 profile Map, specify the burst profile to each subscriber station start points Figure 12-1: 802.16 Downlink Frame Structure Chapter Twelve having an optional TDMA uplink segment following the TDM portion of each downlink data frame. A range of modulation and coding schemes are available (including QPSK, 16-QAM, 64-QAM) and each subscriber station negotiates a scheme with the base station, in line with its particular needs for efficiency (depending on data rate) and robustness (depending on signal propagation environment/signal strength). The result is that, within a single downlink frame transmitted by the base station (Figure 12-1), individual data bursts destined to individual subscriber stations will be transmitted with different coding and modulation schemes - a technique known as adaptive burst profiling. Achievable data rates for different modulation methods are shown in Table 12-3. Table 12-3: 802.16 Bit Rate vs. Channel Width and Modulation Method Channel width Bit rate (Mbps) per modulation method (MHz) 16-QAM 64-QAM 134.4 PHY Layer for 2-11 GHz Spectrum The differing propagation characteristics in the 2-11 GHz range, compared to the EHF range up to 66 GHz, require an air interface that can accommodate significant multi-path propagation in a non line-of-site operating environment. Three optional PHY layer specifications are defined in the 802.16a standard, which addresses both licensed and unlicensed spectrum, as shown in Table 12-4. Most of the key parameters of the 802. 16a standard are common to its higher frequency predecessor, as shown in the Table 12-5. In addition, 802.16a also supports advanced antenna systems. As described in the Section "Wireless LAN Antennas, p. 55", advanced antenna systems or smart antennas can improve link robustness by suppressing interference and increasing overall system gain. As the cost Wireless MAN Standards Table 12-4: 802.16a Optional Air Interfaces Air interface Summary WirelessMAN SC2 A single carrier modulation format, providing interoperability with the 10-66 G