In this paper, we propose a multi-user downlink system for two users based on the orthogonal time frequency space (OTFS) modulation scheme. The design leverages the generalized singular value decomposition (GSVD) of the channels between the base station and the two users, applying precoding and detection matrices based on the right and left singular vectors, respectively. We derive the analytical expressions for three scenarios and present the corresponding simulation results. These results demonstrate that, in terms of bit error rate (BER), the proposed system outperforms the conventional multi-user OTFS system in two scenarios when using minimum mean square error (MMSE) equalizers or precoder, both for perfect channel state information and for a scenario with channel estimation errors. In the third scenario, the design is equivalent to zero-forcing (ZF) precoding at the transmitter.

In the last years, it has been shown that nonlinear (NL) orthogonal frequency division multiplexing (OFDM) can outperform linear OFDM because the nonlinear distortion has useful information about the transmitted signals. However, only a maximum likelihood (ML) receiver can exploit this information. Despite its potential, the optimal ML receiver for NL OFDM is highly complex, and its performance is difficult to simulate. Moreover, although some theoretical performance bounds exist for OFDM transmissions involving many subcarriers and high signal-to-noise ratio (SNR), the behavior of NL OFDM under more practical SNR conditions remains insufficiently explored. This paper focuses on optimal detection methods for NL OFDM systems. We present a comprehensive analysis of how the distortion received on different subcarriers contributes to the signal of a specific subcarrier and derive a performance bound for the ML detection of NL OFDM that is applicable across a broad range of SNR values. Furthermore, we propose a practical iterative decision-directed receiver that achieves significant performance improvements over linear OFDM in both uncoded and coded setups.11This work supported by Fundação para a Ciência e Tecnologia and Instituto de Telecomunicações under the project UIDB/50008/2020, DOI:10.54499/UIDB/50008/2020, and Universidade Lusófona.

Nonlinearities are usually something to be avoided in orthogonal frequency division multiplexing (OFDM) transceivers. However, it has been demonstrated that nonlinear (NL) OFDM can outperform linear OFDM, since NL distortion has useful information on the transmitted signals that can be exploited by the optimum maximum likelihood receiver. The optimum ML receiver for NL OFDM is too complex and it is not easy to obtain its performance, although some theoretical performance bounds are available for scenarios with a large number of subcarriers and signal-to-noise ratio (SNR). However, the optimum performance of NL OFDM in small-to-moderate SNR values is still not well studied. In this paper, we consider the optimum detection of NL OFDM schemes, where we take advantage of the NL distortion to improve performance. We present a detailed study of the contributions to the signal associated with a given subcarrier from the distortion received at the other subcarriers. This allowed us to obtain a bound on the maximum likelihood (ML) performance of NL OFDM that is valid over a wide range of SNR values subcarriers and demonstrates that the performance gains observed in the asymptotic SNR region also take place for lower SNRs. Additionally, we propose a practical, iterative decision-directed receiver that exhibits large performance gains over linear OFDM in both uncoded and coded scenarios.

In the last years, it has been shown that nonlinear (NL) orthogonal frequency division multiplexing (OFDM) schemes can outperform linear OFDM. This can be explained by the fact that nonlinear distortion has useful information on the transmitted signals that can be employed for detection purposes. Although the performance gains of NL OFDM can be theoretically evaluated in the asymptotic signal-to-noise ratio (SNR) region and when the number of sub carriers is large, the performance in small-to-moderate SNR values is still not clear. Moreover, receivers that try to exploit those potential gains involve very high complexity. In this paper, we present a bound on the optimum performance of NL OFDM valid for a wide range of SNR values. Moreover, a novel receiver structure that approximates the optimum performance is proposed1,

Extremely large antenna arrays (ELAAs) are being envisioned for sixth-generation (6G) to increase spatial selectivity and achieve higher capacities. The use of larger arrays combined with higher carrier frequencies increases the near-field region, where the channel model must account for the curvature of the electromagnetic waves, and the plane wave regime is inaccurate. When conventional matched-filtering (MF) beamforming is de-signed for near-field, a beam-focusing effect is observed around the focal point. This can be used to mitigate the impact of a malicious user and increase the physical layer security (PLS). In this paper, we propose an optimized beamforming design for uplink near-field communications that accounts not only for the positioning of the legitimate user but also for the position of the jammer. It is shown that the jamming rejection capabilities of the proposed beamforming are superior to that of conventional MF.1

