Heung-Gyoon Ryu

Dummy sequence insertion (DSI) for PAPR reduction in the OFDM communication system

OFDM communications system is very useful for the high data rate transmission. However, power efficiency of HPA (high power amplifier) is very poor, since there should be a large back-off because of the high PAPR (peak-to-average power ratio) of the OFDM signal. Otherwise, OFDM signal may be distorted by the nonlinear HPA. In this paper, we propose a DSI (dummy sequence insertion) method for PAPR reduction. The complementary sequence and the combination of the correlation sequence belong to the dummy sequence. Flipping technique can be combined into DSI method to effectively lower the PAPR reduction. Also, PAPR threshold technique escalates the processing speed. Unlike the conventional PTS (partial transmit sequence) and SLM (selected mapping) methods, the specified dummy sequence is inserted only for PAPR reduction and any kind of side information is not necessary. So, BER of the DSI method is independent of the error in the dummy data sequence. Although the DSI method is not better than the conventional PTS and block coding methods with respect to the PAPR reduction performance, the DSI method improves the BER performance than the conventional PTS and is even more spectrally efficient than the conventional block coding for PAPR reduction.

An improved ICI reduction method in OFDM communication system

Orthogonal frequency division multiplexing (OFDM) is a promising technique for the broadband wireless communication system. However, the inter-sub-carrier-interference (ICI) produced by the phase noise of transceiver local oscillator is a serious problem. Bit error rate (BER) performance is degraded because the orthogonal properties between the sub-carriers are broken down. In this paper, ICI self-cancellation of data-conjugate method is studied to reduce ICI effectively. CPE (common phase error), ICI and CIR (carrier to interference power ratio) are derived and discussed by the linear approximation of the phase noise. Then, the system performance of the data-conjugate method is compared with those of the original OFDM and the conventional data-conversion method. As results, it can be shown that CPE becomes zero in the OFDM of the data-conjugate method. Besides, in the OFDM system with phase noise, the data-conjugate method can make remarkable improvement of the BER performance and it is better than the data-conversion method and the original OFDM with or without convolution coding.

Phase noise analysis of the OFDM communication system by the standard frequency deviation

This paper analyzes phase noise effects on the performance of an OFDM communication system by the linear approximation approach. We newly present a generalized phase noise power spectrum density (PSD) model using the normalized phase noise power spectrum model-Lorentzian model. Also, we derive the relationship between the phase noise PSDs parameter f/sub 3dB/ and the standard frequency deviation f/sub d/ of phase noise probability density function (PDF), and the relationship between phase noise variance /spl sigma//sub /spl phi///sup 2/ and f/sub d/. Analytical BER results closely match with simulation results of the OFDM communication system using QPSK and 16QAM modulation format. The performance of OFDM system is evaluated when the bandwidth B/sub s/=10 MHz and phase noise is included. When the standard frequency deviation f/sub d/ are 5 Hz, 8 Hz. and 12 Hz, the SNR to meet the BER=10/sup -4/ in AWGN channel experiences the power penalty by about 0.6 dB, 1.0 dB and 1.7 dB in the QPSK scheme and about 1.9 dB, 3.2 dB and 67 dB in 16QAM modulation than the OFDM communication system without phase noise. Furthermore, the BER performance of the QPSK-OFDM communication system is considerably degraded because of the BER error floor if the phase noise standard frequency deviation f/sub d/ becomes greater than 30 Hz.

PAPR reduction of the low complexity phase weighting method in OFDM communication system

Orthogonal frequency division multiplexing (OFDM) has been used in many communication systems. However, the major drawback of OFDM system is high peak-to-average-power ratio (PAPR). In this paper, we introduce low complexity phase weighting method to reduce PAPR of OFDM signals. Proposed method is derived from property of discrete Fourier transform (DFT) of periodical sequences. PAPR can be reduced efficiently by phase weighting method with low complexity. The simulation results show the same PAPR reduction efficiency of proposed method in comparison with partial transmit sequence (PTS) or sub-block phase weighting (SPW). The proposed technique can reduce 2.15 dB of PAPR with two phase-factors and 3.95 dB of PAPR with four phase-factors. The analysis on complexity presents remarkable improvement of low complexity phase weighting method. Proposed method odd case can save 75% and 87.5% number of multiplications of PTS method and SPW method separately. The other case of proposed method, even case, can save near 60% number of additions of PTS and over 70% of SPW.

A new PAPR reduction scheme: SPW (subblock phase weighting)

OFDM (orthogonal frequency division multiplexing) has become a popular technique owing to the the high spectrum efficiency and channel robustness. However, one of the serious drawbacks of OFDM signals is the high PAPR (peak to average power ratio) which causes nonlinear distortion and power penalty at the transmitter. In this paper, a new SPW (subblock phase weighting) method is studied for PAPR reduction. PAPR is reduced by using only one IFFT block in the SPW method. Also, the insertion methods on the side information about the subblock phase control weighting to reduce PAPR are proposed for the correct demodulation of OFDM symbols. Compared with the conventional methods, the proposed SPW scheme reduces the computational complexity of PAPR by using the adaptive PAPR threshold method. A new SPW method is examined. The CCDF (complementary cumulative distributed function) of PAPR is evaluated in the cases that both feedback and feedforward type of side information insertion are devised into the basic SPW method. PAPR reduction efficiency grows up as the number of subblocks is increased. However, there is a slight loss of the spectral efficiency due to the side information insertion. The feasibility of the proposed method and the validity of side information insertion method are proved.

