Transmitted Orthogonal Frequency Division Multiplexing Signal Research Articles

Dual high-order QAM-modulated mm-wave signal transmission in the Q-band enabled by simple IM/DD architecture and bandpass delta-sigma modulation.

The intensity-modulation (IM)/direct-detection (DD) systems have been proven effective and low-cost due to their simple system architecture. However, the Mach-Zehnder modulator (MZM) of the IM/DD systems only reserves its driving signal intensity. Therefore, the IM/DD systems are generally unable to transmit vector signals and have a restricted spectrum efficiency and channel capacity. Similarly, the radio-over-fiber (RoF) transmission systems based on IM/DD are limited by their simple architecture and generally cannot transmit high-order quadrature amplitude modulation (QAM) signals, which hinders the improvement of their spectrum efficiency. To address the challenges, we propose a novel, to the best of our knowledge, scheme to simultaneously transmit the dual independent high-order QAM-modulated millimeter-wave (mm-wave) signals in the RoF system with a simple IM/DD architecture, enabled by precoding-based optical carrier suppression (OCS) modulation and bandpass delta-sigma modulation (BP-DSM). The dual independent signals can carry different information, which increases channel capacity and improves spectrum efficiency and system flexibility. Based on our proposed scheme, we experimentally demonstrate the dual 512-QAM mm-wave signal transmission in the Q-band (33-50 GHz) under three different scenarios: 1) dual single-carrier (SC) signal transmission, 2) dual orthogonal-frequency-division-multiplexing (OFDM) signal transmission, and 3) hybrid SC and OFDM signal transmission. We achieve high-fidelity transmission of dual 512-QAM vector signals over a 5 km single-mode fiber (SMF) and a 1-m single-input single-output (SISO) wireless link operating in the Q-band, with the bit error rates (BERs) of all three scenarios below the hard decision forward error correction (HD-FEC) threshold of 3.8 × 10-3. To the best of our knowledge, this is the first time dual high-order QAM-modulated mm-wave signal transmission has been achieved in a RoF system with a simple IM/DD architecture.

4096-QAM OFDM THz-Over-Fiber MIMO Transmission Using Delta-Sigma Modulation

A terahertz (THz)-over-fiber multiple-input multiple-output (MIMO) transmission of 4096-ary quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) supported by 46-GBaud quadrature phase shift keying (QPSK) signal at 320 GHz is experimentally demonstrated in this letter. By employing the polarization division multiplexing (PDM) technique and 1-bit delta-sigma modulation (DSM) quantization, we successfully implement 4.6-GHz 4096-QAM OFDM signal transmission over 20 km of standard single-mode fiber (SSMF) and a 3-m MIMO wireless link, and satisfy the soft-decision forward error correction (SD-FEC) threshold of 1.0 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> with 15% overhead, which corresponds to a net rate of 84.4 Gbps for THz-over-fiber transmission. To the best of our knowledge, fusing OFDM and PDM, this is the highest rate achievable for DSM applications in THz communication. This experimental demonstration can provide a promising solution for high-order QAM signals in a fiber-wireless seamless network in future 6G networks.

High security optical OFDM transmission scheme with four-dimensional region joint encryption based on power division multiplexing.

In this paper, a high security chaotic encryption scheme for orthogonal frequency division multiplexing (OFDM) transmission system is proposed by using power division multiplexing (PDM) technology and four-dimensional region joint encryption. The scheme uses PDM to realize simultaneous transmission of multiple user information, which can achieve a good compromise among system capacity, spectral efficiency and user fairness. In addition, bit cycle encryption, constellation rotation disturbance (CRD) and region joint constellation disturbance (RJCD) are used to realize four-dimensional region joint encryption, effectively improving the physical layer security. The masking factor is generated by the mapping of two-level chaotic systems, which can enhance the nonlinear dynamics and improve the sensitivity of encrypted system. A 11.76 Gb/s OFDM signal transmission over 25 km standard single-mode fiber (SSMF) is experimentally demonstrated. At the forward-error correction (FEC) bit error rate (BER) limit -3.8×10-3, the proposed receiver optical power based on quadrature phase shift keying (QPSK) without encryption, QPSK with encryption, variant-8quadrature amplitude modulation (V-8QAM) without encryption and V-8QAM with encryption are about -13.5dBm, -13.6dBm, -12.2dBm, and -12.1dBm. The key space is up to 10128. The results show that this scheme not only improves the security of the system and the ability to resist attackers, but also improves the system capacity and has the potential to serve more users. It has a good application prospect in the future optical network.

