flowchart TB
A[ru_thread] --> RFin>block rx_rf] --> feprx
feprx --> half-slot --> end_feprx
feprx --> second-thread -- block_end_feprx --> end_feprx>feprx]
end_feprx --> rx_nr_prach_ru
rx_nr_prach_ru -- block_queue --> resp_L1>resp L1]
resp_L1 -- async launch --> rx_func
resp_L1 -- immediate return --> RFin
subgraph rxfunc
rx_func_implem[rx_func]
subgraph rxfuncbeg
handle_nr_slot_ind
--> rnti_to_remove-mgmt
--> L1_nr_prach_procedures
--> apply_nr_rotation_RX
end
subgraph phy_procedures_gNB_uespec_RX
fill_ul_rb_mask
--> pucch(decode each gNB->pucch)
-->nr_fill_ul_indication
--> nr_ulsch_procedures
--> nr_ulsch_decoding
--> segInParallel[[all segments decode in parallel]]
--> barrier_end_of_ulsch_decoding
end
subgraph NR_UL_indication
handle_nr_rach
--> handle_nr_uci
--> handle_nr_ulsch
--> gNB_dlsch_ulsch_scheduler
--> NR_Schedule_response
end
rx_func_implem --> rxfuncbeg
rxfuncbeg --> phy_procedures_gNB_uespec_RX
phy_procedures_gNB_uespec_RX --> NR_UL_indication
-- block_queue --> L1_tx_free2>L1 tx free]
-- async launch --> tx_func
L1_tx_free2 -- send_msg --> rsp((resp_L1))
end
rx_func --> rxfunc
subgraph tx
direction LR
subgraph tx_func2
phy_procedures_gNB_TX
--> dcitop[nr_generate dci top]
--> nr_generate_csi_rs
--> apply_nr_rotation_TX
-- send_msg --> end_tx_func((L1_tx_out))
end
subgraph tx_reorder_thread
L1_tx_out>L1_tx_out]
--> reorder{re order} --> reorder
reorder --> ru_tx_func
reorder --> L1_tx_free((L1_tx_free))
ru_tx_func --> feptx_prec
--> feptx_ofdm
end
tx_func2 --> tx_reorder_thread
end
This tuto for 5G gNB design, with Open Cells main {: .text-center}
The infinite loop:
Collect radio signal samples from RF board
all SDR processing is triggered by I/Q sample reception and it's date (timestamp)
TX I/Q samples will have a date in the future, compared to RX timestamp
called for each 5G NR slot
it blocks until data is available
the internal time comes from the RF board sampling numbers
(each sample has a incremental number representing a very accurate timing)
raw incoming data is in buffer called "rxdata"
We derivate frame number, slot number, ... from the RX timestamp
{: .func2}
only compute frame numbre, slot number, ... {: .func3}
main processing for both UL and DL
start by calling oai_subframe_ind() that trigger processing in pnf_p7_subframe_ind() purpose ???
all the context is in the passed structure UL_INFO
the context is not very clear: there is a mutex on it,
but not actual coherency (see below handle_nr_rach() assumes data is up-to-date)
The first part (in NR_UL_indication, uses the data computed by the lower part (phy_procedures_gNB_uespec_RX), but for the previous slot
Then, phy_procedures_gNB_uespec_RX will hereafter replace the data for the next run
This is very tricky and not thread safe at all.
