How can data rate be increased without increasing bandwidth?

To transmit data faster without widening the radio frequency bandwidth, the transmitter must pack more bits of information into each transmitted symbol. This is accomplished by using a more efficient digital code or modulation scheme, such as moving from QPSK (2 bits per symbol) to 16-QAM (4 bits per symbol). Many candidates incorrectly guess that forward error correction (FEC) would increase the data rate. However, FEC actually decreases the effective user data rate because it adds redundant parity bits to the data stream, taking up valuable space to ensure signal reliability rather than speed.