Modulation Symbols Research Articles

Multi-RF and Generalized Single-RF Combination Models for Spatial Modulation and NOMA Technologies

Inspired by the individual advantages of spatial modulation (SM), space shift keying (SSK), and non-orthogonal multiple access (NOMA) technologies, this research designs NOMA-SM/SSK amalgamation models with a general number of users. Based on the number of RF chains, they are classified into multi-RF (MRF) and single-RF (SRF) models. We first present a spectral-efficient MRF model and then propose a new energy-efficient generalized SRF model denoted as SRFGen. In SRFGen, all users jointly choose a shared transmit antenna (TA) and independently select their modulation symbols. The SRFGen's flexibility produces several special cases covering the literature on SRF models and resolves their limits. Among the scenarios, an SRF NOMA-SSK supports multi <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</i> SSK signals and is proposed to reduce interference and detection processes. Then, introduce a low complexity TA index detector and an adaptive and fair power allocation algorithm. The spectral efficiency (SE) of the MRF and SRFGen is analyzed with lower bounds for both Gaussian and finite-alphabet inputs. Using the SE analysis and bit-error-rate (BER) simulation, the NOMA-SM/SSK models are evaluated regarding users’ rates, sum-rate, ergodic capacity, and BER. The results show that the MRF grants high SE, whereas the SRFGen provides flexibility in trading-off features like SE, BER, users’ fairness, and detection complexity. The outcomes also reveal the superior performance of the presented SRF models compared to the literature SRF models. Besides, the results reinforce the superiority of NOMA-SM/SSK over OMA-SM.

Integrated Spatial Modulation and STBC-VBLAST Design toward Efficient MIMO Transmission.

In this contribution, the concept of spatial modulation (SM) is firstly integrated into the structure of space-time block codes (STBC)-aided vertical Bell-labs layered space-time (VBLAST) systems, in order to strike a balanced tradeoff among bit error ratio (BER), spectral efficiency and computational complexity. First of all, in order to enhance the BER performance of STBC-VBLAST, we advocate an effective transmit power allocation (TPA) scheme with negligible implementation costs, while dividing the STBC and VBLAST layers with alleviated interference, so as to facilitate combination with SM. Then, we further utilize the unique structure of SM for enhancing the spectral efficiency of original STBC-VBLAST, wherein the information is conveyed by not only the amplitude/phase modulation (APM) symbols but also the antenna indices. In addition, constellation sets of STBC symbols are specifically designed to be rotated to make full use of the degrees of freedom. Finally, the performance advantages of the above-mentioned structures over traditional STBC-VBLAST are demonstrated by the theoretical derivation of a closed-form expression for the union bound on the bit error probability for various spectral efficiencies, and they are supported by simulation results.

On the Kurtosis of Modulation Formats for Characterizing the Nonlinear Fiber Propagation

Knowing only two high-order statistical moments of modulation symbols, often represented by the fourth moment called “kurtosis”, the overestimation of nonlinear interference (NLI) in a Gaussian noise (GN) model due to Gaussian signaling assumption can be corrected through an enhanced GN (EGN) model. However, in some modern optical communication systems where the transmitted modulation symbols are statistically correlated, such as in systems that use probabilistic constellation shaping (PCS) with finite-length sphere shaping, the kurtosis-based EGN model produces significant inaccuracies in analytical prediction of NLI. In this paper, we show that for correlated modulation symbols, the NLI can be more accurately estimated by substituting a statistical measure called windowed kurtosis into the EGN model, instead of the conventional kurtosis. Remarkably, the optimal window length for windowed kurtosis is found to be consistent with the self-phase modulation (SPM) and cross-phase modulation (XPM) characteristic times in various system configurations. The findings can be used in practice to analytically evaluate and design NLI-tolerant modulation formats.

APPLICATION OF DETERMINATED NOISE IN A RADIO COMMUNICATION SYSTEM WITH DIGITAL MODULATION

The article describes a communication system with digital pseudo-random modulation. The system is one of the variants of spread spectrum radio communication systems. This communication system uses deterministic noise as the spreading sequence. The noise immunity of the considered communication system can be changed by scaling the spreading factor — the length of the modulation symbol. By increasing the spreading factor, not only an increase in the noise immunity of the system is achieved, but also the possibility of separating subscribers simultaneously operating in a common frequency band. It has been established that for communication systems with digital pseudo-random modulation, the length of the modulation symbol (the value of the spreading factor) and the number of subscribers to be separated are related by a quadratic dependence.

