Spectral Sidelobes Research Articles

Experimental Demonstration of Bidirectional OFDM/OQAM-MIMO Signal Over a Multicore Fiber System

A bidirectional transmission and massive multi-input multi-output (MIMO) enabled radio over a multicore fiber system with centralized optical carrier delivery is experimentally investigated in this paper. Optical carriers for upstream are delivered from the central office to the remote antenna unit via the inner core for the coreless implementation. In our experiment, as one of the fifth-generation (5G) waveform candidates, orthogonal frequency division multiplexing using offset quadrature amplitude modulation (OFDM/OQAM) is adopted in both uplink and downlink to increase the spectral efficiency and side lobes suppression ratio. An advanced $2\times 2$ MIMO-OFDM/OQAM channel estimation algorithm is optimally designed to equalize the hybrid optical and wireless MIMO channels. The experimental results show that bidirectional transmission of 4.46 Gb/s $2\times 2$ MIMO-OFDM/16-OQAM could be achieved over a 20-km seven-core fiber and a 0.4-m wireless link. The proposed scheme proves that the multicore fiber can realize the transparent transmission of bidirectional radio signals and effectively simplify the infrastructures in the future 5G cellular systems.

A High Data-Rate Energy-Efficient Triple-Channel UWB-Based Cognitive Radio

This paper presents a fully integrated ultra wideband (UWB)-based cognitive radio (CR) transceiver for $1\,{\text{Gb/s}}$ data-rate energy-efficient short-range wireless connectivity by scavenging triple discrete inactive frequency bands in 3.1–10.6 $\,{\text{GHz}}$ ISM band. The transceiver including receivers (RXs), transmitters (TXs), spectrum sensors, and synthesizers is implemented in $1\,{\text{V}}$ $65\,{\text{nm}}$ standard CMOS technology. A novel pulse generator (PG) enables the TX to meet UWB emission mask and to reduce spectral sidelobe peak by ${ with low power consumption of $3.6\,{\text{mW}}$ . A dual-mode RX front-end provides quadrature analog correlation (QAC) for analog domain matched filtering during communication and analog wavelet-based energy detection (ED) during spectrum sensing (SS) without changing circuitry. The transceiver achieves the minimum energy consumption of $59.7\,{\text{pJ/b}}$ with $1.97 \times {10}^{-4}\,{\text{bit error rate (BER)}}$ and the maximum energy consumption of $102.3\,{\text{pJ/b}}$ with $1.25 \times {10}^{-3}\,{\text{BER}}$ . Die area is $4.6\,{{\text{mm}}^{2}}$ with on-die phase-locked loops (PLLs) and pads.

Experimental validation of orthogonal frequency division multiplexing with peak‐to‐average power ratio reduction and out‐band distortion control using software defined radio

Orthogonal frequency division multiplexing (OFDM) is an emerging technology in recent wireless communication standards where high data rate is required at low latency and better spectral efficiency. OFDM signals have a generic problem of high peak-to-average power ratio (PAPR) due to the superimposition of the data-modulated subcarriers. This high peak signals may result in amplifier non-linearity which creates many problems such as performance degradation and out-of-band distortion. Hence, the authors addressed this problem by designing and implementing four new PAPR reduction schemes such as phase modulation, rail clipping, sample and hold approach and threshold methods based on amplitude clipping to reduce PAPR and improve the band gap between spectral sidelobes to main lobe significantly. For practical proof of the suggested concepts, they have chosen software-defined radio as an experimental setup in which universal software radio peripheral N210 employed as hardware and GNU radio as software. Experimental results are analysed in terms of significant PAPR reduction and out-of-band spectral leakage control. Besides these, motivation, background and relevant mathematical analysis with rigid justification are presented in this study.

Correlatively Precoded OFDM With Reduced PAPR

Correlative precoders have been proposed for orthogonal frequency-division multiplexing (OFDM) to provide either improved spectral sidelobe suppression, which yields very compact signal spectrum, or enhanced intercarrier interference (ICI) self-cancelation, which renders robustness against carrier frequency offset (CFO). However, these signaling features are commonly traded off by increasing the peak-to-average power ratio (PAPR) of the correlatively precoded waveform and not accomplished concurrently. To overcome this shortcoming, new correlative precoders are designed in this paper to endow correlatively precoded OFDM waveforms with reduced PAPR while preserving enhanced ICI self-cancelation or improved spectral sidelobe suppression, either respectively or jointly. Numerical results show that correlative precoders can be properly designed to achieve joint performance characteristics in PAPR reduction, spectral sidelobe suppression, and ICI self-cancelation.