Multiple Input Multiple Output (MIMO) architectures are now widely adopted in wireless systems, providing substantial capacity benefits by harnessing spatial diversity and multiplexing. Nonetheless, the large peak-to-average power ratio (PAPR) associated with common pre-processing techniques, like singular value decomposition (SVD), increases the system's susceptibility to nonlinear distortion. Conventional receiver designs that mitigate this distortion often neglect the fact that it has useful information on the transmitted data. Maximum likelihood (ML) detection offers the capability to take advantage of the nonlinear distortion, but its inherent complexity is prohibitively high. This paper introduces a new MIMO receiver design aimed at exploiting the diversity introduced by the transmitter nonlinearities. It also provides an approximate bound on the achievable ML bit error rate (BER) performance. Our results indicate that the proposed receiver can have a performance close to the ML receiver with just a few iterations.1

In satellite communications power consumption and reliability are key concerns. The unreliable nature of the wireless channel can cause erroneous transmissions, which must be compensated using retransmissions, at the expense of more power. This issue is aggravated by the low amplification efficiency of high data rate schemes such as Orthogonal Frequency Division Multiplexing (OFDM). Ideally, retransmissions could be avoided entirely by increasing transmission power, however, this would drastically lower the overall energy efficiency of the system. Sophisticated Hybrid Automatic Repeat Request (HARQ) techniques can simultaneously reduce the need for retransmissions and increase the success likelihood of each retransmission attempt (a critical aspect for delay-constraint services over satellite links), while minimizing average consumed power. In this work, we propose a novel HARQ technique where retransmitted signals are submitted to a soft clipping operation that lowers the Peak-to-Average Power Ratio (PAPR) of the OFDM signal, thereby increasing the amplification efficiency. A Generalized Approximate Message Passing (GAMP) based receiver makes use of the additional distortion in the retransmission to increase the likelihood of a successful decoding. It is shown that this approach can outperform Chase Combining (CC) schemes even for Super Quadrature Amplitude Modulation (Super-QAM), while decreasing the power expended during retransmissions.

Large multiple antenna systems have been widely used in 5G for improved spectral efficiencies. For 6G, even bigger arrays are being proposed to obtain larger gains and/or to reduce interference levels. When the physical size of the array is large compared to the propagation distance, communication might occur in the near-field regime, which requires channel models that capture the curvature of the spherical waves. In this paper, we consider the near-field communication regime and study the jamming rejection features of an extremely large array (ELAA). Due to the focusing effect available in the near-field, it is shown that the jamming rejection features in this region are much stronger than the ones associated with far-field. Therefore, near-field beam-focusing constitutes an interesting physical layer security (PLS) technique for 6G communications.1

Non-linear distortion imposed to orthogonal frequency division multiplexing (OFDM) signals from non-linear power amplifiers (PAs), has always been a challenging problem in wireless communications. This is due to the high peak-to-average power ratio (PAPR) of OFDM signals. To overcome this issue, receiver side compensation techniques based on distortion cancellation and iterative detection have been proposed. However, these techniques require knowledge of the non-linearity (NL) parameters. Therefore, in this paper we focus on NL parameter estimation and propose a least squares (LS) based estimation technique by transmitting a training signal. We study the performance of the proposed technique for the solid state power amplifier (SSPA) model and the Bussgang model. The studies are done considering both perfect channel knowledge and imperfect channel knowledge at the receiver. The results show that the proposed technique has good accuracy and performance for distortion cancellation and iterative detection based receivers such as the Bussgang receiver. The results also indicate that the estimation technique is more sensitive to channel estimation error than the additive Gaussian noise.

