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Communication Between gNB and UE Nodes

Packet communication is central to the 5G new radio (NR) interface. This topic presents the communication flow between the 5G base station (gNB) and user equipment (UE) nodes, explaining the uplink (UL) and downlink (DL) transmission.

In-Band and Out-of-Band Packet

System-level simulation involves the transmission of various packet types in both UL and DL directions. NR nodes send some packets out-of-band, delivering these packets directly at the destination (bypassing the channel) without using any time-frequency resources. NR nodes transmit the remaining packets in-band, which involves the application of channel and use of time-frequency resources.

UL Transmission

This figure illustrates the UL transmission.

UL Transmission Procedures

The UL transmission consists of these packets.

  • Sounding reference signal (SRS) — The UE node sends SRSs to the gNB node as in-band packets. The gNB node estimates these channel state information (CSI) parameters from the received SRSs.

    • Transmitted precoding matrix indicator (TPMI) — The TPMI parameter indicates the precoding for the UL transmission preferred by the gNB node.

    • UL Rank indicator (RI) — The UL RI parameter defines the number of layers the gNB node can successfully receive. The current implementation of the UL transmission fixes the ULRI to 1.

    • Modulation scheme and code rate (MCS) — The MCS parameter provides information about the acceptable highest MCS for the UL transmission. Use the FixedMCSIndexUL name-value argument of the configureScheduler object function to set the MCS for UL transmission.

    The estimated UL CSI helps the gNB node allocate the resources judiciously and make effective scheduling decisions. The gNB node created by the nrGNB object automatically sets the connection parameters related to the SRS. For more information about SRS configuration, see CSI-RS and sounding reference signal (SRS) configuration.

  • Buffer status report (BSR) — Through BSRs, the UE node provides the gNB node with information about the amount of data in the UE node transmit buffer. The UE node transmits a BSR with a predefined periodicity as an out-of-band packet. You can use the connectUE object function of the nrGNB object to set the periodicity of the BSR report.

  • Scheduling grant — Upon receiving the BSR from the UE node, the base station provides grants (an out-of-band packet) to the UE node for the UL transmission.

  • UL data transmission — This is an in-band packet. The UE node transmits the UL data over the physical uplink shared channel (PUSCH) when it receives the scheduling grant.

DL Transmission

This figure illustrates the DL transmission.

DL Transmission

The DL transmission consists of these packets.

  • CSI reference signal (RS) — The gNB node sends CSI-RSs to the UE node. The UE node uses these reference signals to measure the DL channel characteristics.

    The nrGNB object automatically sets the connection parameters related to the CSI-RS. For more information about the CSI-RS configuration, see CSI-RS and sounding reference signal (SRS) configuration.

  • CSI feedback — The UE node estimates these CSI parameters from the received CSI.

    • DL PMI — The DL PMI parameter indicates the precoding for the DL transmission preferred by the UE node.

    • DL RI — The DL RI parameter defines the number of layers the UE node can successfully receive.

    • Channel quality indicator (CQI) — The CQI parameter indicates the quality of the channel. The CQI value provides information about the highest MCS for the DL transmission. Use the FixedMCSIndexDL name-value argument of the configureScheduler object function to set the MCS for DL transmission.

    The UE node then reports back to the gNB node with the estimated PMI, RI, and CQI. For more information about the CSI parameters, see the 5G NR Downlink CSI Reporting example.

  • DL data transmission — The gNB node schedules the DL transmissions and uses the CSI parameters to provide the UE node with the transmission parameters, such as modulation scheme, code rate, and number of transmission layers. The gNB node sends the UE node a DL scheduling assignment packet, informing the UE of a forthcoming DL transmission.

    You can use the configureScheduler function of the nrGNB object to configure the scheduling parameters.

  • Hybrid automatic repeat request (HARQ) acknowledgment — The UE node generates a positive acknowledgment for the successful reception of the DL data or a negative acknowledgment for erroneous reception.

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