Multiplexing Communication Systems Research Articles

Robust QoS-Aware Cross-layer Design of Adaptive Modulation Transmission on OFDM Systems in High-Speed Railway

In this paper, we consider an orthogonal frequency division multiplexing communication system that adopts frame-by-frame transmission in high-speed railway (HSR) scenario. Due to the increase in demand for the QoS sensitive services, an efficient QoS-aware transmission strategy that improves the system performance is required urgently. Many efforts have been devoted to addressing this problem with the assumption of block fading channel in a frame duration. However, due to the frequent channel quality variation in a frame duration and serious inter channel interference in HSR scenario, the throughput of the QoS sensitive services degrades severely. Hence, a robust cross-layer transmission strategy that combines adaptive modulation (AM) scheme with truncated automatic repeat request protocol is proposed. In this cross-layer formulation, the normalized average throughput is optimized subject to the average power and the packet loss rate (PLR) requirements. First, we derive the closed form average bit error rate that represents the PLR requirement at the physical layer. Second, we obtain the solution of robust AM scheme and power allocation policy in the case of continuous rate. Third, we present the adaptive bits and power allocation scheme in the case of discrete rate, which can be implemented in practice. Finally, the performance of the proposed transmission strategy is evaluated by extensive simulations. Comparing with the constant transmit power AM scheme, the throughput increases by 20%, which demonstrates that the proposed robust cross-layer design is suitable for the HSR communication systems.

Optimum Resource Allocation in Orthogonal Frequency Division Multiplexing Communication System using Fuzzy Rule Base System and Particle Swarm Optimization and Comparison with Conventional other Techniques

OFDM is a technique that is chosen for high data rate communication and is important for 4th generation communication systems. To provide best possible quality of service, intelligent allocation of resources such as power, bandwidths are necessary. Various algorithms have been proposed for allocation of resources. Fuzzy Rule Based System (FRBS) and Particle Swarm Optimization (PSO) algorithm are used for optimization of code rate, modulation and power. FRBS is used to choose the best possible modulation and code rate and PSO is used for better power allocation.

Improved superimposed training sequence-based timing synchronization for space optical orthogonal frequency division multiplexing system

This paper investigates the timing synchronization problem of a space optical orthogonal frequency division multiplexing (OOFDM) communication system. First, based on the good autocorrelation property of gen- eralized chirp-like sequence, a training sequence is constructed to fit the non-negative light intensity signal requirement of the OOFDM system, of which the front and rear portions are cyclical and the whole is mirror- symmetric. No longer a periodic-repetition structure, the mirror-symmetric structure can effectively avoid the side lobe of objective function and reduce the complexity of correlation calculation, and thereby can improve the synchronization performance. Then, the constructed training sequence is superimposed on a complete data symbol for transmission to efficiently utilize transmitting power and spectrum resources of the communication system. At the receiver, the position of timing synchronization is estimated using maximum-likelihood algorithm and the correlation between the local sequence and the received signal. Simulation results show that, compared with several existing methods, the proposed timing synchronization method achieves better synchronization performances under both strong and weak atmospheric turbulence channels. © 2015 Society of Photo-Optical

BER analysis of hybrid carrier system based on WFRFT with carrier frequency offset

Carrier frequency offset (CFO) due to Doppler shift or misaligned frequency between the transmitter and the receiver has a serious impact on both orthogonal frequency division multiplexing (OFDM) and single carrier (SC) communication systems. Inter-carrier interference coefficients of OFDM system produced by CFO have been proposed and applied in analysing exact bit error rate (BER) upon diverse channel models. In this Letter, interference of hybrid carrier communication system based on weighted-type fractional Fourier transform impaired by CFO is investigated, which is a weighted form of multi-carrier and SC schemes, and then exact BER expression of BPSK modulation HC system is derived and verified by simulations.

