Software defined radio is a radio communication system approach where components traditionally implemented in hardware (such as mixers, filters, amplifiers, modulators/demodulators) are instead implemented by means of software on a programmable processing platform. As an academic concept and research field, it investigates the principles, architectures, algorithms, and technologies required to realize highly flexible, reconfigurable, and adaptable wireless communication systems through the digital processing of radio signals. Key characteristics include the ability to dynamically alter radio parameters, modulation schemes, and protocols via software updates, enabling support for multiple communication standards and environments on a single hardware platform. Its significance lies in providing a versatile methodology and research platform for developing advanced wireless systems, facilitating innovation in areas such as dynamic spectrum access, cognitive radio, and rapid deployment of new communication technologies.