High Spectral Efficiency Transmission Research Articles

10.51-Tbit/s multi-channel IF-over-fiber transmission with SDM/WDM/SCM for beyond-5G mobile fronthaul accommodating ultra-high-density antennas

In beyond-5G and 6G mobile communication systems, it is expected that massive numbers of antennas are densely deployed to provide a stable and high-speed communication environment. To efficiently accommodate high-density antennas, high-capacity mobile fronthaul (MFH) is also required. Intermediate-frequency-over-fiber (IFoF) is considered as one of the attractive solutions for realizing high-capacity MFH due to its high spectral efficiency and multi-channel transmission capability. In this paper, we demonstrate the highest-capacity IFoF transmission with combinations of space division multiplexing, wavelength division multiplexing, and subcarrier multiplexing; 4608 channels of 64-quadrature amplitude modulation orthogonal frequency division multiplexing signals with a total net bit rate of 10.51 Tbit/s (4608×380.16MHz/ch×6bit/s/Hz) are successfully transmitted over a 10.1-km uncoupled 12-core multi-core fiber.

MIMO-OFDM channel estimation based on minimum error entropy criterion under non-Gaussian environment

ABSTRACT Using the orthogonal frequency division multiplexing (OFDM) technique, a multiple-input multiple-output (MIMO) system can provide high spectral efficiency and high data transmission over a fading channel. To achieve the best performance in a MIMO-OFDM system, high accuracy in channel estimation is a very important factor which leads to appropriate receiver design. Therefore, in the most channel estimation algorithms, the mean square error (MSE) is the main criterion for noise minimisation which is robust in the case of Gaussian noise. However, in telecommunication systems which do not have noise with Gaussian distribution, the MSE criterion is not appropriate. So as to tackle this problem, a robust adaptive filtering algorithm was proposed using minimum error entropy (MEE) criterion and improved least square (ILS) approach which by far is better than MMSE criterion to robust channel estimation. In this work, impulsive noise is used for non-Gaussian environment simulation. Furthermore, MEE is employed to attenuate the impulsive noise, and ILS is adopted to reduce LS estimation variance. Compared to the MSE-based algorithms, simulation results indicate that the proposed method outperforms the channel estimation in a non-Gaussian environment.

Complementary Polarization-Diversity Coherent Receiver for Self-Coherent Homodyne Detection With Rapid Polarization Tracking

There has been renewed interest in coherent detection to provide high spectral efficiency transmission for short-reach optical interconnects. However, the expensive high-stable laser sources may preclude its potential application. To balance the high performance and low cost for short-reach optical networks, the self-coherent homodyne receiver has been investigated, where the dual-polarization (DP) signal and the remote local oscillator (LO) originating from the same laser source co-propagate over a duplex fiber, enabling a remarkable tolerance of laser linewidth. However, the fast evolution of the state of polarization (SOP) of remotely delivered LO becomes problematic for self-coherent homodyne detection systems. To combat the polarization wandering of the remote LO, the complicated adaptive/automatic polarization controller (APC) has been deployed with only up to a few hundred rad/s polarization tracking speed. In this paper, we propose a complementary polarization-diversity coherent receiver (C-PDCR) for self-coherent homodyne detection using remote LO. The proposed C-PDCR features rapid polarization tracking for remote LO utilizing electronic digital signal processing (DSP). The robustness of the proposed C-PDCR is verified and demonstrated with a 1.08-Tb/s line rate (net rate 769.4 Gb/s over 45-GHz electrical bandwidth) using DP PCS-256QAM signal under rapid polarization rotation rate up to 314 krad/s in the experiment, which is more than 3-orders improvement of magnitude compared to the prior reports.

