Carrier Frequency Offset Compensation Research Articles

The Feasibility of Mobile Physical-Layer Network Coding with BPSK Modulation

This paper considers applying physical-layer network coding (PNC) to orthogonal frequency division multiplexing (OFDM) modulated mobile ad-hoc networks (MANETs) to resolve the outstanding issue of short contact time between nodes due to their mobility. Ideally, PNC enables data exchange twice as fast as the traditional scheduling, and thus, it is a potential performance booster in MANETs. However, the application of PNC in MANETs is challenged by the carrier frequency offset (CFO) problem inherently caused by node-motion-induced Doppler shifts and asynchronous oscillators. CFO induces intercarrier interference (ICI) that degrades PNC performance. In this paper, we investigate the CFO/ICI impact on the signal-to-interference-and-noise ratio (SINR) and bit error rate (BER) in the signal detection of PNC in a two-way relay channel (TWRC) based on binary phase-shift keying (BPSK) modulation. We find that PNC with power control suffers, at most, a 3 dB SINR penalty compared with generic point-to-point communications in both the flat-fading and the frequency-selective channels. We also find that a belief propagation (BP) algorithm could be employed in the signal detection of PNC to effectively tackle ICI and reduce its impact on the BER of PNC. For CFO compensation in PNC, we propose a method that amounts to positioning the relay's local oscillator frequency at the middle of the received frequencies from the two end nodes in the TWRC. Importantly, we show that 1) this compensation method can theoretically maximize the worst SINR in PNC and that 2) in the case of similar CFO of the two uplinks in TWRC, it allows PNC to achieve a BER at the relay close to that in the ideal case, i.e., in point-to-point communications without CFO. Overall, this paper demonstrates that mobile PNC is feasible in general, laying the foundation for future studies.

Self-Cancellation of Interchannel Interference for OFDM Systems in Amplify-and-Forward Relay Communication Networks

We propose a method to compensate the effects of multiple carrier frequency offsets (CFOs) for orthogonal frequency division multiplexing (OFDM) systems. The multiple CFOs, we considered, are caused by several spatially separated frequency-unsynchronized transmitters as in amplify-and-forward relay communication networks or wireless ad hoc sensor networks. Despite its appealing features, OFDM is extremely sensitive to CFO, especially to multiple CFOs. At the receiver, the residual CFO which appears due to incomplete compensation of multiple CFOs destroys orthogonality among subcarriers and brings interchannel interference (ICI). In the proposed scheme, we compensate the CFOs of the received signal by the predesigned value, which results in cancellation of the opposite ICI by each other. This is based on the property that the spectrum of ICIs shown in a subcarrier has similar magnitudes and opposite signs from the fact that spectrum shapes of the ICI are of sinc function. Simulation results are shown to demonstrate the effectiveness of the proposed method.

Time–Frequency Multiplex Transceiver Design with RX IQ Imbalance, CFO, and Multipath Channel Estimation and Compensation for Multicarrier Systems

The low complexity time---frequency multiplex estimator and equalizer transceiver design are proposed to combat the RF impairment problems for the zero-IF transceiver of multi-carrier systems. Moreover, the proposed preamble can estimate and compensate the receiver (Rx) in-phase and quadrature-phase (IQ) imbalance, carrier frequency offset (CFO), and channel impulse response parameters. There are two parts in the proposed system. Firstly, all parameters of impairments are estimated by the proposed time---frequency multiplex estimator design with the iterative cancellation scheme. Secondly, the iterative group pair equalizer with the estimated parameters is proposed to eliminate the RF impairments effect and detect the transmitted signal with low complexity advantage. Simulation results confirm that the proposed estimator can provide the reliable performance over the severe IQ imbalance, CFO, and multipath fading channel environments.

Efficient Transmission of Encrypted Images with OFDM in the Presence of Carrier Frequency Offset

In this paper, the impact of carrier frequency offset (CFO) and CFO compensation on the transmission of encrypted images with different orthogonal frequency division multiplexing (OFDM) versions is...

Interference Mitigation in MC-CDMA System with CFO compensation based Modified MMSE-FDE Technique

carrier Code Division Multiple Access (MC-CDMA) has been an attractive technique for wireless communication because it offers high data rate, high spectral efficiency, large system capacity, less transmit power and less implementation complexity. But due to Carrier Frequency Offset (CFO), which comes into account due to asynchrony between transmitter and receiver local oscillators or by Doppler Effect due to high mobility, orthogonality among the subcarriers gets effected resulting in Inter Carrier Interference (ICI) which leads to rapid degradation in performance of MC-CDMA system. In this paper, the performance of Uplink MC-CDMA system with Zero forcing- frequency domain Equalization (ZF-FDE), conventional MMSE equalizer and Modified MMSE-FDE using Walsh-Hadamard spreading sequence in multipath fading channel has been analyzed. Simulation has been performed using 32 subcarriers and 64 subcarriers for these equalization techniques. Simulation results confirm excellent performance of the Modified MMSE-FDE which outperforms the conventional MMSE equalization and ZF- FDE. The further improvement in performance is achieved by employing 64 subcarriers instead of 32 subcarriers. Numerical equations and simulation results are given to support the analysis. Keywords-CDMA, CFO, MMSE-FDE, ZF, ICI, Walsh-Hadamard Sequence.

