Intercarrier Interference Mitigation Research Articles

Spectrally Efficient Non-Orthogonal Discrete Multi-Tone Transmission for Underwater Wireless Optical Communication With Low-Complexity High Performance ICI Mitigation

In this paper, a novel spectrally efficient non-orthogonal discrete multi-tone (NODMT) system is proposed for the bandwidth-starved underwater wireless optical communication (UWOC) to compensate for the insufficient bandwidth of components. Different from other non-orthogonal systems with complex-valued correlation matrices, the designed NODMT system has a real-valued correlation matrix, which is significant in reducing the complexity of decoders, e.g., the complexity of the proposed NODMT with iterative detection (ID) algorithm can be reduced by 50% as compared to the conventional non-orthogonal system with ID. However, in circumstances of higher spectrum efficiency (SE), the non-orthogonality is intensified and inter-carrier interference (ICI) becomes severer, leading to the sharp deterioration of ID decoding capability. The traditional sphere decoding (SD) algorithm cannot solve this issue because Cholesky decomposition only works with the positive definite matrix. Therefore, we propose a novel SD algorithm based on QR decomposition, named QRSD, which effectively compensates for the insufficient decoding capability of the ID algorithm in the case of higher SE. By combining ID and QRSD methods, over 40% complexity reduction and nearly identical BER performance with QRSD are achieved. Through a proof-of-concept UWOC experiment, a 3.31-Gbps NODMT transmission with up to 24.44% data rate promotion is demonstrated, and the feasibility / effectiveness of the proposed ID-QRSD algorithm in different turbidity water is verified. Moreover, we have experimentally realized 75-m/2.66-Gbps and 60-m/3.02-Gbps underwater transmission with 15% and 25% bandwidth savings, respectively. This UWOC system with high SE shows great potential in the bandwidth-limited UWOC and underwater internet of things (UIoT).

BER Performance of Pre-Coded Space-Time Conjugate Two-Path OFDM Systems

It’s critical to design a robust wireless communication system that achieves an improved bit error rate (BER) in mobile channels. In this paper, a 2x2 space-time (ST) orthogonal frequency division multiplexing (OFDM) system is proposed with conjugate cancellation (CC) two-path transmission scheme. The 2x2 STCC-OFDM system features in the excellent inter-carrier interference (ICI) mitigation in the mobile channels with a better BER than that of a conventional 2x2 ST-OFDM system. Five orthogonal transforms are employed as the precoder to enhance the BER further, such as Zadoff-Chu transform (ZCT), slant transform (ST), discrete Hartley transform (DHT), Haar transform (HT), and Walsh–Hadamard transform (WHT). ZCT row wise precoder provide the lowest peak-to-average power ratio and good BER performance. Simulations demonstrate that the proposed STCC-OFDM systems are robust to various channel environments Although the STCC-OFDM systems improve BER with the cost of reducing bandwidth efficiency when compared to a ST-OFDM, they can be compensated by a higher QAM constellation size.

Frequency reversal-based inter-carrier interference mitigation for underwater acoustic OFDM communications

Orthogonal-frequency-division-multiplexing (OFDM) modulation has been intensively studied in underwater acoustic (UWA) communications recently for its ability to tackle large delay spread channels. Nevertheless, the inter-carrier interference (ICI) caused by Doppler spread of UWA channels degrades the system performance significantly. This paper proposes the concept of frequency reversal (FR) dual to time reversal (TR), as well as a feasible FR scheme. FR can compress the frequency spread caused by Doppler, and hence eliminate the ICI in OFDM communications. Furthermore, FR has similar advantages as TR: 1) low computational complexity and 2) robustness against complicated and changeable channels. Numerical simulation verifies the effectiveness of the proposed FR scheme as an ICI mitigation technique for UWA OFDM communications.

An Efficient ICI Cancellation Scheme to Mitigate the Effect of ICI on OFDM Systems.