Design and Analysis of 25 and 35 Gbps Optical Communication System based on APDCDM Technique

and 35 Gbps Absolute Pole Duty cycle division multiplexing (APDCDM) based communication system is designed successfully. A technique for estimation of BER is developed for four users APDCDM based optical communication system, referring to the data recovery rules. The performance parameters like SNR, Q-factor and BER are calculated for the transmission rate

Frequency-Domain Iterative SDFE for MIMO-OFDM Systems

This paper proposes a frequency-domain iterative soft decision feedback equalization (SDFE) scheme for multiple-input---multiple-output orthogonal frequency division multiplexing communication systems in multi-path fading channels. Error propagation can seriously affect the performance of the adaptive decision feedback equalization. We propose an iterative SDFE to not only combat error propagation but also be suitable for multilevel modulation systems. Under a Gaussian approximation of the a priori information and the SDFE output, the output of an equivalent AWGN channel and expressions for the a posteriori log likelihood ratio (LLR) and the extrinsic LLR are derived for multilevel modulation. In addition, we also investigate the iterative receivers by employing irregular low-density parity check codes (LDPC) and turbo codes. The performance of the LDPC-coded iterative receiver is better than that of the turbo-coded iterative receiver. Some simulation examples are given to show the effectiveness and comparisons of the proposed receiver.

On the Performance of Coherent OFDM Systems in Free-Space Optical Communications

This paper presents a new model of a coherent orthogonal frequency division multiplexing free space optical communication (OFDM-FSO) system that can be tuned with the virtual local oscillator, under the Gamma-Gamma atmospheric turbulence channel, not only considering the intensity scintillation but considering the phase noise caused by atmospheric turbulence effect on the performance of the system as well. The mapping mode of the OFDM signal uses 16-quadrature amplitude modulation (QAM) with better performance. On this basis, we derived the closed form of the symbol error rate and interrupt probability, respectively. In this paper, we analyzed under the premise of the different number of subcarriers, the influence of each parameter of atmospheric channel on the bit error, and the communication interrupt performance of the OFDM-FSO system, respectively. The results show that the coherent OFDM-FSO system can obtain higher sensitivity and can overcome the bad atmosphere influence to obtain good performance of the system.

On the Use of Alpha-Stable Distributions in Noise Modeling for PLC

Power-line communication (PLC) deals with the challenge of transmitting data over a channel impaired by background and impulsive noises. In this paper, we provide evidence that the marginal distribution of the noise found in PLC systems exhibits some statistical properties that can be well captured by using α-stable distribution. Motivated by this result, we present a practical application of the α-stable model to synthesize noise in the power line. Furthermore, we also show that this family of distributions is a suitable alternative in order to model the PLC background noise that is commonly assumed as Gaussian. Finally, we study the performance of an orthogonal frequency-division multiplexing communication system under both kinds of background noise (i.e., Gaussian and α-stable). From these experiments, we conclude that if highly impulsive background traffic is mistakenly assumed as Gaussian in evaluation studies, the system performance is largely overestimated.

Performance Evaluation of Pilot-Assisted PAPR Reduction Technique in Optical OFDM Systems

The pilot-assisted peak-to-average power ratio (PAPR) reduction technique proposed for optical orthogonal frequency division multiplexing (OFDM) communication systems is evaluated empirically and theoretically in this letter. The PAPR reduction is achieved by rotating the phase of data symbols with P iterations of randomly generated pilot symbol sequence. The results of our hardware implementation and analysis show a close agreement to that of computer simulations. In comparison with basic OFDM, where no PAPR reduction technique is implemented, experimental PAPR reduction gain of pilot-assisted OFDM at a complementary cumulative distribution function of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> with P = 5 is ~2 dB. This gain is ~0.2 dB less than that of analytical results. The experimental results also show that the pilot-assisted technique does not cause any significant deterioration of the bit error performance.

Demonstration of simultaneous mode conversion and demultiplexing for mode and wavelength division multiplexing systems based on tilted few-mode fiber Bragg gratings.

We experimentally demonstrate mode conversion by exploiting optical reflection of tilted few-mode fiber Bragg grating (FM-FBG). Mode conversions from LP(01) mode to higher symmetric and asymmetric modes are achieved, and more than 99.5% conversion efficiency from LP(01) to LP(11) mode is obtained using a 1.6°-tilted FM-FBG. Influences of the weakly tilted FM-FBG parameters on the property of mode conversion is analyzed and discussed. A simultaneous mode conversion and demultiplexing scheme for 4-mode × 3-wavelength multiplexing transmission is proposed and the modal crosstalk is analyzed based on the transmission spectra of the tilted FM-FBGs. The proposed approach shows potential applications in mode and wavelength division multiplexing communication systems.