Threshold IBO of HPA in the predistorted OFDM communication system

Orthogonal frequency division modulation (OFDM) system is very useful for the multi-path channel and highly bandwidth-efficient system. However, OFDM signal shows the high peak to average power ratio (PAPR) so that nonlinear distortion can happen in the high power amplifier (HPA). This paper studies BER performance variation according to input back off (IBO) values when the predistorter is used for the compensation for the HPA nonlinearity. In the case of the solid state power amplifier (SSPA) of nonlinearity parameter p=0.5 and IBO=0 dB, the system with predistorter is poorer than the one without predistorter. But this situation is inverted, if IBO is increased from 0 dB to 3 dB. So, we can find that there is a threshold value of IBO for the predistorter to effectively compensate for the nonlinear SSPA when the sub-carrier number and p value are changed. This threshold IBO value is proportional to the nonlinearity parameter p value but it is nearly independent of the sub-carrier number because amplitude and distribution are normalized. The OFDM system with predistorter can be improved only when thee IBO value is higher than the threshold value.

PAPR reduction using soft clipping and ACI rejection in OFDM system

OFDM (orthogonal frequency division multiplexing) is usefully applied for DAB (digital audio broadcasting) and DVB (digital video broadcasting) systems due to its high spectral efficiency and robustness to ISI (intersymbol interference) and multipath fading. However, the OFDM signal has a large PAPR (peak-to-average power ratio), which results in significant nonlinear distortion when it passes through an HPA (high power amplifier) such as a TWTA (traveling wave tube amplifier) and a SSPA (solid state power amplifier). We propose a new PAPR reduction method using soft clipping and filtering. Unlike conventional hard clipping, soft clipping has non-zero slope in the clip region. Next, filtering, which uses additional FFT and IFFT processes, is applied in order to reject the out-of-band clip noise. Although the filtering removes the out-of-band noise, it oppositely increases PAPR by a small amount. Since an HPA increases the power consumption according to the linearity range, soft clipping for an SSPA lowers power consumption and is more efficient than the conventional hard clipping method. By the results of the proposed method, the wanted PAPR can be obtained and there is no out-of-band radiation caused by the clipping process.

Improvement of power efficiency of HPA by the PAPR reduction and predistortion

In this paper, we propose the PAPR (peak to average power ratio) reduction methods of SLM (selected mapping), and PTS (partial transmit sequence) to decrease the nonlinear distortion and to improve the power efficiency of the nonlinear HPA (high power amplifier) in the multi-code CDMA (code division multiple access) system. The phase rotation factors transmitted as side information in PTS and SLM methods play an important role in the BER performance. So, we newly derive the theoretical BER equation when the effects of side information are considered in the multi-code CDMA communication system. Simulation results show that the PTS reduces the PAPR by about 2 dB while SLM can reduce by about 3 dB, compared with the original MC-CDMA when QPSK is used for 32 branches. Also, we find the BER performance of multi-code CDMA system with and without predistorter in front of the HPA. When predistorter is used, 3 dB IBO (input backoff) is required in SLM method, and 5 dB IBO in PTS method to closely match with the linear amplifier. Without PAPR reduction method, more than 7 dB is required. Finally, we show that the simulation results are very close to the theoretical results.

Nonlinear analysis of the phase noise in the OFDM communication system

For the detailed analysis of OFDM system, we present a novel approach of the nonlinear approximation including the second order term of phase noise. Phase noise of PLL (phase locked-loop) frequency synthesizer is analyzed when the commercial components of the PLL are considered for the practical analysis. It is shown that BER results by analytical approach closely match with simulation results of the OFDM communication system using QPSK and 16QAM modulation. DF (degradation factor) that is the ratio of SNR without phase noise ((S/N)/sub without{/spl Phi/}/) to SNR with phase noise ((S/N)/sub with{/spl Phi/}/) is used for the performance measure. When the phase noise variances controlled by the PLL closed- loop bandwidth are 0.005, 0.008, and 0.012 respectively, it can be shown that there are about 0.6 dB, 1.0 dB and 1.7 dB power penalties in QPSK scheme and about 1.9 dB, 3.2 dB, and 6.7 dB in 16QAM scheme to meet BER=10/sup -4/ in AWGN channel than OFDM communication system without phase noise. Furthermore, BER performance of QPSK-OFDM communication system is considerably degraded when phase noise variance is bigger than 0.03. Difference between the results of this work and the previous work is shown more obvious if the phase noise variance exceeds 0.02.

Analysis and minimization of phase noise of the digital hybrid PLL frequency synthesizer

This paper analyzes the phase noise of the digital hybrid phase locked loop (DH-PLL) frequency synthesizer that can give high speed frequency synthesis. Because noise generated by the D/A converter is added to the output phase noise, total phase noise is increased unlike in the conventional PLL. So, the aim of this paper is to minimize output phase noise for pure signal synthesis. Mathematical models of three noise sources such as input reference noise, D/A converter noise due to the quantization error and VCO noise are derived and analysed to get the minimum phase noise. Phase noise analysis of the DH-PLL is studied by the closed loop bandwidth and frequency synthesis division ratio (N). From the analysis and simulation results, we can deduce that the DH-PLL system has optimum performance for phase noise and switching speed. Schematic simulation results by the practical devices are verified to be consistent with the analysis results.