Multicarrier-based frequency splitting simultaneous wireless information and power transmission (FS-SWIPT) signals are very prone to nonlinear effects due to the combination of high-power energy harvesting (EH) subcarriers with low-power data subcarriers. In this article, we study the impact of nonlinear effects in FS-SWIPT signals, as well as ways to minimize these effects on the data transmission performance. We start by analytically characterizing the impact of nonlinear devices on the signals, showing that the nonlinear effects on data subcarriers are exacerbated by a factor close to the ratio between the power spectral densities of EH and data terms. This suggests that conventional receivers, that treat nonlinear distortion as an additional noise term, can have very poor performance. Then, we present an iterative receiver that iteratively estimates and cancels nonlinear distortion, and is able to have a performance close to the linear case. Finally, we study the optimum performance of FS-SWIPT signals with strong nonlinear distortion levels, showing that, by using optimum receivers, the nonlinear distortion term can be used to improve the performance of FS-SWIPT, even outperforming the linear case.

In wireless communication systems, the achievable data rates are always dependent on the characteristics of the wireless channel. Although a system is designed for an expected channel link quality, the channel may deteriorate for certain periods of time due to the changing nature of the propagation environment. A weak channel may lead to transmission errors, which can be corrected by employing Hybrid Automatic Repeat Requests (HARQ). In particular, type II HARQ protocols have become an appealing option since they only involve the transmission of additional parity bits, instead of repeating the original transmission. These additional parity bits can be obtained by puncturing a low rate channel code. However, finding an optimum pattern of puncturing for a specific code is non-trivial. In this work, we propose a retransmission scheme that considers various Low-Density Parity-Check (LDPC) codes of different rates that can be concatenated to perform the encoding and decoding operations, resulting in a lower encoding overhead for successful transmissions, while maintaining efficient decoding. We show that this technique can maintain high throughput and goodput, and a low outage probability, even when the received signal power is suddenly reduced due to the loss of the Line-of-sight (LOS) component.

Multiple-input, multiple-output (MIMO) techniques usually involve some kind of pre-processing at the transmitter, namely when singular value decomposition (SVD) techniques are employed. However, the precoding operations in general and the ones associated to SVD techniques in particular can substantially increase the envelope fluctuations of the transmitted signal, which leads to an increased sensitivity to transmitter nonlinearities and inherent performance degradation. However, recent research demonstrates that nonlinear distortion can be seen as useful information, provided that an optimum detection is considered. In this work, we present an analytical approach to obtain the asymptotic optimum performance of MIMO-SVD systems with strong nonlinear distortion effects1.

The design of a wireless catheterization laboratory (CathLab) presents considerable challenges, especially for intravascular ultrasound (IVUS) systems. In fact, the huge bit rates, together with the highly demanding quality of service (QoS) requirements such as high reliability, very low latency and secure communications, make the design of a wireless CathLab particularly difficult, especially due to the limited signal processing capabilities at the transmitter side. In this paper, we consider the design of a secure wireless CathLab system using a physical layer security scheme that takes advantage of the Multi-Input Multi-Output (MIMO) channel characteristics. It is shown that the physical layer security scheme can have excellent secrecy characteristics, even for an eavesdropper close to the transmitter or receiver. Moreover, the signal processing requirements are very small, especially for the data transmission phase, making this technique particularly interesting for a wireless IVUS scenario.

The need for higher data rates in wireless communications has resulted in a significant effort in what regards to the design of transmission techniques and waveforms. This design can be very challenging thanks to the strong trade-off between the spectral efficiency and the energy efficiency, which comes from the fact that spectral-efficient waveforms usually presents a very large peak-to-average power ratio (PAPR), requiring a linear amplification for preserving the linearity of the transmitted signals. Nevertheless, obtaining spectral- and energy-efficient wireless communications will undoubtedly be one of the main challenges of both 5G and beyond 5G (B5G) systems. In this work, we propose a new amplification scheme named quantized digital amplification (QDA). The QDA is based on the decomposition of a high-PAPR signal into constant-envelope components and allows to obtain very high energy efficiencies, being suitable for spectrally-efficient transmission techniques such as orthogonal frequency-division multiplexing (OFDM). By presenting a set of results of the QDA’s prototype, we demonstrate that it is possible to obtain amplification efficiencies way above the ones of the state-of-the art amplification schemes, while preserving the linearity of the transmitted signals.1

