Baseband Orthogonal Frequency Division Multiplexing Research Articles

Joint Analog and Digital Transceiver Design for Wideband Full Duplex MIMO Systems

In this paper, we propose a wideband Full Duplex (FD) Multiple-Input Multiple-Output (MIMO) communication system comprising of an FD MIMO node simultaneously communicating with two multi-antenna UpLink (UL) and DownLink (DL) nodes utilizing the same time and frequency resources. To suppress the strong Self-Interference (SI) signal due to simultaneous transmission and reception in FD MIMO systems, we propose a joint design of Analog and Digital (A/D) cancellation as well as transmit and receive beamforming capitalizing on baseband Orthogonal Frequency-Division Multiplexing (OFDM) signal modeling. Considering practical transmitter impairments, we present a multi-tap wideband analog canceller architecture whose number of taps does not scale with the number of transceiver antennas and multipath SI components. We also propose a novel adaptive digital cancellation based on truncated singular value decomposition that reduces the residual SI signal estimation parameters. To maximize the FD sum rate, a joint optimization framework is presented for A/D cancellation and digital beamforming. Finally, our extensive waveform simulation results demonstrate that the proposed wideband FD MIMO design exhibits higher SI cancellation capability with reduced complexity compared to existing cancellation techniques, resulting in improved achievable rate performance.

Constant-Envelope OFDM for Power-Efficient and Nonlinearity-Tolerant Heterodyne MMW-RoF System With Envelope Detection

The radio over fiber (RoF) system using optical heterodyne (OH) for broadband up-conversion and envelope detection (ED) for phase-noise-insensitive down-conversion stands out as a simple, robust, and cost-effective solution for exploiting the abundant bandwidth resource at the millimeter-wave (MMW) band. In this paper, we dedicate efforts to experimentally investigate such an MMW-RoF system based on the constant-envelope orthogonal frequency division multiplexing (OFDM) (CE-OFDM) technique. A critical benefit of employing CE-OFDM is its excellent tolerance to the nonlinear distortions, which are essential for the wide acceptance of analog RoF systems. Also, thanks to the analog angle modulation transforming the OFDM signal to a constant envelope signal, the carrier-suppressed double-sideband (CS-DSB) modulation can be implemented for encoding the CE-OFDM signal onto the optical carrier, leading to significant improvements in power efficiencies of both fiber and radiofrequency (RF) links. In experiments, a 60-GHz OH-ED MMW-RoF system is established, of which the distances of fiber and MMW wireless transmissions are 10 km and 1.2 m respectively. Under the case of 2-GHz 64-quadrature amplitude modulation (QAM) OFDM baseband signal, the achieved experimental results show that the proposed OH-ED MMW-RoF system outperforms its counterpart using the conventional OFDM in terms of nonlinearity insensitivity, RF and optical power efficiency enhancements (indicated by over 11.5-dB and 3.2-dB reductions in the lowest transmit RF and receive optical powers for satisfying the 8% error-vector-magnitude (EVM) limit). In addition, when the QAM order of the 2.5-GHz OFDM baseband signal reaches 256, the CS-DSB CE-OFDM scheme enables an acceptable EVM performance of 4.43%.

An SNR-improved Transmitter of Delta-sigma Modulation Supported Ultra-High-Order QAM Signal for Fronthaul/WiFi Applications

In this paper, we proposed an improved mobile fronthaul architecture employing the SNR-improved delta-sigma digitization scheme and 4-level pulse amplitude modulation (PAM-4) format. Different from traditional 2-bit quantization, this scheme deploys two-fold delta-sigma quantization, which uses 1-bit delta-sigma modulator to quantize the signal, and another 1-bit delta-sigma modulator to quantize the in-band noise. A significant reduction of the in-band noise can be achieved only applying a differentiator, bringing about a much better noise shaping performance. Meanwhile, the two 1-bit streams can be combined and transmitted in a PAM-4 manner through an intensity modulation direct detection (IM-DD) channel. We have successfully experimentally demonstrated the digitization and transmission of 65536 quadrature amplitude modulation (QAM) baseband orthogonal frequency division multiplexing (OFDM) signal with sampling rate of 1.25 GSa/s over 20-km fiber with standard 10-Gbaud PAM-4 signal, with a signal to noise ratio (SNR) of 57.7 dB. Compared to the conventional 2-bit quantized OFDM scheme, experimental results show that a significant SNR improvement of 17.7 dB has been achieved by the proposed scheme. In addition, we also realized a transmission of 16384-QAM intermediate frequency (IF) signal at the center frequency of 3.5 GHz (with an SNR of 49.6 dB). An improvement of 15.9 dB can also be maintained.

