Active Subcarriers Research Articles

Design and Analysis of Grouping Active Subcarrier Frequency-Time Index Modulation for Differential Chaos Shift Keying Communication System

Design and Analysis of Grouping Active Subcarrier Frequency-Time Index Modulation for Differential Chaos Shift Keying Communication System

Coordinate Interleaved OFDM With Repeated In-Phase/Quadrature Index Modulation

Orthogonal frequency division multiplexing with index modulation (OFDM-IM), which transmits information bits through ordinary constellation symbols and indices of active subcarriers, is a promising multicarrier transmission scheme and has attracted the attention of researchers due to numerous benefits such as flexibility and simplicity. Nonetheless, OFDM-IM cannot satisfy the needs of future wireless communication services such as superior reliability, high data rates, and low complexity. In this article, we propose a novel OFDM-IM scheme named coordinate interleaved OFDM with repeated in-phase/quadrature IM (CI-OFDM-RIQIM), which provides superior error performance and enhanced spectral efficiency due to its diversity order of two and clever subcarrier activation pattern (SAP) detection mechanism, respectively. In addition, CI-OFDM-RIQIM is further extended to coordinate interleaved OFDM with in-phase/quadrature IM (CI-OFDM-IQIM) by doubling information bits transmitted by IM. Furthermore, log-likelihood ratio (LLR) based low-complexity detectors are designed for both proposed schemes. Theoretical analyses are performed and an upper bound on the bit error probability is derived. Comprehensive computer simulations under perfect and imperfect channel state information (CSI), are conducted to compare the proposed and reference schemes. It is shown that CI-OFDM-RIQIM and CI-OFDM-IQIM show superior results and can be considered promising candidates for next-generation wireless communication systems.

Secure Index with OFDM-IM-based Chaotic System

Orthogonal Frequency Division Multiplexing-Index Modulation (OFDM-IM) based on chaos is a com- munication technique that uses chaotic systems to enhance the security of OFDM-IM systems. In this technique, chaotic signals are used to modulate the index values of the subcarriers and the data using DCSK, resulting in a more robust and efficient transmission. OFDM-IM uses the activated index as an information-carrying unit, which increases data rate and spectral efficiency. In this paper, we propose a secure index strategy for our system using a logistic map and Henon map to generate the chaotic signals, considering additive white Gaussian noise (AWGN) and the Raleigh fading channel. A lookup table based on the chaotic system was created using the indices of a randomized index modulation scheme with DCSK modulation. Even if the active subcarriers and their symbols are correctly calculated until the opening frame, the randomized index modulation prevents eavesdroppers from accurately determining message bits.

Index modulation-based efficient technique for underwater wireless optical communications

This paper presents and experimentally demonstrates energy and spectral efficient index modulation (IM) scheme for underwater wireless optical communications (UWOC). The proposed scheme utilizes IM with direct-current optical orthogonal frequency division multiplexing which is based on discrete Hartley transform (DHT-OFDM) instead of conventional discrete Fourier transform-based OFDM (DFT-OFDM). Although DHT-OFDM has been previously studied in various applications such as optical and wireless communications, this is the first work that practically investigates it with the UWOC system. In this paper, we first demonstrate that DHT-OFDM can inherit all advantages of conventional DFT-OFDM with less computational complexity and higher robustness to the inter-carrier interference (ICI). Therefore, the DFT process is employed at receiving end instead of DHT to guarantee low-complexity frequency-domain estimation and equalization processing and ensure that shorter required pilot overheads are required to track the UWOC channel effects. Furthermore, IM-DHT-OFDM has been presented to provide better flexibility of spectral and energy efficiency (SE/EE) to UWOC and offers higher robustness to the ICI effect as the transmitted data is modulated and physically transmitted via some active subcarriers while others must be kept idle. For instance, IM-DHT-OFDM provides up to 50% spectral efficiency improvement compared to DFT-OFDM and DHT-OFDM with better performance over the UWOC system. Simulation and experimental tests, performed over 2-meter UWOC, are provided to demonstrate the outperformance of the proposed schemes against the existing benchmarks in terms of bit error rate (BER), achieved SE, peak-to-average ratio (PAPR), and computational complexity.

