Orthogonal Frequency Division Multiplexing Modulation Research Articles

Clipping Noise Mitigation in Optical OFDM Using Decision-Directed Signal Reconstruction

In this paper, the performance of a new decision-directed signal reconstruction (DDSR) algorithm with optimal reconstruction thresholds for use with optical orthogonal frequency division multiplexing (OFDM) modulation is investigated. Clipping noise remains one of the main drawbacks in many practical optical OFDM transmission systems. It has been shown in previous studies that a time-domain based DDSR algorithm can be used to effectively reduce the clipping noise. However, in all existing works studying this DDSR algorithm, the decision threshold used to direct the time-domain signal reconstruction is fixed and equal to the level at which the transmitted signal is clipped. In this paper, it is shown that, due to the influences of the unavoidable noise at the receiver, fixing the DDSR threshold at the clipping level is not an optimal solution and can severely limit the advantages of using this algorithm. A detailed theoretical analysis is performed to investigate the key factors that influence the choices of the optimal DDSR thresholds. The presented simulation results are used to verify the accuracy of the theoretical analysis and also show that the performance of the DDSR algorithm is significantly improved by using the obtained optimal thresholds.

LDPC Decoding With Low Complexity for OFDM Index Modulation

In this paper, the posterior probabilities of the low-density parity check (LDPC) coded bits are derived for the orthogonal frequency division multiplexing index modulation (OFDM-IM) system to reduce the implementation complexity of LDPC decoding without performance loss. Specifically, the complexity of LDPC decoding is reduced by decomposing the posterior probability of the coded bit into multiple likelihood probabilities, each of which is obtained for each subcarrier of the corresponding OFDM-IM subblock. The results of the complexity analysis indicate that the proposed approach significantly reduces the implementation complexity of LDPC decoding for OFDM-IM compared to the conventional approach. Furthermore, the simulation results confirm that the proposed approach provides the same bit error rate performance as the conventional approach.

A Frequency-Diversity System With Diversity Encoder and OFDM Modulation

In this work, a diversity encoder maps vectors of N modulated symbols to vectors of N diversity symbols using an orthogonal matrix. Each modulated symbol appears in N diversity symbols. An Orthogonal-Frequency-Division-Multiplexing (OFDM) modulator converts the diversity vectors to OFDM symbols. The number of subcarriers in the OFDM symbol equals N. Each diversity symbol modulates one subcarrier; therefore, each modulated symbol is transmitted by N orthogonal subcarriers. The diversity bandwidth of the modulated symbols is N times the subchannel bandwidth. The modulated symbols experience independent fading gains on each subchannel since the diversity bandwidth (OFDM bandwidth) is chosen to be multiple of the coherence bandwidth of the transmission channel. In the receiver, an OFDM demodulator converts back the OFDM symbols to vectors of diversity symbols. The diversity detector maps the diversity vectors to vectors of modulated symbols. The noise samples at the output of the diversity detector are uncorrelated. Maximum likelihood and linear decorrelator detectors are used as diversity detectors. The transmitter and the receiver of this work are implemented using Field-Programable-Gate-Array (FPGA) technology. The performance of the implemented system is the same as the performance of the N channels diversity system with the Maximal-Ratio-Combiner (MRC) receiver.

Li-Fi delivering broad band and multimedia content under dimming in train

This paper focuses in delivering broad band and media content to numerous users while traveling in the train through light fidelity (Li-Fi), an innovative technology under Visible Light Communication (VLC). In our daily life, there is increasing traffic request for open Radio Frequency (RF) to support the users. In RF, even recurrence and spatial reuse of spectrum not able to support in conveying broad band and multimedia content to the internet users. For high-capacity mobile data networks, the VLC can balance the RF-based mobile communication systems. VLC conserves energy through dimming. Dimmed Light Emitting Diode (LED) luminaries provide visual comfort with appropriate optics while accessing internet. In the proposed work, LED dimming is achieved through digital dimming modulation technique, Variable Delta Sigma Modulation. The broad band and multimedia content can be delivered faster through Reverse Polarity Optical Orthogonal Frequency Division Multiplexing modulator to the internet users under dimming with better bit error rate and bit rate.

