Guard Interval Length Research Articles

A New OFDM System for IIR Channels

In this paper, we propose a new orthogonal frequency division multiplexing (OFDM) system for an infinite impulse response (IIR) channel with the form of B(z)/A(z) for two polynomials A(z) and B(z). Different from the conventional OFDM transmission over a finite impulse response (FIR) channel, a guard interval of an OFDM symbol is added such that the corresponding part at receiver is the cyclic prefix (CP) of the received OFDM symbol. The guard interval and CP lengths are the same and not smaller than the orders of polynomials A(z) and B(z). The OFDM symbol without the guard interval is the same as the conventional OFDM symbol without the CP. At the receiver, the IIR channel is then converted to N intersymbol interference (ISI) free subchannels, where N is the number of subcarriers of an OFDM symbol.

RETRACTED: Research on multiple classified signal detection algorithms based on mimo-ofdm system

OFDM is a high-speed signal transmission technology in wireless environment. In wireless channels, the frequency response curve is generally non-flat. MIMO technology can effectively increase system capacity, improve system performance, and improve network coverage and transmission reliability. OFDM technology can effectively resist intersymbol crosstalk caused by multipath propagation by adding a certain length of redundant guard interval in front of transmission symbols. According to the channel estimation and signal detection algorithms in MIMOOFDM systems, this paper will choose a better algorithm to implement MIMOOFDM systems and channel estimation and signal detection algorithms. Theoretical analysis and simulation results show that the optimal detection algorithm has the best performance, but its complexity is exponentially related to the number of antennas and modulation order. The architecture for dense texture analysis is developed based on Field Programmable Gate Array FPGA device which is a general engine for high-speed image processing. In order to maximize the performance, our architecture is designed for minimizing consumed resources. As explained in subsection our architecture is composed of two main operations, namely the AH based sum and difference of images and the calculus of texture features

Performance Analysis of MIMO-OFDM System Based on SVD scheme under multi path selective fading channel

We consider in this paper a MIMO-OFDM wireless communication system based on pre-coding and SVD techniques, which the system has the feedback of CSI to the transmitted side. The multi path channel, delay spread, Rayleigh fading and channel noise are assumed in the simulation of the proposed system. The zero forcing method is applied for detection the data received. The goal of our study is to evaluate the BER performance and see it with different case. The BER resulted shows the improvement by using the SVD decomposition for the associated system of MIMO-OFDM. Particularly in the complex conditions of wireless channel where we considered ten paths for propagation the signals between transmitter and receiver sides. And also the contribution is appeared when the system studied keeps a good BER performance when the data rate is increased by using more transmit antenna. As we evaluated the BER of MIMO-OFDM system done for various sub-carriers and in two guard interval lengths for the OFDM system

Energy‐efficient orthogonal frequency division multiplexing scheme based on time–frequency joint channel estimation

Time-domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) enjoys the higher spectrum efficiency and faster synchronisation than the classical cyclic prefix OFDM (CP-OFDM). However, TDS-OFDM suffers from performance degradation especially under severely fading channels with long delays. To solve these problems, the authors propose an energy-efficient OFDM scheme called time-frequency-training orthogonal frequency division multiplexing (TFT- OFDM) based on the time-frequency joint channel estimation under the framework of compressive sensing (CS). The power of the guard interval (GI) in the proposed scheme can be reduced to achieve higher energy efficiency, which is infeasible for CP-OFDM. This method first utilises the time-domain pseudo noise sequence to acquire partial support information of the channel, and then some frequency-domain pilots are used for the exact channel estimation. Simulation results show that TFT- OFDM with CS can achieve much higher energy efficiency than the classical CP-OFDM, and outperforms the conventional OFDM schemes in both static and mobile environments. Moreover, for the channel with long delay spread, the TFT-OFDM scheme with CS can demonstrate robustness and much better performance than the conventional OFDM schemes. In this way, the TFT-OFDM scheme can use the same GI length for larger broadcasting coverage and hence further achieve higher energy efficiency.