Besides traditional orthogonal multiple access (OMA), 5G and other future wireless communication systems will also involve non-orthogonal multiple access (NOMA). In that context, the combination of NOMA systems with the popular orthogonal frequency-division multiplexing (OFDM) will arise naturally in highly spectral efficient communications. Since OFDM signals are very prone to nonlinear distortion effects, the receiver design of NOMA-OFDM systems should be modified accordingly. In this work, we consider an efficient, nonlinear power amplification for NOMA-OFDM systems and we propose different reception strategies to cope with the nonlinear distortion. It is shown that the performance of conventional power-domain NOMA is very poor under nonlinear transmission scenarios. However, it is also demonstrated that Bussgang receivers combined with optimum detection schemes might be an excellent alternative1.

The pre-processing required at the transmitter side by many MIMO (Multi-Input, Multi-Output) schemes can lead to signals with substantial envelope fluctuations, making them very prone to nonlinear distortion effects. These nonlinear distortion effects can lead to significant performance degradation and signals with high irreducible error floor when conventional receivers are employed. However, the nonlinear distortion has information on the transmitted signals than can be employed to improve the performance, provided that an optimum receiver is employed. Nevertheless, the huge complexity of optimum receivers precludes its use in most practical scenarios. In this paper we consider MIMO schemes with strong nonlinear effects at the transmitter side and we propose an iterative receiver that is able to cope with nonlinear effects, allowing excellent performance even with strong nonlinear distortion effects. In fact, our results show that MIMO schemes with strong nonlinear effects can even perform better than linear MIMO system.

Non-orthogonal multiple access (NOMA) is being considered a key technology for 5G systems. In this work, we consider power-domain NOMA schemes for downlink orthogonal frequency-division multiplexing (OFDM) transmissions. Moreover, we consider the use of an efficient, nonlinear power amplification. We analyze the nonlinear distortion effects associated with the power amplifier and show that the nonlinear distortion can be severe, preventing the use of conventional NOMA receivers. In that context, we propose a receiver that estimates and cancels the nonlinear distortion and enables the implementation of power-domain NOMA schemes in strongly nonlinear OFDM scenarios.

Massive multiple-input multiple-output (mMIMO) systems are very important for 5G since they are one of the main enablers of the targeted huge capacity gains. On the other hand, the power consumption at the mobile terminals should be as low as possible, making single-carrier waveforms, such as single carrier with frequency-domain equalization (SC-FDE), particularly interesting, since the transmit signals can have a low peak-to-average power ratio (PAPR), allowing a highly efficient power amplification. However, the combination of mMIMO systems with SC-FDE modulations gives rise to implementation difficulties at the receiver side. On one hand, the equalization procedure should avoid matrix inversions, but this might lead to performance degradation. On the other hand, low-resolution analog-to-digital converters (ADCs) should be employed in each receive branch to reduce the implementation complexity, which might lead to severe nonlinear distortion effects. In this work, we study analytically the performance of mMIMO systems based on SC-FDE schemes employing low-complexity, iterative FDE receivers, and low-resolution ADCs. It is shown that the performance degradation associated to low-resolution ADCs can be tolerable if the number of receive antennas is larger than the number of transmit antennas, even when low-complexity FDE receivers are employed.

Single-carrier with frequency-domain equalization (SC-FDE) modulations are a natural choice for the uplink of massive multiple intput, multiple output (MIMO) systems, due to the low peak-to-average power ratio (PAPR) of their signals. However, equalization techniques should avoid matrix inversions. On the other hand, energy-efficient analog-to-digital converters (ADCs) should be used in each receive branch to reduce the power consumption and the implementation complexity, which might lead to strong nonlinear distortion effects. In this work, we present the analytical performance evaluation of nonlinear massive MIMO systems based on SC-FDE systems employing low-complexity, iterative FDE receivers1.

