 to as nn-QAM, where NN indi- cates the number of states per Hertz. The number of bits per symbol time is k, where 2k = NN. So, 4 bits/Hz is equivalent to 16-QAM, 6 bits/Hz is equiva- lent to 64-QAM, and 8 bits/Hz is equivalent to 256-QAM. As you can see, QAM has vastly improved throughput as compared to earlier techniques such as 2B1Q, which provided only 2 bits/Hz. Quadrature Phase-Shift Keying (QPSK)-QPSK is equivalent to 4-QAM, with which you get 2 bits per symbol time. QPSK is designed to operate in harsh environments, such as over-the-air transmission and cable TV return paths. Because of its robustness and relatively low complexity, QPSK is widely used in cases such as direct broadcast satellite. Although QPSK does not provide as many bits per second as some other schemes, it ensures qual- ity in implementations where interference could be a problem. Carrierless Amplitude Phase Modulation (CAP)-CAP combines ampli- tude and phase modulation, and it is one of the early techniques used for ADSL. However, portions of the band over which ADSL operates conduct noise from exterior devices such as ham radios and CB radios, SO if these devices are operating while you're on a call over an ADSL line, you experi- ence static in the voice call or corrupted bits in a data session. Consequently, CAP is no longer the preferred technique with ADSL because it provides a rather low quality of service. (ADSL is discussed in Chapter 2, "Traditional Transmission Media," and in Chapter 12, "Broadband Access Alternatives.") Multicarrier Modulation Schemes There are two multicarrier schemes: Orthogonal Frequency Division Multiplexing (OFDM)-OFDM, which is growing in importance, is used in European digital over-the-air broadcast and in many new and emerging wireless broadband solutions, including 802.11a, 802.11g, 802.16x, 802.20x, and Super 3G, and it is the basis of 4G Data Flow and 5G visions. It is also used in xDSL, where it is known as DMT. OFDM is a combination of two key principles: multicarrier transmission and adap- tive 