Orthogonal Frequency Division Multiplexing System In Terms Research Articles

Robust Transmission using Efficient Adaptive Modulation and Coding in Modern OFDM Systems

Abstract: OFDM (Orthogonal Frequency Division Multiplexing) as a special multi-carrier transmission technology has good resistance to narrow-band interference and frequency selective fading ability. Using adaptive modulation and coding in an OFDM systems in comparison with traditional fixed modulation techniques, it becomes possible to take full advantage of spectrum resources enhancing bandwidth efficiency and system capacity making it suitable for the high-speed and reliable mobile communication systems in the future. In AMC (Adaptive Modulation and Coding) technique, modulation method, coding rate, and/or other signal transmission parameters are dynamically adapted to the channel conditions to increase the system performance in terms of Bit Error Rate (BER) and throughput (bps) in various conditions such as channel mismatch, Doppler spreads, fading, etc. The purpose of this paper is to investigate the effect of adaptive modulation and coding in an OFDM system in terms of bit error rate and SNR over fading communication channels. In this paper, we have considered fixed and adaptive modulation technique in an OFDM system over AWGN and RICEAN channel with convolutional and BCH coding. An OFDM system model is simulated in MATLAB programming environment to obtain the simulation result. From the comparative simulation result analysis in terms of BER v/s SNR, it can be concluded that Adaptive modulation with Convolutional coding over RICEAN channel significantly reduces bit error rate giving better trade-off between SNR and BER than all other options considered here.

High-security UFMC optical transmission system of seven-core fiber based on updating the 3D discrete chaotic model.

In this paper, we propose a high-security universal filtered multicarrier (UFMC) transmission system based on a novel, to the best of our knowledge, three-dimensional (3D) discrete chaotic model. The cascade of counters and the 3D discrete chaotic system is used to generate three masking factors and update these factors to encrypt the bitstream, constellation, and the information of the subcarriers. Using this structure, the key space is 10270 and the key is updated in a timely manner. In this work, the encryption scheme solves the dynamic degradation of chaotic cryptography. Experimental results show that there is no significant difference in error performance in comparison with before encryption. Compared with the orthogonal frequency division multiplexing (OFDM) system, the spectrum efficiency of the UFMC system is higher, which reduces out-of-band leakage and interference between services. Considering the complexity constraints of the receiver, the inter-symbol interference caused by fiber dispersion is alleviated by optimizing the distribution of data subbands, and the bit error rate (BER) performance is improved. It is shown that the UFMC system obtains 1.9 dB gain compared with the OFDM system in terms of receiver sensitivity when the BER is 10-3. This encryption scheme has a promising application prospect in short-distance optical access systems in the future.

Performance Analysis of FS-FBMC/OQAM System Using Turbo and LDPC Codes

Filter Bank Multicarrier (FBMC) with offset QAM (OQAM) is a possible waveform candidate for 5G and future wireless systems and standards. FBMC is realized using either polyphase network (PPN)-FFT or Frequency Spread (FS)-FBMC. In this paper, channel coding schemes such as turbo codes and low-density parity check (LDPC) codes were investigated in terms of their peak-to-average power ratio (PAPR) and bit-error-rate (BER) reduction performances for FS-FBMC/OQAM and Orthogonal Frequency Division Multiplexing (OFDM) systems in additive white Gaussian noise (AWGN) channel. Simulation results show that turbo coded and LDPC coded FS-FBMC/OQAM systems performed better than turbo coded and LDPC coded OFDM systems in terms of BER performance. Also, LDPC coded FS-FBMC/OQAM system because of its adequate BER performance, reasonable PAPR performance, and reduced computational complexity was found to be an appropriate choice among other systems for 5G and future wireless networks. It was also found that incorporating channel coding resulted in a very large coding gain with only a minor rise in the system's PAPR value.