This paper considers massive multiple-input, multiple output (MIMO) schemes for single-carrier with frequency-domain equalization (SC-FDE) modulations. We present low-complexity turbo multi-user receivers based on the maximum ratio combining (MRC) and equal gain combining (EGC) techniques. Our receivers are implemented in the frequency-domain and allow excellent performances, even for moderate numbers of antennas.

In this paper, we consider massive multiple input multiple output systems that employ linear precoding techniques and are impaired by transmitter-side nonlinearities and channel estimation errors. We present an analytical method for the statistical characterization of the transmitted signals and develop a general framework to obtain analytically, the performance of different precoding techniques under the assumption of imperfect channel state information and for different types of nonlinearities. Regardless of the particular nonlinear transmission scenario, it is shown that the performance penalty associated to the nonlinear distortion can be tolerable if the number of transmit antennas is greater than the number of independent data streams.

Massive multiple input, multiple output (MIMO) schemes have been considered to support the physical layer of 5G systems and its combination with single-carrier with frequency-domain equalization (SC-FDE) schemes is particularly interesting for the uplink. However, the receiver complexity increases with the number of antennas, and it is important to have low complexity massive MIMO schemes. In this paper we consider the receiver design for the uplink of massive MIMO schemes where SC-FDE techniques are employed by the user terminals. To achieve this, we empoly low resolution analog-to- digital converters (ADCs) at each receive branch of the BS, combined with low complexity FDE techniques. It is shown that, although the nonlinear distortion levels inherent to the use of low resolution ADCs can be very high, we can have excellent performance, even with low complexity FDE receivers, provided that the number of receiver antennas is higher than the number of user terminals.

Orthogonal frequency division multiplexing (OFDM) schemes have high envelope fluctuations and peak- to-average power ratio (PAPR), making them very prone to nonlinear distortion effects, which can affect significantly the performance when conventional receivers are employed. However, it was recently shown that strong nonlinear distortion effects on OFDM signals do not necessarily lead to performance degradation. In fact, nonlinear OFDM schemes can outperform linear ones when optimum maximum likelihood (ML) receivers are employed. In this paper, we considered OFDM schemes with strong nonlinear distortion effects and we proposed a low- complexity detection scheme able to approach the optimum ML performance. Our technique is based on the fireworks algorithm (FWA) and allows excellent trade-offs between performance and complexity.

The use of massive multiple input multiple output (MIMO) techniques combined with orthogonal frequency division multiplexing (OFDM) schemes in the downlink of broadband wireless systems is considered in this paper. It is assumed that the transmitter supports up to R simultaneous links and employs T≫R antenna elements, and we study the feasibility of very low complexity transmitters with low resolution digital-to-analog converters (DACs). It is shown that the nonlinear distortion levels for detection purposes decrease with the ratio T/R, which means that we can employ very low resolution quantizers.

In this paper we consider massive multiple input multiple output (MIMO) schemes where each transmitter branch has very low resolution digital to analog converters (DACs). We present an analytical method for obtaining the statistical characterization of the transmitted signals that allows a simple and accurate performance evaluation even when there is imperfect channel state information (CSI). It is shown that, although the performance penalty associated to the use of low resolution DACs can be considerable, it can be mitigated by employing more antennas at the transmitter than at the receiver.

In this paper, we consider a T \times R massive MIMO system with up to R simultaneous links (T>> R>>1) and nonlinear amplifiers in each transmitter branch. The impact of a nonlinear amplification is analytically studied and accurate expressions for the spectral characterization of the transmitted signals, as well as the bit-error-rate (BER) are derived. The accuracy of our analysis is validated by simulations. It is also shown that the nonlinear distortion levels for detection purposes decrease with the ratio T/R, which means that the impact of the nonlinear distortion effects on the system's performance can be alleviated provided that T>>R. Therefore, the combination of massive MIMO systems with nonlinear, highly efficient power amplifiers can be an interesting option for low-cost, broadband wireless systems.

