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

82 MC-CDMA and MC-DS-CDMAmaximum number of active users K. The OFDM symbol duration increases and the loss inspectral efficiency due to the guard interval decreases with the M-Modification. Moreover,the tighter sub-carrier spacing enables one to guarantee flat fading per sub-channel inpropagation scenarios with a small coherence bandwidth. With the M-Modification, eachuser transmits simultaneously M>1 data symbols per OFDM symbol.The total number of sub-carriers of the modified MC-CDMA system isN c = ML. (2.91)Each user exploits the total of N c sub-carriers for data transmission. The OFDM symbolduration (including the guard interval) increases toT ′s = T g + MLT c , (2.92)where it can be observed that the loss in spectral efficiency due to the guard intervaldecreases with increasing M. The maximum number of active users is still K = L.The data symbol index m, m = 0,...,M − 1, is introduced in order to distinguish theM simultaneously transmitted data symbols d m(k) of user k. The number M is upper-limitedby the coherence time (t) c of the channel. To exploit frequency diversity optimally,the components of the sequences s m ,m= 0,...,M − 1, transmitted in the same OFDMsymbol, are interleaved over the frequency. The interleaving is carried out prior to OFDM.2.1.9.2 Parallel User Groups (Q-Modification)With an increasing number of active users K, the number of required spreading codesand, thus, the spreading code length L increase. Since L and K determine the complexityof the receiver, both values have to be kept as small as possible. The Q-Modificationintroduces an OFDMA component (see Chapter 3) on sub-carrier level and with thatreduces the receiver complexity by reducing the spreading code length per user, whilemaintaining constant the maximum number of active users K and the number of subcarriersN c . The MC-CDMA transmitter with Q-Modification is shown in Figure 2-12user group 1. . .user group Qd (0)d (L−1)d (K−L)d (K−1)spreaderc (0). . .spreaderc (L−1)spreaderc (0). . .spreaderc (L−1)+s 0. . .s Q −1+serial-to-parallelconverterserial-to-parallelconverter0 0. . .L − 10. . .L − 1frequency interleaver. . .N c −1OFDMxFigure 2-12Q-Modification