A precoding scheme for DFT-based OFDM to suppress sidelobes

In spectrum pooling scenario, the spectral leakage of DFT-based OFDM signal can be divided into in-band-out-of-subband (IBOSB) radiation and out-of-band (OOB) radiation. A precoding scheme is proposed to suppress the IBOSB sidelobes. The precoding design is based on the generalized eigenvalue problem. Simulation results demonstrate that the proposed scheme can suppress the sidelobes significantly in contrast to the uncoded schemes. At the same time, the bit-error-rate (BER) performance of the precoded system over multipath fading channel is improved due to the frequency diversity.     

Dual symbol optimization‐based partial transmit sequence technique for PAPR reduction in WOLA‐OFDM waveform

Weighted overlap and add‐orthogonal frequency division multiplexing (WOLA‐OFDM) is a new waveform proposed recently for meeting the requirements of fifth generation (5G) telecommunication standards. In spite of being a serious 5G waveform candidate, WOLA‐OFDM is exposed to the problem of high peak to average power ratio (PAPR) similar to the other waveforms in which multicarrier transmission strategy is employed. Due to the overlapping nature of WOLA‐OFDM waveform, where the extension of the current symbol is overlapped with the extension of the previous symbol, it will not be efficient to apply conventional PTS (C‐PTS) directly to the WOLA‐OFDM waveform. Therefore, in this paper, we propose dual symbol optimization‐based partial transmit sequence (DSO‐PTS) technique for PAPR reduction in WOLA‐OFDM waveform. In our proposed technique, two adjacent symbols are jointly considered when searching for the optimal data block with minimum PAPR unlike the C‐PTS where the adjacent symbols are optimized individually. In the simulations, our proposed DSO‐PTS technique, C‐PTS, and GreenOFDM that is developed recently by modifying the conventional selective mapping (SLM) method are compared with each other with regard to PAPR reduction performance for different search numbers (SNs). In addition, the effects of DSO‐PTS, C‐PTS, and GreenOFDM on the amount of out of band (OOB) radiation in the power spectral density (PSD) graph of WOLA‐OFDM employing solid state power amplifier (SSPA) is measured for different SNs and input back off (IBO) values. According to the simulation results, our proposed DSO‐PTS technique clearly demonstrates a superior PAPR reduction and PSD performance.     

OFDM系统干扰控制的联合预编码

为研究正交频分复用(OFDM)系统的带外(OOB)辐射和高峰值平均功率比(PAPR)两个问题,阐述了两个问题的成因和传统技术的优势与不足,介绍了常见的预编码矩阵.根据OFDM系统原理,对OOB辐射和PAPR问题进行建模,构建新的预编码系统模型.最后对传统OFDM、预编码、联合预编码三者进行了比较,并展望了预编码干扰控制需进一步研究的方向.     

Improved Algorithms for Cancellation Carrier Optimization to Suppress the OFDM OOB Spectrum

Cancellation carriers (CCs) are known to be effective for suppressing the out-of-band (OOB) spectrum of an orthogonal frequency-division multiplexing (OFDM) signal and are useful to realize OFDM-based overlay systems. We show that the known minimum-mean-square-error (MMSE) algorithm is not sufficient to reduced the OOB spectral peaks in the vicinity of transmission band boundaries compared to a direct minmax algorithm and show that a weighted MMSE (WMMSE) algorithm can attain almost the same suppression performance as the direct min-max algorithm with the same computational cost as the MMSE algorithm.     

Out-of-band emissions of digital transmissions using Kahn EER technique

The Kahn envelope elimination and restoration (EER) technique allows for linear RF power amplification by combining nonlinear, but efficient, RF and AF power amplifiers (PAs). In order to use the EER technique for digital signals, a coordinate transform from the original Cartesian in-phase and quadrature mode into a polar mode has to take place, yielding an envelope (or amplitude) and a PM RF signal. This coordinate transform is extremely nonlinear and thereby broadens the spectra of the original signals. In the final PA stage, both signals are recombined. However, since this recombination process is imperfect, out-of-band (OOB) emissions come up, also known as adjacent channel power or spectral regrowth. In this paper, the impact of the broadening of the amplitude and phase signals on OOB emissions is investigated with respect to imperfect restoration due to signal delays and limited bandwidth of the amplitude path. It is shown that the amount of OOB emissions can significantly be reduced if the modulation scheme shows a "hole" at the origin in its vector diagram.     