Inter-carrier interference (ICI) emerges in orthogonal frequency division multiplexing (OFDM) systems used for mobile communication as a consequence of the Doppler Effect's loss of orthogonality among subcarriers. Inter-Carrier Interference (ICI), which affects every subcarrier, drastically lowers performance. The performance of OFDM systems may be enhanced using a variety of ICI mitigation strategies. Comparable subcarrier frequency offsets are guaranteed by the premise that the OFDM transmission bandwidth is suitably modest in the majority of ICI mitigation strategies, on the other hand. The frequency offsets between each subcarrier might change, hence a wideband OFDM system in a situation with high mobility is investigated. Furthermore, the suggested ICI cancellation approach, Total ICI Cancellation, does not reduce bandwidth efficiency or transmission rate. As an example, the Total ICI Cancellation approach uses the ICI matrix's orthogonality to provide perfect ICI cancellation and a significant boost in BER at a linearly increasing cost. The suggested technique, which matches the BER performance of a wideband OFDM system without ICI, offers the best BER performance possible in the presence of frequency offset and time shifts in the channel, according to simulation findings in the AWGN and multipath fading channels.

Turbo DFE Assisted Time-Frequency Packing for Probabilistically Shaped Terabit Superchannels

We present a probabilistically shaped (PS) time-frequency-packing (TFP) wavelength-division multiplexing superchannel system employing higher order modulation (HoM) formats, with an objective to improve the spectral efficiency (SE). However, TFP systems suffer from inter-symbol interference (ISI) and/or inter-carrier interference (ICI). Additionally, the bandwidth limitations of the wavelength selective switches in the fiber link may cause severe ISI for the edge sub-channels (SCs) in a superchannel. Mitigation of such interferences is particularly challenging for HoM systems, since the implementation of the well-known turbo equalization schemes, such as the Bahl-Cocke-Jelinek-Raviv equalizer, is computationally challenging for larger constellations. In this paper, we investigate an expectation propagation based, computationally efficient, turbo decision feedback equalizer for ISI cancellation, in tandem with a parallel interference cancellation based ICI mitigation. By optimizing the parameters in the shaping and the packing dimensions, we show through our numerical results that the proposed PS-TFP superchannels enabling 1.8 Tbps data rates offer up to 1.05 dB packing gain over an unpacked system using the same modulation format, and a 1.15 dB shaping gain over an unshaped system that uses a lower modulation order to achieve the same SE.

Fast Fading Channel Estimation for OFDM Systems with Complexity Reduction

Channel estimation plays a significant role in the Inter-carrier interference (ISI) mitigation and symbol detection in Orthogonal frequency division multiplexing (OFDM) systems. As the speed of mobile terminal increases, the channel changes rapidly, causing fast fading which will degrade the system performance. In this paper, a low complexity fast fading channel estimation method suited for scattered pilot OFDM systems is proposed. This method is characterized by estimating channel matrix directly in the frequency domain, which can significantly reduce the channel estimation complexity with good system performance. Simulation results verify its superiority over the existing estimation methods.

Decision Feedback Kurtosis Minimum Crosstalk Mitigation in Super-Nyquist Multiband CAP Systems

In multiuser access visible light communication (VLC) systems, many multiplex techniques can be utilized to fully exploit the limited bandwidth. In frequency-division multiplexing systems, one possible solution is to dynamically allocate limited bandwidth resources for users to support multiple-service access. To enable high-speed transmissions, the super-Nyquist multiband concept has been proposed in recent years. Specifically, the center frequency spacing is narrowed, and the subcarriers are nonorthogonally overlapped to enable more services and users in multiband systems. Studies have shown that this strategy can support higher spectral efficiency (SE) while simultaneously, introducing intersymbol interference (ISI) and intercarrier interference (ICI). Therefore, a well-designed ICI mitigation scheme is required. In multiband carrierless amplitude phase ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> -CAP) modulation systems, it is technically challenging to achieve the SE beyond one baud/s/Hz under the pre-forward error correction (pre-FEC) threshold without increasing the modulation order. Inspired by the distinctive pulse shaping filter (PSF) structure in a non-orthogonal <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> -CAP scheme, we propose the iterative decision feedback kurtosis minimum (DFKM) scheme and achieve the super-Nyquist SE of approximately 1.06 baud/s/Hz. In contrast to existing ICI mitigation schemes, the theoretical framework reveals that symbol-level crosstalk filters are indispensable to reap the benefit of super-Nyquist transmission, not limited to VLC.

