Clipping Noise Research Articles

Effect of number of sub-carriers, cyclic prefix and analogue to digital converter parameters on coherent optical orthogonal frequency division multiplexing modem's transmission performance

The number of sub-carriers, cyclic prefix (CP), and the characteristics of the analogue to digital converter (ADC), are significant parameters influencing the transmission capacity and distance of an optical orthogonal frequency division multiplexing (OOFDM) modem. Numerical simulations of coherent OOFDM modems are undertaken to investigate the effect of the number of sub-carriers, the CP length and ADC associated parameters such as the sampling speed, clipping ratio and quantisation bit on the system performance over single mode fibre (SMF) links for data rates up to 80 Gb/s. The use of a large number of sub-carriers is more effective in combating fibre dispersion when compared with employing a long CP. Moreover in the presence of fibre non-linearities identifying the optimum number of sub-carriers is crucial factor for the modem performance. For various signal data rates up to 40 Gb/s, a set of data rate and transmission distance-dependent optimum ADC parameters are identified. These parameters give rise to negligible clipping and quantisation noise, moreover ADC sampling speed can increase dispersion tolerance while transmitting over SMF links.

Single-symbol ML decoding for orthogonal and quasi-orthogonal STBC in clipped MIMO-OFDM systems using a clipping noise model with Gaussian approximation

An efficient way to reduce the peak-to-average power ratio (PAPR) in OFDM systems is clipping. After the clipping in an MIMO-OFDM system, the additive noise may not be white. In this paper, we develop fast (single-symbol) ML decoding algorithms for orthogonal space-time block codes (OSTBC) and (linearly transformed) quasi orthogonal space- time block codes (QOSTBC) in clipped MIMO-OFDM systems by using a clipping noise model with Gaussian approximation. By using the statistics of the clipping distortions, our newly developed fast ML decoding algorithms improve the performance for clipped MIMO-OFDM systems with OSTBC and QOSTBC while the decoding complexities are not increased. Simulation results are presented to illustrate the improvement.

Analysis of Clipping Noise and Tone-Reservation Algorithms for Peak Reduction in OFDM Systems

Orthogonal frequency division multiplexing (OFDM) suffers from a high peak-to-average power ratio (PAR). Tone reservation is a popular PAR reduction technique that uses a set of reserved tones to design a peak-canceling signal. In a previous paper by Krongold and Jones, an active-set approach was developed to efficiently compute the peak-canceling signal. In this paper, we consider the use of clipping noise, which is generated when the OFDM signal is clipped at a predefined threshold, to design the peak-canceling signal. To this end, the clipping noise is analyzed as a series of parabolic pulses under tone-reservation constraints. The single-pulse case and the multiple-pulse case are treated. The analysis explains peak regrowth and the constancy of the clipping noise power spectrum over the whole OFDM band. Moreover, the clipping noise at the end of several clipping and filtering iterations is shown to be approximately proportional to that generated in the first iteration. The constant of proportionality is estimated via the level-crossing theory for high clipping thresholds. Using this analysis, a constant-scaling algorithm and an adaptive-scaling algorithm are proposed for tone reservation. These algorithms scale the filtered first-iteration clipping noise to compensate for peaks that are above the threshold. The simulation results show that the proposed algorithms achieve a larger peak reduction and lower complexity than the active-set algorithm.

Proposal of an A/D Converter Clipping Noise Suppression Technique for High-Sensitivity Carrier-Sensing of Cognitive Radio Transceiver

A technique for suppressing the clipping noise of an analogue-to-digital converter (ADC) is proposed to realize a cognitive radio transceiver that offers high sensitivity carrier-sensing. When a large bandwidth cognitive radio transceiver performs carrier-sensing, it must receive a radio wave that includes many primary user transmissions. The radio wave may have high peak-to-average power ratio (PAPR) and clipping noise may be generated. Clipping noise becomes an obstacle to the achievement of high-sensitivity carrier-sensing. In the proposed technique, the original values of the samples clipped by an ADC are estimated by interpolation. Polynomial spline interpolation to the clipped signal is performed in the first step, and then SINC function interpolation is applied to the spline interpolated signal. The performance was evaluated using the signals with various PAPR. It has been found that suppression performance has a dependency on the number of samples clipped at once rather than on PAPR. Although there is an upper limit for the number of samples clipped at once that can be compensated with high accuracy, about 20dB suppression of clipping noise was achieved with the medium degree of clipping.