Nonlinear Channel Estimation for Minimally Sampled Baseband OFDM Systems

The orthogonal frequency-division multiplexing (OFDM) is a widely used modulation scheme in modern digital communication systems. An OFDM signal can be susceptible to channel nonlinearities due to its high peak-to-average power ratio (PAR). This makes estimation and compensation of nonlinear channels particularly vital for OFDM systems. In this paper, we consider identifying cubically nonlinear channels in baseband OFDM systems using minimally sampled input and output signals. Unlike nonlinear channels in bandpass OFDM systems, which can be modeled by a complex baseband equivalent Volterra series with only odd order terms, the nonlinear channel in a baseband OFDM system requires a real Volterra series with both even and odd order terms. The minimally sampled input and output signals of the baseband OFDM system are used to estimate the frequency-domain Volterra kernels of the nonlinear channel. Due to the possible spectral regrowth caused by the channel nonlinearities, the minimally sampled output signal could be aliased. Nevertheless, by conducting higher-order statistical analyses on the OFDM signals, we show that closed form expressions of the Volterra kernels can still be derived. As a result of the closed form expressions, a simple estimate for the Volterra kernels is attained. Furthermore, the derived Volterra kernel estimate is shown to achieve the minimum mean square error (MMSE). Therefore, the proposed approach indeed provides an efficient yet accurate way to estimate nonlinear channels in baseband OFDM systems.

Feasibility of Modeling Orthogonal Frequency-Division Multiplexing Communication Signals with Unsupervised Generative Adversarial Network.

High-quality recordings of radio frequency (RF) emissions from commercial communication hardware in realistic environments are often needed to develop and assess spectrum-sharing technologies and practices, e.g., for training and testing spectrum sensing algorithms and for interference testing. Unfortunately, the time-consuming, expensive nature of such data collections together with data-sharing restrictions pose significant challenges that limit data set availability. Furthermore, developing accurate models of real-world RF emissions from first principles is often very difficult because system parameters and implementation details are at best only partially known, and complex system dynamics are difficult to characterize. Hence, there is a need for flexible, data-driven methods that can leverage existing data sets to synthesize additional similar waveforms. One promising machine-learning approach is unsupervised deep generative modeling with generative adversarial networks (GANs). To date, GANs for RF communication signals have not been studied thoroughly. In this paper, we present the first in-depth investigation of generated signal fidelity for GANs trained with baseband orthogonal frequency-division multiplexing (OFDM) signals, where each subcarrier is digitally modulated with quadrature amplitude modulation (QAM). Building on prior GAN methods, we developed two novel GAN models and evaluated their performance using simulated data sets with known ground truth. Specifically, we investigated model performance with respect to increasing data set complexity over a range of OFDM parameters and conditions, including fading channels. The findings presented here inform the feasibility of use cases and provide a foundation for further investigations into deep generative models for RF communication signals.

A novel method for PAPR reduction of the OFDM signal using nonlinear scaling and FM

Orthogonal frequency division multiplexing (OFDM) has been adopted as standard beginning with the 4th generation mobile communication system because of its high-bit-rate transmission capability under frequency selective fading channel conditions. However, a major disadvantage of OFDM is the non-constant envelope signal with a high peak-to-average power ratio (PAPR). The high peak signal in OFDM is distorted through a nonlinear amplifier, which causes bit error ratio (BER) reduction. Many techniques have been developed for reducing PAPR at the cost of inefficient bandwidth usage or throughput because of the additional information about PAPR reduction. We propose a novel method, in which the high peak signal above the threshold of the nonlinear amplification region is nonlinearly downscaled to lower the PAPR. The time slot location and scaling ratio for where and how the high peak baseband OFDM signal is downscaled are transmitted using frequency modulation (FM) combined with OFDM, which requires less additional bandwidth than the previously proposed methods. Simulation results show that the proposed novel method provides a lower PAPR and elicits a better BER performance compared with other conventional methods, because it reduces the PAPR by nonlinear scaling and restores the pre-distorted signal using FM.

