therefore we will not describe it further. In ASK, the carrier amplitude A may take one of 1' A, 2' A values to represent one or more bits of data. Similarly, in PSK the carrier phase O may take one of O l' 0 2' O values to represent one or more bits of data. As noted above, a combination of the two may also be used: specific values for both A and O may be used to represent two or more bits of data. This technique is known as quadrature amplitude modulation (QAM). ASK is commonly used in wired broadband systems such as Ethernet, and PSK and QAM techniques are used both in wired (xDSL) and wireless (mobile and satellite) telecommunications systems and applications. Broadband Telecommunications Technologies and Management The design choice generally depends on the application and the complexity of the modulation technique, as well as the required bit error rate (BER) performance of the method with respect to the signal to noise ratio (SNR) at the receiver. Amplitude Shift Keying ASK techniques use different amplitude levels to represent O's and 1's. For example a two- level ASK system is shown in Figure 7.1. As before the transmitted signal can be written with its carrier frequency as a cosine waveform, with a time-varying amplitude A(t). The information signal is embedded in the carrier's amplitude with A(t) = a representing a 1 and A(t) = b representing a 0. As shown in Figure 7.1, the carrier frequency is modulated by these two different amplitudes, and one bit of information is transmitted at every transmission time interval. The receiver side extracts the information by measuring the amplitude of the received carrier signal and comparing it to a reference level to detect whether a 1 or a 0 has been trans- mitted. In some systems one of the levels is set to zero, and the resulting ASK modulation technique is referred to as on-off keying (OOK). This method is used widely in optical fibre telecommunications. Figure 7.2 shows the binary ASK signal constellation, where the x-axis shows the amplitude of the signal, 