A Novel Carrier Frequency Offset Estimation for OFDM Systems Over Time-Varying Multipath Channels

This paper introduces a novel method to estimate fractional carrier frequency offset (CFO) under time-varying multipath channels in OFDM systems based on the approximation of Basis Expansion Model (BEM). Due to the symmetrical character of the BEM basis, we propose a new design of training sequence with trailing zeros. This training sequence can be used at the receiver to estimate the frequency offset without channel estimation only in one OFDM symbol. We also compare our method with Beek’s ML estimator and Lv’s fine synchronization estimator. The numerical results demonstrate that the proposed method provides an improved performance when the training sequence has a short length.

Joint Carrier-Frequency Offset and Channel Estimation for MIMO-OFDM Transmission

In this paper, we consider a multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) system operating over frequency-selective fading channels. We propose a novel scheme for joint carrier-frequency offset (CFO) and channel estimation based on the expectation–maximization (EM) algorithm. Furthermore, the Cramer-Rao bounds (CRBs) for both CFO and channel estimators are exploited to evaluate the performance of the proposed scheme. Computer simulations show that the proposed algorithm achieves almost ideal performance compared with the CRBs for both channel and frequency offset estimations.

Maximum-Likelihood Estimator for Coarse Carrier Frequency Offset Estimation in OFDM Systems

Orthogonal frequency division multiplexing (OFDM) systems are more sensitive to carrier frequency offset (CFO) compared to the conventional single carrier systems. CFO destroys the orthogonality among subcarriers, resulting in inter-carrier interference (ICI) and degrading system performance. To mitigate the effect of the CFO, it has to be estimated and compensated before the demodulation. The CFO can be divided into an integer part and a fractional part. In this paper, we investigate a maximum-likelihood estimator (MLE) for estimating the integer part of the CFO in OFDM systems, which requires only one OFDM block as the pilot symbols. To reduce the computational complexity of the MLE and improve the bandwidth efficiency, a suboptimum estimator (Sub MLE) is studied. Based on the hypothesis testing method, a threshold Sub MLE (T-Sub MLE) is proposed to further reduce the computational complexity. The performance analysis of the proposed T-Sub MLE is obtained and the analytical results match the simulation results well. Numerical results show that the proposed estimators are effective and reliable in both additive white Gaussian noise (AWGN) and frequency-selective fading channels in OFDM systems.

Effect of Carrier Frequency Offset on the Channel Capacity in Multiuser OFDM-FDMA Systems

Orthogonal Frequency Division Multiplexing (OFDM) is a very robust transmission procedure in multipath and frequency selective radio channels. A Frequency Division Multiple Access (FDMA) resource allocation technique offers the opportunity of a detailed link adaptation scheme. The combination of these transmission- and multiple access techniques in OFDM-FDMA is an ideal and very strong candidate for the downlink of future fourth generation (4G) mobile communication systems. This technical combination offers high cell capacities by exploiting the inherent multiuser diversity effect of the system. To apply OFDM-FDMA in the uplink, the time and carrier synchronization accuracy becomes very important. Non-ideal synchronization of the user signals to the carrier frequency of the base station leads to intercarrier interferences (ICI). In this paper, an analytical model for the ICI consideration in the uplink of a multiuser OFDM-FDMA based system is derived. The impact of the carrier frequency offset (CFO) on the performance of a cellular multiuser system with respect to different subcarrier allocation schemes is analyzed.

Evaluation of Performance Improvement Capabilities of PAPR-reducing Methods

One of the major drawbacks of multicarrier modulation is the large envelope fluctuations which either require an inefficient use of high power amplifiers or decrease the system performance. Peak-to-average power ratio (PAPR) is a very well known measure of the envelope fluctuations and has become the cost function used to evaluate and design multicarrier systems. Several PAPR-reducing techniques have been proposed with the aim to alleviate back-off specifications or increase the system performance. Besides the fact that these techniques have varying PAPR-reduction capabilities, power, bandwidth and complexity requirements, it is interesting to notice that the performance of a system employing these techniques has not been fully analyzed. In this paper we, first, develop a theoretical framework for both PAPR and the distortion introduced by a nonlinearity, and then simulate an OFDM system employing several well known PAPR-reducing techniques from the literature. By means of the theoretical analysis and the simulation results we will show the relation between PAPR and the performance of OFDM systems when a clipping device is present and we will evaluate the real performance improvement capabilities of the PAPR-reducing methods. The agreement between the theoretical and the simulation results demonstrate the validity of the analysis.

A Joint OFDM Channel Estimation and ICI Cancellation for Double Selective Channels

Orthogonal frequency division multiplexing (OFDM) systems suffer significantly from inter-carrier interference (ICI) caused by double selective channels. In this paper, we develop a two-stage hybrid channel estimation and ICI cancellation structure for OFDM. The double selective channels are approximated by an improved Basis Expansion Model (BEM), which is more accurate than the conventional BEM when the normalized Doppler frequency is smaller than one. Based on this improved model, a new ICI cancellation scheme is proposed to reduce ICI impact. Simulation results show that the framework performs well.

