Modulated Orthogonal Frequency Division Multiplexing Signals Research Articles

A Novel 64 QAM-OFDM Optical Access System Based on Bit Reconstruction

This paper proposes a novel orthogonal frequency division multiplexing (OFDM) optical access scheme based on bit reconstruction. In this method, correlation is introduced into the data information of optical line terminals (OLT) through the logical coding circuits and partition mapping. Even after passing through the optical fibre channel, the strong correlation after bit reconstruction can still be used in the optical network unit (ONU) for reliable decoding. In the simulation experiments, a 60 Gbit/s bit reconstruction 64 quadrature amplitude modulation (QAM) OFDM signal was successfully transmitted over a 10/20 km single-mode fibre (SMF). The simulation results show that the proposed scheme can effectively achieve reliable transmission with gains of about 1.3 dB and 0.51 dB at a 20% soft decision-forward error correction (SD-FEC) threshold, respectively. The proposed scheme is a promising candidate for a next-generation passive optical network (NGPON) solution.

Photonics-Assisted Analog Wideband Self-Interference Cancellation for In-Band Full-Duplex MIMO Systems With Adaptive Digital Amplitude and Delay Pre-Matching

A photonics-assisted analog wideband RF self-interference (SI) cancellation and frequency downconversion approach for in-band full-duplex (IBFD) multiple-input multiple-output (MIMO) systems with adaptive digital amplitude and delay pre-matching is proposed based on a dual-parallel Mach–Zehnder modulator (DP-MZM). In each MIMO receiving antenna, the received signal, including different SI signals from different transmitting antennas and the signal of interest, is applied to one arm of the upper dual-drive Mach–Zehnder modulator (DD-MZM) of the DP-MZM, the reference signal is applied to the other arm of the upper DD-MZM, and the local oscillator signal is applied to one arm of the lower DD-MZM. The SI signals are canceled in the optical domain in the upper DD-MZM and the frequency downconversion is achieved after photodetection. To cancel the SI signals, the analog reference signal is constructed in the digital domain, while the amplitude and delay of the constructed reference are adjusted digitally by upsampling with high accuracy. Experiments are performed when two different SI signals are employed. The genetic algorithm or least squares algorithm is combined with segmented search respectively for the SI signal reconstruction and amplitude and delay pre-matching. A cancellation depth of around 20 dB is achieved for the 1-Gbaud 16 quadrature-amplitude modulation orthogonal frequency-division multiplexing signal.

Performance Comparison of Photonic Downconversion for Uplink Mobile Fronthaul

This study compares the performance of photonic downconversion technology using single and dual wavelength modulation methods using an optical phase modulator. We compared the system characteristics, including the conversion gain, phase noise, and spurious-free dynamic range, of the two approaches. To demonstrate the proof-of-concept, we transmitted high-order quadrature amplitude modulation orthogonal frequency-division multiplexing signals in the 43 and 52 GHz bands over the system. The results confirm the potential of the system as a promising solution for uplink mobile fronthaul in high-frequency bands, such as in 5G-Advanced networks.

Linearization for Microwave Photonic OFDM Transmission Systems Using an Iterative Algorithm Based on FEC Mechanism

A digital linearization algorithm is proposed to reduce the third-order intermodulation distortion (IMD3) for the microwave photonic (MWP) orthogonal frequency-division multiplexing (OFDM) transmission system with intensity modulation and direct detection. Forward error correction (FEC) is added to the OFDM signal to suppress the IMD3 in conjunction with an iterative operation. The distorted OFDM signal from the MWP system goes through iterative operations including fast Fourier transform, demodulation, error correction, signal reconstruction, and interference cancellation until the output converges. Compared with the non-iterative method and the iterative method without FEC mechanism, the proposed method has a significant advantage in linearizing signals with large nonlinearity and low signal-to-noise ratio. 200-Msym/s 64 quadrature-amplitude modulation (64-QAM) OFDM signals with a large peak-to-peak voltage of 650 mV are amplified by a power amplifier and then input to the MWP link and linearized by the FEC-based iterative method. The results show that the error vector magnitudes (EVMs) of the OFDM signals can be optimized from 13.1% to 2.9% and from 13.4% to 6.3% when the output <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E_b/N₀</i> from the MWP link is about 35 dB and 13.5 dB, respectively. Besides, a 3-GHz 64-QAM OFDM signal is successfully transmitted through a 25-km standard single-mode fiber with an EVM of 3.9% after linearization.