Understanding Normalized Mean Squared Error in Power Amplifier Linearization

This letter provides a detailed analysis of the normalized mean squared error (NMSE) of an ideal orthogonal frequency-division multiplexing (OFDM) system, subject to soft clipping of the power amplifier (PA) output. Based on the central limit theorem, the OFDM baseband input is modeled as white Gaussian noise, and based on the Bussgang theorem, the NMSE is studied in some detail. Asymptotes describing the NMSE for the considered system are derived, when the system is working in the linear operation mode and when working under compression, respectively. Detailed yet simple expressions between the NMSE, average output power, saturation voltage of the PA, and the noise level of the additive channel or thermal noise are derived, which are believed to provide insight into the system's behavior.

Spectrally efficient digital mobile fronthaul with discrete cosine transform and multi-band quantization.

A spectrally efficient digital mobile fronthaul (MFH) with discrete cosine transform and multi-band quantization (DCT-MBQ) is first proposed and experimentally demonstrated. In this approach, the baseband orthogonal frequency division multiplexing (OFDM) symbols are segmented into individual blocks and independently transformed by DCT to obtain a set of transform coefficients. Different from digitizing each OFDM sample with the same number of quantization bits (QBs) in previous digital MFH, MBQ is introduced to separately digitize different parts of the coefficients with different numbers of QBs according to the significance of these coefficients. Furthermore, the part of transform coefficients with little significance is discarded. Thus, the total number of required QBs for digitizing an OFDM symbol can be greatly reduced and, thereby, high bandwidth efficiency can be achieved. In the experiment, the proposed DCT-MBQ-based digital MFH transmission is demonstrated over a 25Gb/s four-level pulse amplitude modulation intensity modulation-direct detection optical link supporting up to 16×100 MHz 5G new radio carriers with 1024 QAM, achieving an error vector magnitude of 0.65%. The equivalent common public radio interface rates from 55 to 158Gb/s are supported.

HRO-OFDM Scheme Design and Optimization for a Hybrid RF/VLC Baseband System

A hybrid radio frequency (RF) and visible light communication (VLC) system can provide better data rate performance and user experience, and only the hybrid RF/VLC systems that use independent RF access points (APs) and VLC APs have been currently considered. However, a hybrid design in baseband can more centrally allocate various resources. In this paper, a new hybrid RF/VLC orthogonal frequency division multiplexing (OFDM) baseband is presented. Different from conventional OFDM baseband, the new baseband combines both RF link and VLC link in the physical layer. IQ modulation is used in RF link, whereas intensity modulation and direct detection are used in VLC link. Furthermore, the proposed hybrid of radio and optical OFDM (HRO-OFDM) scheme can better combine the different features in RF and VLC link. Based on the new hybrid baseband and HRO-OFDM, we improve the conventional optimization and consider both power allocation and bandwidth allocation. Compared with some conventional designs like RF OFDM baseband design, VLC OFDM baseband design and hybrid RF/VLC design, results show that our new design can achieve better performance and adaptively allocate bandwidth in RF/VLC links under the different distances between user and the baseband.

Covet underwater acoustic communication based on spread spectrum orthogonal frequency division multiplexing (OFDM)

A covert underwater acoustic communication (UAC) method is realized with a dolphin whistle as the information carrier. The proposed method jointly utilizes the spread spectrum and orthogonal frequency division multiplexing (OFDM) techniques. The original dolphin whistle is represented as the complex baseband OFDM transmitted signal and the phase of its discrete Fourier transform (DFT) coefficients are modulated by a spread spectrum code to carry information. The audio quality and waveform similarity are used as two covert effect criterion. In simulations, the influences of the code length and modulation parameters are investigated. It is verified that the communication performance and camouflage effect are a pair of contradictions. Therefore, the best compromise formula should be chosen according to the actual demands.

