Orthogonal Frequency Division Multiplexing With Index Modulation System Research Articles

Spectral efficiency improvement in cooperative RIS aided OFDM-IM system with hybrid indexing

Increasing demand for wireless communication leads to significant challenges in achieving high spectral efficiency (SE) and energy efficiency (EE). Orthogonal Frequency Division Multiplexing with Index Modulation (OFDM-IM) is one of the considerations, although it has the inherent disadvantage of a trade-off between SE and EE. In this work, frequency-dependent control on reconfigurable intelligent surfaces (RIS) and indexing to activate or deactivate the elements of RIS have been proposed to improve the SE of OFDM-IM systems. The Upper bound pairwise error probability has been derived and confirmed by simulation. The proposed work enhances the SE and EE compared to the conventional OFDM-IM systems. For instance, SE loss of 0.5bpcu in an OFDM-IM system with quadrature phase shift keying (QPSK) compared to an OFDM system has been compensated by the proposed work by maintaining EE.

Strategic Optimization and Application Analysis of Orthogonal Frequency Division Multiplexing Index Modulation Systems

Orthogonal Frequency Division Multiplexing (OFDM) is a type of Multi-Carrier Modulation that operates by dividing a single channel into numerous orthogonal subchannels. By doing this, it facilitates the transformation of high-speed digital signals into parallel subdata streams. These streams are then modulated onto the subchannels, thereby enabling efficient transmission. OFDM with Index Modulation (OFDM-IM) further enhances this process by combining the benefits of OFDM with the high spectral efficiency of spatial modulation techniques, leading to significant improvements in the bit-error rate performance of the system. Despite the advantages of OFDM-IM systems, several challenges exist. For instance, the presence of inconsistent sampling rates can result in amplitude distortion. Furthermore, a high peak-to-average power ratio can adversely impact the efficiency of the RF power amplifier. These problems necessitate the exploration of diverse solutions to improve the performance of OFDM-IM systems. The focus of this thesis is to investigate and assess different strategies aimed at addressing these issues within OFDM-IM systems. We will explore these strategies, compare their effectiveness, and offer comprehensive recommendations on their optimal applications based on specific system requirements and scenarios. In doing so, the goal is to pave the way for more effective and efficient implementation of OFDM-IM systems in diverse contexts.

Optimized Greedy Detection for OFDM-IM Systems

Recently, a new transmission method has been proposed for power-efficient wireless communication systems called Orthogonal Frequency Division Multiplexing with Index Modulation (OFDM-IM). This method offers high spectral efficiency with good error performance. The greedy detection (GD) scheme of subcarrier indices is often used to reduce the complexity of the OFDM-IM system in comparison to other relevant detectors. In this letter, an optimized greedy detection (OGD) scheme is proposed. The received subblock is divided into mini-subblocks to decrease the risk of error. The Total Symbol Error Probability (TSEP) is derived using a closed-form expression to evaluate the performance. Simulation and analytic results demonstrate the superiority of the proposed detector.

A Modified Selective Mapping Technique for Peak-to-Average Power Ratio Reduction in Orthogonal Frequency Division Multiplexing with Index Modulation Systems

In this study, we propose a modified selective mapping (SLM) technique to reduce the Peak-to-Average Power Ratio (PAPR) of Orthogonal Frequency Division Multiplexing with Index Modulation (OFDM-IM) systems. The main reason for using Reed–Muller and Polar codes is to protect the information bits to be transmitted while providing error correction capability and improved PAPR performance to the transmitted signal. Although OFDM-IM technology is a popular communication transmission technology recently, one of its disadvantages is that the transmitted signal has a high PAPR. The proposed method uses the Hamming weight distribution of the specific order Reed–Muller code to create a corresponding lookup table to select the subcarriers that are active in the OFDM-IM system. Moreover, it can correct errors in the information used. The proposed method also uses Polar codes as a channel encoder of the OFDM-IM system to analyze its performance in improving the PAPR. The simulation results show that method can not only improve the PAPR performance of OFDM-IM systems but also provide OFDM-IM transmission-signal error correction capacity.

Y-Shaped Net-Based Signal Detection for OFDM-IM Systems

This letter presents the power of deep learning for signal detection in orthogonal frequency division multiplexing with index modulation (OFDM-IM) systems. In this letter, we propose the Y-shaped net-based scheme with fully connected layers (Y-FC) and with bidirectional long short-term memory units (Y-BLSTM) to further optimize the data reception. In the Y-shaped scheme, the active indices and the constellation demapping are learned by two parallel sub-neural networks respectively, the output features are then concatenated and flattened to obtain binary bits. The performance is evaluated under various system parameters, such as modulation methods, number of hidden neural nodes and channel states. Simulation results show that compared with the data-driven signal detection with direct input-output, the proposed scheme achieves several dB of bit-error-rate performance improvement with almost the same running time.

