Orthogonal Frequency Division Multiplexing Communication Systems Research Articles

Low Complexity Precoded Greedy Power Allocation Algorithms for OFDM Communication Systems

In this paper, an enhanced greedy bit and power allocation algorithms for orthogonal frequency division multiplexing (OFDM) communication systems are introduced. These algorithms combine low complexity greedy power allocation algorithms with a simplified maximum ratio combining (MRC) precoding technique at the transmitter for maximizing the average data throughput of OFDM communication systems. Results of computer simulations show that precoding is an effective technique for improving the throughput performance of the proposed bit and power allocation algorithms.

Low-Complexity Distortionless Techniques for Peak Power Reduction in OFDM Communication Systems

A high peak-to-average power ratio (PAPR) is one of the major drawbacks to using orthogonal frequency division multiplexing (OFDM) modulation. The three most effective distortionless techniques for PAPR reduction are partial transmit sequence (PTS), selective mapping (SLM), and tone reservation (TR). However, the high computational complexity due to the inverse discrete Fourier transform (IDFT) is a problem with these approaches. Implementation of these techniques typically employ direct computation of the IDFT, which is not the most efficient solution. In this paper, we consider the development and performance analysis of these distortionless techniques in conjunction with low-complexity IFFT algorithms to reduce the PAPR of the OFDM signal. Recently, proposed IFFT-based techniques are shown to substantially reduce the computational complexity and improve PAPR performance.

Research on the Doppler Shift Mitigation Scheme for OFDM Underwater Acoustic Communication

Doppler shift decreases the estimation accuracy of carrier frequency and may result in divergent estimation. It thus degrades the performance of orthogonal frequency division multiplexing (OFDM) underwater acoustic (UWA) communication system. In this paper, the two-step Doppler shift mitigation scheme[4] was adopted to mitigate the Doppler effect. The innovation lied in the second step in which an enhanced carrier frequency offset (CFO) two-time estimation approach on basis of the pilot was proposed to improve the Doppler shift mitigation scheme. In addition, the new Doppler shift mitigation scheme with the proposed approach was implemented in the OFDM communication system under the simulated multipath-spread channel environment. The experiment results show that the new scheme is able to improve the performance of OFDM UWA communication significantly.

Unified Error-Rate Analysis of OFDM over Time-Varying Channels

Orthogonal frequency division multiplexing (OFDM) transmission over mobile communication channels is very sensitive to time variations due to Doppler spreading. Time variations within the transmission time of one OFDM block destroy the orthogonality among the subcarriers giving rise to interchannel interference (ICI) which degrades the performance of OFDM over fast fading channels. Error rate analysis of OFDM in the presence of ICI is not trivial due to the cross-correlation between the ICI and the useful signal components. Based on accurate statistical models of the ICI, this paper presents a new accurate non-direct analytical method which simplifies greatly performance evaluation of OFDM in the presence of ICI. This results in new unified expressions for bit and symbol error rates of multilevel quadrature amplitude modulation (MQAM) and M-ary phase shift keying (MPSK) based OFDM communication systems over fast Rayleigh multipath fading channels. These results are further extended to include error rate analysis of OFDM with transmit diversity techniques.

Intercarrier Interference Cancellation for Time-Reversed Orthogonal Frequency Division Multiplexing Systems

Time-reversed orthogonal frequency division multiplexing (OFDM) communications arouse much interest in the bandlimited underwater acoustic channel. In this paper, a time-reversed OFDM communication system combined with an intercarrier interference (ICI) self cancellation scheme is proposed to reduce the ICI due to carrier frequency offset. Simulation results demonstrate that time-reversed OFDM systems using the proposed scheme can effectively reduce ICI effects, increase the signal-to-noise ratio (SNR) with low computation complexity, and hence give rise to performance improvement.

Resource Management Techniques for OFDM Systems with the Presence of Inter-Carrier Interference

The aim of the paper is to study the problems of resource management in the presence of inter-carrier interference (ICI) and multipath fading channel for orthogonal frequency division multiplexing (OFDM) systems. OFDM is a promising technique for the broadband wireless communication systems. However, the OFDM communication system is sensitive to ICI which arises because of Doppler spread and carrier frequency offset (CFO). To solve these problems, an optimization method has been exploited, and a computationally efficient method using numerical optimization techniques is proposed. The simulation results show that these derived optimal solutions and proposed suboptimal algorithms as compared with the uniform power allocation algorithm or conventional water-filling algorithm can significantly improve the performance of the OFDM systems.