A Novel Technique for Channel Estimation and Equalization for High Mobility OFDM Systems

Orthogonal frequency division multiplexing (OFDM) technique, when used in wireless environments, is known to be robust against frequency selective fading. However, when the channel shows time selective fading, rapid variations destroy the subcarrier orthogonality and introduce inter-carrier interference (ICI). The use of ICI mitigation schemes requires the availability of channel state information (CSI) at the receiver, which is a non-trivial task in fast fading systems. In our work, we have addressed the problem of estimation of rapidly varying channels for OFDM systems. The channel is modeled using complex exponentials as basis functions and the estimation process makes use of the cyclic prefix (CP) part available in OFDM symbols as training. The system is viewed as a state space model and Kalman filter is employed to estimate the channel. Following this, a time domain ICI mitigation filter that maximizes the received SINR (signal to interference plus noise ratio) is employed for equalization. This method performs considerably well in terms of MSE as well as BER at very high Doppler spreads.

Blind Joint Symbol Detection and DOA Estimation for OFDM System with Antenna Array

An analysis of the received signal of Orthogonal frequency division multiplexing (OFDM) system with array antennas shows that the received signal has trilinear model characteristics. Trilinear decomposition-based joint symbol detection and direction of arrival (DOA) estimation for OFDM system with antenna array is proposed in this paper. The simulation results reveal that the symbol detection performance of the proposed algorithm is very close to the post-FFT receiver with perfect channel state information; DOA estimation performance is very close to least squares method, and even this algorithm supports small sample sizes. Finally this algorithm does not require the channel fading information, DOA and training sequence or pilot information, so it has blind characteristics.

A Decorrelating-MRC Based Space–Time Multiuser Receiver for Time-Hopping UWB System

In this paper, we propose a two-stage linear multiuser detector (LMD) for ultra wideband (UWB) multiple single-input multi-output (M-SIMO) system and multipath fading environment. Time-hopping (TH) and antipodal pulse amplitude modulation (PAM) are employed for the multiple access system. The decorrelating detector is first employed at the front end of each receive antenna to eliminate the multi-user interference (MUI), then a set of maximum-ratio-combiners (MRC) are proceeded to maximize the signal-to-noise power ratio (SNR) for each user. Since the channel information is crucial for the Decorrelating-MRC (D-MRC) receiver, we develop a subspace-based blind M-SIMO channel estimation method. The effect of channel estimation error on system performance is extensively evaluated. It is also verified from the analytical and numerical results that by exploiting both spatial and temporal diversities, the D-MRC receiver dramatically improves system performance even without additional coding. Moreover, we demonstrate that both the D-MRC receiver and subspace-based blind channel estimator are computationally feasible and near-far resistant.

Time-varying Channel Estimation and Symbol Detection Using Superimposed Training in OFDM Systems

In mobile orthogonal frequency division multiplexing (OFDM) systems, time-varying channels result in severe intercarrier interference (ICI), and greatly degrade the system performance. So, it is necessary to estimate the accurate channel for equalization of received symbols. But, the conventional pilot-assisted channel estimation scheme consumes valuable bandwidth. In this paper, we adopt superimposed training approach for OFDM systems to estimate the time-varying channel, which is approximated by a basis expansion model (BEM). The proposed scheme is an extension of the superimposed training approach previously proposed for time-invariant channels in OFDM systems. At the same time, we employ an iterative best linear unbiased estimator (BLUE) to minimize the mean square error (MSE) of the coefficient estimates and improve the system performance. Simulation results prove the effectiveness of the proposed scheme in fast time-varying scenario.

On Composite Leakage Current Maximization

Sub-threshold, gate and reverse biased junction band-to-band tunneling leakage currents depend on the logic inputs of a CMOS circuit. In this paper, we consider all leakage currents together and generate pattern with the objective of maximizing the overall leakage current to avoid any optimism in leakage current estimation. The computation involves Boolean reasoning on a pre-characterized set of interconnected gates. This problem is known to be computationally intractable. We propose a heuristic with reduced complexity by looking for a lower and an upper bound instead. The bounds tighten progressively with computation and converge asymptotically on a provably exact solution. By appropriately setting the objective function, the same algorithm may also be applied to find the pattern that minimizes the leakage power in the system idle state, which is considered to be ∼50% of the total power consumed in the current technology generations.

A Novel Scheme for Power Amplifier Predistortion Based on Indirect Learning Architecture

Polynomial predistortion techniques for power amplifier (PA) non-linearities based on indirect learning architecture (IDLA) are widely used. The benefit of the IDLA leaves unnecessary the assumption of a model for PA, corresponding parameters estimation and inverse construction. In this paper, a novel scheme based on IDLA for PA predistortion is proposed. It has better power spectral density (PSD) and relative mean square error (RMSE) performances than the conventional IDLA-based methods especially when the PA non-liearity is more severe. Simulations and theoretical analysis verify the good performances of the proposed scheme.

