An equalizer and memory mapping predistorter for nonlinearly amplified signals transmitted by coded OFDM systems in multipath fading channel

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Journal Article
Wireless Personal Communications, 2003, 25 (2), pp. 101 - 115
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We present an improved channel equalizing scheme to be used in Coded OFDM systems over an ISI channel. The scheme uses Turbo Trellis Coded Modulation (TTCM) which retains a powerful error correction capability while elegantly incorporating turbo coding into the Trellis Coded Modulation (TCM). In OFDM over wireless communications, M-ary Quadrature Amplitude Modulation (M-QAM) schemes have been widely used to achieve high bandwidth efficiency. However, M-QAM signals require linear amplification to avoid spectral regrowth and severe performance degradation due to their large envelope fluctuations. Moreover, the OFDM transmitter shapes the signal with a linear pulse shaping filter. The combination of a linear filter with a High Power Amplifier (HPA) forms a nonlinear system with memory, which severely degrades performance. We propose an efficient one-tap-equalizer using a modified LMS algorithm and a novel adaptive predistortion technique to compensate for the nonlinear distortion caused by the high power amplifier cascaded with a linear filter in TTCM-OFDM systems. The performance of this method is compared with methods that have been used previously with OFDM. A computer simulation confirms that the proposed approach is superior to the conventional methods.
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