Extended Space Index Modulation

In this letter, a new scheme called extended space index modulation (ESIM) is proposed and investigated. Different from previous works, both the signal constellation and index domains are exploited, and the performance of the spectral efficiency and the reliability can be further enhanced. Specifically, in the ESIM, an antenna index (AI) vector set is designed to convey more extra index bits, which consists of two AI vector subsets. The two AI vector subsets, which are respectively used to modulate the quadrature amplitude modulation (QAM) and secondary QAM (S-QAM) symbols, include the AI vectors having two non-zero elements from <inline-formula> <tex-math notation="LaTeX">$\{(1,1),(1,j),(j,1),(j,j)\}$ </tex-math></inline-formula> to activate two transmit antennas and the AI vectors having only one non-zero element equaling to &#x201C;1&#x201D; to activate one transmit antenna. Furthermore, the S-QAM constellation is designed to enhance the squared Minimum Euclidean Distance. Simulation results are provided to show a close match with analytical results particularly in the high signal-to-noise ratio (SNR) region and demonstrate that significant performance gain can be achieved in the proposed ESIM.

Low-Complexity GSM Detection Based on Maximum Ratio Combining

Generalized spatial modulation (GSM) technology is an extension of spatial modulation (SM) technology, and one of its main advantages is to further improve band efficiency. However, the multiple active antennas for transmission also brings the demodulation difficulties at the receiver. To solve the problem of high computational complexity of the optimal maximum likelihood (ML) detection, two sub-optimal detection algorithms are proposed through reducing the number of transmit antenna combinations (TACs) detected at the receiver. One is the maximum ratio combining detection algorithm based on repetitive sorting strategy, termed as (MRC-RS), which uses MRC repetitive sorting strategy to select the most likely TACs in detection. The other is the maximum ratio combining detection algorithm, which is based on the iterative idea of the orthogonal matching pursuit, termed the MRC-MP algorithm. The MRC-MP algorithm reduces the number of TACs through finite iterations to reduce the computational complexity. For M-QAM constellation, a hard-limited maximum likelihood (HLML) detection algorithm is introduced to calculate the modulation symbol. For the M-PSK constellation, a low-complexity maximum likelihood (LCML) algorithm is introduced to calculate the modulation symbol. The computational complexity of these two algorithms for calculating the modulation symbol are independent of modulation order. The simulation results show that for GSM systems with a large number of TACs, the proposed two algorithms not only achieve almost the same bit error rate (BER) performance as the ML algorithm, but also can greatly reduce the computational complexity.

Space–time line coded spatial modulation

In this letter, a novel multiple-input multiple-output transceiver technique, named space–time line coded spatial modulation (STLC-SM) is proposed, where two modulation symbols are transmitted through one of the multiple transmit antennas determined by the incoming information bits after constellation-rotated STLC encoding. The receiver decodes the transmitted modulation symbols and the activated antenna index through an optimal joint maximum-likelihood detector. As a main result, the closed-form upper-bound of bit-error-rate of the proposed STLC-SM system in general antenna configurations is mathematically analysed. Finally, it is validated that the STLC-SM significantly outperforms the conventional STLC in terms of the bit-error-rate.

Symbol division multiplexing in optical fiber communication systems.

Modern emerging data services and applications have put forward an ever-increasing bandwidth requirement for fiber-optic communication channels. To this end, we propose a novel symbol division multiplexing technology (SDM) by multiplexing/de-multiplexing of multiple quadrature amplitude modulation (QAM) symbols onto one complex constellation point. In our SDM scheme, every 7-bit 128QAM symbol is multiplexed per complex valued signal sequentially according to the optimal many-to-one mapping law, forming a 32QAM in the constellation and achieving an extra 40% gain for symbol capacity in an optical discrete multi-tone transmission system. The experiments prove that the SDM-32QAM successfully mitigates the signal impairments induced by fiber chromatic dispersion and Kerr nonlinearity, thus leading to 3.91-dB superior receiver power sensitivity and 2-dB enhancement of systematic tolerance to fiber nonlinear effect. The results highly motivate a fundamental paradigm in multiplexing techniques for optical fiber communication systems.

