What is a cognitive radio and how does it use SDR to dynamically access available spectrum?

A cognitive radio is an intelligent wireless communication system that uses software defined radio (SDR) technology to autonomously sense the surrounding radio frequency environment, learn from observations, and dynamically adapt its transmission parameters (frequency, bandwidth, power, modulation, protocol) to access available spectrum without causing harmful interference to primary (licensed) users. The cognitive radio cycle consists of four phases: spectrum sensing (monitoring the RF environment to detect which frequencies are currently unused by primary users), spectrum analysis (evaluating the available spectrum holes for quality, bandwidth, and expected availability duration), spectrum decision (selecting the best available spectrum and transmission parameters based on the application's quality of service requirements), and spectrum mobility (vacating a frequency band when a primary user is detected and seamlessly moving to another available band). The enabling technology is SDR: only a software-reconfigurable radio can change its operating frequency, bandwidth, modulation, and protocol on the fly without hardware modification. Spectrum sensing techniques include energy detection (simplest, comparing received power to a threshold), matched filter detection (correlating with known primary user signal features for best sensitivity), and cyclostationary feature detection (exploiting the periodic statistical properties of modulated signals to distinguish signals from noise).