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

92 MC-CDMA and MC-DS-CDMA10 010 −110 −2BER10 −310 −4MRTEGTZFCE, a th = 0.175Quasi MMSE10 −50 2 4 6 8 10 12 14 16 18 20E b /N 0 in dBFigure 2-21 BER versus SNR for MC-CDMA with different pre-equalization techniques in theuplink: fully loaded system; no FEC codingIn Figure 2-21, the BER versus the SNR for an MC-CDMA system with differentpre-equalization techniques in the uplink is shown. The system is fully loaded. It can beobserved that with pre-equalization a fully loaded MC-CDMA system can achieve promisingresults in the uplink. Quasi MMSE pre-equalization and controlled pre-equalizationwith a threshold of a th = 0.175 outperform other pre-equalization techniques.When assuming that the information about the uplink channel for pre-equalization isonly available at the beginning of each transmission frame, the performance of the systemdegrades with increasing frame duration due to the time variation of the channel. A typicalscenario would be that at the beginning of each frame a feedback channel provides thetransmitter with the required channel state information. Of importance for the selection ofa proper frame duration is that it is smaller than the coherence time of the channel. Theinfluence of the frame length for an MC-CDMA system with controlled pre-equalizationis shown in Figure 2-22. The Doppler frequency is 26 Hz and the OFDM symbol durationis 13.6 µs.In Figure 2-23, the performance of an MC-CDMA system with controlled pre-equalizationand an update of the channel coefficients at the beginning of each OFDM frame is shown fordifferent system loads. An OFDM frame consists of 200 OFDM symbols.In the following, the performance of different combined equalization techniques ina multi-user MC-CDMA uplink is shown. The system is fully loaded, i.e. L users areactive per subsystem. Walsh–Hadamard codes of length L = 8 are used. At the receiver,a soft interference canceller with three iterations is applied. The ETSI BRAN ChannelE [36] is taken as the propagation channel. Figure 2-24 illustrates the BER versus SNRperformance of a fully loaded rate 1/2 coded uplink MC-CDMA system.