UE shall need to check all possible combination of PDCCH locations, PDCCH formats, and DCI formats and act on those message with correct CRCs (taking into account that the CRC is scrambled with UE identity). Carrying out such a 'blind decoding' of all the possible combinations would require the UE to make many PDCCH decoding attempts in every subframe.
For large system bandwidths, with a large number of possible PDCCH locations, it would be significant burden, leading to excessive power consumption in the UE receiver. The alternative approach defined for LTE to reduce this burden. For each UE a limited set of CCE locations where a PDCCH may be placed. The set of CCE locations in which the UE may find its PDCCH can be considered as search space. In LTE the search space is a different size for each PDCCH format. Moreover, separate dedicated and common search spaces are defined, where a dedicated search space is configured for each UE individually, while all UEs are informed of the extent of the common search space.
The physical downlink control channel carries scheduling assignments and other control information. A physical control channel is transmitted on an aggregation of one or several consecutive control channel elements (CCEs), where a control channel element corresponds to 9 resource element groups (REG). Each REG in turn has four Resource Elements (REs). The number of resource-element groups not assigned to PCFICH or PHICH is N(Reg) .
The CCEs available in the system are numbered from 0 and N(cce) - 1, where N(cce) = floor(N(Reg) 9) . PDCCH is modulated with QPSK modulation.
|PDCCH format||Number of CCEs||Number of REG||No of Res 1 REG = 4 REs||Number of PDCCH bits 1 symbols contains 2 bits|