Secure OFDM transmission scheme based on chaotic encryption and noise-masking key distribution.

In this Letter, we propose a secure orthogonal frequency division multiplexing (OFDM) transmission scheme based on chaotic encryption and noise-masking key distribution. With the implementation of a three-dimensional digital chaotic system, the security performance is effectively enhanced by scrambling the phase, symbol, and subcarrier frequency of the OFDM signal. The proposed noise-masking key distribution can mask the key information of the chaotic system into noise and transmit it with the chaotic encrypted signal simultaneously. By this mechanism, the legal receiver can realize uninterrupted authentication and decryption even if the key is constantly updated. Transmission of a 62.2-Gb/s quadrature phase shift keying (QPSK) and 124.4-Gb/s 16 quadrature amplitude modulation (16QAM) OFDM signal over a 2-km 7-core fiber using the proposed scheme is experimentally demonstrated. The results show that the proposed scheme can realize security enhancement and cost-effective key distribution without significant bit error ratio (BER) performance degradation.

High-speed fiber-wireless-fiber system in the 100-GHz band using a photonics-enabled receiver and optical phase modulator.

This Letter describes a high-speed yet simple fiber-wireless-fiber system in the 100-GHz band using a photonics-enabled receiver and optical phase modulator. At the antenna site, a millimeter-wave signal is down-converted to the microwave band using an electronic mixer with a local oscillator signal generated remotely using photonic technology. The down-converted signal is converted to an optical signal using an optical phase modulator. At the receiver, a simple direct detection of the phase-modulated signal is performed using optical filtering technology. To demonstrate the proof-of-concept, we successfully transmitted a 64-quadrature amplitude modulation orthogonal frequency-division multiplexing signal with a record line rate of 80 Gb/s (net data rate of 55 Gb/s) over a system consisting of two radio-over-fiber links and a 5 m wireless link in the 100-GHz band. The proposed system with simple antenna sites and optical transceivers can facilitate the deployment of ultra-dense small cells in high-frequency bands in beyond-5G networks.

Seamless Fiber–Wireless System in W-Band Using Optical Phase Modulation and Self-Homodyne Receiver

We propose and demonstrate a seamless fiber–wireless system in the W-band using a simple transmitter, antenna site, and radio receiver. The transmitter utilizes an optical phase modulator instead of an optical in-phase/quadrature modulator for data modulation. A high-output optical-to-radio converter is fabricated and used for radio signal generation at the antenna site. The receiver employs a self-homodyne detector (SHD) that does not require a separate local oscillator source to receive and down-convert the signal to the microwave band. For a proof-of-concept demonstration, we successfully transmitted a high-order quadrature amplitude modulation orthogonal frequency-division multiplexing signal with a bandwidth of up to 4 GHz over the system. The performance of the signal transmission over the system using an envelope detector was also evaluated. Owing to its better received sensitivity, the SHD receiver exhibited better performance. The proposed system can facilitate the deployment of seamless access networks in high-frequency bands in future mobile networks.

Joint Phase Noise Compensation in Dual-Comb Assisted OFDM Receiver

Optical frequency combs (OFC) have revolutionized the field of optics ranging from metrology to radio-frequency photonics. One of the main advantages of an OFC is its spectral purity and broadband phase coherence. An increase in comb linewidth corresponds to elevated phase noise and system performance degradation that mandates larger processing complexity. A specialized processing scheme relying on Vernier OFC pair seeded by a single laser has highly correlated tones over the entire processing band. Recognizing this fact, this report describes a low complexity joint phase noise compensation scheme for orthogonal frequency division multiplexing (OFDM) receiver assisted by dual combs. 4-QAM modulated OFDM signals with 60 subcarriers were demodulated using a dual-comb OFDM receiver. The experiment measured the error vector magnitude (EVM) to validate the receiver performance. The new joint phase noise compensation algorithm can effectively suppress phase noise for high order tones without pilot channels and was shown to be robust under variable SNR levels.