A novel multicarrier index keying‐aided non‐orthogonal multiple access systems in presence of uncertain channel state information

SummaryThis paper investigates the performance of novel multicarrier index keying‐aided non‐orthogonal multiple access (MCIK‐NOMA) system. Unlike the classical scheme, the proposed system sends the information through both index domain and constellation domain symbols thereby increases the spectral efficiency. The impact of channel state information (CSI) uncertainty on the system's performance is investigated. More specifically, the performance of MCIK‐NOMA under three different CSI conditions is analyzed: perfect CSI, MMSE‐based variable CSI, and fixed CSI uncertainty. This paper also discusses the optimum power difference needed for the successful separation of symbols at the SIC receiver by alternating the power ratio in the proposed system. The influence of selecting different active subcarriers and modulation techniques on the system's performance is studied in detail. The simulation results show that the proposed system achieves better BER and spectral efficiency and outperforms the existing systems.

Peak‐to‐average power ratio of selective mapping technology involving a phase generation mechanism based on polar codes for orthogonal frequency division multiplexing with index modulation systems

AbstractIn this paper, a novel selective mapping (SLM) method is proposed for reducing the peak‐to‐average power ratio (PAPR) of orthogonal frequency division multiplexing with index modulation (OFDM‐IM) systems. In the proposed method, polar codes are used to construct the SLM phase generation mechanism. This systematic structural design is superior to conventional methods in which phases are randomly generated. Moreover, polar codes and Reed–Muller codes are selected to protect the input data of the OFDM‐IM system. The weight distribution of the Reed–Muller codes is then used to determine the location data of the active subcarriers in the OFDM‐IM system, and these data are used to determine the error correction capacity. Simulation results indicated that the PAPR improvement achieved using the proposed method is similar to that achieved with conventional SLM methods. Moreover, linear block codes are used to construct the active subcarrier selection and SLM phase generation mechanisms to ensure that both mechanisms have systematic structural designs.

Performance analysis of FBMC/OQAM systems in frequency-selective channels

Filter bank multicarrier (FBMC) systems based on offset QAM modulation (FBMC/OQAM) represent an interesting alternative to the widely used OFDM scheme for machine type communications or cognitive radio. In this paper the performance of FBMC/OQAM systems exploiting a single-tap receiver in frequency-selective Rayleigh fading channels, is analyzed. In particular, the SER and the asymptotic SER (i.e., for a large number of subcarriers) for any prototype filter, is derived. Moreover, a simple closed-form approximate expression of the asymptotic SER is obtained by taking into account, for each active subcarrier, only intersymbol interference terms due to the considered subcarrier and the adjacent subacarriers, and intercarrier interference terms due to the adjacent subcarriers. The numerical results show that the obtained SER expression can predict the actual performance of the system while the asymptotic SER expression provides a useful upper-bound. Moreover, the curves obtained by exploiting the derived simple closed-form approximate expression can be used to predict the asymptotic upper-bound when the considered prototype filter is adopted.

Broad Learning System-Based Detector for OFDM With Index Modulation

This letter presents a broad learning system (BLS) based orthogonal frequency division multiplexing with index modulation (OFDM-IM) detector called BLS-IM, which achieves bit-error-rate performance improvement with less training time. In BLS-IM, in order to help the network detect the index of the active subcarrier, the received data needs to be preprocessed into polar coordinates first. The processed data is then fed into the BLS network. In particular, the BLS is built in the form of a flat network where the initial signal bits are transferred among the feature nodes. At the same time, the structure is extended in a broad sense in terms of enhanced nodes. The proposed BLS-IM detector has an advantage in training time because it avoids the time-consuming training process caused by a large number of connection parameters in the layers. Simulation results demonstrate that BLS-IM is capable of achieving better detection performance with less training time than the existing algorithms.

A novel interference signal superposition algorithm for providing secrecy to subcarrier number modulation‐based orthogonal frequency division multiplexing systems

