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

The correct answer is that the type of digital modulation that contains four symbols with 2 bits in each symbol is Quadrature Phase Shift Keying (QPSK). In QPSK, each of the four distinct symbols represents a unique combination of two bits, allowing for efficient transmission of data.

QPSK achieves this by utilizing phase shifts of a carrier signal, where each phase correlates to a different pair of bits. For instance, four possible phase states can encode the bit pairs (00, 01, 10, and 11). This modulation scheme effectively doubles the data rate compared to binary phase shift keying (BPSK), which only utilizes two phase states and therefore can only transmit one bit per symbol.

Other modulation types listed convey different characteristics: Quadrature amplitude modulation (QAM) combines both amplitude and phase variations, while amplitude modulation (AM) focuses on varying the amplitude of the carrier wave without phase consideration. Phase shift keying (PSK) in its basic form typically involves either two symbols (for binary PSK) or more complex arrangements but does not specifically define four symbols for 2 bits each.