Receive-diversity-aided power-fading compensation for C-band IM/DD OFDM systems.

A receive-diversity-aided power-fading compensation (RDA-PFC) scheme is proposed and demonstrated to eliminate the chromatic dispersion (CD)-induced power fading for C-band double-sideband (DSB) intensity modulation and direct detection (IM/DD) orthogonal frequency division multiplexing (OFDM) systems. By combining the responses before and after a dispersive element using a maximal-ratio combining (MRC) algorithm, the CD-induced power fading dips within the signal bandwidth of around 50 GHz can be effectively compensated for, which results in an up to 17.6-dB signal-to-noise ratio (SNR) improvement for the fading subcarriers after transmission over 10 km of standard single-mode fiber (SSMF). Using the 16 quadrature amplitude modulation (QAM) format, a diversity receiver with the proposed RDA-PFC scheme can support 170.6-Gbit/s OFDM signal transmission over a 10-km SSMF and reduces the bit error rate (BER) by more than an order of magnitude compared with a conventional receiver. Moreover, 208.1-Gbit/s adaptive bit and power loading OFDM signal transmission over a 10-km SSMF is realized by the proposed RDA-PFC scheme, which improves the capacity by 15.3% compared with the case without RDA-PFC at a BER of 3.8 × 10-3. The proposed RDA-PFC scheme shows great potential in CD-induced power-fading compensation for high-speed IM/DD OFDM systems.

65,536-QAM OFDM signal transmission over a fiber-THz system at 320 GHz with delta-sigma modulation.

The transmission of a 65,536-ary quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) signal supported by a hybrid fiber-terahertz (THz) multiple-input multiple-output (MIMO) system at 320 GHz is experimentally demonstrated in this Letter. We adopt the polarization division multiplexing (PDM) technique to double the spectral efficiency. Based on a 23-GBaud 16-QAM link, 2-bit delta-sigma modulation (DSM) quantization enables 65,536-QAM OFDM signal transmission over a 20-km standard single-mode fiber (SSMF) and a 3-m 2 × 2 MIMO wireless delivery, and satisfies the hard-decision forward error correction (HD-FEC) threshold of 3.8 × 10-3, corresponding to a net rate of 60.5 Gbit/s for THz-over-fiber transport. Meanwhile, below the fronthaul error vector magnitude (EVM) threshold of 0.34%, a maximum signal-to-noise ratio (SNR) of 52.6 dB is achieved. To the best of our knowledge, this is the highest modulation order achievable for DSM applications in THz communication.

CNN-Attention-DNN Design for CFO Estimation of Non-Pilot-Assisted OFDM System

In this letter, we propose a novel attention mechanism inspired convolutional neural network (CNN)-deep neural network (DNN) called CAD model architecture for carrier frequency offset (CFO) estimation in orthogonal frequency division multiplexing (OFDM) systems. The hyperparameters used in the proposed CAD model are optimized using the Bayesian approach. The proposed CFO estimation scheme is robust to small symbol timing offset errors and does not require apriori knowledge of modulation format, channel, and transmitted OFDM signal parameters. Simulations performed over the realistic frequency-selective channel models with and without power amplifier non-linearity indicate that the proposed CAD architecture outperforms the conventional statistical-based and recurrent neural network (RNN)-based models. Lastly, the proposed CAD model is validated on the radio frequency testbed.

STO Estimation for OFDM System Using CDM

In this letter, we propose a novel scheme for symbol timing offset (STO) estimation by using a convolutional neural network (CNN)-deep neural network (DNN) model (CDM) architecture for the orthogonal frequency division multiplexing (OFDM) system over different fading channel models. The proposed scheme estimates STO in the presence of carrier frequency offset (CFO) and without prior knowledge of the channel, modulation format, and transmitted OFDM signal parameters. The CDM architecture achieves better estimation accuracy gain compared to that of statistical-based and pilot-assisted CNN-based methods. Finally, the proposed CDM is validated over a radio frequency testbed, and the desired constellation diagram is obtained.

