ortant to mention as it represents a different type of open source license, which is incidentally used for one of the most popular open source RAN implementa- tions. It is a modified version of the Apache license, with one significant difference - it allows contributing parties to charge royalties based on patents for commercial exploita- tion of the software. Therefore, some members of the open source community do not consider OAI as "true" open source. In particular, the OAI Public License contains the following terms not present in the Apache license: 3.2 Grant of Patent License for purposes other than study and research: For purposes other than study, testing and research, and subject to the terms and conditions of this License, You commit to be prepared to negotiate a non-exclusive, non-transferable, non-assignable license of Essential Patents with each Contributor and/or the Licensor on Fair, Reasonable and Non-Discriminatory ("FRAND") terms and conditions for the use of the Work or Contribution(s) incorporated within the Work. 6.3.4 Software-Defined Radio One important difference between open source RAN and most other open source projects (e.g. Linux) is that RAN cannot be entirely implemented in software, as it has to interact with the physical world through radio. While the desire is to move as much functionality to software as possible, the radio hardware will always be a part of any wireless commu- nications system. Ideally, such radio should be fully configurable by software, which is the concept of SDR, developed long before open source RAN was considered. Without SDR, neither virtual RAN (described in Section 6.2) nor open source RAN would have been possible. Here we illustrate the concept by using the ideal SDR scheme, in which everything but the RF and A/D (or D/A) is realized in software, as shown in Figure 6.3.1. In the ideal SDR receiver scheme an A/D converter is attached to an antenna. The output from A/D is sent to the digital signal processor, which runs the software implementing the rest o