Analysis of Doppler and Multipath on Orthogonal Chirp Division Multiplexing in Shallow Water Acoustic Channel

In the scenario of underwater acoustic communication, Orthogonal Frequency Division Multiplexing (OFDM) is widely used for its high spectral efficiency and high transmission rate. However, OFDM is sensitive to Doppler shift, which will generate inter-carrier interference to degrade bit error rate performance. Orthogonal Chirp Division Multiplexing (OCDM) consists of multiplexing a number of chirp waveforms that are mutually orthogonal with each other and share the same bandwidth and time slot, achieving good robustness against multipath and Doppler shift. This study reviews the concepts of Orthogonal Chirp Division Multiplexing and combines this multicarrier communication scheme with an underwater acoustic channel. Unlike previous studies analyzing within the signal domain, this study formulates the mathematical model of OCDM in shallow water acoustic channel and illustrates the reason why OCDM can outperform OFDM in both Doppler robustness and multipath robustness, which is a necessary and solid complement for OCDM in the further development of underwater acoustic communication.

An investigation on fronthaul and millimeter wave technologies for 5G

.The growing demand for communications and the emergence of new applications have put the capacity, transmission rate and latency of communications systems to a more severe test. In this context, 5G communication technology has emerged. 5G applications are divided into three typical application scenarios: eMBB, URLLC and mMTC. In order to support these applications, the key technologies of 5G need to be studied in depth. This paper firstly investigates the networking architecture of 5G bearer networks, and carries out a detailed analysis and comparison of 5G fronthaul technology, including the optical fibre direct connection solution, passive WDM solution, active WDM/OTN solution and WDM-PON solution. Secondly, this paper introduces 5G millimetre wave technology to achieve large capacity and high spectral efficiency transmission, including direct intensity modulation method, external modulation method, optical heterodyne method, optical injection locking method and optical phase-locked loop method. Finally, this paper provides an outlook on 5G fronthaul technology and millimetre wave technology.

Reconfigurable Intelligent Surface-Assisted Uplink Sparse Code Multiple Access

Reconfigurable intelligent surface-empowered communication (RIS) and sparse code multiple access (SCMA) are promising candidates for future generations of wireless networks. The former enhances the transmission environments, whereas the latter provides a high spectral efficiency transmission. This letter proposes, for the first time, a low-cost design for RIS-assisted uplink SCMA (SCMA-RIS) scheme to improve the conventional SCMA spectrum efficiency. The message passing algorithm (MPA) is utilized and modified to decode the SCMA-RIS transmitted signals. Moreover, a low-complexity decoder for the SCMA-RIS scheme is proposed to significantly reduce the MPA decoding complexity and improve the bit error rate performance of the conventional SCMA. Monte Carlo simulations and complexity analysis are presented, which support the findings.

On the Complexity Reduction of Uplink Sparse Code Multiple Access for Spatial Modulation

Multi-user spatial modulation (SM) assisted by sparse code multiple access (SCMA) has been recently proposed to provide uplink high spectral efficiency transmission. The message passing algorithm (MPA) is employed to detect the transmitted signals, which suffers from high complexity. This paper proposes three low-complexity algorithms for the first time to the SM-SCMA. The first algorithm is referred to as successive user detection (SUD), while the second algorithm is the modified version of SUD, namely modified SUD (MSUD). Then, for the first time, the tree-search of the SM-SCMA is constructed. Based on that tree-search, another variant of the sphere decoder (SD) is proposed for the SM-SCMA, referred to as fixed-complexity SD (FCSD). SUD provides a benchmark for decoding complexity at the expense of bit-error-rate (BER) performance. Further, MSUD slightly increases the complexity of SUD with a significant improvement in BER performance. Finally, FCSD provides a near-optimum BER with a considerable reduction of the complexity compared to the MPA decoder and also supports parallel hardware implementation. The proposed algorithms provide flexible design choices for practical implementation based on system design demands. The complexity analysis and Monte-Carlo simulations of the BER are provided for the proposed algorithms.

