Orthogonal Frequency Division Multiplexing Receiver Research Articles

Symbol error rate performance of nonlinear OFDM receiver with peak value threshold over frequency selective fading channel

Nonlinear blanking and clipping methods are widely used in Orthogonal Frequency Division Multiplexing (OFDM) receiver to mitigate impulse interference. To quantitatively analyze the reliability performance of nonlinear OFDM receivers with pulse blanking and clipping based on peak value threshold, the symbol error rate (SER) performance of nonlinear OFDM receiver over frequency selective Rayleigh and Ricean fading channels is presented. Firstly, the analytical expressions of instantaneous output signal-to-interference and noise ratio (SINR) for nonlinear OFDM receivers with regular method, peak value blanking and peak value clipping are derived. Then, the SER performance of nonlinear OFDM receiver over frequency selective Rayleigh and Ricean fading channels is given based on the SINR expressions. Finally, simulation results are demonstrated to show good agreement with theoretical results. It has been observed that the peak value blanking method has achieved the best SER performance, and the inter-carrier interference based on the peak value blanking and peak value clipping will lead to error floor.

Exploiting Cyclic Prefix for Turbo-OFDM Receiver Design

In an orthogonal frequency division multiplexing (OFDM) system, the cyclic prefix (CP) has traditionally been not useful for data detection at the receiver. This paper presents a low-complexity turbo-OFDM receiver to exploit all of the extra information offered by the CP to improve the data detection and channel estimation performance. In the receiver, two iterative processes, namely, the outer iteration based on the turbo equalization principle and the inner iteration based on the expectation–maximization algorithm, are proposed. The CP observation, which contains extra information about data and channel, is represented by a Forney-style factor graph and incorporated into the inner iteration for joint channel estimation and data detection, using a Gaussian message passing process. The extrinsic messages of the data enhanced by exploiting the CP are then exchanged in the outer iteration for joint channel estimation, data detection, and decoding to further improve a system performance. To effectively implement the receiver, several approximations are introduced to reduce the computational complexity to be logarithmic in the length of the OFDM symbol $N$ , i.e., $\mathcal O(NlogN)$ per OFDM symbol. Extensive simulation results show that the bit error rate performance of the proposed low-complexity turbo-OFDM receiver is superior to that of existing turbo-OFDM receivers with exploiting the channel information in the CP only or without exploiting the CP, as well as conventional OFDM receivers, in particular for time-varying channels.

OFDM Receiver for Fast Time-Varying Channels Using Block-Sparse Bayesian Learning

We propose an iterative algorithm for orthogonal frequency-division multiplexing (OFDM) receivers operating over fast time-varying channels. The design relies on the assumptions that the channel response can be characterized by a few nonnegligible separable multipath components and that the temporal variation of each component gain can be well characterized with a basis expansion model (BEM) using a small number of terms. As a result, the channel estimation problem is posed as that of estimating a vector of complex coefficients that exhibits a block-sparse structure, which we solve with tools from block-sparse Bayesian learning (BSBL). Using variational Bayesian inference, we embed the channel estimator in a receiver structure that performs iterative channel and noise precision estimation, intercarrier interference (ICI) cancelation, detection, and decoding. Simulation results illustrate the superior performance of the proposed receiver over state-of-the-art receivers.

OFDM Receiver Structure for a Subband-Selective Mitigation of Time-Variant Interference

System performance of orthogonal frequency-division multiplexing (OFDM) degrades in the presence of time-variant frequency-selective interference. In the literature, several pulse-blanking concepts have been introduced over the past few years, which are promising ways to mitigate impulsive interference for OFDM reception. However, blanking yields an attenuation of the signal and intercarrier interference (ICI). In this paper, a receiver structure is proposed, which consists of a frequency-selective pulse-blanking unit, a blanking mitigation unit, and an iterative ICI cancelation unit. The L-band Digital Aeronautical Communication System 1 (L-DACS 1) is regarded as an application that suffers from interference from the impulsive Distance Measuring Equipment (DME) system whose frequency bands partly overlap. It is shown that, by using the proposed mitigation scheme for impulsive interference, a reliable reception is possible, even in strong interference situations.