High-Dimensional Feature Based Non-Coherent Detection for Multi-Intensity Modulated Ultraviolet Communications

Ultraviolet communication (UVC) has been regarded as a promising supplement for overloaded conventional wireless communications. One challenge lies in the communication deterioration caused by the UV-photon scattering induced inter-symbol-interference (ISI), which will be even worse when encountering multilevel pulse amplitude modulation (multi-PAM) symbols. To address this ISI, traditional coherent detection methods (e.g., maximum-likelihood sequence detection, MLSD) require high computational complexities for UV channel estimation and sequential detection space formation, thereby making them less attractive. Current non-coherent detection, which simply combines the ISI-insensitive UV signal features (e.g., the rising edge) into a one-dimensional (1D) metric, cannot guarantee reliable communication accuracy. In this work, a novel high-dimensional (HD) non-coherent detection scheme is proposed, leveraging a HD construction of the ISI-insensitive UV signal features. By doing so, we transform the ISI caused sequential detection into an ISI-released HD detection framework, which avoids complex channel estimation and sequential detection space computation. Then, to compute the detection surface, a UV feature based unsupervised learning approach is designed. We deduce the theoretical bit error rate (BER), and prove that the proposed HD non-coherent detection method has a lower BER than that of the current 1D non-coherent scheme. Simulation results validate our results, and more importantly, demonstrate a BER that approaches that of the state-of-the-art coherent MLSD ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$&lt; $</tex-math></inline-formula> 1 dB in SNR at BER = <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$4.5\times 10^{-3}$</tex-math></inline-formula> , the 7% overhead forward-error-correction limit), and also a reduction of computational complexity by at least two orders of magnitude.

Neural Network Detection for Bandwidth-Limited Non-Orthogonal Multiband CAP UVLC System

In this paper, we propose a novel sparse data-to-symbol neural network (SDSNN) receiver for bandwidth-limited underwater visible light communication (UVLC) based on non-orthogonal multi-band carrierless amplitude and phase modulation (NM-CAP). Bandwidth limited NM-CAP signals usually carry severe inter-symbol interference (ISI) and inter-band interference (IBI). The SDSNN receiver directly converts the received NM-CAP data with ISI and IBI into quadrature amplitude modulation symbols without distortion for each sub-band. In contrast, the conventional receiver requires the least mean square (LMS) equalizer to cancel ISI, and the subcarrier component extraction with complex independent component analysis (SCE-ICA) to cancel IBI, respectively. SDSNN provides a novel receiving structure to replace post-equalization, matched filtering, and SCE-ICA. A blue-LED based UVLC system has been demonstrated utilizing NM-CAP16 with 3 sub-bands. The experimental results show that NM-CAP with the SDSNN receiver case reaches the highest spectral efficiency, where an enhancement of 43%, 20%, 6% has been measured over the orthogonal multi-band CAP case, NM-CAP with LMS equalizer case, and NM-CAP with joint LMS equalizer and SCE-ICA case, respectively. Compared with joint LMS equalizer and SCE-ICA case, the proposed SDSNN receiver can achieve 98% reduction of computational complexity.

Golden codeword‐based generalized spatial modulation

SummaryConventional‐generalized spatial modulation (C‐GSM) is an multiple‐input multiple‐output (MIMO) spatial modulation (SM) scheme that transmits the same complex amplitude/phase modulation symbol from multiple active transmit antennas. By using two active transmit antennas during a transmission interval, C‐GSM is capable of achieving high spectral efficiencies. However, as the size of the signal constellation/domain increases, the minimum Euclidean distances (ED) between the symbols decrease. Hence, this leads to a degradation of the average bit error rate (ABER) performance of the scheme. Hence, motivated to improve the error performance of C‐GSM schemes, first, this study proposes a new scheme called Golden codeword‐based generalized spatial modulation (GCW‐GSM), which combines C‐GSM and the Golden code to take advantage of the benefits of both while avoiding their drawbacks. The proposed scheme uses conventional quadrature amplitude modulation (C‐QAM), and it is investigated over Rayleigh frequency‐flat fading channels with additive white Gaussian noise. Second, a closed‐form expression of the theoretical average bit error probability of the proposed GCW‐GSM scheme is formulated. Compared to simulation results, it proves to be increasingly tight at high signal‐to‐noise ratio values. Finally, the proposed scheme performance over the Rayleigh‐frequency‐flat fading channel is compared to that over the Rician channel. An improvement in the error performance of with spectral efficiency of is seen in C‐16QAM GCW‐GSM over C‐256QAM conventional‐spatial modulation and C‐256QAM C‐GSM schemes at a bit error rate (BER) of . The C‐16QAM GCW‐GSM scheme exhibits an improvement in the error performance of with spectral efficiency of over the C‐64QAM conventional quadrature spatial modulation scheme at a BER of . Finally, the 16‐QAM GCW‐GSM under Rayleigh fading has a performance gain of 3 dB over 16‐QAM GCW‐GSM under Rician fading with a Rician K factor of 3.

