Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

248 ApplicationsTable 5-17Applications and QoS requirementsService category Transported service QoS requirementsrtPSReal-time polling serviceUGSUnsolicited grant serviceErtPSExtended real-timepolling servicenrtPSNon-real-time pollingserviceBEBest effort serviceStreaming audio or videoVoIPVoice with activity detection(VoIP)File transfer protocol(FTP)Data transfer, web,browsing, etc.– Maximum reserved rate– Maximum sustained rate– Maximum latency tolerance– Traffic priority– Maximum sustained rate– Maximum latency tolerance– Jitter tolerance– Maximum reserved rate– Maximum sustained rate– Maximum latency tolerance– Jitter tolerance– Traffic priority– Maximum reserved rate– Maximum sustained rate– Traffic priority– Maximum sustained rate– Traffic priorityMAC Frame StructureThe basic IEEE 802.16 OFDM frame format [23], supporting adaptive modulation, issketched in Figure 5-26. The same frame format is used for the FDD and TDD mode,where for the latter case the TDM portion can be extended to support the uplink TDMAmode. Each frame starts with a preamble of known sequences and a frame control header(FCH) which is protected by the most robust PHY mode (BPSK1/2). The FCH transmitsthe OFDM burst profile. The first burst after the FCH is made of two parts, broadcastand data. The broadcast part is made of the downlink map (DL map), uplink map (ULmap), and downlink and uplink channel descriptors (DCD and UCD). The DL map transmitsinformation regarding the contents of the DL frame with respect to the employedmodulation scheme. Each frame starts with the most robust modulation and ends up withthe most spectral efficient one. Note that the UL map transmits the uplink grant informationassigned for each terminal station. Within the so-called downlink and uplink channeldescriptors several additional pieces of information are broadcasted, which deal with thebase station and the terminal station dedicated information, such as BS-identifier, TSidentifier,supported frame length (2.5, 8, and 20 ms), PHY type (SC or OFDM), TDD orFDD, modulation and FEC supported, burst concatenation, etc.The frame format in the case of TDD for an OFDMA system [24] is illustratedin Figure 5-27. As in the case of the generic OFDM frame, each OFDMA frame isdivided into downlink and uplink sub-frames, separated by transmit/receive (respectivelyreceive/transmit) time gaps TTG (respectively RTG). This is necessary for a TDD schemefor avoiding downlink and uplink transmission collisions. Note that each burst transmitsa given number of MAC-PDUs, as sketched in Figure 5-26.