Carrier Frequency Offset Compensation Scheme Research Articles

Joint low‐complexity nonlinear equalization and carrier frequency offset compensation for multiple‐input multiple‐output orthogonal frequency division multiplexing communication systems

AbstractThe conventional zero forcing (ZF) equalizer suffers from the noise enhancement problem and the increasing complexity with the increase of the number of subcarriers. On the other hand, the minimum mean square error (MMSE) equalizer mitigates the noise enhancement, but it needs estimation of the signal‐to‐noise ratio (SNR) to work properly. In addition, the Joint Low‐complexity Regularized ZF (JLRZF) equalization and carrier frequency offset compensation scheme mitigates the noise enhancement using a constant parameter called the regularization parameter without SNR estimation. In this paper, we present a JLRZF with successive iInterference cancelation (JLRZF‐SIC) scheme for multiple‐input multiple‐output (MIMO) discrete Fourier transform orthogonal frequency division multiplexing (DFT‐OFDM) system to cope with the above‐mentioned problems. The proposed JLRZF‐SIC scheme jointly performs the equalization and the carrier frequency offset (CFO) compensation processes in two steps. The coupling nature of the MIMO channel is canceled in the first step. A regularization term is added in the second step to avoid the inter‐symbol‐interference (ISI). Simulation results show that the proposed JLRZF‐SIC scheme improves the system performance with low complexity, especially in the presence of estimation errors.

Joint low‐complexity equalization and CFO estimation and compensation for UWA‐OFDM communication systems

SummaryFrequency synchronization has a great importance in preserving the performance of the underwater acoustic (UWA) orthogonal frequency division multiplexing (OFDM) systems. The carrier frequency offset (CFO) estimation can be blind or data‐aided. In this paper, the Zadoff‐Chu (ZC) sequences are used for OFDM synchronization in UWA communications, and they are compared with different data‐aided algorithms. We propose a low‐complexity algorithm for CFO estimation based on ZC sequences. Also, a joint equalization and CFO compensation scheme for UWA‐OFDM communication systems is presented. Simulation results demonstrate that the proposed CFO estimation algorithm allows estimation of the CFO accurately with a simple implementation in comparison with the traditional schemes. Also, the performance of the UWA‐OFDM system can be preserved in the presence of frequency offsets.

Filter bank SCFDMA: an efficient uplink strategy for future communication systems

This paper develops a generalized system model for the precoded multicarrier communication system, using basic multirate building blocks. Mathematical analysis of the proposed model is carried out, and the results are utilized in developing an efficient uplink wireless communication standard filter bank single carrier frequency division multiple access. The proposed system combines the low peak to average power ratio (PAPR) advantage of a single carrier communication system with the reduced out of band emission (OBE) of filter bank multicarrier (FBMC) scheme. The sensitivity of the proposed system to carrier frequency offset (CFO) is analyzed, and the results are utilized in developing a CFO compensation scheme with reduced complexity. A Nyquist filter design approach, which strikes a balance between OBE and tail size, is developed and is incorporated into the proposed system to enhance the OBE and PAPR characteristics. The instantaneous power of the proposed system is theoretically analyzed using characteristic function based approach, and the effectiveness of modifications is substantiated. A detailed simulation study is carried out to validate the performance of the proposals.

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.

Joint Low-Complexity Equalization and Carrier Frequency Offsets Compensation Scheme for MIMO SC-FDMA Systems

Due to their noise amplification, conventional Zero-Forcing (ZF) equalizers are not suited for interference-limited environments such as the Single-Carrier Frequency Division Multiple Access (SC-FDMA) in the presence of Carrier-Frequency Offsets (CFOs) . Moreover, they suffer increasing complexity with the number of subcarriers and in particular with Multiple-Input Multiple-Output (MIMO) systems. In this letter, we propose a Joint Low-Complexity Regularized ZF (JLRZF) equalizer for MIMO SC-FDMA systems to cope with these problems. The main objective of this equalizer is to avoid the direct matrix inversion by performing it in two steps to reduce the complexity. We add a regularization term in the second step to avoid the noise amplification. From the obtained simulation results, the proposed scheme is able to enhance the system performance with lower complexity and sufficient robustness to estimation errors.

