CORESET is a common resource set that is set of multiple physical resources (Specific in NR downlink resource grid) and set of parameters that are used to carry PDCCH/DCI information. The PDDCH/DCI information will be the same as LTE.                               Unlike LTE there is no PCFICH in 5G as PCFICH gives information about PDCCH OFDM symbol in the time domain and in the frequency domain, there is no need to specify as it spreads across the whole channel bandwidth.

       But in 5G, the frequency region should be specific, and It can be signaled by the RRC signaling message.    

Time Domain resources
PCFICH indicator(No of OFDM symbol) CFI.
MaxCORSETDuration in RRC signaling message (Max 3).
Frequency domain resources
there is no need to specify as it spreads across whole channel bandwidth.
Frequencies domain resources messages are signaled by RRC, and each bit corresponds to a group of 6RBs.

       The network can define a common control region and UE specific control region. The number of CORESET is limited to 3 per BWP (Bandwidth part) including both common and UE specific CORESET.

=> Frequency allocation in CORESET configuration can be contiguous or non-contiguous.

=> In the time domain, CORESET configuration spans 1 to 3 consecutive OFDM symbols.

=> REs in CORESET are organized in REGs (RE Groups).

=> Each REG consists of 12 REs of one OFDM symbol in one RB.

Parameters of CORESET are as follows.
RE (Resource Element)The smallest unit of resource grid, 1 subcarrier x 1 OFDM symbol
REG (Resource Element Group)Made of 1 RB (Resource Block) i.e. 12 REs x 1 OFDM symbol
REG Bundles1 REG bundle is made of multiple REGs, Bundle size is specified by parameter "L".
CCE (Control Channel Element)One CCE is made of multiple REGs.
Aggregation levelIt indicates the number of allocated CCEs for PDCCH. It can be 1/2/4/8/16.

The Time-domain and frequency-domain parameters of CORESET are defined in TS 38.211 document. RRC signaling message consists of the following fields.
=>  NRBCORESET: The number of RBs in the frequency domain in CORESET.
=>  NSymbCORESET: The number of symbols in the time domain in CORESET. This can be               1/2/3.
=>  NREGCORESET: The number of REGs in CORESET.
=>  L: REG bundle size

RRC parameter structure of CORESET:

1- ControlResourceID: Bit 0 identifies common coreset and bit 1 identifies coreset for dedicated signaling. This ID should be unique for all BWP.

2- FrequencyDomainResources: Each bit corresponds to the group of 6RBs in the frequency domain.

3- maxCORESETDuartion: Contiguous time duration of the CORESET in OFDM symbols.

4- CCE-REG-MappingType: Mapping Method of CCE to REG. CCE aggregation level could be 1,2.4,8 and 18.

Use of CORESET in NR PDCCH channel

         A PDCCH channel is confined to one CORESET and transmitted with its own DMRS (Demodulation Reference Signal). Hence UE specific beam-forming of the control channel is possible.
=> PDCCH channel is carried by 1/2/4/8/16 CCEs (Control Channel Elements) to carry various DCI payload sizes or coding rates.
=> Each CCE consists of 6 REGs.
=> The CCE to REG mapping for CORESET can be interleaved (to support frequency diversity) or non-interleaved (for localized beam-forming).

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