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

320 Additional Techniques for Capacity and Flexibility Enhancementa Rayleigh fading channel. It is important to note that the performance of OFDM-CDMis comparable to the performance of a fully loaded MC-CDMA scheme in the downlinkand that the performance of OFDM is comparable to the performance of OFDMAor MC-TDMA with perfect interleaving. The transmission bandwidth of the systems isB = 2 MHz and the carrier frequency is located at 2 GHz. The number of sub-carriersis 512. The guard interval duration is 20 µs. As channel codes, punctured convolutionalcodes with memory 6 and variable code rate R between 1/3 and 4/5 are applied. QPSK isused for symbol mapping. The depth of the interleaver is equal to 24 subsequent OFDMsymbols, so that time and frequency interleaving is applied. SFBC is realized with M = 2transmit antennas. There is no correlation between the antennas in the space–frequencyblock coded OFDM system. In the case of OFDM-CDM, short Hadamard codes of lengthL = 8 are applied for spreading. The mobile radio channel is modeled as an uncorrelatedRayleigh fading channel, assuming perfect interleaving in the frequency and timedirection.In Figure 6-22, the BER versus the SNR per bit is shown for space–frequency blockcoding in OFDM and OFDM-CDM systems. The results are presented for OFDM-CDMwith different single-symbol detection techniques. No FEC coding is used. The correspondingresults without SFBC are shown in Figure 2-14.10 010 −110 −2BER10 −310 −4MRCEGCZFMMSE eq.OFDM10 −50246810E b/N 0in dB1214161820Figure 6-22 BER versus SNR for space–frequency block coding applied to OFDM andOFDM-CDM with different single-symbol detection techniques: no FEC coding