In LTE each scheduling grant is defined based on fixed size control channel elements (CCE). The four different CCE aggregation levels are defined for the transmission of a control channel. Whatever LTE scheduler schedules for PDCCH, that should be mapped to the CCE size by applying different coding rates.
For example if LTE scheduler schedules Format 1A DCI of length 28bits then Add CRC to 28bits (28bits + 16 bits) = 44bits , Now this 44 bits can be mapped to different CCE as follows.
PDCCH format 0 (CCE size = 72 bits) 44bits are converted into 72 bits using coding rate 44/72.
PDCCH format 1 (CCE size = 144 bits) 44 bits are converted into 144 bits using coding rate 44/144.
PDCCH format 2 (CCE size = 288 bits) 44 bits are converted into 288 bits using coding rate 44/288.
PDCCH format 3 (CCE size = 576 bits) 44 bits are converted into 576 bits using coding rate 44/576.
It is to be noted that the combination of PDCCH formats and CCE aggregation levels with code rates >3/4 (0.75) is not supported. All the PDCCHs transmitted in a subframe are multiplexed together, and then scrambled with a Cell Specific sequence prior to QPSK modulation.