A Low-Complexity Pilot-Based Frequency-Domain Channel Estimation for ICI Mitigation in OFDM Systems

A low-complexity pilot pattern and a frequency-domain channel estimation method for Inter-Carrier Interference (ICI) mitigation is proposed for Orthogonal Frequency Division Multiple Access (OFDM) systems. The proposed method exploits the band structure of the coupling matrix to perform an ICI-free channel estimation in the frequency domain. This ICI-free estimation relies on some conditions imposed over the pilot pattern that simplify the complexity of channel estimation significantly, since its complexity is the same as classical least squares (LS) channel estimation used in low mobility scenarios. Then, the ICI is removed by using a modified version of Minimum Mean Square Error (MMSE) equalization, which reduces the computational complexity considerably. This modified MMSE equalization relies on the sparse and banded structure of the coupling matrix and on a low complexity variant of the Cholesky decomposition, which is named LDLH factorization. It is shown that the proposed method greatly improves the Bit Error Rate (BER) in the high Signal-to-Noise Ratio (SNR) regime.

Design of Time-Frequency Packed WDM Superchannel Transmission Systems

We consider time and frequency-packing (TFP) wavelength division-multiplexing (WDM) superchannel systems for longhaul optical fiber communication. Employing very high baud rates in conjunction with higher-order modulation formats is challenging due to practical constraints. TFP superchannels therefore become an attractive choice to facilitate high data rates with improved spectral efficiency. However, TFP introduces inter-symbol interference (ISI) and/or inter-carrier interference (ICI) that necessitate efficient interference handling techniques. Moreover, phase noise (PN) due to wide laser linewidth (LLW) may cause significant signal distortion in WDM transmission, and the presence of ISI and ICI in TFP systems further complicates carrier phase recovery (CPR). In this article, we propose a spectrally efficient TFP superchannel design equipped with powerful interference cancellation and PN mitigation techniques, targeting Terabit-per-second (Tbps) data rates. First, we propose a joint ISI and ICI channel estimation algorithm coupled with polarization-recovery and a coarse PN cancellation method. Second, we investigate two iterative CPR algorithms to mitigate the distortion due to the residual PN. Third, a combination of successive and parallel interference cancellation methods for ICI mitigation is investigated in tandem with turbo ISI equalization. The effectiveness of the proposed algorithms is demonstrated through computer simulations of a coded TFP superchannel system in the presence of linear fiber impairments, targeting a throughput of 1.2 Tbps. Numerical results suggest that the proposed TFP design offers more than 2 dB performance gains and as high as 960 km transmission distance improvement over existing competitive super-Nyquist designs. Simulation results also indicate that the proposed design exhibits excellent tolerance to high LLWs and aggressive optical filtering stemming from cascaded reconfigurable optical add-drop multiplexers in the fiber link.

Performance evaluation of ICI self-cancellation schemes in fractional wavelet-based OFDM system

The widely used multi carrier modulation technique in the current wireless scenario is orthogonal frequency division multiplexing (OFDM). It is severely affected by frequency offsets and leads to inter carrier interference (ICI). Fractional wavelet transforms (FrWT)-based OFDM model is proposed to mitigate the effect of ICI by its orthogonal fractional wavelets. The performance of FrWT involved OFDM model is comparatively analysed through various mapping schemes of ICI self-cancellation in order to mitigate frequency offsets. This system overcomes the drawback of ICI self-cancellation and shows improved bandwidth efficiency without any cyclic prefix. Simulation results are carried out for the proposed model and compared with exiting Fourier transform and wavelet transform-based OFDM system. The analysis is performed for a range of normalised frequency offsets values. The proposed model performance is tested with various mapping schemes against a frequency offset of 0.05. It is proved that FrWT-based OFDM with ICI self-cancellation gives reduced bit error rate (BER) of 10–5 at 8 dB signal to noise ratio (SNR). From various graphs, it states that weighted conjugated transformation results in effective mitigation of ICI with BER achievement of 10-4.4 at frequency offset of 0.1.

ICI and PAPR enhancement in MIMO-OFDM system using RNS coding

The Inter-Carrier-Interference (ICI) and the Peak-to-Average-Power Ratio (PAPR) of the transmitted signal are considered as bottlenecks in the utilization of Multiple-Input-Multiple-Output Orthogonal Frequency Division Multiplexing (MIMO-OFDM) systems, which caused a significant performance degradation. In this paper Residue Numbers as a coding scheme is impeded in MIMO-OFDM systems, where the PAPR values are analysed and compared to non-RNS coding systems, and the ICI levels are measured and evaluated with respect to the conventional ICI mitigation techniques as pulse shaping, windowing and self-cancellation techniques implemented in MIMO-OFDM system. The ICI errors, Bit-Error-Rate (BER) and the Complementary Cumulative Distribution Function (CCDF) for MIMO-OFDM system with Residue Number System (RNS) coding are analysed and evaluated. The results demonstrated a performance enhancement of the transmission model over the system without RNS implementation.