AbstractOrthogonal Frequency Division Multiplexing with Subcarrier Number Modulation (OFDM‐SNM) is a transmission scheme that aims to convey additional data bits by exploiting the number of active subcarriers. Even though OFDM‐SNM creates many advantages, such as low bit error rate (BER), high spectral efficiency, and high reliability, it suffers from the security vulnerabilities against eavesdroppers and malicious wire‐tappers. The reason why OFDM‐SNM is vulnerable against security threats is that the nature of subcarrier number‐based additional data transmission differs from the conventional data modulation techniques, which makes the existing physical layer security (PLS) applications to be inapplicable for OFDM‐SNM. Therefore, novel PLS techniques must be developed for OFDM‐SNM to secure the data that is transmitted by exploiting the number of subcarriers. In this paper, a novel interference signal superposition security algorithm for OFDM‐SNM is proposed at the physical level to overcome its security vulnerabilities. The proposed security application aims to provide the data secrecy by embedding an artificial interference signal into the data that are meant to be conveyed by the active subcarriers. This artificial interference signal is created by exploiting the channel state information of the legitimate user, and deliberately included into the transmitted data to create a confusion for those who try to infiltrate the communication system, such as eavesdroppers or malicious wire‐tappers. In this paper, the transmission of data for both the legitimate user and eavesdropper is operated by M‐ary PSK/QAM modulation over a Rayleigh fading channel. The theoretical calculations and simulation results prove that the proposed security application for OFDM‐SNM is a strong candidate to provide the security requirements of such cases in future wireless networks, where providing data secrecy is among the top priorities.

Reactive jammer detection in OFDM with index modulation

Detection of jamming attacks is an important tool to improve the resource efficiency of jammer resilient communication networks. Detecting reactive jammers is especially difficult since the attacker is cognitive and focuses only on the used channels. Orthogonal frequency division multiplexing with index modulation (OFDM-IM) consists of active and passive subcarriers. Only active subcarriers carry modulated signals while passive subcarriers are left unused. In OFDM-IM systems, information bits are also dynamically embedded in the indices of these active subcarriers. As a result, remaining passive subcarriers cause instantaneously changing and unused holes in the spectrum that a reactive jammer cannot escape from attacking. In this paper, we propose an OFDM-IM-based detection scheme to improve the detection performance against reactive jammers. The proposed method exploits the dynamically changing empty OFDM-IM subcarriers to improve detection performance. A detection mechanism that is based on the variance of received signals is considered to identify the jammed subcarriers reliably and with low complexity. We assumed a destructive and elusive reactive jammer model that applies a zero-mean Gaussian jamming signal to the occupied channels. The performance of the variance detector is investigated analytically for OFDM-IM and OFDM-based systems under the given jammer model. The results showed that passive subcarriers of OFDM-IM inherently provide a better detection performance compared to the classical OFDM. Lastly, the analytical results are verified via simulations against both full-band and partial-band reactive jammers. Also, the effect of noise and the jamming power on the detection performance is investigated via extensive simulations.

OFDM Radar With Subcarrier Aliasing—Reducing the ADC Sampling Frequency Without Losing Range Resolution

Many radar applications require a high range resolution, entailing the necessity for a large signal bandwidth. When employing an orthogonal frequency-division multiplexing (OFDM) radar system in its standard implementation, the radar system must operate at a high sampling frequency to ensure covering the entire signal bandwidth, thus imposing significant challenges on its implementation. Therefore, methods for reducing the sampling frequency are topic of recent research. In this work, a method for reducing the analog-to-digital converter (ADC) sampling frequency at the receiver without losing range resolution is proposed. This is achieved by activating only a fraction of the subcarriers at the transmitter, while simultaneously allowing for systematic aliasing due to undersampling in the receiver. This aliasing is designed such that the active subcarriers do not interfere with each other, which ensures a simple separation of them in the frequency domain. The proposed concept, denoted as subcarrier aliasing (SA), allows reducing the ADC sampling frequency in the receiver, but lacks in reducing the digital-to-analog converter (DAC) sampling frequency in the transmitter. However, the key asset of SA-OFDM lies in the properties of its waveform, making it especially suitable for joint radar and communication applications over other waveforms, e.g., stepped-carrier OFDM.

Enhancing diversity of OFDM with joint spread spectrum and subcarrier index modulations

This paper proposes a novel spread spectrum and sub-carrier index modulation (SS-SIM) scheme, which is integrated to orthogonal frequency division multiplexing (OFDM) framework to enhance the diversity over the conventional IM schemes. Particularly, the resulting scheme, called SS-SIM-OFDM, jointly employs both spread spectrum and sub-carrier index modulations to form a precoding vector which is then used to spread an M-ary complex symbol across all active sub-carriers. As a result, the proposed scheme enables a novel transmission of three signal domains: SS and sub-carrier indices, and a single M-ary symbol. For practical implementations, two reduced-complexity near-optimal detectors are proposed, which have complexities less depending on the M-ary modulation size. Then, the bit error probability and its upper bound are analyzed to gain an insight into the diversity gain, which is shown to be strongly affected by the order of sub-carrier indices. Based on this observation, we propose two novel sub-carrier index mapping methods, which significantly increase the diversity gain of SS-SIM-OFDM. Finally, simulation results show that our scheme achieves better error performance than the benchmarks at the cost of lower spectral efficiency compared to classical OFDM and OFDM-IM, which can carry multiple M-ary symbols.