The use of massive multiple input multiple output (MIMO) techniques combined with orthogonal frequency division multiplexing (OFDM) modulations is being proposed for future broadband wireless systems such as 5G cellular networks. However, although these combination brings large capacity gains, it also suffers from two important drawbacks: the high implementation complexity inherent to the large number of antenna elements and the high sensitivity to nonlinear distortion effects due to the large peak-to-average power ratio (PAPR) of OFDM signals. In this paper, we study the capacity of massive MIMO-OFDM systems with strong nonlinear distortion effects. We derive theoretical expressions for the channel capacity considering different downlink scenarios where a base station with T nonlinear transmitting branches communicate with R receive antennas. It is shown that, although nonlinear distortion effects can reduce substantially the system capacity, this capacity loss can be reduced by increasing the number of transmit antennas (i.e., by using T≫ R).1.

Multiple input multiple output (MIMO) schemes combined with orthogonal frequency division multiplexing (OFDM) modulations are widely employed in broadband wireless systems. However, the transmitted signals are prone to nonlinear distortion effects due to their high envelope fluctuations. In this paper, we present a simple analytical method for studying nonlinear effects in MIMO-OFDM systems. Our method allows accurate results for the power spectral density (PSD) of the transmitted signals, as well as the signal- to- interference (SIR) levels at the detection level. It is shown that the robustness to nonlinear distortion effects increases with the number of transmit antennas (for a fixed number of independent data streams).

The combination of massive multiple input multiple output (MIMO) schemes with orthogonal frequency division multiplexing (OFDM) modulations are being considered for 5th generation (5G) wireless broadband systems. However, both the high complexity associated to massive MIMO transceivers as well as the amplification problems of OFDM constitute important drawbacks of such combination. In this paper we propose a reduced complexity, energy efficient massive MIMO OFDM scheme. For that purpose, we consider a maximal ratio transmit (MRT) technique combined with a simple time-domain clipping operation. We present an analytical method for obtaining both the power spectral density (PSD) of the transmitted signals, as well as the signal-to-interference (SIR) at the detection level. It is shown that having a large number of transmit antennas not only provides the known power and capacity gains, but also increases the robustness to nonlinear distortion effects.

BS (Base Station) cooperation schemes allow for an improved overall system's capacity and spectral efficiency. This is due to the possibility of MTs (Mobile Terminals) in adjacent cells sharing the same physical channel. The BSs send a quantized version of the received signals to a central unit that performs the separation of the signals associated to different MTs. In this paper we consider the uplink of BS cooperation schemes with SCFDE (Single-Carrier with Frequency domain Equalization) modulations and receivers based on the IB-DFE (Iterative Block Decision Feedback Equalization) concept for the multi-user detection process. We design robuts receivers that take advantage of the statistical characteristics of the quantization noise.

The study of nonlinear effects on orthogonal frequency division multiplexing (OFDM) signals can be made taking advantage of their Gaussian characteristics. This is relatively simple and straightforward for smooth, polynomial nonlinear characteristics, since the required number of intermodulation products (IMPs) can easily be obtained. However, the situation is more complex for clipping characteristics, since they are not differentiable and their polynomial approximation is a function of the clipping level. In this paper we consider the effects of clipping on sampled OFDM signals and we define an equivalent nonlinear characteristic that is polynomial, with small degree, whose output has the same spectral characteristics of a clipping operation. Our results show that we can use this equivalent nonlinear characteristic for studying the impact of the effects of clipping on sampled OFDM signals, allowing very accurate estimates of the power spectral density (PSD) of the output signals, as well as the corresponding signal-to-interference ratio (SIR). It can also be employed to obtain the performance of optimum receivers for clipped OFDM.

Orthogonal frequency division multiplexing (OFDM) modulations are very popular in wireless communications due to their flexibility and good performance over severely time-dispersive channels. However, OFDM signals have large peak-to-average power ratio (PAPR), which leads to amplification problems. Constant envelope OFDM (CE-OFDM) techniques use an OFDM signal to modulate the phase of a given carrier, allowing a 0 dB PAPR, with all inherent advantages. However, the phase modulator is a nonlinear device that can lead to performance degradation. In this paper, we take advantage of the Gaussian characteristics of OFDM signals to characterize analytically CE-OFDM signals, both in terms of power spectral density (PSD) and optimum asymptotic performance.