Frequency-domain subcarrier diversity receiver for discrete Hartley transform OFDM systems

In recent years, there has been an increasing interest in the use of the discrete Hartley transform (DHT) for orthogonal frequency-division multiplexing (OFDM). Previous studies have not reported an improvement in bit error rate (BER) when using a DHT-based OFDM without substantial complexity increase due to the fact that the DHT cannot diagonalize a circulant channel matrix but rather produces a unique structured matrix that has only a main diagonal and anti-diagonal. In this paper, the coupling between symmetric carriers is exploited by introducing a frequency-domain subcarrier diversity receiver for DHT-based OFDM systems. The proposed simple receiver structure takes advantage of the coupling to enhance the BER performance of the DHT-OFDM system by increasing the diversity gain between subcarriers. A detailed statistical and performance analysis of the system is presented, in which closed-form expressions of the average BER were derived. The proposed system performance was compared to the conventional discrete Fourier transform OFDM (DFT-OFDM), the generalized DHT-OFDM, and the DHT-precoded OFDM systems in terms of average BER and peak-to-average power ratio (PAPR). The proposed DHT-OFDM system outperforms the generalized DHT-OFDM system by 12 dB and outperforms the conventional DFT-OFDM and the DHT-precoded OFDM systems by 17 dB at an average BER of 10−5. In terms of PAPR, the proposed DHT-OFDM system suffers from an approximately 5.5, 2, and 1 dB increase in PAPR when compared to the DHT-precoded OFDM, the conventional DFT-OFDM, and the generalized DHT-OFDM systems, respectively.

Spectral Efficiency Enhancement Based on Sparsely Indexed Modulation for Green Radio Communication

Jointly enhancing both energy efficiency (EE) and spectrum efficiency (SE) of modulation schemes becomes one of the main issues for 5G mobile communications. Recently, an indexed modulation (IM) technique provides an interesting tradeoff between EE and SE. Data can be conveyed through the combination of subcarriers pattern that can be divided between activated/non-activated subcarriers in the frequency domain. Maximum SE can be attained at half subcarrier activation, hence producing symbols with half energy of the conventional orthogonal frequency-division multiplexing (OFDM) system. In this paper, alternatively, the new concept of sparsely indexing modulation (SIM) on overall subcarrier space is clarified. Sparse (few) subcarrier activations provide much higher EE, while the combinatorial indexing of the sparse subcarriers on the overall subcarriers as a single group spans huge combinatorial space that provides approximately the same SE of the plain OFDM system. The fallacy of indexing difficulty on overall subcarrier space without grouping is resolved. Moreover, a further SE improvement is suggested by introducing permutation-based indexing and combinatorial indexing on over-complete dictionaries. Sparsely indexing represents the cornerstone, which enables compressive sensing tools to enforce IM gains. Based on the conducted simulations, the proposed SIM scheme outperforms the conventional OFDM system in terms of the error performance, the peak-to-average power ratio, and the EE with the same spectral efficiency without channel coding complexity. The proposed SIM scheme is considered one of the energy saving-oriented modulations.

Power Spectral Analysis of UW-OFDM Systems

Unique word- (UW-) orthogonal frequency division multiplexing (OFDM) is a new multicarrier technique that is shown to be superior to the cyclic prefix- (CP-) OFDM system in terms of bit-error-rate performance in fading channels. Furthermore, in the literature, it is shown by simulations that UW-OFDM has a considerably lower out-of-band (OOB) radiation compared with CP-OFDM. In this paper, we derive an analytical expression to investigate the spectral characteristics of UW-OFDM to be able to determine the effect of the different system parameters on the OOB radiation. In addition, we include in our analysis the effects of the interpolation filter and the unique word. We show that the OOB radiation in UW-OFDM is mainly determined by the number of non-modulated subcarriers reserved as the guard band, and the ratio of the UW sequence length to the number of subcarriers. By increasing either of these two parameters, we show that we can suppress the OOB radiation to any amount. This is an important advantage of UW-OFDM over classic CP-OFDM systems, and shows that UW-OFDM is an excellent candidate for cognitive radio systems.