Capacity and outage performance of multiple AF relays based cooperative OFDM-QIM system

In this paper, we propose a multiple amplify-and-forward (AF) relays based cooperative orthogonal frequency division multiplexing (OFDM) with quadrature index modulation (C-OFDM-QIM) system to enhance the spectral efficiency and the transmission reliability, where two independent subcarrier subsets are activated to convey the real and imaginary symbol vectors, respectively, and the per-subcarrier relay selection (RS) scheme is adopted to select the relay with the maximum end-to-end signal-to-noise ratio (SNR) for each active subcarrier. Both fixed-gain AF and variable-gain AF protocols are investigated at the selected relays. Based on the order statistics and the max-flow min-cut theory, the closed-form average outage probability (AOP) and the average capacity are derived, respectively. Numerical results demonstrate that the theoretical AOP is correct and accurate and the proposed C-OFDM-QIM system outperforms the traditional cooperative OFDM with index modulation (C-OFDM-IM) system in the spectral efficiency, the outage performance, and the average capacity.

Coordinate Interleaved OFDM With Power Distribution Index Modulation

Orthogonal frequency division multiplexing with index modulation (OFDM-IM), which transmits information bits via <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$M$ </tex-math></inline-formula> -ary constellation symbols and indices of the active subcarriers, is a promising OFDM-based multicarrier transmission scheme. Recently, several new schemes have been proposed by utilizing the flexibility of the OFDM-IM to meet the diverse needs of 5G networks. Coordinate interleaved OFDM-IM (CI-OFDM-IM) that conveys the real and imaginary part of the data symbols over different subcarriers and OFDM with power distribution IM (OFDM-PIM) that transmits the data symbols over high and low power subcarriers, are two OFDM-IM based schemes with additional diversity gain and improved bit error rate (BER) performance. In this letter, we propose a novel transmission scheme named as coordinate interleaved OFDM with power distribution IM (CI-OFDM-PIM), which applies coordinate interleaving and power distribution in subcarriers to achieve a higher diversity gain. The average bit error probability (ABEP) for the CI-OFDM-PIM is derived and the superior error performance of the proposed scheme over benchmarks are shown via computer simulations.

General Form of the Power Spectral Density of Multicarrier Signals

Multicarrier modulation has been adopted in many modern communication systems because of its many appealing properties. Its relatively high out-of-band radiation has spurred many techniques to shape the power spectral density (PSD) of multicarrier signals, including spectral precoding and pulse shaping. We derive a general expression for the PSD which is valid for subcarrier-specific pulses and for spectral precoders operating over multiple blocks. The only statistical assumptions on the vector-valued sequence modulating the active subcarriers are wide-sense cyclostationarity and properness. The applicability of the result to a number of popular multicarrier formats is discussed.

Asymptotic performance of FBMC-PAM systems in frequency-selective Rayleigh fading channels

This paper deals with the performance analysis of the recently introduced FBMC-PAM system, a multicarrier scheme that represents a promising alternative to OFDM to meet the requirements of cognitive radio or machine type communications. An analytical expression of the symbol error-rate (SER) of the single-tap sub-channel (SC) equalizer designed as the inverse of the complex gain that multiplies the symbol of interest at the output of the filter matched to each subcarrier, is derived. Specifically, an asymptotic (i.e., for a large number of subcarriers) analytical expression of the SER of the SC receiver operating in frequency-selective Rayleigh fading channel is obtained and compared via simulation results. Moreover, a simple approximate closed-form expression of the asymptotic SER of the SC receiver is obtained by taking into account, for each active subcarrier, only inter-symbol interference (ISI) terms due to the considered subcarrier and adjacent subcarriers, and, moreover, inter-carrier interference (ICI) terms due to adjacent subcarriers.