A Low Complexity Multi-Carrier System Over Doubly Selective Channels Using Virtual-Trajectories Receiver

Current communication standards are designed to operate under either frequency or time selectivity. However, the rising demand of higher data rates combined with the mobility of the transceivers has led to the necessity of transmitting under doubly selective channels. In these channels, both single-carrier and multi-carrier approaches find high interference conditions, where the performance in terms of spectral efficiency and bit error rate is low. The state-of-the-art solutions involve high complexity and/or low compatibility making them infeasible. In this paper, a modified orthogonal frequency-division multiplexing (OFDM) communication system based on the virtual decomposition of the channel is proposed. The system is fully compatible with the standard OFDM transmitters and maintains several blocks of the standard OFDM receivers. This novel system substantially surpasses the performance of current approaches while requiring low computational complexity.

Architecture of Channel Estimators for OFDM-Based IEEE 802.11ad WLAN

Channel estimation is an important issue in orthogonal frequency division multiplexing (OFDM) systems to improve the performance of the system. For OFDM based IEEE 802.11ad wireless local area network (WLAN), two types of symbols – training symbols and pilot symbols – can be used for channel estimation in the OFDM receiver. We analyze the algorithms of channel estimation methods based on symbol types, training symbols and pilot symbols respectively, under variation of channel environment. Based on the simulation results, the performance evaluation for channel estimation methods is then performed. These estimation methods are applied to design hardware architectures for channel estimators and implemented using a 65-nm CMOS standard library. The channel estimation field based channel estimator uses the training symbols for estimation, and the pilotbased channel estimation uses the pilot symbols for estimation. The hardware cost of estimators are also performed in this work.

SIMULASI TEKNIK MODULASI OFDM QPSK DENGAN MENGGUNAKAN MATLAB

This paper provides a brief explanation of the processing steps involved in Orthogonal Frequency Division Multiplexing (OFDM) with Quadrature Phase Shift Keying (QPSK) modulation technique implemented as a simulation program in MatLab. Input data of the simulation program in the form of random bit stream or text can be selected by users. The process conducted in the simulation is divided into three consecutive steps, processes in the OFDM transmitter, in transmission channel and in the OFDM receiver. The result of computation and the wave form involved in every processing steps will be displayed during the simulation. The final result shows, how the input data go through processing steps in the OFDM transmitter, transmission channel with noise and finally received by OFDM receiver will produce the output, random bit stream or text, exactly similar with input data

A low complexity SI sequence estimator for pilot-aided SLM–OFDM systems

Selected mapping (SLM) is a well-known method for reducing peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) systems. However, as a consequence of implementing SLM, OFDM receivers often require estimation of some side information (SI) in order to achieve successful data recovery. Existing SI estimation schemes have very high computational complexities that put additional constraints on limited resources and increase system complexity. To address this problem, an alternative SLM approach that facilitates estimation of SI in the form of phase detection is presented. Simulations show that this modified SLM approach produces similar PAPR reduction performance when compared to conventional SLM. With no amplifier distortion and in the presence of non-linear power amplifier distortion, the proposed SI estimation approach achieves similar data recovery performance as both standard SLM–OFDM (with perfect SI estimation) and also when SI estimation is implemented through the use of an existing frequency-domain correlation (FDC) decision metric. In addition, the proposed method significantly reduces computational complexity compared with the FDC scheme and an ML estimation scheme.

A Novel Differential Detection for Differential OFDM Systems With High Mobility

In conventional differential detection (CDD), the previous received symbol is used as the phase reference for the current received symbol, therefore the channel variation due to fast fading causes an error floor. We propose a novel iterative decision-directed differential detection (iD4) algorithm for orthogonal frequency division multiplexing (OFDM) systems employing differential PSK modulation in each subcarrier. The core of iD4 is a low complexity time and frequency 2-D estimator which can effectively estimate the channel variation experienced by adjacent time domain symbols. This estimated channel variation is then compensated in the differential detection process. Both theoretical analysis and computer simulations show that significant performance improvement can be achieved using iD4 over the CDD for OFDM receivers in fast fading channels.