ICI Mitigation in OFDM System Using Negative Peak Augmented Raised Cosine Power Pulse

Orthogonal frequency division multiplexing (OFDM) is a digital modulation technique generally which is utilized in modern wireless applications to support high data rates. The carrier frequency offset (CFO), which models the frequency discrepancy between the transmitter and receiver oscillators, gives rise to inter carrier interference (ICI), along with destroying the orthogonality between the OFDM subcarriers. Thus the alleviation of an ICI power is extremely important in an OFDM system. Pulse shaping functions is a standout amongst the most generally utilized solutions with the end goal of lessening the ICI power. In this paper, an Adaptive pulse shaping filter based on the concept of Negative Peak Augmentation is proposed to mitigate the ICI and to enhance the performance of the OFDM system. The simulation results demonstrate the impact of the proposed pulse shaping function and performance investigation in terms of SIR, SNR and BER is carried out to show the superiority of the proposed pulse shaping function.

Analysis and Mitigation of ICI Due to Gain Adjustment in OFDM Systems

Automatic gain control (AGC) is widely adopted in orthogonal frequency division multiplexing (OFDM) systems to compensate the significant variation of the received signal. In OFDM systems, the duration of the cyclic prefix (CP) is usually equal to the delay spread of the channel to minimize the system overhead. The inter-symbol interference (ISI) between consecutive OFDM symbols spreads over a large portion of the CP. The CP contaminated by the ISI should not be used to estimate the received power. If the duration of the remaining CP is shorter than the settling time of the AGC, then the gain adjustment will be carried out within the useful portion of the OFDM symbol. The subcarrier orthogonality is lost and the system performance is degraded by the resulting inter-carrier interference (ICI). In this paper, we propose a simple and efficient scheme to mitigate the ICI. The proposed scheme can be easily implemented in actual systems because of its low computational complexity. The simulation results show that the proposed scheme can increase the signal-to-interference-plus-noise ratio significantly compared to the traditional scheme.

Adaptive Windowing for ICI Mitigation in Vehicular Communications

The performance of orthogonal frequency division multiplexing systems in vehicular environments suffers from intercarrier interference (ICI) and the inherent non stationarity of the channel statistics. Receiver windowing constitutes an effective technique for enhancing the banded structure of the frequency-domain channel matrix, thus improving the effectiveness of a banded equalizer for ICI mitigation. However, its optimality has been verified only for stationary channels with perfectly known statistics. In non stationary channels, the second-order statistics have to be tracked and the optimal performance can be achieved at the expense of cubic complexity over the number of the subcarriers. To overcome this limitation, an adaptive windowing technique is proposed that is able to track directly an optimal receiver window in terms of average signal-to-interference noise ratio, requiring only linear complexity. Extensive simulation results verify both the ability of the proposed approach to track the time varying channel statistics and its increased robustness to channel estimation errors that are common in vehicular environments.

Location-Aware ICI Reduction in MIMO-OFDM Downlinks for High-Speed Railway Communication Systems

High mobility may destroy the orthogonality of subcarriers in OFDM systems, resulting in inter- carrier interference (ICI), which may greatly reduce the service quantity of high-speed railway (HSR) wireless communications. This paper focuses on ICI mitigation in the HSR downlinks with distributed transmit antennas.For such a system, its key feature is that the ICIs are caused by multiple carrier frequency offsets corresponding to multiple transmit antennas. Meanwhile, the channel of HSR is fast time varying, which is another big challenge in the system design. In order to get a good performance, low complexity real-time ICI reduction is necessary. To this end, we first analyzed the property of the ICI matrix in AWGN and Rician scenarios, respectively. Then, we propose corresponding low complexity ICI reduction methods based on location information. For evaluating the effectiveness of the proposed method, the expectation and variance of remaining interference after ICI reduction is analyzed with respect to Rician K-factor. In addition, the service quantity and the bandwidth and computation cost are also discussed. Numerical results are presented to verify our theoretical analysis and the effectiveness of proposed ICI reduction methods. One important observation is that our proposed ICI mitigation method can achieve almost the same service quantity with that obtained on the case without ICI at 300km/h,that is, ICI has been completely eliminated. Numerical results also show that the scenarios with Rician K-factors over 30dB can be considered as AWGN scenarios, which may provide valuable insights on future system designs.

