Cyclic Prefix OFDM Research Articles

Optical Channel Selection Avoiding DIPP in DSB-RFoF Fronthaul Interface.

The paper presents a method of selecting an optical channel for transporting the double-sideband radio-frequency-over-fiber (DSB-RFoF) radio signal over the optical fronthaul path, avoiding the dispersion-induced power penalty (DIPP) phenomenon. The presented method complements the possibilities of a short-range optical network working in the flexible dense wavelength division multiplexing (DWDM) format, where chromatic dispersion compensation is not applied. As part of the study, calculations were made that indicate the limitations of the proposed method and allow for the development of an algorithm for effective optical channel selection in the presence of the DIPP phenomenon experienced in the optical link working in the intensity modulation–direct detection (IM-DD) technique. Calculations were made for three types of single-mode optical fibers and for selected microwave radio carriers that are used in current systems or will be used in next-generation wireless communication systems. In order to verify the calculations and theoretical considerations, a computer simulation was performed for two types of optical fibers and for two selected radio carriers. In the modulated radio signal, the cyclic-prefix orthogonal frequency division multiplexing (CP-OFDM) format and the 5G numerology were used.

Mutual Interference of Automotive OFDM Radars—Analysis and Countermeasures

Due to its noise-like signal property, digital orthogonal frequency-division multiplexing (OFDM) radars are often assumed to be robust against interference. While a lot of research has been carried out for interference between different modulation schemes, the mechanisms of interference from OFDM to OFDM radars have been barely addressed. This paper provides a thorough analysis of mutual OFDM to OFDM interference based on radar measurements using a 4x4 77 GHz multiple-input multiple-output (MIMO) OFDM radar prototype. The effects of interference are described both qualitatively and quantitatively for cyclic-prefix and stepped-carrier OFDM. Second, it is shown that conventional mitigation methods in the spectrogram are not suitable due to the random coding of cyclic-prefix OFDM. As an alternative, the application of adaptive beamforming is proposed and two realization possibilities are provided. Finally, new mitigation strategies in the modulation domain are proposed. They allow to shape interference to specific range-Doppler cells, yielding an interference-free range-velocity map for the area of interest. Additionally, the method may be used as the basis to enable simple conventional interference mitigation strategies.

Filtered orthogonal frequency division multiplexing for improved 5G systems

Wireless communications became an integrated part of the human life. Fifth generation (5G) is the modern communication which provides enhanced mobile broadband (eMBB), ultra reliable low latency communications (URLLC), and massive machine type communication (mMTC). Thus, 5G have to provide coverage to multi-numerology devices, therefore, modulation and access schemes are suggested in the literature such as cyclic prefix orthogonal frequency division multiplexing (CP-OFDM) and filtered OFDM (f-OFDM). CP-OFDM suffers from the high out of band emission which limited the multi-numerology applications. In f-OFDM, the out of band emission can be suppressed to an accepted extent such that different numerologies can be coexisting. On the other hand, f-OFDM can be more improved by using a proper filtering approach. In this paper three different filters are suggested based windowed-sinc function; Hanning, Hamming, and Blackman. Simulation results show that the proposed filters are promising for high spectral efficiency and out of band emission rejection. Furthermore, the bit error rate, error vector magnitude, and power spectral density are further improved with respect to CP-OFDM scheme but some trade-off is present. Overall, the suggested windowed-sinc filters are outperforming the traditional CP-OFDM. As a conclusion, the suggested windnowed-sinc filters have no limitations on the modulation order or the number of subcarriers utilized in the system.

Precoding for PAPR Reduction in UW-OFDM

Unique Word Orthogonal Frequency Division multiplexing (UW-OFDM) is a variant of the multicarrier technique that has shown better bit error performance compared to cyclic prefixed (CP-)OFDM. Like other multicarrier techniques, UW-OFDM suffers from the high peak-to-average-power ratio (PAPR) problem, which is an obstacle ahead of practical implementation of UW-OFDM. In this letter, the generator matrix, which is inherent to UW-OFDM, is not only used to create the specific UW-OFDM system, but also to reduce the PAPR. To do so, a suitable optimization problem is developed, and solved using the Procrustes method. The main advantage of the proposed method is that the built-in generator matrix should be calculated only once for each system setting, and the PAPR reduction (PAPR-R) is achieved without any overhead redundancy. Simulation results are provided for evaluating the PAPR-R performance of the proposed method.