Improved Performance in the Detection of ACO-OFDM Modulated Signals Using Deep Learning Modules

Free space optical communication (FSO) is widely deployed to transmit high data rates for rapid communication traffic increase. Asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) modulation is a very efficient FSO communication technique in terms of transmitted optical power. However, its performance is limited by atmospheric turbulence. When the channel includes strong turbulence or is non-deterministic, the bit error rate (BER) increases. To reach optimal performance, the ACO-OFDM decoder needs to know accurate channel state information (CSI). We propose novel detection using different deep learning (DL) algorithms. Our DL models are compared with minimum mean square error (MMSE) detection methods in different turbulent channels and improve performance especially for non-stationary and non-deterministic channels. Our models yield performance very close to that of the MMSE estimator when the channel is characterized by weak or strong turbulence and is stationary. However, when the channel is non-stationary and variable, our DL model succeeds in improving the performance of the system and decreasing the signal to noise ratio (SNR) by more than 8 dB compared to that of the MMSE estimator, and it succeeds in recovering the received data without needing to know accurate CSI. Our DL decoders also show notable speed and energy efficiency improvement.

Diffused-Line-of-Sight Communication for Mobile and Fixed Underwater Nodes

The misalignment of mobile underwater wireless optical communication (UWOC) systems, compounded by turbulence in underwater scenarios, is a practical problem that can be resolved through various means. This work describes a pulse-position-modulation-based (PPM-based) diffused-line-of-sight UWOC system that offers a solution to this issue. PPM is found to be power-efficient and, in terms of bit-error-ratio performance, outperforms on-off keying modulation and orthogonal frequency-division multiplexing modulation in complex dynamic underwater channels. Through indoor experiments and outdoor deployment, we validated the robustness of the proposed PPM-based mobile UWOC system. This work sheds light on the practical implementation of UWOC networks for relieving the strict pointing-acquisition-and-tracking requirements when an underwater apparatus is transmitting or receiving signals on the fly.

Flexible OFDM Transceiver for Underwater Acoustic Channel: Modeling, Implementation and Parameter Tuning

Acoustic signals are a first choice in underwater communication since sound waves face very low attenuation in water compared to radio frequency (RF). However, the tendency of receiver to detect multipath signals (due to signals bouncing off surface or the bottom) induces large delay spreads and thus strong channel frequency selectivity. Additionally, rapid spatial and temporal variations in underwater acoustic (UWA) channel makes it hostile for communication systems based on single carrier modulation. Multicarrier modulation techniques, on the other hand, can greatly improve bandwidth utilization and help deal with time dispersal effects. Orthogonal frequency division multiplexing (OFDM) is a proven multicarrier communication system having capabilities to cope with frequency selectivity and delay spreads effectively. OFDM has started to get attention for being a simpler alternative to high complexity and high maintenance single carrier systems in UWA communication systems. This work proposes a Matlab model of an OFDM transceiver along with UWA channel characterization based on Rician shadowed fading model as it perfectly characterizes the way in which a shallow UWA channel behaves. Eventually, the proposed design allows implementation of various OFDM modulation methods and to perform Monte Carlo simulations for bit error rate comparisons together with the ability to tune multiple UWA channel parameters.

NVIS Multicarrier Modulations for Remote-Sensor Applications.