{: .func3}
This block processes data already decoded and stored in structures behind UL_INFO {: .func4}
- handle_nr_rach()
process data from RACH primary detection
if the input is a UE RACH detection {: .func4}- nr_schedule_msg2() {: .func4}
- handle_nr_uci()
handles uplink control information, i.e., for the moment HARQ feedback. {: .func4} - handle_nr_ulsch()
handles ulsch data prepared by nr_fill_indication() {: .func4} - gNB_dlsch_ulsch_scheduler ()
the scheduler is called here, see dedicated chapter {: .func4} - NR_Schedule_response()
process as per the scheduler decided {: .func4}
???? {: .func4}
- nr_decode_pucch0()
actual CCH channel decoding form rxdataF (rx data in frequency domain)
populates UL_INFO.uci_ind, actual uci data is in gNB->pucch
{: .func4} - nr_rx_pusch()
{: .func4}- extracts data from rxdataF (frequency transformed received data) {: .func4}
- nr_pusch_channel_estimation() {: .func4}
- nr_ulsch_extract_rbs_single() {: .func4}
- nr_ulsch_scale_channel() {: .func4}
- nr_ulsch_channel_level() {: .func4}
- nr_ulsch_channel_compensation() {: .func4}
- nr_ulsch_compute_llr()
this function creates the "likelyhood ratios"
{: .func4}
- nr_ulsch_procedures()
{: .func4}
- actual ULsch decoding {: .func4}
- nr_ulsch_unscrambling() {: .func4}
- nr_ulsch_decoding() {: .func4}
- nr_fill_indication()
populate the data for the next call to "NR_UL_indication()"
it would be better to call NR_UL_indication() now instead of before (on previous slot) {: .func4}
- nr_common_signal_procedures()
generate common signals {: .func4} - nr_generate_dci_top() generate DCI: the scheduling informtion for each UE in both DL and UL {: .func4}
- nr_generate_pdsch()
generate DL shared channel (user data) {: .func4}
tx precoding {: .func3}
do the inverse DFT {: .func3}
send radio signal samples to the RF board
the samples numbers are the future time for these samples emission on-air
{: .func3}
The main scheduler function is called by the chain: nr_ul_indication()=>gNB_dlsch_ulsch_scheduler()
It calls sub functions to process each physical channel (rach, ...)
The scheduler uses and internal map of used RB: vrb_map and vrb_map_UL, so each specific channel scheduler can see the already filled RB in each subframe (the function gNB_dlsch_ulsch_scheduler() clears these two arrays when it starts)
The scheduler also calls "run_pdcp()", as this is not a autonomous thread, it needs to be called here to update traffic requests (DL) and to propagate waiting UL to upper layers
After calling run_pdcp, it updates "rlc" time data but it doesn't actually process rlc
it sends a iiti message to activate the thread for RRC, the answer will be asynchronous in ????
Calls schedule_nr_mib() that fills MIB,
Calls schedule_nr_prach() which schedules the (fixed) PRACH region one frame in advance.
Calls nr_csi_meas_reporting() to check when to schedule CSI in PUCCH.
Calls nr_schedule_RA(): checks RA process 0's state. Schedules Msg.2 via nr_generate_Msg2() if an RA process is ongoing, and pre-allocates the Msg. 3 for PUSCH as well.
Calls nr_schedule_ulsch(): It is divided into the "preprocessor" and the "postprocessor": the first makes the scheduling decisions, the second fills nFAPI structures to indicate to the PHY what it is supposed to do. To signal which users have how many resources, the preprocessor populates the NR_sched_pusch_t (for values changing every TTI, e.g., frequency domain allocation) and NR_sched_pusch_save_t (for values changing less frequently, at least in FR1 [to my understanding], e.g., DMRS fields when the time domain allocation stays between TTIs) structures. Furthermore, the preprocessor is an exchangeable module that schedules differently based on a particular use-case/deployment type, e.g., one user for phytest [in nr_ul_preprocessor_phytest()], multiple users in FR1 [nr_fr1_ulsch_preprocessor()], or maybe FR2 [does not exist yet]:
- calls preprocessor via pre_processor_ul(): the preprocessor is responsible
for allocating CCEs (using allocate_nr_CCEs()) and deciding on resource
allocation for the UEs including TB size. Note that we do not yet have
scheduling requests. What it typically does:
- check whether the current frame/slot plus K2 is an UL slot, and return if not.
- Find first free start RB in vrb_map_UL, and as many free consecutive RBs as possible.
- Either set up resource allocation directly (e.g., for a single UE,
phytest), or call into a function to perform actual resource allocation.