Flexible Numerology in 5G NR: Interference Quantification and Proper Selection Depending on the Scenario

The 3rd Generation Partnership Project (3GPP) adopted cyclic prefix OFDM (CP-OFDM) for both uplink and downlink communications (although DFT-s-OFDM is also allowed in the uplink) in 5G New Radio (NR) Release 15. However, due to the variety of proposed deployment options and scenarios, a single numerology will not be enough to fulfil all performance requirements. A scalable OFDM numerology was required to enable diverse services on a wide range of frequencies and deployments, and finding the right numerology for each scenario is of special relevance for the proper functioning of 5G NR. Using a simulator calibrated according to the parameters established for NR performance by the 3GPP, this paper presents the performance evaluation of NR for the main 5G scenarios and different CP-OFDM numerologies and device speeds. Results show that increasing subcarrier spacing boosts the strength of the system against intercarrier interference (ICI) caused my Doppler spread; however, to increase subcarrier spacing, the CP must be reduced proportionally, which makes intersymbol interference (ISI) and ICI caused by insufficient CP have a more predominant effect. Therefore, it is necessary to quantify the total interference of the system, in order to determine the proper numerology for each scenario, which will depend on all the factors mentioned above, and not only on the operation band, as suggested in the standardization process. All this allows concluding that the choice of the appropriate numerology for a particular system depends not only on the band of operation but also on the deployment scenario and the speed of the user equipment (UE). Likewise, it is concluded that it is even possible to use more than one numerology for the same scenario.

X-Transform Time-Domain Synchronous IM-OFDM-SS for Underwater Acoustic Communication

Time-domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) is presented to enhance the spectral and energy efficiency over the conventional cyclic prefix OFDM and zero padding OFDM, but the interbloc interference (IBI) between the data block and adjacent pilot symbols deteriorates its performance. The IBI is considered as the main TDS-OFDM drawback in the harsh long-delay underwater acoustic (UWA) multipath channel. This article proposes a new scheme called X-transform time-domain synchronous index modulation (IM) OFDM to eliminate the TDS-OFDM IBI, explore the multipath diversity, and reduce the peak-to-average power ratio (PAPR). Particularly, X-transform, which merges the discrete Hartley transform and discrete Fourier transform in a very low-complexity unitary matrix, and IM techniques have been employed to remove the effects of IBI existing in underwater acoustic (UWA) TDS-OFDM. Moreover, compressive sensing has been adopted to guarantee a reliable UWA channel estimation based on a guard interval IBI-free region. The proposed scheme is evaluated over simulated and experimental UWA channels. The computer simulation and the real experiment show the outperformance of the proposed scheme in terms of bit error rate, computational complexity, PAPR, energy efficiency, and spectral efficiency over its conventional benchmarks.

DMRS-based channel estimation for railway communications in tunnel environments

The aim of this paper is to address the Demodulation Reference Signal (DMRS) based channel estimation issue in railway tunnel scenarios. Thus, we propose a deterministic model to accurately generate the time-varying channel response. And to investigate the impact of these particular environments on a Cyclic-Prefix Orthogonal Frequency Division Multiplexing (CP-OFDM) system, we first explore the pilot patterns proposed in the 5G specification, then we apply a thresholding based channel estimation algorithm to extract the demodulated data symbols. By comparing the Bit Error Rate (BER) and the latency of seven slot configurations at different train speeds, the results allow us to make some recommendations depending on the intended railway application and the required Quality of Service (QoS) level.