Integration system of photon-assisted W-band millimeter wave wireless communication and free space optical communication based on OFDM signal transmission

Abstract In this paper, we propose and experimentally demonstrate a novel multi-access orthogonal frequency division multiplexing (OFDM) signal transmission system for free space optical communication and photon-assisted W-band millimeter-wave (mm-wave) communication. This new structure can combine the advantages of the free space optical communication and the W-band mm-wave wireless communication. Besides, the OFDM signal with high spectral efficiency and anti-interference capability can improve the data transmission rate and flexibility. Furthermore, it can adapt to the requirements of different communication scenarios. Utilizing the intra-symbol frequency-domain averaging (ISFA) technique and the discrete Fourier transform-spread (DFT-Spread) technique, 1Gbaud 16-ary quadrature amplitude modulation (16QAM) OFDM signal is transmitted successfully with the distance of 100 m for single-mode fiber (SMF), 100 m for FSO link and 4 m for wireless. The BER is under the soft-decision Forward Error Correction (SD-FEC) threshold of 2 × 10−2. To our knowledge, this is the first time to realize the transmission of OFDM signal in the integration system of free space optical communication and photon-assisted W-band mm-wave communication.

Optical Comb Assisted OFDM RF Receiver

Orthogonal frequency-division multiplexing (OFDM) is a widely used signaling format in wireless communication and ultrawide band radio-frequency (RF) applications. An OFDM transceiver relies on a discrete Fourier transform (DFT) processor that matches both the resolution and the bandwidth of wireless link. A conventional wideband OFDM receiver necessarily relies on high-speed analog-to-digital converters (ADC) followed by a Fourier processing core. Both fundamental and practical ADC considerations that include bandwidth-resolution tradeoff impose strict limits on high precision digitization of the wideband OFDM signal. In addition to the quantization barrier, computational complexity of the high-speed DFT processor leads to high dissipation levels in conventional receivers. This report demonstrates an alternative OFDM receiver architecture that eliminates the need for high-speed ADC and digital DFT processor. The new design is based on hybrid, optoelectronic DFT processor, capable of generating DFT coefficients of wideband RF signal in real time. We demonstrate demodulation of a 4-quadratic amplitude modulation OFDM signal with 60 subcarriers positioned within 4–15.8 GHz band and separated by 200 MHz. The error vector magnitude is measured for all channels to characterize the performance of the comb-assisted OFDM RF receiver.

Enhanced noise tolerance for 10 Gb/s Bi-directional cross-wavelength reuse colorless WDM-PON by using spectrally shaped OFDM signals

Abstract Spectrally shaped orthogonal frequency division multiplexing (OFDM) signal for symmetric 10 Gb/s cross-wavelength reuse reflective semiconductor optical amplifier (RSOA) based colorless wavelength division multiplexed passive optical network (WDM-PON) is proposed and further analyzed to support broadband services of next generation high speed optical access networks. The generated OFDM signal has subcarriers in separate frequency ranges for downstream and upstream, such that the re-modulation noise can be effectively minimized in upstream data receiver. Moreover, the cross wavelength reuse approach improves the tolerance against Rayleigh backscattering noise due to the propagation of different wavelengths in the same feeder fiber. The proposed WDM-PON is successfully demonstrated for 25 km fiber with 16-QAM (quadrature amplitude modulation) OFDM signal having bandwidth of 2.5 GHz for 10 Gb/s operation and subcarrier frequencies in 3–5.5 GHz and DC-2.5 GHz for downstream (DS) and upstream (US) transmission respectively. The result shows that the proposed scheme maintains a good bit error rate (BER) performance below the forward error correction (FEC) limit of 3.8 × 10−3 at acceptable receiver sensitivity and provides a high resilience against re-modulation and Rayleigh backscattering noises as well as chromatic dispersion.

