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Abstract
Signal clipping is a well-established method employed in orthogonal frequency division multiplexing (OFDM) systems to mitigate peak-to-average power ratio. The utilization of this technique is widespread in electronic devices with limited power or resource capabilities due to its high efficiency and low complexity. While clipping effectively diminishes nonlinear distortion stemming from power amplifiers (PAs), it introduces additional distortion known as clipping distortion. The optimization of system performance, considering both clipping distortions and the nonlinearity of PAs, remains an unresolved challenge due to the intricate modeling of PAs. In this article, we undertake an analysis of PA nonlinearity utilizing the Bessel–Fourier PA model and simplify its power expression through intermodulation product analysis. We mathematically derive expressions for the receiver signal-to-noise ratio and system symbol error rate (SER) for nonlinear clipped OFDM systems. Using these derivations, we explore the optimal system configuration required to achieve the lower bound of SER in practical OFDM systems, taking into account both PA nonlinearity and clipping distortion. The results and methodologies presented in this article contribute to an improved comprehension of system-level optimization in nonlinear OFDM systems employing clipping technology.
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Power amplifiers (PAs) lead to nonlinear distortion in index modulation assisted OFDM (OFDM-IM) systems when the peak-to-average power ratio (PAPR) is high. But PAPR reduction in OFDM-IM is more complicated than that in traditional OFDM owing to the frequent switch of subcarrier states between active and idle. When implementing OFDM-IM on resource-limited platforms like battery-powered Internet of Things (IoT) devices, PAPR cannot always be ideally eliminated due to the complexity of the reduction algorithm and the limitation of computation resources. Hence, performance optimization under non-negligible PAPR becomes an important issue. This paper performs symbol error rate (SER) optimization in OFDM-IM systems with high PAPR. First, we derive a low complexity expression of receiver signal-to-noise ratio (SNR) in nonlinear OFDM-IM systems. Second, we find that SER in OFDM-IM is affected by the nonlinear distortion in both constellation mapping and index estimation. With this observation, we derive a closed-form expression of SER and prove that there is a lower bound. Third, we do SER optimization and identify the optimal system operating point with respect to the minimum SER. Finally, we validate our analysis through simulations.
Interleaved Frequency Division Multiple Access (IFDMA) has the salient advantage of lower Peak-to-Average Power Ratio (PAPR) than its competitors like Orthogonal FDMA (OFDMA). A recent research effort of ours put forth a new IFDMA transceiver design significantly less complex than conventional IFDMA transceivers. The new IFDMA transceiver design reduces the complexity by exploiting a certain correspondence between the IFDMA signal processing and the Cooley-Tukey IFFT/FFT algorithmic structure so that IFDMA streams can be inserted/extracted at different stages of an IFFT/FFT module according to the sizes of the streams. Although our prior work has laid down the theoretical foundation for the new IFDMA transceiver’s structure, the practical realization of the transceiver on specific hardware with resource constraints has not been carefully investigated. This paper is an attempt to fill the gap. Specifically, this paper puts forth a heuristic algorithm called multi-priority scheduling (MPS) to schedule the execution of the butterfly computations in the IFDMA transceiver with the constraint of a limited number of hardware processors. The resulting FFT computation, referred to as MPS-FFT, has a much lower computation time than conventional FFT computation when applied to the IFDMA signal processing. Importantly, we derive a lower bound for the optimal IFDMA FFT computation time to benchmark MPS-FFT. Our experimental results indicate that when the number of hardware processors is a power of two: 1) MPS-FFT has near-optimal computation time; 2) MPS-FFT incurs less than 44.13% of the computation time of the conventional pipelined FFT.
Transparent relay is a frequently used solution to enhance signal coverage with low implementation cost. However, the nonlinear distortion caused by cascaded amplification in the relay systems degenerates the system performance, increases the analysis complexity, and brings challenges to the system-level optimization. This paper analytically studies the system-level optimization of symbol error rate (SER) performance in nonlinear OFDM relay systems, where both power amplifiers (PAs) in transmitter and transponder show clear nonlinearity. Firstly, the system SER is expressed as a function of receiver signal-to-noise ratio (SNR), and the latter is further simplified to a polynomial-based function of PA parameters and system operating points. Secondly, the asymptotic behavior and Hessian Matrix (HM) of the receiver SNR are analyzed, and it is found that the SNR has an upper bound when dual nonlinearity of the transmitter PA and transponder PA are considered. Thirdly, this paper derives the analytical expressions of the optimal PA operating points corresponding to the optimal SER performance for the first time. Finally, the expression of the SNR upper bound is presented, and the derived functions are found to have the merit of low complexity, which suits the application in transparent relay systems with limited computational resources.
