## Symbol error performance for QPSK

#### Many textbooks present results for the symbol error rate for QPSK which indicate a worse performance than for BPSK. This is to be expected since the symbol states are closer together. It must be remembered, however, that QPSK is conveying two bits of information for every symbol and the likelihood of both bits being detected in error is much smaller than only one bit being in error (assuming Gray coding of the symbol states). Taking this into account, the bit error probability will be less than the symbol error probability for QPSK, and as we already know is the same as the bit error rate for BPSK. Because of the good bandwidth efficiency and noise performance, QPSK and its variants are currently the most widely used modulation types in both wired and wireless modems. |