Introduces Intercarrier Interference Research Articles

Joint CFO and channel estimation using pilot aided interpolation for high performance MIMO-OFDM

ABSTRACT Multiple Input Multiple Output (MIMO) system with orthogonal frequency division multiplexing (OFDM) is a promising physical layer candidate for the current and future wireless standards. MIMO increases system throughput while OFDM maximizes spectrum efficiency. To reduce the impacts of frequency selective fading and increase the capacity, MIMO requires precise channel state information (CSI). The presence of carrier frequency offset (CFO) introduces intercarrier interference (ICI) among the subcarriers which degrades the system performance. MIMO channel estimation is done under the assumption of perfect frequency synchronisation between the transmitter and receiver, or zero CFO. However, such an assumption is incorrect for time-varying and frequency-selective wireless channels, and the presence of CFO has an impact on channel estimation accuracy. In the existing methods, CFO and channel estimations are carried out independently using separate training sequences which reduces the spectral efficiency. As a result, the pilot aided interpolation approach is proposed in this paper for the joint estimate and compensation of CFO and channel effects using the same set of pilot tones. Theresults show that the proposed method offers precise CFO and channel estimation which improves the bit error rate performance and carrier to interference power ratio (CIR) of MIMO-OFDM system.

Comparison of SPS and FA methods for Blind Carrier Frequency Offset Estimation in OFDM systems

In orthogonal frequency-division multiplexing (OFDM) systems, synchronization issues are of great importance since synchronization errors might destroy the orthogonality among all subcarriers and, therefore, introduce intercarrier interference (ICI) and intersymbol interference (ISI). Several schemes of frequency offset estimation in OFDM systems have been investigated. This paper compares performance and computational complexity of Smoothing Power Spectrum (SPS) and Frequency Analysis (FA) methods for blind carrier frequency offset (CFO) estimation in OFDM systems.

Signal modulation techniques in Non-orthogonal waveform for future wireless communication system

<span>In the future generation of the wireless communication system, the utilization of bandwidth needs to be managed efficiently. Due to that, research on enhancing the efficiency of the bandwidth has recently attracted numerous interests focusing more on the non-orthogonal waveform generation and detection. Even though non-orthogonal waveform is promising a higher efficiency, it introduces inter-carrier interference (ICI) at the transmitter due to the subcarrier overlapping. In this paper, the signaling technique for non-orthogonal waveform generation and detection is discussed. This paper also proposed the use of accumulator (ACC) to improve the system performance of the signal modulation in the non-orthogonal waveform for the next generation of the wireless communication system. The result is represented in an EXtrinsic Information Transfer (EXIT) chart to show the effects of adding ACC into the non-orthogonal system.</span>

Spectrum Efficient MIMO-FBMC System Using Filter Output Truncation

Due to the use of an appropriately designed pulse shaping prototype filter, filter bank multicarrier (FBMC) system can achieve low out of band (OoB) emissions and is also robust to the channel and synchronization errors. However, it comes at a cost of long filter tails which may reduce the spectral efficiency significantly when the block size is small. Filter output truncation (FOT) can reduce the overhead by discarding the filter tails but may also significantly destroy the orthogonality of FBMC system, by introducing inter carrier interference (ICI) and inter symbol interference (ISI) terms in the received signal. As a result, the signal to interference ratio (SIR) is degraded. In addition, the presence of intrinsic interference terms in FBMC also proves to be an obstacle in combining multiple input multiple output (MIMO) with FBMC. In this paper, we present a theoretical analysis on the effect of FOT in an MIMO-FBMC system. First, we derive the matrix model of MIMO-FBMC system which is subsequently used to analyze the impact of finite filter length and FOT on the system performance. The analysis reveals that FOT can avoid the overhead in time domain but also introduces extra interference in the received symbols. To combat the interference terms, we then propose a compensation algorithm that considers odd and even overlapping factors as two separate cases, where the signals are interfered by the truncation in different ways. The general form of the compensation algorithm can compensate all the symbols in a MIMO-FBMC block and can improve the SIR values of each symbol for better detection at the receiver. It is also shown that the proposed algorithm requires no overhead and can still achieve a comparable BER performance to the case with no filter truncation.

