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

112 Hybrid Multiple Access SchemesOFDMA with frequency hopping has a close relationship with MC-CDMA. We knowthat MC-CDMA is based on spreading the signal bandwidth using direct sequence spreadingwith processing gain P G . In OFDMA, frequency assignments can be specified witha code according to a frequency hopping (FH) pattern, where the number of hops can beslow. Both schemes employ OFDM for chip transmission.3.2.1.5 General OFDMA TransceiverA general conceptual block diagram of an OFDMA transceiver for the uplink of a multiusercellular system is illustrated in Figure 3-3. The terminal station is synchronized tothe base station. The transmitter of the terminal station extracts from the demodulateddownlink received data MAC messages with information about sub-carrier allocation,frequency hopping pattern, power control messages and timing, and further clock and frequencysynchronization information. Synchronization of the terminal station is achievedby using the MAC control messages to perform time synchronization and using frequencyinformation issued from the terminal station demodulator (the recovered basestation system clock). The MAC control messages are processed by the MAC managementblock to instruct the terminal station modulator on the transmission resources assigned toit and to tune the access performed to the radio frequency channel. The pilot symbols areinserted to ease the channel estimation task at the base station.At the base station, the received signals issued by all terminal stations are demodulatedby the use of an FFT as a conventional OFDM receiver, assisted by the MAC layermanagement block.TS ReceiverBS TransmitterMACmessagesClock,frequencyDownlinkMedium Access ControllerClock,frequencyMAC- Sub-carrier allocation- Timing- Power control, RangingSynchronizationSynchronizationInterleavingEncodingSymbol mappingFramingMulti-carrier modulator(IFFT)Terminal StationOFDMA TransmitterSub-carrier shapingRF up-converterPilotinsertionRFoutputUplinkRFinputRF down-converterMatched filteringBase StationOFDMA ReceiverMulti-carrier demodulator(FFT)Equalization, DemappingDeinterleaving, DecodingChannelestimationFigure 3-3General OFDMA conceptual transceiver