Autonomous driving in 5G: Mitigating interference in OFDM-based vehicular communications

The nonlinearity of high power amplifiers (HPAs) exhibits amplitude and phase distortions, which cause loss of orthogonality among the subcarriers in orthogonal frequency division multiplexing (OFDM) systems, and hence, inter-carrier interference (ICI) is introduced in the transmitted signal. The ICI power is proportional to the amplitude of the signal at the amplifier input and it may cause a considerable bit error rate (BER) degradation. Due to the ICI is a random process and has unknown distribution function, it is difficult to evaluate the theoretical BER when the ICI is taken into account. This paper evaluates the BER analytically using sequence development approach (SDA) for OFDM systems perturbed by the HPA. In comparison to the state-of-the-art technique, the evaluation of the BER using this approach achieves superior approximation at the expense of additional computational complexity.

The performance of Link Adaptation (LA) under the influence of inter carrier interference (ICI), which is cause by carrier frequency offset (CFO) and Doppler frequency spread due to mobility, in orthogonal frequency division multiplexing (OFDM) based wireless systems is analyzed in this work. LA maximizes throughput while maintaining a target bit error rate (BER) or Block Error Rate (BLER) at the receiver using the signal to noise ratio (SNR) fed back from the receiver to the transmitter. Since ICI power is proportional to the signal strength, the implications of such an impairment on LA OFDM systems is not obvious and has not received much attention till now. It is seen that when ICI is not captured in the SNR measured then the LA system fails to meet the target error rate. However, it is shown that with suitable modification to the SNR switching threshold for changing the adaptive modulation can over the problem. It is also noted that ICI severely reduces the spectral efficiency of OFDM systems even when LA is used.

It is well known that Orthogonal Frequency Division Multiplexing (OFDM) systems suffer from intercarrier interference (ICI) in mobile environment due to loss of orthogonality among subcarriers caused by Doppler shifts. There exist many ICI mitigation techniques in the literature to improve the performance of OFDM systems. However, most of the existing ICI mitigation techniques assume the OFDM transmission bandwidth is narrow enough that the frequency offsets on all subcarriers are identical. In a wideband OFDM transmission or a non-contiguous OFDM spanning over large bandwidth, the Doppler shifts on different subcarriers are different, especially in high speed aerial vehicle communication systems. In this paper, we analyze the wideband OFDM system in high mobility environment where the frequency offsets vary from subcarrier to subcarrier. We then propose a novel ICI cancellation scheme to eliminate the ICI effect and offer the wideband OFDM system significantly improved BER performance. Simulation results in AWGN channel and multipath fading channel confirm the effectiveness of the proposed scheme in the presence of frequency offset and time variations in the channel, offering the best BER performance available which matches the BER performance of wideband OFDM system without ICI. To our knowledge, this paper is the first to address the ICI problem of varying frequency offsets across subcarriers in wideband OFDM system.

One of the widely adopted multicarrier modulation technique in wireless communication system is Orthogonal Frequency Division Multiplexing (OFDM), because of its enhanced robustness against channel dispersion. However, Inter Carrier Interference (ICI) caused by time varying wireless channel and Peak to Average Power Ratio (PAPR) due to the presence of a large number of subcarriers with varying amplitude are the critical issues faced in OFDM. One of the efficient ICI self-cancellation scheme is correlative precoder but it introduces increased PAPR. To overcome this shortcoming a new form of circulantly precoded OFDM system is implemented to enhance ICI reduce and also employed with Selective Mapping (SLM) which is effective in PAPR reduction and uncomplicated. BER performance of correlative and circulantly precoded OFDM system is evaluated for LMMSE and OLMMSE equalizers. Simulation results show that circulantly precoded OFDM with SLM can achieve jointly better ICI self cancellation and PAPR reduction than correlative precoding technique over time varying channel.

Orthogonal Frequency Division Multiplexing (OFDM) has been considered as a strong candidate for next generation wireless communication systems. Compared to traditional OFDM, Single Carrier OFDM (SC-OFDM) has demonstrated excellent bit error rate (BER) performance, as well as low peak to average power ratio (PAPR). Similar to other multi-carrier transmission technologies, SC-OFDM suffers significant performance degradation resulting from intercarrier interference (ICI) in high mobility environments. Existing techniques for OFDM can be directly adopted in SC-OFDM to improve performance, however, this improved performance comes at costs such as decreased throughput. In this paper, we analyze the effect of ICI on an SC-OFDM system and propose a novel modulation scheme. The proposed Magnitude-Keyed Modulation (MKM) modulation provides SC-OFDM system immunity to ICI and with an easy implementation it significantly outperforms OFDM, SC-OFDM and MC-CDMA systems with Phase Shift Keying (PSK) modulation and Quadrature Amplitude Modulation (QAM) in severe ICI environment. Analysis also illustrates the proposed SC-OFDM system with MKM modulation maintains low PAPR compared to traditional OFDM and SC-OFDM systems with PSK and QAM modulations. Simulation results for different modulation schemes in various ICI environments confirm the effectiveness of the proposed system.