A Very Simple Algorithm of Sequential IQ Imbalance and Carrier Frequency Offset Compensation in Coherent Optical OFDM

In this work, a hardware efficient algorithm for sequential in-phase (I) and quadrature (Q) imbalance (IQ imbalance) and carrier frequency offset (CFO) compensation under chromatic dispersion (CD) and phase noise (PN) environment is proposed. Two identical orthogonal frequency division multiplexing (OFDM) symbols, which are namely training sequences (TSs), are used to acquire CFO and IQ imbalance coefficients. The CFO is obtained by calculating phase differences between the two TSs. To achieve the image frequency interference factors which are caused by an IQ imbalance effect, each symbol of the TS is modulated on only a half of all the subcarriers while the remaining subcarriers are modulated with zeros. By doing this, the IQ imbalance coefficients are directly estimated without recursive calculation requirements. This brings a low complexity to implementation in hardware. The performances of the modeling system are evaluated by a numerical simulation method where the error vector magnitude (EVM), the bit error ratio (BER), and the mean square error (MSE) quantities are used as performance indicators. The numerical simulation results are showed that the performance of the modeling system is enormously improved even when highly dispersive channels and phase noise are considered.

Low Complexity CFO Compensation in Uplink OFDMA Systems With Receiver Windowing

Orthogonal frequency division multiple access (OFDMA) systems in the uplink suffer from multiple access interference (MAI) due to their high sensitivity to frequency misalignments between different users. In this paper, we propose the application of time domain receiver windowing methods to confine the leakage caused by multiple carrier frequency offsets (CFOs) to a few neighboring subcarriers with almost no additional computational burden. The CFO effects can be translated into a linear system of equations with a coefficient matrix that is called interference matrix. As a result of receiver windowing, we can approximate the interference matrix with a quasi-banded one by neglecting its small elements outside a certain bandwidth. This allows us to propose a class of low complexity CFO compensation techniques. These techniques are applicable to the generalized subcarrier allocation scheme (G-CAS). The complexity reduction in the proposed solutions is substantial when compared to the existing ones in the literature. It is worth mentioning that this substantial complexity reduction is achieved in expense of some bandwidth efficiency loss. Hence, there exists a tradeoff between bandwidth efficiency and complexity. Solutions based on both zero forcing (ZF) and minimum mean squared error (MMSE) criteria are proposed and compared. Simulation results demonstrating the effectiveness of the proposed algorithms in approaching the optimal performance are also presented.

A Comparative Analysis of CFO Estimation Techniques in OFDM Systems

OFDM systems are sensitive to Carrier Frequency Offsets (CFO) that results in performance's degradation. Researchers have proposed various CFO compensation techniques. Some estimates the CFO's value then compensates for it using preamble methods. Others estimate the CFO value using the inherent construction of the time-domain OFDM symbol. Our proposed technique changes the conventional structure of the OFDM symbol to allow estimating the CFO values over a wide range. CFO estimation range, accuracy, transmitted data-rate, system complexity, and the compatibility of a specific estimator to be used in different applications are the most important parameters to be considered in researches. In this paper, a comparative analysis between preamble techniques (Moose, Training Symbol) and the OFDM internal construction-based estimators including blind and proposed schemes will be discussed. Simulations show the trade-offs between those techniques.

Frequency domain CFO compensation for FBMC systems

The frequency-domain compensation of the deleterious effects introduced by the residual carrier frequency offset (CFO) still present on the received signal after rough time-domain compensation in a filter bank multicarrier (FBMC) receiver is considered. The compensation method depends not only on the particular FBMC modulation technique but also on the chosen receiver structure. The paper is dedicated to the case where the recently proposed frequency-despreading receiver, one of the most promising solutions, is used in the OFDM/OQAM transceiver. Specifically, this receiver exploits some samples of an extended FFT and, as shown in the paper, it can nearly perfectly compensate the CFO effects on the received signal by adapting the weights of the linear combination of the considered samples. A detailed analysis of the frequency-despreading structure and of the proposed CFO-compensation technique is presented by using as performance measure the signal-to-interference ratio and the bit-error-rate.