Orthogonal frequency division multiplexing (OFDM) modulations are very popular in wireless communications due to their flexibility and good performance over severely time-dispersive channels. However, OFDM signals have large peak-to-average power ratio (PAPR), which leads to amplification problems. Constant envelope OFDM (CE-OFDM) techniques use an OFDM signal to modulate the phase of a given carrier, allowing a 0 dB PAPR, with all inherent advantages. However, the phase modulator is a nonlinear device that can lead to performance degradation. In this paper, we take advantage of the Gaussian characteristics of OFDM signals to characterize analytically CE-OFDM signals, both in terms of power spectral density (PSD) and optimum asymptotic performance.

Orthogonal frequency division multiplexing (OFDM) schemes are being considered for both optical fibre and wireless optical communications mainly because their ability to cope with the high inter‐symbol interference levels associated to dispersive channels. DC biased optical OFDM (DCO‐OFDM) is a promising technique that meet the particularities of intensity modulated OFDM schemes. To reduce the required DC component OFDM signals need to be submitted to an asymmetric clipping. However, the resulting non‐linear distortion effects can lead to significant performance degradation and/or undesirable out‐of‐band radiation. Therefore it is important to evaluate the impact of an asymmetric clipping on DC‐biased optical OFDM signals. In this work, the authors take advantage of the Gaussian‐like nature of OFDM signals and they study analytically the impact of an asymmetric clipping operation in the performance of DCO‐OFDM schemes. The authors present accurate theoretical expressions for the power spectral density and the non‐linear distortion at the subcarrier level, which can be used for obtaining the bit error rate of conventional receivers. The authors also study the optimum performance of these non‐linear OFDM schemes and the authors show that for an optimum maximum likelihood detection the non‐linear distortion instead of leading to performance degradation can actually improve it, even outperforming conventional, linear OFDM schemes.

It is widely recognized that orthogonal frequency division multiplexing (OFDM) signals are very prone to nonlinear distortion effects, which can lead to significant performance degradation. However, recent results have showed that nonlinear distortion effects do not necessarily mean performance degradation and can actually lead to performance improvements relative to conventional linear OFDM schemes. In this paper, we consider the effects of bandpass memoryless nonlinear devices on OFDM signals and study the optimum asymptotic performance for both nondispersive channels and severely time-dispersive channels. We present analytical methods for obtaining the Euclidean distance between two OFDM signals that are subjected to different nonlinear characteristics. These results are then employed for obtaining the average asymptotic gain relative to conventional linear OFDM schemes in nondispersive channels and the gain distribution for severely time-dispersive channels with Rayleigh-distributed fading on the different multipath components. Our analytical results, which are shown to be very accurate, indicate that the optimum detection of OFDM schemes with strong nonlinear distortion effects allows significant gains when compared with conventional linear OFDM schemes.

Amplify-and-forward relay techniques combined with OFDM (Orthogonal Frequency Division Multiplexing) modulations are being considered for broadband wireless systems since they allow simple relay systems suitable for severely time-dispersive channels. However, since they rely in OFDM-based signals, these techniques are prone to nonlinear distortion effects. In this paper we consider OFDM- based amplify-and- forward relay systems with strong nonlinear distortion effects at both the transmitter and the relay node and we present a simple, but accurate, analytical method for characterizing the distortion levels at the subcarrier level.

In this paper, we present a theoretical analysis for obtaining the minimum Euclidean distance between two nonlinearly distorted OFDM (Orthogonal Frequency Division Multiplexing) signals. This analysis takes advantage of the Gaussian behavior of OFDM signals with a large number of subcarriers and is a key step to get the asymptotic gain of the optimum receiver. We consider both polar and Cartesian memoryless nonlinearities, showing that in general the optimum performance of nonlinear OFDM is better than linear OFDM.