Amplitude Clipping and Iterative Reconstruction of STBC/SFBC-OFDM Signals

As with orthogonal frequency-division multiplexing (OFDM), one main disadvantage of multi-input multi-output (MIMO) OFDM is the prohibitively large peak-to-average power ratio (PAPR) of transmitted signals on different antennas, which can be reduced by a deliberate amplitude clipping. However, clipping causes distortion that degrades the system performance. In this letter, we propose clipping noise mitigation techniques for space-time and space-frequency block coded (STBC/SFBC)-OFDMs. A new SFBC transmitter for clipped OFDM is proposed, and the optimum equalizer is derived for reconstruction of the clipped signals. Simulation results show that the proposed receivers effectively recover contaminated STBC/SFBC-OFDM signals with a moderate computational complexity.

Turbo Decision Aided Receivers for Clipping Noise Mitigation in Coded OFDM

Orthogonal frequency division multiplexing (OFDM) is the modulation technique used in most of the high-rate communication standards. However, OFDM signals exhibit high peak average to power ratio (PAPR) that makes them particularly sensitive to nonlinear distortions caused by high-power amplifiers. Hence, the amplifier needs to operate at large output backoff, thereby decreasing the average efficiency of the transmitter. One way to reduce PAPR consists in clipping the amplitude of the OFDM signal introducing an additional noise that degrades the overall system performance. In that case, the receiver needs to set up an algorithm that compensates this clipping noise. In this paper, we propose three new iterative receivers with growing complexity and performance that operate at severe clipping: the first and simplest receiver uses a Viterbi algorithm as channel decoder whereas the other two implement a soft-input soft-output (SISO) decoder. Each soft receiver is analyzed through EXIT charts for different mappings. Finally, the performances of the receivers are simulated on both short time-varying channel and AWGN channel.

Mitigating the Clipping Noise by Using the Oversampling Scheme in OFDM Systems

Mitigating the Clipping Noise by Using the Oversampling Scheme in OFDM Systems

Clipping noise mitigation for channel estimation in OFDM systems

This letter provides a new technique to mitigate the clipping noise on pilot symbols in clipped and pilot-aided orthogonal frequency division multiplexing (OFDM) systems. The basic principle is to directly filter the clipping noise on the positions of pilots in the frequency domain before the insertion of pilots. Simulation results show that the new technique can effectively improve channel estimation and system performance

Reducing the Clipping Noise in OFDM Systems by Using Oversampling Scheme

In an Orthogonal Frequency Division Multiplexing (OFDM) systems, the Peak to Average power Ratio (PAR) is high. The clipping signal scheme is a useful and simple method to reduce the PAR. However, it introduces additional noise that degrades the systems performance. We propose an oversampling scheme to deal with the received signal in order to reduce the clipping noise by using finite impulse response (FIR) filter. Coefficients of the filter are obtained by correlation function of the received signal and the oversampling information at receiver. The performance of the proposed technique is evaluated for frequency selective channel. Results show that the proposed scheme can mitigate the clipping noise significantly for OFDM systems and in order to maintain the system's capacity, the clipping ratio should be larger than 2.5.

A simplified clipping and filtering technique for PAR reduction in OFDM systems

The existing iterative clipping and filtering techniques require several iterations to mitigate the peak regrowth. In this letter, we analyze the conventional clipping and filtering using a parabolic approximation of the clipping pulse. We show that the clipping noise obtained after several clipping and filtering iterations is approximately proportional to that generated in the first iteration. Therefore, we scale the clipping noise generated in the first iteration to get a new clipping and filtering technique that, with three fast Fourier transform/inverse fast Fourier transform (FFT/IFFT) operations, obtains the same PAR reduction as that of the existing iterative techniques with 2K+1 FFT/IFFT operations, where K represents the number of iterations.