RIS-Enabled SISO Localization Under User Mobility and Spatial-Wideband Effects

Reconfigurable intelligent surface (RIS) is a promising technological enabler for the 6th generation (6G) of wireless systems with applications in localization and communication. In this paper, we consider the problem of positioning a single-antenna user in 3D space based on the received signal from a single-antenna base station and reflected signal from an RIS by taking into account the mobility of the user and spatial-wideband (WB) effects. To do so, we first derive the spatial-WB channel model under the far-field assumption, for orthogonal frequency-division multiplexing signal transmission with the user having a constant velocity. We derive the Cramer Rao bounds to serve as a benchmark. Furthermore, we devise a low-complexity estimator that attains the bounds in high signal-to-noise ratios. Our estimator neglects the spatial-WB effects and deals with the user mobility by estimating the radial velocities and compensating for their effects in an iterative fashion. We show that the spatial-WB effects can degrade the localization accuracy for large RIS sizes and large signal bandwidths as the direction of arrival or departure deviate from the RIS normal. In particular, for a 64X64 RIS, the proposed estimator is resilient against the spatial-WB effects up to 140 MHz bandwidth. Regarding user mobility, our results suggest that the velocity of the user influences neither the bounds nor the accuracy of our estimator. Specifically, we observe that the state of the user with a high speed (42 m/s) can be estimated virtually with the same accuracy as a static user.

A Novel Companding Technique to Reduce High Peak to Average Power Ratio in OFDM Systems

The reduction of the high peak-to-average-power ratio (PAPR) is important to the efficiency of the orthogonal frequency division multiplexing (OFDM) technique. Excessive PAPR contributes to nonlinear clipping induced harmonic distortions that reduce system reliability. In this article, a new technique for decreasing the high PAPR in OFDM with minimum effects on the system performance is proposed. The technique uses the image adjust (IMADJS) function to reduce the high PAPR of transmitted OFDM signals by compressing large signals and expanding small signals. In comparison, the IMADJS strategy has the advantage of maintaining a constant average power level before and after companding. A comparative analysis is provided between the proposed (IMADJS) technique and well-known companding techniques such as μ-law, absolute exponential (AEXP), and the new error function (NERF). The comparison is based on PAPR, bit error rate (BER), power spectral density (PSD), and average power performance metrics. Simulation results confirm that the IMADJS technique significantly improved the drawbacks of the PAPR. Furthermore, the PAPR is reduced by 2.81dB. The IMADJS technique has less impact on the original power spectrum than on other companding schemes. The average out-of-band radiation of the IMADJS technique reaches about -50dB at the frequency of 0MHz. In contrast, the average of the original OFDM signal is that the out-of-band radiation reaches around -52dB. The AEXP and NERF companding techniques reach about -46dB, while the μ-law companding technique hits about -37dB.

Hybrid wideband multipath self-interference cancellation with an LMS pre-adaptive filter for in-band full-duplex OFDM signal transmission.

A wideband multipath self-interference cancellation (SIC) system employing both dual-drive Mach-Zehnder modulator-based analog SIC and least mean square (LMS) algorithm-based pre-adaptive filter digital SIC is proposed and demonstrated for the cancellation of multipath self-interference (SI) and facilitation of in-band full-duplex (IBFD) orthogonal frequency-division multiplexing (OFDM) signal transmission. The multipath effect is an unavoidable challenge in SIC due to the dynamic and unpredictable properties in each path, as well as the need for separate matching components for compensating for each path. In this Letter, an LMS algorithm-based adaptive filter is used as a pre-equalizer to adapt and generate the matching signal to the closest approximate of the multipath SI signal. The adaptation is based on the minimization of the error signal generated from the matching signal and multipath SI signal in the LMS algorithm. With the introduction of the LMS adaptive filter to the analog SIC, an additional 9 dB cancellation improvement is obtained, resulting in a total of 32 dB cancellation depth over a cancellation bandwidth of 2.7 GHz at a center frequency of 1.65 GHz. To the best of our knowledge, the achieved performance is by far the widest cancellation bandwidth in a multipath SIC system, which is essential in a large bandwidth and high data rate transmission system. With the help of the proposed LMS adaptive filter digital SIC assisted analog SIC located at the remote node, power-efficient IBFD transmission of an OFDM signal through a 25 km fiber is experimentally demonstrated with a 6 dB bit error rate and 8% error vector magnitude improvements.

