II 00 00 00 01 00 01 01 00 01 CHAPTER 3.4 Channel protection fundamentals 4-QAM (i.e., a higher minimum tolerated SNR is flexibilities and simplifies network planning. Its value has needed). become even more apparent with recent increasing The LP bit stream is essentially either 4-QAM demands for delivery of different services over [Figure 3.4-14 (top)] or 16-QAM [Figure 3.4-14 heterogeneous networks, where communication channels (bottom)] modulated. Thus, it carries two or four bits between the sender and the clients are extremely diverse and has the same data rate as the corresponding non- in available bandwidths and channel noise. hierarchical modulation method. The noise sensitivity is For example, suppose that two digital TV programs comparable to that of the whole constellation [16-QAM are to be transmitted simultaneously. With non- in Figure 3.4-14 (top) or 64-QAM in Figure 3.4-14 hierarchical modulation, the two programs must be (bottom)] Note that the total rate of the HP and LP bit broadcast over two separate frequency channels: 4-QAM streams is equal to the rate of the whole nonhierarchical can be used for the first channel (achieving a data rate of constellation (16-QAM or 64-QAM). two bits per modulation signal) and 16-QAM for the The HP bit stream is obviously more robust to channel second channel (with a data rate of four bits per noise than the LP bit stream; indeed, a transition of the modulation signal). With hierarchical modulation carrier phase (due to channel noise) from one digital state ("4-QAM in 64-QAM"), only one channel is needed: the to the other within a quadrant is more likely to occur than first program can be transmitted as an HP bit stream (at a transition to a state in another quadrant. However, the a data rate of two bits per modulation signal), while the robustness of the HP and LP bit streams can be further second TV program can be transmitted as an LP bit improved by channel coding (i.e., by adding error pro- stream (at a data rate of four bits per modulation signal). tection