Constellation expanded active interference cancellation technique for suppression of out-of-band radiation in OFDM based cognitive radios

Cognitive radio is a novel technology to address the problem of spectrum under-utilization, by allowing the secondary users to dynamically use the spectrum of the primary users. OFDM is a potential candidate for the physical layer of cognitive radio, due to its inherent features of spectrum shaping, spectrum analysis and robustness to multipath. OFDM based cognitive radio, however suffers from out-of-band radiation (OOBR). In cognitive radio, suppression of OOBR is much more important as the interference to the primary users needs to be kept to the minimum possible level. In the literature, time and frequency characteristics of the transmitted signal have been exploited to minimize the OOBR. In this paper, we investigate the possibility of combining merits of the constellation expansion (CE) technique and the active interference cancellation (AIC) technique for suppression of OOBR. We show through simulations that the joint technique based on CE and AIC achieves much better OOBR reduction, with a slight degradation in error performance.     

基于脉冲成形技术降低OFDM峰均比

正交频分复用（OFDM）利用相互正交的多个子载波来传输信息,具有较高的频谱利用率和良好的抗多径干扰能力,适用于高速数据传输,成为第四代移动通信的核心技术之一。但OFDM系统中一个最主要的缺点在于OFDM信号的峰值平均功率比（PAPR）很大,这容易导致系统线性功放的效率降低。研究了将脉冲成形应用到OFDM系统来降低PAPR的技术,以及脉冲波形的选取。仿真结果表明该方法能有效降低OFDM系统的PAPR,而且不影响系统的BER性能。     

Outage Analysis of OFDM‐Based Cognitive AF Relay Network in the Presence of Narrowband Interference

Orthogonal frequency‐division multiplexing (OFDM) is one of the most widely used technologies in current wireless communication systems and standards. Cognitive radio (CR) provides a robust solution to the problem of spectrum congestion as it offers opportunistic usage of frequency bands that are not occupied by primary users. Due to the underlying sensing, spectrum shaping, scaling, and interoperable capabilities of OFDM, it has been adapted as a best transmission technology for CR wireless systems. However, the performance of an OFDM‐based CR wireless system is affected by the existence of narrowband interference (NBI) from other users. Further, due to carrier frequency offset in NBI sources, NBI energy may spread over all subcarriers of an OFDM signal. In this paper, a fixed Amplify‐and‐Forward (AF) relay that operates at a frequency band that is different from that of direct mode is introduced to suppress the effect of NBI. Analytical expressions are derived for outage probability in direct, AF‐relay, and incremental relaying modes. The outage performance of the proposed AF relay–based CR network is proven to be better than that of direct mode.     

一种降低OFDM信号PAPR的新方法

正交频分复用(OFDM)技术的一个缺点是信号的高峰值平均功率比(PAPR)大大降低了系统中线性功放的效率。本文提出一种新的基于Nyquist脉冲成形的PAPR抑制方法。这种方法基于选择适当的Nyquist脉冲波形集合对OFDM的各个子载波进行脉冲成形。分析论证了该方法的PAPR上限值和所用的脉冲成形波形集合。仿真结果表明该方法能有效降低任意子载波数目的OFDM信号的PAPR值,而且与已有方法相比应用更为简单。因此脉冲成形技术不仅能对传输信号进行频谱成形来提高系统的频带利用率,而且还可以减小信号的PAPR值。     

Improved Nyquist-I Pulses to Enhance the Performance of OFDM-Based Systems

Wireless Personal Communications - Pulse shaping is used in orthogonal frequency division multiplexing (OFDM) based systems to reduce inter-carrier interference (ICI) power and peak-to-average...     

OFDM调制新方案及其脉冲成形滤波器的设计

本文提出一种新的正交频分复用(OFDM)调制方案,结合了两种OFDM系统的特点,既利用了脉冲成形技术的优势,又能采用保护间隔.分析了频率偏移存在时脉冲成形滤波器与载波间干扰的关系,并给出一个设计实例的仿真.数值分析和仿真结果表明,该设计能显著抑制载波间干扰.     

DFT扩展OFDM系统中降低信号峰均比的频域成型方法

OFDM(正交频分复用)技术的主要缺点之一是其发送信号具有较高的峰均比。DFT(离散傅里叶变换)扩展OFDM系统的发送方案可以有效降低信号的峰均比。在该系统中,频域成型方法不仅对系统有无码间干扰的特性和抗噪声性能有重要影响,而且对发送信号的峰均比也有重要影响。文中首先在加性高斯白噪声信道下推导了无码间干扰和接收机最大输出信噪比条件下发送频域成型函数及接收频域成型函数所要满足的条件,然后根据该条件设计了几种频域成型方法。仿真结果表明,频域成型方法能进一步有效降低发送信号的峰均比。     

Sidelobe suppression in OFDM-based spectrum sharing systems using adaptive symbol transition

In this letter, we introduce a new method for orthogonal frequency division multiplexing (OFDM) sidelobe suppression. An extension is added to OFDM symbols that is calculated using optimization methods to minimize adjacent channel interference (ACI) while keeping the extension power at an acceptable level. Using this technique, interference to adjacent signals is reduced significantly at the cost of a small decrease in the useful symbol energy. The proposed method can be used by cognitive radio (CR) systems to shape the spectrum of OFDM signals and to minimize interference to licensed users (LU), or to reduce the size of guard bands used in conventional OFDM systems.     

