A Software-Defined Radio (SDR) system is a radio communication system where components that have typically been implemented in hardware (i.e. mixers, filters, amplifiers, modulators/demodulators, detectors. etc.) are instead implemented using software on a personal computer or other embedded computing devices. While the concept of SDR is not new, the rapidly evolving capabilities of digital electronics are making practical many processes that were once only theoretically possible.
Software Defined Radio (SDR) is already influencing radio design, but this has not yet been a subject for standardization in ETSI. In particular it is not clear if any regulatory changes are required to support widespread use of SDR in mobile terminals.
With the development of technology, Software Defined Radio equipment is becoming interesting for use in radio networks as the equipment cost penalty is decreasing whilst equipment lifetime is getting longer. In some markets, such as national security and public safety, there is a requirement for longer equipment lifetimes which might be met by the use of SDR.
Cognitive Radio is likely to be based on SDR-platforms, even though this is not a necessity. The group of technologies for Cognitive Radio and for Software Defined Radio are all technologies for Reconfigurable Radio Systems (RRS). Such systems exploit the capabilities of reconfigurable radio and networks and self-adaptation to a dynamically changing environment, with the aim to ensure end-to-end connectivity.
Reconfigurable Radio System standardization, with the scope of improved spectral utilisation and inter-operator coexistence, using flexible usage modes and a variety of technologies, is not yet adequately addressed by any standardization body.
Source: http://www.etsi.org/WebSite/technologies/RRS.aspx (ETSI)