Despite the advantage of power efficiency and spectral efficiency, spatial modulation (SM) technology makes communication systems sensitive to the transmission impairment. This paper analytically investigates the performance of SM multiple-input multiple-output (MIMO) systems where power amplifier (PA) shows clear nonlinearity. Firstly, the system model is presented with nonlinear distortion included. Secondly, the system symbol error rate (SER) is newly derived to a polynomial based function of system parameters. Thirdly, the asymptotical behaviour of SER is analysed, and it is found that SER in nonlinear SM-MIMO systems has a lower limit. Finally, the analytical expressions of SER lower bond are presented. The analysis methodology and derived findings can be a useful reference for the designers of SM-MIMO systems.
In this article, we analytically investigate the performance of a downlink multiuser (MU) multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) system with hardware impairments. Specifically, we focus on the joint impact of nonideal oscillators and nonlinear high-power amplifiers, which can be quantified in terms of multiplicative and additive nonlinear distortions as well as phase noise, which causes inter-channel interference (ICI). We first derive the signal-to-distortion-plus-interference-plus-noise ratio (SDINR) and then utilize the SDINR to derive the closed-form expressions for symbol-error-rate and the average capacity of the aforementioned system. Our analysis shows that the statistics of an additive nonlinear distortion and additive white Gaussian noise are not affected by phase noise. However, the phase noise itself significantly deteriorates the performance of the MU-MIMO-OFDM system even in the absence of the nonlinear distortions. We further show that nonlinear distortion and ICI cause an irreducible error floor which considerably degrades the system error performance and also adversely affects the total capacity of the MU-MIMO-OFDM system. Comparisons are drawn between the performance of an ideal MU-MIMO-OFDM system and a MU-MIMO-OFDM system in presence of above impairments. In the end, we present the simulation results to validate the efficacy of our theoretical analysis.
While achieving reduced/good peak-to-average power (PAPR) in orthogonal frequency division multiplexing (OFDM) systems is attractive, this must not be performed at the expense of the transmitted signal with over-reduced signal power as it leads to degraded bit error ratio (BER). We introduce a uniform distribution approach to solving the PAPR reduction problem of OFDM signals and then use Lagrange multiplier (LM) optimization to minimize the number of iterations involved in an adaptive fashion. Due to the nonlinear attenuation of the PAPR reduction scheme, we compensate the output signal using a correlation factor that minimizes the error floor in the in-band distortion of the clipped signal using minimum mean square error (MMSE) method so as to improve the BER performance. Three different methods are introduced each enabling PAPR reduction by clipping followed by filtering with no direct dependency on a clipping ratio parameter. We find that our approach significantly reduces the PAPR of the OFDM signals (especially with LM optimization) better than the conventional adaptive iterative clipping and filtering operating without LM optimization. Based on our proposed methods, we additionally outline two simple steps for achieving perfect PAPR reduction (i.e. 0dB). We also evaluate the performance of the three new models over high power amplifier (HPA) for completeness; the HPA is found to induce negligible BER degradation effects on the processed signal compared to the unprocessed signal.
Orthogonal frequency division multiplexing (OFDM) offers spectral efficiency advantage, however, it is limited by peak-to-average power (PAPR) problem. The PAPR can be reduced using iterative clipping and filtering (ICF) scheme but requires that the same signals are iteratively clipped with a fixed clipping threshold at different clipping iterations. This method warrants that fast-Fourier transform (FFT)/inverse IFFT (IFFT) blocks must be driven in the order of iterations many times to attain a desired PAPR threshold which expends the system power and expands processing time. Using a second-order cone program, the number of iterations required to attain the desired PAPR threshold was reduced. This optimized ICF (OICF) was later simplified using Lagrange Multiplier Optimization (LMO). In this study, we apply an adaptive clipping threshold to the LMO scheme to improve the performance of the simplified OICF (SOICF). Our results show significant reduction of the PAPR problem compared to the earlier SOICF schenme albeit with some degradation in the BER performance that can be under 1.0 dB depending on the chosen clipping threshold. In addition, we also illustrated the results of the theoretical relationships and performances between the error vector magnitude (EVM) and PAPR, between clipping ratio (CR) and EVM, and lastly the inter-dependencies of EVM, PAPR, the number of OFDM subcarriers and the CR.
