 roof top antenna gains). Hierarchical modulation can be combined with quality/resolution scalable video coders. Then, the LP bit stream plays the role of the enhancement layer, which improves the quality/resolution of the HP (base layer) bit stream. Depending on transmission conditions, the receiver will be able to decode at the higher or lower quality/resolution level. Another application is simulcast of the high definition TV formats, together with the standard definition formats. (Transmitting the standard definition together with the high definition formats is necessary because all the receivers do not have screens that support the latter Figure 3.4-15 A nonuniform 16-QAM constellation with formats.) Here, the HP bit stream carries the standard a = b/a = 2. definition TV formats, and thus will be available to all Channel protection fundamentals CHAPTER 3.4 receivers, whereas the LP bit stream carries the high (NACK) to indicate a transmission error. The efficiency definition TV formats only. of an ARQ scheme is measured by its reliability and Comparisons between hierarchical and non- throughput. The reliability is the probability that the hierarchical modulation in different scenarios can be receiver accepts a word that contains an undetectable found in [42]. error. The throughput is the ratio of the average number of bits successfully accepted per unit of time to 3.4.5 Automatic repeat request, the total number of bits that could be transmitted per unit of time [29]. In the following, we overview the most hybrid FEC/ARQ important ARQ schemes. Details can be found in [29] and [50]. In this section, we present error protection techniques that use retransmissions. Here we assume the presence 3.4.5.1.1 Stop-and-wait ARQ of a feedback channel from the receiver to the transmitter. We first describe pure ARQ techniques, In stop-and-wait ARQ, the transmitter sends a codeword which are based on error detection and retransmission of and waits for an acknowledgment for that codeword. If the corrupted packets. Then we 