Design and EXIT Chart Analysis of a Doubly Iterative Receiver for Mitigating Impulsive Interference in OFDM Systems

In this paper, a doubly iterative receiver for orthogonal frequency-division multiplexing (OFDM) systems is designed to mitigate impulsive interference based on pulse blanking. Although being a very efficient countermeasure, pulse blanking introduces intercarrier interference in OFDM systems leading to system performance degradation. The doubly iterative receiver is designed to overcome this drawback. The inner loop combines iterative demodulation and decoding to cancel intercarrier interference. To make soft information available to the outer loop, the equivalent noise power is derived to employ soft demodulation. The outer loop improves the performance of pulse blanking by performing a hypothesis test, which is used to decide if peaks in the received signal are due to impulsive interference or the large peak-to-average-power ratio common to OFDM signals. This allows to optimize the blanking threshold through maximizing the signal-to-interference-and-noise ratio. To evaluate performance and complexity of the proposed receiver, extrinsic information transfer (EXIT) chart analysis is carried out, where EXIT functions for pulse blanker, demodulator, and decoder are derived. An EXIT chart-based trellis search approach is introduced to calculate the loop schedule achieving target bit-error-rate performance with minimized complexity. Numerical results show a considerable bit-error-rate performance improvement as well as a remarkable complexity reduction.

Suppressing Alignment: Joint PAPR and Out-of-Band Power Leakage Reduction for OFDM-Based Systems

Orthogonal frequency division multiplexing (OFDM) inherently suffers from two major drawbacks: high out-of-band (OOB) power leakage and high peak-to-average power ratio (PAPR). This paper proposes a novel approach called suppressing alignment for the joint reduction of the OOB power leakage and PAPR. The proposed approach exploits the temporal degrees of freedom provided by the cyclic prefix(CP), a necessary redundancy in OFDM systems, to generate a suppressing signal, that when added to the OFDM symbol, results in marked reduction in both the OOB power leakage and PAPR. Additionally, and in order to not cause any interference to the information data carried by the OFDM symbol, the proposed approach utilizes the wireless channel to perfectly align the suppressing signal with the CP duration at the OFDM receiver. Essentially, maintaining a bit error rate (BER) performance similar to legacy OFDM without requiring any change in the receiver structure.

Minimization of PAPR in OFDM Using SFBC and MIMO

Peak-to-Average Power Ratio (PAPR) reduction is one of the main key factors in orthogonal frequency division multiplexing (OFDM) systems. From the various existing PAPR reduction techniques, Neural Network (NN) has been one of the efficient and powerful techniques in reducing the PAPR due to its good generalization properties with flexible modeling and learning capabilities. In this paper, we propose a new method that uses NNs trained on the active constellation extension (ACE) signals to reduce the PAPR of OFDM signals. It employs a receiver NN unit, at the OFDM receiver side for achieving significant bit error rate (BER) improvement with low computational complexity. To reduce the time complexity SFBC (Space frequency block coding) block is used in NN and multiple input and multiple output (MIMO) technique is introduced.

Low Power OFDM Receiver Exploiting Data Sparseness and DFT Symmetry

An undersampling technique appropriate for Orthogonal Frequency Division Multiplexing (OFDM) that can be implemented with low complexity hardware is presented. The OFDM receiver can operate in undersampling mode, 75% of the time. One of the advantages of the proposed scheme is the reduction down to the half of the power consumed by the Analog Digital Converter (ADC) and the Fast Fourier Transform (FFT). The FFT memory requirements for sample storage can also be reduced and its operating speed can be increased. Simulations were performed for two Quadrature Amplitude Modulation orders (16-QAM and 32-QAM), two options for the FFT size (1024 and 4096), two alternative input symbol structures for the inverse FFT (IFFT), and several sparseness levels and samples substitution options. The Symbol Error Rate (SER) and image reconstruction examples are used to show that a full reconstruction or a very low error can be achieved. Although the proposed undersampling method is evaluated for wired channel, it can also be used without any modification to wireless Single Input Single Output (SISO) systems (with an expected SER degradation). It can also be used with Multiple Input Multiple Output (MIMO) systems when an appropriate arrangement of the IFFT input symbols is adopted.