Transforming the Statistical Distribution of Wireless OFDM Signal for PAPR Reduction

To reduce the high peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) signal, a novel nonlinear companding transform (CT) scheme is proposed in this paper. This scheme can reallocate both the amplitude/power as well as statistical distribution of the companded signal more reasonably and flexibly than existing CT methods with low computational complexity. By choosing an appropriate companding parameter, it can provide more effective PAPR reduction but at the price of a minimal amount of bit-error-rate (BER) performance degradation caused by the companding distortion. The closed-form expressions including the achievable PAPR gain, signal attenuation factor, and corresponding selection criteria for the companding parameter were derived. Computer simulations demonstrate that the scheme significantly improves the overall performances of OFDM system in terms of PAPR, BER and bandwidth efficiency under the multipath fading channel or with the high power amplifier.

Simplified performance estimation of ISM-band, OFDM-based WSNs according to the sensitivity/SINAD parameters

A novel method is proposed to estimate committed information rate (CIR) variations in typical orthogonal frequency division multiplexing (OFDM) wireless local area networks (WLANs) that are applied in wireless sensor networks (WSNs) and operate within the industrial, scientific and medical (ISM) frequency bands. The method is based on the observation of a phenomenon of which the significance has not previously been recognized nor documented; here termed the service level differential zone (SLDZ). This method, which conforms to the ITU-T Y.1564 test methodology, provides the means to set a committed information rate (CIR) reference for IEEE 802.11a/g/n OFDM systems in terms of committed throughput bandwidth between a test node and an access point (AP) at a specific range. An analytical approach is presented to determine the relationship between the maximum operating range (in metres) of a wireless sensor network for a specific committed throughput bandwidth, and its link budget (in dB). The most significant contributions of this paper are the analytical tools to determine wireless network capabilities, variations and performance in a simplified method, which does not require specialized measurement equipment. With these it becomes possible for industrial technicians and engineers (who are not necessarily information technology (IT) network experts) to field analyze OFDM WLANs and so qualify their performance in terms of Y.1564 specified service level agreement (SLA) requirements, as well as in terms of the widely acceptable sensitivity/SINAD parameters.

Fuzzy Logic Based Modified Adaptive Modulation Implementation for Performance Enhancement in OFDM Systems

Adaptive modulation is one of the recent technologies used to improve future communication systems. Many adaptive modulation techniques have been developed for the improving the performance of Orthogonal Frequency Division Multiplexing (OFDM) system in terms of high data rates and error free delivery of data. But uncertain nature of wireless channel reduces the performance of OFDM system with fixed modulation techniques. In this paper, modified adaptive modulation technique has been proposed which adapts to the nature of communication channel based upon present modulation order, code rate, BER and SNR characterizing uncertain nature of communication channel by using a Fuzzy Inference System which further enhances the performance of OFDM systems in terms of high transmission data rate and error free delivery of data.

Weighted-noise threshold based channel estimation for OFDM systems

Orthogonal frequency division multiplexing (OFDM) technology is the key to evolving telecommunication standards including 3GPP-LTE Advanced and WiMAX. Reliability of any OFDM system increases with improved mean square error performance (MSE) of its channel estimator (CE). Particularly, a least squares (LS) based CE incorporating a time-domain denoising threshold, enables better MSE performance, while avoiding the need for a-priori knowledge of channel statistics (KCS). Existing optimal time-domain thresholds exhibit suboptimal behavior for completely unavailable KCS environments. This is because they involve consistent estimation of one or more KCS parameters, and corresponding estimation errors introduce severe degradation in MSE performance of the CE. To overcome the MSE degradation, this paper proposes a weighted-noise threshold, by introducing a modified hypothesis-testing-problem (HTP) interpretation. Derivation of resulting analytical MSE expression is also provided. Results of OFDM system simulations carried out in rayleigh faded ITU-TU6 and WiMAX-SUI4 channel environments with U-shaped power spectral densities, are presented. The performance results show that, compared to many of the existing thresholds, the proposed threshold renders better MSE performance to the CE and higher reliability to the OFDM system in terms of better bit error rate (BER) performance.