MIMO spread spectrum GFDM communication systems with index modulation

In the fourth-generation communication system (4G), orthogonal frequency division multiplexing (OFDM) is widely used. However, due to the cyclic prefix (CP) being added, the low spectrum efficiency and high out-of-band radiation (OOB) cause OFDM not to be suitable for the beyond fifth-generation (B5G) communication system. Therefore, to meet the rigorous demands in B5G for the Internet of Things (IoT) and massive machine-type communications (MMTC), generalized frequency division multiplexing (GFDM) technology has received significant attention. On the other hand, index modulation (IM) can provide flexible settings of different parameters to achieve the ideal system goals. Therefore, this paper considers the combination of spread spectrum (SS) and IM under the framework of GFDM, called SS-IM-GFDM. In this scheme, the information bits are jointly conveyed by the indices of spreading codes and the conventional M-ary modulated symbols, which increase the spectrum efficiency and improve the bit error rate performance. Furthermore, a low-complexity maximum likelihood (ML) detector is proposed to recover the transmitted signal effectively at the receiver. Finally, the space-time block code (STBC) is employed to construct the multi-input multi-output SS-GFDM-IM (MIMO SS-GFDM-IM) system to improve the system's reliability. The simulation results show that the proposed scheme is superior to the existing systems.     

Analysis of a two-dimensional radome of arbitrarily curved surface

The transmission effect of a dielectric shell on electromagnetic radiation is analyzed by a modal cylindrical-wave spectrum technique. This method takes into consideration the curvature effect, which is generally ignored in classical approaches such as the ray method and the plane-wave spectrum analysis. The technique is applied to the analysis of radomes of elliptic shape. Numerical results are presented     

Reduction of out-of-band radiation in OFDM systems by insertion of cancellation carriers

Orthogonal frequency-division multiplexing (OFDM) suffers from high out-of-band radiation. In this letter, we investigate a new method for sidelobe suppression characterized by the insertion of a few so-called cancellation carriers (CCs) at both sides of the OFDM spectrum. These special carriers are modulated with complex weighting factors which are optimized such that the sidelobes of the CCs cancel the sidelobes of the transmit signal. With this technique a significant reduction of out-of-band radiation is achieved at the cost of a small degradation in system performance.     

Optimization of linear arrays for broadband signals

The effect of broadband signals on the far-zone field of long arrays is investigated in terms of the energy radiation pattern. A generalized directive gain is introduced which depends on the signal spectrum. With maximum directivity as a criterion it is shown that the problem of optimizing a long isotropic array with respect to a given signal can be associated with a classical eigenvalue problem. The concept of array-bandwidth is introduced. Computed curves for directivity and directive gain are given in terms of this parameter and signal bandwidth. It is shown that while little improvement in directivity is achieved by optimizing the distribution coefficients of an array, a substantial reduction in sidelobe level is obtained when the signal bandwidth is comparable to the array bandwidth.     

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.     

An optimal linear and elliptical antenna array design using black widow optimisation for 5G communication

This research illustrates a precise linear and elliptical antenna array design for synthesising the optimal far-field radiation pattern in the fifth-generation (5G) communication spectrum using a meta-heuristic optimisation technique known as black widow optimisation (BWO). 5G communication is an emerging technology with revolutionary changes in the wireless communication system with ultra-high data rate, enhanced capacity, low latency and good quality of service. An accurate antenna array design for an ideal far-field radiation pattern synthesis with a suppressed side lobe level (SLL) value and half power beam width (HPBW) is the most crucial aspect of 5G communications. A suppressed SLL is necessary to reduce interference in the entire side lobe region, whereas a low HPBW is required for long-distance communication. Here, the BWO is employed to find the optimal feeding current to each array element to lower the SLL and the HPBW value. The BWO algorithm sustains impeccable equity between the exploration and exploitation stages to impact different potential regions of the search space and generate new solutions to attain the global optima by evading the trap of local optima. The design examples of the linear antenna array (LAA) and elliptical antenna array (EAA) are illustrated in this article by applying the optimal feeding currents to each array element. Compared to the uniform antenna array and methodologies described in the recently published literature, the results obtained utilising the BWO algorithm for designing the LAAs and EAAs demonstrate a substantial development in the reduction of SLL and HPBW.     

Participation of electron-phonon interaction in the ultrafast self-modulation of absorption of light in GaAs. Relation of modulation of absorption with the spectrum of stimulated radiation in GaAs

Ultrafast (varying for ～ 1 ps) self-modulation of the absorption spectrum of light takes place during the picosecond-scale photogeneration of charge carriers and intense intrinsic stimulated radiation in GaAs. With the modulation, formation of local amplifications of absorption in the spectrum (juts), which are attributed to local depletion of electron populations in the conduction band, is implied. It is found experimentally that the location of the juts in the spectrum is repeated over the interval determined by the energy of the longitudinal optical (LO) phonon and masses of the electron and heavy hole. This circumstance confirms the previous assumption about the substantial role of the electron-(LO phonon) interaction in ultrafast self-modulation of the absorption spectrum. The previously established notion of the relation of the shape of modulation of the absorption spectrum with the shape of the time-integrated spectrum of intrinsic picosecond radiation is also expanded to the case when ultrafast self-modulation of the absorption spectrum manifests itself.     

