Modulated Orthogonal Frequency Division Multiplexing Systems Research Articles

Evaluating Performance: A Study of Encrypted vs. Unencoded Signals in SISO-OFDM with LS and MMSE Estimations

The continuous progress in digital communication has played a crucial role in meeting the increasing need for faster data rates. Orthogonal Frequency Division Multiplexing (OFDM), a pivotal methodology in this progression, attains improved data rates through the efficient utilisation of densely packed carriers within a specified channel bandwidth. This article focuses on the investigation of channel estimation in OFDM systems. the study of search lies in its examination of the performance consequences associated with the incorporation or lack thereof of a convolutional encoder in OFDM systems and looks at how well two well-known channel estimation algorithms, Least Square (LS) and Minimum Mean Square Error (MMSE), work in 4-Quadrature amplitude modulation (4-QAM) OFDM systems with and without a convolutional encoder with a comprehensive evaluation of the efficacy of the OFDM system across various channel conditions. It uses MATLAB implementations as its main tool. The findings of the study indicate that the MMSE algorithm, despite its higher complexity, exhibits superior performance in comparison to the LS algorithm when combined with a convolutional encoder. The gain in terms of bit error rate (BER) improvement approximately 12 dB. This represents the logarithmic scale improvement in BER from BER1(uncoded) to BER2(coded) at the same the energy per bit to noise power spectral density ratio (Eb/N0) of 40 dB.

Iterative Compensation for Clipping Noise in Spatial Modulation OFDM Systems

Clipping is an effective technique to adjust the peak-to-average power ratio (PAPR) of spatial modulation (SM) orthogonal frequency-division multiplexing (OFDM) systems. In this paper, an iterative soft compensation method is proposed for mitigating the effect of the clipping noise. In the proposed method, the sparsity and relativity of all transmit antennas are exploited, and the mean and power of the clipping noise are estimated based on the a priori information. Furthermore, the attenuation factor in each iteration is updated to yield a more accurate system model. Simulation results show that the proposed method is capable of providing considerable bit error rate (BER) performance gains over conventional compensation methods for both balanced and unbalanced antenna configurations.

An Improved Variational Inference Approach to Iterative OFDM Receiver Design for Superimposed Training-Based AF Relay Networks

Using variational Bayesian approximation (VBA), we propose a general framework for joint channel estimation, demodulation, and decoding in an orthogonal frequency-division multiplexing (OFDM) amplify-and-forward (AF) relay network. AF relaying is adopted due to its simplicity and ease of implementation, and both cases of fixed- and variable-gain AF protocols are addressed. Unlike the classical approach where a number of subcarriers are solely dedicated to pilots, partial data-dependent superimposed training is considered here, which preserves the spectral efficiency. Assuming a bit-interleaved coded modulation OFDM system at the source, a modified formulation of VBA is proposed, which is, in fact, a Bayesian framework of turbo data detection of OFDM under channel estimation errors. More precisely, by an appropriate exploitation of the inherent “soft” information on channel and data available from the VBA formalism, we derive a modified iterative receiver, which reduces the impact of channel uncertainty by averaging the “soft data decision” over the posterior distribution of the unknown channel at each decoding iteration. The proposed modified VBA approach is contrasted to the conventional VBA and to the mismatched VBA approach, where, in the latter, the unknown channel is just replaced by its imperfect estimate. We show that conventional and mismatched VBA are suboptimal and can be viewed as a particular case of the proposed VBA method. In addition, we show that the VBA approach makes a nice connection between the classical techniques such as pilot-only channel estimation and the expectation–maximization algorithm, which can be viewed as a special case of VBA. By comparison with state-of-the-art VBA-based estimators through numerical analysis, we highlight the superiority of our modified VBA method and demonstrate a notable performance improvement in terms of the bit error rate.

基于副载波外差检测的副载波-正交频分复用系统误码性能研究

基于副载波外差检测的副载波-正交频分复用系统误码性能研究

CSI feedback-based CS for underwater acoustic adaptive modulation OFDM system with channel prediction

In this paper, we investigate the performance of adaptive modulation (AM) orthogonal frequency division multiplexing (OFDM) system in underwater acoustic (UWA) communications. The aim is to solve the problem of large feedback overhead for channel state information (CSI) in every subcarrier. A novel CSI feedback scheme is proposed based on the theory of compressed sensing (CS). We propose a feedback from the receiver that only feedback the sparse channel parameters. Additionally, prediction of the channel state is proposed every several symbols to realize the AM in practice. We describe a linear channel prediction algorithm which is used in adaptive transmission. This system has been tested in the real underwater acoustic channel. The linear channel prediction makes the AM transmission techniques more feasible for acoustic channel communications. The simulation and experiment show that significant improvements can be obtained both in bit error rate (BER) and throughput in the AM scheme compared with the fixed Quadrature Phase Shift Keying (QPSK) modulation scheme. Moreover, the performance with standard CS outperforms the Discrete Cosine Transform (DCT) method.