Twofold peak-to-average power ratio reduction technique in direct-detection optical orthogonal frequency division multiplexing system

High peak-to-average power ratio (PAPR) of the orthogonal frequency division multiplexing (OFDM) signal is one of the limitations to the transmission performance of an optical OFDM system. Many PAPR reduction techniques have been proposed in previous works. However, most of them only consider a single scheme while overlooking the complementary features of these techniques. We propose a twofold PAPR reduction technique called Hadamard transform combined with a partial transmit sequence (PTS) (HTCP). The proposed HTCP scheme combines the merits of two complementary techniques, i.e., Hadamard transform and PTS, to improve the performance of the optical OFDM system in terms of PAPR and bit error rate (BER). Furthermore, the side information generated in PTS is transmitted by pilot sequences which increase the utilization of subcarriers. Least square estimation is used to estimate the pilot signal’s phase to recover the side information. The HTCP scheme is theoretically analyzed in a direct-detection optical OFDM system. Simulation results show that the HTCP scheme has a better performance with regards to PAPR and BER compared with the case of applying only the Hadamard transform or PTS technique.

Performance on ICI Self-Cancellation in FFT-OFDM and DCT-OFDM System

In orthogonal frequency division multiplexing (OFDM) system, the existence of frequency offset in AWGN channel affects the orthogonality among the subcarriers and consequently introduces the intercarrier interference (ICI). The paper investigates new ICI self-cancellation technique to mitigate the effect of ICI in FFT-OFDM and compares it to DCT based OFDM system in terms of bit error rate (BER) and carrier to interference ratio (CIR). The proposed method for group size three results in a significant 20 dB improved CIR in FFT-OFDM. In terms of BER, proposed ICI self-cancellation technique outperforms the other self-cancellation techniques in FFT-OFDM. Also, this paper investigates outperforming BER and CIR improvement by using DCT-OFDM without applying self-cancellation techniques, due to its energy compaction property.

Hadamard Transform Combined with PTS for PAPR Reduction in Direct-Detection Optical OFDM System

High Peak-to-Average Power Ratio (PAPR) of the Orthogonal Frequency Division Multiplexing (OFDM) signal is one of the limitations to the transmission performance of optical OFDM system. In this paper, we propose a novel hybrid PAPR reduction technique called Hadamard Transform Combined with Partial transmit sequence (HTCP). The proposed HTCP scheme combines the merits of two complementary techniques, i.e., Hadamard transform and Partial Transmit Sequence (PTS), to improve the performance of the optical OFDM system in terms of PAPR and Bit Error Rate (BER). Simulation results show that the HTCP scheme has better performance with regard to PAPR and BER compared with the case of applying only the Hadamard transform or PTS technique.

Polar-Based OFDM and SC-FDE Links Toward Energy-Efficient Gbps Transmission Under IM-DD Optical System Constraints [Invited

Recent studies have investigated the use of orthogonal frequency division multiplexing (OFDM) and single carrier frequency-domain equalization (SC-FDE) for gigabyte per second (Gpbs) short-range optical wired and wireless access networks based on direct intensitymodulation with direct detection (IM-DD). OFDM systems have an inherent high peak-to-average power ratio (PAPR). SC-FDE systems have lower PAPR and thus outperform comparable OFDM systems in terms of bit-error performance under various practical considerations, mainly the limited dynamic range of operation at the transmitter. In RF-based OFDM and SC-FDE systems, the output signal is bipolar, complex, and cannot be transmitted in IM-DD systems. Therefore, several power efficient schemes, however spectrally inefficient, have been proposed to make positive OFDM and SC-FDE signals, e.g., asymmetrically clipped optical OFDM (ACO-OFDM) and repetition and clipping optical SCFDE (RCO-SCFDE). To increase the data rate, novel OFDM and SC-FDE signal formats, called polar OFDM (P-OFDM) and polar SC-FDE (P-SC-FDE), based on a polar representation of complex symbols, are proposed. The proposed format offers twice as much spectral efficiency as state-of-the art unipolar OFDM and SC-FDE formats. Moreover, using a power allocation approach on the time-domain positive samples, the PAPR is further reduced, and the numerical evaluation of the bit-error performance under optical source power and dynamic range constraints demonstrates superior results toward Gpbs transmission.

