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

190 Implementation IssuesEqualizationDe-mapping and soft metric derivationReliabilityestimation1/H g 00Reliabilityestimation1/H 1 g 1QuantizationQuantizationFFT••P/STo decoder••••Reliabilityestimation1/H Nc−1 g Nc−1QuantizationFigure 4-51Channel equalization and soft metric derivationbe optimally converted to soft metric information. Thus, the channel-corrected data haveto be combined with the reliability information exploiting channel state information foreach sub-carrier so that each encoded bit has an associated soft metric value and a harddecision, which are provided to the Viterbi decoder.As shown in Figure 4-51, after channel correction, i.e. equalization, a reliability informationis provided for each mapped bit of the constellation. This reliability informationcorresponds to the minimum distance from the nearest decision boundary that affects thedecision of the current bit. This metric corresponds to LLR values (see Section 2.1.8) [70]after it is multiplied with the corresponding value of the SNR of each sub-carrier γ n =4|H n | 2 /σn 2 . Finally, after quantization (typically 3–4 bits for amplitude and 1 bit for thesign), these soft values are submitted to the channel decoder.4.6 Adaptive Techniques in Multi-Carrier TransmissionAs shown in Chapter 1, the signal suffers especially from time and frequency selectivityof the radio channel. Co-channel and adjacent channel interference (CCI and ACI) arefurther impairments that are present in cellular environments due to the high frequencyre-use. Each terminal station may have different channel conditions. For instance, theterminal stations located near the base station receive the highest power which results ina high carrier-to-noise and -interference power ratio C/(N + I). However, the terminalstation at the cell border has a lower C/(N + I).In order to exploit the channel characteristics and to use the spectrum in an efficientway, several adaptive techniques can be applied, namely adaptive FEC coding, adaptivemodulation, and adaptive power leveling. Note that the criteria for these adaptive