01110. Solution: Turning to Table 5.2 and 5.4, we see the output when different bits are input. Using these tables, with input 00 11 10, we find the output shown in Figure E5.1. 3Acos(Wct) 3Acos(Wct) Acos(Wct) 4 ASK Acos(Wet+00) Acos(Wct + 4 PSK Figure E5.1 Output of 4-ASK and 4-PSK Choosing a Modulation Method Bandpass modulation offers a lot of choices. You first have to choose from among ASK, PSK, FSK, and QAM. Once you've made that choice, you've got to pick a num- ber: will it be, for example, 4-ASK, 8-ASK, or 16-ASK? In this section, I'll briefly discuss the most common criteria for choosing the modulation scheme that's best for your communication system. Let's start by taking a look at the different modulation types (ASK, PSK, FSK). Rather than tell you what to use, I'm going to tell you what to avoid using. In ASK, all the information is in the amplitude. Let's say your communication channel creates amplitude distortions. In that case, ASK is not the modulation you want. That's because the channel messes up the part of your signal containing the information. You don't want that. Chapter Five In PSK, all the information is in the phase. Let's say your channel introduces phase distortions and you are unable to remove these phase distortions at the receiver. In this case, you don't want to use PSK, because the channel is distorting the part of the signal with information. Finally, in FSK, you've got information in the frequencies. If the channel causes significant frequency distortions that you can't correct, then you don't want to use FSK. FSK has another drawback. It takes up more bandwidth than either PSK or ASK. In ASK and PSK, you always send the information signal at the same frequency-it's always sent around W (e.g., Table 5.4). But in FSK, you send the signal at a lot of different frequencies (e.g., around W + AWO or W + AW1 (Table 5.5)). This means you've got to have more frequencies available to you if you want to send an FSK signal. Let's say you've decided on PSK, but should you use BPSK or 4-PSK