Efficient ICI Self‐Cancellation Scheme for OFDM Systems

In this paper, we present a new inter‐carrier interference (ICI) self‐cancellation scheme — namely, ISC scheme — for orthogonal frequency‐division multiplexing systems to reduce the ICI generated from phase noise (PHN) and residual frequency offset (RFO). The proposed scheme comprises a new ICI cancellation mapping (ICM) scheme at the transmitter and an appropriate method of combining the received signals at the receiver. In the proposed scheme, the transmitted signal is transformed into a real signal through the new ICM using the real property of the transmitted signal; the fast‐varying PHN and RFO are estimated and compensated. Therefore, the ICI caused by fast‐varying PHN and RFO is significantly suppressed. We also derive the carrier‐to‐interference power ratio (CIR) of the proposed scheme by using the symmetric conjugate property of the ICI weighting function and then compare it with those of conventional schemes. Through simulation results, we show that the proposed ISC scheme has a higher CIR and better bit error rate performance than the conventional schemes.     

MIMO-OFDM系统ICI自消除改进算法

现有的ICI自消除算法在提高系统载干比(CIR)的同时会降低系统的抗噪声性能。针对这一问题,提出一种在发送端对数据进行复数加权共轭运算的ICI自消除改进算法,这种算法造成的相位旋转最小,可以有效减少接收信号因相位旋转而导致的相位误差,从而提高系统的抗噪声性能。仿真结果表明,所提算法具有与已有算法相近的ICI抑制能力,但比现有算法具有更好的抗噪声性能。     

A new ICI mitigation method with generalized data‐allocation for OFDM systems

This paper describes an ICI mitigation method based on the generalized data‐allocation of (1, ?β) for orthogonal frequency division multiplexing systems. To improve the performance of the ICI mitigation for the higher‐frequency offset, we propose an efficient search algorithm to generate the sub‐optimal parameter β for maximizing the carrier‐to‐interference ratio (CIR). The CIR and bit error rate performances of the proposed method were derived in this paper. The performances with different carrier frequency offset scenarios were evaluated by computer simulations. According to the simulation results, the performance of the proposed ICI mitigation scheme is better than that of the conventional ICI self‐cancellation scheme and is nearly the same as that of the ICI self‐cancellation scheme for the optimal parameter β. Additionally, the proposed ICI mitigation scheme has a dramatically reduced hardware complexity in comparison with the ICI self‐cancellation scheme for the optimal parameter β. Copyright © 2009 John Wiley & Sons, Ltd.     

OFDM系统中ICI自消除算法研究

OFDM系统对频率偏移极为敏感,频率偏移将导致子载波之间失去正交性,于是产生子载波干扰（ICI）,从而降低系统性能。通过对有效抑制ICI的方法—ICI自消除算法进行研究,分析了ICI自消除算法对OFDM系统的影响。在软件无线电（GNU Radio）平台上搭建该系统,并在实际环境中运行。研究结果表明：与传统OFDM系统相比,ICI自消除算法使OFDM系统的误码率得到改善。     

正交频分复用信号峰均比抑制技术研究

正交频分复用(OFDM)系统存在峰值平均功率比(PAPR)较高的缺点.通过对PAPR相关特性的深入讨论分析,针对峰值抵消波峰因子降低(PC?CFR)算法引入的非线性失真较大的问题,提出了一种可降低信号非线性失真的峰均比抑制算法,即PC?CFR算法与部分传输序列(PTS)算法联合改进的PAPR抑制方案.仿真结果表明,该方...     

OFDM系统中相位噪声的影响及ICI自消除方案

相位噪声也能引起信道间干扰( ICI) ,本文系统的分析了采用ICI自消除方案前后相位噪声对OFDM系统的影响,得出了载波干扰比和信噪比的近似计算公式,在AWGN信道上仿真了系统的误码性能。从仿真结果看,ICI自消除的方案能够显著提高系统的载波干扰比和误码率     

Intercarrier interference self-cancellation scheme for OFDM mobilecommunication systems

For orthogonal frequency-division multiplexing (OFDM) communication systems, the frequency offsets in mobile radio channels distort the orthogonality between subcarriers resulting in intercarrier interference (ICI). This paper studies an efficient ICI cancellation method termed ICI self-cancellation scheme. The scheme works in two very simple steps. At the transmitter side, one data symbol is modulated onto a group of adjacent subcarriers with a group of weighting coefficients. The weighting coefficients are designed so that the ICI caused by the channel frequency errors can be minimized. At the receiver side, by linearly combining the received signals on these subcarriers with proposed coefficients, the residual ICI contained in the received signals can then be further reduced. The carrier-to-interference power ratio (CIR) can be increased by 15 and 30 dB when the group size is two or three, respectively, for a channel with a constant frequency offset. Although the redundant modulation causes a reduction in bandwidth efficiency, it can be compensated, for example, by using larger signal alphabet sizes. Simulations show that OFDM systems using the proposed ICI self-cancellation scheme perform much better than standard systems while having the same bandwidth efficiency in multipath mobile radio channels with large Doppler frequencies     