A Novel Chaos-Based Encryption Approach for Future-Generation Passive Optical Networks Using SHA-2

A physical layer enhanced secure future generation passive optical network (PON) based on chaotic signal scrambling and the secure hash algorithm (SHA) is proposed and demonstrated. In this paper, the architecture of a wavelength-division multiplexed orthogonal frequency division multiplexing (OFDM) PON system based on a centralized light source using direct detection is analyzed. A logistic chaos map is employed here for the scrambling of OFDM symbols in the frequency and time domains. Before transmitting the signal over optical fiber, a message- digest of scrambled and descrambled OFDM signals is computed and compared at the optical line terminal and optical network unit, respectively, to verify the actual data recovered. For the downstream channel, a 10 Gbps 32 quadrature amplitude modulation OFDM signal is successfully transmitted over a distance of 50 km single-mode fiber within the limits of the peak-to-average-power value. The results show that the proposed scheme can protect the system from bad actors and eavesdroppers, while chaos encryption with SHA-2 provides a robust and promising secure strategy for future-generation PON systems.

SQNR Improvement Enabled by Nonuniform DAC Output Levels for IM-DD OFDM Systems

We propose and demonstrate via both simulation and experiment a scheme for a low-bit resolution digital-to-analog convertor (DAC) to improve the signal-to-quantification noise ratio (SQNR) of an intensity-modulated direct-detection (IM-DD) orthogonal frequency division multiplexing (OFDM) signal. In this scheme, the discrete output levels of a DAC are nonuniform and properly assigned based on the distribution of the OFDM signal. Such nonuniformity allows a better exploitation of output levels for the OFDM signal so that low-level waveforms that appear much more frequently experience mitigated quantization noise. To calculate these optimized output levels with the distribution of an OFDM signal, a numerical algorithm is proposed and verified. Our proposed scheme is proven effective in the simulations for Gaussian-distributed signals and brings over 3.6-dB and ∼2.5-dB improvements on SQNR and receiver sensitivity, respectively. The feasibility is also verified by a proof-of-concept experiment with ∼7.4-Gbps quadrature phase shift keying (QPSK)-modulated OFDM signals output by a 3-bit DAC. Our scheme enables transmission of such signals with no further optimization with ∼−25-dBm receiver sensitivity, while it is impossible using a conventional way. Both simulation and experimental results validate our scheme as a promising way to be implemented in low-bit resolution DACs for IM-DD OFDM applications.

High-Order Moments and Gaussianity of Single-Carrier and OFDM Signals

This paper analyzes the high-order moments of linearly modulated single-carrier as well as orthogonal frequency-division multiplexing (OFDM) signals assuming that the modulated symbols that form resulting continuous signal waveform are statistically independent. For a single-carrier case with the ideal low-pass filter used for pulse shaping, closed-form expressions of the moments with up to the eighth-order are derived. The proposed approach is then extended to Gaussianity assessment of OFDM signals; we show how the moments of OFDM signals can approach to those of Gaussian distribution as the number of subcarriers increases for a given modulation constraint on each subcarrier. Also reported is the peculiar nonstationary behavior of the fourth-order moments in the case of BPSK modulated OFDM signals.

A Low-Cost 16 Quadrature Amplitude Modulation Direct-Detection–Orthogonal Frequency-Division Multiplexing Radio-over-Fiber System Using Low-Cost Direct-Modulation Laser to Generate Optical mm-Wave

This article demonstrates a novel scheme to generate 16 quadrature amplitude modulation orthogonal frequency-division multiplexing signals for radio-over-fiber systems using a low-cost direct-modulation laser to generate an optical millimeter-wave. Mathematical analysis of that system is also investigated. The fiber Bragg grating is employed because the repetitive frequency of the radio frequency source and the bandwidth of the optical modulator are largely reduced, and the architecture of the radio-over-fiber system is simpler. Because no expensive broadband external modulator is used, the overall system is considered a low-cost solution. The simple structure and low cost of the radio-over-fiber system is attractive for the future cost-effective systems.

