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What is it ?
What is OpenRAN ? There are many different definition and the scope of definition keep changing and expanding, so it gets more and more difficult to clearly define it. I just want to define it by sticking to the literal meaning of the name. Literally OpenRAN mean 'Open' the 'RAN(Radio Access Network)'. If you know what RAN is, OpenRAN can be illustrated as below. The issue is to figure out the meaning of 'OPEN' in this context.
The meaning of 'OPEN' in this context can be summarized by a few keywords as listed below.
OpenRAN is not a thing that is invented overnight. It is the result of gradual evolution of the RAN architecture. This RAN evolution process can be summarized in illustration as shown below. The architecture shown at stage (E) is what we call OpenRAN.
In the illustration shown above, I used the term 'RAN : Upper' and 'RAN : Lower'. There are many options to split RAN protocol stack into two parts. By 'RAN Upper', I meant the higher layer of RAN protocol stack and by 'RAN Lower' I mean the lower layer of RAN protocol stack. One example of this kind of split is CU-DU split which is defined in 3GPP NR specification which can be illustrated below. The RAN Upper corresponds to CU and RAN Lower corresponds to DU (NOTE : Check this page for the details of CU-DU separation)
Why OpenRAN ?
What is the motivation for OpenRAN ? The answer would vary depending on who you ask. One example list of the motivation is as shown below. To me, this seems to be more with respect to business side. There would be different list of motivation with focus on technical aspect as well.
Reference : Specification
 3GPP 38.473-NG-RAN; F1 Application Protocol (F1AP)