5G NR : PUSCH/PDSCH re-transmissions

Jyothi
2 min readFeb 14, 2024

PUSCH re-transmissions

In 5G NR, PUSCH messages can be sent either using dynamic grant allocations or UL-configured grant allocations. gNB can request a UE to re-transmit the PUSCH. These re-transmission requests are initiated using PDCCH in case of any allocation type.

In the case of dynamic grants, gNB sends the DCI with NDI (new data indicator) same as the previous value. It will not toggle the value. UE should re-transmit the same PUSCH.

In the case of UL-configured grants, gNB sends the DCI with NDI as 1, UE should re-transmit the same PUSCH.

In the case of re-transmissions, gNB can specify the MCS value or it can specify the MCS reserved values (27 to 31 or 28 to 31 based on the selected MCS Index table). If gNB specifies the non-reserved MCS value, UE can use it as it. If gNB specifies the reserved MCS value, UE should use the latest valid MCS value used for that PUSCH. To support this UE should save the previously used MCS value along with transport block information.

PDSCH re-transmissions

In 5G NR, Similar to PUSCH, PDSCH messages can be re-transmitted.

In the case of dynamic grants, DCI will contain the NDI (new data indicator) value which is the same as the previous value. It will not toggle the value.

In the case of SPS, DCI will have NDI as 1.

In the case of re-transmissions, DCI can contain the specific MCS value or MCS reserved values (27 to 31 or 28 to 31 based on the selected MCS Index table). If the MCS value is non-reserved, it can be used as it, but if it is a reserved value, gNB should use the latest received non-reserved MCS value.

I have a question here there are multiple reserved MCS values. what is the difference between these multiple reserved values? I didn’t find any specific details for it in 3GPP 38.214 standard.

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