一种基于BOC信号的导航发射信道预失真技术

由于卫星导航系统中导航卫星发射信道的记忆非线性效应,使带宽受限的二进制偏移载波（BOC）调制信号失真加剧,导航系统性能将受到严重影响。针对这个问题,建立并分析了导航发射信道模型,提出将等效简化信道模型的主体部分（HPA及其前后置滤波器）等效为Wiener—Hammerstein模型的思想,同时设计了一种针对此模型的基于直接学习结构的自适应LMS预失真方案。仿真结果表明,该方案能有效抑制BOC信号的带外频谱扩展,优化星座图,可以很好地消除导航信道对BOC信号的失真影响,在卫星导航系统中有着重要的意义和良好的应用前景。     

A multilevel formulation of the method of moments [EM scattering]

The modeling of electromagnetic scattering and radiation problems using the method of moments (MoM) is limited to resonant frequencies because of the extensive computational requirements of solving large matrix equations. In this study, a multilevel formulation of MoM which allows substantial computational savings and, thus, extends the application of MoM to higher frequencies is presented. Using a hierarchy of discretization levels, the multilevel technique extracts different modal components of the solution by focusing on a specific portion of the spectrum of the solution at a given level. The fundamental features of this process for the MoM solution of the electric field integral equation (EFIE) are developed and implemented. This multilevel MoM allows the rapid evaluation of the current distributions on a variety of 2-D scatterers with thousands of unknowns in fewer than ten cycles and in fractions of the normal CPU times. The method is stable, fast, suitable for multiple excitations, and adaptable as a `solve' module for almost any MoM code     

OFDM信号的频谱分析及应用PCC对系统的改进

使用极性交替的sinc信号表示OFDM系统的各个正交子载波,这样的表示方法与用多项式抵消编码技术(PCC)来消除载波间干扰(ICI)的作用相一致。传统的OFDM系统信号频率滚降慢、带外辐射大。文中通过对系统的频谱分析和计算机仿真,证实了PCC技术可以改善信号频谱,大幅度降低信号带外辐射。     

固体壁面红外辐射特性的研究

在飞行器红外隐身技术中,研究固体壁面的红外辐射特性十分重要,该红外辐射特性又与法向全发射率和红外辐射光谱的测量密切相关。本文介绍用参比法测量法向全发射率及用光栅单色仪测量红外辐射光谱的实验方法并对测试结果进行分析。     

On the RF/DSP design for efficiency of OFDM transmitters

In this paper, a system-level RF/digital signal processing (DSP) design approach of power-efficient orthogonal frequency-division multiplexing (OFDM) transmitters is proposed. A DSP-based low-IF architecture, which allows a significant enhancement of their power and spectrum efficiencies, is proposed. The cascade of the peak-to-average power ratio (PAPR) reduction technique, predistortion technique, and the in-phase and quadrature modulation led to impressive improvement in the power efficiency and effective linear output power of the OFDM transmitter. Measurement results carried out on an IEEE 802.11a transmitter designed and built for this experiment are presented in terms of error vector magnitude (EVM), adjacent channel leakage ratio, and power efficiency. The power stage of this transmitter uses a heterojunction bipolar InGaP transistor operating in a deeply class AB. The cascade of the PAPR reduction and baseband predistortion processing modules results in the reduction of the power backoff operation point by approximately 10 dB accompanied by a relative increase in the wireless local area network transmitter power efficiency by roughly 400% while meeting the emission mask spectrum and EVM levels demanded by the 802.11a standard.     

Probe radiation cancellation in wideband probe-fed microstriparrays

The dual probe-fed patch technique, which is used to suppress unwanted probe radiation in wideband probe-fed microstrip patches, is extended for the case of a balanced two-patch sub-array. To demonstrate this technique, two S-band microstrip arrays were fabricated: with and without the balanced two-patch sub-arrays. The balanced array showed a reduction in the cross-polarisation peak level by nearly 10 dB. The unwanted radiation is effectively suppressed (20 dB cross-polarisation isolation) over a bandwidth of greater than 6.3%     

Blind IB-MSNR Beamforming for DSSS-based Communication Systems

The spread spectrum technique is widely used and effective in combating narrowband interference with a moderate power as well as co-channel interference (CCI) and noise. However, it requires a configuration with long signature sequence for interference suppression, which is practically impossible. In addition, a system will exhibit a substantial degradation in performance due to both strong interference and desired coherent signals introduced by multipaths. In this paper, a novel detector is first proposed to determine the number of strong interferers for direct sequence spread spectrum (DSSS) based communication systems. Furthermore, the blind interference-blocked (IB) maximum signal-to-noise ratio (MSNR) beamforming is then proposed to collect desired coherent signals and to suppress strong interference. Theoretical analysis and numerical results demonstrate that the proposed IB-MSNR beamformer can achieve nearly the same performance as the optimal non-blind beamformer.