On Deploying Secondary Networks in Co-Channel Bands with DTV Networks

This paper explores the feasibility and scope of co-channel coexistence between a low data rate Orthogonal Frequency Division Multiplexing (OFDM) secondary network and a digital television (DTV) broadcast network operating in the same area. Since DTV receivers do not transmit, neither the presence nor the impact of interference from the secondary network on these DTV receivers can be known. Consequently, conventional spectrum sensing techniques cannot be employed effectively. This paper proposes to compensate the lack of feedback by combining the available information about population distribution in the region - ranging from coarse population density estimates to accurate geospatial maps - with better estimation of interference using a data-driven technique called Ordinary Kriging. Radio Environment Maps constructed using the Kriging method from field-reported observations from the secondary nodes are used to obtain better accuracy than the conventional statistical models. The estimates are then used to compute communication parameters for each secondary link in terms of power, number of active subcarriers, and active time slots, while keeping the average interference in the DTV network within threshold bounds. Thus, the viability of such co-channel license-exempt operation of secondary networks in the DTV black and grey spaces for low data rate applications in densely populated urban areas and higher data rate capabilities for sparsely populated rural areas is established.

A novel orthogonal frequency division multiplexing with index modulation waveform with carrier frequency offset resistance and low peak‐to‐average power ratio

SummaryIn this paper, we propose a novel orthogonal frequency division multiplexing (OFDM) scheme with high carrier frequency offset (CFO) resistance and low peak‐to‐average power ratio (PAPR). In this scheme, we consider a hybrid model with two subblock types, namely, pilot subblocks and standard subblocks. In pilot subblocks, active subcarriers are utilized for PAPR reduction while inactive carriers generated by the index modulation (IM) are utilized for the coarse CFO estimation. For standard subblocks, we consider unique subcarrier activation patterns (SAPs) with high‐diversity IM to enhance the bit error performance of the overall system. Additionally, the inactive data tones in standard subblocks are utilized for fine CFO estimation, which enhances the CFO estimation quite significantly. Furthermore, in this paper, we show that proposed hybrid OFDM‐IM (H‐OFDM‐IM) scheme can outperform conventional OFDM‐IM and classical OFDM both in CFO estimation and PAPR reduction without requiring transmission of any side information. Finally, we show both mathematically and through computer simulations that proposed H‐OFDM‐IM can achieve a satisfactory bit error rate (BER) performance under high CFO scenarios.

Group‐indexed orthogonal frequency division multiplexing index modulation aided performance trade off

In this study, a novel group-indexed orthogonal frequency division multiplexing index modulation (OFDM-IM) scheme is proposed to achieve a tradeoff between spectral efficiency (SE) and bit-error-rate (BER) performance. In the proposed scheme, the total subcarriers in a group are divided into subgroups, and additional bits are transmitted by subgroup indexing, unlike the conventional OFDM-IM scheme, which uses index bits to select active subcarriers. With the proposed scheme, the additional degree of freedom provided by the number of active subgroups selected provides a tradeoff between spectral efficiency and BER performance. Decoding is performed in steps to reduce computional complexity in the decoder design. Simulaton results show that the number of active subgroups selected influences the proposed scheme's performance in terms of energy efficiency, spectral efficiency, and BER performance.

Coherent Optical OFDM With Index Modulation for Long-Haul Transmission in Optical Communication Systems

In this paper, it is proposed to enhance coherent optical orthogonal frequency-division multiplexing (OFDM) system by using index modulation in terms of bit error rate (BER) and spectral efficiency for different levels of the fiber nonlinearity. In the coherent optical OFDM with index modulation system (CO-OFDM-IM), the information is conveyed not only through activated subcarriers but also by indices of the active subcarrier that are dynamically updated at the transmitted symbol rates. Unlike the conventional coherent optical OFDM (CO-OFDM) system, adjusting the number of the active subcarriers in the CO-OFDM-IM system provides improved BER performance and reduction the signal peak-to-average-ratios, when the fiber nonlinearity becomes significant. The simulation results illustrate that the proposed CO-OFDM-IM system has better BER performance when the proposed CO-OFDM-IM system and the CO-OFDM system have similar spectral efficiency. Also, the CO-OFDM-IM system can provide better spectral efficiency than the CO-OFDM system at the low-to-medium order modulation such as BPSK, QPSK, and 16QAM.