Low-Complexity Selective Mapping Methods for Multicarrier Faster-Than-Nyquist Signaling

Multicarrier faster-than-Nyquist (MFTN) signaling is a high spectral efficiency transmission scheme, which is promising in the future communication. One of the major problems of MFTN signaling is high peak-to-average power ratio (PAPR). Therefore, PAPR reduction is necessary in MFTN systems. Generally, four times oversampling is used in PAPR reduction methods for MFTN signaling to make the discrete time MFTN signaling approximate the peak of continuous time MFTN signaling. However, generating four times oversampled discrete time MFTN signaling will increase the computational complexity of PAPR reduction methods. In this paper, we propose three new low-complexity PAPR reduction methods based on selective mapping (SLM) for MFTN signaling. The first step of proposed methods is using discrete time MFTN signaling with oversampling factor lower than four as reference discrete signal. Then, three new methods utilize picking, reconstructing, and filtering, respectively, to acquire the peak power of four times oversampled MFTN signaling. It is shown that the proposed methods can significantly reduce the computational complexity while almost without PAPR reduction performance loss.

Overhead-optimization of pilot-based digital signal processing for flexible high spectral efficiency transmission.

We present a low-complexity fully pilot-based digital signal processing (DSP) chain designed for high spectral efficiency optical transmission systems. We study the performance of the individual pilot algorithms in simulations before demonstrating transmission of a 51×24 Gbaud PM-64QAM superchannel over distances reaching 1000 km. We present an overhead optimization technique using the system achievable information rate to find the optimal balance between increased performance and throughput reduction from adding additional DSP pilots. Using the optimal overhead of 2.4%, we report 9.3 (8.3) bits/s/Hz spectral efficiency, or equivalently 11.9 (10.6) Tb/s superchannel throughput, after 480 (960) km of transmission over 80 km spans with EDFA-only amplification. Moreover, we show that the optimum overhead depends only weakly on transmission distance, concluding that back-to-back optimization is sufficient for all studied distances. Our results show that pilot-based DSP combined with overhead optimization can increase the robustness and performance of systems using advanced modulation formats while still maintaining state-of-the-art spectral efficiency and multi-Tb/s throughput.

Constant Modulus Secure Beamforming for Multicast Massive MIMO Wiretap Channels

Massive MIMO attains high spectral and power efficiency transmission by leveraging a large number of transmit antennas. However, to capture the benefits of massive MIMO, each antenna should be accompanied with a dedicated RF chain, and consequently, the hardware costs would scale up tremendously with the increase of the antennas. Cheap implementations of massive MIMO have recently gained considerable attention, and constant modulus (CM) signaling is seen as a promising solution, owing to its low peak-to-average power ratio (PAPR). This paper investigates the physical-layer (PHY) security in massive MIMO with an emphasis on the CM signaling. In particular, we consider a transmitter with massive antennas broadcast common confidential information to a group of legitimate receivers, and a number of eavesdroppers overhear the transmission and attempt to intercept the information. Our goal is to design the CM beamforming at the transmitter so that the multicast secrecy rate is maximized. This secrecy rate maximization (SRM) problem is generally NP-hard. To tackle it, two tractable approaches are developed. The first one employs the semidefinite relaxation (SDR) technique and the Charnes–Copper transformation to obtain a convex relaxation of the SRM problem. However, due to the dimension lifting of SDR, this approach is feasible only for small to medium antenna sizes. The second approach leverages the Dinkelbach method to work directly over the beamformer domain; a custom-build nonconvex alternating direction method of multipliers (ADMM) algorithm is proposed to efficiently perform each Dinkelbach update. Simulation results demonstrate that the second approach is computationally more efficient and can achieve nearly optimal performance when the number of antennas is large.

Experimental demonstration of IDMA‐OFDM for visible light communications

We experimentally demonstrate a multicarrier interleave division multiple access (IDMA) scheme for bidirectional visible light communications using orthogonal frequency division multiplexing (OFDM), which has the advantages of good tolerance against multi-path, high spectral efficiency and excellent transmission performance. The generated IDMA signals are modulated on OFDM subcarriers and the different users are separated by the interleavers. When compared with orthogonal frequency domain multiple access, IDMA-OFDM offers increased transmission spans of 30 and 50 cm at a bit error rate (BER) of 10 -3 in the cases of two and four users, respectively.