Blind CFO estimation based on decision directed MUSIC technique for interleaved uplink OFDMA

This paper deals with blind carrier frequency offset (CFO) estimation based on the multiple signal classification (MUSIC) criterion with Taylor series expansion for interleaved orthogonal frequency division multiple access uplink systems. An efficient estimator is suggested which finds a new CFO vector based on the Taylor series expansion of the one initially given in the presence of CFO. The problem of finding the new CFO vector is formulated as the closed form of a generalized eigenvalue problem, which allows one to readily solve it. Due to more accurate residual CFO estimate, which can provide more accurate residual CFO compensation, this paper also presents a decision-directed MUSIC scheme to improve the CFO estimation performance. The proposed estimator not only can estimate CFO, but also entail less computational load. Several computer simulation results are provided for illustrating the effectiveness of the proposed blind CFO estimator.

送電線用ディジタル電力線搬送における周波数オフセット補償方式

The transmission performance of a digital power line carrier system with adaptive equalizer degrades in the presence of the carrier frequency offset (CFO). Therefore, accurate CFO estimation and compensation is necessary. In this paper, we propose a CFO estimation method suitable for a digital power line carrier system with adaptive equalizer in the presence of delayed propagation paths. The proposed CFO estimation method estimates the CFO from the measured phase rotation per symbol which is obtained by the measurement of autocorrelation of a 4PSK training sequence. By setting the time separation for autocorrelation measurement to the training sequence length, the accurate CFO estimation is achieved in the presence of delayed propagation paths. In order to achieve sufficiently accurate CFO estimation in a low SNR condition, averaging the instantaneous autocorrelation measurements is done by a simple first-order filter. It was confirmed by computer simulation that even in the presence of delayed propagation paths, by using the proposed CFO estimation method, a digital power line carrier system with adaptive equalizer achieves a BER performance close to the case of no CFO.

PARAFAC-based frequency synchronization for SC-FDMA uplink transmissions

Frequency synchronization is a critical requirement for single-carrier frequency division multiple-access (SC-FDMA) uplink transmissions. Based on the key observation that the received signal at the base station can be formulated as the parallel factor analysis model, we present a joint carrier frequency offset (CFO) estimation and compensation scheme for SC-FDMA uplink transmissions with interleaved sub-carrier allocation. The proposed scheme, employing identical training symbols, guarantees the identifiability of CFO estimation, allows the system to operate on full load, and it is readily extendible to multi-antenna base station (BS) reception. Comparisons with existing approaches in the literature reveal that the proposed algorithms provide lower complexity and superior performance.

FBMC receiver for multi-user asynchronous transmission on fragmented spectrum

Relaxed synchronization and access to fragmented spectrum are considered for future generations of wireless networks. Frequency division multiple access for filter bank multicarrier (FBMC) modulation provides promising performance without strict synchronization requirements contrary to conventional orthogonal frequency division multiplexing (OFDM). The architecture of a FBMC receiver suitable for this scenario is considered. Carrier frequency offset (CFO) compensation is combined with intercarrier interference (ICI) cancellation and performs well under very large frequency offsets. Channel estimation and interpolation had to be adapted and proved effective even for heavily fragmented spectrum usage. Channel equalization can sustain large delay spread. Because all the receiver baseband signal processing functionalities are proposed in the frequency domain, the overall architecture is suitable for multiuser asynchronous transmission on fragmented spectrum.

Low‐complexity joint regularised equalisation and carrier frequency offsets compensation scheme for single‐carrier frequency division multiple access system

Since the conventional zero-forcing receiver does not operate satisfactorily in interference-limited environments, because of its noise amplification; the statistics of the transmitted data and the additive noise are required for the minimum-mean-square error receiver; the potential of the regularised receiver is proposed and investigated in this study to cope with these problems. In this study, the authors introduce an efficient low-complexity joint regularised equalisation and carrier frequency offset compensation (LJREC) scheme for single-carrier frequency division multiple access system. The proposed LJREC scheme avoids the noise amplification problem and the estimation of the signal-to-noise ratio and the interference matrices of other users. From the obtained simulation results, the proposed scheme enhances the system performance with lower complexity and sufficient robustness to the estimation errors.

Multi-User Interference Cancellation Schemes for Carrier Frequency Offset Compensation in Uplink OFDMA

Each user in the uplink of an Division Multiple Access (OFDMA) system may experience a different carrier frequency offset (CFO). These uncorrected CFOs destroy the orthogonality among subcarriers, causing inter-carrier interference and multi-user interference, which degrade the system performance severely. In this paper, novel time-domain multi-user interference cancellation schemes for OFDMA uplink are proposed. They employ an architecture with multiple OFDMA-demodulators to compensate for the impacts of multi-user CFOs at the base station's side. Analytical and numerical evaluations show that the proposed schemes achieve a significant performance gain compared to the conventional receiver and a reference frequency-domain multi-user interference cancellation scheme. In a particular scenario, a maximum CFO of up to 40% of the subcarrier spacing can be tolerated, and the CFO-free performance is maintained in the OFDMA uplink.

