Which type of digital modulation contains four symbols with 2 bits in each symbol?

Quadrature Phase Shift Keying (QPSK) is indeed the type of digital modulation that contains four symbols with 2 bits in each symbol. In QPSK, each symbol can represent two bits, allowing for a total of four unique combinations: 00, 01, 10, and 11. This capability of encoding two bits per symbol effectively doubles the data rate compared to simple Phase Shift Keying (PSK), which only encodes one bit per symbol.

QPSK achieves this by varying the phase of the carrier wave, specifically using four distinct phase angles to represent the four different combinations. This modulation technique is advantageous in digital communications because it optimizes bandwidth efficiency while maintaining resilience against signal degradation in noisy environments.

Other modulation types mentioned, such as Amplitude Modulation (AM) and Frequency Modulation (FM), do not follow the same principle as QPSK, as they primarily vary the amplitude and frequency of the carrier signal respectively, rather than utilizing phase shifts to encode multiple bits effectively. While Phase Shift Keying (PSK) does use phase to encode information, it typically does so with only two symbols representing one bit each, and thus does not reach the capacity offered by QPSK.