In Orthogonal Frequency Division Multiplexing (OFDM) transmitter systems, nonlinear distortion is a major impairment caused by power amplifier (PA). The distortion is constituted by numerous noise-like inter-modulation products (IMPs). Although the overall behavior of nonlinear IMP distortion has been well studied, in large multi-carrier contexts, such as in OFDM, the knowledge of individual IMPs and their quantitative interrelationships, together with the derivation of behavioural knowledge and the predictability of consequences arising from such interrelationships, is a fairly unexplored topic. This is the subject of this paper. The powers of the very numerous individual IMPs generated are analytically derived based on a Bessel-Fourier PA behavioural model, and it is proven that these powers have only a small number of distinct powers. Proven also is that the ratios of these power values are constants, and that these constants are invariant with the particular PA's nonlinearity characteristics or operating point. Further, it is proven that specific types of IMPs dominate nonlinear distortion effects. Finally, it is shown that IMP power spectra can be derived from a simple product of two polynomials, which are quadrature functions of frequency and PA operating point, respectively. Application possibilities for findings presented in this paper, such as their potential to contribute efficiency improvements in OFDM system modulation fidelity calculations (over 1100 times faster than conventional methods), are also outlined in the conclusions.
Clipping is one of the simplest peak-to-average power ratio (PAPR) reduction
schemes for orthogonal frequency division multiplexing (OFDM). Deliberately
clipping the transmission signal degrades system performance, and clipping
mitigation is required at the receiver for information restoration. In this
work, we acknowledge the sparse nature of the clipping signal and propose a
low-complexity Bayesian clipping estimation scheme. The proposed scheme
utilizes a priori information about the sparsity rate and noise variance for
enhanced recovery. At the same time, the proposed scheme is robust against
inaccurate estimates of the clipping signal statistics. The undistorted phase
property of the clipped signal, as well as the clipping likelihood, is utilized
for enhanced reconstruction. Further, motivated by the nature of modern
OFDM-based communication systems, we extend our clipping reconstruction
approach to multiple antenna receivers, and multi-user OFDM. We also address
the problem of channel estimation from pilots contaminated by the clipping
distortion. Numerical findings are presented, that depict favourable results
for the proposed scheme compared to the established sparse reconstruction
schemes.
This paper presents a practical performance analysis of an orthogonal space-time block code (OSTBC) based multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) system in the presence of nonlinear power amplifiers (PAs). The equivalent single-input single-output (SISO) scalar model is introduced and this then is used to construct a model for the nonlinear MIMO-OFDM system. Based on an unique inter-modulation product (IMP) analysis, it is shown how the receiver signal-to-noise ratio (SNR) may be derived to a polynomial based function of the system operating point. It is also shown that expressions for the asymptotical behavior of this SNR, when the linear and highly nonlinear PA operations are applied, may also be derived. Based on these derivations, it is further shown that the optimal system operating point, which corresponds to the maximum receiver SNR and SEP lower bound, can be identified and be derived to a polynomial based expression. Additionally, a general expression for the different integral expressions of SEP for the phase shift keying (PSK) and quadrature amplitude modulation (QAM) signals is found. By means of these analytical results, it was found possible to advance analytical explanations of observed behaviour in simulations and also to provide a practical analysis of performance optimization of nonlinear MIMO-OFDM systems.
The high Peak to Average power Ration (PAR) levels of Orthogonal Frequency Domain Multiplexing (OFDM) signals forces to the utilization of linear power amplifiers. However, linear amplifiers have low power efficiency which is problematic considering that battery life is a critical resource in mobile systems; also linear amplifiers have a clipping effect when considered as a limiter. Amplifier nonlinearities affect drastically the performance of OFDM systems. In this paper, the effects of Clipping and amplifier nonlinearities are modeled in an OFDM system. We showed that the distortion due to these effects is highly related to the dynamic range it self rather than the clipping level or the saturation level of the nonlinear amplifier. Computer simulations of the OFDM system using Matlab are completely matched with the deduced model in terms of OFDM signal quality metrics such as BER, ACPR and EVM.
A new modified Bessel-Fourier (MBF) nonlinear RF power amplifier (PA) memoryless behavioural model is proposed. The accuracy and behavioural prediction performance
especially of its low third order model is better than the best published low order models in use today. Besides inheriting all the key properties of the BF model, surpassing its low order and matching its high-order accuracy and performance, the new model comes with two additional attributes of ‘margin of reliability’ beyond the dynamic range set by the extraction measurements and model order ‘monotonic extensibility.