Simplified Approach to Optimized Iterative Clipping and Filtering for PAPR Reduction of OFDM Signals

Iterative clipping and filtering (ICF) is a well-known technique to reduce the peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) signals. Recently, Wang and Luo investigated the clipped signal and proposed a modified algorithm called optimized ICF (OICF). This is an optimal algorithm since it can achieve the required PAPR reduction with minimum in-band distortion and far fewer iterations. However, OICF needs to solve a convex optimization problem with O(N3) complexity, where N represents the number of subcarriers. In this paper, instead of analyzing the clipped signal, we study the clipping noise and propose a simplified OICF algorithm. In the new algorithm, solving the convex optimization problem is approximated by some simple algebraic operations and the computational complexity reduces to O(N). Simulation results show that after three iterations, the original OICF algorithm can achieve the desired PAPR while the simplified one exhibits almost the same performance: for a 128-subcarrier and quadrature phase shift keying (QPSK) modulated OFDM system, the PAPR-reduction performance difference between the two algorithms are 5×10-3dB at a 10-4 clipping probability and the bit-error-rate performance difference is 6×10-3 dB at a 10-7 error probability.

An Evaluation of Coded Wavelet for Multicarrier Modulation with OFDM

Orthogonal frequency division multiplexing (OFDM) is pronounced in wireless communication systems. Methods for improving the performance of the OFDM-based systems are mostly sought. A method of doing this involves error correction coding and another, a better multicarrier modulation kernel. In this work, convolutional error correction coding with interleaving is introduced in wavelet multicarrier modulation OFDM system (wavelet-OFDM) to improve the performance of multicarrier systems as the signal traverses the multipath and noisy transmission channels. This is compared with FFT-based multicarrier modulation (FFT-OFDM). Results show that coded wavelet-OFDM saves more than a half of the transmit power than the uncoded wavelet. Also it will be shown that, the interleaved and non-interleaved coded wavelet-OFDM well outperform interleaved coded and non-interleaved coded FFT-OFDM systems respectively.

Combining block-coded modulation codes and improved constellation extended schemes to reduce peak-to-average power ratio in orthogonal frequency-division multiplexing systems

Orthogonal frequency-division multiplexing (OFDM) has been recommended as the standard for numerous wireless transmission systems. However, OFDM still has deficiencies that must be improved, such as a high peak-to-average power ratio (PAPR) occurring in the transmission signal. A constellation extended scheme (CES) is one technology for reducing high PAPR. The PAPR performance of the CES is directly correlated to the amount of candidate signals, high PAPR in OFDM systems will be significantly reduced as the candidate signals increase. However, because calculating the PAPR values of candidate signals requires more inverse fast Fourier transform operations, the CES hardware circuit is difficult to achieve. Therefore this study combines the CES, partial transmit sequences (PTSs) and block-coded modulation (BCM) codes to propose a BCM–CES–PTS technology, which simultaneously improves the high computation of the CES, the necessary side information delivered in PTS and the lack of error correction capabilities in the transmitted signal itself, applying this to a 16-QAM modulation OFDM system. In addition, this study proposes two structures of the CES by arranging the extended constellation points in symmetrical and asymmetrical forms, called S-BCM–CES–PTS and AS-BCM–CES–PTS, respectively. The generator matrices of the two structures were proposed to simplify the division of the circuit into sub-blocks.

Exact Error Performance Analysis of Arbitrary 2-D Modulation OFDM Systems with Carrier Frequency Offset

In an orthogonal frequency division multiplexing (OFDM) system, carrier frequency offset (CFO) causes intercarrier interference (ICI) which significantly degrades the system error performance. In this paper we provide a closed-form expression to evaluate the exact error probabilities of arbitrary 2-D modulation OFDM systems with CFO, and analyze the effect of CFO on error performance.

