5G NR: DCI Formats

5G NR: DCI Formats

Introduction: 

         In this blog of our 5g series, we discussed downlink control information or DCI. we will look at its content on how it is encoded and modulated then mapped to the 5g new radio slot etc.

DCI: 

=> downlink control information or DCI carries control information used to schedule user data PD-SCH on the downlink and PU SCH on the uplink.

=> it is carried by the PDC CH or physical downlink control channel.

=> it indicates the location in time and frequency of the data that is scheduled for transmission.

=> the modulation and coding schemes used the number of antenna ports or layers as well as other aspects such as HARQ.

=> the user equipment needs to decode the DCI before they can decode downlink data or transmit uplink data depending on the content of the DCI.

=> one or more of several formats can be used.

 => Format 0 is for uplink grant meaning that it contains information that pertains to data the UE is about to transmit on the uplink.

=> Format 1 is for downlink allocation this means it includes information about the way data was sent to the UE.

=>For both uplink and downlink information there are two possible formats one with

underscore zero(0) and one with an underscore one(1).

=> The format with underscore zero is called the fallback format it is more compact than the full format with underscore one because it doesn't include all options and therefore it trades off less scheduling flexibility for reduced control overhead.

=> finally format 2 addresses the information needed for groups of UEs and TPC commands.

=> Downlink control information uses polar code for error protection this is the main difference with encoding in LTE where tell binding convolutional encoding was used.

=> another difference with LTE where that the CRC used here is longer at 24 bits instead

of 16 for LTE.

=> the CRC value is crumbled with a UE identifier called the radio network temporary identifier(RNTI) in order to indicate that which UE the message is intended for.

=> After encoding downlink control information is scrambled with QPSK modulated and mapped to resource blocks with a very specific pattern.

=>UE must look for PDCCH and decode the PDCCH to get the required DCI information for further processing.

=> There are several significant differences with LTE:

1-  first the PDC CH may not spend the complete 5g bandwidth whereas in LTE it always does this. it is important because of the bandwidth may be much larger up to 400 Mhz in 5g and UEs in 5G or not required to support large bandwidth.

2- PDCCH in 5g supports device-specific beamforming this means, control information can be beamed toward a particular UE, this is possible because of the PDCCH has associated DMRS or demodulation reference symbols which undergo the same beamforming. it is similar to the concept of EPDCCH that was introduced late in LTE deployment.

    

 note that P DCCH is mapped to a corset or control resource set a concept that defines the location of a control region within the 5G resource grid.

Examples:

let us now look at two concrete examples of DCI usage first for downlink data scheduling.

For Downlink:

=>  The UE looks for the pc CH and if a match is found meaning that a block decoded with a CRC that matches the RNTi of a UE. it passes the DCI and extracts all information about where in time and frequency data is located and how data was sent to the UE, with this information, the UE can grab the relevant parts of the 5g grids.

=>Performs channel estimation equalization inverse rate matching and decoding to

retrieve the downlink data packet.

For uplink:

 => for the uplink transmission downlink control information carrying an uplink grant. comes in response to a scheduling request from the UE when the gNB received the scheduling request, it makes all the decisions about when and how the UE should transmit the data that is ready for transmission.

=> Those parameters include beside the time and frequency location and modulation and coding scheme other information such as precoding which comes in the form of an index that points to a table of possible precoding matrices.

=>After decoding the control information for the uplink grant remember this would be format 0_0 and format 0_1. The UE transmits uplink data according to those parameters.

=> To understand how downlink information is mapped to the 5g grid, we must introduce two new concepts.

1- resource element groups
2- control channel elements or CCE

1-Resource element group:
    The resource element group is simply a block of 12 resource elements by one symbol. this is the basic unit used to define CCEs.

2- control channel elements or CCE

=> one control channel element corresponds to six resource element groups this means that one CCE includes six times 12 resource elements that equals 72.

1CCE = 6x12 = 72 resource elements

54 are available for the PDCCH itself.

18 are reserved for associated DMRS or demodulation reference symbols.

 =>one PDCCH is mapped to one or more CCEs. the standard defines several aggregation

levels as in LTE except for the introduction of a new level of 16 which was not available in LTE. 

=> The higher the aggregation level the more resources are used but the more possibility for redundancy enhance.