 three channel coders you provided in (a), (b), and (c). 7. Study Figure Q6.1. Now sketch the output of the modulator when (1) the modulator is BPSK and (2) the modulator is QPSK. Channel Modulator Modulator 111 101 Single parity check bit Figure Q6.1 3 bit 4 bit A channel coder and modulator Chapter Channel Coding and Decoding: Part 2-Convolutional Coding and Decoding n Chapter 6, we took a careful look at block coders and decoders. We're now going to look at another class of channel coder and decoder, something called a convolu- tional coder and decoder. Just like the name indicates, convolutional coders and decoders are a little bit more complicated than block coders and decoders. 7.1 Convolutional Coders At the transmitter side, convolutional coders, like block coders, take each set of k bits and put out a set of n bits. That means an extra (n - k) bits are introduced. At the receiver side, a convolutional decoder uses the extra bits to detect and correct bit errors. There is one main difference between convolutional coders and block coders. In block coders, you take a block of k input bits and you put out a block of n output bits. The n output bits only depend on the incoming k input bits. Convolutional coders have what could be described as a greater appreciation of history. In convolutional coders, each k-bit block that comes in is mapped to an n-bit output. But with convolutional coders, the n-bit output depends on the current k-bit block that came in, and also on the previous K blocks of k bits that came in. This is seen in Figure 7.1. Rather than elaborate in general, let's go to an example. 7.1.1 Our Example We'll take a look at a convolutional coder which takes in blocks of k = 1 bit at a time and puts out a chunk of n = 2 bits. Now, if this were a block coder, we might say, "If a 0 comes in, map it to a (00), and if a 1 comes in, map it to a (1 1)." But it's not a block coder. This convolutional coder makes its decision not just considering the current k = 1 bit but also by considering the previous K