Massive MIMO With Downlink Energy Efficiency Operation in Industrial Internet of Things

In this article, we present high energy-efficiency (EE) operation schemes for downlink orthogonal frequency division multiplexing (OFDM)-based massive multiple-input multiple-output (MIMO) systems in industrial Internet of Things (IIoT) networks. We derive the exact closed-form expressions for some important downlink operation parameters, and based on the parameters, we propose the methods of systematic operation to satisfy the required EE metric with low latency. The operation parameters include the number of service antennas, downlink transmission power, and related coverage. To increase the EE of downlink OFDM signal transmission, it is also well known that peak-to-average power ratio (PAPR) reduction is particularly important, and we apply the clipping PAPR reduction technique for the purpose to increase the power amplifier efficiency, and thus we also determine the level of clipping to satisfy the EE requirement. We use two representative linear precoding schemes for massive MIMO, such as maximum ratio (MR) precoding and zero-forcing (ZF) precoding, and show that the amount of clipping distortion of both MR precoding and ZF precoding is proportional to the distortion from the corresponding IIoT device's signal and distortions from other IIoT devices' signals. Simulation results are provided to validate the analysis and related schemes.

Noise and Distortion Reduction in OFDM Radio-Over-Fiber Link by Graded-Index Plastic Optical Fiber

We demonstrate that a graded-index plastic optical fiber (GI POF) enables higher-quality radio-over-fiber (RoF) transmission of an orthogonal frequency-division multiplexing (OFDM) signal, compared with a conventional multimode fiber, for short-distance household applications (< 100 m). It is shown that the improvement in OFDM signal transmission, using the GI POF, can be attributed to the reduction in noise and intermodulation distortion, owing to the external optical feedback from the transmission fibers. The low noise and low distortion in the signal transmission are mainly due to the strong mode coupling in the GI POF, which is closely related to the microscopic heterogeneous structures in core polymer materials. The developed GI POF will be invaluable for RoF networks in households as a solution for indoor broadband wireless communications in the upcoming 5G era.

Companding Schemes for Reducing PAPR in OFDM System: A Review

AbstractHigh PAPR poses a large hindrance to high data rate Orthogonal Frequency Division Multiplexing (OFDM) signals. A companding transform converts the Probability distribution function (PDF) of the transmitted OFDM signal in a way which reduces the Peak-to-average power-ratio (PAPR). In this paper, we have performed the study of companding techniques and analyzes the PAPR and BER performances of these schemes. Here, we have considered μ-law companding, exponential companding, trapezoidal companding (TC) and trigonal companding for comparison. In this paper, we have demonstrated that μ-law companding, exponential companding and trigonal companding schemes have no adaptability in designing the companding function, and along these lines an exchange off between BER performance and PAPR reduction ability is unrealistic to accomplish. However, TC scheme has two parameters (a and b) to design the companding function and hence a trade-off between PAPR reduction capability and BER performance is possible to achieve. BER performance and PAPR reduction capabilities of the scheme under consideration are evaluated and compared through computer simulations. We have also presented an approach to obtain the minimum BER and PAPR of TC and found out the values of design parameters (a and b) for which TC scheme has minimum BER and PAPR.

Polar Coded OFDM Signal Transmission at the W-Band in Millimeter-Wave System

We experimentally demonstrate the transmission of orthogonal frequency division multiplexing (OFDM) signals with polar codes FEC in a W-band MMW system. The BER performances of the M-QAM OFDM signal with FEC and without FEC are investigated, respectively. The experimental results show that the polar codes have a net gain of 7.9 dB compare with that of the LDPC code (7.4 dB). To the best of our knowledge, this is the first demonstration that the polar codes FEC are used in a W-band MMW system with OFDM modulation.

A Communication Method between High-speed UUV and Distributed Intelligent Nodes

In this paper, a doppler distortion compensation method for underwater transmission of OFDM (Orthogonal Frequency Division Multiplexing) signals is proposed, which is used to realize real-time underwater acoustic communication between high-speed UUV (Unmanned Underwater Vehicle) and intelligent nodes. The method decomposes channel variations based on a functions with a set of known equations. OFDM symbol reconstruction is performed after the Taylor(T) FFT (Fast Fourier Transformation) process. We design an adaptive stochastic gradient descent algorithm based on MMSE criterion to learn the combiner weights for differentially coherent detection, thereby achieving adaptive channel equalization in the underwater acoustic channel. Comprehensive data and experimental data from the recent mobile acoustic communication delta are used to demonstrate the feasibility of this approach. The method can realize high-speed underwater acoustic communication, provide data sample basis for long-term big data analysis, and realize real-time communication result analysis.

