Residual Carrier Frequency Offset Research Articles

多基站协作通信系统的频偏补偿策略

In multi-base station cooperative communication systems, cooperative base stations (BSs) transmit to multiple user equipments (UEs) at the same time slots and carriers, in order to obtain spatial diversity or multiplexing gain. Therefore, a UE may receive several signals associated with different carrier frequency offsets (CFOs) simultaneously. It is impossible for BSs or UEs to pre- or post-compensate for the multiple CFOs respectively. We propose a joint pre- and post-compensation algorithm associated with relaxed residual CFOs. Simulation results show that the proposed algorithm is capable of substantially mitigating the CFOs of multi-base station cooperative systems though it cannot eliminate the multiple CFOs completely.

Simple Carrier Recovery Approach for RF-Pilot-Assisted PDM-CO-OFDM Systems

For RF-pilot-assisted PDM-CO-OFDM systems, we propose and demonstrate a carrier recovery method applying a simple moving average filter (MAF) to extract the central RF-pilot for phase noise compensation. Because only two additions per output sample would be required to implement such a MAF, its computational complexity is rather simple compared to any other direct-form finite impulse response (FIR) filter. To handle its weak side-lobe attenuation, which would cause the spectrally nearby OFDM signal to interfere with the extracted pilot signal, we further propose using multiple MAFs in cascade to enhance the side-lobe attenuation. We evaluate the performance of a 16-QAM and 40-Gbps PDM-CO-OFDM signal, in terms of the bandwidth tolerance, optical signal to noise ratio (OSNR) tolerance, nonlinear tolerance, linewidth tolerance, and residual carrier frequency offset (RFO) tolerance, with different filters including the simple MAF, cascaded MAFs, and the previously-demonstrated multi-stage decimation and interpolation filter (MDIF). We've found that 1) all the filters exhibit similar performance to the ideal brick-wall filter in terms of noise and nonlinear tolerances, 2) the MAF1 exhibits the worse tolerances against linewidth and RFO, and 3) the MDIF has to be carefully designed to enhance its tolerances against both the linewidth and RFO.

Reduction of Intercarrier Interference Based on Window Shaping in OFDM RoF Systems

In this letter, we theoretically investigate the intercarrier interference (ICI) that is induced by carrier frequency offset (CFO) and sampling frequency offset (SFO) for orthogonal frequency division multiplexing (OFDM) radio-over-fiber (RoF) system. A window shaping technique in the time domain at the OFDM receiver is presented, which can improve the tolerance to the frequency offset and reduce ICI. Compared with the conventional demodulation scheme, the simulation shows that the utilization of the window shaping method can increase the resistance to residual CFO to about eight times at power penalty of 2 dB. It can also improve the tolerance to SFO. A 4.65 Gb/s 4QAM OFDM RoF system over 40 km fiber link and 60 GHz wireless link is demonstrated experimentally. The bit error rate (BER) performance of the scheme is measured, with and without the proposed window shaping method, respectively. About 0.9 dB improvement of the required receiver power at a BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> shows the feasibility of the window shaping method for ICI reduction in OFDM RoF system.

Symbol Detection of IDMA Systems in the Presence of Carrier Frequency Offsets

In this paper we investigate the detection algorithms for interleave division multiple access (IDMA) systems in the presence of carrier frequency offsets (CFOs). The existing IDMA detection algorithm is designed under the zero CFO assumption and its performance will be degraded when the CFOs are present. We first extend the existing algorithm to the nonzero CFO case by utilizing effective channel coefficients which take the CFO effects into account. Then we turn to a more practical scenario with imperfect CFO estimates. We propose an algorithm that can cope with the residual CFO effects by integrating the CFO updating into the iterative receiver. Signal-to-interference-plus-noise ratio analysis and simulations show the feasibility and superiority of our proposed algorithms.