Spectrally Efficient OFDM Pilot Waveform for Channel Estimation

Pilot-aided channel estimation is essential to orthogonal frequency division multiplexing systems and standards. Under the system setup that data and pilot waveforms are processed individually for enhancing the spectral efficiency of the composite waveform, pilot patterns are designed here to jointly optimize channel estimation on quasistatic multipath channels and provide extremely high spectral compactness by suppressing spectral sidelobes. Specifically, a general constraint on pilot pattern is developed to endow the pilot waveform with very small spectral sidelobes and thus compact spectrum. Using the constraint, systematic multiple-stage design procedures are proposed to develop the desired pilot patterns achieving Cramer Rao bound for channel estimation on quasistatic multipath channels while providing fast decaying spectral sidelobes. Among the demonstrated pilot patterns achieving optimum channel estimation, the pilot waveforms using $\mathcal {A}_{I}$ - and $\mathcal {B}_{I}$ -typed pilot patterns developed from the procedures are shown to exhibit higher spectral compactness than conventional pilot waveforms. Besides, $\mathcal {C}_{I}$ -typed pilot pattern that exploits extremely high spectral compactness without resort to channel estimation optimization is also developed. By sacrificing a slight loss in channel estimation, the pilot waveforms using $ \mathcal {C}_{I}$ -typed pilot pattern are shown to exhibit spectrums approaching the Nyquist spectrum.

Spectrally Precoded OFDM Without Guard Insertion

Orthogonal frequency-division multiplexing (OFDM) signaling format is commonly associated with guard subintervals to reject interblock interference (IBI) and enable simple one-tap frequency-domain equalization (FDE) at the receiver when received over dispersive channels. Despite the advantages, the insertion of guard subintervals expands the signal spectrum and thus trades off the spectral efficiency significantly. In this paper, the nonguarded OFDM (NG-OFDM) signaling format, which is exactly OFDM without inserting guard subintervals, is considered to avoid the spectrum expansion and combined with spectral precoding to further achieve high spectrum compactness. In particular, spectral precoders are designed for NG-OFDM to provide very small spectral sidelobes, and thus extremely compact signal spectrum, and associate the transmitted waveform with low peak-to-average power ratio. To reject IBI among received NG-OFDM blocks over dispersive channels, a guard subinterval in each received block is removed at the receiver, and this turns the resultant receiver as nonorthogonal over subcarrier subchannels, thus resulting in intercarrier interference (ICI). The corresponding channel capacity and linear minimum-mean-square-error (LMMSE) data estimation are analyzed. It is shown that the proposed spectrally precoded NG-OFDM (SP-NG-OFDM) system can provide channel capacity very close to nonprecoded and spectrally precoded OFDM using the cyclic prefix (CP-OFDM) when the removal guard ratio is small, and much higher channel capacity per unit bandwidth than spectrally precoded CP-OFDM conveying the same data rate provided that a small out-of-band power proportion is required. When a sufficiently small precoding rate is adopted, the SP-NG-OFDM receiver, using guard removal and LMMSE-FDE, is also shown to counteract ICI effectively and exploit frequency diversity over dispersive channels.

Enhanced OFDM for fragmented spectrum use in 5G systems

ABSTRACTMost of the recent and emerging wireless systems have selected orthogonal frequency division multiplexing (OFDM) scheme as the basis for the physical layer due to its flexibility and robustness. OFDM is commonly considered also as the first candidate technology for advanced cognitive radio, dynamic spectrum use and fragmented coexistence scenarios, including the 5G system development. Nevertheless, OFDM has the problem of high‐power spectral sidelobes around the active subcarriers, which limit its feasibility in fragmented spectrum use and asynchronous frequency division multiple access operation. Therefore, various sidelobe suppression techniques have been proposed in the literature to mitigate these effects. This paper investigates four different suppression techniques: time‐domain windowing, cancellation carrier method, subcarrier weighting and polynomial cancellation coding, as well as their combinations. These methods are elaborated for a practical heterogeneous fragmented spectrum use scenario based on 5‐MHz 3GPP Long‐Term Evolution (LTE), proposing various enhancements to achieve effective suppression in narrow gaps with affordable complexity. In addition to the spectral characteristics, also, the possible side effects are evaluated in details, considering peak‐to‐average power ratio characteristics, error rate performance, computational complexity and resources usage. Copyright © 2014 John Wiley & Sons, Ltd.