Wireless channel model using stochastic high‐level Petri nets for cross‐layer performance analysis in orthogonal frequency‐division multiplexing system

In this study, the authors form a wireless channel model for orthogonal frequency-division multiplexing (OFDM) systems with stochastic high-level Petri net (SHLPN) formalism in order to simplify the cross-layer performance analysis of modern wireless systems. Compared with existing finite state Markov channel model whose state space grows exponentially with the number of OFDM subchannels, the author's proposed SHLPN model uses state aggregation technique to deal with this problem. Closed-form expressions to calculate the transition probabilities among the compound markings of the SHLPN model are provided. When applied to derive the performance measures for OFDM system in terms of the average throughput, average delay and packet dropping probability, the SHLPN model can accurately capture the correlated time-varying nature of wireless channels. Simulation is performed to show that the numerical results offered by the proposed model are more accurate compared with other simplified channel models for avoiding state space complexity.

BER Analysis and Compensation for the Effects of Polynomial HPA Non-linearity in MIMO OFDM Systems Over Fading Channel

The nonlinearities caused by the High Power Amplifier (HPA) has a crucial effect on the performance of Multiple Input Multiple Output (MIMO) with Orthogonal Frequency Division Multiplexing (OFDM) systems. In this paper, we investigate the performance of Vertical Bell Laboratories Layered Space-Time (VBLAST) MIMO OFDM systems in the presence of non HPA which is modeled as a NL polynomial model. Specifically, we present a theoretical analysis of NL distortion effects in VBLAST MIMO OFDM systems in terms of error probability and system capacity. Therefore, we propose a nonlinear distortion compensator based on Neural Network (NN) accompanied with Minimum Mean Square Error receiver which corrects this nonlinearity. In the fact, the correction is done at the transmitter after HPA and at the receiver level after OFDM demodulation in frequency and time domain. Also, we present the structure of the NN adapted to both the predistortion and frequency domain. Simulation results show the comparison between the theoretical and simulated of nonlinear distortion effects in MIMO-OFDM transmission systems, as well as the performance of different methods of compensation proposed.

Low complexity Wizard amplitude shaping system for peak‐to‐average power ratio reduction

Orthogonal frequency division multiplexing (OFDM) is a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme for wide-band digital communication. Despite its advantages, it has a major drawback of its high peak-to-average power ratio (PAPR) value which degrades its efficiency and increases its cost. In this study, two proposed systems are discussed to reduce the PAPR value. They depend on a signal conversion to an approximated triangle signal by means of a proposed Wizard amplitude shaping encoder. The whole characteristics mathematical analysis are presented for the proposed systems. In addition, the complexity evaluations, reliability, in-band distortion; in terms of error vector magnitude, time and frequency domain behaviours are explained. The two proposed systems are compared with the conventional OFDM system in terms of PAPR and bit error rate under additive white Gaussian noise channel and multipath fading channels. Furthermore, the impacts of the proposed schemes design parameters, are studied.

An Efficient Nonlinear Companding Transform for Reducing PAPR of OFDM Signals

In this paper, an efficient nonlinear companding scheme is proposed to reduce the peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) signals. By transforming the statistics of original signals into a specified distribution form along with remaining an unchanged average output power level, this scheme can achieve significant reduction in PAPR as well as an improved BER performance simultaneously. Moreover, by properly adjusting the transform parameters, more design flexibility in companding form can also be given by this scheme to satisfy various system requirements. The theoretical analysis results regarding the transform gain, companding distortion and the selection criteria of parameters are derived. It is proved that this scheme may lead to less signal distortion compared to existing companding schemes. Simulation results demonstrate that the proposed scheme can substantially offer better overall performance of OFDM system in terms of PAPR reduction, BER performance and bandwidth efficiency.