A Novel Shaping Scheme for PAPR Reduction in Single-Carrier Modulation

A novel PAPR shaping technique intended for systems with high spectral efficiency and single carrier transmission is presented. This technique applies puncturing of error correction redundancy bits to achieve a desired Markovian symbol transmission probability distribution, which avoids symbol sequences with high peak values. For a 16-QAM system with transmission rate of 3 bit/symbol and a root raised cosine (RRC) pulse shape filter with roll-off factor 0.1 and duration of six symbols, an overall gain of about 2.2 dB is demonstrated by simulations using pragmatic binary turbo code for error correction. The technique can be used to improve any single carrier communication link limited by the high power amplifier (HPA) peak power.

41.5-Tb/s Transmission Over 549 km of Field Deployed Fiber Using Throughput Optimized Probabilistic-Shaped 144QAM

We demonstrate high spectral efficiency transmission over 549 km of field deployed single-mode fiber using probabilistic-shaped 144QAM. We achieved 41.5 Tb/s over the C-band at a spectral efficiency of 9.02 b/s/Hz using 32-Gbaud channels at a channel spacing of 33.33 GHz, and 38.1 Tb/s at a spectral efficiency of 8.28 b/s/Hz using 48-Gbaud channels at a channel spacing of 50 GHz. To the best of our knowledge, these are the highest total capacities and spectral efficiencies reported in a metro field environment using C-band only. In high spectral efficiency transmission, it is necessary to optimize back-to-back performance in order to maximize the link loss margin. Our results are enabled by the joint optimization of constellation shaping and coding overhead to minimize the gap to Shannon's capacity, transmitter- and receiver-side digital backpropagation, signal clipping optimization, and I/Q imbalance compensation.

Paired SSB optical OFDM channels for high spectral efficient signal transmission over DWDM networks

A new high spectral efficient SSB-OOFDM DWDM transmission system has been experimentally demonstrated. The proposed transmitter employs paired optical channels consisting of two SSB modulated OFDM signals using opposite sidebands in order to allow an efficient use of the spectrum with optical carriers separation under 10GHz. Moreover, different paired channels are multiplexed into the 25GHz grid DWDM fiber transmission link. Optical carrier spacing of 8.75GHz in paired channels has been demonstrated allowing 40.8Gb/s signal transmission rate over a 25GHz paired channel bandwidth.

Energy-Efficiency of Millimeter-Wave Full-Duplex Relaying Systems: Challenges and Solutions

Full-duplex (FD) relaying is a promising solution for fifth generation (5G) wireless communications due to its potential to provide high spectral efficiency (SE) transmission. However, FD relay nodes consume much higher power than half-duplex relay nodes, especially in millimeter (mm)-wave band. Therefore, energy-efficiency (EE) is an important issue to address for mm-wave FD relaying systems, which highlights the green evolution of 5G wireless communications. Existing FD relaying related research is SE-oriented, which is not efficient in cutting carbon footprint. This particle is different in that it addresses the critical EE challenges in implementing FD relaying in mm-wave systems, including low drain efficiency of power amplifier, high circuit power consumption, and additional power required by FD relays to mitigate self-interference. Based on the features of mm-wave communications, we outline a number of promising EE-oriented solutions for designing FD relaying enabled systems, including adaptive self-interference cancellation, transmission power adaptation, hybrid relaying mode selection, multi-input-multi-output (MIMO), and massive MIMO FD relaying. Some EE-oriented future research is also envisaged for mm-wave FD relaying systems.

Nonlinear inverse synthesis for high spectral efficiency transmission in optical fibers.