The number of Internet of Things (IoT) devices has experienced a large growth during the last decade, as well as the data volume gathered from remote sensors. Satellites are still a suitable communication method and may be preferable for a remote ubiquitous sensor network (USN), which sometimes are located in places without much communications infrastructure where coverage is the principal drawback. Alternatively, the proposed solution for this article aims at a near-vertical incidence skywave (NVIS) channel for high frequencies (HF) with a low-cost platform, allowing a low-power transmissions coverage area up to 250 km for USN. The HF standards are focused on generic communication channels not being robust for NVIS communications. In this article we study and test an alternative based on orthogonal frequency-division multiplexing (OFDM) modulations to make them more robust and less dependent on the channel NVIS communications. For that purpose, we test the HF standard modulations and a designed OFDM modulation to prove the robustness of each. This study has been tested between Barcelona and Tarragona, using different transmission power levels and modulation orders.

Downlink data transmission for high‐speed trains in 5G communication systems

In this study, a new transmitter–receiver approach for high-speed train (HST) downlink orthogonal frequency division multiplexing (OFDM) data transmission and detection in fifth generation (5G) communication systems is proposed. The proposed method uses a new Doppler shift frequency (DSF) estimation method, proposed in this study, to calculate the channel tap DSFs at the HST receiver. These estimated DSFs are used to calculate the channel taps’ angle of departure (AoD). Using the projected AoDs, the transmitter beamformer adjusts its coefficients to suppress the non-line-of-sight (NLoS) channel taps and a DSF compensation block is used to mitigate the line-of-sight channel tap DSF. Simulation results indicate the fidelity of the proposed DSF estimation method and the proposed transmitter–receiver approach. Due to the accuracy of the proposed DSF estimation method and the effectiveness of the beamformer in suppressing the NLoS channel taps, the approach of using a classic digital modulation and demodulation method instead of the OFDM scheme is appraised. Simulation results indicate that this shift from the OFDM modulation to the classic modulation results in a data detection approach which has acceptable accuracy, low computational complexity, and high spectral efficiency.

4-QAM OFDM Data Switching via Free Carrier Absorption in Si-Rich SiC Waveguide

The all-optical cross-wavelength quadrature amplitude modulation orthogonal frequency-division multiplexing (QAM-OFDM) data switching in the silicon rich silicon carbide (Si-rich SiC) micro-ring (μ-ring) waveguide is demonstrated for the first time by using the two-photon absorption (TPA) induced free carrier absorption (FCA) process. With enlarging the allowable 4-QAM OFDM data bandwidth to 1.16 GHz, the all-optical 4-QAM OFDM pump-to-probe switching with allowable bandwidth of 1.16 GHz can achieve a raw data rate up to 2.32 Gbit/s with forward error correction (FEC) qualified error vector magnitude (EVM), signal-to-noise ratio (SNR), and bit-err-ratio (BER) of 31.9%, 9.92 dB, and 8.7 × 10−4, respectively. With upgrading the bandwidth of electrical pre-amplifier to 1.5 GHz, the wavelength-converted probe data can successfully pass the FEC criterion with qualified SNR of 9.49 dB, BER of 1.4 × 10−3 and EVM of 33.5%, to demonstrate the all-optical switching data rate approaching 3 Gbit/s.

Nonlinear Noise Compensation in an Integration System of Fiber-Wireless Communication and Free Space Optical Communication

We propose and experiment an integration system of fiber-wireless communication and free space optical (FSO) communication based on nonlinear noise compensation. This integration system, combining the benefits of FSO communication and high-frequency wireless communication, as well as the high-frequency efficiency and anti-interference to multipath fading of the multicarriers orthogonal frequency division multiplexing (OFDM) modulation, can increase the capacity and flexibility of the system. The nonlinear equalizer is employed to compensate the distortion caused by the beating between OFDM signal subcarriers in the combination system. Up to 1-Gbaud 16QAM discrete multitone (DMT) OFDM signal can be first transmitted over the distance of 100-m single-mode fiber (SMF), then delivered over the distance of 100-m FSO link, and finally, transmitted over the distance of 4 m wireless. The bit-error-ratio (BER) of the OFDM signal in the integration system can be less than the hard-decision forward-error-correction (HD-FEC) threshold of 3.8 × 10−3. The experiment shows that the nonlinear equalizer can work well compatible with the combination system and can improve the transmission performance by 0.7-dB receiving sensitivity in the combination system.

