Physical Downlink Control Channel (PDCCH) & its simulation

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PDCCH conveys Downlink Control Information (DCI). It carries various information such as downlink scheduling assignments, resource allocation among multiple users, transport format, HARQ related information, and control information related to spatial multiplexing.

Resources for PDCCH in resource grid are allocated in terms of control channel elements (CCE), that consists of 9 resource element groups (REG). Each REG consists of 4 Resource Elements (RE). So effectively, one CCE contains 36 RE in a resource grid. As PDCCH uses QPSK modulation so each RE carries 2 bits and therefore each CCE carries 72 bits.

These different numbers of CCEs are used in network and termed as Aggregation levels.

Higher the aggregation level means higher number of CCE required and therefore greater number of Resource Elements (RE) are used for PDCCH, which will impact the resources left for shared channel (PDSCH) to transmit the information/data bits and hence impacts the cell/user throughput.
Whenever the radio conditions are not good, normally higher aggregation level is used to transmit the control information over PDCCH. Therefore, it is imperative that radio conditions should be kept good to achieve higher efficiency in terms of resource allocation in radio environment.
Number of OFDM symbols (time domain) to be defined for PDCCH is derived from PCFICH (Physical control format indicator). e.g. There is one OFDM symbol used for PDCCH in 5MHz carrier bandwidth where it is two OFDM symbols in case of 1.4MHz carrier bandwidth.
Resource allocation for PDCCH is simulated below based on Test model 1.1 defined by 3GPP for different bandwidths.