for high-data-rate applications. However, it is the most robust of all the PSK techniques because it takes a tremendous amount of distortion for the demodulator to reach an incorrect decision. For this rea- son, it is the best solution under noisy conditions. It is commonly used with the DSSS version of the original 802.11 radio link. (DSSS is discussed later in this chapter.) Quadrature Phase-Shift Keying (QPSK)-QPSK introduces four different phase shifts-0 degrees, 90 degrees, 180 degrees, and 270 degrees-provid- ing two bits per symbol time. QPSK can operate in harsh environments, such as with over-the-air transmission. Because of its robustness and rela- tively low complexity, QPSK is widely used in situations such as with Direct Broadcast Satellite (DBS). It is also used with 802.11x WLANs. In 802.11b, it is used when operating at 5.5Mbps and 11Mbps, and in 802.11a and 802.11g, it is used when operating at 12Mbps or 18Mbps. Differential Phase-Shift Keying (DPSK)-DPSK is a form of phase-shift keying used for digital transmission in which the phase of the carrier is dis- cretely varied in relation to the phase of the immediately preceding signal element and in accordance with the data being transmitted. The demodula- tor determines the changes in the phase of the received signal rather than the phase itself. DPSK can be significantly simpler to implement than ordi- nary PSK because there is no need for the demodulator to have a copy of the reference signal to determine the exact phase of the received signal. However, it produces more erroneous demodulations as a result. Differen- tial Binary PSK (DBPSK) is used in low-speed 802.11 WLANs (operating at 1Mbps), and Differential Quadrature PSK (DQPSK) is used with the extended-rate 2Mbps 802.11 WLANs. Quadrature Amplitude Modulation (QAM)-QAM modulates multiple levels of amplitude or uses both amplitude and phase to yield a higher spec- tral efficiency. As discussed in Chapter 5, various levels of QAM are referred to as nn-QAM, where NN indicates the number of sta