Residual Frequency Offset Research Articles

Bit Error Rate Approximation of MIMO-OFDM Systems with Carrier Frequency Offset and Channel Estimation Errors

The bit error rate (BER) of multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems with carrier frequency offset and channel estimation errors is analyzed in this paper. Intercarrier interference (ICI) and interantenna interference (IAI) due to the residual frequency offsets are analyzed, and the average signal-to-interference-and-noise ratio (SINR) is derived. The BER of equal gain combining (EGC) and maximal ratio combining (MRC) with MIMO-OFDM is also derived. The simulation results demonstrate the accuracy of the theoretical analysis.

Sampling Frequency Offset Estimation for Pilot-Aided OFDM Systems in Mobile Environment

In orthogonal frequency division multiplexing (OFDM) systems, most of the conventional sampling frequency offset (SFO) estimation methods work under the assumption of time-invariant or slow time-variant channels. In mobile environment, the time-variant channel significantly degrades the accuracy of SFO estimation. To solve the problem, we first analyze the properties of time-variant channels. If terminal moves within some tens of the wavelength of radio frequency (RF) signal, channel path delay almost remains unchanged. For most practical OFDM systems, our analysis indicates that channel path delay can be regarded as unchanged during the interval of some tens of OFDM symbols in time-variant channels. Based on the analysis, we propose a novel SFO estimation method for pilot-aided OFDM systems. Different from the conventional methods, the proposed method estimates SFO by detecting the variation of the symbol timing error caused by SFO. The detection is finished by implementing correlation between the channel impulse responses (CIRs) estimated by different OFDM symbols. Performance of the proposed method is simulated and compared with that of two conventional post-FFT methods. Numerical results show that, the proposed SFO estimation method performs better than the conventional methods not only in time-variant channels, but also at low SNRs and large residual carrier frequency offsets (CFOs).

Improved schemes for tracking residual frequency offset in DVB-T systems

Carrier frequency offset (CFO) synchronization is a crucial issue in the implementation of orthogonal frequency division multiplexing (OFDM) systems. Synchronization is performed in two stages: acquisition and tracking. After a first estimation and correction of the CFO performed in the acquisition stage, there still remains a residual frequency offset (RFO) due to real system conditions. This paper presents two new schemes for RFO tracking affecting a digital video broadcasting (DVB-T) system. Due to the nature of DVB-T, where data transmission is performed continuously, RFO tracking needs to be performed accurately during all the data length in order to correct the ICI and the phase rotation impairment produced by this RFO. Comparisons of the novel algorithms with two previously reported estimators in terms of mean square error (MSE), complexity and speed of convergence are performed. The proposed schemes improve performance and/or speed of convergence compared to the conventional estimators with a reduction of the computational complexity. Furthermore, a combined technique for RFO tracking involving the two new schemes is proposed. This method obtains the best results in terms of accuracy, convergence and complexity.

Additional Insight on SAGE-based Carrier and Residual Frequency Offset Estimations in OFDM Systems

Insight on estimations of the space-alternating generalized expectation-maximization (SAGE)-based carrier ? and residual $${\epsilon}$$ frequency offset with the inclusion of inter-carrier interference (ICI) power using Cramer-Rao's lower bound (CRLB) theorem and a linear model method is given. Additional simulation results are given. Detailed discussions are presented.

BER Degradation of MC-CDMA at High SNR with MMSE Equalization and Residual Frequency Offset

Multicarrier Code Division Multiple Access (MC-CDMA) is an attractive technique for high speed wireless data transmission in view of its advantages over orthogonal frequency division multiplexing. In this paper, we analyze the performance of fully loaded downlink MC-CDMA systems with minimum mean square error (MMSE) equalizer in the presence of residual frequency offset (RFO) in multipath Rayleigh fading channels. We first show that as the SNR is increased beyond a value, referred as threshold SNR, the performance degrades. We then analyze the cause for this behavior and propose a remedy to prevent the degradation by regularizing the coefficient(s) of the equalizer, and use the regularized equalizer for SNRs beyond the threshold value. We suggest two methods for estimating this SNR, one gives close to the true value but requires the knowledge of RFO and the channel state information (CSI), while the other gives an approximate value but requires only CSI. We show that the regularization based on the approximate value also prevents the degradation, but the performance at higher SNRs is slightly poorer compared to that with the better estimate. Numerical and simulation results are provided to support the analysis.

Estimation of residual carrier and sampling frequency offsets in OFDM-SDMA uplink transmissions

This paper investigates the estimation of multiple residual carrier frequency offsets (RCFOs) and sampling frequency offsets (SFOs) in the uplink of a multiuser OFDM network with space division multiple access (SDMA). The proposed solutions are based on the presence of some dedicated pilot tones within the signal spectrum, which are traditionally employed in OFDM systems to track channel variations and residual frequency errors. The main obstacle is the large number of parameters involved in the estimation process, which makes the uplink synchronization a rather challenging task. A practical solution to this problem relies on the separation of the received signals before the estimation procedure is started. This way, the RCFOs and SFOs of different users are estimated independently with affordable complexity. We propose two alternative approaches for users' separation. The former is based on the use of orthogonal pilot sequences, while the latter exploits the distinct spatial signatures of the uplink signals. Computer simulations and theoretical analysis are employed to assess the performance of the proposed schemes and to make comparison with existing alternatives.