Design and implementation of application‐specific instruction‐set processor design for high‐throughput multi‐standard wireless orthogonal frequency division multiplexing baseband processor

The two implementation choices for the baseband part of wireless radios are the application-specific platforms (e.g. application-specific integrated circuits (ASICs)) and the programmable processors (e.g. digital signal processors (DSPs)). An application-specific instruction-set processor (ASIP) is a customised processor that bridges the gap between the two platforms. In this work, a novel implementation of the signal processing part of an orthogonal frequency division multiplexing (OFDM) baseband processor using three ASIPs is presented. The ASIPs provide novel architectures for the symbol chain, including fast Fourier transform, channel estimation subsystem and synchronisation subsystem. This design provides a close to DSPs level of flexibility, making it suitable for supporting all the modes of a large number of OFDM standards. In the meantime, the system maintains a performance level comparable to ASICs. This is demonstrated by providing post-layout results for 0.13 μm Taiwan semiconductor manufacturing company complementary metal-oxide semiconductor technology.

Real-Time 10.4-Gb/s Single-Band Optical 256/64/16QAM Receiver for OFDM-PON

In this letter, a 10.4-Gb/s real-time baseband orthogonal frequency division multiplexing (OFDM) receiver utilizing 256/64/16-quadrature amplitude modulation (QAM) adaptive modulation format, incorporating several key functionalities, including symbol synchronization, channel equalization, as well as sampling clock frequency synchronization is successfully implemented using a commercial field programmable gate array chip and a sample rate of 5-GS/s analog-to-digital converter. The performance of the real-time baseband OFDM receiver is experimentally investigated in an intensity-modulation and direct-detection optical communication system. The offline generated 10.4-Gb/s adaptively modulated 256/64/16QAM encoded optical OFDM signal with a high spectral efficiency of 4.84 bit/s/Hz can be successful transmitted over 20-km standard single mode fiber with a bit error rate <;3.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . In addition, the frame miss probability is real-time measured in the receiver to obtain an optimal threshold value for the symbol synchronization.

Detecting OFDM Signals in Alpha-Stable Noise

This paper analyzes various receiver schemes for orthogonal frequency division multiplexing (OFDM) transmission in impulsive noise. We consider Rayleigh block-fading and model the noise process by additive white symmetric α-stable noise (AWSαSN). We begin by discussing maximum-likelihood (ML) detection performance of baseband OFDM. Though optimal, the computational cost increases exponentially with the number of carriers. Alternatively, one may evaluate soft-estimates of the transmitted symbol block and employ carrier-wise detection to lower computational complexity. We analyze such schemes under the frameworks of M-estimation and compressed sensing theory. Moving on, we highlight important rules that ensure the passband AWSαSN process is converted to a baseband form suitable for the discussed schemes. Finally, it is shown that linear passband-to-baseband conversion actually reduces the signal-to-noise ratio (SNR) at the receiver and that all these rules may be avoided by applying an estimation scheme directly on the passband samples.

Experimental investigations in transmission performance of real-time long-reach adaptively modulated direct-detection optical-orthogonal frequency division multiplexing systems

A real-time base-band orthogonal frequency division multiplexing (OFDM) transceiver with symbol synchronization, channel equalization, sampling clock frequency synchronization, and adaptive modulation technique is successfully implemented by field programmable gate arrays and a 2.5-GSps digital-to-analog converter and analog-to-digital converter. The real-time optical OFDM signal at a raw bit rate of 5.156 Gbps within about 1.1-GHz bandwidth transmission over 100-km standard single-mode fiber (SSMF) is experimentally investigated in a simple intensity-modulation and direct-detection system. The experimental results show that the real-time system has a good bit error rate (BER) performance by using an adaptive modulation technique according to the conditions on the subchannels. After 100-km SSMF transmission, at a BER of 1×10−3, the power penalty is <1 dB. Moreover, there is a negligible penalty between the off-line and real-time digital signal processing results.

I/Q Channel Separated Baseband OFDM Optical Transmission Using Orthogonal Polarizations in IM/DD System

We have proposed baseband orthogonal frequency division multiplexing (OFDM) optical transmission technique using I/Q channel separation based on dual polarization with intensity modulation/direct detection (IM/DD). We experimentally verified performance of a proposed system in compare with intermediate frequency (IF)-upconverted OFDM system. In order to obtain better performance, adaptively modulated optical OFDM signal was used in this study. As a result, experimentally achieved maximum throughput after 50 km single mode fiber transmission was 28 Gbps with only using 4 GHz signal bandwidth. The proposed technique has relatively simple hardware structure and reduces computational complexity compared to other methods (IF upconversion or Hermitian symmetry) in IM/DD-based polarization multiplexed OFDM system.

