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NON-LINEARITY DISTORTION MITIGATION OF DOWNLINK-LTE SYSTEM USING MODIFIED AMPLITUDE CLIPPING AND FREQUENCY DOMAIN RANDOMIZATION

Authors: Montadar Abas Taher --- Abidaoun Hamdan Shallal --- Ilham Hameed Qaddoori
Journal: DIYALA JOURNAL OF ENGINEERING SCIENCES مجلة ديالى للعلوم الهندسية ISSN: 19998716/26166909 Year: 2015 Volume: 8 Issue: 4 Pages: 613-617
Publisher: Diyala University جامعة ديالى

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Abstract

Wireless telecommunication systems are almost the most dominant field of the communication systems context nowadays especially the long-term evolution (LTE) system. Orthogonal frequency division multiplexing (OFDM), has become the base for current and future communication systems, because of its capability to combat multipath and fading channels and its competency for high data rate transmissions. However, coherent combination of subcarriers leads to large power peaks with respect to the average power level; this is the so-called peak power ratio (PPR). Various methodologies are available in the literature, the simple approach is the amplitude clipping. However, amplitude clipping causes in-band distortion and out-of-band-radiation, thus, the bit error rate (BER) performance will degrades dramatically. Taher et al (2014) suggested a new amplitude-clipping algorithm, where the core-clipping function was replaced with a non-distorting function, but the proficiency of reducing the PPR was not in the good extent. In this paper, the new clipping function will be supported by frequency-domain randomization operation, such that the coherent combination of the OFDM subcarriers will not stay in the same order, leading to lower PPR at the output of the power amplifier. Results show that the hybrid combination of the randomization process with the new clipping function produces lower values of PPRs. Thus, the proposed approach has gotten 1.5 dB more reduction magnitude in the PPR with respect to Taher et al scheme. The complementary cumulative distribution function (CCDF) was the tool to monitor the PPR performance behavior.


Article
POWER ENVELOPE VARIATION IMPROVEMENT OF DOWNLINK LTE SYSTEM USING COMPLEX NUMBER MANIPULATION APPROACH

Authors: Montadar Abas Taher --- Abdulmunem Ahmed Mohammed
Journal: DIYALA JOURNAL OF ENGINEERING SCIENCES مجلة ديالى للعلوم الهندسية ISSN: 19998716/26166909 Year: 2015 Volume: 8 Issue: 4 Pages: 618-623
Publisher: Diyala University جامعة ديالى

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Abstract

Today the world is witnessing very significant developments in the world of digital communications. For that, the subscribers are demanding for quality and speed for the available services. To achieve these objectives, must be accurate and low impurity communication devices available. The orthogonal frequency division multiplexing (OFDM) scheme, is the downlink modulation scheme utilized in the long term evolution (LTE) system. Because of the coherent addition of the subcarriers, the output power variation will show high peaks compared with the average, known as Crest factor, thus, the amplification gain is degraded and intermodulation distortion will appears. Different sorts of schemes have been introduced to mitigate these problems, but either on the expense of complexity or the bit error rate (BER) performance degradation. In this paper, a novel simple to implement algorithm suggested by the authors to mitigate the high Crest factor. The suggested approach is based on complex numbers manipulation, by exchanging the real and imaginary parts, to destroy the coherence order of the OFDM subcarriers, leading to lower crest factor. Results show that the computational complexity and the reduction gain are improved significantly.

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