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Advanced Transmission Techniques in WiMAX Part 4

Tham khảo tài liệu 'advanced transmission techniques in wimax part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 66 Advanced Transmission Techniques in WiMAX Fig. 3. Transmitter of MIMO SFBC-OFDM employing C334 code controlled. Maximal amplitude that does not result in increase in BER depends on both the baseband modulation scheme and the in-band nonlinear distortion introduced by HPA. Figure 4 shows maximum allowed amplitude vs. IBO for various modulations. All curves fulfill the following conditiomBER R BERconv i.e. BER of TR based SFBC-OFDM system is lower or equal to that of the conventional system. This figure can be used by system designer as upper bound for the amplitude of the reserved tones in the different system setups. As it can be appreciated these results are in compliance with our previous assumptions. We can go for higher amplitudes of peak-reduction tones and achieve large out-of-band radiation reduction without BER penalty when QPSK and 16 QAM or coded 64 QAM are adopted for the transmission. The presumptions of the amplitude constraints when uncoded 64 QAM is used are of more relevance especially for lower IBO. In other words when applying the uncoded higher modulation schemes e.g. 64 QAM the amplitude of the correcting tones is constrained to the very low power leading to poorer performance of the proposed method performing at the low IBO. However it should be noted that for low IBO achieved BER of the original system is very poor characterized by the occurrence of the error floor thus this performance is not of our interest. Because of this designer must go for the higher IBO. Figure 5 shows the PSD of original and TR-reduced OFDM signals when a soft limiter operating at IBOs of 4dB or 5dB is present at the output of the transmitter. In order to prevent the BER performance degradation resulting from the broken space orthogonality among transmitted signals the maximum amplitude Y is constrained to be Y 0.2. That corresponds to the power of reserved tones being more than 14 dB lower than the average signal power. It allows for obtaining the reduction in .