Parallel Iterative Inter-carrier Interference Cancellation in Underwater Acoustic Orthogonal Frequency Division Multiplexing

In underwater acoustic orthogonal frequency division multiplexing (OFDM) communication system, channel variations would introduce inter-carrier interference (ICI). This gets more severe as mobile speed, carrier frequency or OFDM symbol duration increases. In this paper, we propose a kind of parallel iterative ICI cancellation algorithm to combat that problem. The proposed method extracts channel variations from the adjacent OFDM symbols, and constantly improves the estimation accuracy of ICI through iteration. Finally, simulation and trial in real lake channel verify the availability and stability of the algorithm in the underwater acoustic time varying channel. In addition, this algorithm avoids the matrix inversion, reducing the complexity of the algorithm, and has the obvious advantage in accuracy and speed.

Quality of Service Aware Inter Carrier Interference Mitigation and Antenna Selection Schemes for Beyond 4G Systems

Future broadband wireless communication systems demand high quality of service (QoS) for anytime anywhere multimedia applications. The standards which use orthogonal frequency division multiplexing (OFDM) coupled with multi input multi output (MIMO) are expected to rule the future wireless world. Time selective nature of the channel introduces inter carrier interference (ICI), which is the major performance limiting parameter in OFDM based systems. ICI causes loss in spectral efficiency and results in poor bit error rate (BER) performance, affecting the QoS of MIMO-OFDM systems. The conventional single input single output (SISO)-OFDM-flexible subcarrier spacing (FSS) system offers better performance than the fixed subcarrier spacing systems in terms of ICI mitigation. But BER and spectral efficiency performance of SISO-OFDM-FSS is not good enough to satisfy the requirements of future wireless broadband services. To improve the BER performance, SISO-OFDM system is replaced by space frequency block coded (SFBC)-OFDM system, which adds spatial and frequency diversity benefits to the conventional system. More number of antennas in the MIMO scheme increases the hardware cost, computational complexity and percentage of overhead. In the present study, to improve the spectral efficiency and to reduce the complexity and cost, optimal transmit antenna selection (OTAS) is combined with the SFBC-OFDM-FSS scheme. The simulation results prove that the proposed SFBC-OFDM-FSS-OTAS scheme offers better QoS than the conventional SISO-OFDM-FSS scheme.

A New Blind SFO Estimation Algorithm for NG-DSL Systems

Next generation digital subscriber line (NG-DSL) gains high efficiency by taking scheme of base-band orthogonal frequency-division multiplexing (OFDM) modulation with very large subcarrier number and very high modulation levels. However, it requires high sampling synchronization precisions since sampling frequency offset (SFO) will introduce inter-carrier interference (ICI) and degrade performance. To achieve a higher synchronization, we propose a new blind SFO estimation algorithm by segmenting the OFDM symbols and then estimate SFO in iterations. The new algorithm has higher precision and faster convergence speed than the traditional pilot-aided algorithms under the condition of large subcarrier number and high modulation levels.