LMMSE Channel Estimation Algorithm Based on Channel Autocorrelation Minimization for LTE-Advanced with Adaptive Guard Interval

In this paper, we propose an adaptive guard interval (GI) for LTE-advanced using channel impulse response (CIR) length estimation. The CIR length is estimated from the channel autocorrelation function. This estimate can be used to generate linear minimum mean square error filter coefficients for data subcarriers channel estimation and to adjust the GI length adaptively. The simulation results show that the improvement of mean square error for signal to noise ratio of 30 dB with CIR length estimation is about 10 dB when compared to conventional methods. Also, due to adaptive GI, the relative spectral efficiency can be increased by 0.11 compared to the conventional system with constant GI.

Compressive Sensing Based Channel Estimation for OFDM Systems Under Long Delay Channels

Time-domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) has advantages in spectral efficiency and synchronization. However, its iterative interference cancellation algorithm will suffer from performance loss especially under severely fading channels with long delays and has difficulty supporting high-order modulations like 256 QAM, which may not accommodate the emerging ultra-high definition television service. To solve this problem, a channel estimation method for OFDM under the framework of compressive sensing (CS) is proposed in this paper. Firstly, by exploiting the signal structure of recently proposed time-frequency training OFDM scheme, the auxiliary channel information is obtained. Secondly, we propose the auxiliary information based subspace pursuit (A-SP) algorithm to utilize a very small amount of frequency-domain pilots embedded in the OFDM block for the exact channel estimation. Moreover, the obtained auxiliary channel information is adopted to reduce the complexity of the classical SP algorithm. Simulation results demonstrate that the CS-based OFDM outperforms the conventional dual pseudo noise padded OFDM and CS-based TDS-OFDM schemes in both static and mobile environments, especially when the channel length is close to or even larger than the guard interval length, where the conventional schemes fail to work completely.

DFT-based offset-QAM OFDM for optical communications

We experimentally demonstrate and numerically investigate a discrete-Fourier-transform (DFT) based offset quadrature-amplitude-modulation (offset-QAM) orthogonal frequency division multiplexing (OFDM) system. We investigate the scheme using a set of square-root-raised-cosine functions and a set of super-Gaussian functions as signal spectra. It is shown that offset-QAM OFDM exhibits negligible penalty for all investigated spectra, in contrast to rectangular-function based Nyquist FDM (N-FDM) and sinc-function based conventional OFDM (C-OFDM). The required guard interval (GI) length for dispersion compensation in offset-QAM OFDM is analyzed and shown to scale with twice the subcarrier spacing rather than the full OFDM bandwidth. Experimental results show that 38-Gb/s offset-16QAM OFDM supports 600-km fiber transmission with negligible penalty in the absence of GI while a GI length of eight is required in C-OFDM. Further numerical simulations show that by avoiding the GI, 112-Gb/s polarization multiplexed offset-4QAM OFDM can achieve 23% increase in net data rate over C-OFDM under the same transmission reach. We also discuss the design of the pulse-shaping filter in the DFT-based implementation and show that when compared to N-FDM, the required memory length of the filter for pulse shaping can be reduced from 60 to 2 in offset-QAM OFDM regardless of the fiber length.

Application of 3GPP LTE and IEEE 802.11p Systems to Ship Ad-Hoc Network with the Existence of ISI

In order to provide high data rate and real time services under maritime environment, link-level performance of ship ad-hoc network (SANET) based on 3GPP LTE and IEEE 802.11p (WAVE) specifications are investigated and discussed in this paper. The measured maritime channel, whose delay spread is longer than the length of guard interval (GI) of both 3GPP LTE and IEEE 802.11p specifications, is adopted for the link-level simulations. For the purpose of eliminating inter-symbol interference (ISI) due to insufficient GI length, double antenna pattern (DAP) scheme and advanced time-domain decision-feedback equalizer (DFE) are proposed for LTE and WAVE systems, respectively. The proposed DFE removes the ISI in a same manner as the residual inter-symbol interference cancellation (RISIC) algorithm, but the inter-carrier interference (ICI) is reduced via cyclicity removal instead of cyclicity restoration used in the RISIC algorithm. Compared with existing schemes, our proposed DFE is a robust technique to overcome the severe ISI channel which has a comparatively large delay spread. Based on simulation results, not only comparisons between systems are discussed, but also some reformative suggestions are given.

