What is a direct conversion (zero-IF) receiver architecture?

Direct conversion receiver directly converts RF to baseband without intermediate frequency, mixing with LO at carrier frequency. I/Q demodulation produces in-phase and quadrature baseband signals. Advantages: No image frequency problem, no IF filters, lower component count, easier integration. Challenges: DC offset (LO-to-RF leakage), I/Q imbalance, 1/f noise at baseband, LO pulling, and second-order distortion. Widely used in modern wireless (Wi-Fi, cellular) due to integration advantages. Requires calibration techniques for DC offset and I/Q balance.