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.

Iterative Hard Thresholding Algorithm-Based Detector for Compressed OFDM-IM Systems

A low-complexity iterative hard thresholding (IHT) algorithm-based detector is proposed for the compressed orthogonal frequency division multiplexing with index modulation (OFDM-IM) system. To improve recovery accuracy and reduce the number of iterations, detection is embedded into the iterations of the IHT algorithm, and the block energy information is used to identify subblocks that are more likely to be error-recovered. Compared with the existing methods for improving spectral efficiency, the new method does not expand the detection search space and not need extra pseudo-inverse calculations during iterations. Theoretical and simulation results show that the proposed scheme can considerably reduce computational complexity while improving spectral efficiency and preserving significant BER performance gain.

Toward Spectral and Energy Efficient 5G Networks Using Relayed OFDM With Index Modulation

Next generation wireless networks are expected to provide much higher data throughput and reliable connections for a far larger number of wireless service subscribers and machine-type nodes, which results in increasingly stringent requirements of spectral efficiency (SE) and energy efficiency (EE). Orthogonal frequency-division multiplexing with index modulation (OFDM-IM) stands out as a promising solution to satisfy the SE requirement with a reasonable increase in system complexity. However, the EE of OFDM-IM is still required to be enhanced. Moreover, diversity gain is difficult to harvest from the frequency domain without affecting the SE for OFDM-IM systems, which hinders further reliability enhancement. In this regard, relay-assisted OFDM-IM, as a promising joint paradigm to achieve both high SE and EE, was proposed and has been studied since last year. The objectives of this study are to summarize the recent achievements of this joint paradigm, articulate its pros and cons, and reveal the corresponding challenges and future work. More importantly, we provide a full picture and insights into the implementation of this new paradigm in next generation networks.

A SLM Scheme for PAPR Reduction in Polar Coded OFDM-IM Systems Without Using Side Information

In the present article, we propose a novel selected mapping (SLM) scheme based on a Polar coding technique for peak to average power ratio (PAPR) reduction in orthogonal frequency division multiplexing with index modulation (OFDM-IM) systems. The proposed scheme by utilizing random frozen bits in polar codes generates different number of candidate sequences. Then the candidate sequence with the smallest PAPR is selected for transmission. The proposed scheme can be considered as one block in an OFDM-IM system that performs both PAPR reduction and error correction. For a fair comparison, the performance of an OFDM-IM system based on the proposed scheme is compared with a system that carries out the same operations with two separate blocks where a Polar encoder is cascaded with the one for PAPR reduction (such as conventional SLM or PTS), and a Polar coded OFDM-IM system without any PAPR reduction scheme. Moreover, based on our proposed SLM scheme, a novel receiver without using side information (SI) is proposed. This SI free receiver is based on the successive cancellation list (SCL) polar decoding scheme. Simulation results show that as the Polar code can bring both error correction and PAPR reduction capabilities by using our proposed scheme. Also, the proposed SI free receiver can achieve similar bit error rate (BER) performance as that of the ideal case in both additive Gaussian white noise (AWGN) and frequency selective channels.

Noncoherent Detection for Improving the Spectral Efficiency of OFDM-IM Systems

Today, Orthogonal Frequency Division Multiplexing (OFDM) is the most preferred and commonly used scheme for all communication systems. A new multicarrier technique called OFDM with Index Modulation (OFDM-IM) has been developed recently where the indices of the active subcarriers are used to transmit information in addition to the constellation symbols. In this article, system models of OFDM-IM and the two generalized schemes of OFDM-IM called OFDM with Generalized Index Modulation 1 (OFDM-GIM1) and OFDM with Generalized Index Modulation 2 (OFDM-GIM2) are studied. Here, noncoherent detection has been introduced for the two generalized schemes—OFDM-GIM1 and OFDM-GIM2 with the need for increasing the spectral efficiency. The probability of index error (Pie) is derived for OFDM-IM, OFDM-GIM1 and OFDM-GIM2. The developed noncoherent generalized schemes have reduced Pie and better performance with minimum Signal-to-Noise Ratio (SNR). This is clear from the simulation results that OFDM-GIM1 has less Pie than the other OFDM-IM schemes and it is around 10−1 when the required SNR is low about 8 dB.