Currently, this is done using pf_ul() which implements a basic
proportional fair scheduler:
- for every UE, check for retransmission and allocate as necessary
- Calculate DMRS stuff (nr_set_pusch_semi_static())
- Calculate the PF coefficient and put eligible UEs into a list
- Allocate resources to the UE(s) with the highest coefficient
- Mark used resources in vrb_map_UL.
- loop through all users: get a HARQ process as indicated through the preprocessor, update statistics, fill nFAPI structures directly for PUSCH, and call config_uldci() and fill_dci_pdu_rel15() for DCI filling and PDCCH messages.
Calls nr_schedule_ue_spec(). It is divided into the "preprocessor" and the "postprocessor": the first makes the scheduling decisions, the second fills nFAPI structures to indicate to the PHY what it is supposed to do. To signal which users have how many resources, the preprocessor populates the NR_UE_sched_ctrl_t structure of affected users. In particular, the field rbSize decides whether a user is to be allocated. Furthermore, the preprocessor is an exchangeable module that schedules differently based on a particular use-case/deployment type, e.g., one user for phytest [in nr_preprocessor_phytest()], multiple users in FR1 [nr_fr1_dlsch_preprocessor()], or maybe FR2 [does not exist yet].
- calls preprocessor via pre_processor_dl(): the preprocessor is responsible
for allocating CCEs and PUCCH (using allocate_nr_CCEs() and
nr_acknack_scheduling()) and deciding on the frequency/time domain
allocation including the TB size. What it typically does:
- Check available resources in the vrb_map
- Checks the quantity of waiting data in RLC
- Either set up resource allocation directly (e.g., for a single UE,
phytest), or call into a function to perform actual resource allocation.
Currently, this is done using pf_dl() which implements a basic
proportional fair scheduler:
- for every UE, check for retransmission and allocate as necessary
- Calculate the PF coefficient and put eligible UEs into a list
- Allocate resources to the UE(s) with the highest coefficient
- Mark taken resources in the vrb_map
- loop through all users: check if a new TA is necessary. Then, if a user has allocated resources, update statistics (round, sent bytes), update HARQ process information, and fill nFAPI structures (allocate a DCI and PDCCH messages, TX_req, ...)
RRC is a regular thread with itti loop on queue: TASK_RRC_GNB it receives it's configuration in message NRRRC_CONFIGURATION_REQ, then real time mesages for all events: S1/NGAP events, X2AP messages and RRC_SUBFRAME_PROCESS
RRC_SUBFRAME_PROCESS message is send each subframe
how does it communicate to scheduler ?
RLC code is new implementation, not using OAI mechanisms: it is implemented directly on pthreads, ignoring OAI common functions.
It is a library, running in thread RRC but also in PHY layer threads and some bits in pdcp running thread or F1 interface threads.
RLC data is isolated and encapsulated.
It is stored under a global var: nr_rlc_ue_manager
The init function rlc_module_init() populates this global variable.
A small effort could lead us to return the pointer to the caller of rlc_module_init() (internal type: nr_rlc_ue_manager_internal_t)
but it returns void.
It could return the initialized pointer (as FILE* fopen() for example), then the RLC layer could have multiple instances in one process.
Even, a future evolution could remove this global rlc layer: rlc can be only a library that we create a instance for each UE because it doesn't shareany data between UEs.
When adding a UE, external code have to call add_rlc_srb() and/or add_rlc_drb(), to remove it: rrc_rlc_remove_ue()
Inside UE, channels called drd or srb can be created: ??? and deleted: rrc_rlc_config_req()
nr_rlc_tick() must be called periodically to manage the internal timers
successful_delivery() and max_retx_reached(): in ??? trigger, the RLC sends a itti message to RRC: RLC_SDU_INDICATION (neutralized by #if 0 right now)
Incoming data to be transmitted is forwarded to RLC by PDCP via rlc_data_req().
