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

72 MC-CDMA and MC-DS-CDMA2.1.6 Pre-EqualizationIf information about the actual channel is aprioriknown at the transmitter, pre-equalizationcan be applied at the transmitter such that the signal at the receiver appears nondistorted andan estimation of the channel at the receiver is not necessary. Information about the channelstate can, for example, be made available in TDD schemes if the TDD slots are short enoughsuch that the channel of an uplink and a subsequent downlink slot can be considered asconstant and the transceiver can use the channel state information obtained from previouslyreceived data.An application scenario of pre-equalization in a TDD mobile radio system would be thatthe terminal station sends pilot symbols in the uplink which are used in the base stationfor channel estimation and detection of the uplink data symbols. The estimated channelstate is used for pre-equalization of the downlink data to be transmitted to the terminalstation. Thus, no channel estimation is necessary in the terminal station which reduces itscomplexity. Only the base station has to estimate the channel, i.e. the complexity can beshifted to the base station.A further application scenario of pre-equalization in a TDD mobile radio system wouldbe that the base station sends pilot symbols in the downlink to the terminal station, whichperforms channel estimation. In the uplink, the terminal station applies pre-equalizationwith the intention to get quasi-orthogonal user signals at the base station receiver antenna.This results in a high spectral efficiency in the uplink, since MAI can be avoided. Moreover,complex uplink channel estimation is not necessary.The accuracy of pre-equalization can be increased by using prediction of the channelstate in the transmitter where channel state information from the past is filtered.Pre-equalization is performed by multiplying the symbols on each sub-channel withan assigned pre-equalization coefficient before transmission [10, 23, 37, 45, 47]. Theselection criteria for the equalization coefficients is to compensate the channel fading asfar as possible, such that the signal at the receiver antenna seems to be only affected byAWGN. In Figure 2-9, an OFDM transmitter with pre-equalization is illustrated whichresults with a spreading operation in an MC-SS transmitter.2.1.6.1 DownlinkIn a multi-carrier system in the downlink (e.g. SS-MC-MA) the pre-equalization operationis given bys = Gs, (2.57)where the source symbols S l before pre-equalization are represented by the vector s and Gis the diagonal L × L pre-equalization matrix with elements G l,l . In the case of spreading,symbolmapperspreaderspre-equalizerGsOFDMFigure 2-9OFDM or MC-SS transmitter with pre-equalization