A Novel Adaptive Detection Scheme based on Parallel Interference Cancellation for V-BLAST System

In this paper, in order to reduce the computational complexity of the detector of vertical Bell Laboratories layered space-time (V-BLAST) system over time-varying channels, an adaptive detection scheme is proposed based on parallel interference cancellation (PIC). The presented scheme has three units, which are primary adaptive detector, detection ordering determiner, and adaptive PIC detector. The proposed scheme can employ many of known adaptive algorithms for detection of V-BLAST system. In this paper, we present computational complexity of the proposed scheme using LMS, RLS, APA and AMSER adaptive algorithms and evaluate its performance with numerical simulations.

Uplink Channel Estimation for Multi-user OFDM-based Systems

In this paper we propose a simple, yet flexible and efficient, channel estimator for the uplink in broadband orthogonal frequency division multiplexing (OFDM) systems. The processing is performed in the time-domain, by extracting the Channel’s Impulse Response (CIR) for each user from a joint training signal. In this OFDM system, the pilot sequence we advocate, where all users share the same pilot sub-carriers, consists of one OFDM-symbol endowed with time-shifted properties per user, which isolates each user’s CIR and is robust against multi-user interference. The feasibility of our approach is substantiated by system simulation results obtained using BRAN-A broadband mobile wireless channel model.

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.

Low-Complexity PAPR Reduction of MC-SS Signals in PAN

A peak-to-average power ratio (PAPR) reduction scheme with low complexity is proposed for the multicarrier spread spectrum (MC-SS) system in personal area network (PAN). Traditional clipping and filtering scheme requires a high oversampling rate to meet the emission mask requirements. This would cause high power consumption for mobile PAN devices in personal network. To solve the problem, upsampling is introduced between clipping and filtering in this paper to reduce the oversampling rate. A simplified implementation structure is also derived for the proposed scheme. Simulation results show that its complexity is about 65% of the conventional scheme while achieving satisfying performance.

Cognitive Radio with Single Carrier TDCS and Multicarrier OFDM Approach with V-BLAST Receiver in Rayleigh Fading Channel

This article presents the performance comparison of TDCS and OFDM based cognitive radio for MIMO system using VBLAST receiver architecture to reconstruct the transmitted data. The interference avoidance performance in terms of BER and bitrate are improved by adding multiple antennas to the system and the use of V-BLAST technique at the receiver. The results show the most promising interference avoidance technique combined with MIMO V-BLAST architecture to be applied in the CR system.

Robust Transmission Over Fast Fading Channels on the Basis of OFDM-MFSK

We present an OFDM-based transmission scheme which is suitable for robust transmission in fast fading environments, where a reliable channel estimate is impossible or very difficult to obtain. Our scheme is based on the combination of noncoherently detected M-ary frequency shift keying (MFSK) and orthogonal frequency division multiplexing (OFDM). Noncoherent detection of OFDM-MFSK allows an arbitrary phase choice for all subcarriers in the transmitter. One possibility to exploit this degree of freedom is to choose the subcarrier phases such, that the peak-to-average power ratio (PARR) is reduced. A second possibility is to use the subcarrier phases to transmit additional data. This can be done by differentially modulating the subcarriers that are occupied by the OFDM-MFSK scheme. Both possibilities do not affect the robustness of the underlying noncoherently detected OFDM-MFSK modulation.

Finite-state Markov Model for Lognormal,Chi-square (Central), Chi-square (Non-central), and <Emphasis Type="Italic">K</Emphasis>-distributions

This paper formulates a finite-state Markov channel model to represent received signal-to-noise (SNR) ratios having lognormal, K-distribution, chi-square (central) and chi-square (non-central) distributions in a slow fading channel. The range of the SNRs is partitioned into a finite number of states following earlier works in literature. Performance measures like level crossing rates, steady-state probabilities, transition probabilities, and state-time durations are derived, and numerical results are plotted and discussed for the FSMC models for all the distributions.

Spectrum Efficiency Evaluation for MRC Diversity Schemes Under Different Adaptation Policies Over Generalized Rayleigh Fading Channels

In this paper, closed-form expressions for the capacities per unit bandwidth for Generalized Rayleigh fading channels are derived for optimal power adaptation, constant transmit power, channel inversion with fixed rate, and truncated channel inversion adaptation policies. The closed-form solutions are derived for the single antenna reception (without diversity combining) and MRC diversity reception cases. Optimal power adaptation policy provides the highest capacity over the other adaptation policies both with and without diversity combining. Truncated channel inversion policy suffers a large capacity penalty relative to the optimal power adaptation policy as the number of degrees of freedom is increased. However, with increase in diversity, the capacity penalty for the truncated channel inversion policy decreases. Capacity gains are more prominent for channel inversion with fixed rate policy as compared to the other adaptation policies.