coding 5G SA, add documentation

doc/SW_archi.md

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+This tuto for 5G gNB design, with Open Cells main
+
+# Top file: executables/ocp-gnb.c
+
+the function main() initializes the data from configuration file
+
+# The main thread is in ru_thread()
+The infinite loop:
+## rx_rf()
+  Collect radio signal samples from RF board  
+    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"
+### nr_fep_full()
+"front end processing" of uplink signal  
+performs DFT on the signal  
+same function (duplicates): phy_procedures_gNB_common_RX()
+it computes the buffer rxdataF (for frequency) from rxdata (samples over time)
+### gNB_top()
+only compute frame numbre, slot number, ...
+### ocp_rxtx()
+main processing for both UL and DL  
+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 replace the data for the next run  
+
+This is very tricky and not thread safe at all.
+
+#### NR_UL_indication()  
+This block processes data already decoded and stored in structures behind UL_INFO
+
+* handle_nr_rach()  
+process data from RACH primary detection  
+if the input is a UE RACH detection
+    * nr_schedule_msg2()
+* handle_nr_uci()  
+????	      
+* handle_nr_ulsch()  
+handles ulsch data prepared by nr_fill_indication()
+* gNB_dlsch_ulsch_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  
+* NR_Schedule_response()  
+process as per the scheduler decided
+
+#### L1_nr_prach_procedures()  
+????
+#### phy_procedures_gNB_uespec_RX()
+* nr_decode_pucch0()  
+actual CCH channel decoding  
+* nr_rx_pusch()  
+    * extracts data from rxdataF (frequency transformed received data)  
+    * nr_pusch_channel_estimation()
+    * nr_ulsch_extract_rbs_single()  
+    * nr_ulsch_scale_channel()
+    * nr_ulsch_channel_level()
+    * nr_ulsch_channel_compensation()
+    * nr_ulsch_compute_llr()  
+this function creates the "likelyhood ratios"  
+* nr_ulsch_procedures()
+    * actual ULsch decoding
+    * nr_ulsch_unscrambling()
+    * nr_ulsch_decoding()
+    * nr_fill_indication()   
+populated the data for the next call to "NR_UL_indication()"
+
+#### phy_procedures_gNB_TX()
+* nr_common_signal_procedures()  
+generate common signals
+* nr_generate_dci_top()  
+generate DCI: the scheduling informtion for each UE in both DL and UL
+* nr_generate_pdsch()  
+generate DL shared channel (user data)
+
+### nr_feptx_prec()
+tx precoding
+### nr_feptx0
+do the inverse DFT
+### tx_rf()
+send radio signal samples to the RF board  
+the samples numbers are the future time for these samples emission on-air