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

174 Implementation IssuesA common approach is that each user transmits a known user-specific reference sequenceat a certain period of time. These user-specific sequences are used to estimate the channelimpulse response h (k) (τ, t) by evaluating the correlation function of the known referencesequences with the received sequences.In the case of a synchronous uplink, a similar approach as described above for theMC-CDMA uplink can be applied to MC-DS-CDMA.4.3.10 Channel Estimation in MIMO-OFDM SystemsThe basic concept of applying OFDM in multiple input multiple output (MIMO) systems,i.e. employing multiple transmit and receive antennas, is described in Chapter 6.Regarding channel estimation, pilot-based or decision directed channel estimation withmultiple antennas can simultaneously estimate multiple channel transfer functions if thechannel has moderate delay spread, such that the correlations between adjacent subcarriersare high. Based on the assumption that fading on adjacent sub-carriers is equal,it is possible to decouple the channel transfer functions corresponding to the differenttransmit antennas as long as transmitted data on the different antennas are knownat the receiver by either pilot symbols, or the decoded data can be used to generatethe reference data [3, 10, 47, 49, 59]. Making use of the fact that the channeldelay profiles of the various channels in a MIMO scheme should have similar delayprofiles, this knowledge allows to improve the accuracy of the channel estimator further[50].Blind channel estimation in MIMO-OFDM systems can be achieved using statisticalmethods and separating signals from the different transmit antennas by performing aperiodic pre-coding of the individual data streams prior to transmission [6]. Each transmitantenna is assigned a different pre-coding sequence.4.4 Channel Coding and DecodingChannel coding is an inherent part of any multi-carrier system. By using channel stateinformation (CSI) in a maximum likelihood type FEC decoding process a high diversityand, hence, high coding gain can be achieved, especially in fading channels [1, 24].Therefore, it is crucial to choose the encoder in such a way that it enables the exploitationof soft information for decoding. Furthermore, flexibility on the coding scheme to derivedifferent code rates (e.g. for unequal error protection) from the same mother code isalways preferred. This flexibility may allow one to adapt the transmission scheme todifferent transmission conditions.The following channel coding schemes have been proposed for multi-carrier transmission[1, 16, 17, 19, 20, 24]:– punctured convolutional coding;– concatenated coding (e.g. inner convolutional and outer block code, i.e. ReedSolomon code);– Turbo coding (block or convolutional); and recently the– low density parity check code (LDPC).