Multiple carrier frequency offsets tracking in co‐operative space‐frequency block‐coded orthogonal frequency division multiplexing systems

This study addresses the problem of carrier frequency offset (CFO) tracking in co-operative space-frequency block-coded orthogonal frequency division multiplexing (OFDM) systems with multiple CFOs. Considering that the inserted pilot tones are decayed by data subcarriers in the presence of multiple CFOs, a novel recursive residual CFO tracking (R-RCFOTr) algorithm is proposed. This method first removes CFO-induced inter-carrier interference from data subcarriers, and then updates the residual CFO (RCFO) estimation of each OFDM block recursively. When used in conjunction with a multiple CFOs estimator, the proposed R-RCFOTr can effectively mitigate the impacts from the multiple RCFOs with affordable complexity. Finally, simulation results are provided to validate the effectiveness of our proposed R-RCFOTr algorithm, which has performance close to that of perfect CFO estimation at moderate and high signal-to-noise ratio, and significantly outperforms conventional CFO tracking algorithm for large CFOs.

Blind Residual Carrier Frequency Offset and Timing Offset Estimation Using Improved Minimum Output Variance Approaches in Uplink MC-CDMA Systems

This paper deals with blind residual carrier frequency offset (CFO) and timing offset (TO) estimation based on minimum output variance (MOV) criterion without devoting training sequence and pilot symbol for uplink multicarrier-code division multiple access (MC-CDMA) systems. It has been shown that the performance of MOV estimator is degraded because the effect of noise and wideband spread-spectrum signals. In conjunction with spreading code-aided and subspace projection techniques, we present an improved code-aided MOV (ICMOV) estimator to achieve the high accuracy estimate for the noisy case. Due to more accuracy residual CFO estimate can provide more accuracy residual TO estimate, this paper also present a decision-directed ICMOV (DD-ICMOV) approach to improve the estimation performance of ICMOV estimator. Computer simulations are provided for illustrating the effectiveness of the proposed approaches.

RLS-Based Estimation and Tracking of Frequency Offset and Channel Coefficients in MIMO-OFDM Systems

In this paper, we investigate the problem of carrier frequency-offset (CFO) synchronization and channel estimation in multiple-input multiple-output orthogonal frequency-division multiplexing systems operating over unknown frequency-selective fading channels. We first propose a novel joint CFO and channel estimator, assuming time-domain training blocks are available. The proposed joint estimator consists of two recursive least-square (RLS) algorithms which iterate their estimated CFO and CIR values. We then derive a more precise pilot-aided RLS algorithm to estimate the residual frequency synchronization errors or track small CFO changes. With this, the accuracy of channel estimation is also enhanced. The analysis and simulation results show that, the proposed estimation and tracking scheme which is fully compatible with the existing standards is able to attain fast convergence, high stability, and ideal performances as compared with relevant Cramer---Rao bounds in all ranges of signal-to-noise ratio. Moreover, it can work well for wide tracking range up to ±0.5 of the subcarrier spacing.

Estimation and Compensation of Inter Carrier Interference in WiMAX Physical Layer Under Various Channel Models

WiMAX is Wireless Interoperability for Microwave Access has emerged as a promising solution for transmission of higher data rates for fixed and mobile applications. IEEE 802.16d and e are the standards proposed. To attain higher data rates the Multi Carrier System with Multiple Input and Multiple Output MIMO is incorporated in the WiMAX. And all these sub carriers are considered to be orthogonal to each other. As the number of sub carriers is increased there is no guarantee of sustained orthogonality, i.e. at some point the carriers are not independent to each other, and hence where the orthogonality can be loosed which leads to interference and also owing to the synchronization between transmitter and receiver local oscillator, it causes interference known as Inter Carrier Interference (ICI).In this scheme at the transmitter side the modulated data and a few predefined pilot symbols are mapped onto the non neighboring sub carriers with weighting coefficients of +1 and -1. With the aid of pilot symbols the frequency offset is exactly estimated by using Maximum Likelihood Estimation MLE and hence can be minimized. At demodulation stage the received signals are linearly combined along with their weighted oefficients and pilot symbols, called as Pilot Aided Self Cancellation Method PASCS. The simulations are carried out on Stanford University Interim (SUI)channels. The simulation results shows that by incorporating this method into WiMAX systems it performs better when the Line Of Sight (LOS) component is present in the transmission and also it improves the Bit Error Rate (BER) and Carrier to Interference Ratio (CIR). The CIR can be improved 20 dB. In this paper the effectiveness of PASCS scheme is compared with the Self Cancellation Method (SCM). It provides accurate estimation of frequency offset and when residual CFO is less significant the ICI can be diminished successfully.