Exact Signal Model and New Carrier Frequency Offset Compensation Scheme for OFDM

Carrier frequency offset (CFO) causes spectrum misalignment of transmitter and receiver filters. This misalignment leads to energy loss and distortion of received signal, resulting in performance degradation of the whole system. However, these issues are often overlooked by existing works. This paper presents an exact signal model for OFDM that takes into account the CFO-induced spectral misalignment and related aliasing effects. Incorporating these practical issues, we also propose a new preamble structure and a new CFO compensation approach to mitigate their negative effects. Theoretical analysis and simulation results substantiate the importance of the exact signal model and the advantage of the proposed CFO compensation scheme.

Equalization and Carrier Frequency Offsets Compensation for the SC-FDMA System

Recently, the Single-Carrier Frequency Division Multiple Access (SC-FDMA) system has attracted the attention as an efficient alternative to the Orthogonal Division Multiple Access (OFDMA) system in the uplink communications. In this system, the Carrier Frequency Offsets (CFOs) disrupt the orthogonality between subcarriers, and give rise to Inter-Carrier Interference (ICI), and Multiple Access Interference (MAI) among users. In this paper, the impact of the CFOs on the performance of the Discrete Fourier Transform SC-FDMA (DFT-SC-FDMA) and the Discrete Cosine Transform SC-FDMA (DCT-SC-FDMA) systems is investigated. Based on the Minimum Mean Square Error (MMSE) criterion, a new low-complexity joint equalization and CFOs compensation scheme is proposed. It is refered to as the MMSE scheme. The MMSE weights of the proposed scheme are derived taking into account the MAI and the noise. Furthermore, a hybrid scheme comprising the proposed MMSE scheme and a Parallel Interference Cancellation (PIC) stage is also suggested and investigated to further enhance the performance of interleaved subcarriers mapping systems. From the obtained simulation results, it is clear that CFOs disrupt the orthogonality between the subcarriers in SC-FDMA systems and degrade the Bit Error Rate (BER) performance. The proposed compensation schemes are able to enhance the system performance, even in the presence of the estimation errors.

Carrier Frequency Offsets Problem in DCT‐SC‐FDMA System: Investigation and Compensation

The Single‐Carrier Frequency Division Multiple Access (SC‐FDMA) system is a well‐known system, which has recently become a preferred choice for uplink channels. In this system, the Carrier Frequency Offsets (CFOs) disrupt the orthogonality between subcarriers and give rise to Intercarrier Interference (ICI), and Multiple Access Interference (MAI) among users. In this paper, the impact of the CFOs on the performance of the Discrete Cosine Transform (DCT) SC‐FDMA (DCT‐SC‐FDMA) system is investigated. Then, a new low‐complexity joint equalization and CFOs compensation scheme is proposed to cancel the interference in frequency domain. The Minimum Mean Square Error (MMSE) equalizer is utilized in the proposed scheme. A hybrid scheme comprising MMSE equalization, CFOs compensation, and Parallel Interference Cancellation (PIC) is also suggested and investigated for further enhancement of the performance of the DCT‐SC‐FDMA system with interleaved subcarriers assignment. For simplicity, this scheme will be referred to as the MMSE+PIC scheme. From the obtained simulation results, it is found that the proposed schemes are able to enhance the system performance, even in the presence of the estimation errors.