PAPR Reduction in All-optical OFDM Systems Based on Phase Pre-emphasis

This paper investigates the peak-to-average power ratio (PAPR) theory in all-optical orthogonal frequency division multiplexing (OFDM) optical fibre communication systems. We find out that phase pre-emphasis could effectively reduce PAPR in all-optical OFDM communication systems which employ intensity modulation-direct detection (IM-DD) method. An equation is developed and proposed to calculate suitable phasing values for pre-emphasis. Furthermore, we find out that phase pre-emphasis cannot reduce PAPR effectively in all-optical OFDM systems that employ Phase Shift Keying (PSK) or Quadracture Amplitude Modulation (QAM) method.

Using turbo iterative receiver to mitigate RPI effect in MIMO OFDM communication system

The received power imbalance (RPI) effect exists in multiple antennas wireless communications system due to design flaw, operator's negligence etc. The range of RPI can be from -10dB to 0dB and if the RPI information is not available at the receiver terminal, it can implement turbo iterative receiver by concatenating multi-input and multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) demodulation (MIMO OFDM demodulation, MOD) serially by channel decoder such as low density parity check (LDPC) decoder or Convolution code (CC) decoder to mitigate the RPI effect and to maintain the system performance at certain acceptable level.

A Novel Predistorter Design for Nonlinear Power Amplifier with Memory Effects in OFDM Communication Systems Using Orthogonal Polynomials

Orthogonal frequency division multiplexing (OFDM) signals have high peak-to-average power ratio (PAPR) and cause large nonlinear distortions in power amplifiers (PAs). Memory effects in PAs also become no longer ignorable for the wide bandwidth of OFDM signals. Digital baseband predistorter is a highly efficient technique to compensate the nonlinear distortions. But it usually has many parameters and takes long time to converge. This paper presents a novel predistorter design using a set of orthogonal polynomials to increase the convergence speed and the compensation quality. Because OFDM signals are approximately complex Gaussian distributed, the complex Hermite polynomials which have a closed-form expression can be used as a set of orthogonal polynomials for OFDM signals. A differential envelope model is adopted in the predistorter design to compensate nonlinear PAs with memory effects. This model is superior to other predistorter models in parameter number to calculate. We inspect the proposed predistorter performance by using an OFDM signal referred to the IEEE 802.11a WLAN standard. Simulation results show that the proposed predistorter is efficient in compensating memory PAs. It is also demonstrated that the proposal acquires a faster convergence speed and a better compensation effect than conventional predistorters.

Computationally efficient channel estimation and inter-carrier interference suppression for OFDM communication systems

This article puts forward a novel channel estimation and inter-carrier interference (ICI) suppression method for time-varying orthogonal frequency division multiplexing (OFDM) systems. The proposed adaptive finite impulse response (FIR) filter utilizes the time domain correlation of the subcarriers. Based on the one order auto-regressive (AR) model, a modified Kalman filter is exploited to track the channel variance. By solving the Yule-Walker equation, the coefficient of the AR process can be obtained from zero order Bessel function. However, when the channel is assumed not perfectly known, the above criterion will lead to an impractical application. To deal with it, the coefficient of the AR process is taken as a constant. Subsequently a new state space model is deduced and found. Simulation results show that the proposed method can work effectively at the speed of 144 km/h with a phase noise of −85 dBc/Hz at 100 kHz offset. The proposed method yields an improvement bit error rate (BER) compared with three of the existing algorithms with much lower complexity.