Nonlinear phase distortion can severely degrade the performance of optical signals, both in terms of spectral characteristics and BER (Bit Error Rate) performance. This is especially serious for multicarrier optical signals. In this paper we study analytically the impact of nonlinear phase distortion on Coherent Optical Orthogonal Frequency Division Multiplexing (CO-OFDM) schemes with large number of subcarriers. The nonlinearly-distorted signal can be decomposed as the sum of useful and self-interference components. Conventional CO-OFDM implementations treat the self-interference as an undesirable noise term that leads to performance degradation. However, it is shown that this distortion component has information on the transmitted signals that can be employed to improve the performance. We show that the nonlinear phase distortion can lead to performance improvements relatively to the ideal linear case, contrary to what one could expect. We also present a sub-optimum receiver that is able to take advantage of this potential performance improvement.

In this paper, we present a theoretical analysis for obtaining the minimum Euclidean distance between two nonlinearly distorted OFDM signals (Orthogonal Frequency Division Multiplexing). This analysis takes advantage of the Gaussian-like behavior of OFDM signals with a large number of subcarriers and can be employed to provide the asymptotic gain of the optimum receiver. This approach is then employed with several different nonlinear characteristics, showing that in general the optimum performance of nonlinear OFDM is better than linear OFDM.

Recent results indicate that the optimum performance of uncoded OFDM (Orthogonal Frequency Division Multiplexing) in the presence of strong nonlinear distortion effects can be better than with linear transmitters. However, the uncoded OFDM performance is not very useful since typically OFDM schemes are combined with channel coding (also denoted coded OFDM), with substantially different uncoded and coded OFDM performances. In this paper we consider the optimum performance of coded OFDM schemes in the presence of strong nonlinear distortion effects. It is shown that nonlinear distortion effects not only do not lead to performance degradation, but they also might lead to substantial performance gains relatively to coded OFDM with linear transmitters.

In this paper we consider the optimum detection of OFDM (Orthogonal Frequency Division Multiplexing) signals with strong nonlinear distortion effects. It is shown that the optimum performance with strong nonlinear distortion effects is not as bad as one might expect and can even be better than the performance with conventional, linear transmitters. To achieve these excellent performances we should employ receivers able to take advantage of the information associated to transmitted data symbols that is inherent to the nonlinear distortion component, in opposition to traditional OFDM implementations where nonlinear distortion effects are regarded as an undesirable noise-like component. We study the achievable gains of the optimum receiver both analytically and by simulation. Since the complexity of optimum receivers is extremely high when we have nonlinear distortion effects, even for OFDM signals with a small number of subcarriers, we propose several sub-optimum receivers and evaluate their performance. Our sub-optimal receivers allow remarkable performance improvements, being able to reduce significantly the gap between the optimum performance and the performance of typical OFDM receivers.

In this paper we consider the optimum ML (Maximum-Likelihood) detection for OFDM signals (Orthogonal Frequency Division Multiplexing) with iterative clipping and filtering. It is shown that the nonlinear distortion does not necessarily mean significant performance degradation and, in fact, the optimum performance could even better than the performance with ideal, linear transmitters. We also present sub-optimum receivers that allow remarkable performance improvements, being able to reduce significantly the gap between the optimum performance and the performance of typical OFDM receivers.

The high envelope fluctuations of OFDM signals (Orthogonal Frequency Division Multiplexing) make them very prone to nonlinear distortion effects. In typical OFDM implementations the nonlinear distortion component is regarded as a noise-like term that leads to performance degradation. To achieve optimum performance we should employ a ML (Maximum Likelihood) receiver where we take into account all the information associated to the transmitted signals that is in the nonlinearly-distorted signal. Although the performance of an ML is substantially better than traditional receivers, its complexity is prohibitively high. In this paper we consider nonlinearly distorted OFDM schemes and we present sub-optimum receivers that try to approach the ML performance. It is shown that, contrarily to what was expectable, the ML performance with nonlinear transmitters can be better than with ideal, linear transmitters. Our suboptimal receivers allow remarkable performance improvements, being able to reduce significantly the gap between the ML performance and the performance of typical OFDM receivers.