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

188 Implementation IssuesK ModFrom encoder(serial bits)S/Pb 0••2 m QAM-MappingIb m −1QK ModFigure 4-49Signal mapping block diagramgeneral principle of modulation schemes is illustrated in Figure 4-49, which is valid forboth uplink and downlink.For the downlink, high-order modulation can be used such as 4-QAM (m = 2) up to64-QAM (m = 6). For the uplink, more robust 4-QAM and 16-QAM are preferred. Typically,constellation mappings are based on Gray mapping, where adjacent constellationpoints differ by only one bit. Table 4-6 defines the constellation for 4-QAM modulation.In this table, b l ,l = 0,...,m− 1, denotes the modulation bit order after serial-to-parallelconversion.The complex modulated symbol takes the value I + jQ from the 2 m point constellation(see Figure 4-50). In the case of transmission of mixed constellations in the downlinkframe, i.e. adaptive modulation (from 4-QAM up to 64-QAM), a constant transmit powershould be guaranteed. Unlike the uplink transmission, this would provide the advantagethat the downlink interference from all base stations has a quasi-constant behavior. Therefore,the output complex values are formed by multiplying the resulting I + jQ value by anormalization factor K MOD , as shown in Figure 4-49. The normalization K MOD dependson the modulation as prescribed in Table 4-7.Symbol mapping can also be performed differentially as with D-QPSK applied in theDAB standard [16]. Differential modulation avoids the necessity of estimating the carrierphase. Instead, the received signal is compared to the phase of the preceding symbol [70].However, since one wrong decision results in two decision errors, differential modulationperforms worse than non-differential modulation with accurate knowledge of the channelin the receiver. Differential demodulation can be improved by applying a two-dimensionaldemodulation, where the correlations of the channel in the time and frequency directionsare taken into account in the demodulation [30].Table 4-6 Bit mapping with 4-QAMb 0 b 1 I Q0 0 1 10 1 1 −11 0 −1 11 1 −1 −1