AN EFFICIENT CFO ESTIMATION AND ICI REDUCTION IN MC-CDMA SYSTEM THROUGH CONJUGATE WEIGHTED MUSIC BASED RPRCC SCHEME

Multicarrier code division multiple access (MC-CDMA) is one of the most attractive method for future broadband wireless systems and long term evolution (LTE) standard. The performance of MC-CDMA systems is greatly deteriorated by carrier frequency offset (CFO) which is due to oscillator instability and Doppler shifts. CFO leads to loss of orthogonality between the subcarriers and introduces intercarrier interference (ICI). Among the numerous ICI cancellation techniques, the proposed receiver phase rotated conjugate cancellation (RPRCC) offers greater ICI reduction compared to other methods. The distributed nature of networks and concurrent transmission from multiple users in uplink, RPRCC needs multiple frequency offset estimation at the receiver. Multiple signal classification (MUSIC) algorithm is the best suited CFO estimation technique in multiuser environment. However the performance of MUSIC algorithm is inferior for closely spaced CFOs. To achieve precise CFO estimation and greater ICI reduction, RPRCC with conjugate weighted MUSIC based CFO estimation is proposed in this paper. The simulation results depict that the ICI power reduction capability and BER performance of the suggested method outperforms the conventional techniques in both properly and closely spaced CFOs.

Low Complexity ICI Mitigation for MIMO-OFDM in Time-Varying Channels

Time-varying channels destroy the orthogonality among subcarriers in orthogonal frequency division multiplexing (OFDM) systems, and introduce intercarrier interference (ICI). Lots of efforts have been devoted to mitigate ICI in OFDM systems, with different frame structures and channel models, but the computational complexity of the methods is usually very high. In multiple-input multiple-output (MIMO) systems, the complexity is even higher. In this paper, a low-complexity ICI mitigation method is proposed for MIMO-OFDM systems under the assumption of linear time-varying channels. It reduces the complexity of ICI compensation from $ {O(K^{3}(N^{3}+MN^{2}+MN)+NK\log (K))}$ to $ {O(K(N^{3}+2MN^{2}+2MN+2M^{2}+N\log (K)))}$ , where ${K}$ is the number of subcarriers, ${M}$ the number of transmitters, and ${N}$ the number of receivers. It requires channel estimation based on the linear time-varying channel model, and no transmission overhead is needed. The proposed algorithm applies to all OFDM systems as long as linear time-varying channel estimation is applicable. Time-domain synchronous-OFDM naturally suits for the proposed ICI mitigation algorithm because its receiver is able to easily estimate linear time-varying channels. Simulation with QPSK and 16QAM modulation demonstrates the performance of the proposed method, in comparison with no ICI mitigation case. It shows that 2 dB signal to noise gain is achieved when the uncoded bit error rate is $ {10^{-3}}$ and the normalized Doppler frequency is 0.1.

Compressed Channel Estimation With Position-Based ICI Elimination for High-Mobility SIMO-OFDM Systems

Orthogonal frequency-division multiplexing (OFDM) is widely adopted for providing reliable and high-data-rate communication in high-speed train (HST) systems. However, with increasing train mobility, the resulting large Doppler shift introduces intercarrier interference (ICI) in OFDM systems and greatly degrades channel estimation accuracy. Therefore, it is necessary and important to investigate reliable channel estimation and ICI mitigation methods in high-mobility environments. In this paper, we consider a typical HST communication system and show that the ICI caused by large Doppler shifts can be mitigated by exploiting the train position information and the sparsity of the basis expansion model (BEM)-based channel model. Then, we show that for the complex-exponential BEM (CE-BEM)-based channel model, the ICI can be completely eliminated to get the ICI-free pilots at each receive antenna. After that, we propose a new pilot pattern design algorithm to reduce system coherence to improve the compressed-sensing-based channel estimation accuracy. The proposed optimal pilot pattern is independent of the number of receive antennas, the Doppler shifts, train position, or train speed. Simulation results confirm the effectiveness of the proposed scheme in high-mobility environments. The results also show that the proposed scheme is robust to the speed of the moving train.

