tes per Hertz. The num- ber of bits per symbol time is k, where 2k = nn. Therefore, 4 bits per Hertz = 24 = 16-QAM, 6 bits per Hertz = 26 = 64-QAM, and 8 bits per Hertz = 28 = 256-QAM. QAM techniques are used in many applications, including digi- tal cable television, cable modems, DSL modems, digital satellite systems, 802.11 (Wi-Fi), 802.16 (WiMax), and 3G W-CDMA/HSDPA systems. Multicarrier Modulation Techniques Multicarrier modulation techniques use an aggregate amount of bandwidth and divide it into subbands. Each subband is then encoded by using a single-carrier Spectrum Utilization technique, and the bitstreams from the subbands are bonded together at the receiver. OFDM, discussed later in this chapter, is an increasingly popular multi- carrier technique. It is used in European digital over-the-air broadcast and in many new and emerging wireless broadband solutions, including 802.11a, 802.11g, 802.16x, 802.20x, and Super 3G; it is the basis of the 4G and 5G visions. Spectrum Utilization The wireless spectrum is limited. Wireless is like having one invisible cable in the sky that the whole world has to share. This is one of the major limitations of wire- less, and we need techniques for efficiently using the spectrum that we do have. There are several steps in reusing spectrum. The first step in spectrum reuse is to apply space division-that is, carve up the service area into smaller coverage areas. The key purpose of space division is to reuse frequencies across the coverage areas, or cells. The second step is to apply a multiple-access technique to allow the sharing of spectrum by multiple users. After you have delineated the space and combined multiple conversations onto one channel, you can apply spread spectrum, duplexing, and compression techniques to use the bandwidth even more efficiently. The following sections discuss these steps in more detail. Using the Available Radio Spectrum The first key characteristic of a radio transmission system is how it makes use of the available radio spectrum, or band