Analytical performance evaluation of a class of receivers with joint equalisation and residual carrier frequency offset estimation

ABSTRACTIn this paper, we consider a single‐carrier modulation combined with frequency‐domain equalisation block transmission with residual frequency errors. We employ a frequency‐domain iterative decision feedback equalisation (DFE) denoted iterative block‐decision feedback equalisation (IB‐DFE), which is combined with a decision‐directed carrier frequency offset (CFO) estimator. This estimator produces a CFO estimate within each iteration of the equaliser, which is used to compensate for the CFO. As the analytical study of the IB‐DFE is very hard because of the feedback chain, IB‐DFE receivers with joint symbol detection and carrier frequency synchronisation are also very difficult to tackle. To address this problem, we propose three new models. One models the decision‐directed CFO estimator, another models the IB‐DFE and finally one makes use of the previous two models to jointly characterise equalisation and carrier frequency synchronisation. In addition, a novel graphical tool for the convergence evaluation of the iterative receiver is included. Finally, we validate our models using Monte Carlo simulations and conclude that they are accurate for small and moderate residual CFO values. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Improved Detection of Uplink OFDM-IDMA Signals with Carrier Frequency Offsets

This letter proposes an improved detection scheme to mitigate the influence of carrier frequency offsets (CFOs) in uplink orthogonal frequency division multiplexing-interleave division multiple access (OFDM-IDMA) systems. The basic principle is to iteratively estimate and cancel the combined interference from multiple users and CFOs at the receiver so that the additional interference due to the residual CFOs from other users can be suppressed. Simulation results show that the proposed scheme can effectively eliminate the interference and significantly improve the system performance in the presence of CFOs.

Capacity Analysis and Pilot-Data Power Allocation for MIMO-OFDM With Transmitter and Receiver IQ Imbalances and Residual Carrier Frequency Offset

We analyze the ergodic capacity and capacity bound of a multiple-input–multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) system suffering from frequency-selective in-phase and quadrature-phase (IQ) imbalances along with residual carrier frequency offset (CFO). In the literature, capacity analysis of systems with IQ imbalance has been done only for the single antenna case and has considered either frequency-flat IQ imbalances at both transmitter and receiver sides without CFO or only receiver-side IQ imbalances with CFO. Additionally, these works dealt with IQ imbalance by treating the mirror tone interference as an additional source of noise, which is not optimal. In this paper, we consider a generalized system model that includes a multiantenna system with frequency-selective IQ imbalances at both the transmitter and the receiver sides, in addition to the presence of CFO, and perform capacity analysis considering joint detection of the signal and its self-interference. In addition, for the imperfect channel estimation case, existing works in the literature optimize for pilot spacings and pilot designs. Pilot-data power allocation is an additional degree of freedom that is available to further optimize the system and has not been addressed before. Therefore, we analyze the pilot-data power allocation that maximizes the capacity bound and obtain a closed-form solution for the optimal power allocation at high signal-to-noise ratio (SNR) regime. We then study how the IQ imbalance and residual CFO severity affect capacity, symbol error rate (SER), and capacity bound maximizing power allocation. Finally, we derive the optimal power allocation when the channel is slow varying and remains constant over a block of several OFDM symbols. The derived results are validated through Monte Carlo simulations.