Focusing gain analysis of time-reversal precoding in MISO OFDM communication systems

Emerging communication systems can benefit from time-reversal (TR) technology thanks to its good spatio-temporal signal focusing effect. The recent advances in low-cost wideband devices fabrication further leverage the use of TR wideband communication systems. The TR is generally carried out in the time domain and the focusing effect of TR comes from the use of a high transmit rate back-off factor (BOF), which is the signal up-sampling rate. In spite of the widely-used orthogonal frequency-division multiplexing (OFDM) modulation, few works have investigated the frequency-domain (FD) TR precoding in combination with OFDM communication systems. Furthermore, most existing works on FD/TR precoding rely on multiple-antenna technology at the transmitter to create the focusing effect. In this paper, we investigate the focusing gain provided by TR precoding in multiple-input single-output (MISO) OFDM systems. In particular, we compare the communication performance of such system at the intended position and that at the unintended position. Based on the analysis, we demonstrate that increasing the BOF and/or the number of transmit antennas significantly improves the focusing effect at the intended position. In contrast, the unintended positions receive less useful power. We derive approximated mean-square-error (MSE) expressions of equalized received signals at both intended and unintended positions. The subsequent focusing gain is presented as a function of the BOF and the number of antennas, enabling us to gain insights in the contributions of each parameter to the system performance. Numerical simulations with multi-path Rayleigh fading channels are carried out to validate the MSE expressions.

Study on An Improved Accuracy Algorithm Based on Channel Estimation in 5G Application

5G has ultra-high spectrum utilization, energy efficiency and resource utilization. Orthogonal frequency division multiplexing modulation is a key technology in order to achieve the characteristics of 5G communication such as ultra-high spectrum utilization. In recent years, a pseudo-random sequence (PN) filled with cyclic prefix time-domain synchronous orthogonal frequency division multiplexing modulation technology has emerged as a modulation technology in high-performance 5G communication systems. And the technology has been maturely applied in the field of terrestrial digital multimedia wireless broadcasting because of its higher channel estimation and synchronization performance. At the same time, 5G has higher requirements in terms of transmission rate and so on, so it has stricter performance requirements for transmission delay control and timing recovery. However, the TDS-OFDM system in 5G application does not allocate pilot subcarriers, which are used for system channel estimation and various parameters. Since the autocorrelation function of the time-domain PN sequence filled in the TDS-OFDM system is not an ideal shock function, when the phase difference with the received sequence is not 0, the autocorrelation value is not 0 as well. When used as a channel estimation, it will cause mutual interference of various multipath components, reduce the accuracy of the estimation, and may also cause some weak multipath Loss of components. This paper proposes an improved channel estimation scheme for TDS-OFDM based systems in 5G application, which can effectively improve the accuracy of channel estimation in the case of high signal-to-noise ratio.

Bit Error Probability and Capacity Bound of OFDM Systems in Deterministic Doubly-Selective Channels

Doubly-selective channels, such as those that occur when the transmitter and the receiver move relative to each other at high speeds, are a key scenario for fifth generation (5G) cellular systems, which are mostly based in the use of the orthogonal frequency-division multiplexing (OFDM) modulation. In this paper, we consider an OFDM system using quadrature amplitude modulation (QAM) symbols and we show that, when transmitting over deterministic doubly-selective channels, the inter-carrier-interference (ICI) affecting a symbol can be well approximated by a complex-valued normal distribution. Based on this, we derive a lower bound for the link capacity using the Shannon-Hartley theorem. Finally, we provide an approximation of the bit error probability (BEP) using the well-known BEP expressions for Gray-coded QAM constellations over additive white Gaussian noise (AWGN) channels, and show numerical results that confirm that the proposed BEP expression approximates accurately the bit error ratio (BER) of the OFDM system for standardized channel models. The proposed closed-form analytical expressions for the capacity and the BEP do not only allow for discarding the need of computationally-costly Monte-Carlo system simulations, but also provide a theoretical framework to optimize the system parameters directly impacting on the achievable performance.