Carrier Frequency Offsets Problem in DCT-SC-FDMA System: Investigation and Compensation

The Single-Carrier Frequency Division Multiple Access (SC-FDMA) system is a well-known system, which has recently become a preferred choice for uplink channels. In this system, the Carrier Frequency Offsets (CFOs) disrupt the orthogonality between subcarriers and give rise to Intercarrier Interference (ICI), and Multiple Access Interference (MAI) among users. In this paper, the impact of the CFOs on the performance of the Discrete Cosine Transform (DCT) SC-FDMA (DCT-SC-FDMA) system is investigated. Then, a new low-complexity joint equalization and CFOs compensation scheme is proposed to cancel the interference in frequency domain. The Minimum Mean Square Error (MMSE) equalizer is utilized in the proposed scheme. A hybrid scheme comprising MMSE equalization, CFOs compensation, and Parallel Interference Cancellation (PIC) is also suggested and investigated for further enhancement of the performance of the DCT-SC-FDMA system with interleaved subcarriers assignment. For simplicity, this scheme will be referred to as the MMSE+PIC scheme. From the obtained simulation results, it is found that the proposed schemes are able to enhance the system performance, even in the presence of the estimation errors.

Performance Analysis of a Low-Complexity CFO Compensation Scheme for OFDMA Uplink

Similar to orthogonal frequency-division multiplexing (OFDM) systems, orthogonal frequency-division multiple access (OFDMA) is vulnerable to carrier frequency offset (CFO). Since the CFO of each user is different, CFO compensation in OFDMA uplink is much more involved than that in OFDM systems. It has been shown that the zero-forcing (ZF) compensation method is a simple yet effective remedy; however, it requires the inversion of a large matrix and the computational complexity can be very high. Recently, we have developed a low-complexity iterative method to alleviate this problem. In this paper, we consider the theoretical aspect of the algorithm. We specifically analyze the output signal-to-interference-plus-noise-ratio (SINR) of the algorithm. Two approaches are used for the analysis; one is simple but approximated, and the other is complicated but exact. The convergence problem is also discussed. In addition to the analysis, we propose a pre-compensation (PC) method enhancing the performance of the algorithm. Simulations show that our analysis is accurate and the PC method is effective.

A low-complexity zero-forcing CFO compensation scheme for OFDMA uplink systems

Similar to the conventional orthogonal frequencydivision multiplexing (OFDM) system, an OFDM multiple access (OFDMA) system will have a carrier frequency offset (CFO) problem. Since CFOs of all users are different, CFO compensation in the OFDMA uplink system is much more involved. A simple, yet efficient, method is the zero-forcing (ZF) compensation method. However, it involves an inverse of an <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> times <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> CFO-induced ICI matrix, where <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> is the number of subcarriers. Thus, the complexity can become very high when <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> is large, a case commonly seen in OFDMA systems. In this work, we propose a low-complexity ZF method to overcome the problem. The main idea is to use Newton's method to solve matrix inversion iteratively. We explore the structure of the CFOinduced ICI matrix and develop a method that can implement Newton's method with fast Fourier transforms (FFTs). As a result, the required computational complexity is significantly reduced from <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</i> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) to <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</i> (2 <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> log <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> ). Simulations show that, with only three iterations, the proposed method can have similar performance to the direct ZF method.

A frequency offset compensation scheme for the received signal under the influence of Rician fading channels with a direct component shifted by Doppler effect

AbstractA scheme that estimates and compensates for the frequency offset from the phase rotation of pilot symbols is proposed as a carrier frequency offset compensation scheme that periodically inserts pilot symbols in the transmitted signal. Since the frequency offset is estimated and compensated for at a frequency close to the direct path wave when a conventional scheme is applied in a Rician fading environment where the direct path wave is Doppler shifted, the Doppler spread appears to widen further, the estimation error of the fading distortion increases, and the error rate performance degrades. In this paper, we propose a frequency offset compensation scheme that estimates the center frequency of the Doppler spread and reduces the effects of the Doppler‐shifted direct path wave by (1) calculating the signal power in the frequency domain by taking the discrete Fourier transform (DFT) of the pilot symbols and (2) smoothing the signal power in a frequency window. Computer simulations demonstrate the superior error rate performance of the proposed scheme compared to the conventional scheme and clearly show its effectiveness. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 85(12): 65–75, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.10006