Spectrally Precoded DFT-Based OFDM and OFDMA With Oversampling

Spectrally precoded orthogonal frequency-division multiplexing (SP-OFDM) and SP-OFDM multiple access (SP-OFDMA) are bandwidth-efficient rectangularly pulsed multicarrier signaling formats that can provide power spectral sidelobes decaying asymptotically as f -2L-2 , with L being a preassigned positive-integer design parameter, which results in a compact spectrum. The existing spectral precoders for SP-OFDM and SP-OFDMA are designed based on the rectangularly pulsed analog multicarrier waveform representation that has an unlimited power spectrum; thus, the corresponding waveforms cannot be exactly synthesized in a discrete Fourier transform (DFT) structure. Consequently, these spectral precoders are subject to modification in DFT-based OFDMA and OFDM systems in which signaling waveforms are digitally synthesized with the use of DFT and provide discrete approximation embodiments to analog multicarrier waveform counterparts. In this paper, a general constraint on spectral precoding and the corresponding spectral precoders are developed for DFT-based OFDMA and OFDM with oversampling to ensure the desirable spectral property that the resultant spectrally precoded DFT-based waveforms provide powerspectral-sidelobe envelope bounds decaying asymptotically as f -2L-2 . It is analytically shown that the precoders suitable to DFT-based waveforms can be simply obtained by properly transforming the previously designed precoders suitable to corresponding analog multicarrier waveforms with an appropriate diagonal phase-rotation matrix, while yielding similar spectral performance and peak-to-average power ratio (PAPR) characteristics.

A HYBRID PRECODING TECHNIQUE FOR REDUCING BOTH PAPR AND SIDELOBE POWER IN COGNITIVE RADIO NETWORKS

Orthogonal Frequency Division Multiplexing (OFDM) is a recognized transmission technique for Cognitive Radio (CR) networks, which is called non-contiguous OFDM (NC-OFDM). While this technique has two major drawbacks, Out OF Band (OOB) leakage due to high spectral sidelobe and high Peak to Average Power Ratio (PAPR), which cause interference with Primary Users (PU). Many solutions were proposed for solving either of these problems individually or together. In this paper, a hybrid precoding technique for reducing both PAPR and sidelobe power is proposed. This technique is based on using Zadoff-Chu sequences with Singular Value Decomposition (SVD) optimization method to minimize the power interference with PU. Simulation results cover the PAPR, sidelobe reduction, and Bit Error Rate (BER) for the proposed technique.

Spectral Precoding for Cyclic-Prefixed OFDMA With Interleaved Subcarrier Allocation

Spectrally precoded orthogonal frequency-division multiple access (OFDMA) using cyclic prefix and interleaved subcarrier allocation (IOFDMA) is a useful rectangularly pulsed OFDMA signaling format for it exploits high spectral compactness and frequency diversity characteristics. In this paper, two new spectral precoders of the largest precoding rate are developed for IOFDMA in order to provide improved spectral compactness and reduce implementation complexity, respectively. Specifically, a new correlative precoder is developed and shown to minimize the dominant spectral coefficient among all correlative precoders of the largest precoding rate and thus can construct IOFDMA signals providing extremely high spectral compactness. Besides, a multiple-stage spectral precoding technique is proposed to construct a low-complexity orthogonally precoded IOFDMA with power spectral sidelobes decaying as f^{-2L-2} for L>1, where L is the number of stages. The corresponding orthogonally precoded IOFDMA signals conveying independent zero-mean data are shown to provide the same power spectrum as existing orthogonally precoded IOFDMA signals of the largest precoding rate while requiring much less complexity.

Low-Complexity Spectrum Shaping for OFDM-Based Cognitive Radio Systems

Orthogonal frequency-division multiplexing (OFDM) is an ideal transmission technique for dynamic spectrum access in cognitive radio (CR) systems. In this letter, we propose low-complexity spectrum shaping to enable fast decaying of power spectral sidelobes and enhance spectral compactness for OFDM-based CR systems. Based on a basic scheme mapping antipodal symbol pairs onto adjacent subcarriers, we present two modified schemes to further balance sidelobe suppression and system throughput. Compared with existing spectrum shaping schemes, ours exhibit their advantage of both simplicity and flexibility.

