fficiency is of great importance [12]. Quadrature Amplitude Modulation It can be noted from Figure 7.16 that when the number of points in a PSK signal constellations increase, such as in 16-PSK, the distance between constellation points becomes very small. A combination of both phase and amplitude levels can be used to increase inter-constellation point distance, thereby reducing the probability that additive noise may cause the detection boundary to be crossed and result in a detection error. Following from the transmit signal rep- resentation, A(t)cos[@ t+((p(t)], both A(t) and Q(t) may vary as a function of time and be used to carry transmitted data information. This technique is known as QAM. One commonly used QAM technique is 16-QAM. This constellation uses 3 different amplitude levels and 12 different phase levels for a total of 16 constellation points. Each point in the 16-QAM signal constellation represents 4 bits, enabling the transmission of 4 bits during each time interval. Figure 7.19 compares 16-QAM with 16-PSK signal constellations. It can be observed that for a similar signal power, inter-constellation symbol distance is larger for 16-QAM, and therefore the probability that a symbol is erroneously detected is lower. This is verified by the BER performance graph for the two systems shown in Figure 7.20, as approximately 4 dB separates the performances at BER of 10-4. As might be expected, 16-QAM is used much more commonly than 16-PSK. 16-QAM is used in wireless standards such as 3G WCDMA and the digital subscriber line (DSL) family of standards for fixed telephone line systems. Higher order modulations such as 64-QAM and 256-QAM have also been standardised and are used in wireless and fixed applications. The 64-QAM constellation facilitates transmission of 6 bits per symbol and Broadband Telecommunications Technologies and Management 16-PSK 16-QAM Figure 7.19 16-PSK and 16-QAM signal constellations 1.0E+00 1.0E-01 16-PSK 1.0E-02 16QAM 1.0E-03 1.0E-04 1.0E-05 Eb/No (dB) Figure 7.20 16-PSK and 16-QAM 