1. DCI Format 1A in LTE
NOVEMBER 27, 20144COMMENTS
DCI or the downlink control information format 1A is one of the DCIs used for scheduling a
PDSCH codeword. DCI format 1A is only used for scheduling a single codeword/transport block
on PDSCH. The DCI format 1A is specifically used when the resource allocation type used is RAT2.
In RAT2 there are two different modes of physical resource block allocation, one, it can be
localized, where in the resource blocks are contiguous and the second mode is distributed
resource block allocation, where the resource blocks (RB) are spread throughout the system
bandwidth using a fixed pattern (probably a separate article on RAT2 distributed type would
be better ) . One more significant use of the DCI format 1A is to initiate a RACH process
using PDCCH order, which is a non-contention based RACH. Below are the different contents of
DCI format 1A and their significance,
1. Format 1A/0 differentiator, which is a 1 bit field, this field helps the UE to decide if the
current DCI format is 1 or 0, because all the fields are almost the same otherwise
2. 1 bit flag to indicated if the current PDSCH scheduling is with RA type 2 localized or
distributed
3. Resource block assignment bits, the number of bits here depends on the system bandwidth,
for eg: its 13 for5Mhz. The resource block assignment bits tell theUE about number of
resource blocks, RB start index in frequency domain (Refer section 7.1.6.3 in 36.213 for
more details on how to arrive at number of resource blocks and the RB start index from
resource block assignment bits) This field is also called as RB coding inFAPI convention
Now if the RA type 2 is distributed, then the MSB in the resource block assignment
indicates the N-Gap value (Which is a specific term used only in deciding the pattern of
RB distribution, N-Gap can be either 0 or 1) and the remaining bits give the actual RB
coding
4. 5 bit modulation and coding scheme index, also called as MCS_Index, this is again a
important parameter, from which the UE comes to know about the modulation scheme (if
QPSK, 16 or 64 QAM), the transport block size index (I_TBS) which along with the number of
RB , it uses to find the transport block size for the current PDSCH
5. HARQ process number, which is 3 bit for FDD and 4 bit for TDD
6. New data indicator bit, which indicates the UE if the current transmission of PDSCH is a new
transmission or are transmission. This is a 1 bit field and the bit being toggled or not
toggled, indicates if this is a fresh or retransmission to the UE
This field has a special meaning in case of system bandwidth being higher than 10Mhz
and RA type is distributed and the DCI is carrying either of P-RNTI, SI-RNTI or RA-RNTI,
then this field indicates the N-Gap value. The main reason for this is to give more bits in
the RB coding field for higher system bandwidths
2. 7. A 2 bit redundancy version field, which indicates the retransmission version, if its a 1st, 2nd,
3rd or 4th
8. 2 bit field for PUCCH transmit power control (TPC), which will tell theUE to either step
up/down or remain at the same PUCCH transmit power for the HARQ indication for the
currentPDSCH
The TPC field has a special meaning too, when the current DCI carries a SI/P/RA-RNTI,
which is as below,
The LSB of the TPC field indicates the number of RB, that UE should use to determine
the transport block size for this RNTIs. The number of RB will be 2 if the LSB is 0 and 3
if it is 1
9. Downlink assignment index, worth 2 bits, only applicable for TDD systems. To be honest I
am not aware of how this field is to be understood by the UE
Now if the DCI 1A is to be used for initiating a non-contention based RACH using PDCCH order
then the below fields are applicable instead of the above one’s,
1. Same as above
2. 1 bit flag to indicated if the current PDSCH scheduling is with RA type 2 localized or
distributed always set to 0
3. Resource block assignment bits all set to 1 always
4. 6 bit preamble index, that UE shall use to transmit the PRACH
5. 4 bit PRACH mask index
6. All the remaining fields are set to 0
So thats all about DCI format 1A. Please do leave a comment for any doubts or question or
081905490149 mas fadli
DCI is used for controlling downlink resource allocation on the PDSCH channel, or uplink
resource allocation on the PUSCH channel, or transmit power control commands for either the
PUSCH or PUCCH channels. The various DCI formats and what they are used for are listed in the
table below:
3. Summary of DCI Formats
DCI
Format
Application
Downlink
Resource
Allocation
1
Scheduling of one PDSCH codeword
Single antenna port transmission or transmit diversity
1A
Compact scheduling of single PDSCH codeword or random access
initiation
Single antenna port transmission or transfer diversity
1B
Compact scheduling of single PDSCH codeword with precoding
information
Closed loop spatial multiplexing using a single layer
1C
Very compact Resource Block scheduling of single PDSCH codeword
Single antenna port transmission or transmit diversity
1D
Compact scheduling of single PDSCH codeword with precoding and
power offset information.
Muti-user MIMO
2
Scheduling of one or two PDSCH codewords
Closed loop spatial multiplexing or transmit diversity
2A
Scheduling of one or two PDSCH codewords
Open loop spatial multiplexing or transmit diversity
2B
Scheduling of one or two PDSCH codewords
Dual or single layer transmission using antenna ports 7 and 8
2C
Scheduling of one or two PDSCH codewords
Up to 8 layer transmission using antenna ports 7 to 14
6-1A
Compact scheduling of one PDSCH codeword in one cell
Random access procedure initiated by a PDCCH order
6-1B Scheduling of one PDSCH codeword in one cell
0
Scheduling of one PUSCH codeword
Closed loop spatial multiplexing
4. Uplink
Resource
Allocation
4 Scheduling of one or two PUSCH codewords
6-0A Scheduling of PUSCH in one UI cell
6-0B Scheduling of PUSCH in one UI cell
Transmit
Power
Control
3 TPC commands for PUCCH and PUSCH with 2 bit power adjustments
3A TPC commands for PUCCH and PUSCH with 1 bit power adjustments
Sidelink
Resource
Allocation
5 Scheduling of PSCCH, PSSCH