On the Phase Sequences and Permutation Functions in the SLM Scheme for OFDM-IM Systems

In orthogonal frequency division multiplexing with index modulation (OFDM-IM), the active subcarriers can convey information bits by modulated symbols as well as their indices. OFDM-IM has attracted a great deal of attention from researchers by virtue of high energy efficiency. Unfortunately, OFDM-IM has inherited large peak-to-average power ratio (PAPR) of the classical OFDM signal, but there are few works dealing with it. Selected mapping (SLM) is a promising PAPR reduction technique that is distortionless, has good PAPR reducing capability, and can be easily adapted in OFDM-IM systems. In SLM for OFDM-IM systems, the phase sequences rotate the OFDM-IM block in the frequency domain before the inverse fast Fourier transform (IFFT) is applied. Also, permutation in the frequency domain can be easily introduced before multiplication of phase sequences in SLM. In this paper, we investigate the phase sequence set (PSS) and permutation functions in the SLM scheme for OFDM-IM systems; First, the efficiency of the permutation is analyzed as a function of the number of active subcarriers. Second, the optimal conditions for the PSS and permutation functions are derived, which is verified through simulations.

Channel estimation for OFDM-IM systems

Orthogonal frequency division multiplexing with index modulation (OFDM-IM) has been recently proposed to increase the spectral efficiency and improve the error performance of multicarrier communication systems. However, all the OFDM-IM systems assume that the perfect channel state information is available at the receiver. Nevertheless, channel estimation is a challenging problem in practical wireless communication systems for coherent detection at the receiver. In this paper, a novel method based on the pilot symbol-aided channel estimation technique is proposed and evaluated for OFDM-IM systems. Pilot symbols, which are placed equidistantly, allow the regeneration of the response of the channel so that pilot symbol spacing can fulfill the sampling theorem criterion. Our results shows that the low-pass interpolation and SPLINE techniques perform the best among all the channel estimation algorithms in terms of bit error rate and mean square error performance.

Deep Learning-Based Detector for OFDM-IM

This letter presents the first attempt of exploiting deep learning (DL) in the signal detection of orthogonal frequency division multiplexing with index modulation (OFDM-IM) systems. Particularly, we propose a novel DL-based detector termed as DeepIM, which employs a deep neural network with fully connected layers to recover data bits in an OFDM-IM system. To enhance the performance of DeepIM, the received signal and channel vectors are pre-processed based on the domain knowledge before entering the network. Using datasets collected by simulations, DeepIM is first trained offline to minimize the bit error rate (BER) and then the trained model is deployed for the online signal detection of OFDM-IM. Simulation results show that DeepIM can achieve a near-optimal BER with a lower runtime than existing hand-crafted detectors.

OFDM-IM Based Dual-Hop System Using Fixed-Gain Amplify-and-Forward Relay With Pre-Processing Capability

Orthogonal frequency-division multiplexing with index modulation (OFDM-IM) has recently attracted many researchers’ attention due to its superior spectrum efficiency and reliability compared to the traditional OFDM. Cooperative decode-and-forward relaying has been incorporated with OFDM-IM, which provides a higher energy efficiency and better network coverage. However, it might not be feasible in realistic applications, owing to the high system complexity and transmission delay rendered by complex decoding and channel estimation procedures. Therefore, in this paper, we propose a fixed-gain (FG) amplify-and-forward (AF) relay-assisted OFDM-IM system, which does not need to perform complex decoding and channel estimation at the relay, but only requires a pre-processing capability at the relay, e.g., cyclic prefix removal and re-insertion. Therefore, the system complexity can be reduced and the forwarding delay as well as power consumption caused by processing at the relay also decline. We analyze the average outage probability, block error rate, and achievable rate of the proposed system and verify all analysis by numerical results. The proposed FG AF relay-assisted OFDM-IM provides a simple solution to the implementation of OFDM-IM in new network paradigms, where nodes are simple, power limited, and/or complexity limited.

Distributed Processing for Multi-Relay Assisted OFDM With Index Modulation

Orthogonal frequency-division multiplexing with index modulation (OFDM-IM) has become a high-profile modulation scheme for the fifth-generation (5G) wireless communications and has thus been extended to multi-hop scenarios in order to improve the network coverage and energy efficiency. However, the extension of OFDM-IM to multi-relay cooperative networks is not trivial, since it is required that a complete OFDM block should be received and decoded as an entity in one node. This requirement prevents the employment of multiple relays to forward fragmented OFDM blocks on individual subcarriers. In this regard, we propose a distributed processing scheme for multi-relay assisted OFDM-IM, by which multiple relays are selected to forward signals in a per-subcarrier manner to provide optimal error performance for two-hop decode-and-forward (DF) OFDM-IM systems. Specifically, a single selected relay only needs to decode partial information carried on certain active subcarriers and forward just as for traditional OFDM systems without IM. After receiving all signals on active subcarriers forwarded by different relays, the destination can reconstruct the complete OFDM block and retrieve the full information. We analyze the average block error rate and modulation capacity of the two-hop OFDM-IM system employing the proposed distributed DF protocol and verify the analysis by numerical simulations.

