LTE中PUCCH Format 2: PUCCH Capacity Calculations

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LTE中PUCCH Format 2: PUCCH Capacity Calculations

In this blog the capacity of the PUCCH for format 2 is assessed.

The following table shows the PUCCH formats used for channel feedback. The channel feedback could carry channel quality indicator (CQI), Precoding Matrix Indicator (PMI) and Rank Indicator (RI), depending on transmission mode configured for the UE. Code Division Multiplexing  and Frequency Division Multiplexing is used to multiplex UE’s on the same RB (more accurately RB-pairs) configured for PUCCH resources.

Format 2 carries CQI, PMI and ACK/NACKs. The multiplexing capacity could be 4, 6 or 12, depending on parameter settings. In this example, for illustrative purposes only, let’s assume that the multiplexing capacity is 4

As explained in the previous blog, CQI-PMI-ConfigIndex determines how often the UE is supposed to report CQI and PMI  on the PUCCH. RI-ConfigIndex determines how often the UE is supposed to report RI on the PUCCH.

CQI, PMI and RI are transmitted to the eNodeB in format 2, 2a or 2b in the PUCCH as shown below (the picture below assumes a bandwidth of 10 MHz for the UL and 1 resource block for the PUCCH).

If we assume that CQI-PMI-Configindex is within the range 17 £ ICQI/PMI £ 36, then, the reporting period is 20 sub-frames or 20 ms.

Mapping of cqi-pmi-ConfigIndex to Subframe Periodicity

If we assume that RI-Configindex is within the range 322 £ ICQI/PMI £ 482, then, the rank indicator reporting period is 20 sub-frames*4 or 80 ms.

Mapping of ri-ConfigIndex to Subframe Periodicity

nRBCQI is a parameter that defines the number of resource blocks for CQI periodic reporting (Format 2, 2a or 2b). If we assume its value is 2, then, the number of users than can report CQI periodically in the PUCCH is:

NRBCQI * no. of UE multiplexed per sub-frame *periodicity of CQIreporting

=  (2RB)(4 UE/RB)(20) = 160 UE per CQI reporting period. 

See picture below. The number of UE per PUCCH can be increased by modifying the following parameters:

a)     Increasing NRBCQI (but the PUSCH capacity will decrease)

b)    Decreasing the periodicity of CQI reporting (decreasing CQI-PMIConfigIndex) but this might affect our uplink throughput.

c)     Increasing the number of UE multiplexed per RB.

It is up to the operator to decide how to play with this values and achieve the goals planned.

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