Modeling of low coherence interferometry using broadband multi-Gaussian light sources

Using a low coherence interferometry (LCI) model, a comparison of broadband single-Gaussian and multi-Gaussian light sources has been undertaken. For single-Gaussian sources, the axial resolution improves with the source bandwidth, confirming the coherence length relation that the resolution for single Gaussian sources improves with increasing spectral bandwidth. However, narrow bandwidth light sources result in interferograms with overlapping strata peaks and the loss of individual strata information. For multiple-Gaussian sources with the same bandwidth, spectral side lobes increase, reducing A-scan reliability to show accurate layer information without eliminating the side lobes. The simulations show the conditions needed for the resolution of strata information for broadband light sources using both single and multiple Gaussian models. The potential to use the model to study optical coherence tomography (OCT) light sources including super luminescent diodes (SLDs), as reviewed in this paper, as well as optical delay lines and sample structures could better characterize these LCI and OCT elements. Forecasting misinformation in the interferogram may allow preliminary corrections. With improvement to the LCI-OCT model, more applications are envisaged.

A novel multicarrier CDMA transmission scheme for cognitive radios with sidelobe suppression

SUMMARYNoncontiguous multicarrier code division multiple access (MC‐CDMA) is an effective multiple access method for cognitive radio systems that can deploy the noncontiguous vacant parts of a certain spectrum shared with the users of a primary system. However, the large spectral sidelobes of the Fourier transform based implementation of the MC‐CDMA interfere with the adjacent primary transmission. In this paper, a novel complex signature sequence set is proposed for synchronous downlink MC‐CDMA based cognitive radio networks to suppress the sidelobes. To do so, the minimization of the sidelobes power is developed as an eigen‐optimization problem that is optimally solved by using the eigenvalue decomposition method. The optimal complex signature sequences are chosen as the eigenvectors attained by the eigenvalue decomposition of a symmetric matrix that is dependent of the spectral characteristics of the primary users. Simulation results show that by a slight decrease in the maximum number of active users compared with full load, the sidelobes can be considerably suppressed. The effect of cyclic prefix length, the bandwidth of the primary system and the number of active users on the sidelobe suppression performance is analyzed through numerical simulations. Copyright © 2012 John Wiley & Sons, Ltd.

Novel spreading codes for multicarrier code division multiple access based cognitive radio networks with sidelobe suppression

ABSTRACTThe deployment of multicarrier code division multiple access as the transmission scheme of the spectrum overlay based cognitive radio (CR) networks faces two challenging issues: (i) the need for spreading codes with arbitrary length and (ii) the interfering effect of the leaked power (due to spectral sidelobes) to the adjacent spectral band used by the primary system. To meet these challenges, we propose two novel complex spreading code sets. For this purpose, a cost function is defined as the ratio of the power leaked to the adjacent primary band to the power transmitted to the band allowed for CR operation. The cost function is shown to be convertible to a trace ratio problem. The two complex spreading code sets are determined by deploying two different standard solutions. The first is the conventional but approximate solution based on generalized eigenvalue decomposition (GEVD) method, and the other is an iterative algorithm that converges to the optimal solution and hence outperforms the GEVD based solution. Simulation results show that by a slight decrease in number of users, the code set yielded by iterative algorithm suppresses the leaked power to almost zero. The applicability of the proposed code sets in different scenarios is discussed. Copyright © 2012 John Wiley & Sons, Ltd.

Spectrally Precoded OFDM and OFDMA with Cyclic Prefix and Unconstrained Guard Ratios

Spectrally precoded orthogonal frequency-division multiplexing with cyclic prefix (SP-CP-OFDM) and SP-CP-OFDM multiple access (SP-CP-OFDMA) are investigated in this paper. A general constraint on spectral precoding is developed for both SP-CP-OFDM and SP-CP-OFDMA signal formats with unconstrained guard ratios and arbitrary input data statistics to ensure very small power spectral sidelobes decaying asymptotically as f-2J-2, with J a predesigned integer-valued parameter. In the SP-CP-OFDM signaling format, new spectral precoders are devised to meet the constraint. It is shown that the proposed SP-CP-OFDM signals can provide much smaller spectral sidelobes than the previously designed SP-CP-OFDM signals with block partitioning and constrained guard ratios. In the SP-CP-OFDMA signaling format, a general procedure is proposed to facilitate the precoder design for SP-CP-OFDMA signals with arbitrary subcarrier allocation for each user and some spectral precoders are accordingly devised. It is shown that the proposed SP-CP-OFDMA signals can suppress sidelobe powers effectively and thus achieve higher spectral compactness than nonprecoded rectangularly-pulsed CP-OFDMA.