Impact of Equalizer step size in Underwater Acoustic Communication Channel

The Underwater Acoustic Channel (UAC) is a time variant channel and its multipath effects create ISI. This is one of the most important obstacles in the UAC channel which reduces the transmission rate. To remove this obstacle, a proper filter has to be designed in the receiver section. In this article, optimal step size for equalizer is computed and compared the results with the known techniques namely Decision Feedback Equalizer with interleave division multiple access (DFE IDMA) and Cyclic Prefix - Orthogonal Frequency Division Multiplexing (CP-OFDM) Equalizer. Channels are modeled using ray tracing methods. The various factors considered are ambient noise, attenuation loss, bottom and surface loss. The overall path loss for channels is computed by summing up the attenuation loss, surface and bottom loss. Simulation results evident that for short range UAC channel, the BER in the order of 10-2 is achieved using proposed methodology with least Eb/No compared to standard DFE method.

PAPR reduction using twin symbol hybrid optimization-based PTS and multi-chaotic-DFT sequence-based encryption in CP-OFDM system

Orthogonal frequency division multiplexing (OFDM) is considered as one of the most significant transmission methodologies of the recent past. Moreover, it permits easy demodulation and modulation. To find the new OFDM-based waveform to be used in fifth generation which is one of the foremost open issues for wireless networks of the next generation. In addition, the OFDM is affected by the maximum Peak-to-Average Power Ratio (PAPR). In order to minimize these problems, this paper proposed a Twin Symbol Hybrid Optimization used as a basis of the Partial Transmit Sequence (TSHO-PTS) method of Cyclic Prefix-OFDM (CP-OFDM). This CP-OFDM achieves the requirements of 5G telecommunication standards. Moreover, the exhaustive searching for optimal phase factors might increase the computational cost of PTS. To beat this problem, a hybrid version of slap swarm optimization (SSO) and Bald Eagle Search (BES) algorithm is introduced to investigate the phase factor optimally by the PTS method. Digital chaotic sequences are used to ensure the physical layer security during the data transmission scheme for the Discrete Fourier Transform Spread OFDM (DFT-S-OFDM) subcarrier allocation. The simulation takes place in the MATLAB platform, and the performances are evaluated by several performance metrics like Complementary cumulative distribution function (CCDF), Bit Error Rate (BER), and computational complexity. The performance of the proposed model is compared with various existing approaches and previous works. From the implemented results, the proposed strategy achieved less (5 dB) PAPR, minimum (10–8) BER, less processing time (0.18 s) than the existing schemes, and hence the complexity also very low (7%) than others.

Novel Iterative Clipping and Error Filtering Methods for Efficient PAPR Reduction in 5G and Beyond

The physical-layer radio access of 5G New Radio (NR) and other modern wireless networks builds on the cyclic prefix (CP) orthogonal frequency-division multiplexing (OFDM), known to suffer from the high peak-to-average power ratio (PAPR) challenge. In this article, novel PAPR reduction methods are developed, referred to as the iterative clipping and weighted error filtering (ICWEF) approach. To this end, clipping noise is separated from the data signal in frequency domain and properly tailored frequency-selective clipping noise filtering is adopted to control the tradeoff between PAPR reduction and transmitted signal quality. Furthermore, as 5G NR networks support adopting different OFDM numerologies at different bandwidth parts within one channel bandwidth, the ICWEF approach is also extended to take into account and suppress the resulting inter-numerology interference-something that most existing state-of-the-art methods do not consider. To facilitate comprehensive performance evaluations, a software-defined radio based prototyping testbed including a high-power base station power amplifier is also developed and used for assessing the performance of PAPR reduction solutions. The proposed ICWEF-based PAPR reduction concept is thereon thoroughly validated with extensive numerical and experimental results and shown to outperform the existing state-of-the-art reference solutions.

A Quadrature Amplitude Modulation Receiver Model with Matched Filters

In this study, it is mathematically shown that the detector inputs obtained using the matched filters in the Quadrature Amplitude Modulation (QAM) receiver are the same expressions as those obtained using the integrators if a rectangular pulse is used in the transmitter and the channel is Additive White Gaussian Noise (AWGN). Under the assumption that there is a perfect synchronization between the transmitter and the receiver. QAM is intensively used at the present time for the data transmission on subchannels in orthogonal frequency-division multiplexing (OFDM) communication systems such as digital subscriber lines (DSLs) modems, wireless local area networks (WLANs) and cellular wireless communication systems (e.g. 4G LTE). Cyclic-prefix OFDM is deployed in 4G LTE systems and the rectangular pulse is used in the time domain to shape all subcarriers of OFDM.

