Draft: Add F1 Handover procedures

• Clean up at RRC and MAC
• Add DU handover procedures, completely handle CellGroupConfig at DU
• Add CU handover procedures

Instructions: build with telnet support (./build_oai --build-lib telnetsrv), run as described in !2716 and load CI telnet module at CU (--telnetsrv --telnetsrv.shrmod ci) , trigger handover through telnet: echo ci trigger_f1_ho | nc 127.0.0.1 9090 && echo

TBD:

• need to rewrite history/commits and acknowledge @rmagueta and @batuhanduyuler
• need to gracefully handle ho_context structure, e.g., reconfiguration failure (reestablishment), UE context release request
• update feature set
• make traffic work after reestablishment (RFsim)
• make Quectel handover work in TDD
• CGI needs to incorporate gNB-DU
• validation of measurement configs (in itself, on DU connection)
• make Quectel handover work in FDD: done by ABS and works for them
• make Quectel handover work with different configs, e.g., one DU no 256 QAM, the other with
• make CI test with Quectel:
• Test DU synchronization: done by ABS and works for them, to be reproduced in CI
• write documentation, including steps and informative text from !2716
• test auto-trigger handover
• various TODO commits to be addressed
• need to refactor ho_context structure: prepare for inter-CU handover

Things not implemented/open

• CI test: no good set up as of now.
• measurement gaps [needed for inter-frequency handover -> not supported yet]
• A/B preamble lists
• refactoring and handling of CGI in config: suggestion is to add cgi = { gnb_du_id = 0xe; cell_id = 0x01; bit_width = 8; };
• timers at RRC: e.g., T304 to check if UE does handover in time and possibly react [unrelated to handover: release timers for auto-release, etc]
• cell-handling at CU: 1:1 DU-cell