A NEW APPROACH FOR MOVING TARGET DETECTION USING BARTLETT METHOD FOR SPECTRAL ESTIMATION

Moving Target Detection (MTD) is an automated radar signal and data processing system, which is designed to improve the performance of radar systems in the presence of various forms of clutter. Consequently, it provides high probability of detection (Pd) for an acceptable probability of false alarm (Pia). It employs coherent, linear Doppler filtering, adaptive thresholding and a fine ground clutter map to reject ground clutter, rain clutter, birds, and interference. The current MTDs relay on the Fast Fourier Transform (FFT) of the total received data sequence to estimate the clutter power spectrum and consequently reduce or remove its effect on the detection performance. Direct FFT leads to high sidelobes level and requires a significant large computation time. The high level sidelobes increases the false alarm probability at the output of the Doppler filters bank. A solution for reducing the effect of spectral sidelobes is the utilization of window functions. However, this solution leads to widening the main spectral lobe and reduce the Doppler resolution. Also it increases the hardware complexity of the system. In the present work, Bartlett method for spectral estimation which depends on dividing the received data sequence into a number, K, of nonoverlapping segments and averaging the calculated FFT for each segment over K, is applied in the MTD instead of the direct FFT for typical ground based radar. The proposed method enhances the target detection capabilities, by providing higher detection probabilities, lower false alarm rates and an additional gain of 7-10 dB in the improvement factor, in the presence of ground and weather clutter, compared to the traditional one. This is because the sidelobe levels obtained are very small in magnitude. This in turns facilitates the realization of the Doppler filters bank without using additional weighting. The obtained performance can be achieved by applying the direct FFT with weighting function to the total received data sequence, which leads to a more hardware complexity and long time calculations compared to the proposed method. Computer simulation results are presented to support the superiority of the proposed technique.

Spectral Nulling on Transmit via Nonlinear FM Radar Waveforms

Modern radars operate in electromagnetic environments crowded with other RF users. Adaptive spectral nulling on transmit can reduce interference from and to these other RF users. An analytical theory is developed for spectral nulling by a minimal adjustment, or offset, to the phase of the radar pulse that maintains constant pulse amplitude. Numerical examples show that a small time-varying phase offset designed by the proposed method can produce deep spectral nulls at one or more chosen frequencies in the radar's spectral sidelobes with little other effect on the pulse spectrum or ambiguity function.

Spectral precoding for constant-envelope OFDM

Spectrally precoded orthogonal frequency-division multiplexing (OFDM) and constant-envelope OFDM (CE-OFDM) are two modified OFDM formats that enhance the signaling features of OFDM, while allowing for the efficient implementation by discrete Fourier transform and the insertion of guard intervals to counteract channel dispersion. The spectrally precoded OFDM signal provides very small power spectral sidelobes decaying asymptotically as f-2J-2 with J integer-valued design parameter, thereby suppressing sidelobe powers effectively and achieving higher spectral efficiency than nonprecoded rectangularly-pulsed OFDM which has power spectral sidelobes decaying asymptotically as f-2. The CE-OFDM signal exhibits a unity peak-to-average power ratio, and thus enables the efficient use of transmitter's power amplifier. In this paper, a specific CE-OFDM block signaling format is proposed to incorporate spectral precoding with an aim to suppressing sidelobe powers more effectively than conventional nonprecoded CE-OFDM. The general constraint on spectral precoding is developed to ensure that the power spectral sidelobes of the spectrally precoded CE-OFDM block signal with arbitrary data statistics decay asymptotically as f-2J-2. Some spectral codes previously found for spectrally precoded OFDM are shown to meet the constraint and thus suited for spectrally precoded CE-OFDM. Specifically, spectral performance is analyzed for the spectrally precoded CE-OFDM block signal with independent and identically distributed data to illustrate the prevailing spectral advantage. The corresponding receiver is also developed to demodulate the spectrally precoded CE-OFDM block signal over dispersive channels.

Broad-bandgap and low-sidelobe surface plasmon polariton reflector with Bragg-grating-based MIM waveguide

Surface plasmon polariton reflector (SPPR) based on metal-insulator-metal (MIM) Bragg grating waveguide is numerically studied. A quasi-chirped technique is applied to the engraved grooves in the surface of the MIM waveguide, and a new kind of broad-bandgap SPPR is achieved. Meanwhile, by optimizing the profile of gap width between the metal and dielectric, the spectral sidelobe of SPPR is effectively suppressed and thus the performance o f the SPPR is further improved.

Asymptotic spectral behavior of spectrally precoded OFDM signal with arbitrary input statistics

In this letter, we study the asymptotic spectral behavior of the spectrally precoded orthogonal frequency-division multiplexing (SP-OFDM) signal with arbitrary input statistics. It is shown that all constraints and spectral precoders previously developed for SP-OFDM with independent and identically distributed and zero-mean data provide fast decaying spectral sidelobes for SP-OFDM with arbitrary input statistics.