短波FH/OFDM通信系统抗跟踪式干扰性能分析

对于短波多载波跳频通信系统,跟踪式干扰是一种有效的干扰模式。分析了正交频分复用( OFDM)部分子信道干扰与符号误码率的关系,通过仿真获得了最佳跟踪干扰的部分时间参数,结合OFDM频谱结构与干扰频谱关系,推导了高斯信道条件下部分频带干扰和多音干扰时系统误码率,分析了短波FH/OFDM通信系统抗跟踪式干扰的误码率性能,仿真结果表明：跟踪干扰时间窗口对误码率的影响与部分频带干扰因子和信干比有关;在部分频带干扰与部分时间干扰之间,存在等效的干扰效果区域;多音干扰因子越大,系统所受影响越大,跟踪干扰时间窗口对误码率的影响与多音干扰因子和信干比有关,针对OFDM符号的多音干扰影响要远大于部分频带干扰的影响;跳频与OFDM技术的结合、提高载波跳速、减小跟踪式干扰对OFDM符号的影响,也是消除多音干扰的重要手段。     

Robust Cognitive‐Radio‐Based OFDM Architecture with Adaptive Traffic Allocation in Time and Frequency

Cognitive radio (CR) has been proposed as an effective technology for flexible use of the radio spectrum. The interference between primary users and CR users, however, becomes a critical problem when they are using adjacent frequency channels with different transmission power levels. In this paper, a robust CR orthogonal frequency division multiplexing (OFDM) architecture, which can effectively suppress interference to nearby primary users and overcome adjacent channel interference (ACI) to the CR user, is proposed. This new approach is characterized by adaptive data repetition for subcarriers under heavy ACI, and adaptive time spreading for subcarriers near the borders of the CR user's spectrum. The data repetition scheme provides extra power gain against the ACI coming from primary users. Time spreading guarantees an acceptable interference level to nearby primary users. By computer simulation, we demonstrate that, under a CR environment, the proposed CR OFDM architecture outperforms conventional OFDM systems in terms of throughput and BER performance.     

ICI Reduction in OFDM Systems Using Phase Modified Sinc Pulse

This paper is focused on the problem of reducing the intercarrier-interference (ICI) power in the transmission over Orthogonal Frequency Division Multiplexing Systems (OFDM) using pulse shaping methods. A new pulse was proposed here and it was investigated in terms of ICI interference. It appears to be suitable for transmission in OFDM systems with carrier frequency offset. The results obtained by calculations show that the performance improvements are significant for reducing average intercarrier-interference (ICI) power and increasing the ratio of average signal power to average ICI power (SIR).     

Deepen spectrum notch and reduce PAPR in OFDM systems via combining subcarriers precoding with tone reservation

In orthogonal frequency division multiplexing (OFDM) systems,spectrum notch can be generated to mitigate interference to other narrow-band services by turning off corresponding subcarriers.However,the inter-carrier interference may limit considerably the produced notch depth.Another problem in OFDM is the high peak-to-average power ratio (PAPR).In this paper,a technique combining subcarriers precoding with tone reservation is proposed to deepen spectrum notch and simultaneously reduce peak-to-average power ratio.Simulation results show that this presented joint algorithm can produce satisfied results to solve these problems at the expense of a moderate loss in bit error rate (BER) performance.     

基于OFDM的无人机雷达通信一体化设计方法

为了最大程度地利用资源和减小电磁干扰,无人机平台的雷达通信一体化设计有着尤为重要的意义。正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)信号具有较高的频谱利用率、较强的抗干扰性能、易于数字化处理等优点,在雷达通信一体化系统备受关注。然而,OFDM信号在雷达和通信系统中对多普勒频偏较为敏感,针对多普勒频偏引起的子载波间干扰(Inter-Carrier Interference,ICI)问题,本文设计出一种RS-OFDM雷达通信一体化信号,并提出一种ICI抑制的信号处理算法,其中基于多普勒估计和校正实现OFDM的无ICI处理。仿真结果表明:所提方法在目标成像性能上优于传统方法,且对多普勒频偏具有较强的鲁棒性,而且并无明显增加计算复杂度。      

