {
cqi-ReportModeAperiodic rm30,
nomPDSCH-RS-EPRE-Offset 0,
cqi-ReportPeriodic setup : {
cqi-PUCCH-ResourceIndex 12,
cqi-pmi-ConfigIndex 8,
cqi-FormatIndicatorPeriodic widebandCQI : NULL,
ri-ConfigIndex 483,
simultaneousAckNackAndCQI TRUE
}
},
soundingRS-UL-ConfigDedicated setup : {
srs-Bandwidth bw0,
srs-HoppingBandwidth hbw0,
freqDomainPosition 0,
duration TRUE,
srs-ConfigIndex 356,
transmissionComb 0,
cyclicShift cs1
},
schedulingRequestConfig setup : {
sr-PUCCH-ResourceIndex 0,
sr-ConfigIndex 97,
dsr-TransMax n4
}
}
}