and QoS for mobile networking. 802.16h MAC enhancements to enable coexistence of licence exempt 802.16 based systems and primary users in licensed bands. These different air interfaces are supported by a common MAC layer, which has been designed to provide the key requirements for metropolitan area networking - scalability, service type flexibility and quality of service. 802.16 PHY Layer PHY Layer for 10-66 GHz Spectrum In this extremely high frequency range, RF propagation requires for all practical purposes a line-of-sight between transmitter and receiver. Given this limitation, it is not necessary to consider the use of complex techniques such as OFDM to overcome multi-path effects, which occur in a non-line- of-site environment, and the Working Group therefore selected a simple single carrier (SC) modulation technique for this interface (Table 12-2). In downlink transmissions (from the base station (BS) to subscriber stations (SS)), time division multiplexing (TDM) is used, with time slots Wireless MAN Standards Table 12-2: 802.16 Key Parameters Parameter 802.16 standard RF band 10-66 GHz Modulation Single carrier modulation (SC) (QPSK, 16- & 64-QAM) Data rate Peak data rates to 134 Mbps Channelisation 20, 25 or 28 MHz channel widths Duplex method TDD and FDD, as well as half-duplex via TDMA Network topology Point-to-multipoint Bandwidth allocation Grant per subscriber station (GPSS) - see the Section "802.11 PHY Layer, p.310" allocated to individual subscribers. This enables bandwidth to be guaranteed to latency sensitive services. In the uplink direction (from SS to BS) time division multiple access (TDMA) is used. Duplexing of up and downlinks can be achieved either by time division or frequency division duplexing (TDD or FDD). Half-duplex subscribers, transmitting and receiving on the same channel, are also supported by TDM portion Broadcast TDMA portion control (DIUC 1) (DIUC 2) (DIUC n-1) Downlink Uplink (DIUC n) (DIUC n+1) (DIUC n+2) (DIUC m) Downlink Interval Usage Codes (DIUC), in the Downlink Burst