적응 등화기를 이용한 적응 OFDM 시스템 성능분석

본 논문에서 OFDM 시스템의 성능분석을 Matlab 시뮬레이터를 이용하여 시뮬레이션 하였다. 적응 OFDM 시스템에 채널 추정 알고리즘을 적용하고 멀티미디어 무선통신 다중경로 페이딩 채널환경에서 시스템에 미치는 영향을 분석하고 추정된 결과를 이용하여 적응 등화기를 적용한 적응 OFDM 시스템의 성능을 BER을 통하여 분석하였다. 분석결과, 채널상태에 따라 가변을 취하는 적응변조 OFDM 시스템의 경우 일반적인 시스템에 비해 (BER=<TEX>$10^{-1}$</TEX>)를 기준으로 약 7dB 성능의 이득이 있음을 확인하였다. 따라서 고속 멀티미디어 무선통신 채널환경에서 효율적인 전송을 위해 제안한 적응 시스템이 효율적임을 확인 하였다. In this paper, the performance of OFDM (Orthogonal Frequency Division Multiplexing) was assessed by using computer simulations performed using Matlab. We analyzed channel estimation algorithm for adaptive modulation techniques and effect of system using designed simulator in Multimedia wireless communication multipath fading channel environment. Also, we analyzed performance of adaptive OFDM systems that apply adaptive equalizer using guided result through BER. In result, in case of adaptive modulation OFDM systems that modulation mode changes according to channel state, we knew that adaptive modulation OFDM systems have gains of about 7dB performance than general system (BER=<TEX>$10^{-1}$</TEX>). Thus we know that adaptive OFDM propose systems is required for efficient transmission in the high speed Multimedia wireless communication channel environment.

Performance Improvement of the DSRC System Using a Novel S and Π-Decision Demapper

Based on the constellation diagram of the different modulations, a novel S and Π-decision rule is designed for the analog demapper of the orthogonal frequency-division multiplexing (OFDM) systems. The dedicated short-range communications (DSRC) system is chosen as an OFDM platform to compare the performance among the proposed S and Π-decision decoder, hard-decision and soft-decision decoders. Simulation results demonstrate that both the complexity and performance of S and Π-decision demapper used for M-ary quad-rature amplitude modulation (QAM) OFDM system can be greatly improved. The number of decisions be-tween the received symbol and constellation points can be simplified to look up table times for M-ary QAM OFDM system.

Unified models for adaptive OFDM systems when QAM or PSK modulation is applied

AbstractIn this paper, the solutions of the major bit‐loading problems that are found in the literature, are unified into simple equations (models). These equations give an insight of the affection of the channel over a multicarrier system. Moreover, the models allow the transition from the solution of one loading problem to the solution of the others. The models are dependent on the modulation type that is used. We examine the quadrature amplitude modulation (QAM) modulated orthogonal frequency division multiplexing (OFDM), and the phase shift keying (PSK)‐modulated OFDM systems. For each modulation type, the continuous throughput model is extracted. The continuous throughput model refers to a system that its subcarriers can be loaded with non‐integer number of bits. The non‐integer bit distribution can then be rounded to integers, using a rounding method. The models except from the calculation of the optimised quantities (total data rate, total power and symbol error rate) without the need of running a loading algorithm to its extent, provide a means to manipulate a whole multicarrier system in a way similar to single‐carrier system. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Integer QP relaxation-based algorithms for intercarrier-interference reduction in OFDM systems

Orthogonal frequency-division multiplexing (OFDM) modulation can be utilized to deal with severe channel conditions without complex equalization. However, in a fast-fading channel, Doppler spread caused by user mobility destroys the orthogonality among subcarriers, prompting intercarrier interference (ICI). In this paper, the OFDM ICI reduction problem is formulated as a combinatorial optimization problem. Two relaxation methods are proposed to relax the maximum-likelihood detection problem into convex quadratic programming (QP) problems. To further reduce computational complexity, the QP problems are solved by limiting the search to the two-dimensional subspace. A low-bit descent search can also be employed to improve the system performance. The extension to higher-order quadrature amplitude modulation (QAM) OFDM systems is also addressed. Performance results are given which demonstrate that the integer QP relaxation-based algorithms provide excellent performance with reasonable computational complexity.

LDPC coded unitary space-time modulated OFDM system in broadband mobile channel

A unitary space-time modulated orthogonal frequency division multiplexing (OFDM) system using low density parity check (LDPC) code for increasing the system performance is proposed. The proposed system can obtain space-time diversity using unitary space-time modulation without linear operation like space-time block code (STBC), and large coding gain using LDPC code.