The IMP distortion occurring with the amplification of OFDM signals through a nonlinear RF power amplifier (PA) is analysed with a view to establishing internal relations and impairment significance of different types and combinations - composition categories- of IMPS. Through this, a novel estimation method was deduced to calculate power spectral densities (PSD) of nonlinear distortion. This method also helps expose some PA independent and operating point (OP) independent characteristics of PSD of nonlinear distortion. The simulation and analysis is based on the use of an IEEE 802.11a OFDM signal and a nonlinear PA model is derived from envelope characteristic measurements of an X-band and S-band GaN power amplifier.
Memory effects in power amplifier nonlinearity are frequently an impediment to amplifier linearization. In this paper, memory effects in a cellular phone power amplifier are characterized via time domain measurements, with waveforms including steps, triangular waveforms and CDMA signals. Memory effects found with different waveforms are shown to be consistent, and in agreement with measurements made with sinusoids in two-tone tests. The results are analyzed by considering gain modulation by a parameter varying at the baseband frequency in response to the signal envelope. For pseudorandom input signals, such as for CDMA, cross-correlation of gain residues with the signal envelope yields an impulse response associated with the memory effects.
Multipair (MP) massive multiple-input–multiple-output (MIMO) two-way relaying system is a promising solution that is able to provide excellent spectral efficiency performance. It consists in multiple pairs (
$2K$
) of single-antenna user terminals that exchange information through of a relay using a large number of antennas
$M_{t}\gg 2K$
. The relay is equipped with a large number of energy-efficient power amplifiers (PAs) and ensure the multipair two-way forwarding. In this article, we first present an analytical study focused on the effects of PAs on the considered system over block fading Rayleigh channel. We derive closed-form expressions for the symbol error rate. Then, we develop a new efficient distributed learning approach for MP two-way massive MIMO. Specifically, we design the MP two-way massive MIMO system by leveraging the concept of the autoencoder, whereas the end-to-end communication system is designed using one deep neural network (NN). Interestingly, the proposed scheme, referred to as AE-MP-mMIMO, is suited for varying block fading Rayleigh channel scenarios. Indeed, we propose to adopt one stage precoder/decoder for each UE and two-stage precoding scheme for the relay: 1) an NN-Tx is used to address the PA nonlinearity and 2) a linear zero-forcing precoder is adopted to remove the multiuser interference. The NN-Tx and the UE's precoder/decoder are trained off-line and when the channel varies, only the zero-forcing precoder changes on-line. Numerical simulations show the capability of the proposed approach to achieve competitive performance with a significantly lower complexity compared to existing literature.
The nonlinear distortions attributed by radio frequency power amplifiers (PAs) are inevitable in millimeter-wave (mmWave) systems due to the high frequency and large bandwidth. This nonlinearity induces multiplicative distortion and intercarrier interference in mmWave multiple-input-multiple-output (MIMO) orthogonal frequency-division multiplexing systems, which in collusion with a frequency-selective channel and a carrier frequency offset (CFO) brings great challenges to signal detection. Furthermore, the nonlinearity destroys orthogonality of the training pilots and degrades the estimation accuracy of the channel gains and the CFO. Also, the target posterior distribution for data detection becomes analytically intractable due to the nonlinearity. In this article, we present an importance-sampling-based framework for estimating the CFO in the presence of PA impairment, which utilizes few pilots and initializes the channel estimator. We propose a particle-filter-based iterative algorithm for data detection, where the intractable posterior distribution is approximated by weighted random measures. Furthermore, a sequential-maximum-likelihood-based semiblind channel estimator in the presence of nonlinearity is proposed. Performances of the estimator and the detector are evaluated by means of normalized mean square error, mean square error, and bit error rate. To validate the efficacy of the channel and the CFO estimator, the Cramer–Rao bound (CRB) and the weighted Bayesian CRB are derived, respectively.