ICI Mitigation by Estimation of Double Carrier Frequency Offsets in High-Speed-Railway Communication Systems for Smart Cities

The urbanization and the fourth industrial revolution lead to the explosion of smart cities. One of the most prior problems to solve in digital cities is transportation infrastructures and high speed railway (HSR) is an effective solution. In communication aspect of HSR, the passenger's demand and transportation management networks require high-speed data services with reliable connections. Orthogonal frequency division multiplexing (OFDM) is a modulation method in the advanced communication systems to provide broadband communications services. OFDM is substantial against inter-symbol-interference due to long symbol duration, but it is very sensitive to doppler effect that happens when the speed of the train is getting much faster. In addition, inter-carrier interference (ICI) caused by high doppler frequency shift has a severe impact on OFDM in case of high channel variations. In this paper, we propose an ICI mitigation method by utilizing the estimation and pre-compensation of high doppler shifts in HSR communication systems for smart cities. The estimate of the doppler shift is based on a preamble frame of data in communication link between EnodeB and user equipment. The simulation results show that the performance of system has been improved using the proposed model.

OFDM for payload communications of UAS: channel estimation and ICI mitigation

For real-time transmission of high-bandwidth sensor data from an unmanned aircraft vehicle (UAV) to the ground station (GS) in an unmanned aircraft system, a high-speed payload communication system is essential. While orthogonal frequency-division multiplexing (OFDM) is very effective for terrestrial wideband systems, possible higher speed of UAVs or higher carrier frequencies can increase the Doppler shift, and therefore increase the inter-carrier interference (ICI) in OFDM systems. In these scenarios, the complete frequency-domain channel matrix should be estimated and used for ICI mitigation and data recovery. In this study, a time-domain pilot-based channel estimation scheme is described to estimate the entire frequency-domain channel matrix for moderate-to-high Doppler OFDM systems. Simulation results indicate that the new scheme provides better performance compared with channel estimation schemes designed for low Doppler scenarios that partially estimate the frequency-domain channel matrix. The comparison is also presented with the other full channel estimation scheme, named autoregressive scheme, showing a comparable performance with much lower complexity for the new scheme.

Design of a novel hybrid intercarrier interference mitigation technique through wavelet implication in an OFDM system

Orthogonal Frequency Division Multiplexing (OFDM) is the most preferred and widely used multiplexing technique in current wireless environment for sidelining the spectrum scarcity problem by splitting a signal into N signals and subsequently modulating them through several orthogonal subcarriers (which bears high frequency). With several great features, OFDM is severely affected by undesirable effects of frequency offset errors and Local Oscillator (LO) frequency synchronization errors. This paper proposes an approach that devises a new hybrid technique, which is a combination of Maximum Likelihood Estimation (MLE) and Self Cancellation (SC) techniques through wavelet implication, to enhance BER performance of the OFDM system. A comparative analysis of SC, MLE, and wavelet-based hybrid InterCarrier Interference (ICI) cancellation techniques was conducted using 64-QAM and differential offset (0.02 and 0.01) for differential data bits (4 Mb and 16 Mb) to justify the outstanding results. The simulation results showed a significant byte-error-rate improvement over different legacy techniques based on fast Fourier transform.

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.

ICI Mitigation for Dual-Carrier Superchannel Transmission Based on m-PSK and m-QAM Formats

We propose an intercarrier interference (ICI) mitigation scheme for dual-carrier superchannel transmission. Using the modified decision directed least mean square (DD-LMS) based multi-input multi-output (MIMO) processing, we can simultaneously realize both polarization division demultiplexing and ICI mitigation for PDM-m-PSK and PDM-m-QAM formats. Thanks to the innovative modification of frequency-shift operation, the MIMO equalization enables synchronous output of dual-carrier signals with robust performance against frequency offset, laser phase noise, as well as polarization mode dispersion (PMD). The experimental back-to-back transmission of PDM-16QAM signal has 8.3-dB OSNR improvement using the proposed MIMO processing. Three dual-carrier 160-Gb/s PDM-16QAM superchannels within 25-GHz grid are experimentally transmitted over 640-km standard single-mode fiber (SSMF), leading to net spectral efficiency of 5.7b/s/Hz.