Fine frequency synchronization in the uplink of multiuser OFDM systems

This contribution considers the estimation and compensation of residual multiple carrier frequency offsets (CFOs) and sampling frequency offsets (SFOs) in the uplink of multiuser OFDM systems. Based on the observation of a user-dependent phase rotation, an approximate maximum-likelihood (ML) estimator is proposed in order to estimate the CFOs and SFOs and to track the phase in a post-synchronization step. By using a Taylor approximation, the estimation complexity is reduced. Bit error-ratio (BER) simulations show that the system performance degradation due to (small) frequency mismatches is negligible if the proposed method is applied.

Pilot-Assisted Fine Frequency Synchronization for OFDM-Based DVB Receivers

After coarse synchronization in orthogonal frequency division multiplexing (OFDM) systems, there might still be a residual frequency offset (RFO) and a sampling clock frequency offset (SFO), which seriously degrade the performance of the systems. This paper proposes a simple way of jointly estimating the RFO and SFO by using a continual pilot (CP) defined in an OFDM-based digital video broadcasting (DVB) system. For deriving an unbiased joint RFO and SFO estimator in the current DVB system, a CP subset is selected to simultaneously remove the effect of RFO and SFO. We can observe by simulation that the proposed frequency estimator endowed with a properly selected CP subset is unbiased and performs robustly.

Decision-aided compensation of severe phase-impairment-induced inter-carrier interference in frequency-selective OFDM

A new, reduced complexity algorithm is proposed for compensating the Inter-Carrier Interference (ICI) caused by severe PHase Noise (PHN) and Residual Frequency Offset (RFO) in OFDM systems. The algorithm estimates and compensates the most significant terms of the frequency domain ICI process, which are optimally selected via a Minimum Mean Squared Error (MMSE) criterion. The algorithm requires minimal knowledge of the phase process statistics, the estimation of which is also considered. The scheme outperforms previously proposed compensation methods of similar complexity, when severe phase impairments are present.

Computationally Efficient Joint Frequency Synchronization for OFDM-Based DVB-T

This letter proposes a computationally efficient way of jointly estimating the residual frequency offset (RFO) and sampling frequency offset (SFO) by using a continual pilot (CP) defined in OFDM-based DVB-T system. In order to devise an unbiased joint frequency estimator in the current DVB-T system, a CP subset is selected to offset the effects of RFO and SFO simultaneously.

Cramér-Rao Bound for SAGE-based Residual Frequency Offset Estimation Algorithm in OFDM Systems

The Cramer-Rao bound (CRB) is a powerful tool in estimation theory as it gives a performance lower bound for parameter estimation problems. In this paper, a much tighter CRB for Lee’s residual frequency offset (RFO) estimation method (IEEE Transactions on Communications 54:765, 2006) is first given. The tighter low bound is obtained by considering the ICI that affects the performance of space-alternating generalized expectation-maximization (SAGE) based RFO estimator. It can be concluded that the performance of SAGE based RFO estimation method decreases as the normalized RFO increases and increases with the increasing of signal-to-noise (SNR). Simulation results show that the proposed CRB of SAGE based RFO estimator is extremely tight. It approximates closely the MSE performance obtained by Monte Carlo simulation.

Pilot-aided frequency offset tracking scheme for OFDM-based DVB-T

The main disadvantage of orthogonal frequency division multiplexing (OFDM) systems is its sensitivity to carrier frequency offset and timing offset. This paper proposes a simple way of improving the performance of a residual frequency offset (RFO) estimator in OFDM-based digital video broadcasting (DVB) system. To demonstrate the efficiency of the proposed RFO estimator, comparisons are made with conventional estimator in terms of mean square error (MSE) performance, estimation range, and complexity.

An Improved Sampling Frequency Offset Estimator for OFDM-Based Digital Radio Mondiale Systems

After coarse synchronization in orthogonal frequency division multiplexing (OFDM) systems, there might still be a residual frequency offset (RFO) and a sampling clock frequency offset (SFO), which seriously degrade the performance of the systems. This paper suggests a simple way of estimating the SFO in OFDM-based digital radio mondiale (DRM) system. To demonstrate the efficiency of the proposed SFO estimator, comparisons are made with other existing estimators in terms of mean square error (MSE) performance, estimation range, and complexity.