Multi-carrier code division multiple access (MC-CDMA)has been considered as a strong candidate for next generation wireless communication system due to its excellent performance in multi-path fading channel and simple receiver structure. However, like all the multi-carrier transmission technologies such as OFDM, the inter-carrier interference (ICI) produced by the frequency offset between the transmitter and receiver local oscillators or by Doppler shift due to high mobility causes significant BER (bit error rate) performance degradation in MC-CDMA system. Many ICI cancellation methods such as windowing and frequency domain coding have been proposed in the literature to cancel ICI and improve the BER performance for multi-carrier transmission technologies. However, existing ICI cancellation methods do not cancel ICI entirely and the BER performance after ICI cancellation is still much worse than the BER performance of original system without ICI. Moreover, popular ICI cancellation methods like ICI self-cancellation reduce ICI at the price of lowering the transmission rate and reducing the bandwidth efficiency. Other frequency-domain coding methods do not reduce the data rate, but produce less reduction in ICI as well. In this paper, we propose a novel ICI cancellation scheme that can eliminate the ICI entirely and offer a MC-CDMA mobile system with the same BER performance of a MC-CDMA system without ICI. More importantly, the proposed ICI cancellation scheme (namely Total ICI Cancellation) does not lower the transmission rate or reduce the bandwidth efficiency. Specifically, by exploiting frequency offset quantization, the proposed scheme takes advantage of the orthogonality of the ICI matrix and offers perfect ICI cancellation and significant BER improvement at linearly growing cost. Simulation results in AWGN channel and multi-path fading channel confirm the excellent performance of the proposed Total ICI Cancellation scheme in the presence of frequency offset or time variations in the channel, outperforming existing ICI cancellation methods.

In high‐mobility scenarios, the time variation of mobile radio channels leads to a loss of orthogonality among subcarriers in orthogonal frequency division multiplexing (OFDM) systems, resulting in intercarrier interference (ICI) and performance deterioration. Conventional channel estimation schemes are usually based on pilot tones, which are distributed in each OFDM symbol to estimate the channel variation. Hence, the channel estimator itself suffers from ICI. In this study, a new estimation scheme, which does not suffer from ICI, is proposed to estimate the channel variation within OFDM symbols. The main idea is to zero‐pad (ZP) the OFDM symbol in the time domain. Then, in the middle of the ZP interval, an impulse signal is inserted as a pilot sample, which is used to estimate the channel at the pilot signal in the OFDM symbol. Finally, a linear model is used to estimate the channel variation over an OFDM symbol. Additionally, we derive the mean squared error (MSE) of the proposed estimation technique under the constraint that the channel varies linearly within OFDM symbols. Simulation results show that our scheme can achieve a substantial improvement in the bit error rate (BER) performance of OFDM, in spite of the OFDM symbol length being increased. Moreover, in many cases, the new scheme can achieve the same BER performance as the perfect knowledge of channel state information (CSI). Theoretical analysis and numerical simulations show that our scheme achieves excellent performance with much lower computational complexity.