Carrier frequency offset estimation for OFDMA uplink using null sub-carriers

Carrier synchronization in OFDMA uplink systems comprises estimation and compensation of carrier frequency offsets (CFOs) corresponding to different users. We propose a null sub-carrier-based CFO estimation algorithm for OFDMA uplink systems, employing generalized or sub-band sub-carrier allocation scheme. We analyze the effect of location and number of null sub-carriers on the performance of the algorithm, in the presence of multi-user interference (MUI) and channel frequency selectivity. We calculate the acquisition range of the proposed estimation algorithm and prove that the identifiability of CFO estimation is guaranteed in the presence of MUI and channel nulls. We also evaluate the computational complexity and the performance of the proposed estimation algorithm and, compare it with some of the existing approaches in the literature.

A Receive Beamforming Technique for an LTE-based Mobile Relay Station with Antenna Array

A mobile relay station (MRS) with soft handover experiences performance degradation because the received signal at the MRS located between two base stations (BSs) is the sum of these two signals with different carrier frequency offsets (CFOs). In this paper, two receive beamforming techniques with CFO compensation (a joint receive beamforming (JB) technique and a spatially separated receive beamforming (SB) technique) are proposed for the MRS with a uniform linear array (ULA) in orthogonal frequency-division multiplexing (OFDM) systems. The SB technique is shown to be robust against CFO errors and can be effectively used for the MRS in a multipath fading channel. The performance of the proposed techniques is evaluated via simulation in a Long Term Evolution (LTE) environment.

Improving the OFDMA Performance with Common Carrier Frequency Offset Correction

In an OFDMA system, different carrier frequency offsets (CFOs) are possibly raised due to the mismatch of local oscillators and the Doppler effect because of multiple-antenna channels. In spite of an initial CFO synchronization scheme able to be applied at transmitters, the compensation of residual CFOs is required at the receiver in order to eliminate the inter-carrier interference (ICI) for individual subscribers, especially with an interleaved subcarrier allocation scheme. Unfortunately, conventional ICI mitigation methods cannot simultaneously remove the CFOs caused by multiple subscribers, and therefore, the multiuser interference (MUI) remains. In this case, a common CFO (CCFO) existing among the multiple CFOs is found in relation to the overall OFDMA system performance. In this paper, a CCFO estimation method is proposed at the OFDMA receiver, and the CCFO is then corrected to reach the minimum weighted mean square error (MSE) performance. Numerical results show that new OFDMA receivers after correcting the estimated CCFO significantly improve the overall bit error rate (BER) performance over the conventional receivers.

Alamouti Coding Scheme for AF Relaying With Doppler Shifts

In this paper, we propose an Alamouti-code-based relaying scheme for frequency asynchronous amplify-and-forward (AF) relay networks. Both the oscillator frequency offsets and the Doppler shifts among the distributed nodes are considered in our design. We employ orthogonal frequency-division multiplexing (OFDM) modulation at the source node and let the two relay nodes implement only simple operations, such as time reversal, conjugation, and amplification. We show that without Doppler shifts, the multiple carrier frequency offsets (CFOs) can be directly compensated at the destination, and the received signals exhibit an Alamouti-like structure. We further prove that full spatial diversity can be achieved by the fast symbol-wise detection when the oscillator frequency offset between the relay nodes is smaller than a certain threshold, which yields lower decoding complexity compared with the existing schemes. In the case with Doppler shifts, where the direct CFO compensation becomes impossible, we develop a repetition-aided Alamouti coding approach, by which full diversity can be nearly achieved from the fast symbol-wise detection. Numerical results are provided to corroborate the proposed studies.

Joint Maximum Likelihood Estimation of CFO, Noise Power, and SNR in OFDM Systems

Estimation of noise power and signal-to-noise ratio (SNR) are fundamental tasks in wireless communications. Existing methods to recover these parameters in orthogonal frequency-division multiplexing (OFDM) are derived by following heuristic arguments and assuming perfect carrier frequency offset (CFO) synchronization. Hence, it is currently unknown how they compare with an optimum scheme performing joint maximum likelihood (ML) estimation of CFO, noise power and SNR. In the present work, the joint ML estimator of all these parameters is found by exploiting the repetitive structure of a training preamble composed of several identical parts. It turns out that CFO recovery is the first task that needs to be performed. After CFO compensation, the ML estimation of noise power and SNR reduces to a scheme that is available in the literature, but with a computational saving greater than 60% with respect to the original formulation. To assess the ultimate accuracy achievable by the ML scheme, novel expressions of the Cramer-Rao bound for the joint estimation of all unknown parameters are provided.