Design and EXIT Chart Analysis of a Doubly Iterative Receiver for Mitigating Impulsive Interference in OFDM Systems

In this paper, a doubly iterative receiver for orthogonal frequency-division multiplexing (OFDM) systems is designed to mitigate impulsive interference based on pulse blanking. Although being a very efficient countermeasure, pulse blanking introduces intercarrier interference in OFDM systems leading to system performance degradation. The doubly iterative receiver is designed to overcome this drawback. The inner loop combines iterative demodulation and decoding to cancel intercarrier interference. To make soft information available to the outer loop, the equivalent noise power is derived to employ soft demodulation. The outer loop improves the performance of pulse blanking by performing a hypothesis test, which is used to decide if peaks in the received signal are due to impulsive interference or the large peak-to-average-power ratio common to OFDM signals. This allows to optimize the blanking threshold through maximizing the signal-to-interference-and-noise ratio. To evaluate performance and complexity of the proposed receiver, extrinsic information transfer (EXIT) chart analysis is carried out, where EXIT functions for pulse blanker, demodulator, and decoder are derived. An EXIT chart-based trellis search approach is introduced to calculate the loop schedule achieving target bit-error-rate performance with minimized complexity. Numerical results show a considerable bit-error-rate performance improvement as well as a remarkable complexity reduction.

Analysis of improved pulse-shaped RPRCC for synchronous and asynchronous MC-CDMA system under multi-user CFO estimation techniques

Long-term evolution (LTE) is one of the emerging fourth-generation (4G) standards for high-speed wireless networks. Due to the high spectral efficiency, large system capacity and robustness to multi-path fading, multi-carrier code division multiple access (MC-CDMA) system is a highly apt standard for 4G. However, MC-CDMA systems are more susceptible to carrier frequency offset (CFO) due to the discrepancy between the carrier frequencies of transmitter and receiver local oscillators. CFO effects loss of orthogonality among the subcarriers and introduces intercarrier interference (ICI). To reduce ICI and to confine the interference only to a limited number of neighbouring subcarriers, an improved pulse shaping with receiver phase rotated conjugate cancellation (RPRCC) scheme is proposed in this paper. Simulation results are provided to validate the performance of the proposed method using different spreading codes such as Walsh-Hadamard code, m-sequence and gold code in both synchronous and asynchronous transmissions.

Improved Carrier Frequency Offset Estimation Algorithms for PUCCH Format 1 in LTE-Advanced

Physical Uplink Control Channel (PUCCH) is an important narrow-band channel in LTE-Advanced system, which carries the uplink control information, such as acknowledge/non-acknowledge (ACK/NACK) bits, downlink channel quality indicator (CQI), HARQ-ACK feedback information, and it requires much higher transmission quality than the other channels. With the aid of orthogonal CDMA, multiple users share the same time-frequency resource, the multiuser interference has been the main factor that limits the transmission quality of the control information [1]. In the high-speed mobile scenario, there exists a large Doppler frequency shift, which leads to loss of code orthogonality and also introduces inter-carrier interference (ICI) within a single SCFDMA symbol. Besides, the multiuser interference in high-speed mobile environment is severe. In this paper, the carrier frequency offset estimation algorithms are analyzed, and a double-iteration carrier frequency offset (CFO) estimation algorithm is proved to perform the best for PUCCH Format 1 in high-speed mobile scenario. The estimation process is achieved by coarse estimation and fine estimation, which enlarges the estimation range and improves the estimation accuracy.

A Novel CFO Suppression Algorithm for OFDMA Uplink Communication System

In orthogonal frequency division multiple access uplink system, orthogonal multiple subcarriers are assigned to different users for parallel high data rate communications. However, carrier frequency offset (CFO), which is due to oscillator mismatches and/or Doppler shift, will destroy the orthogonality among subcarriers and introduce intercarrier interference (ICI) as well as multiple-access interference (MAI). Thus, system performance will be seriously degraded. To overcome this problem, it is of great importance to do research on suppression of the interferences caused by CFO. Especially, for the application services of the fast time-varying WiMAX, LTE and vehicular communication, the most important property of the suppression algorithm of the multiple CFOs effects is the efficient and high speed processing to compensate the ICI as well as MAI due to the multiple CFOs effects. In this paper, we propose a novel fast time domain inverse matrix receiver to suppress the interference of multiple CFOs. Compared with the conventional frequency domain direct ZF inverse matrix method, which has high complexity in obtaining the ICI matrix and its inverse matrix, the proposed method has very low complexity in obtaining the interference matrix. Furthermore, the signal after interference suppression is a frequency domain signal. Thus the FFT processing in conventional receiver can be skipped and receiver complexity can be simplified. Simulation results show that the proposed simple system works very fast and it does not show the serious error floor phenomenon in high SNR range and this algorithm has almost the same performance to the frequency domain direct ZF inverse matrix method.

