Simulation of BPSK, QPSK, 2FSK, and 16QAM Modulation and Demodulation in Communication Experiments with Constellation Diagram Observation

Digital modulation techniques form the core components of modern communication systems, where different modulation schemes exhibit distinct characteristics in spectral efficiency and noise resistance. Simulation experiments allow intuitive comparison of four typical modulation methods: BPSK, QPSK, 2FSK, and 16QAM. In code implementation, these modulations can be simulated using MATLAB's communication toolbox functions like pskmod, fskmod, and qammod with appropriate parameter configurations.

BPSK (Binary Phase Shift Keying) represents the most fundamental binary modulation, where each symbol carries 1 bit of information through two phase states (0° and 180°). Its constellation diagram shows only two symmetric points on the real axis, demonstrating the strongest interference resistance but lowest spectral efficiency. Implementation typically involves mapping binary data to phase shifts using cosine and sine waveform generators.

QPSK (Quadrature Phase Shift Keying) extends phase states to four positions (45°, 135°, 225°, 315°), enabling 2-bit transmission per symbol. The constellation displays four points uniformly distributed on the unit circle, doubling spectral efficiency while maintaining good noise immunity. Code implementation often utilizes I/Q modulation with orthogonal carriers, where bit pairs are mapped to specific phase angles.

2FSK (Frequency Shift Keying) employs frequency variations to carry information, resulting in constellation points dispersed at different frequency locations in the two-dimensional plane. This modulation exhibits lower sensitivity to frequency offsets but requires wider bandwidth. Simulation code typically generates two distinct frequencies corresponding to binary symbols using voltage-controlled oscillators or direct frequency synthesis.

16QAM (16-Quadrature Amplitude Modulation) as a high-order modulation scheme displays a 4×4 square lattice pattern in the constellation diagram, transmitting 4 bits per symbol. While achieving the highest spectral efficiency, the reduced distance between constellation points increases sensitivity to noise and interference. Implementation involves combining amplitude and phase modulation through predefined I/Q value lookup tables.

Constellation diagram observation provides intuitive assessment of modulation quality: under ideal conditions, all sample points should cluster tightly around constellation points; phase noise causes rotational spreading; amplitude noise results in radial dispersion; while frequency offset induces overall constellation rotation. These characteristics are crucial for communication system debugging and performance analysis, and can be visualized using scatterplot functions in simulation software with error vector magnitude (EVM) calculations.