or specify the outgoing route in the traffic generator (like option “-I”
in ping command).
## Split 6 DL 4G
The contract describes to reuse the uplink existing if4p5 and to develop
is this work the downlink “functional split 6”.
The customer required after signature to develop also the uplink
functional split 6. This is accepted, as long as the whole work is
research with no delivery completeness warranty.
### Simulation
To be able to verify the new features and to help in all future
developments, Open Cells added and improved the Rf board simulator
during this contract.
We added the channel modeling simulation, that offer to simulate various
3GPP defined channels.
### Main loop
The main log is in RF simulator is in
`targets/RT/USER/lte-ru.c and targets/RT/USER/lte-enb.c`
As this piece of SW is very complex and doesn’t meet our goals
(functional split 6), a cleaned version replaces these 2 files in
executables/ocp-main.c (openair1/SCHED/prach\_procedures.c is also
replaced by this new file as it only launching the RACH actual work in a
way not compatible with our FS6).
The main loop cadences the I/Q samples reception, signal processing and
I/Q samples sending.
The main loop uses extensively function pointers to call the right
processing function depending on the split case.
A lot of OAI reduntant global variables contains the same semantic data: time,frame, subframe.
The reworked main loop take care of a uniq variable that comes directly from harware: RF board sampling number.
To use OAI, we need to set all OAI variables that derivates from this timestamp value. The function setAllfromTS() implements this.
### Splitted main level
When FS6 is actived, a main loop for DU (du_fs6()) a main loop for CU case replaces the uniq eNB main loop.
Each of these main loops calls initialization of OAI LTE data and the FS6 transport layer initialization.
Then, it runs a infinite loop on: set time, call UL and DL. The time comes from the RF board, so the DU sends the time to the CU.
This is enough for RF board dialog, but the FS6 is higher in SW layers,
we need to cut higher functions inside downlink and uplink procedures.
As much as possible, the FS6 code is in the directory OPENAIR_DIR/executables. When a given OAI piece of code is small or need complex changes, it is reworked in the file fs6-main.c. The functions naming keeps the OAI function name, adding suffix _fromsplit() or _tosplit().
When this organization would lead to large code copy, it is better to insert modifications in OAI code. This is done in two files:
- openair1/SCHED/phy_procedures_lte_eNb.c: to send signaling channels computation results
- the function sendFs6Ulharq() centralizes all signaling channels forwarding to CU
- openair1/PHY/LTE_TRANSPORT/ulsch_decoding.c: to deal with FS6 user plane split
- sendFs6Ul() is used once to forward user plane to CU
### DownLink
The main procedure is phy\_procedures\_eNB\_TX()
This is building the common channels (beacon, multi-UE signaling).
The FS6 split breaks this function into pieces:
* The multi-UE signals, built by common\_signal\_procedures(),
* These functions run in the DU, nevertheless all context has to be sent
(it is also needed partially for UL spitting)
* Run in the DU also to meet the requirement of pushing
in DU the data encoded with large redundancy (>3 redundancy)
* the per UE data: pdsch\_procedures() needs further splitting:
* dlsch\_encoding\_all() that makes the encoding: turbo code
and lte\_rate\_matching\_turbo() that will be in the DU (some
unlikely cases can reach redundancy up to x3, when MCS is very
low (negative SINR cases)).
* dlsch\_encoding() output needs to be transmitted between the
DU and the CU for functional split 6.
* dlsch\_scrambling() that will go in the DU
* dlsch\_modulation() that will go in the DU
The du user plane data is made of expanded bit in OAI at FS6 split level. 1 pair of functions compact back these bits into 8bits/byte before sending data and expand it again in the DU data reception (functions: fs6Dl(un)pack()).
### Uplink
The uplink require configuration that is part of the DL transmission.
It interprets the signalling to extract the RACH and the per UE data
channels.
Ocp-main.c:rxtx() calls directly the entry procedure
phy\_procedures\_eNB\_uespec\_RX() calls:
* rx\_ulsch() that demodulate and extract soft bits per UE.
* This function runs in the DU
* the output data will be processes in the DU, so it needs to be
transmitted to the DU
* ulsch\_decoding() that do lte\_rate\_matching\_turbo\_rx()