Channel Estimation for OFDM Systems over Doubly Selective Channels: A Distributed Compressive Sensing Based Approach

Channel estimation for an orthogonal frequency-division multiplexing (OFDM) broadband system over a doubly selective channel is very challenging. This is mainly due to the significant Doppler shift, which results in a time-frequency doubly-selective (DS) channel. The DS channel features a large number of channel coefficients, which introduces inter-carrier interference (ICI) and forces the need for allocating a large number of pilot subcarriers. To tackle this problem, in this paper we propose a novel channel estimation scheme based on distributed compressive sensing (DCS) theory. Taking advantage of the basis expansion model (BEM) and the channel sparsity in the delay domain, we transform the original DS channel into a novel two-dimensional channel model, where several jointly sparse BEM coefficient vectors become the estimation goal. Then a special decoupling form originating from a novel sparse pilot pattern is designed for such estimation, which results in an ICI-free structure and enables the DCS application to make joint estimation of these vectors accurately. Combined with a smoothing treatment process, the proposed scheme can achieve significantly higher estimation accuracy than the existing ones, although with a much smaller number of pilot subcarriers. Theoretical analysis and simulation results both confirm its performance merits.

窄带电力线通信中对抗突发脉冲的信道估计方法研究

在窄带电力线通信系统中，时域突发脉冲是影响性能的最主要干扰之一。该系统中采用OFDM多载波方式，接收机在频域进行处理时，时域突发脉冲在FFT之后会扩展到整个频段上，进而严重影响系统的性能。因此，接收端需要采取有效的干扰消除机制。置零算法因其操作简单被广泛应用，然而，这种非线性操作会引入载波间干扰(ICI)，导致系统信道估计准确性降低和BER性能变差。本文在此种条件下提出一种新的信道估计方法，迭代消除信道估计中的ICI部分，来降低突发脉冲的影响。仿真结果表明本文提出的改进方案可以有效提高信道估计的准确性，并使系统性能相应提升。 The additive impulsive noise is one of the serious interferences in narrow band power line communication (NB-PLC). With the widely used OFDM (Orthogonal Frequency Division Multiplexing) technology, the influence of the impulsive noise can spread to every subcarrier after DFT in the receiver, which degrades the communication per- formance seriously. Therefore, a simple and effective interference cancelation scheme is required, for example, blanking nonlinearity has been widely used to suppress the impulsive noise due to its convenient operation. However, the nonli- near operation introduces inter-carrier interference (ICI), resulting in degradation in both system performance and the precision of channel estimation. In this paper, a new channel estimation method for OFDM transmission is proposed to reduce the affection of impulsive noise in power line channel, where the ICI part in the estimated channel response function can be iteratively cancelled. The simulation results show that the proposed method can improve the precision of channel estimation and reduce the bit error ratio (BER) significantly at the same time.