基于并行干扰抵消的OFDM/OQAM系统中的信号检测方法

在多径衰落信道中,基于交错正交幅度调制的正交频分复用(OFDM with Offset QAM,OFDM/OQAM)系统使用迫零均衡器进行信号检测时,不能完全消除信道复数特性和滤波器实数正交特性引入的时域符号间干扰和频域子载波间干扰,及信道估计误差导致的误码率性能损失。该文利用对数据初始判决并重构相邻载波及符号间干扰的思想,通过分析采用迫零均衡信号检测时的残余干扰与信道估计误差干扰,提出了一种基于并行干扰抵消和迫零均衡器结合的OFDM/OQAM信号检测方法,并在IEEE 802.22技术标准的两种典型多径衰落信道中进行了计算机仿真与比较研究。仿真结果表明,与基于迫零均衡的检测方法相比,基于并行干扰抵消的迭代信号检测方法在误码率为1%时,可获得1 dB至2 dB的性能提升。     

OFDM信号伪逆谱分析及参数估计

提出一种基于伪逆谱的OFDM 信号盲检测方法,方法充分利用了OFDM信号伪逆谱谱线的隐含周期性,具有较强的抑制加性干扰和乘性干扰的能力,能够在较低的信噪比条件下快速准确地检测OFDM信号,可以精确估计出OFDM信号的子载波频率间隔、符号周期、信号带宽、子载波数目,具有运算量小、精度高、适于硬件实现等特点。仿真结果进一步说明了该检测方法的有效性。     

Bandwidth Overlap Factor for Feasible Coexistence of LTE-A and Point to Multipoint LMDS Systems

The necessity to share the radio spectrum becomes highly significant in the recent time due to emerging new wireless technologies and, the cost and difficulty in attaining new frequency spectrum licenses. However, spectrum sharing leads to substantial interference between the close systems especially in the same frequency band. In this paper, we present a spectrum sharing scheme to investigate coexistence feasibility results in interference from orthogonal frequency division multiplexing (OFDM)-based Long Term Evolution-Advanced (LTE-A) into point to multipoint (P-MP) local multipoint distribution service (LMDS). This method proposes a spectrum frequency mask for the OFDM-based LTE-A according to bandwidth overlapping method with the P-MP LMDS system. The relative power spectral density of the proposed spectrum emission mask due to bandwidth overlap has been mathematically derived in a more compact closed-form. It shows smoother and 1.7 dB less than that of traditional OFDM mask in the main spectrum bandwidth which in turn leads to reduce the interference effects to LMDS system. The coexistence coordination studies are made at the frequency of 3500 MHz, on the base of co-primary operation and co-located systems. The numerical results demonstrate that the difference in amount of the required carrier frequency for safe coexistence increases as LMDS sector size increases. In addition, OFDM system with small channel bandwidth is much more affected than the wider channel bandwidth in terms of the necessary minimum guard bandwidth for harmless coexistence when the LMDS cell size changes from 0.5 to 3 km².     

基于高阶累积量的OFDM信号调制识别技术

为解决非合作通信系统中OFDM（Orthogonal Frequency Division Multiplexing正交频分复用技术）信号的调制识别问题,提出一种基于四阶累积量的OFDM信号调制识别新方法。该方法利用OFDM信号时域包络具有渐近高斯性的特点,对已有基于累积量的识别方法进行改进,通过计算截获信号的复中频信号模值的四阶累积量,提取OFDM信号与单载波信号的分类特征量,对OFDM调制信号进行识别。该算法与已有的基于累积量的识别方法相比,计算量大大减小,且具有较好的识别效果。仿真实验表明SNR高于-2dB时,正确识别率大于99%,证明了该算法的有效性。     

OFDM系统中成形脉冲抑制ICI方法研究

OFDM系统面临的一个主要问题是对频率偏移的敏感性,频率偏移将导致子载波间干扰（ICI）的产生,从而大大降低系统性能。本文主要分析频偏存在时脉冲成形与子载波间干扰的关系,给出各种成形脉冲性能的评价标准,并提出在此标准下性能最佳的成形脉冲。     

A novel spectrum sharing scheme for cognitive radios

A novel approach, which combines spectrum adaptation and orthogonal frequency division multiplexing (OFDM), is proposed to share the licensed spectrum dynamically for cognitive radio systems. Given spectrum sensing and channel estimation information by the receiver, an improved model due to signal power thresholds is adopted to achieve spectrum adaptation for unlicensed users. In order to efficiently allocate the unlicensed signal power, a dynamic power allocation algorithm is also proposed. Simulation results indicate that the propositional scheme solves the partial interference problem of interference temperature model (ITM) and improves the spectrum utilization.     

Blind OFDM symbol synchronization in ISI channels

We present a new algorithm for blind symbol synchronization in orthogonal frequency division multiplexing (OFDM) systems. The new algorithm declares symbol synchronization when a certain autocorrelation matrix, constructed from the received signal, achieves minimum rank. Unlike previously proposed blind algorithms, the new rank method guarantees correct symbol synchronization, even in the presence of intersymbol interference. Also, it does not assume that the OFDM time samples are i.i.d. In particular, the rank method works even with OFDM systems that employ pulse shaping. The increased complexity of the algorithm would be acceptable for systems, such as fixed-receiver broadcast systems, that require guaranteed synchronization under all conditions.