Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier digital modulation technique employed extensively in 4G LTE, 5G NR, and modern Wi-Fi networks. It divides a high-speed data stream across hundreds of closely spaced, overlapping subcarriers whose frequency spacing Δf = 1/T_sym satisfies the mathematical orthogonality condition. This orthogonality ensures that at each subcarrier's center frequency, all other subcarriers produce a spectral null, completely eliminating inter-carrier interference (ICI) without spectral guard bands.