Performance of Gaussian and hyperbolic scattering channels in mobile OFDM systems

This paper estimates the inter-carrier-interference power P ICI of Gaussian and hyperbolic scattering channels in a mobile orthogonal frequency division multiplexing (OFDM) system by using their angle-of-arrival (AoA) probability density functions (pdf). The P ICI difference of the two channels is estimated and plotted as functions of ( τ , ζ ) , ( τ , ϵ ) and ( τ , f d T s ) , where τ is the ratio of the coverage distance D and effective radius R eff , ζ frequency tracking, f d maximum Doppler spread, T s symbol duration and ϵ travelling direction of a user equipment. Experimental P ICI difference for ϵ = 0 , ζ = 0 and f d T s = 0.05 is obtained and plotted with the simulated results for comparison purposes. The ICI power is also plotted against ( f d T s , ϵ ) , ( f d T s , ζ ) , and ( ζ , ϵ ) for specific values of τ . Detailed analyses on the performance of the hyperbolic and Gaussian AoA pdfs are given. Relative merits of each pdf are discussed. Closed-form bit error rate (BER) expressions of OFDM communication systems in an added white Gaussian noise (AWGN) channel and Rayleigh fading environment using bipolar phase shift keying (BPSK), OFDM-BPSK, are derived as functions of P ICI and phase noise to validate the effectiveness of the approximate Gaussian and hyperbolic AoA pdfs. Future work is also outlined.

부반송파간 간섭과 도플러 효과에 강한 WFMT 변조 방식의 성능 분석

OFDM(Orthogonal Frequency Division Multiplexing)은 고속 통신 시스템에 매우 효과적이다. 그러나 OFDM은 단일 반송파 시스템과 달리 높은 PAPR(Peak to Average Power Ratio)과 심각한 ICI(Inter Subcarrier Interference) 문제를 갖는다. 특히 ICI는 OFDM 신호들의 부반송파 사이에 직교성을 깨드려 시스템의 성능을 저하시킨다. 본 논문에서는 ICI에 대해 좋은 성능을 보이는 WFMT(Wavelet Filtered Multitone) 시스템을 연구하고, 위상 잡음과 반송 주파수 오프셋에 대한 성능 분석한다. WFMT 시스템은 멀티 채널 신호를 생성하고 분석하기 위하여 필터 뱅크 기술과 웨이블렛 이론을 기반으로 한다. 그러므로 WFMT 시스템은 필터 뱅크 시스템의 장점을 유지한다. 이 논문에서는 반송 주파수와 위상 잡음으로 인한 WFMT 시스템과 OFDM 시스템의 ICI와 ISI(Inter Symbol Interference)를 비교한다. 또한, 이 시스템들에서 PAPR 성능과 도플러 효과가 미치는 영향을 수식적으로 분석하고, HPA와 위상 잡음, 도플러 효과가 시스템에 존재할 때 각 시스템들의 BER 성능을 보인다. OFDM is very effective for the high speed communication system. However, OFDM has the problems of high PAPR and serious ICI, which is different from the single carrier system. Especially, ICI problem caused by phase noise, Doppler effect and frequency offset is main reason for poor performance since it breaks down the orthogonality between subcarriers in OFDM communication system. Therefore, WFMT modulation scheme is studied and the effect of the carrier frequency offset and phase noise is analyzed. WFMT modulation scheme is based on the wavelet theory and complex filter banks for synthesis and analysis of multichannel signal. WFHT modulation scheme keeps on the advantage of filter banks system and can be easily possible for the implementation of filter banks. In this paper, we compare ISI and ICI distortions of the WFMT and OFDM system due to the carrier frequency offset and phase noise. Also, we analyze the PAPR and BER performances in the HPA and ICI situation caused by the Doppler frequency shift and the frequency offset.

SPOCS: Application Specific Signal Processor for OFDM Communication Systems

This paper presents an Application-Specific Signal Processor (ASSP) for Orthogonal Frequency Division Multiplexing (OFDM) Communication Systems, called SPOCS. The instruction set and its architecture are specially designed for OFDM systems, such as Fast Fourier Transform (FFT), scrambling/descrambling, puncturing, convolutional encoding, interleaving/deinterleaving, etc. SPOCS employs the optimized Data Processing Unit (DPU) to support the proposed instructions and the FFT Address Generation Unit (FAGU) to automatically calculate input/output data addresses. In addition, the proposed Bit Manipulation Unit (BMU) supports efficient bit manipulation operations. SPOCS has been synthesized using the SEC 0.18 μm standard cell library and has a much smaller area than commercial DSP chips. SPOCS can reduce the number of clock cycles over 8%~53% for FFT and about 48%~84% for scrambling, convolutional encoding and interleaving compared with existing DSP chips. SPOCS can support various OFDM communication standards, such as Wireless Local Area Network (WLAN), Digital Audio Broadcasting (DAB), Digital Video Broadcasting-Terrestrial (DVB-T), etc.