Quasi-Orthogonal Time Division Multiplexing and Its Performances in Rayleigh Fading Channels

This paper proposes an efficient transmission scheme, Quasi-Orthogonal Time Division Multiplexing (QOTDM), which employs the shift orthogonal property of the pulse function with raised-cosine spectral shape, and the signal waveforms are quasi-orthogonal in time domain. Comparing to orthogonal frequency division multiplexing (OFDM), QOTDM is less sensitive to carrier frequency offset and power amplifier nonlinearities while keeping a similar spectral efficiency with OFDM due to single-carrier characteristics. QOTDM is a suitable consideration for the downlink transmission such as in satellite communications. An upper bound of sample error probability (SER) is derived to evaluate the performance of QOTDM. Comparisons of QOTDM and OFDM in Rayleigh fading channels show that the proposed QOTDM system is better than that of OFDM system in terms of bit error rate (BER) in high Eb/No regions.

PAPR 감소와 주파수 다이버시티 효과를 갖는 FD-CI-OFDM 시스템의 성능 평가

높은 데이터 전송 속도를 갖는 OFDM(Orthogonal Frequency Division Multiplexing) 통신 시스템은 시간 영역에서 높은 PAPR(Peak-to-Average Power Ratio)이 발생되는 중요한 단점이 있다. 이 때문에 송신 신호의 비선형 왜곡을 일으켜 통신 성능을 매우 나쁘게 된다. CI(Carrier Interferometry) - OFDM 시스템은 각각 병렬 데이터를 N개의 모든 부반송파와 서로 직교하는 위상 성분에 실어 전송함으로써 첨두값이 낮추는 효과를 얻는다. 또한, 한 데이터를 N개의 부반송파에 나누어 전송하기 때문에 주파수 다이버시티(FD: Frequency Diversity) 효과를 얻는다. 그러므로 CI-OFDM은 PAPR 저감과 BER 개선에 우수한 성능을 보인다. 그러나 CI-OFDM을 실제로 구현할 때, 송신기에서 모든 데이터마다 각각 다른 위상 성분들과 반송파에 실려 동시에 전송되기 때문에 수신기에서 독립적인 위상 성분들을 차례로 보상할 수 있는가가 매우 중요한 문제점이 된다. 이 논문에서는 Walsh Hadamard 시퀀스를 사용하여 각 병렬 데이터를 구분하고, 동시에 기존 CI-OFDM의 특성을 갖는 개선된 CI-OFDM, 즉 FD-CI-OFDM 시스템을 제안하고 성능을 평가한다 컴퓨터 시뮬레이션을 통하여 그 성능이 기존 OFDM이나 CI-OFDM보다 개선됨을 보인다. Orthogonal frequency division multiplexing(OFDM) that is very useful for the high-speed communication system has serious problem of high peak-to-average power ratio(PAPR) in time domain. Because of this, the non-linear distortion can be produced and system performance gets worse. CI-OFDM system can get the peak power lowered. In this CI-OFDM system, each parallel data is distributed into N all sub-carriers and conveyed by the orthogonal phase factor. Also, CI-OFDM shows frequency diversity effect. Therefore, CI-OFDM system is better than ordinary OFDM system in terms of BER performance and the PAPR reduction. When it is implemented, however, there is a serious problem whether it can separate and compensate the phase factor in order in the receiver. Because all bits are transmitted simultaneously over all subcarriers and each other phase factors in transmitter. In this paper, we propose and evaluate a FD-CI-OFDM system that is a version of improved CI-OFDM. This is designed by use of the Walsh Hadamard sequence. The FD-CI-OFDM shows better performance than ordinary OFDM and CI-OFDM system.