Adaptive OFDM Waveform Design for Cognitive Radar in Signal-Dependent Clutter

The problem of adaptive orthogonal frequency division multiplexing (OFDM) waveform design in the case of stochastic extended targets in the presence of strong and nonstationary interferences is an essential part of a cognitive radar system. In this article, a new approach is developed based on maximizing the mutual information between the random target impulse response and the received echoes with respect to the transmitted waveform. We use the advantage of OFDM signals and present an adaptive structure to match the complex weights of OFDM signal with the radar scene for making complete use of the transmission power. The interference reflections are utilized rather than eliminating them to design the weights of the transmitted OFDM signal. The optimization algorithm for designing transmitted signals is based on adaptive learning whose essential information is obtained by a feedback between receiver and transmitter. This feedback includes essential target and interference characteristics which extracted from the reflected echoes. Based on changes in interference features, the weights of the transmitted OFDM signal on each subcarrier are designed and transmitted in the next coherent processing interval. Experimental results reveal that the proposed approach achieves a higher rate of performance compared to nonadaptive waveform design approaches.

A Clipping-Noise Compression Method to Reduce PAPR of OFDM Signals

Iterative clipping and filtering (ICF) is widely employed in the orthogonal frequency-division multiplexing (OFDM) systems to reduce the peak-to-average power ratio (PAPR). However, the conventional ICF-based methods usually involve numerous transformations between the frequency domain and the time domain and run into numbers of iteration. This letter proposes a clipping-noise compression method, in which the transmitted OFDM signal is modified in the time domain by clipping and noise compression. The computational complexity is greatly reduced owing to the fact that only one fast Fourier transform is needed in the approach. Furthermore, the proposed method exhibits better PAPR reduction and bit error rate (BER) performance in the simulations compared with the conventional ICF-based methods.

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.

32-GS/s 6-bit DAC based on SiGe technology for IM-DD OFDM systems with non-uniform quantization.

In this paper, we design and fabricate a 32-GS/s 6-bit digital-to-analog convertor (DAC) based on SiGe technology. The DAC uses double sampling technique and segment current steering architecture to achieve high dynamic linearity. A corrector circuit is proposed to suppress the duty cycle error for linearity optimization. The spurious free dynamic range (SFDR) is above 31.5 dBc over the Nyquist bandwidth at the sampling rate of 32-GS/s. A full-rate input interface is integrated to realize data exchange with field-programmable gate array (FPGA). The experimental results show that the net data rate of 50-Gb/s orthogonal frequency division multiplexing (OFDM) signals with non-uniform quantization can be generated by the designed and fabricated DAC. The OFDM signal transmission over 10-km standard single mode fiber (SSMF) with 64-quadrature amplitude modulation (QAM) and 128-QAM formats have been successfully achieved.

In-Service BER Based Estimation of OFDM PAPR and CFO-induced Peak Phase Deviation

High Peak-to-Average Power Ratio (PAPR) of the transmitted OFDM signal is a well-known major drawback of the Orthogonal Frequency-Division Multiplexing (OFDM), so the High-Power Amplifier (HPA) is therefore necessary to operate in its linear region, i.e. with large back-off between the operating input power and its saturation region, so introducing not only in-band distortion, but also the adjacent channel interference. Specifically with the Long-Term Evolution (LTE) systems downlink, some sort of PAPR reduction, such as e.g. clipping, must be utilized. Considering that in many practical situations, determining PAPR demands complex test equipment, such as e.g. Vector Signal Analyzer (VSA), which might not be available, in this paper, we develop a simple Bit-Error-Rate (BER) based model for the (residual) PAPR estimation, by applying link abstraction, i.e. considering the easy measurable BER degradation due to HPA non-linearity, as if it were the consequence of the according level of additive white Gaussian noise (AWGN) abstracting the HPA distortion, while considering high Signal-to-Noise Ratio (SNR) and long enough cyclic prefix (CP), thus neglecting (real) additive noise and time dispersion (i.e. multipath fading). Moreover, the out-of-service BER testing, which requires network operator to interrupt its revenue-generating traffic, can be substituted by in-service BER estimation from in-phase and quadrature-phase eye diagram closures, measured on live traffic, by means of a simple oscilloscope. The analytical model is verified by the appropriate Monte-Carlo simulations.