A Frequency Offset Estimation and Compensation Scheme for MB-OFDM UWB Modem

We present a carrier and sampling frequency offset estimation and compensation scheme for a multi-band orthogonal frequency division multiplexing (MB-OFDM) ultra-wideband (UWB) modem. We first perform initial carrier frequency offset (CFO) estimation and compensation during the preamble period, and then conduct the estimation and compensation of the residual CFO and sampling frequency offset (SFO) during the payload period. The proposed design scheme reduces the logic gate count of the frequency offset compensation block by about 10%, while it gives almost the same performance at the packet error rate (PER) of 10-4 in the CM1 channel. The frequency offset estimation and compensation block is implemented using 90nm CMOS technology and tested.

Performance analysis of distributed ZF beamforming in the presence of CFO

We study the effects of residual carrier frequency offset (CFO) on the performance of the distributed zero-forcing (ZF) beamformer. Coordinated transmissions, where multiple cells cooperate to simultaneously transmit toward one or multiple receivers, have gained much attention as a means to provide the spectral efficiency and data rate targeted by emerging standards. Such schemes exploit multiple transmitters to create a virtual array of antennas to mitigate the co-channel interference and provide the gains of multi-antenna systems. Here, we focus on a distributed scenario where the transmit nodes share the same data but have only the local knowledge of the channels. Considering the distributed nature of such schemes, time/frequency synchronization among the cooperating transmitters is required to guarantee good performance. However, due to the Doppler effect and the non-idealities inherent to the local oscillator embedded in each wireless transceiver, the carrier frequency at each transmitter deviates from the desired one. Even when the transmitters perform frequency synchronization before transmission, a residual CFO is to be expected that degrades the performance of the system due to the in-phase misalignment of the incoming streams. This paper presents the losses of the signal-to-noise ratio gain analytically and the diversity order semi-numerically of the distributed ZF beamformer for the ideal case and in the presence of a residual CFO. We illustrate our results and their accuracy through simulations.

Effects of carrier frequency offset on the average effective SNR and the average BER for asynchronous MC‐CDMA uplink systems with a guard period

AbstractThis paper presents an analysis of asynchronous multicarrier‐code division multiple access (MC‐CDMA) systems over frequency‐selective multipath fading channels when the carrier frequency offsets (CFOs) of all users are random variables and the frequency offset for the desired user is compensated. The effect of residual CFO on the average bit error rate (BER) is evaluated by the semi‐analytical method, then the approximated BER performance is obtained as a closed‐form expression. Moreover, the signal‐to‐noise ratio (SNR) loss caused by residual CFO is evaluated. Derived results show that the performance degradation due to residual frequency offset is negligible if the estimation error of CFO for the desired user is less than the normalized value of 10−1. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Sensitivity analysis of offset QAM multicarrier systems to residual carrier frequency and timing offsets

This paper presents a sensitivity analysis of filter bank multicarrier (FBMC) systems to carrier frequency offset (CFO) and timing offset. Although the emphasis of the paper is on the class of FBMC systems that are based on offset quadrature amplitude modulation (OQAM), the developed results are found applicable to cosine modulated-based FBMC systems as well. We assume that coarse CFO and timing offset estimations and compensations have already been performed so that the residual CFO and timing offset are small. A simple closed-form approximation to the signal-to-interference ratio (SIR) is derived. This approximation is compared numerically with the exact SIR and confirmed to be accurate for a variety of FBMC systems with different prototype filters. The derived SIR is characterized by a pair of coefficients that are only dependent on the underlying prototype filter, thus, allows the designer to pick from the choices of the prototype filter the one that has minimum sensitivity to CFO and/or timing offset.