Pilot-free dynamic phase and amplitude estimations for wireless ICI self-cancellation coded OFDM systems

OFDM has the advantage over the conventional single-carrier modulation schemes in the presence of frequency-selective fadings. Nevertheless, intercarrier-interference (ICI) due to Doppler frequency drift, phase offset, local oscillator frequency drift, and sampling clock offset will be a severe problem in the wireless OFDM systems. Previous ICI self-cancellation coding schemes can greatly reduce the ICI, but they are very sensitive to the phase ambiguity, which is due to the composite effect of the phase offset, the multipath fading and the local oscillator frequency drift. In this paper, a novel receiver which combines the current ICI self-cancellation coding techniques with a new pilot-free joint phase/amplitude estimation and symbol detection scheme is proposed. Based on the energy modulation or the irregular symbol constellation, our new technique does not have any requirement of pilot symbols and it can operate on all kinds of phase error ranges. The proposed scheme is promising in comparison with other existing methods at different noise levels through OFDM simulations.     

Joint phase/amplitude estimation and symbol detection for wireless ICI self-cancellation coded OFDM systems

OFDM has been applied in the current wireless local-area networks and digital video broadcasting systems since it has the advantage over the conventional single-carrier modulation schemes when the frequency-selective fadings are present. Nevertheless, intercarrier-interference (ICI) due to Doppler frequency drift, phase offset, local oscillator frequency drift, and multipath fading will be a severe problem in OFDM systems. Previous ICI self-cancellation coding schemes can greatly reduce the ICI, but they are very sensitive to the phase ambiguity, which is due to the composite effect of the phase offset, the multipath fading and the local frequency drift. In this paper, the phase ambiguity and amplitude ambiguity problems in ICI self-cancellation coded OFDM receivers will be formulated and discussed. Then, a novel receiver which combines the current ICI self-cancellation coding techniques with a new expectation-maximization-based joint phase/amplitude estimation and symbol detection scheme is proposed. The outstanding performance of this proposed scheme is shown and compared with other existing methods at different noise levels through OFDM simulations.     

Optimization of Peak to Average Power Reduction in OFDM

Orthogonal frequency division multiplexing (OFDM) a multicarrier system [1, 2] provides base for all advanced wireless communication system. The performance of OFDM is degraded by peak-to-averagepower ratio (PAPR). High PAPR requires high power amplifiers (HPAs). The nonlinearity of the HPA exhibits amplitude and section distortions, that cause loss of orthogonality among the subcarriers, and hence, intercarrier interference (ICI) is introduced inside the transmitted signal. Not only that, high PAPR put together lands up in in-band distortion and out-of-band radiation. Rather than using HPA’s, the only way to improve performance of OFDM system is to reduce PAPR. The PAPR reduction of OFDM system gives fair reduction in PAPR under partial transmits sequence (PTS) and DCT-SLM techniques. Here in this paper we proposed a combination of PTS and DCT-SLM and an algorithm to cut back the PAPR. This hybrid combined technique reduces PAPR effectively and minimizes the complexity of PTS technique.     

General ICI self-cancellation scheme for OFDM systems

One of the challenges in designing orthogonal frequency-division multiplexing (OFDM) systems is their inherent sensitivity to any frequency shift in the signal. A frequency offset between the local oscillators at the transmitter and receiver causes a single frequency shift in the signal, while a time-varying channel can cause a spread of frequency shifts known as the Doppler spread. Frequency shifts ruin the orthogonality of OFDM subcarriers and cause intercarrier interference (ICI); therefore, quickly diminishing the performance of the system. ICI self-cancellation schemes have been proposed to reduce the sensitivity of OFDM systems to frequency shifts. These schemes use signal processing and frequency domain coding to reduce the amount of ICI generated as a result of frequency shifts, with little additional computational complexity. These methods can be used as an alternative to the fine frequency-offset estimation methods to battle oscillator frequency offset or simply be used as an ICI mitigation technique when the system is operating over time-varying channels. We propose a general ICI self-cancellation scheme that can be implemented through windowing at the transmitter and receiver. We show that the previously proposed self-cancellation schemes are equivalent to special cases of this method. Through theoretical analysis of the signal-to-interference ratio and bit-error rate and the use of Monte Carlo simulations, we demonstrate that the proposed system considerably outperforms the existing systems in the presence of frequency offset or time variations in the channel. We consider both coherent and noncoherent systems.     