At the transport layer, in downlink (DL) at the gNB and uplink (UL) at UE, the scheduler relies on knowing the quantity of data awaiting transmissionis, therefore is using the following MAC/RLC interface functions:
mac_rlc_data_req()to retrieve data bytes to be transmitted by RLC and to fill the MAC SDUmac_rlc_status_ind()to request and set the number of bytes scheduled for transmission by the RLC
Subsequently, the scheduler issues commands to lower layers.
In the RX chain, in downlink (DL) at the UE and uplink (UL) at gNB, the transport layer pushes data into RLC through mac_rlc_data_ind(). Following this, RLC forwards the data to PDCP by invoking pdcp_data_ind() via a complex internal callback mechanism (deliver_sdu()).
The PDCP implementation is secured by a general mutex, akin to the design of the RLC layer. This setup ensures that PDCP data remains isolated and encapsulated.
Initialization of the PDCP layer follows a structure similar to that of the RLC layer. The function nr_pdcp_layer_init() initializes PDCP, while a second initialization function, pdcp_module_init(), must also be invoked.
To manage UE connections, nr_pdcp_add_srbs() is employed for adding UE SRBs in PDCP, while nr_pdcp_remove_UE() is used for their removal. Similarly, nr_pdcp_add_drbs() adds UE DRBs in PDCP, with nr_pdcp_remove_UE() handling their removal.
On the Tx side (downlink in gNB), the entry functions nr_pdcp_data_req_drb() and nr_pdcp_data_req_srb() are called by the upper layer. The upper layer could be GTP or a PDCP internal thread like enb_tun_read_thread(), which reads directly from the Linux socket if the 3GPP core implementation is skipped. The PDCP internals for nr_pdcp_data_req_srb() and nr_pdcp_data_req_drb() are thread-safe. Within these functions, the PDCP manager protects access to the SDU receiving function of PDCP (recv_sdu() callback, corresponding to nr_pdcp_entity_recv_pdu() for DRBs) using mutex. When necessary, the PDCP layer pushes this data to RLC by calling rlc_data_req().
At the Rx side, pdcp_data_ind() serves as the entry point for receiving data from RLC. Within pdcp_data_ind(), the PDCP manager mutex protects access to the PDU receiving function of PDCP (recv_pdu() callback corresponding to nr_pdcp_entity_recv_pdu() for DRBs). Following this, the deliver_sdu_drb() function dispatches the received data to the SDAP sublayer.
nr_pdcp_config_set_security(): sets the keys for AS security of a UE
A sequence diagram of the traffic flow across PDCP and RLC layers. By default, data traffic is directed towards AM DRBs.
This is the flow for downlink, involving MAC and upper layers:
sequenceDiagram
title Downlink AM DRB traffic flow
box Purple gNB
participant GG as GTP / TUN
participant SG as SDAP
participant PG as PDCP
participant RG as RLC
participant MG as MAC
end
GG->>SG: sdap_data_req
note over SG: nr_sdap_tx_entity
SG->>PG: nr_pdcp_data_req_drb
note over PG: deliver_pdu_drb_gnb
note over PG: nr_pdcp_entity_process_sdu
note over PG: enqueue_rlc_data_req
PG->>RG: rlc_data_req via rlc_data_req_thread
note over RG: nr_rlc_entity_am_recv_sdu
MG-->>RG: mac_rlc_data_req
note over RG: nr_rlc_entity_am_generate_pdu
note over RG: generate_tx_pdu
note over RG: serialize_sdu
RG-->>MG: PDU to MAC
MG-->MUE: UL TX / RX procedures
box Blue UE
participant MUE as MAC
participant RUE as RLC
participant PUE as PDCP
participant SUE as SDAP
participant GUE as GTP / TUN
end
MUE-->>RUE: SDU to RLC
RUE->>PUE: pdcp_data_ind
note over PUE: nr_pdcp_entity_recv_pdu
note over PUE: deliver_sdu_drb
PUE->>SUE: sdap_data_ind
note over SUE: nr_sdap_rx_entity
SUE->>GUE: send to GTP-U
and for uplink:
sequenceDiagram
title Uplink AM DRB traffic flow
box Blue UE
participant GUE as GTP / TUN
participant SUE as SDAP
