1.0E+00 4-ASK 8-ASK 16-ASK 1.0E-01 1.0E-02 1.0E-03 1.0E-04 1.0E-05 Eb/No (dB) Figure 7.9 BER performance of multi-level ASK techniques in an optical system. Source: Ref. [8] Note that again an error is detected only if any of the detection boundaries are crossed. Furthermore, it is possible that a transmitted C may be received as a if the added noise is large, and the combined signal and noise amplitudes cross the (a+b)/2 threshold. This will result in two bit errors as a transmitted '11' is detected as a '00'. This probability is however negligible compared with the more common single-bit error events. Also note that to achieve the same error rate with 4-ASK, the amount of transmission power needs to be larger than that needed with 2-ASK. The reason is that to maintain the same detection boundary, the signal constella- tion points need to be larger in amplitude, resulting in larger necessary transmission powers. Similarly 8-ASK, 16-ASK and SO on may be designed, each of which requires significantly larger transmission power. The BER performances of different ASK techniques are compared in Figure 7.9. Note that to obtain a similar BER of 10-4, 4-ASK requires nearly 5 dB (3 times) more power compared with 2-ASK. This is clearly inefficient, but nevertheless presents the telecommunications engineer with a mechanism to transmit more information in an available bandwidth. Therefore if transmission power is not a constraint, then 4-ASK allows the system to transmit at twice the rate of 2-ASK. ASK modulation techniques are simple to implement and relatively simple to detect. However, they are less efficient in their BER performance compared with PSK techniques and are only used in applications that are not sensitive to power limitations, such as transmissions over wired lines used for telephony or local area networks. Broadband Telecommunications Technologies and Management Pulse Amplitude Modulation ASK techniques are widely used in Ethernet systems where they are better known as pulse-amplitude modulation (PAM). For 