Digital Mobile Fronthaul Based on Performance Enhanced Multi-Stage Noise-Shaping Delta-Sigma Modulator

A novel topology multi-stage noise-shaping (MASH) delta-sigma modulator is proposed for 20-km digital mobile fronthaul (MFH) in this article. In the proposed MASH structure, a newly designed feedback unit is combined with a traditional fourth-order sturdy MASH structure to enhance the noise-shaping capacity. The detailed comparison between the conventional fourth-order single delta-sigma modulator (SDSM) and the proposed new topology MASH is presented in a 512/1024 quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) transmission system with the bandwidth of 1.125 GHz. The OFDM signal is quantized to two bits by SDSM/MASH analog-to-digital conversion (ADC), and this digitized signal is transmitted over 20-km single mode fiber (SMF) in 20-Gbaud 4-level pulse amplitude modulation (PAM4) intensity modulation direct detection (IM/DD) system. The signal to noise ratios (SNRs) of the retrieved OFDM signal utilizing the proposed new topology MASH and the fourth-order SDSM ADCs are 38.7dB and 34.5dB, respectively. In the case of 1024-QAM PAM4 system, the error vector magnitude (EVM) floors of the proposed new topology MASH and the conventional fourth-order SDSM schemes are 1.64% and 1.96% over 20-km SMF transmission at off-line digital signal processing (DSP) reception, and 1.2 dB receiver sensitivity improvement is achieved.

ZF and MMSE Detectors Performances of a Massive MIMO System Combined with OFDM and M-QAM Modulation

This paper presents a least squares channel estimation (LSCE) in the UpLink transmission for a Massive MIMO systems in 5G wireless communications, combined with Orthogonal Frequency Division Multiplexing and higher-order M-QAM modulation. The Mean Square Error (MSE) of the LSCE is computed, and the performance of ZF, MMSE and V-BLAST detectors is also evaluated. Using 128-QAM modulation (i.e., increasing the noise system sensibility) with 1024 subcarriers and increasing the Base Station antennas array decreases more the Bit Error Rate (BER) for the ZF, MMSE and V-BLAST detectors with respect to the MSE. The performance degradation both ZF and MMSE compared to V-BLAST is negligible in a high receive antennas; hence, faster data rates and higher spectral efficiency levels for the communication systems. Consequently, the ZF, MMSE and V-BLAST detectors achieve a better BER with respect to channel estimation.

Temperature and Noise Dependence of Tri-Mode VCSEL Carried 120-Gbit/s QAM-OFDM Data in Back-to-Back and OM5-MMF Links

The transmission performances of the tri-mode vertical cavity surface-emitting laser (VCSEL) carrying 16-quadrature amplitude modulation orthogonal frequency division multiplexing (16-QAM OFDM) data streams at a data rate beyond 100 Gbit/s are demonstrated in OM5 and OM4 multi-mode fibers (MMFs) over 100 m. By using the double oxide-confined aperture with a diameter of 5 μm, the tri-mode VCSEL provides three main transverse modes at a power of 2 mW and a quantum efficiency of 0.4 mW/mA, in which these main transverse modes individually contribute to 53.1%, 37.6%, and 9.3% of the output power from the tri-mode VCSEL for delivering the QAM-OFDM data. With optimizing the bias current of the tri-mode VCSEL to 8 mA (32 Ith) and pre-leveling the QAM-OFDM subcarrier amplitudes at a slope of 0.5 dB/GHz in the back-to-back (BtB) transmission case, the raw data rate up to 120 Gbit/s can be achieved under the FEC receiving criterion. In particular, the performance of the tri-mode VCSEL operating at the temperature of up to 85°C in BtB case is also demonstrated, while its lasing spectrum translates from tri-mode to quasi-single-mode because of the heating induced carrier dissipation and index reprofiling process. After propagating through the 100-m OM5-MMF, the pre-leveled 16-QAM OFDM data carried by the tri-mode VCSEL guarantees the 104-Gbit/s FEC-certified transmission with a receiving power penalty of 3.8 dB.