IMD 저감 방식을 기반으로 하는 OFDM 통신 시스템

Orthogonal Frequency Division Multiplexing(OFDM) 시스템은 주파수 선택적 페이딩에 강하고 높은 대역 효율을 갖는 통신 시스템이다. OFDM 신호는 높은 PAPR로 인하여 비선형 증폭기에서 왜곡이 생긴다. 그래서 비선형 왜곡 성분인 IMD(Inter-Modulation Distortion)를 저감하는 것이 중요하다. IMD 저감 방식은 비선형 왜곡에 대하여 PAPR(Peak-To-Average Power Ratio) 저감 방식보다 BER 성능을 좋게할 수 있지만, 송신기에 FFT(Fast Fourier Transform)가 추가되어 시스템 복잡도를 증가시킨다. 본 논문에서는 IMD 저감 방식을 기반으로 하는 OFDM 통신 시스템의 BER 분석과 복잡도를 저감하기 위한 연구를 하였다. SPW(Sub-Block Phase Weighting)을 적용한 IMD 저감 방식에서 계산량 감소를 위한 새로운 방식을 제안하고 기존의 방식과 계산 복잡도를 비교한다. SPW 방식은 입력 데이터를 여러 개의 서브 블록으로 나누고 위상 회전 벡터를 곱해줌으로써 PAPR을 감소하거나 IMD를 저감한다. 제안된 방식은 BER 성능 면에서 기존 방식과 비슷하지만, PAPR처럼 시간 영역에서 IMD의 전력량을 계산하므로 시스템의 계산 복잡도와 시스템 크기를 줄일 수 있는 장점이 있다. OFDM system has very good high spectral efficiency and the robustness to the frequency-selective fading. Because of the high PAPR, OFDM signals can be distorted in nonlinear HPA(High Power Amplifier). So, to overcome the nonlinear distortion, it is very important to reduce the IMD value. With respect to the BER performance, IMD reduction method is better than the PAPR reduction method. However, IMD reduction method has much more system complexity because of the additional FFT processor in transmitter. In this paper, we study the OFDM communication system based on the IMD reduction method using SPW method. A new IMD reduction method is proposed to reduce the computational complexity. SPW method is to divide the input OFDM data into several sub-blocks and to multiply phase weighting values with each sub-blocks for the reduction of PAPR or IMD. Unlike the conventional method, the system size and computational complexity can be reduced.

IQ Imbalance와 위상 잡음을 고려한 OFDM 통신 시스템의 성능 분석

OFDM(Orthogonal Frequency Division Multiplexing) 시스템은 이동성이 있는 고속 데이터 전송에 적합한 전송 방법으로 널리 알려져 있지만, 위상 잡음과 직교 불균형(IQ imbalance)에 매우 민감한 특성을 보이는 단점이 있다. 그러므로, 본 논문에서는 위상 잡음 특성에 따른 위상 잡음 분산을 계산한 다음, 직교 불균형이 있는 OFDM 시스템의 통신 성능을 분석하였다. 시뮬레이션 결과, 위상 잡음의 분산과 직교 불균형의 크기 변화에 따라 통신 성능이 변화하는 것을 분석하였다. 분석 결과, 진폭 불균형이 <TEX>${\varepsilon}$</TEX>=0.2; 0.3; 0.4이고, 위상 불균형이 <TEX>${\phi}=10^0$</TEX>이고, 위상 잡음의 분산 <TEX>${\sigma}^2=0.012$</TEX>일 때, BER은 <TEX>$10^{-5}$</TEX>에서 각각 2.88 dB, 3.61 dB, 4.69 dB의 SNR penalty가 존재한다. OFDM system is an excellent high speed transmission method but it is seriously sensitive to the phase noise and IQ imbalance. Therefore, in this paper, we analyze the communication performance of the OFDM communication system with IQ imbalance and phase noise. Phase noise's variance can be calculated by integral calculus of phase noise power spectrum. From simulation results, it can be shown that the BER performances show different change according to the phase noise variance and IQ imbalance amount. When amplitude imbalance is <TEX>${\varepsilon}$</TEX>=0.2; 0.3; 0.4 and phase imbalance is <TEX>${\phi}=10^0$</TEX>, and distribution of phase noise is <TEX>${\sigma}^2=0.012$</TEX>, BER is degraded by 2.88 dB, 3.61 dB, 4.09 dB in <TEX>$10^{-5}$</TEX> in the respect of the SNR penalty.

