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

Signal Constellation, Mapping, De-Mapping, and Equalization 18710 0 410 −1OFDM-CDM, CCOFDM, CCOFDM, TC, I = 256OFDM, TC, I = 102410 −2BER10 −310 −410 −51 2 35E b /N 0 in dB6 7 8Figure 4-48 Performance of OFDM-CDM with classical convolutional codes versus OFDM withTurbo codes and interleaver size 256 and 1024 respectivelyMC-CDMA and SS-MC-MA can be considered as special cases of OFDM-CDM. InMC-CDMA, CDM is applied for user separation and in SS-MC-MA different CDM blocksof spread symbols are assigned to different users [41].The OFDM-CDM receiver applies single-symbol detection or more complex multisymboldetection techniques which correspond to single-user or multi-user detectiontechniques respectively in the case of MC-CDMA. The reader is referred to Section 2.1.5for a description of the different detection techniques.In Figure 4-48, the performance of OFDM-CDM using classical convolutional codes(CC) is compared with the performance of OFDM using Turbo codes. The BER versusthe SNR for code rate 1/2 and QPSK symbol mapping is shown. Results are given forOFDM-CDM with soft IC after the first iteration and for OFDM using Turbo codes withinterleaver sizes I = 256 and I = 1024 and iterative decoding with 10 iterations. As areference, the performance of OFDM with classical convolutional codes is given. It can beobserved that OFDM-CDM with soft IC and classical convolutional codes can outperformOFDM with Turbo codes.4.5 Signal Constellation, Mapping, De-Mapping, and Equalization4.5.1 Signal Constellation and MappingThe modulation employed in multi-carrier systems is usually based on quadrature amplitudemodulation (QAM) with 2 m constellation points, where m is the number of bitstransmitted per modulated symbol and M = 2 m is the number of constellation points. The