LTE Physical Uplink Control Channel (PUCCH)

Physical Uplink Control Channel (PUCCH) carries Uplink Control Information (UCI) which is basically bits and pieces of information that eNB requires from UE in order to understand what UE (user equipment) needs and carries other information like channel quality that UE is seeing in downlink,  etc

UCI is divided into three main sub branches i.e. Channel State Information (CSI), Scheduling Requests (SR) and HARQ ACK/NACK shown with some details as below

Note: 3GPP Release 8 and Release 9 UEs cannot transmit PUCCH and PUSCH on same TTI. If UE has to transmit UCI and data simultaneously within the same TTI, then PUSCH is used for UCI transmission but Release 10 allows simultaneous transmission of PUSCH and PUCCH

3GPP has defined different PUCCH formats to transfer different combinations of the information as can be seen in below table

A single PUCCH transmission always occupies 2 Resource Blocks
Frequency diversity of those two RBs is ensured by slot based PUCCH frequency hopping (See figure below) Each pair of RBs allocated to PUCCH can be used simultaneously by multiple UEs by
using different cyclic shifts and different orthogonal spreading codes

PUCCH Capacity Calculations

Remember every  RRC Connected UE needs to send CQI reports periodically!

Basic formula to calculate how many UEs (RRC Connected) are supported by eNB is

Number of UEs = Number Resource blocks used for CQI reporting x UEs Multiplexed per Resource Blocks x CQI reporting periodicity

The CQI reporting periodicity (in subframes) are determined based on the parameter
cqi-pmi-ConfigIndex given in Table 7.2.2-1A for FDD in 3GPP TS 36.213

Lets assume we have reserved  5 resource blocks (eNB Parameter) for CQI reporting and 6 UEs can be multiplexed per resource blocks. Also lets assume that CQI periodicity is 40 ms then the total number of RRC Connected UEs that eNB can support is as below

Number of UEs= 5 * 6 * 40 = 1200

Note: This is not the only limitation for how many RRC Connected users eNB can support. The above capacity calculation is just to explain the requirement atleast from UE CSI reporting perspective