Lexicographic Codebook Design for OFDM With Index Modulation

In this paper, we propose a novel codebook design scheme for orthogonal frequency-division multiplexing with index modulation (OFDM-IM) to improve system performance. The optimization process can be implemented efficiently by the lexicographic ordering principle. By applying the proposed codebook design, all subcarrier activation patterns with a fixed number of active subcarriers will be explored. Furthermore, as the number of active subcarriers is fixed, the computational complexity for estimation at the receiver is reduced and the zero-active subcarrier dilemma is solved without involving complex higher layer transmission protocols. It is found that the codebook design can potentially provide a tradeoff between diversity and transmission rate. We investigate the diversity mechanism and formulate three diversity-rate optimization problems for the proposed OFDM-IM system. Based on the genetic algorithm, the method of solving these formulated optimization problems is provided and verified to be effective. Then, we analyze the average block error rate and bit error rate of the OFDM-IM systems applying the codebook design. Finally, all analyses are numerically verified by the Monte Carlo simulations. In addition, a series of comparisons are provided, by which the superiority of the codebook design is confirmed.

Outage Performance of Two-Hop OFDM With Index Modulation and Multi-Carrier Relay Selections

In this letter, we propose a two-hop orthogonal frequency-division multiplexing with index modulation (OFDM-IM) system, which is assisted by multiple relays and employs multi-carrier relay selection to enhance the system reliability. In particular, two commonly used multi-carrier relay selection schemes are considered, i.e., bulk and per-subcarrier relay selections, and the decode-and-forward relaying is adopted as the forwarding protocol for the proposed system. The analysis of average outage probability of the proposed system is given in closed form and substantiated by numerical results generated by Monte Carlo simulations. By the analytical and numerical results provided, we can observe that the full diversity (equal to the number of relays) can be achieved by applying both bulk and per-subcarrier relay selections in two-hop OFDM-IM systems.

Low-Complexity LLR Calculation for OFDM With Index Modulation

Orthogonal frequency division multiplexing with index modulation (OFDM-IM) is a promising multicarrier transmission technique. In OFDM-IM, only a subset of subcarriers are activated to transmit modulated symbols and the indices of the activated subcarriers are used to convey information bits implicitly. In the coded OFDM-IM system, we need to calculate the log likelihood ratio (LLR) value of each bit, whose computational complexity is extremely high due to the dependence between the subcarriers caused by the subcarrier-index modulation. In this letter, we propose a novel simplified LLR calculation algorithm for OFDM-IM. Simulation results show that the proposed algorithm achieves near-optimal coded bit error rate performance with considerably low computational complexity.

Effect of Subcarrier Activation Ratio on the Performance of OFDM-IM over Rayleigh Fading Channel

Orthogonal frequency division multiplexing with index modulation (OFDM-IM) is acquiring increasing attention due to its advantages over classical OFDM. These advantages include superior BER performance and transmit power saving since only a subset of the subcarriers is activated to transmit $M$ -QAM symbols and the remaining subcarriers are turned OFF. The vast majority of the literature studies the OFDM-IM by assuming certain subcarrier activation ratios, $r$ , together with other system parameters without figuring out the possible variation in system performance at other $r$ values. In this letter, the effect of $r$ and $M$ on the spectral efficiency (SE) and BER of OFDM-IM is investigated. An inequality is developed to maximize SE while power saving is maintained. Moreover, the BER performance of an OFDM-IM system exploiting a maximum subcarrier energy detector over multipath Rayleigh channel is analyzed. An approximate average BER expression is derived and the effect of $r$ and $M$ is discussed. Computer simulation has showed the validity of the derived expressions.

On the Optimal Tone Spacing for Interference Mitigation in OFDM-IM Systems

Orthogonal frequency division multiplexing with index modulation (OFDM-IM) has been recently proposed as an efficient technique to improve the error performance and enhance the spectral efficiency achieved by the classical OFDM. In this letter, we minimize the presence of intercarrier and intersymbol interference, experienced by OFDM-IM systems under mobility conditions and multipath scenarios, by selecting the appropriate tone spacing between adjacent subcarriers. Finally, we prove that the optimal value of tone spacing increases the system capacity, occupying only the necessary amount of bandwidth, and provide closed-form expressions for the interference power per active subcarrier.