openairinterface5g/doc/nr-ue-design.md

PHY and MAC Interface

The PHY sends scheduling requests and data indication to MAC via nr_ue_dl_indication for DL path and nr_ue_ul_indication() for UL path and the MAC sends scheduling configuration to PHY via nr_ue_scheduled_response(). The following diagram shows the interaction for PDCCH and PDSCH reception.

sequenceDiagram
    PHY->>+MAC: Requests for PDCCH config (via nr_ue_dl_indication)
    MAC->>+PHY: Schedules PDCCH reception (via nr_ue_scheduled_response)
    PHY->>+MAC: Indicates decoded DCI(s) (via nr_ue_dl_indication)
    MAC->>+PHY: Schedules PDSCH reception (via nr_ue_scheduled_response)

Multi-threading Design

The UE_thread function in nr-ue.c is the main top level thread that interacts with the radio unit. Once the thread spawns, it starts the 'Initial Synchronization'. Once its complete, the regular processing of slots commences.

The UE exits when at any point in operation it gets out of synchronization. When the command line option --non-stop is used, the UE goes to 'Initial Synchronization' mode when it loses synchronization with gNB. However, this feature is not fully implemented and it is a work in progress at the time of writing this documentation. This will be the default behavior (not a command line option) when the feature is fully implemented.

Initial Synchronization Block

graph TD
    A(Start) -->|UE_thread| B["readFrame<br/>--Reads samples worth 2 frames"]
    B --> |Tpool thread| C["UE_synch<br/>--PSS & SSS detection<br/>--PBCH decode"]
    B --> |UE_thread| D["readFrame<br/>--trash samples to unblock radio"]
    C --> |Tpool thread| E[syncInFrame<br/>--shift first sample to start of frame]
    D --> |UE_thread| E

Regular Slot Processing

graph TD
    E[syncInFrame<br/>--shift first sample to start of frame] -->|UE_thread| F["trx_read_func (slot n)"]
    F --> |Tpool thread| G["processSlotTX (slot n+4)<br/>--PUSCH encode<br/>--PUCCH encode<br/>--trx_write_func"]
    F --> |UE_thread| H["UE_processing (slot n)<br/>--PDCCH decode<br/>--PDSCH decode"]
    G --> |Tpool thread| I(Merge)
    H --> |UE_thread| I(Merge)
    I --> |Go to next slot<br/>UE_thread| F