Optimization on semi-blind channel estimation for MIMO-OFDM systems

An orthogonal frequency division multiplexing (OFDM) is one of the effective techniques used in wireless communication. In OFDM systems, channel impairments due to multipath dispersive spreading can cause deep fades in wireless channels. Thus, the OFDM receiver requires channel state information when coherent detection is involved. Therefore, to overcome the impact of channel fades good channel estimation (CE) methods are needed in OFDM systems. And one of these CE methods is a semi-blind CE. However, the semi-blind method requires a large number of processing operations. In order to avoid the high computing complexity of the existing method, scaled least square (SLS) technique is applied to improve the performance of the semi-blind channel estimator which require less knowledge of the channel second-order statistics and have better performance than the least square (LS) which used in semi-blind CE. Simulation results shows, this proposed method of semi-blind CE has the capacity of elevating CE performance in multiple-input multiple-output (MIMO) OFDM systems.

A Windowing Technique for Optimal Time-Frequency Concentration and ACI Rejection in OFDM-Based Systems

In this paper, we introduce a windowing technique, which provides optimal time-frequency containment and maximal adjacent channel interference (ACI) rejection for orthogonal frequency-division multiplexing (OFDM)-based systems. Instead of using a single pulse shape function for all subcarriers, multiple functions are considered in order to maximize the time-frequency containment of the OFDM waveform. The main strategy is to concentrate the spectrum of windowing functions into a given bandwidth while achieving maximum suppression in the out-of-band region. This is achieved by employing prolate-based windowing functions which give optimal spectral concentration for time-limited pulse shapes. The windowing functions are designed per-subcarrier basis in order to exploit available concentration band for each subcarrier. In addition, the proposed concept is considered for the receive filtering in the presence of ACI. It is shown that the optimal spectral concentration property also maximizes ACI rejection for OFDM receivers.

Visible Light Communications Using OFDM and Multiple LEDs

Visible-light communication (VLC) systems leverage solid-state illumination devices to create high-speed communication links. Orthogonal frequency-division multiplexing (OFDM) has been considered for these intensity-modulated/direct-detection (IM/DD) optical channels, however, it suffers from high peak-to-average power ratio (PAPR). Moreover, the implementation of linear, power-efficient, high-current, wideband drivers is challenging. In this paper, the concept of spatial summing is developed where wideband, high PAPR OFDM signals are partitioned into many low-PAPR narrowband signals that are transmitted from multiple LEDs. The signals from different LEDs are allowed to sum in space before being detected by a conventional OFDM receiver. Spatial optical-OFDM (SO-OFDM) is proposed in which filtered subsets of carriers are emitted by each LED. In addition, low-PAPR optical single-carrier FDMA (OSC-FDMA) is developed where different collections of LEDs act as virtual users in a multiple-access scheme. The different variations of SO-OFDM and OSC-FDMA are compared to conventional DC-biased optical (DCO) OFDM and are shown to achieve lower PAPR and more robustness to LED nonlinearities leading to error rate performance gains at high signal-to-noise ratios.