Decision Feedback Frequency Offset Estimation and Tracking for General ICI Self-Cancellation Based OFDM Systems

Inter-carrier interference (ICI) self-cancellation schemes were often employed in many OFDM systems as a simple and effective approach to suppress ICI caused by carrier frequency errors. The same procedure, however, can not perform very well at high frequency offsets. We propose a simple decision feedback scheme based on the general ICI self-cancellation scheme to perform estimation and tracking of the carrier frequency offsets. A system with the scheme does not consume additional bandwidth since it used the same data symbols employed for ICI cancellation for the estimation. After an initial estimation is completed, the scheme switches to the tracking mode to carry out the estimation of deviations in the frequency offsets. Finally this fine-tuned estimate is applied to the ICI self-cancellation scheme concurrently for frequency offset correction and hence improved the system performance greatly. Simulation results showed that our scheme allowed up to 9% of random variations in the frequency offset. The effectiveness of our scheme is further verified by calculating the bit error rate performance of various OFDM receivers.     

SFBC MIMO-OFDM peak-to-average power ratio reduction by polyphase interleaving and inversion

Many PAPR reduction schemes have been proposed for OFDM systems. Among these, the signal scrambling methods such as the partial transmit sequences (PTS) (S. H. Muller, et al., 1997) and selective mapping (SLM) (R. W. Bauml, et al., 1996) are attractive as they obtain better PAPR property by modifying OFDM signals without distortion. These schemes can also be applied to a SFBC MIMO-OFDM system, which is advantageous for dispersive channels, in a straightforward way by performing signal scrambling on data sequence before it is distributed to the transmit antennas according to employed encoding scheme. Note however that in the case of PTS PAPR reduction in the time domain is not possible, which leads to prohibitively large complexity of such scheme. In this letter, we introduce more effective approach, the polyphase interleaving and inversion (PII) PAPR scheme and its reduced complexity version (RC-PII), which is designed to suppress peaks in SFBC-OFDM, transmit diversity.     

Novel Low-Complexity Partial Transmit Sequences Scheme for PAPR Reduction in OFDM Systems Using Adaptive Differential Evolution Algorithm

A low-complexity partial transmit sequence (PTS) technique for reducing the peak-to-average power ratio (PAPR) of an orthogonal frequency division multiplexing signal is presented. However, the conventional PTS scheme requires an exhaustive searching over all combinations of allowed phase factors. Consequently, the computational complexity increases exponentially with the number of the subblocks. This paper presents a novel approach to the PAPR problem to reduce computational complexity based on the relationship between phase weighing factors and transmitted bit vectors. In this paper, we aim to obtain the desirable PAPR reduction with the low computational complexity. Since the process of searching the optimal phase factors can be categorized as combinatorial optimization with some variables and constraints, we propose a novel scheme, which is based on a stochastic optimization technique called modified differential evolution, to search the optimal combination of phase factors with low complexity. To validate the analytical results, extensive simulations have been conducted, showing that the proposed schemes can achieve significant reduction in computational complexity while keeping good PAPR reduction.     

Conjugate Interleaved Partitioning PTS Scheme for PAPR Reduction of OFDM Signals

The interleaved partitioning partial transmit sequence (IP-PTS) scheme is an attractive technique for peak-to-average power ratio (PAPR) reduction in orthogonal frequency division multiplexing (OFDM) systems. But the PAPR performance of IP-PTS is inferior to that of the adjacent partitioning PTS (AP-PTS) scheme because the candidates generated in IP-PTS are not fully independent. This paper analyzes the independence of candidates in IP-PTS in detail and finds the effective phase factor vectors. In order to improve the PAPR performance of IP-PTS, a conjugate IP-PTS (C-IP-PTS) scheme is proposed. By performing the conjugate operations on some sub-blocks, the number of candidates is increased. Because of the conjugate property of the discrete Fourier transform (DFT), the additional inverse DFT can be avoided. By optimizing the conjugate sequence, the complexity can be further lowered. Simulation results show that C-IP-PTS can obtain better PAPR performance compared with AP-PTS; moreover, the computational complexity of C-IP-PTS is not high.     

A Phase Adjustment Based Partial Transmit Sequence Scheme for PAPR Reduction

The peak-to-average power ratio (PAPR) of an orthogonal frequency division multiplexing signal is normally very large and is crucial for the analog amplifier in the transmitter. It is therefore necessary to reduce the PAPR before the signal is transmitted. In this paper, a PAPR reduction scheme is presented based on the partial transmit sequence (PTS) method. By allowing each subblock of data to be rotated by an arbitrary angle, a simple yet efficient phase search algorithm is proposed. With a prespecified number of iterations, the proposed method gives the best phase vector for a combination of data blocks to achieve the smallest PAPR value. Our simulation results show that the proposed scheme provides significant computational savings while yielding a performance similar to that of the conventional PTS method.     