A Proposed Equivalent Channel Power Delay Profile for a Millimeter Wave Wireless OFDM System and Optimum Guard Interval Evaluation in a Built-Up Area Propagation Scenario

Vertical plane launching ray tracing method has been applied for a real urban scenario and the delay characteristics of received signals in the overall coverage area have been extracted. Using these parameters, an equivalent channel power delay profile (ECPDP) has been introduced for the overall coverage area. For a millimeter wave wireless OFDM system, the optimum guard interval length which maximizes the mean of signal to interference and noise ratio has been simulated. Also, using the proposed ECPDP, the optimum guard interval length has been analytically extracted and compared with the simulated one.

地上デジタル放送用ガードインターバル越えマルチパス等化器の等化特性改善

Most digital terrestrial television broadcasting systems including Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) system use the orthogonal frequency division multiplexing (OFDM) scheme, which is robust against echoes (multipath interference) due to the insertion of a guard interval. However, echoes can occur outside the guard interval and degrade the quality of the received signal, especially in single frequency network (SFN). To overcome this problem, a channel equalizer that performs a Fast Fourier transform (FFT) on several OFDM symbols in combination and then equalizes the frequency characteristics in the frequency domain has been shown to be effective. In this paper, we extend this scheme and propose an equalization algorithm using serially concatenated two-frequency domain equalizers. Simulation results showed that the performance was improved, especially when the delay spread was within the guard interval length. We also added an adaptive control method to the leak process part of the channel estimation. Results of a field experiment in a receiving environment with reflection waves from the mountains further demonstrated the effectiveness of the proposed method.

Transmission of 4-ASK Optical Fast OFDM With Chromatic Dispersion Compensation

We experimentally demonstrate transmission of 20-Gb/s four-amplitude-shifted-keying optical fast orthogonal frequency-division-multiplexing (F-OFDM) over various fiber lengths up to 840 km, and show that symmetric extension, instead of cyclic extension, based guard interval (GI) is required to enable chromatic dispersion (CD) compensation using one-tap equalizers after channel demultiplexing. We characterize the CD tolerance under different GI lengths and investigate the impact of fiber nonlinearity on the system performance.

Channel shortening techniques for differential encoded OFDM

In OFDM systems, the Inter-Symbol Interference (ISI) and the Inter-Block Interference (IBI) are mitigated by using Null-Tones (NTs) and the Guard Interval (GI) redundancy of length higher than the channel size. To preserve a high bit rate, channel shortening is required to reduce the GI length. We propose here a new adaptive method for the blind estimation of the Time domain EQualizer (TEQ) for time varying communication channels. Our contribution is three-fold. First, we exploit the knowledge of the first emitted symbol in a differential encoded OFDM system to derive the initial estimate of the TEQ. Then, we update the TEQ coefficients according to the channel variation by optimizing a criterion exploiting both the GI and NT redundancies and using a fast Generalized Eigen Value (GEV) tracking algorithm. Finally, the optimization of the previous criterion is achieved in such a way we control the Target Impulse Response (TIR) quality and we improve the overall system performance. Simulation results are provided to illustrate the performance of our method and assess our theoretical derivations.