A Novel SR-DCSK-Based Ambient Backscatter Communication System

In order to expand the application of differential chaos shift keying (DCSK) technology in Internet of Things, a novel ambient backscatter communication system is proposed. In this system, the ambient short reference DCSK (SR-DCSK) signal is ultilized as radio frequency (RF) source, and the modulation waveform design can be easily generated by the state transition of the SR-DCSK modulation symbol according to the transmitted information bit, so as to eliminate strong direct-link interference. Furthermore, a noise reduction method based on averaging operation at the detector is designed for enhancing the bit error rate (BER) performance. Theoretical BER expression over Rayleigh fading channel is derived and validated via simulation. Moreover, the performance of the proposed system is compared with those of other DCSK-based systems.

Near-Nyquist-Limit Optical Communication and Ranging Method Based on Waveform Matched PPM

Deep-space optical communication and ranging technologies have attracted much attention for satellite-to-earth and moon-to-earth exploration. In integrated communication and ranging scenarios, non-integer oversampling factors are employed to improve the ranging performance, which can cause accumulated timing errors. We propose a novel pulse position modulation (PPM) symbol decision method based on waveform matching to reduce the impact of accumulated timing errors near the Nyquist sampling limit. Simulation results demonstrate that the ranging accuracy can reach 2.6 and 0.52 mm at sampling rates of 625 Msps and 2.5 Gsps, respectively. The proposed symbol decision method has a gain of over 1.1 dB compared with the traditional method at the bit error rate (BER) of less than 10−6. The experimental results verify that this method can achieve high-precision measurements of distance and reliable transmission of information.

An Effective Spatial Modulation Based Scheme for Indoor VLC Systems

We propose an enhanced spatial modulation (SM)-based scheme for indoor visible light communication systems. This scheme enhances the achievable throughput of conventional SM schemes by transmitting higher order complex modulation symbol, which is decomposed into three different parts. These parts carry the amplitude, phase, and quadrant components of the complex symbol, which are then represented by unipolar pulse amplitude modulation (PAM) symbols. Superposition coding is exploited to allocate a fraction of the total power to each part before they are all multiplexed and transmitted simultaneously, exploiting the entire available bandwidth. At the receiver, a two-step decoding process is proposed to decode the active light emitting diode index before the complex symbol is retrieved. It is shown that at higher spectral efficiency values, the proposed modulation scheme outperforms conventional SM schemes with PAM symbols in terms of average symbol error rate (ASER), and hence, enhancing the system throughput. Furthermore, since the performance of the proposed modulation scheme is sensitive to the power allocation factors, we formulated an ASER optimization problem and propose a sub-optimal solution using successive convex programming (SCP). Notably, the proposed algorithm converges after only few iterations, whilst the performance with the optimized power allocation coefficients outperforms both random and fixed power allocation.

Accurate Frequency Estimation of Signals with Symbol Transition by Using Interpolated Dft

Accurate Frequency Estimation of Signals with Symbol Transition by Using Interpolated Dft

Two-Stage ML Detector Using Absolute Value of IQ Components and SVM for Adaptive OFDM-IM

Among various index modulation technology, adaptive orthogonal frequency division multiplexing with index modulation (A-OFDM-IM) improves reliability by solving the problem of deep fading. However, the maximal likelihood (ML) detector of A-OFDM-IM has high complexity by simultaneously performing active subcarrier detection and quadrature amplitude modulation (QAM) symbol demodulation. A simpler type of ML detector can be applied since the A-OFDM-IM has independent active states of each subcarrier and uses a single QAM constellation. In this paper, we propose two low-complexity detectors for the A-OFDM-IM. The proposed detectors have a two-stage receiving process, active subcarrier detection, and QAM demodulation. In active subcarrier detection, the presence or absence of the QAM symbol is more important than its information. Therefore we derive the thresholds using the absolute value of the in-phase and quadrature components of the received signal in the frequency domain. Using the absolute value, the distribution of noise added to inactive subcarrier is half normal. This is different from noise distribution of the ML detector, and the proposed detectors select the threshold considering the corresponding noise distribution. The first proposed detector uses a threshold derived by the ML estimation method. The second detector estimates the active subcarriers via a support vector machine. The proposed detectors have lower complexity than the ML detector because of the divided receiving process. Moreover, the theoretical analysis and simulation results show that the proposed detectors have better reliability than the ML detector due to different noise distribution and thresholds.