MMSE-Interference Canceling Receiver for QAM-FBMC Systems

Quadrature amplitude modulation-filter-bank multi-carrier (QAM-FBMC) technology, which utilizes a non-orthogonal pulse-shaping filter, has high spectral confinement characteristics and spectral efficiency, and it is considered as a candidate waveform to replace cyclic prefix-orthogonal frequency division multiplexing (CP-OFDM). However, the QAM-FBMC system suffers from performance degradation due to the non-orthogonal filter no matter how well the prototype filter is designed. In this letter, we propose the interference-canceling algorithm for QAM-FBMC with an MMSE receive filter. The proposed algorithm effectively mitigates the performance degradation caused by the non-orthogonal filters of QAM-FBMC systems, and it closely achieves the performance of CP-OFDM system in moderate SNR region. Also, the proposed algorithm performs the symbol-wise detection, which allows practical implementation without system delay.

Enhanced F-OFDM candidate for 5G applications

The demand for high data rate, the generation of Internet of Things (IoT), and various Machine Type Communications (MTC) emerged for a new transmission phenomenon. In other words, it is substantial to communicate without synchronization, or synchronization overhead, with mixed signal types. such specifications cannot be covered by the Fourth Generation (4G) systems, which is based on Cyclic Prefix Orthogonal Frequency Division Multiplexing (CP-OFDM). However, to achieve the specifications of the next generation system, numerous waveform replacements for the CP–OFDM were suggested, Filter Bank Multi–Carrier (FBMC), Generalized Frequency Division Multiplexing (GFDM), Universal Filtered Multi–Carrier (UFMC), and Filtered OFDM (F–OFDM). The filter design occupies essential part in these replacements, thus, in this paper, novel filters are introduced where simulation results show that the proposed filters outperform previous designs in terms of spectral efficiency improved dramatically by releasing the synchronization overhead.

Secrecy Capacity of FBMC-OQAM Modulation Over Frequency Selective Channel

This letter studies the information-theoretic secrecy capacity of an Offset-QAM-based filterbank multicarrier (FBMC-OQAM) communication over a wiretap frequency selective channel. The secrecy capacity is formulated as an optimization problem which has a closed-form solution in the high signal-to-noise ratio (SNR) regime. Two of the most common equalization strategies in FBMC-OQAM are considered, namely, single-tap and multi-tap equalization. For the sake of comparison, we also consider the secrecy capacity of a generic modulation and a cyclic prefix-orthogonal frequency division multiplexing (CP-OFDM) modulation. As a result, we find that FBMC-OQAM is particularly competitive for medium-to-long burst transmissions.

A novel PAPR reduction algorithm for DCO-OFDM/OQAM system in underwater VLC

In the underwater visible light communication (VLC), the photon propagation scattering effect is obvious and the multipath effect is enhanced as the link distance increases. This paper uses orthogonal frequency division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) of filter bank multicarrier technology (FBMC), which is based on time frequency localization (TFL) filter, to overcome inter symbol interference (ISI) and inter carrier interference (ICI). Since light-emitting diode (LED) modulation needs positive modulation signal, we use the direct current (DC) bias to add on OFDM/OQAM signal so that the signal is positive. But, as the OFDM/OQAM is also multicarrier modulation system, the instantaneous fluctuation of OFDM/OQAM signal is bigger than single carrier modulation system and the nonlinear distortion is very serious. Therefore, this paper puts forward the modified overlapped selective mapping (M-OSLM) suppression algorithm to restrain the peak to average power ratio (PAPR), and optimizes the range of data symbol which affects on current data symbol by taking the characteristic of filter bank into consideration so that decrease PAPR. Thus, the instantaneous volatility of OFDM/OQAM signals is reduced in order to select the appropriate DC bias and reduce the clipping distortion. The simulation results show that compared with DC-biased cyclic prefix optical OFDM (DCO-CP-OFDM), DC-biased optical OFDM/OQAM (DCO-OFDM/OQAM) has a better overcoming ability on ISI of underwater wireless optical scattered channel. Meanwhile, the performance of PAPR reduction is greatly improved by M-OSLM algorithm. The signal-to-noise ratio (SNR) is improved by 3 dB averagely compared to DCO-CP-OFDM at the same bit error rate (BER).