A new PTS OFDM scheme with low complexity for PAPR reduction

In this paper, we introduce a new partial transmit sequence (PTS) orthogonal frequency division multiplexing (OFDM) scheme with low computational complexity. In the proposed scheme, 2/sup n/-point inverse fast Fourier transform (IFFT) is divided into two parts. An input symbol sequence is partially transformed using the first l stages of IFFT into an intermediate signal sequence and the intermediate signal sequence is partitioned into a number of intermediate signal subsequences. Then, the remaining n-l stages of IFFT are applied to each of the intermediate signal subsequences and the resulting signal subsequences are summed after being multiplied by each member of a set of W rotating vectors to yield W distinct OFDM signal sequences. The one with the lowest peak to average power ratio (PAPR) among these OFDM signal sequences is selected for transmission. The new PTS OFDM scheme reduces the computational complexity while it shows almost the same performance of PAPR reduction as that of the conventional PTS OFDM scheme.     

A Novel Algorithm of Inter-Subchannel Interference Self-Cancellation for OFDM Systems

Time-varying multipath channels distort the orthogonality between subchannels in orthogonal frequency-division multiplexing (OFDM) transmission. The loss of sub-channel orthogonality causes inter-subchannel interference (ICI), which limits the achievable bit-error probability (BEP) at high signal-to-noise ratio (SNR). In this paper, we propose a simple but very effective ICI self-cancellation algorithm. A pre-processor and a post-processor arc inserted in the transmitter and receiver, respectively. The pre-processor adds diversity to the frequency- domain symbols by time-domain periodical extension, while the post-processor uses this diversity to make most of the ICI self- cancelled. Our algorithm can provide a trade-off between ICI reduction and system throughput by adjusting the length of periodical extension. For the full-extension scheme, we show that the ICI can be completely removed if the channel variation of each path is linear with time within one extended symbol interval. We further propose an equivalent implementation of the post-processor such that the complexity of the receiver is the same as the standard OFDM receiver. This implementation also enables our algorithm to be readily combined with other OFDM algorithms of channel estimation, synchronization, coding, and so on that do not consider the ICI effect. Applying the proposed algorithm of ICI reduction makes these algorithms more applicable in fast-fading channels. To provide more insight on the ICI cancellation, we derive the equivalent channel effect of our algorithm. We also analyze the variance of ICI and observe the density function of the residual ICI in our algorithm, based on which we show a procedure to derive a BEP upper bound. The proposed algorithm is further validated by simulation and the comparison with another ICI self-cancellation algorithm.     

A hybrid technique for the PAPR reduction of NOMA waveform

Non-orthogonal multiple access (NOMA) is a great contender for future cellular modulation due to its desirable properties like massive connectivity, high data rate transmission, and high spectral efficiency. However, its peak-to-average power ratio (PAPR) is significant, which becomes a significant disadvantage for the efficient operability of the NOMA waveform compared to current techniques. Several PAPR reduction algorithms like selective mapping (SLM), partial transmission sequence (PTS), and companding techniques have been proposed to lower the PAPR of multicarrier waveforms (MCWs). PTS reduces the PAPR but has high complexity. On the other hand, SLM has a less complex framework, but its PAPR performance is not as efficient as PTS. Companding methods reduce the PAPR by compressing the signals at the transmitter, which unfortunately reduces the dynamic range of the signal. In this work, we propose a hybrid algorithm (SLM + PTS) with a companding method for the first time for the NOMA waveform, which efficiently reduces the PAPR with low computational complexity. Furthermore, we compare the performances of a host of candidate algorithms like SLM, PTS, hybrid (SLM + PTS), hybrid + A law (SLM–PTS–A law), and hybrid + Mu law (SLM–PTS–Mu law). The results of the experiments show that the hybrid + Mu law did a better job than the existing PAPR reduction algorithms.     

A Hybrid Clustering Approach Based Q-Leach in TDMA to Optimize QOS-Parameters

An Orthogonal frequency part multiplexing suffers from a considerable challenge due to a high peak to average power ratio (PAPR). Hence, an effective method such as partial transmits sequence (PTS) can avert this defiance by limit the design of PAPR. Therefore, an improving PAPR reduction performance via a novel approach is proposed by detaching each subblock into two parts furthermore exchanges the first sample with the final selection in each portion of the subblock to generate a new partitioning scheme. Several typical traditional segmentation schemes are used to analyze and apply the presented algorithm, such as adjacent, interleaving, and pseudo-random schemes. Besides, two scenarios are adopted based on simulation software in which the number of subcarriers is set to 128 and 256. Based on the results, a superior PAPR reduction performance is achieved based on the improved segmentation schemes regarding traditional strategies in both systems. Moreover, the enhanced adjusted PTS scheme poses a low computational complexity compared with that of the conventional schemes.