In linear communication channels, spectral components (modes) defined by the Fourier transform of the signal propagate without interactions with each other. In certain nonlinear channels, such as the one modelled by the classical nonlinear Schrödinger equation, there are nonlinear modes (nonlinear signal spectrum) that also propagate without interacting with each other and without corresponding nonlinear cross talk, effectively, in a linear manner. Here, we describe in a constructive way how to introduce such nonlinear modes for a given input signal. We investigate the performance of the nonlinear inverse synthesis (NIS) method, in which the information is encoded directly onto the continuous part of the nonlinear signal spectrum. This transmission technique, combined with the appropriate distributed Raman amplification, can provide an effective eigenvalue division multiplexing with high spectral efficiency, thanks to highly suppressed channel cross talk. The proposed NIS approach can be integrated with any modulation formats. Here, we demonstrate numerically the feasibility of merging the NIS technique in a burst mode with high spectral efficiency methods, such as orthogonal frequency division multiplexing and Nyquist pulse shaping with advanced modulation formats (e.g., QPSK, 16QAM, and 64QAM), showing a performance improvement up to 4.5 dB, which is comparable to results achievable with multi-step per span digital back propagation.

Two improved SLM methods for PAPR and BER reduction in OFDM–ROF systems

OFDM–ROF (orthogonal frequency division multiplexing–radio over fiber) system has high spectral efficiency and high transmission rate. However, due to the non-linear power amplifier, the high peak-to-average power ratio (PAPR) of the transmission signals is one of the major setbacks in the OFDM–ROF transmission systems. In order to reduce PAPR, this paper proposes two improved selected mapping (SLM) methods without explicit side information (SI): Proposed ED-SLM and Proposed SLM. In the Proposed ED-SLM method, signals will be transmitted without side information which is scrambled by special initial phase sequences and at the receiver an Euclidean phase distance detection (EPD) method is used to detect the SI and finishes the demodulation. In the proposed SLM method, the signals are transformed by Hadamard matrix to reduce PAPR. Then the row vectors of Hadamard matrix are used as phase sequences and superimposed on the data signals. The theory and simulation results show that the two improved SLM methods perform better in PAPR and BER reduction than the conventional SLM method in the OFDM–ROF transmission systems.

Asynchronous Layered Modulation for High Spectral Efficiency Transmission with Lower Sensitivity to Phase Errors

In this paper, an asynchronous double layered modulation scheme is proposed to achieve high spectral efficiency transmission but has lower sensitivity to phase errors. By intentionally introducing a delay offset to the transmitted signals at each layer, the asynchronous double layered format can use lower level quadrature amplitude modulation (QAM) to achieve the same spectral efficiency as the conventional high level QAM modulation. The error probability performance of the asynchronous double layered format is analyzed in the presence of phase errors and the symbol error rate and the bit error rate are derived in the closed-form expression. Both the theoretical analysis and the simulation results show that the asynchronous layered format can achieve the same spectral efficiency as the conventional high level QAM modulation while has lower sensitivity to the phase errors.

Closed-form expressions for nonlinear transmission performance of densely spaced coherent optical OFDM systems

There has been a trend of migration to high spectral efficiency transmission in optical fiber communications for which the frequency guard band between neighboring wavelength channels continues to shrink. In this paper, we derive closed-form analytical expressions for nonlinear system performance of densely spaced coherent optical OFDM (CO-OFDM) systems. The closed-form solutions include the results for the achievable Q factor, optimum launch power density, nonlinear threshold of launch power density, and information spectral efficiency limit. These analytical results clearly identify their dependence on system parameters including fiber dispersion, number of spans, dispersion compensation ratio, and overall bandwidth. The closed-form solution is further substantiated by numerical simulations using distributed nonlinear Schrödinger equation.

Quadrature Duobinary for High-Spectral Efficiency 100G Transmission

This paper proposes and analyzes a quadrature duobinary modulation format with coherent detection for high-spectral efficiency wavelength-division-multiplexing transmission systems at 112 Gb/s per channel. A practical system design of a dual polarization quadrature duobinary (DP-QDB) format with coherent heterodyne detection is analyzed and compared with the DP-differential quadrature phase-shift keying format for feasibility of 112 Gb/s transmission. Using simulation analysis, we show that the relatively narrow spectrum of quadrature duobinary modulation provides an effective practical means for realizing high-spectral efficiency transmission, approaching the theoretical limit of 4 bits/s/Hz, while also providing excellent tolerance to optical filter cascades for a robust optical networking functionality.