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

Channel Estimation 17110 010 −1BU, 3 km/hTU, 30 km/hHT, 150 km/hRA, 250 km/hBER10 −210 −310 −44 5 6 7 8 9 10 11 12 13 14E b /N 0 in dBFigure 4-30downlinkBER versus SNR of MC-CDMA with pilot symbol aided channel estimation in theAlternatively, a pilot symbol design in the time domain is possible by using for eachuser a different fraction of an OFDM symbol [83]. These methods have restrictions onthe maximum length of the channel impulse response τ max . When fulfilling the conditionτ max T sK , (4.72)the system is able to estimate K different channel impulse responses within one OFDMsymbol. Since the OFDM system design typically results in an OFDM symbol duration T smuch larger than τ max , several users can be estimated in one OFDM symbol. The principleof time domain channel estimation in the uplink of an MC-CDMA system with eight usersis illustratively shown in Figure 4-31, where h (k) is the channel impulse response h (k) (τ, t)of user k.The structure for channel estimation shown in Figure 4-31 is obtained by insertingpilot symbols in an OFDM symbol and rotating the phase of the complex pilot symbolsaccording to the user index. This results in a delayed version of the time reference signalfor each user. Since whole OFDM symbols are used for this type of channel estimation,the overhead in pilot symbols is comparable to that of scattering pilots in the frequencydomain, as described at the beginning of this section.Additionally, filtering in the time direction can be introduced, which reduces the overheadon pilot symbols. The distance between OFDM symbols with reference informationis N t OFDM symbols. The filter in the time direction can be designed as described in