In orthogonal frequency-division multiplexing (OFDM) systems, it is generally assumed that the channel response is static in an OFDM symbol period. However, the assumption does not hold in high-mobility environments. As a result, intercarrier interference (ICI) is induced and the system performance is degraded. A simple remedy for this problem is the application of the zero-forcing (ZF) and minimum mean square error (MMSE) equalizers. Unfortunately, the direct ZF method requires the inversion of an NxN ICI matrix, where N is the number of subcarriers. When N is large, the computational complexity can become prohibitively high. As for the direct MMSE method, in addition to an NxN matrix inverse, it requires an extra NxN matrix multiplication, making the required computational complexity higher compared to the direct ZF method. In this dissertation, we first propose a low-complexity ZF method to solve the problem in single-input-single-output (SISO) OFDM systems. The main idea is to explore the special structure inherent in the ICI matrix and to apply Newton's iteration for matrix inversion. With our formulation, fast Fourier transforms (FFTs) can be used in the iterative process, reducing the complexity from O(N^3) to O(Nlog_2 N) . Also, the required number of the iteration is typically one or two. We also analyze the convergence behavior of the proposed method and derive the theoretical output signal-to-interference-noise-ratio (SINR). For the MMSE method, we first reformulate the MMSE solution in a way that the extra matrix multiplication can be avoided. Similar to the ZF method, we then exploit the structure of the ICI matrix and apply Newton's iteration to reduce the complexity of the matrix inversion. For a multiple-input-multiple-output (MIMO) OFDM system, the required complexity of the ZF and MMSE methods becomes more intractable. We then manage to extend the proposed ZF and MMSE methods for SISO-OFDM systems to MIMO-OFDM systems. It turns out that the computational complexity can be reduced even more significantly. Simulation results show that the performance of the proposed methods is almost as good as that of the direct ZF and MMSE methods, while the required computational complexity is reduced dramatically. Finally, we explore the application of the proposed methods in mobility-induced ICI mitigation for OFDM multiple access (OFDMA) systems, and in carrier frequency offset (CFO) induced ICI mitigation for OFDMA uplink systems. As that in OFDM systems, the proposed methods can reduce the required computational complexity, effectively.

Orthogonal frequency division multiplexing (OFDM) is sensitive to carrier frequency offset and channel estimation error, which destroy the subcarrier orthogonality and give rise to intercarrier interference (ICI). This paper addresses the performance degradation due to imperfect parameters in an orthogonal frequency and code division multiplexing (OFDM-CDM) system with multiple transmit antennas. For the performance evaluation, the average bit error rate (BER) impairment due to imperfect channel information and frequency offset is investigated taking into account the effect of time-varying channels. Derived results show that a space-time block coded (STBC) OFDM-CDM system experiences a severe BER degradation regardless of the number of users.

OFDM(Orthogonal Frequency Division Multiplexing) 시스템에서 직교 불균형 문제는 송수신기의 front-end에서 발생하며, 성상도에 영향을 주게 되어 BER(Bit Error Rate)을 증가시킨다. 또한, 위상 잡음은 송수신시 국부 발진기에서 발생되는 잡음으로 각 부반송파의 직교성을 깨뜨림으로써 시스템 성능을 크게 저하시킨다. 기존 방식인 PNS(Phase Noise Suppression) 알고리즘은 이러한 위상 잡음을 효과적으로 제거하는 방법이지만 직교 불균형 이동시에 적용되면 오히려 성능이 감소된다. 본 논문에서는 OFDM 시스템의 수신기에서 하향 변환 시 발생하는 직교 불균형과 위상 잡음의 영향을 분석하고, 수신기 FFT(Fast Fourier Transform) 후단에서 파일럿 심볼을 사용하여 CPE를 먼저 제거하고 직교 불균형과 위상 잡음의 성분을 검출하여 등화기의 판정 기준으로 사용하여 보상하는 방법을 제시하였다. 또, 다른 기존 방식들은 FFT 후단에서 추정하고 피드백 시키거나 프리엠블과 같은 시퀀스를 사용하는 방식이지만, 본 논문에서는 FFT 후단에서 MMSE 등화기만을 사용하여 제거하므로 기존의 방법보다 복잡도가 줄어든다. 기존의 위상 잡음 제거 방식에 ICI(Inter Carrier Interference) 제거 기능을 추가하고 직교 불균형 성분을 추출하여 MMSE(Minimum Mean Square Error) 과정 중에 적응 forgetting factor를 적용하면 성능 개선과 직교 불균형 성분의 영향이 줄어들며 성능이 개선됨을 보인다. In the OFDM system, IQ imbalance problem happens at the RF front-end of transceiver, which degrades the BER(bit error rate) performance because it affects the constellation in the received signal. Also, phase noise is generated in the local oscillator of transceivers and it destroys the orthogonality between the subcarriers. Conventional PNS algorithm is effective for phase noise suppression, but it is not useful anymore when there are jointly IQ(In-phase and Quadrature) imbalance and phase noise. Therefore, in this paper, we analyze the effect of IQ imbalance and phase noise generated in the down-conversion of the receiver. Then, we estimate and compensate the IQ imbalance and phase noise at the same time. Compared with the conventional method that IQ imbalance after IFFT is estimated and compensated in front of FFT via the feedback, this proposed method extracts and compensates effect of IQ imbalance after FFT stage. In case IQ imbalance and phase noise exist at the same time, we can decrease complexity because it is needless to use elimination of IQ imbalance in time domain and training sequences and preambles. Also, this method shows that it reduces the ICI and CPE component using adaptive forgetting factor of MMSE after FFT.

