Which modulation scheme packs data by varying the amplitude and phase of a carrier?

• A. FDM
• B. TDM
• C. QAM
• D. OFDM

Answer: (C)

Varying both the amplitude and the phase of a single carrier is what Quadrature Amplitude Modulation does. By modulating two orthogonal components—in-phase and quadrature—the system can map more bits onto each symbol than by changing amplitude or phase alone. The constellation diagram for QAM shows multiple points in the I-Q plane, with each point representing a specific combination of amplitude and phase. For example, 16-QAM uses 16 points, conveying 4 bits per symbol. This makes QAM a high-efficiency way to encode data directly through amplitude and phase. In contrast, FDM spreads data across different frequencies, TDM splits data into time slots, and OFDM uses many subcarriers—often each modulated with QAM, but the modulation scheme that directly encodes data via amplitude and phase is QAM.