Effect of Carrier Frequency Offset on Single-Carrier CDMA With Frequency-Domain Equalization

Single-carrier code-division multiple access (SC-CDMA) with frequency-domain equalization (FDE) has attracted renewed interest in CDMA due to its lower peak-to-average power ratio (PAPR) and higher frequency diversity than orthogonal frequency-division multiple access (OFDMA), increased robustness to asynchronous multipath and multiuser interference than RAKE-based CDMA, and a reduced need for timing and frequency coordination than time-division multiple access, frequency-division multiple access, and OFDMA. However, existing studies on SC-CDMA are mostly conducted based on an optimistic assumption of perfect carrier frequency estimation; hence, the carrier frequency offset (CFO) effect on the performance of SC-CDMA has not adequately been addressed and is not well understood. In this paper, the system performance of SC-CDMA with FDE is evaluated in terms of the average bit error rate (BER) and signal-to-interference-plus-noise ratio (SINR) obtained through an analytical approach for downlink transmission over a frequency-selective fading channel. The results show that CFO gives rise to three effects: 1) a net phase rotation; 2) fading-like signal power reduction; and 3) intercode interferences (ICIs) due to the loss of code orthogonality. How the CFO-induced BER degradation varies with the number of active users, the number of virtual subcarriers, and constellation size is also investigated. We further show that, in practice, to prevent the BER performance from dramatically deteriorating, the residual CFO after synchronization should be kept within 5% of the virtual subcarrier spacing, irrespective of the constellation size.

Carrier Frequency Offset Tracking in the IEEE 802.16e OFDMA Uplink

The IEEE 802.16e standard for nomadic wireless metropolitan area networks adopts orthogonal frequency-division multiple-access (OFDMA) as an air interface. In these systems, residual carrier frequency offsets (CFOs) between the uplink signals and the base station local oscillator give rise to interchannel interference (ICI) as well as multiple access interference (MAI). Accurate CFO estimation and compensation is thus necessary to avoid a serious degradation of the error-rate performance. In this work, we address the problem of CFO tracking in the IEEE 802.16e uplink and present a closed-loop solution based on the least-squares (LS) principle. In doing so, we exploit a set of pilot tones that are available in each user's subchannel. The resulting scheme can be implemented with affordable complexity and is able to reliably track the CFOs of all active users. When used in conjunction with a frequency offset compensator, it can effectively mitigate ICI and MAI, thereby allowing channel equalization and data detection to follow directly. Numerical simulations are used to demonstrate the effectiveness of the proposed solution in the presence of residual time-varying frequency offsets.

Iterative Frequency Domain Equalization and Carrier Synchronization for Multi-Resolution Constellations

Broadband broadcast and multicast wireless systems usually employ OFDM modulations (Orthogonal Frequency Division Multiplexing) combined with non-uniform hierarchical constellations. However, these schemes are very prone to nonlinear distortion effects and have high carrier synchronization requirements. SC-FDE (Single-Carrier with Frequency-Domain Equalization) is an attractive alternative for OFDM, especially when an efficient power amplification is intended. In this paper we consider the use of SC-FDE schemes combined with non-uniform hierarchical constellations in broadband broadcast and multicast wireless systems. We study the impact of residual CFO (Carrier Frequency Offset) on the performance of multi-resolution schemes and we propose iterative frequency domain receivers with joint detection and carrier synchronization to cope with residual CFO estimation errors (a coarse CFO estimation and compensation is assumed before the equalization procedure). Our results show that while a very high carrier synchronization accuracy is required for the least protected bits, the most protected bits are relatively robust to the CFO. By employing the proposed receiver we increase significantly the robustness to residual CFO estimation errors.

Joint frequency offset tracking and PAPR reduction algorithm in OFDM systems

This paper presents an algorithm that aims to reduce the peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (QFDM) communication systems while maintaining frequency tracking. The algorithm achieves PAPR reduction by applying the complex conjugates of the data symbol obtained from the frequency domain to cancel the phase of the data symbol. A likelihood estimator is used to obtain the sub-carrier phase error due to the residual carrier frequency offset (RCFO) using the same complex conjugates as a pilot signal. Furthermore, a joint time and frequency domain multicarrier phase locked loop (MPLL) is developed to compensate additional frequency offset. Simulation results show that this algorithm is capable of reducing PAPR without impacting the frequency tracking performance.

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).