Terminology » LTE » LTE PUCCH: Physical Uplink Control Channel Explained

LTE PUCCH: Physical Uplink Control Channel Explained

lte 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

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 MHz	1.25	2.5	5	10	15	20
PUSCCH Control region	1	2	4	8	12	16
No. of resource blocks	2	4	8	16	24	32

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 Format	Modulation	Index	No. of bits per subframe	No. of REs occupied(Normal CP)	No. of REs occupied(Extended CP)
1	-	-	-	48+48=96 OR 48+36=84	48+48=96 OR 48+36=84
1a	BPSK	1	1	48+48=96 OR 48+36=84	48+48=96 OR 48+36=84
1b	QPSK	2	2	48+48=96 OR 48+36=84	48+48=96 OR 48+36=84
2	QPSK		20	120
2a	QPSK+BPSK		21	120	Not Applicable
2b	QPSK+QPSK		22	120	Not Applicable

Table-2 LTE PUCCH Formats

LTE PUCCH Format	No. of bits per subframe	Normal CP	Extended CP
1	-	Scheduling Request
1a	1	1 x HARQ-ACK OR	1 x HARQ-ACK +SR
1b	2	1 x HARQ-ACK +SR	2 x HARQ-ACK +SR
2	20	2 x HARQ-ACK +SR
2a	21	CQI CQI OR HARQ-ACK+CQI
2b	22	1 x HARQ-ACK + CQI	-
HARQ-ACK + CQI		2 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.