LTE PUCCH: Physical Uplink Control Channel Explained

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pucch
uplink channel
control information
harq

This page describes the LTE PUCCH (Physical Uplink Control Channel) within the LTE system. This LTE channel is used to carry UCI (Uplink Control Information).

UCI can also be transported using the PUSCH channel.

An LTE UE (User Equipment) can never transmit both PUCCH and PUSCH during the same subframe.

  • If the UE has application data OR RRC signaling, then UCI is carried over PUSCH.
  • If the UE does not have any application data OR RRC signaling, then UCI is carried over PUCCH.

This is a stand-alone uplink physical channel.

This PUCCH control signaling channel comprises the following:

  • HARQ ACK/NACK (Hybrid Automatic Repeat Request Acknowledgement/Negative Acknowledgement)
  • CQI (Channel Quality Indicators)
  • MIMO feedback - RI (Rank Indicator), PMI (Precoding Matrix Indicator)
  • Scheduling requests for uplink transmission
  • BPSK or QPSK used for PUCCH modulation

LTE PUCCH channel LTE PUCCH channel

PUCCH consists of 1 RB (Resource Block) per transmission at one end of the system bandwidth, which is followed by another RB in the following slot (at the opposite end of the channel spectrum). This utilizes frequency diversity with an estimated gain of 2dB.

A PUCCH Control Region comprises every two such RBs.

System BW in MHz1.252.55101520
PUSCCH Control region12481216
No. of resource blocks248162432

The standard specifies 6 LTE PUCCH formats, as mentioned in the table below.

As mentioned, PUCCH format 2a and 2b are not applicable for extended CP (Cyclic Prefix).

LTE PUCCH FormatModulationIndexNo. of bits per subframeNo. of REs occupied(Normal CP)No. of REs occupied(Extended CP)
1---48+48=96 OR 48+36=8448+48=96 OR 48+36=84
1aBPSK1148+48=96 OR 48+36=8448+48=96 OR 48+36=84
1bQPSK2248+48=96 OR 48+36=8448+48=96 OR 48+36=84
2QPSK20120
2aQPSK+BPSK21120Not Applicable
2bQPSK+QPSK22120Not Applicable

Table-2 LTE PUCCH Formats

LTE PUCCH FormatNo. of bits per subframeNormal CPExtended CP
1-Scheduling Request
1a11 x HARQ-ACK OR1 x HARQ-ACK +SR
1b21 x HARQ-ACK +SR2 x HARQ-ACK +SR
2202 x HARQ-ACK +SR
2a21CQI CQI OR HARQ-ACK+CQI
2b221 x HARQ-ACK + CQI-
HARQ-ACK + CQI2 x HARQ-ACK + CQI-

Table-3 Information carried by each PUCCH format

The LTE PUCCH channel is allocated 2 RBs at the edges of the channel BW. Each PUCCH transmission occupies 1 RB on each side of the channel bandwidth. These two RBs are distributed across two time slots. RB numbering for PUCCH starts on the outside edges and increases inwards.

PUCCH is allocated RBs at the edge of the channel BW to avoid fragmenting RBs available to PUSCH.

LTE PUCCH Channel Reference

3GPP TS 36.211

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