New Construction Scheme to Reduce the PAPR of M-QAM OFDM Signal

A novel time domain combination scheme is proposed to reduce the peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) systems. in this paper, a high-order quadrature amplitude modulation (QAM) OFDM signal sequence is constructed with several Quaternary/Binary phase shift Keying (QPSK/BPSK) signal sequences. As this operation can be implemented after the inverse fast fourier transform (IFFT), more candidates can be accommodated by the proposed scheme with fewer numbers of IFFT operations. Although for the same number of candidates, the PAPR performance of our proposed scheme is slightly lower than the conventional selected mapping (C-SLM), our scheme significantly decreases the computational complexity. Simulation results also show that the proposed scheme almost achieves the same bit error rate of the C-SLM when they pass through the nonlinear high power amplifier.

IDRG–PTS scheme with low complexity for peak-to-average power ratio reduction in OFDM systems

This article presents a novel peak-to-average power ratio (PAPR) reduction scheme based on the insertion of dummy random Gaussian (IDRG) subcarriers to the data constellation points in the frequency domain integrated with a partial transmit sequence (PTS) technique. The dummy signals are Gaussian random signals which have the same statistical distribution as that of the output signals of the conventional orthogonal frequency division multiplexing (OFDM) system. The integration of IDRG with PTS technique reduces the overall complexity of the OFDM system. Compared to the conventional PTS which requires several inverse fast Fourier transform (IFFT) operations, the proposed IDRG–PTS scheme requires only half as many IFFT operations with comparable PAPR performance. Simulation results have been examined with 16 quadrate amplitude modulation OFDM signals.

A 90-nm CMOS Power Amplifier for 802.16e (WiMAX) Applications

In this paper, we demonstrate a single-stage 90-nm CMOS power amplifier (PA) for 2.3-2.7-GHz WiMAX (802.16e) band applications. An integrated balun is used to match the output to 50-? load. The PA gain and saturated power ( P SAT) are +18 and +32 dBm, respectively, working from a 3.3-V supply, with a peak power-added efficiency of 48%. A digital-predistortion technique is used to enhance the PA linearity. The measured error vector magnitude for a 64 quadrature amplitude modulation orthogonal frequency-division multiplexing signal is improved from -24 to -30 dB at +25-dBm output power. Compliance with the 802.16e standard 10-MHz WiMAX mask and Federal Communications Commission regulations is demonstrated at + 25 dBm of output power with power efficiency of ~ 25% and with a measured second harmonic level of - 31 dBm/MHz. Using a crest factor reduction technique, mask compliance is achieved at + 27 dBm with tradeoff on EVM = -20 dB, sufficient for quadrature phase-shift keying.

BER Analysis of OFDM Systems Impaired by Phase Noise in Frequency-Selective Fading Channels

In this paper, we study the effect of finite-power, phase-locked loop based phase noise on the bit-error-rate (BER) performance of orthogonal frequency division multiplexing (OFDM) systems in frequency-selective fading channels. It is well known the impact of phase noise on the performance of an OFDM system can be divided into a multiplicative term called common phase error (CPE) and an additive term called intercarrier interference (ICI). Based on the conditional Gaussian approximation technique, we first derive the BER formulas for BPSK, QPSK, 16-QAM, and 64-QAM modulated OFDM signals in frequency-selective Rayleigh fading channels. To further quantify the individual influence of the CPE and the ICI on system performance for different phase noise spectra, we derive the BER expressions for perfect CPE compensation cases. The analytical results obtained for frequency-selective Rayleigh fading channels are then generalized to frequency-selective Rician fading channels. Simulation results not only validate the accuracy of our analysis but also show the dependency of BERs on the shapes of phase noise spectra.

Bit-Based SLM Schemes for PAPR Reduction in QAM Modulated OFDM Signals

In this paper, we propose two bit-based selected mapping (SLM) schemes for reducing peak to average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) signals with quadrature amplitude modulation (QAM), called bitwise SLM (BSLM) and partial bit inverted SLM (PBISLM). Contrary to the conventional SLM which rotates the phases of QAM symbols in the frequency domain, the proposed schemes change the magnitudes as well as the phases of QAM symbols by applying binary phase sequences to the binary data sequence before mapped to QAM symbols. Simulation results show that the proposed schemes have better PAPR reduction performance with shaping gain than the conventional SLM scheme for the QAM modulated OFDM signals, especially for the small number of subcarriers.