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

254 ApplicationsTxMACPDUScramblingFECEncoderMappingTxDiversity(option)OFDM /OFDMA(IFFT)Framing(Pilots,References,Guard time)D/ARFStandardizedSynchronization & Channel EstimationChannel, N/LOSRxMACPDUFECDecoderDe-Mapping&Equaliz.RxDiversity(option)OFDM /OFDMA(FFT)De-ScramblingDe-Framing(Pilots,References,Guard time)A/DRFFigure 5-32PHY layer block diagram overview5.3.4.4 PHY LayerAn overview of the PHY layer transmission and reception functions is depicted inFigure 5-32. The transmit MAC PDUs after scrambling and FEC coding are mapped tothe transmitted modulation constellation. Depending on the transmission mode (OFDMand OFDMA), the modulated data are assigned to the sub-carriers [23].Allocated Carrier FrequenciesIn Table 5-18, some target carrier frequency bands below 11 GHz are listed [23]. Thechannel bandwidths depend on the used carrier frequency.The use of these radio bands provides a physical environment where, due to itswavelength characteristics, line of sight (LOS) is not necessary but multi-path may be significant(the delay spread is similar to DVB-T with up to 0.2 ms). The Doppler effects arenegligible for a fixed positioned terminal station; however, for a mobile terminal stationDoppler shifts will affect the link quality.Multi-Carrier Transmission SchemesThe physical layer of both HIPERMAN and WirelessMan standards support multi-carriertransmission modes. The basic transmission mode is OFDM. Depending on the selectedtime/frequency parameters, the system can support TDMA as well as OFDMA. This flexibilityensures that the system can be optimized for (a) short burst type of applications aswell as more streaming type oriented applications, and for (b) mobile and fixed receptionconditions. The main advantage of using OFDMA with high numbers of sub-carriers withthe same data rate as the OFDM mode is to provide higher coverage, i.e. larger guard timeand a reduction of transmitted power for a mobile terminal station (longer battery life).In the pure OFDM mode, in total 256 sub-carriers will be transmitted at once. The downlinkapplies time division multiplexing (TDM) and the uplink uses time division multipleaccess (TDMA). In the pure OFDMA mode, the channel bandwidth is divided into up to2048 sub-carriers, where each user is assigned to a given group of sub-carriers. However,