Spectral analysis of GFDM modulated signal under nonlinear behavior of power amplifier

Generalized frequency division multiplexing (GFDM) is a flexible non-orthogonal waveform candidate for 5G which can offer some advantages such as low out-of-band emission and high spectral efficiency. This paper investigates the effects of nonlinear behavior of practical power amplifier (PA) on the GFDM spectrum. A closed form expression for power spectral density (PSD) of GFDM signal is extracted. Then, PSD at the output of PA as a function of input power and the coefficients of nonlinear polynomial PA model is derived. In addition, the adjacent channel power (ACP) and ACP ratio, as two important performance metrics, are evaluated. The simulation results confirm the accuracy of derived analytical expressions. Moreover, to validate the performance of GFDM modulation after nonlinear PA, it is compared with orthogonal frequency division multiplexing modulation.

Performances of OSIC Detector of an UpLink OFDM Massive-MIMO System in Rayleigh and Ricain Fading Channels

This paper presents a least squares channel estimation for Massive-Multiple-Input Multiple- Output (Massive-MIMO) systems in next generation of wireless communications denominated 5G, combined with Orthogonal Frequency Division Multiplexing (OFDM) and 64-QAM modulation. In this paper, after determining the estimated channel, we first consider one OFDM symbol and presented the flat fading and the frequency-selective fading in the case when channel taps equal to one and strictly superior to one respectively. In addition to that, we consider the uplink transmission where the transmitter antenna equal to $$N_t=50$$ and a Base Station (BS) equipped with several antenna superior to that of transmitter, in the same way the data detected using a linear detectors Zero-Forcing and Minimum Mean Square Error. Moreover we improve their performance by an ordered successive interference cancellation (OSIC) method. Using two channels fading namely the Rayleigh and the Ricain respectively and evaluate their performance with a system of Massive-MIMO antenna. Increasing the number of channel taps increase the inter-symbol interference allows to degrade the performance of the system both the Rayleigh and Rician channel, even we used a pilot sequence equipowered, equispaced and phase shift orthogonal. Hence increasing the number of antennas arrays at the receiver (i.e BS) decreases more the Bit Error Rate, consequently increasing the performance of the system. the OSIC methods provide a best performance when the number of antenna grow in a smaller range of Signal-to-Noise Ratio.

Set Partition Modulation

In this paper, a novel modulation scheme called set partition modulation (SPM) is proposed. In this scheme, set partitioning and ordered subsets in the set partitions are used to form codewords. We define different SPM variants and depict a practical model for using SPM with orthogonal frequency division multiplexing (OFDM). For the OFDM-SPM schemes, different constellations are used to distinguish between different subsets in a set partition. To achieve good distance properties as well as better error performance for the OFDM-SPM codewords, we define a codebook selection problem and formulate such a problem as a clique problem in graph theory. In this regard, we propose a fast and efficient codebook selection algorithm. We analyze error and achievable rate performance of the proposed schemes and provide asymptotic results for the performance. It is shown that the proposed SPM variants are general schemes, which encompass multi-mode OFDM with index modulation (MM-OFDM-IM) and dual-mode OFDM with index modulation (DM-OFDM-IM) as special cases. It is also shown that OFDM-SPM schemes are capable of exhibiting better error performance and improved achievable rate than conventional OFDM, OFDM-IM, DM-OFDM-IM, and MM-OFDM-IM.