A technique to exploit cooperation for packet retransmission in wireless ad hoc networks

In wireless data communication systems, retransmission of an erroneous packet is inevitable due to the harsh communication environment. In this paper, an efficient retransmission scheme using cooperation from neighboring nodes is investigated. In the cooperative retransmission scheme, an erroneous packet is transmitted to the destination by cooperative nodes which have favorable channels. This cooperative retransmission scheme requires no a priori information of neighboring nodes and has no limitation on the number of cooperating nodes. Distributed beamforming is used to accommodate multiple cooperating nodes. Phase and frequency offsets of cooperating signals are extracted from the NACK message and used to co-phase retransmitted data packets. The outage probability of the cooperative retransmission scheme is analyzed for the case of perfect synchronization and when the offsets are estimated. To reduce the impact of the residual phase and frequency offsets in cooperating signals, a low-rate feedback scheme is also investigated. It is shown that improved outage probability and reduced packet error rate (PER) performance can be achieved even for long data packets. The proposed cooperative retransmission scheme is found to outperform simple retransmission by the source as well as decode-and-forward cooperation.

Improved Fine Frequency Synchronization Method for OFDM-Based DRM

In this letter, a robust pilot-assisted synchronization scheme is proposed for estimation of residual frequency offset (RFO) in OFDM-based digital radio mondiale (DRM) system. The RFO estimator uses the gain reference pilots mainly reserved for the channel tracking in the DRM standard. To demonstrate the efficiency of the proposed RFO estimator, comparisons are made with the conventional RFO estimator using the frequency reference pilots in terms of mean square error (MSE) performance.

A Simple Joint Estimation Method of Residual Frequency Offset and Sampling Frequency Offset for DVB Systems

This letter presents a simple joint estimation method for residual frequency offset (RFO) and sampling frequency offset (STO) in OFDM-based digital video broadcasting (DVB) systems. The proposed method selects a continual pilot (CP) subset from an unsymmetrically and non-uniformly distributed CP set to obtain an unbiased estimator. Simulation results show that the proposed method using a properly selected CP subset is unbiased and performs robustly.

A Fine Frequency Synchronization and Tracking for Mobile WiMAX Broadcasting Systems

In this paper, we suggest a synchronization receiver for MIMO-OFDM based mobile broadcasting system. A MIMO-OFDM receiver with algorithms for tracking of residual frequency offset (RFO) and sampling frequency offset (SFO) is designed and simulated. The proposed synchronization and compensation schemes are applied to IEEE 802.16e WiMAX and its performance is investigated by computer simulations in terms of mean square error (MSE) and bit error rate (BER). The proposed RFO and SFO estimator has a fast convergence and a small residual degradation even for high speed mobile station.

Residual Synchronization Error Elimination in OFDM Baseband Receivers

It is well known that an OFDM receiver is vulnerable to synchronization errors. Despite fine estimations used in the initial acquisition, there are still residual synchronization errors. Though these errors are very small, they severely degrade the bit error rate (BER) performance. In this paper, we propose a residual error elimination scheme for the digital OFDM baseband receiver aiming to improve the overall BER performance. Three improvements on existing schemes are made: a pilot-aided recursive algorithm for joint estimation of the residual carrier frequency and sampling time offsets; a delay-based timing error correction technique, which smoothly adjusts the incoming data stream without resampling disturbance; and a decision-directed channel gain update algorithm based on recursive least-squares criterion, which offers faster convergence and smaller error than the least-mean-squares algorithms. Simulation results show that the proposed scheme works well in the multipath channel, and its performance is close to that of an OFDM system with perfect synchronization parameters.

Inter-Frame, Fine Frequency/Phase Synchronization forSimple Space-Time-Coded OFDM Receivers

This paper proposes an enhanced receiver (Rx) configuration for multiple-input, multiple-output (MIMO) OFDM systems, operating under the composite effect of phase noise (PHN), residual frequency offset (RFO) and the transmission channel, herein modeled as quasi-static but unknown. The proposed Rx identifies the different impairments by exploiting their different time constants and compensates for each one accordingly. It includes a novel inter-frame fine frequency synchronization (FFS) scheme, which is closely coupled to an intra-frame adaptive phase synchronizer/channel estimator. The proposed scheme is evaluated for a 2 times 2, Alamouti space-time code (STC), and is shown to provide significant performance gain. The theory can be employed with any other STC scheme.

Detection of Interleaved OFDMA Uplink Signals in the Presence of Residual Frequency Offset Using the SAGE Algorithm

In this paper, we propose an iterative detection scheme in the presence of residual frequency offsets (RFOs) using the space-alternating generalized expectation-maximization algorithm for an orthogonal-frequency-division-multiple-access uplink system. In the proposed scheme, the expectation step intends to remove intercarrier interference (ICI) due to the RFOs of simultaneous users in the received signals. Then, the maximization step is utilized to estimate the required parameters for each user (i.e., RFO, data symbols, and channel state information) employing the ICI-cancelled signals. Simulation results show that the proposed scheme achieves almost ideal bit-error-rate performance as long as the magnitude of the normalized RFO value is within 0.2