Asymptotic Performance of Box-RLS Decoders Under Imperfect CSI With Optimized Resource Allocation

This paper considers the problem of symbol detection in massive multiple-input multiple-output (MIMO) wireless communication systems. We consider hard-thresholding preceded by two variants of the regularized least squares (RLS) decoder; namely the unconstrained RLS and the RLS with a box constraint, which is called Box-RLS. For all schemes, we focus on the evaluation of the mean squared error (MSE) and the symbol error probability (SEP) for M-ary pulse amplitude modulation (M-PAM) symbols transmitted over a massive MIMO system when the channel is estimated using linear minimum mean squared error (LMMSE) estimator. Under such circumstances, the channel estimation error is Gaussian which allows for the use of the convex Gaussian min-max theorem (CGMT) to derive asymptotic approximations for the MSE and SEP when the system dimensions and the coherence duration grow large with the same pace. The obtained expressions are then leveraged to derive the optimal power distribution between pilot and data under a total transmit energy constraint. In addition, we derive an asymptotic approximation of the goodput for all schemes which is then used to jointly optimize the number of training symbols and their associated power. Numerical results are presented to support the accuracy of the theoretical results.

Rotated Golden Codewords Based Space-Time Line Code Systems

The space-time line code (STLC) has been recently proposed in the literature. In STLC systems, the channel state information is assumed to be known at the transmitter, but not at the receiver. In the conventional STLC systems, the inputs of the STLC encoder are either M-ary phase shift keying symbols or M-ary quadrature amplitude modulation symbols and can achieve diversity order with very low detection complexity. In order to further improve the error performance of the STLC systems, in this paper, rotated Golden codewords based STLC (RGC-STLC) systems are proposed.We extend the 1&#x00D7;2 and 2 &#x00D7; 2 transmit-receive antenna structure in the conventional STLC into 1 &#x00D7; 4 and 2 &#x00D7; 4 transmit-receive antenna structures in the proposed RGC-STLC system. The inputs of the proposed RGC-STLC encoder are the rotated Golden codewords. Compared to the conventional 1 &#x00D7; 2 and 2 &#x00D7; 2 STLC systems, the 1 &#x00D7; 4 and 2 &#x00D7; 4 RGC-STLC systems achieve a higher diversity order. Based on the feature of the rotated Golden codewords, low complexity detection schemes for the 1 &#x00D7; 4 and 2 &#x00D7; 4 RGC-STLC systems are further proposed. Finally, the union bounds on the average symbol error probability for the 1 &#x00D7; 4 and 2 &#x00D7; 4 RGC-STLC systems are derived and shown to match the simulation results very well at high signal-to-noise ratios.

DC-Assisted Stabilization of Internal Oscillations for Improved Symbol Transitions in a Direct Antenna Modulation Transmitter

Internal oscillations in switched antenna transmitters cause undesirable fluctuations of the stored energy in the system, reducing the effectiveness of time-varying broadbanding methods, such as energy-synchronous direct antenna modulation. To mitigate these parasitic oscillations, a modified direct antenna modulation system with an auxiliary DC source is introduced to stabilize energy storage on the antenna. A detailed circuit model for a direct antenna modulation system is used to identify the origin of the oscillations and to justify the selection of the DC source. Measured phase shift keyed waveforms transmitted by the modified system show significant increases in signal fidelity, including a 10-20 dB reduction in error vector magnitude compared to a time-invariant system. Comparison to an equivalent, scalable time-invariant antenna suggests that the switched transmitter behaves as though it has 2-3 times lower radiation Q-factor and 20% higher radiation efficiency.

Differential Transmission Schemes for Generalized Spatial Modulation

Differential modulation schemes are very relevant in receivers having power and processing limitations, as these schemes dispense with the need for knowledge of channel coefficients for symbol detection. Spatial modulation (SM) is a scheme used in multi-antenna transmission scenarios where the data is transmitted in the amplitude, phase and spatial domains through selected antennas. In the coherent domain, generalized SM (GSM) employs multiple antennas in combination during every time slot to enhance the spectral efficiency (SE). In this paper, we propose two differential schemes which activate two or more antennas at a time to transmit the modulated symbol. These schemes achieve higher SE using a lesser number of antennas and lower order modulation schemes instead of the higher number of antennas required for conventional SM schemes based on differential modulation. Simulation studies reveal that the proposed schemes have better bit error rate performance than traditional differential SM schemes. We also derive the analytical union bound for the proposed schemes and is satisfied from medium to high signal-to-noise ratio (SNR) ranges.