SDR or Software Defined Radio, also at times known by its shorter name, Software Radio, is a form of communication through wireless system which allows a computer to define or generate modulation of transmitter, while a receiver is in the form of a computer which recovers incoming signals.
To ensure that the right modulation of the transmitter is selected, you must have right type of computer programs which make the transmitter and receiver function in the computer. The mobile telephone system or a mobile two-way radio system, will have among others, a transmitting system consisting of an audio amplifying unit, microphone, ADC which converts the signals from analog to digital capable of converting audio signals into ASCII data, while a modulator installed within will be programmed to impress the ASCII intelligence to a RF carrier.
Other equipment includes amplifiers to boost the signal carrier to the required levels for transmission apart from antenna to transmission. Similarly the receivwe which will convert SDR signals to reverse the transmission action, an antenna to receive data. For boosting RF signals, a super-heterodyne system that can convert signals into a frequency which is constant for demodulators, audio amplifying equipment, speakers or headsets.
SDR is versatile, all wireless transmissions deploy different protocols. Even similar services like faxing, protocol changes from country of origin to country of receipt, but a SDR if it has all the required software, can be of use anywhere irrespective of the different protocols, which can be made possible by merely launching the required program, while ensuring the batteries are properly charged in case of portable devices.
In simple words, the role of SDR is that of a single radio transceiver which can at the same time be a wireless telephone system, cell-phone, fax, email, pager, apart from videoconferencing unit, all performing in a wireless environment.