Broadband Mode Converter Based on Photonic Crystal Fiber

In this letter, the scheme of a tapered mode-selective coupler based on photonic crystal fiber (PCF) is proposed. Three typical mode converters, such as $\mathrm{LP}_{\text {01}}$ to $\mathrm{LP}_{\text {11a}}$ , $\mathrm{LP}_{\text {01}}$ to $\mathrm{LP}_{\text {11b}}$ , and $\mathrm{LP}_{\text {01}}$ to $\mathrm{LP}_{\text {21}}$ , are described and simulated in detail. The coupling loss and bandwidth is calculated to evaluate those mode converters. The results demonstrate that those mode converters are broadband and low-loss devices. In addition, those mode converters are insensitive to the orthogonal polarizations. They can be fabricated using PCF postprocessing technique and are useful for future mode-division multiplexing optical fiber communication systems.

Analytical bit error rate performance evaluation of an orthogonal frequency division multiplexing power line communication system impaired by impulsive and Gaussian channel noise

An analytical approach is presented to evaluate the bit error rate (BER) performance of a power line (PL) communication system considering the combined influence of impulsive noise and background PL Gaussian noise. Middleton class-A noise model is considered to evaluate the effect of impulsive noise. The analysis is carried out to find the expression of the signal-to-noise ratio and BER considering orthogonal frequency division multiplexing (OFDM) with binary phase shift keying modulation with coherent demodulation of OFDM sub-channels. The results are evaluated numerically considering the multipath transfer function model of PL with non-flat power spectral density of PL background noise over a bandwidth of 0.3–100 MHz. The results are plotted for several system and noise parameters and penalty because of impulsive noise is determined at a BER of 10−6. The computed results show that the system suffers significant power penalty because of impulsive noise which is higher at higher channel bandwidth and can be reduced by increasing the number of OFDM subcarriers to some extent. The analytical results conform well with the simulation results reported earlier.

Interferometric orbital angular momentum mode multiplexer/demultiplexer based on pairs of cylindrical lenses

A novel interferometric configuration for orbital angular momentum (OAM) mode multiplexing/demultiplexing, based on the image transformation property of pairs of cylindrical lenses and without any splitting loss, is proposed. The effectiveness in mode demultiplexing has been experimentally proven by performing bit error rate measurements for a 20 Gbit/s mode division multiplexing communication system where two modes with different values of OAM, corresponding to the angular indices l = 0 and 1, respectively, are transmitted.

Piezoelectric transceiver matching for multiple frequencies

Abstract. Robust data transmission over long ranges with standard ultrasound devices is a challenge. Ultrasound indoor positioning systems in particular require long ranges and a robust data communication link. Fundamentally, a piezoelectric transducer has a narrow bandwidth for high sound pressure level and efficiency and is not suitable for broad-band applications. Moreover, ultrasound attenuation in the air increases quadratically within frequency, and thus ultrasound localization systems are restricted to low frequencies and low bandwidths. This work presents a novel method to match a piezoelectric transceiver for multiple frequencies by using the parallel and the series resonance of the transceiver. The aim is to adjust the amplitudes at different frequencies from different senders to the same level, which is important for orthogonal frequency division multiplex communication systems. Hence, an analog-to-digital converter (ADC) with low dynamic range (low voltage resolution) can be used to measure multiple frequencies with the same resolution. As a result, the optimization decreases the required dynamic range by 6 dB. Consequently, the ADC requires 1 bit fewer to ensure the same resolution for all carrier frequencies.

Symbol Timing Sequence Structure for OFDM-CDMA Communication System under Low SNR

Symbol synchronization offset of orthogonal frequency division multiplexing (OFDM) communication system will cause the rotation of data constellation at receiver even if such offset falls in the range of the appended cyclic prefix. When OFDM modulation is combined with spread spectrum technique, e.g., OFDM code division multiple access (OFDM-CDMA), the problem becomes more serious since OFDM-CDMA communication system generally works under very low signal-to-noise ratio. This paper focuses on symbol timing synchronization of OFDM-CDMA communication system and proposes an effective sequence structure on the basis of circular conjugate-symmetric properties of discrete Fourier transform. The proposed frequency-domain sequence not only can generate real time-domain signal to reduce calculation complexity, but also can resist peak-to-average power ratio of OFDM modulation by exploiting suitable computer search algorithm. The simulation results show a sharp cross-correlation peak can be obtained, which is quite helpful for accurate symbol timing synchronization of OFDM-CDMA communication system.

