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lte_softmodem.md

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For the OAI eNodeB application, the file where main function is placed is {{lte-softmodem.c=/targets/RT/USER/lte-softmodem.c:1363}}.

The main operations executed at the lte-softmodem main function are:

  1. Parse the command line options
  2. Create the data-structures to hold configuration/operation
  3. Initialize the logging system(s)
  4. Fill the configurations
    1. Configure the eNodeB parameters
    2. Configure eNodeB interfaces with RF (or fronthaul) and MAC
      • This topic is discussed at section: Interfaces (MAC and RF/Fronthaul)
  5. Start the thread(s)
    • This topic is discussed at section: eNodeB Start

Interfaces (MAC and RF/Fronthaul)

The openair interface defines a set of functions and variables to interface with the lower (RF) and upper (MAC) laywers.

These objects are initialized at two points in the code, the functions init_eNB() and l2_init().

  1. init_eNB():
  • configres the interface between the eNodeB and RF (or Fronthaul) {{init_eNB=/targets/RT/USER/lte-enb.c:2010}}
  1. l2_init():
  • configures the interface between eNodeB and MAC as shown {{l2_init=/openair2/LAYER2/MAC/main.c:435}}

Diving into the implementation of init_eNB() function it can be seen as an initializer of some important function and variables. Essentially these objects are chosed based on the node type. Currently there are the following node types:

  • eNodeB_3GPP:
    • The default LTE eNodeB application
  • NGFI_RRU_IF5
    • Remote radio unit for a fronthaul transporting time-domain samples.
  • eNodeB_3GPP_BBU
    • Base Band Unit (or Remote Cloud Center) for a fronthaul transporting time-domain samples.
  • NGFI_RRU_IF4p5
    • Remote radio unit for a fronthaul transporting frequency-domain samples.
  • NGFI_RCC_IF4p5
    • Base Band Unit (or Remote Cloud Center) for a fronthaul transporting frequency-domain samples.
  • NGFI_RAU_IF4p5
    • Radio Agregation Unit (kind of a fronthaul router) for a fronthaul transporting frequency-domain samples.

For each of the nodes above the variables/functions composing the RF interface is configurated differently. The following table hilight the configurations made.

Function Description Who Calls Possible Val
eNB->do_prach execute the PRACH decoding procedures check at {{do_prach}} rxtx() do_prach, NULL
eNB->start_rf initialize the Radio interface ex: (USRP,BLADERF) eNB_thread_single() start_rf, NULL
eNB->start_if initialize the Intermediate interface ex: (Fronthaul) eNB_thread_single() start_if, NULL
eNB->proc_tx process the signal to be transmited on air rxtx() proc_tx_full, proc_tx_high, NULL
eNB->proc_uespec_rx eNB RX processin of UE-Specific signals rxtx() phy_procedures_eNB_uespec_RX, NULL
eNB->fep eNB Uplink's DFT (initial SCFDM decode) phy_procedures_eNB_common_RX() eNB_fep_full(), eNB_fep_rru_if5, NULL
eNB->td Uplink decoding of ULSCH ulsch_decoding() ulsch_decoding_data(), NULL
eNB->te Downlink encoding of ULSCH pdsch_procedures() dlsch_encoding(), NULL
eNB->tx_fh Send signal to fronthaul proc_tx_high(), proc_tx_full() tx_fh_if5_mobipass(), tx_fh_if5(), tx_fh_if4p5(), NULL
eNB->rx_fh Receive signal from fronthaul eNB_thread_fh(), eNB_thread_single() rx_rf(), rx_fh_slave(), rx_fh_if5(), rx_fh_if4p5(), NULL
eNB->fh_asynch Get fronhtaul assynchronous messages eNB_thread_asynch_rxtx() fh_if5_asynch_DL(), fh_if4p5_asynch_DL(), fh_if5_asynch_UL(), fh_if4p5_asynch_UL(), NULL
eNB->rfdevice.host_type Configure the type of host this device is configured on - BBU_HOST, RRH_HOST
eNB->ifdevice.host_type Configure the type of host this device is configured on - BBU_HOST, RRH_HOST

With these interface functions configured it is time to start the eNodeB application, the next section will discuss the how this is made

eNodeB Start

The init_eNB calls an important functino (init_eNB_proc()), whch will initialize eNB application.

There is two main operation modes: single and multi-thread.

  • In the single thread mode a thread is created with the following function:
    • eNB_thread_single
  • In the multi-thread mode three main threads are created with the following functions:
    • 1 x eNB_thread_FH
    • 2 x eNB_thread_rxtx

Independent of the main operation mode there are some side threads that are created too:

  • eNB_thread_prach
    • Decode the Physical Random Access Channel
      • prach_procedures:
  • eNB_thread_synch
    • Runs initial synchronization like UE

Also in the case where the node is a Remote Radio Unit the follwing thread is created:

  • eNB_thread_asynch_rxtx

After these threads started the eNB application is prety much ready to go.

eNodeB Single Thread Mode

The eNodeB RF and IF interfaces:

The slave eNodeB synchronization problem

The main loop

eNodeB Multi Thread Mode

  • eNB_thread_rxtx
    • There are two threads of this type per Component Carrier (CC), one process the even subframes the other process the odd.
    • This thread receives the subframe N and sends the subframe N+4
    • Internaly this thread calls:
      • eNB->do_prach
      • phy_procedures_eNB_common_RX
      • proc_uespec_rx
      • proc_tx
  • eNB_thread_FH
    • Actually get the samples from fronthaul, it calls:
      • rx_fh
        • trx_write_func
        • trx_read_func
      • wakeup_slaves
      • wakeup_rxtx