participant PUE as PDCP
participant RUE as RLC
participant MUE as MAC
end
GUE->>SUE: sdap_data_req
note over SUE: nr_sdap_tx_entity
SUE->>PUE: nr_pdcp_data_req_drb
note over PUE: process_sdu
note over PUE: deliver_pdu_drb_ue
note over PUE: enqueue_rlc_data_req
note over PUE: signal to rlc_data_req_thread
PUE->>RUE: rlc_data_req
note over RUE: nr_rlc_entity_am_recv_sdu
RUE-->>MUE: PDU to MAC
note over MUE: nr_ue_get_sdu
MUE-->MG: UL TX / RX procedures
box Purple gNB
participant MG as MAC
participant RG as RLC
participant PG as PDCP
participant SG as SDAP
participant GG as GTP / TUN
end
MG->>RG: mac_rlc_data_ind
note over RG: nr_rlc_entity_am_recv_pdu
note over RG: reception_actions
note over RG: reassemble_and_deliver
note over RG: deliver_sdu
RG->>PG: pdcp_data_ind
note over PG: enqueue_pdcp_data_ind
note over PG: do_pdcp_data_ind via pdcp_data_ind_thread
note over PG: nr_pdcp_entity_recv_pdu
note over PG: deliver_sdu_drb
PG->>SG: sdap_data_ind
note over SG: nr_sdap_rx_entity
SG->>GG: send to GTP-U
Gtp + UDP are two twin threads performing the data plane interface to the core network The design is hybrid: thread and inside other threads calls. It should at least be protected by a mutex.
Gtp thread has a itti interface: queue TASK_GTPV1_U
The interface is about full definition: control messages (create/delet GTP tunnels) and data messages (user plane UL and DL).
PDCP layer push to the GTP queue (outside UDP thread that do almost nothing and work only with GTP thread) is to push a UL packet.
gtp thread calls directly nr_pdcp_data_req_drb(), so it runs inside it's context internal pdcp structures updates
gtpv1u_create_s1u_tunnel(), delete tunnel, ... functions are called inside the other threads, without mutex.
gtpv1uTask(): this creates only the thread, doesn't configure anything gtpv1Init(): creates a listening socket to Linux for a given reception and select a local IP address
this function will replace the xxx_create_tunnel_xxx() for various cases
The parameters are:
- outgoing TEid, associated with outpoing pair(rnti, id)
- incoming packets callback, incoming pair(rnti,id) and a callback function for incoming data
Each call to newGtpuCreateTunnel() creates a outgoing context for a teid (given as function input), a pair(rnti,outgoing id).
Each outgoing packet received on GTP-U ITTI queue must match one pair(rnti,id), so the gtp-u thread can lookup the related TEid and use it to encode the outpoing GTP-U tunneled packet.
newGtpuCreateTunnel() computes and return the incoming teid that will be used for incoming packets. When a incoming packet arrives on this incoming teid, the GTP-U thread calls the defined callback, with the associated pair(rnti, incoming id).
stuff like enb_flag, mui and more important data are not given explicitly by any legacy function (gtpv1u_create_s1u_tunnel), but the legacy and the new interface to lower layer (like pdcp) require this data. We hardcode it in first version.
These teids and "instance", so in a Linux socket: same teid can co-exist for different sockets Remain here a lack to fill: the information given in the legacy funtions is not enough to fullfil the data needed by the callback
Coexistance until full merge with legacy GTP cmake new option: NEW_GTPU to use the new implementation (it changes for the entire executable) It is possible to use both old and new GTP in same executable because the itti task and all functions names are different Current status of new implementation: not tested, X2 not developped, 5G new GTP option not developped, remain issues on data coming from void: muid, enb_flag, ...
NGAP would be a itti thread as is S1AP (+twin thread SCTP that is almost void processing)?
About all messages are exchanged with RRC thread