In the present article, we propose a novel selected mapping (SLM) scheme based on a Polar coding technique for peak to average power ratio (PAPR) reduction in orthogonal frequency division multiplexing with index modulation (OFDM-IM) systems. The proposed scheme by utilizing random frozen bits in polar codes generates different number of candidate sequences. Then the candidate sequence with the smallest PAPR is selected for transmission. The proposed scheme can be considered as one block in an OFDM-IM system that performs both PAPR reduction and error correction. For a fair comparison, the performance of an OFDM-IM system based on the proposed scheme is compared with a system that carries out the same operations with two separate blocks where a Polar encoder is cascaded with the one for PAPR reduction (such as conventional SLM or PTS), and a Polar coded OFDM-IM system without any PAPR reduction scheme. Moreover, based on our proposed SLM scheme, a novel receiver without using side information (SI) is proposed. This SI free receiver is based on the successive cancellation list (SCL) polar decoding scheme. Simulation results show that as the Polar code can bring both error correction and PAPR reduction capabilities by using our proposed scheme. Also, the proposed SI free receiver can achieve similar bit error rate (BER) performance as that of the ideal case in both additive Gaussian white noise (AWGN) and frequency selective channels.
In this correspondence paper, we study the peak-to-average power ratio (PAPR) problem in orthogonal frequency-division multiplexing systems. In conventional clipping and filtering based PAPR reduction techniques, clipping noise is allowed to spread over the whole active passband, thus degrading the transmit signal quality similarly at all active subcarriers. However, since modern radio networks support frequency multiplexing of users and services with highly different quality-of-service expectations, clipping noise from PAPR reduction should be distributed unequally over the corresponding physical resource blocks (PRBs). To facilitate this, we present an efficient PAPR reduction technique, where clipping noise can be flexibly controlled and filtered inside the transmitter passband, allowing to control the transmitted signal quality per PRB. Numerical results are provided in 5G new radio mobile network context, demonstrating the flexibility and efficiency of the proposed method.
The simple method of clipping is widely used to reduce the peak-to-average power ratio of orthogonal frequency division multiplexing (OFDM) signal prior to passing it through the high power amplifier (HPA). However, clipping is a nonlinear process that causes both in-band and out-of-band distortions. Filtering the clipped OFDM signal can eliminate the out-of-band distortion but it can lead to peak regrowth. Therefore, iterative clipping and filtering have been used to both remove the out-of-band interference and suppress the regrowth of the peak power. However, the iterative process will degrade the bit error rate performance further. In this paper, we proposed a semi-analytic scheme based on compressed sensing to reconstruct the iterative clipping noise. Moreover, the proposed scheme is also applicable for reconstructing the total nonlinear distortion of a system so that the OFDM signal passes both the clipper and HPA. Simulation results show that the proposed scheme performs well and the nonlinear distortion can be recovered effectively.
Orthogonal frequency division multiplexing (OFDM) is one of the widely used modulation techniques for nonlinear communication systems. It offers high data rate, spectral efficiency, robustness to multi path fading, simple receiver design, etc. But, it suffers from high peak-to-average power ratio (PAPR) which causes inband signal distortion and out of band spectral regrowth and results in significant reduction in power efficiency when passed through a nonlinear power amplifier. In this paper, a simple method of PAPR reduction, clipping and filtering is investigated. Effects of clipping and filtering on the performance of OFDM signal such as power spectral density, PAPR, and bit-error rate has been investigated through Matlab simulations. The results obtained indicate that at clipping ratio, CR = 1.4 and 1% of CCDF the reduction in PAPR values are 8.8 dB and 5.4 dB for clipped only and both clipped & filtered signals, respectively. The clipping operation also causes degradation in the BER performance. At 10 dB of signal power and at CR = 1.4, BER values are 1.0×10-2 for clipped signal, 5.1×10-3 for both clipped & filtered signals and 7.8×10-4 for unclipped signal. This increase in BER value is due to distortions caused during the process of clipping and filtering.
Many iterative clipping and filtering (ICF) based techniques have been proposed that achieve similar peak-toaverage power ratio (PAPR) reduction of orthogonal frequency division multiplexing (OFDM) signals as the original ICF, but with lower complexity, such as the simplified clipping and filtering (SCF) technique. However, these low complexity methods require numerous complex fast Fourier transform (FFT) operations and parameter calculations. In this letter, we introduce a novel ICF method that uses an optimized mapper based on artificial neural network and SCF techniques. Compared to the conventional ICF based methods, the proposed scheme offers desirable cubic metric (CM) and bit error rate (BER) simulation results with significantly reduced computational complexity.