地上デジタルテレビ波の安定な海上移動受信のための実験と検討

If terrestrial digital broadcasting services are available at ships used for transportation, they will be very useful and convenient for gathering information including during disasters. We studied and investigated to realize stable reception of digital TV waves and re-transmission technology at ships connecting Hiroshima-Matsuyama. As a result, BER degradation of unexpected multi-path waves with over guard interval length, adjacent channel interference, and large level changes of electric fields that are characteristics of mobile ship reception occurred. Therefore, shielding undesired waves using the ship structure and selection of receiving antennas in accordance with electric field strength and so on are taken. Through these countermeasures, digital TV waves can be received at 90% on all lines and one segment reception can be available for the whole cabin of a ship by retransmission.

Variable Guard Interval Based on Maximum Delay Estimation for Adaptive OFDM Systems

In orthogonal frequency division multiplexing (OFDM) system, a guard interval (GI) is used to remove the inter-symbol interference (ISI) due to a multipath channel. It is difficult to set an optimal GI length in the environment whose multipath varies. In this paper, we propose a variable guard interval based on the estimated maximum delay of a multipath channel. The maximum delay is estimated from a channel impulse response (CIR), which is estimated by a preamble symbol. However, since the estimated CIR includes the noise, it is difficult to decide the optimal GI. In order to solve the problem, we introduce the method which selects the path whose signal to noise ratio is high. Additionally, the information of the optimal GI length is required to be transmitted from a receiver to a transmitter. In this paper, we use an acknowledgment (ACK) frame for the feedback of the GI information.

An ML-Based Estimate and the Cramer-Rao Bound for Data-Aided Channel Estimation in KSP-OFDM

We consider the Cramer-Rao bound (CRB) for data-aided channel estimation for OFDM with known symbol padding (KSP-OFDM). The pilot symbols used to estimate the channel are positioned not only in the guard interval but also on some of the OFDM carriers, in order to improve the estimation accuracy for a given guard interval length. As the true CRB is very hard to evaluate, we derive an approximate analytical expression for the CRB, that is, the Gaussian CRB (GCRB), which is accurate for large block sizes. This derivation involves an invertible linear transformation of the received samples, yielding an observation vector of which a number of components are (nearly) independent of the unknown information-bearing data symbols. The low SNR limit of the GCRB is obtained by ignoring the presence of the data symbols in the received signals. At high SNR, the GCRB is mainly determined by the observations that are (nearly) independent of the data symbols; the additional information provided by the other observations is negligible. Both SNR limits are inversely proportional to the SNR. The GCRB is essentially independent of the FFT size and the used pilot sequence, and inversely proportional to the number of pilots. For a given number of pilot symbols, the CRB slightly increases with the guard interval length. Further, a low complexity ML-based channel estimator is derived from the observation subset that is (nearly) independent of the data symbols. Although this estimator exploits only a part of the observation, its mean-squared error (MSE) performance is close the CRB for a large range of SNR. However, at high SNR, the MSE reaches an error floor caused by the residual presence of data symbols in the considered observation subset.

Combination of interference cancellation with channel estimation for OFDM transmission over mobile radio channels

AbstractThis paper proposes a new method, which combines interference cancellation with channel estimation for an orthogonal frequency division multiplexing (OFDM) system in the case of insufficient guard interval (GI) length. To avoid the problem of causing additional errors of conventional cancellers in the presence of strong additive noise, the proposed cancellation scheme uses the channel state information (CSI) and the so‐called remodulation technique to reproduce the interference distortions. Then, these distortions in both the received pilot symbols and the data symbols are iteratively cancelled. Consequently, the channel estimation as well as the data equalisation performances are iteratively enhanced. In the range of high signal‐to‐noise ratio (SNR), the proposed scheme improves significantly the performance of the system. It does not cause any additional errors to the received signal, when the system suffers from very low SNR. In comparison to the case of sufficient GI length, the required SNR can be reduced by 0.5 dB to achieve the same symbol error rate (SER). Moreover, the spectral efficiency of the system can be increased, because the GI length is allowed to be insufficient. The proposed method is applicable to future OFDM‐based wireless local area networks (WLANs). Copyright © 2007 John & Sons, Ltd.