Evaluation and Analysis of FBMC/OQAM Systems Based on Pulse Shaping Filters

Cyclic prefix orthogonal frequency division multiplexing (CP-OFDM) is the most widespread multicarrier modulation scheme. However, the use of a rectangular filter in CP-OFDM causes poor out-of-band (OoB) radiation. Moreover, the CP causes spectral efficiency loss. To overcome these limitations, a filter bank multicarrier (FBMC) was proposed which uses prototype pulse shaping filters that can be adapted to fulfil the system requirements. The choice of the filter is crucial for FBMC/offset quadrature amplitude modulation (OQAM) due to its significant impact on the achieved performance. In this paper, new prototype pulse shaping filters in FBMC/OQAM systems are proposed, aiming to improve the system performance. A number of prototype pulse shaping filters, such as raised cosine pulse (RC), root raised cosine pulse (RRC), better than raised cosine pulse (BTRC), modified Bartlett-Hanning (MBH), improved sinc power pulse (ISP), phase modified sinc pulse (PMSP), parametric linear pulse (PLP), linear combination pulse (LCP), PHYDYAS and Hermite, are considered. The performance of each pulse shaping filter in FBMC/OQAM is evaluated and compared other in terms of the power spectral density (PSD), spectral efficiency, signal-to-interference ratio (SIR), time offset (TO), carrier frequency offset (CFO) and bit error rate (BER) performance over various channels. In addition, as multiple input multiple output (MIMO) is one of the main challenges associated with FBMC, we propose the new approach that use the frequency/time block spreading based on a Walsh-Hadamard (WH) code in MIMO FBMC/OQAM and investigate the combination of the our proposed prototype pulse shaping filters with MIMO systems. The proposed filters are shown to be suitable candidates for FBMC/OQAM systems.

Frequency Domain Multi-Carrier Modulation Based on Prolate Spheroidal Wave Functions

A novel frequency domain multi-carrier modulation (MCM-FD) scheme is proposed based on prolate spheroidal wave functions (PSWFs) for reducing the high complexity in the conventional time domain PSWFs multi-carrier modulation (MCM-PSWFs-TD) schemes. By constructing the relationship between discrete representation and exponential function representation of MCM-PSWFs-TD signals, it can be observed that the orthogonality and parity symmetry of the waveforms of PSWFs in the frequency domain are the same as that in the time domain. Thus, the PSWFs signal can be divided into two groups based on their parity symmetry, while these two groups of PSWFs signals can be processed simultaneously. Based on this concept, signal waveforms with only half spectrum range are invoked in the process of information loading and signal detection for reducing the number of sampling points participated in the signal operation. Compared to the MCM-PSWFs-TD scheme, the proposed MCM-PSWFs-FD scheme is capable of significantly reducing the computational complexity without severely degrading the system performance, such as spectral efficiency (SE), bit error rate performance, signal energy concentration and peak-to-average power ratio (PAPR). Furthermore, the cyclic-prefix orthogonal frequency division multiplexing (CP-OFDM), OFDM with weighted overlap and add (WOLA-OFDM), filter OFDM (F-OFDM), universal filtered multi-carrier (UFMC), as well as the filter bank multi-carrier with offset quadrature amplitude modulation (FBMC-OQAM) are also demonstrated as benchmarks. Simulation results are provided for illustrating that the proposed MCM-PSWFs-FD scheme is capable of striking a favorable tradeoff between the computational complexity and the system performance (i.e. SE, out-of-band energy leakage, adjacent frequency band interference, and PAPR), while the signal waveform design of the MCM-PSWFs-FD scheme is also more concise and flexible than the benchmarks.