A narrow bandpass filter based on two-dimensional photonic crystals with two reflectors

With the rapid development of wavelength division multiplexing technology, narrow bandpass filters have drawn widespread public attention. In this paper, a compact narrow bandpass filter based on two-dimensional photonic crystals is proposed. The transfer characteristics of the filter with a single mode resonator and two reflectors are analyzed by using coupled mode theory. Research results show that the bandwidth of the filter can be controlled by adjusting the distance between the resonator and the two reflectors, which can be applied to the realization of narrow bandpass filters, and even ultra-narrow bandpass filters. Based on the theoretical model mentioned above, we design a narrow bandpass filter based on two-dimensional photonic crystals, which is composed of silicon rods with square lattice in air. Two single mode waveguides are formed by removing two rows of rods. Meanwhile, a point cavity is formed by removing a dielectric column. In order to precisely control the phase change between the resonator and the two reflectors, a phase adjustment region is introduced. We study the transmission spectrum of the structure by the finite-difference time-domain (FDTD) method. We find that the bandwidth of the filter can be narrowed when the phase change between the resonator and the two reflectors satisfies the specific conditions, and the transmission ratio is still high as well. These are consistent with the theoretical analyses. But it is worth noting that there is a difference between the simulation result and theoretical result. This is because in the theoretical analysis, we consider that the propagation constants of the frequencies close to the central frequency are the same. In fact, the propagation constant increases with the increase of frequency, however, this does not affect the central frequency nor its transmission. The performance of the designed filter is analyzed by FDTD, showing that the working frequency is close to 193.40 THz, the bandwidth is smaller than 5.9 GHz, the peak transmittance is up to 94%, and the length of the working area is only 9 m. Compared with the conventional photonic crystal filters, the designed narrow bandpass filer is very compact, and the performance is suitable for dense wavelength division multiplexed communication systems.

Multiplex Communication with Synchronous Shift and Weight Learning in 2D Mesh Neural Network

We have previously proposed a multiplex communication system in a neural network. However, this system is designed to force the network to communicate in a multiplexed manner, in which “codes” or “temporal sequences” are inevitably induced. This means that the network has a main loop and coding/decoding circuits, which are somewhat artificial. In this paper, we show that it is also possible to communicate without these artificial guidance aids by multiplexing in a 2D mesh-type neural network, where learning procedures are used to find paths from an originating neuron to a destination neuron. We also provide statistics from these neural networks to show that random sequences occur more frequently than non-random sequences.

Multimodulus blind equalization algorithm based compensation for I/Q imbalance in OFDM optical fiber communication

The performance enhancement using multimodulus blind equalization algorithm for compensating in-phase/ quadrature-phase (I/Q) mismatch in orthogonal frequency division multiplexing optical communication system is investigated. The mathematical theory of I/Q imbalance is analyzed. Then, a compensation scheme with two-tap equalizer is implemented, with this scheme, the joint compensation of transmitter and receiver with optical channel that contains noise attenuation and chromatic dispersion is realized. Simulation shows that the performance degradation caused by I/Q imbalance can be compensated by the scheme with multimodulus blind equalization algorithm effectively regardless of the modulation order and channel distortion.

Experimental demonstration of half cycle 64-QAM Nyquist-SCM direct-detection optical communication system with data-aided estimation and overlap frequency-domain equalization

A half cycle 64-quadrature amplitude modulation (QAM) Nyquist subcarrier modulation (SCM) polarization division multiplexing (PDM) intensity modulation direct detection optical communication system is experimentally demonstrated. At the receiver, training sequences-based channel estimation and an overlap frequency domain equalization method are proposed to enhance the system performance. The experimental results show that the half cycle 64-QAM Nyquist-SCM PDM signal can be transmitted over 43-km standard single-mode fibers with a bit error rate below the forward error coding threshold of 2.4×10−2.

An arbitrary mode converter with high precision for mode division multiplexing in optical fibers

In this paper, a high-precision arbitrary-mode converter is proposed for mode-division multiplexing optical fiber communication systems. As the kernel of this converter, a phase-only spatial light modulation based on simulated annealing algorithm is used as a spatial spectrum filter. The results demonstrate that our proposed mode converter is of high precision and is capable of converting different modes without any physical change.