Novel Blind CFO Estimator for Uplink Interleaved OFDMA Systems

Abstract In orthogonal frequency-division multiple access (OFDMA) systems, closely spaced and overlapped sub-carriers are divided into groups and assigned to multiple users for simultaneous transmission. However, carrier frequency offsets (CFOs) between the transmitter and the receiver destroy the orthogonality and introduce inter-carrier interference (ICI), resulting in multiple-access interference (MAI). By the inner structure of the received signals, this paper presents a novel CFO estimation algorithm for interleaved OFDMA uplink systems. The proposed algorithm exploits the idea of whitening the spatial covariance matrix of received signals to obtain the CFO estimates of all users. The proposed approach relies only on stationary second-order statistics that are based on a joint diagonalization of a set of covariance matrices. Simulation results illustrate the efficacy of the proposed algorithm.

Iterative Detection and ICI Cancellation for MISO-mode DVB-T2 System with Dual Carrier Frequency Offsets

In the DVB-T2 system with a multiple-input single-output (MISO) transmission mode, Alamouti coded orthogonal frequency division multiplexing (OFDM) signals are transmitted simultaneously from two spatially separated transmitters in a single frequency network (SFN). In such systems, each transmit-receive link may have a distinct carrier frequency offset (CFO) due to the Doppler shift and/or frequency mismatch between the local oscillators. Thus, the received signal experiences dual CFOs. This not only causes dual phase errors in desired data but also introduces inter-carrier interference (ICI), which cannot be removed completely by simply performing a CFO compensation. To overcome this problem, this paper proposes an iterative detection with dual phase errors compensation technique. In addition, we propose a successive-iterative ICI cancellation technique. This technique successively eliminates ICI in the initial iteration by exploiting pre-detected data pairs. Then, in subsequent iterations, it performs a fine interference cancellation using a priori information, iteratively fed back from the channel decoder. In contrast to previous works, the proposed techniques do not require estimates of dual CFOs. Their performances are evaluated via a full DVB-T2 simulator. Simulation results show that the DVB-T2 receiver equipped with the proposed dual phase errors compensation and the successive-iterative ICI cancellation techniques achieves almost the same performance as ideal dual CFOs-free systems, even for large dual CFOs.

Improved least squares channel estimation for orthogonal frequency division multiplexing

The authors consider the design of pilot sequences for channel estimation in the presence of carrier frequency offset (CFO) in systems that employ orthogonal frequency division multiplexing (OFDM). The CFO introduces intercarrier interference (ICI) which degrades the accuracy of the channel estimation. In order to minimise this effect, the authors design pilot sequence that minimises the mean square error (MSE) of the modified least squares (mLS) channel estimator. Since the identical pilot sequence, which minimises this MSE, has high peak-to-average power ratio of the OFDM signal, an alternative approach is proposed for channel estimation. The authors first introduce a new estimator as an alternative to the mLS estimator and design a low PAPR pilot sequences tailored to this new estimator. They show that the proposed procedure completely eliminates the effect of the ICI on the channel estimate. They then extend their design of pilot sequences for realistic sparse channels. Both analytical and computer simulation results presented in this study demonstrate the superiority of the proposed approach over conventional methods for channel estimation in the presence of ICI.

Timing jitter tracking for orthogonal frequency division multiple access system in high doppler spread

In the use of orthogonal frequency division multiple access (OFDMA) in multiple access applications, symbols are transmitted in narrowband sub-channels, which are supposed to be orthogonal. However, the use of a practical oscillator, with its inherent drift because of temperature and other environmental changes, will result in clock jitter which destroys the orthogonality between subcarriers and introduces inter-carrier interference (ICI). To alleviate the problem, this study proposes a novel tracking-based estimation algorithm, using which the effect of clock jitter can be compensated. Instead of modelling the clock jitter as a white noise process, the authors present a more realistic coloured clock jitter model based on practical oscillators. Using the fact that the jitter is coloured and so the jitters in adjacent OFDMA symbols are very roughly related in a linear manner, the authors formulate a new algorithm that models the jitter in the most recent blocks as linearly time varying. The authors then make use of a new objective function based on this model to derive an iterative jitter tracking scheme, so that the jitter in the current block can be better tracked. Simulation results are presented to illustrate the improvement in performance that can be obtained.