 802.16 Task Group TGe addressed the challenge of adding mobility to the 802.16 standard. A study group on Mobile Broadband Wireless Access was set up in 2002 with the aim of providing mobile access at vehicular speeds of up to 125 km/hr using licensed frequency bands. The resulting 802.16e standard, also designated 802.16-2005, was ratified by the IEEE in December 2005 and specifies new modulation and multiple access schemes to enable mobile non line-of- sight operation. 802. 16e enhances the 802.16's original OFDMA air interface by adding a number of new capabilities, as summarised in Table 12-8. The two key concepts underlying the 802.16e enhancements are the fixing of subcarrier spacing independently from channel width (scalable OFDMA) and the use of subchannelization to enable range versus capacity trade-offs. Wireless MAN Standards Table 12-8: 802.16e Enhancements Enhancement Capabilities Constant subcarrier spacing Increased resistance to multipath fading and Doppler spread in mobile transmission is achieved by keeping subcarrier spacing constant, independent of channel width. This results in a "scaling" of the number of subcarriers with channel width. Improved indoor penetration A subset of the available OFDM carriers is used at higher individual power (subchannelisation) to improve indoor reception. Flexibility in coverage versus Subchannelisation in the downlink allows a flexible capacity trade-off between data capacity and operating range. Advanced antenna support NLOS coverage and performance are improved using advanced multi-antenna diversity and adaptive antenna systems as well as MIMO radio technology. Enhanced error correction New coding techniques are used to improve mobile and NLOS performance (e.g. turbo coding and low-density parity check (LDPC)). Faster error recovery Using hybrid-automatic retransmission request (hARQ) to improve error recovery. Scalable OFDMA As described earlier, 802.16a specifies an OFDM air interface with flexible channel widths from 1.75 to 20 MHz. Each channel is divid