In this paper, the effects of nonlinear High Power Amplifier (HPA) with memory on Multi-Input Multi-Output (MIMO) Orthogonal Frequency Division Multiplexing (OFDM) system are analyzed and a theoretical framework is proposed. The memory effects are represented by Generalized Memory Polynomial (GMP) model. Moreover, we derive an analytical formulation with infinite nonlinearity order and infinite memory length. The channel is assumed MIMO frequency selective and it is modeled as a linear dispersive channel with uniform power profile. We show that the decision variables of the nonlinearity distortion can be modeled by the complex attenuation and zero mean additive noise model. The analytical expressions for complex attenuation, mean and variance of nonlinear noise are derived and SER of MIMO-OFDM system using HPA with memory is presented. Excellent agreement between the analytical results and numerical simulations is observed that verifies the accuracy and efficiency of the proposed method.
Simple two-parameter formulas are presented for the functions involved in the amplitude-phase and the quadrature nonlinear models of a TWT amplifier, and are shown to fit measured data very well. Also, a closed-form expression is derived for the output signal of a TWT amplifier excited by two phase-modulated carriers, and an expression containing a single integral is given when more than two such earriers are involved. Finally, a frequencydependent quadrature model is proposed whose parameters are obtainable from single-tone measurements.
This letter considers the use of the tone injection (TI) scheme to reduce the peak-to-average power ratio (PAPR) of an orthogonal frequency division multiplexing (OFDM) signal. TI is a distortionless technique that can reduce PAPR significantly without data rate loss and does not require the extra side information. However, the optimal TI scheme requires an exhaustive search over all combinations of possible permutations of the expanded constellation, which is a potential problem for practical applications. To reduce the computational complexity while still improving PAPR statistics, this letter first formulates the PAPR reduction with TI scheme as a particular combinatorial optimization problem. Next, it proposes the application of the cross-entropy (CE) method to solve the problem. Computer simulation results show that the proposed CE method obtains the desired PAPR reduction with low computational complexity.
In certain multiple-input multiple-output (MIMO) fading environments, the offered channel capacity can be very low even when the MIMO channels are uncorrelated; this effect has been termed as keyhole or pinhole effect. In this paper, we present rigorous error rate analysis of orthogonal space time block codes in Nakagami-m keyhole channels with arbitrary fading parameters. Using a moment-generation-function based approach, we provide the exact expressions of symbol error rate. Furthermore, the closed-form asymptotic expressions are derived, based on which one can easily quantify the diversity gain of the considered system with arbitrary m-fading parameters while existing methods in literature fail to do so. Simulation results are also provided to verify our analytical results.
For PAR reduction in OFDM systems, the clipping-based Active Constellation Extension (ACE) technique is simple and attractive for practical implementation. However, we observe it cannot achieve the minimum PAR when the target clipping level is set below an initially unknown optimum value. To overcome this low clipping ratio problem, we propose a novel ACE algorithm with adaptive clipping control. Simulation results demonstrate that our proposed algorithm can reach the minimum PAR for severely low clipping ratios. In addition, we present the tradeoff between PAR and the loss in E<sub>b</sub>/N<sub>o</sub> over an AWGN channel in terms of the clipping ratio.
It is shown that a simple extension of the conventional behavioral
characterization of amplifier nonlinearity can be used to quantify power
amplifier performance including many memory effects. External variables
that influence the amplifier behavior (such as power supply voltage,
input bias or temperature) are identified. Measurements of gain and
phase (AM-AM and AM-PM conversion) are subsequently made over a range of
these external variables. The variation of the external variables is
explicitly taken into account with linear equivalent circuits at
baseband. The method is shown to be useful for the estimation of bias
circuit effects and self-heating effects
Clipping the OFDM signals in the digital part of the transmitter is one of the simplest methods to reduce the peak factor. However, it suffers from additional clipping distortion, peak regrowth after digital to analog conversion, and out-of-band radiation in the case of oversampled sequence clipping. We use oversampled sequence clipping to combat the effect of peak regrowth and propose a method to reconstruct the clipped samples and mitigate the clipping distortion in the presence of channel noise at the expense of bandwidth expansion. We show through extensive simulations that by slightly increasing the bandwidth of the system, we can significantly improve the performance while limiting the maximum amplitude of the analog signal.
The repeated clipping and frequency domain filtering of an orthogonal frequency division multiplexing (OFDM) signal was studied to reduce peak-to-average power ratio (PAPR) of the transmitted signal. An oversize inverse fast Fourier transform (IFFT) was used to transform the input vector. The clipping was followed by filtering to reduce the out-of-band power. Analysis suggested that the clip-and-filter technique did not increase the out-of-band power and limited the PAPR reduction practical systems to in-band-distortion.