BER analysis of clipping process in the forward link of the OFDM-FDMA communication system

Clipping is a simple and convenient PAPR (peak-to-average power ratio) reduction method to prevent the high-speed OFDM (orthogonal frequency division multiplexing) communication system from nonlinear distortion. However, clipping method inherently produces clip noise so that BER performance can be degraded. Clipping noise can be divided into in-band and out-of-band clipping noise. In this paper, we study the clipping effect on the forward link of the OFDM-FDMA (frequency division multiple access) communication system. BER (bit error rate) performance of OFDM-FDMA system using clipping method is theoretically analyzed. We also investigate the BER performance of the clipped OFDM-FDMA signal including the HPA and quantify the appropriate IBO (input back-off) in HPA. From the simulation results, it can be found that the BER performance of the OFDM-FDMA system is degraded by the clipping process. When clipping ratio is about 2, BER of the clipped OFDM-FDMA with only in-band noise and in-band plus out-of-band noise closely approaches to the BER of OFDM-FDMA without clipping. The simulation results also show that when the number of subcarriers for each user in OFDM-FDMA system is 16, with clip ratio 2 and IBO of 3 dB, the BER curve with in-band plus out-of-band noise is very similar to that of the linear amplifier. So, we can choose the appropriate IBO value and CR (clip ratio) to get the desired BER performance.

Clipping noise mitigation for OFDM by decision-aided reconstruction

Clipping is often used to reduce the large peak-to-mean envelope power ratio of orthogonal frequency-division multiplexing (OFDM) signals. However, it introduces additional noise that degrades the system performance. A technique called decision-aided reconstruction (DAR) is proposed for mitigating the clipping noise. The performance of the proposed technique is evaluated for additive white Gaussian noise and static intersymbol interference channels. The effect of using a realistic channel estimate is also examined. Results show that the DAR technique can mitigate the clipping noise significantly for OFDM systems that have large block sizes.

Asymptotic clipping noise distribution and its impact on M-ary QAM transmission over optical fiber

Performance analysis for a hybrid subcarrier-multiplexed (SCM) AM/QAM transmission over an optical fiber is presented. It is shown that the bit-error-rate (BER) of M-ary QAM in such hybrid systems can be significantly affected due to occasional laser of the SCM signal. Here we analytically determine the asymptotic distribution of the clipping noise by modeling it as a Poisson impulse train. The BER of M-ary QAM is then evaluated in terms of optical modulation indices of M-ary QAM and AM signals which in turn specify signal-to-noise ratio (SNR), impulsive index (clipping index) of the clipping noise, and power ratio of the Gaussian noise to the clipping noise. Numerical examples are given and compared with experimental data with reasonably good agreement for small SNR's. The results have application for estimating the BER's of digital signals for SCM analog/digital transmission over an optical fiber and for employing appropriate error correction codes and/or optimum receiver design for such environments. >

Hybrid transmission of analogue PAL and digital 64-QAM CATV channels

A system experiment for fibre optic hybrid transmission of analogue CATV BK 300 and 1.8 Gbit/s with 64-QAM channels has been realised. It has been shown that additional transmission of 1.8 Gbit/s of compressed digital video (MPEG) using 64-QAM is possible without a significant degradation of the laser clipping rate. Measured bit error rates are compared with simulation results based on impulsive clipping noise.

AM-SCM-PCM lightwave CATV transmission systems

A new approach to a lightwave multichannel CATV transmission system using PCM is proposed and its advantages over conventional schemes are outlined. The principal system noise, i.e. clipping and quantisation noise, both produced by A/D convertors, has been analysed. For a 40 channel 10 bit system and 8 bit system, the received CNRs are 54 and 43 dB, respectively.

Subjective Evaluation of PCM Coded Speech

Subjective quality ratings of pcm coded speech were obtained with the aims of (i) determining the effects of certain coder parameters and their interactions on speech quality, (ii) finding objective measures for predicting perceived distortions, and (iii) providing guidelines for optimizing coder design. Coders with various combinations of four clipping levels, seven step sizes, four bandwidths, and three logarithmic companding laws were simulated. The coders were rated for quality on a 10-point scale by 48 listeners who heard male and female speech processed by the coders. The ratings depended strongly on clipping level and step size, but only weakly on bandwidth. None of the coder parameters interacted strongly with another. Clipping noise power grossly overestimated the extent of perceived overload distortion; instead, clipping percentage is proposed as a much more realistic predictor. Signal-to-granular-noise ratio was a good predictor of perceived granular noise. For a given bit rate, the coder with the highest quality rating was not the coder with minimum total clipping and granular noise power, contrary to traditional wisdom.