Diversity Transmission and Reception of DAPSK for OFDM

This paper studies the diversity-encoded differential amplitude and phase shift keying (DAPSK) for orthogonal frequency-division multiplexing (OFDM) systems with uniformly interleaved subcarrier grouping and multiple receive antennas over independent but not identically distributed (i.n.i.d.) frequency-selective Rayleigh fading channels. An asymptotic maximum-likelihood (AML) receiver is developed for differentially detecting the received diversity-encoded DAPSK OFDM signal. The bit error probability (BEP) upper bound for the AML receiver is analytically formulated by using the residue theorem. Particularly, an approximate BEP upper bound of the AML receiver is derived for large received signal-to-noise power ratios (SNRs). By virtue of this approximate bound, a design criterion is developed to determine the diversity encoding coefficients for the proposed DAPSK OFDM system. Performance results show that the AML receiver for diversity-encoded DAPSK OFDM can provide better error performance than conventional noncoherent OFDM systems under the same data throughput.

OFDM Synchronization Techniques Analysis for IEEE 802. 16d Review

Orthogonal frequency division multiplexing (OFDM) is a reliable technology for high-speed data transmission by virtue of its spectral efficiency and robustness to multi-path fading. These advantages can be achieved only with good synchronization both in time and frequency. This paper introduces efficient synchronization methods for an OFDM based system, IEEE 802.16d. A symbol timing synchronization scheme is critical in the design of an OFDM receiver. Large timing errors can result in loss of orthogonality between subcarrier, ISI and severe bit degradation. To minimize this degradation two kinds of synchronization algorithms are provided: Auto-correlation and cross correlation. This brief discusses these techniques used in synchronization of OFDM and which one is preferred.

Analog implementation of radix-2, 16-FFT processor for OFDM receivers: non-linearity behaviours and system performance analysis

Analog implementations of decoders have been widely studied by considering circuit complexity, as well as power and speed, and their integration with other analog blocks is an extension of analog decoding research. In the front-end blocks of orthogonal frequency-division multiplexing (OFDM) systems, combination of an analog fast Fourier transform (FFT) with an analog decoder is suitable. In this article, the implementation of a 16-symbol FFT processor based on analog complementary metal-oxide-semiconductor current mirrors within circuit and system levels is presented, and the FFT is implemented using a butterfly diagram, where each node is implemented using analog circuits. Implementation details include consideration of effects of transistor mismatch and inherent noises and effects of circuit non-linearity in OFDM system performance. It is shown that not only can transistor inherent noises be measured but also transistor mismatch can be applied as an input-referred noise source that can be used in system- and circuit-level studies. Simulations of a radix-2, 16-symbol FFT show that proposed circuits consume very low power, and impacts of noise, mismatch and non-linearity for each node of this processor are very small.

Symbol Synchronization for OFDM Receivers with FFT Transport Delay Compensation

This paper proposes a new blind approach for time synchronization of orthogonal frequency division multiplexing (OFDM) receivers (RX). It is largely known that the OFDM technique has been successfully applied to a wide variety of digital communications systems over the past several years — IEEE 802.16 WiMax, 3GPP-LTE, IEEE 802.22, DVB T/H, ISDB-T, to name a few. We focus on the synchronization for the ISDB-T digital television system, currently adopted by several South American countries. The proposed approach uses the coarse synchronization to estimate the initial time reference and then, the fine synchronization keeps tracking the transmitter (TX) time reference. The innovation on the proposed approach regards to the closed loop control stabilization of the fine synchronization. It uses a smith predictor and a differential estimator, which estimates the difference between TX and RX clock frequencies. The proposed method allows the RX to track the TX time reference with high precision ([Formula: see text] sample fraction). Thus, the carriers phase rotation issue due to incorrect time reference is minimized, and it does not affect the proper RX IQ symbols demodulation process. The RX internal time reference is adjusted based on pilot symbols, called scattered pilots (SPs) in the context of the ISDB-T standard, which are inserted in the frequency domain at the inverse fast Fourier transform (IFFT) input in the TX. The averaged progressive phase rotation of the received SPs at the fast Fourier transform (FFT) output is used to compute the time misalignment. This misalignment is used to adjust the RX fine time synchronism. Furthermore, the proposed method has been implemented in an ISDB-T RX. The FPGA-based receiver has been evaluated over several multipath, Doppler and AWGN channel models.