f the receiver functionality. An ideal SDR transmitter is similar, in which a digital signal processor generates a stream of bits which are sent to a D/A converter connected to a radio antenna. In practice, on the one hand the ideal scheme outlined above is currently not fully feasible due to hardware limitations. However, on the other hand, in most but not all baseband implementations significant parts of the functionality are implemented in software, which makes them highly configurable and at least partially aligned with the SDR concept. Regardless of how close we are to the ideal SDR implementation, what is important is the availability of low-cost radio modules that have sufficiently large parts of their functionality implemented in software and are therefore highly configurable, making them a good fit for an open source RAN implementation. Incidentally, the SDR concept lands perfectly in the virtualization paradigm (see Section 6.2), as the software component of SDR can be relatively easily virtualized. 6.3 Open Source Smart Software Defined Radio Antenna Output Processing Flexible Channelization Software Hardware Sample Rate Hardware FPGAs Algorithms Conversion Middleware Input CORBA ASICs Virtual Radio Machine Control RF/IF Digital Base Band Front End Processing ANTENNA Waveform Modem Network AMPLIFIER Error Encryption Routing FILTER Correction HARDWARE SOFTWARE Figure 6.3.1 Ideal SDR receiver and transmitter. An important open source project to mention in the context of SDR is the GNU Radio, which is a free software (distributed under the GPL license) development toolkit that pro- vides multiple building blocks that can be used to implement an SDR. 6.3.5 Open Source RAN Projects All the RAN open source projects described below are still in their infancy - while some are more advanced than others, at the time of writing of this book none of them can be used to create a commercial 5G network. Nevertheless, some of them look promising and, in a few years, they may become a viable alternative to traditional 