Polynomial Cancellation Coded DFT-s-OFDM for Low-PAPR Uplink Signaling

The physical layer signaling of the 5G new radio still utilizes cyclic prefix orthogonal frequency division multiplexing (CP-OFDM) and discrete Fourier transform-spread-OFDM (DFT-s-OFDM) to support 5G services for the sake of system-backward compatibility. However, the transmission requirements among these services differ, and this poses a challenge to the adaptability of the waveforms with regard to the peak-to-average power ratio (PAPR) issue. In particular, DFT-s-OFDM serving as a low-PAPR option for uplink signaling still has room for PAPR improvement in cases such as machine-type and device-to-device communications. We propose polynomial cancellation coded (PCC)-DFT-s-OFDM to flexibly reduce the PAPR of conventional DFT-s-OFDM. The principle of the proposed method, including its transform, is analyzed in the time domain. The results show that it can also be regarded as a novel spectral shaping scheme for PAPR reduction. Through a parameter designed for adjusting the cost of spectral efficiency, the proposed method can regulate the extent of improvement compared with the conventional DFT-s-OFDM, not only in the PAPR, but also for the spectral radiation and bit error rate when considering the nonlinearity of the power amplifier. The increase in computational complexity is neglectable owing to the simplicity of generalized PCC, making it apt to be deployed in existing systems.

Efficient Equalization of Time-Varying Channels in MIMO OFDM Systems

The orthogonality of the cyclic prefix orthogonal frequency division multiplexing (CP-OFDM) modulation is ensured as long as the channel can be assumed constant across the duration of one CP-OFDM symbol period. Unfortunately, this assumption may not hold anymore for a large variety of emerging scenarios with mobility, high carrier frequency and multiple carrier frequency offsets. To tackle this issue, we propose a novel equalization structure. In contrast to existing works in the literature, the equalizer is obtained by considering a Taylor approximation of the ideal time-varying channel equalizer function. This results in an extremely simple implementation only consisting of per-subcarrier multiplications and FFT/IFFT operations. The general form of the equalizer is particularized to two specific cases: zero forcing and linear minimum mean squared error. Furthermore, the implementation complexity of the equalizers is computed and an analytical formula is proposed to efficiently evaluate their performance. Finally, numerical results demonstrate the efficiency of the proposed receivers as compared to the ideal one and previous works.

Multiplierless Filtered-OFDM Transmitter for Narrowband IoT Devices

In cyclic-prefix orthogonal frequency-division multiplexing (CP-OFDM)-based radio access, the coexistence of different technologies without precise time-frequency synchronization is limited due to high out-of-band (OOB) emissions. Therefore, the spectrum enhancement techniques play a key role in relaxing the synchronization and power control requirements. This allows a higher degree of opportunistic spectrum use with minimized interference. In addition, all the transmitting devices have to fulfill specific transmitted signal quality requirements, including the maximum OOB radiated signal power. With the orthogonal frequency-division multiplexing (OFDM)-based radio access, some additional signal processing for improved spectrum containment is commonly needed to achieve these requirements. The filtering and time-domain windowing are two fundamentally different approaches for spectrum enhancement. The filtered OFDM (F-OFDM) provides better spectrum localization than the time-windowing schemes [such as windowed overlap-add (WOLA)], with the cost of higher complexity. This article introduces low-complexity solutions for spectrally enhanced narrowband OFDM transmitters based on the use of lookup tables (LUTs). The proposed LUT approach, requiring only memory units and a low number of additions, allows to avoid all computationally expensive operations in online transmitter processing, as it builds the transmitted signal by summing the stored partial waveforms optimized offline. In certain cases, completely multiplication- and summation-free designs are possible. The transmitters of narrowband Internet of Things (NB-IoT) devices are natural applications for the proposed LUT approach, as they require additional digital baseband signal processing to reach the emission requirements. It is shown that the proposed LUT schemes can provide significant savings in real-time computations of NB-IoT devices, while fulfilling the 3GPP requirements.