Analysis of Performance Assessment of Direct-Detection OFDM Optical Receivers With Imperfect RF Demodulation

An analytical method based on the moment generating function (MGF) is proposed for assessing the performance of direct-detection (DD) orthogonal frequency division multiplexing (OFDM) optical receivers with radio-frequency (RF) demodulation. The MGF-based method is a generalization of the method previously reported in the literature for DD baseband OFDM optical receivers. The proposed method relies on the analytical derivation of equivalent filters that describe the combined effect of electrical filtering + RF demodulation + FFT operation + the equalizer of the OFDM receiver for the real and imaginary parts of the signal at the equalizer output. The method takes into account imperfections of the RF demodulator, namely, power and phase imbalance between the RF demodulator arms and different electrical filtering on its arms. Numerical results show excellent agreement between the bit error probability estimates provided by the proposed method and estimates obtained from Monte Carlo simulation, in the absence and presence of receiver imperfections.

Real-Time Optical OFDM Long-Reach PON System Over 100 km SSMF Using a Directly Modulated DFB Laser

A real-time base-band orthogonal frequency division multiplexing (OFDM) transceiver with a simple symbol synchronization and channel estimation scheme is implemented on only one off-the-shelf commercial field programmable gate array (FPGA). The real-time optical OFDM long-reach passive optical network (PON) system with direct detection over 100 km standard single-mode fiber (SSMF) employing a cost-effective directly modulated distributed feedback (DFB) laser has been successfully demonstrated experimentally. The experimental results show that the simplified symbol synchronization with low complexity and high accuracy is effective for long-reach optical OFDM transmission systems. After 100 km SSMF transmission, the power penalty at a bit-error rate of 1e-3 is only about 1.1 dB.

Accurate Sampling Timing Acquisition for Baseband OFDM Power-Line Communication in Non-Gaussian Noise

In this paper, a novel technique is proposed to address the joint sampling timing acquisition for baseband and broadband power-line communication (BB-PLC) systems using Orthogonal-Frequency-Division-Multiplexing (OFDM), including the sampling phase offset (SPO) and the sampling clock offset (SCO). Under pairwise correlation and joint Gaussian assumption of received signals in frequency domain, an approximated form of the log-likelihood function is derived. Instead of a high complexity two-dimension grid-search on the likelihood function, a five-step method is employed for accurate estimations. Several variants are presented in the same framework with different complexities. Unlike conventional pilot-assisted schemes using the extra phase rotations within one OFDM block, the proposed technique turns to the phase rotations between adjacent OFDM blocks. Analytical expressions of the variances and biases are derived. Extensive simulation results indicate significant performance improvements over conventional schemes. Additionally, effects of several noise models including non-Gaussianity, cyclo-stationarity, and temporal correlation are analyzed and simulated. Robustness of the proposed technique against violation of the joint Gaussian assumption is also verified by simulations.

Moment Generating Function for the Rigorous Performance Assessment of Direct-Detection Baseband OFDM Communication Systems

A new moment generating function (MGF) is proposed to assess rigorously the performance of optically preamplified receivers with direct-detection employing baseband orthogonal frequency division multiplexing (OFDM). By comparison with Monte Carlo simulation, the proposed MGF-based method shows very good accuracy when predicting the bit error probability for arbitrary optical and electrical filters, different numbers of OFDM subcarriers, and double sideband and single sideband optical OFDM signals. Its good accuracy when estimating the bit error probability in each individual subcarrier is also demonstrated. The new MGF-based method exhibits a significant improvement of accuracy when compared to the MGF-based method previously reported in the literature, especially for filter bandwidths not exceeding the OFDM signal bandwidth, which is the range corresponding to the optimum filter bandwidth.

Design and implementation of a low-power OFDM receiver for wireless communications

Orthogonal Frequency-Division Multiplexing (OFDM) systems can efficiently combat the issue of frequency-selective fading channels. Therefore, OFDM has become one of the most popular baseband modulation techniques for wireless communications. This paper presents the design and implementation of a low-power OFDM baseband receiver for wireless local area networks (WLAN). The proposed architecture employs low-power register files and resource-sharing techniques, thus consuming less power than many existing works. The proposed design adopts a number of area-efficient hardware structures to further reduce the hardware cost. Finally, because it is designed using 0.18 μm CMOS technology, the core of the proposed chip has approximately 239K gates, and its power consumption is approximately 36 mW at 20 MHz.