Adaptive Antenna Arrays for Interference Cancellation in OFDM Communication Systems With Virtual Carriers

In this paper, we propose adaptive beamforming schemes for orthogonal frequency-division multiplexing (OFDM) communication systems. After applying the discrete-Fourier-transform operation to the output of each antenna, one estimates the signal-plus-interference spatial correlation matrix by selecting the samples at ldquononzero-valuedrdquo subcarriers and the interference-alone spatial correlation matrix by choosing those samples at ldquozero-valuedrdquo subcarriers (also known as virtual carriers). The beamforming weight vector is the ldquolargestrdquo generalized eigenvector of the resulting matrix pencil, wherein the solution is computed in a batch fashion. By using a limited number of samples, the proposed scheme offers competitive performance compared with that of the scheme using optimal beamforming weight vectors that are calculated for each subcarrier under small-angular-spread scenarios. The adaptive solution for calculating the beamforming weight vector is also derived. The proposed schemes are shown to be suitable in scenarios of narrow angular spread with exponential power delay profile. The simulations that are presented here employ the OFDM signal model to illustrate the efficacy of the proposed algorithms.

Simulation on peak-to-average power ratio for orthogonal frequency division multiplexing

Clipping is a simple and convenient PAPR (peak-to-average power ratio) reduction method for high speed OFDM (orthogonal frequency division multiplexing) communication system. In this paper, we propose a new PAPR reduction method for Wireless-MAN(metropolitan area network)-OFDM system based on IEEE 802. 16, which is over-sampling clipping arithmetic. Simulation and performance of the over-samples clipping's PAPR reduction capability, BER effect is given. The simulation indicates that the PAPR of at least 99.9% OFDM symbol is below 6dB after 2 Nyquist rate clipping, and the performance of BER has 1dB SNR (signal noise ratio) loss. The results prove that this method has better capacity to reducing PAPR. So it can be well used in WMAN-OFDM system.

Pilot Patterns for OFDM Communication Systems in a Fast Time-Varying Channel

In this letter, we focus on rearranged pilot patterns for channel estimation in a mobile communication system using Orthogonal Frequency Division Multiplexing (OFDM). The conventional pilot patterns for channel estimation in OFDM systems do not have robust characteristics in time-varying channels. In order to overcome this weakness of the conventional pilot patterns, we propose the pilot patterns with robust mobility for OFDM systems, which can achieve a good error performance in time-varying multi-path fading channels. Simulation results show that the bit error rate (BER) performances of the proposed pilot patterns are better than those of the conventional pilot patterns in fast time-varying fading channels under the same pilot density and data rate.

The Economical PAPR Minization Scheme for Combinative Coding Technique Applied OFDM Communication System

In this paper, we propose a new combinative scheme to combine with parity check and block coding methods for the reduction of the peak to average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) system. In the proposed schemes, the simulation results shown that Peak-to-Average Power Ratio (PAPR) can be reduced by 3.502 dB. The results of this mapped can be shown that PAPR is reduced. The principle of the scheme is illustrated with the specific example of an eight-carriers signal and its computer simulation results. All simulation results have compare with ideal channel case and AWGN case separately; both of cases are shown the PAPR reduced indeed.

An Improved ICI Reduction Method in OFDM Communication System

Orthogonal frequency division multiplexing (OFDM) is a promising technique for the broadband wireless communication system. However, the inter-sub-carrier-interference (ICI) produced by the phase noise of transceiver local oscillator is a serious problem. Bit error rate (BER) performance is degraded because the orthogonal properties between the sub-carriers are broken down. In this paper, ICI self-cancellation of data-conjugate method is studied to reduce ICI effectively. CPE (common phase error), ICI and CIR (carrier to interference power ratio) are derived and discussed by the linear approximation of the phase noise. Then, the system performance of the data-conjugate method is compared with those of the original OFDM and the conventional data-conversion method. As results, it can be shown that CPE becomes zero in the OFDM of the data-conjugate method. Besides, in the OFDM system with phase noise, the data-conjugate method can make remarkable improvement of the BER performance and it is better than the data-conversion method and the original OFDM with or without convolution coding.