Orthogonal frequency-division multiplexing (OFDM) is robust against frequency selective fading because of the increase of the symbol duration. However, the time-varying nature of the channel causes inter-carrier interference (ICI) which destroys the orthogonal of sub-carriers and degrades the system performance severely. To alleviate the detrimental effect of ICI, there is a need for ICI mitigation within one OFDM symbol. We propose an iterative Inter-Carrier Interference (ICI) estimation and cancellation technique for OFDM systems based on regularized constrained total least squares. In the proposed scheme, ICI aren’t treated as additional additive white Gaussian noise (AWGN). The effect of Inter-Carrier Interference (ICI) and inter-symbol interference (ISI) on channel estimation is regarded as perturbation of channel. We propose a novel algorithm for channel estimation o based on regularized constrained total least squares. Computer simulations show that significant improvement can be obtained by the proposed scheme in fast fading channels.

At the physical layer, Underwater Acoustic Communication (UWAC) channels present many challenges for efficient communications, which feature both long delay spreads and serious Doppler effects. The waves and motions at the surface of the sea are the main causes for these effects. The complexity at the receiver increases along with the symbol rate, thereby increasing the burden of the channel. Orthogonal Frequency Division Multiplexing (OFDM) which is a form of multicarrier modulation has been effectively used for UWAC recently because of its ability to handle long dispersive channels reducing the complexity of the channel equalization. However, Inter Carrier Interference (ICI) is introduced by the Doppler effects that destroy the orthogonality of the subcarriers. To overcome the effects of OFDM, an emerging technique known as Filter Bank Multicarrier Communication system (FBMC) is used in UWAC. The high capacity of the transmission bandwidth in FBMC systems can be accomplished by applying Offset Quadrature Amplitude Modulation (OQAM). Because of cyclic prefix deletion, FBMC/OQAM can arrange higher spectral efficiency when compared to OFDM. This paper worked on both OFDM and FBMC/OQAM by considering the losses and noise effects in UWAC environment and explained their performances with respect to Bit Error Rate (BER).

The Performance of the Orthogonal Frequency Division Multiplexing (OFDM) is degrade due to Doppler frequency or frequency drift between transmitter and receiver oscillator which causes frequency offset and leads to Inter-Carrier-Interference (ICI). In this paper, we present a new scheme of self-cancellation for ICI mitigation in OFDM system and compared with the existing methods of self-cancellation in terms of carrier-to-interference ratio (CIR) and bit-error rate (BER). The simulation result shows that the proposed scheme outperforms the existing methods.

Frequency offset (FO) introduces severe inter-symbol and inter-carrier interferences which will degrade the performances of orthogonal frequency division multiplexing (OFDM) system more adversely. To reduce the effect of CFO, an estimation algorithm has been proposed in this work which uses a turbo coding at the precoding section and Space time block coding techniques (alamouti code) for transmitting data. The turbo codes solve dilemma of structure and randomness through concatenation and interleaving and it will reduce bit error rate (BER) by error detection and error correction process. A fading channel has been modeled for evaluating the performance of the algorithm. Maximum likelihood (ML) estimation is used for the estimation that does take conditional probability into account. The offset estimation in OFDM is done by best linear unbiased estimator (BLUE) principle yet it has disadvantages that it is already sub optimal in nature and it is applicable only for linear observation models. So, ML technique is an alternative estimation criterion that can use as starting point of most of the statistical estimation and for nonlinear observations. The proposed system has improved BER while other methods supports BER of 25-30 dB and the proposed system shows improvement in spectral efficiency as well as the Power spectral density.

Performance of both orthogonal frequency division multiplexing (OFDM) and single carrier (SC) systems suffers from a carrier frequency offset (CFO) due to Doppler effect and the carrier instability between the transmitter and the receiver. We investigate the bit error rate (BER) performance degradation of OFDM and SC systems due to the frequency offset in an additive white Gaussian noise (AWGN) channel as well as multipath Rayleigh fading channels. We consider three effects to the BER degradation, i.e., phase shift, useful power decrease and intercarrier interference (ICI). We also derive the approximate expressions of BER under binary phase shift keying (BPSK) and quaternary phase shift keying (QPSK) for both OFDM and SC systems in the presence of CFO. In general, SC is more robust to CFO in the AVVGN channel than OFDM in terms of BER, while both of them suffer similarly from CFO in the multipath Rayleigh fading channels assuming the the same CFO.