Channel Impulse Response Length and Noise Variance Estimation for OFDM Systems with Adaptive Guard Interval

A new algorithm estimating channel impulse response (CIR) length and noise variance for orthogonal frequency-division multiplexing (OFDM) systems with adaptive guard interval (GI) length is proposed. To estimate the CIR length and the noise variance, the different statistical characteristics of the additive noise and the mobile radio channels are exploited. This difference is due to the fact that the variance of the channel coefficients depends on the position within the CIR, whereas the noise variance of each estimated channel tap is equal. Moreover, the channel can vary rapidly, but its length changes more slowly than its coefficients. An auxiliary function is established to distinguish these characteristics. The CIR length and the noise variance are estimated by varying the parameters of this function. The proposed method provides reliable information of the estimated CIR length and the noise variance even at signal-to-noise ratio (SNR) of 0 dB. This information can be applied to an OFDM system with adaptive GI length, where the length of the GI is adapted to the current length of the CIR. The length of the GI can therefore be optimized. Consequently, the spectral efficiency of the system is increased.

Automatic Detection of the Guard Interval Length in OFDM System

Orthogonal Frequency Division Multiplexing (OFDM) has been increasingly used in wireless broadband communication systems. Guard interval (GI) is used in OFDM system to reduce inter-symbol interference (ISI). As selection of the length of guard interval depends on the channel condition, the length of guard interval will change dynamically. Thus there must be a way to synchronize this parameter between the transmitter and the receivers. Traditional OFDM systems require a transmitter to notify its receivers using additional signal and thus result additional bandwidth. In addition, this complicates the initiation process for an OFDM receiver as it has to search through all the possible values of GI length to find the parameter used by the transmitter. To address this problem, this paper gives a low-complexity method, which can detect the length of guard interval at the receiver without any guard interval length information from the transmitter. The introduction of this method will reduce the additional signaling and simplifies OFDM systems. Both theoretical analysis and simulation results show that the proposed method has good performance. The proposed method can give accurate measure of GI length under both AWGN and Rayleigh Channel even when SNR is as low as 3dB.

An adaptive antenna array for single carrier modulation with cyclic prefix

AbstractIn this paper, a method for weight control of an adaptive array is proposed for a system using a cyclic prefix in single carrier wireless communications (SC‐CP). In the SC‐CP system, the delayed wave within the guard interval (GI) can be equalized by discrete frequency domain equalization. Therefore, it is to be expected that the characteristics can be improved with respect to the received signal power by reception of several arriving waves within the GI. On the other hand, reception of several arriving waves leads to generation of zero points in the frequency response so that noise enhancement may be a problem for a discrete frequency domain equalizer with a zero forcing (ZF) reference. Also caused in the SC‐CP system are the intersymbol interference (ISI) and interblock interference (IBI) due to the delayed wave exceeding the GI length. In this paper, a weight control method is proposed that simultaneously realizes enhanced received signal power by reception of several arriving waves and avoidance of noise enhancement due to ZF reference equalizer. At the same time, the weight control method to suppress delayed waves outside the GI is theoretically derived and its characteristics are evaluated by a computer simulation. © 2005 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 88(9): 38–48, 2005; Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/ecja.20189

デジタル放送・伝送方式 付加遅延挿入ＳＦＮ信号のアレイアンテナを用いる受信方式

In the OFDM SFN system that re-broadcasts signals received from the main station to a the relay station, the area where the delay of the received signal is longer than the guard interval length increases. This is in contrast to the SFN system that broadcasts signals simultaneously at both the main station and the relay station. This generates inter-symbol and inter-carrier interference, degrading the bit error rate. We developed a technique that allows the SFN system to re-broadcast signals received from the main station after adding an appropriate delay. In this paper, a receiver using an array antenna and a maximum ratio combining technique for this SFN system is proposed. Theoretical analyses and computer simulation show that this technique improves the bit error rate more than do conventional methods.