Channel estimation for OFDM-IM systems

With the rapid development of the information industry, the amount of data for communications is growing, and people are demanding communication of higher performance and faster speed. The index modulation (M) technique attracts great attention from scholars due to its high spectral and energy efficiency. Recently, a new technique for implementing index modulation using orthogonal frequency division multiplexing (OFDM) subcarriers has been proposed. This technique is called OFDM with index modulation (OFDM-IM). Instead of activating all subcarriers, OFDM-IM technology selectively activates a portion of subcarriers to transmit modulation symbols. This paper first briefly introduces the OFDM - IM system. Then, the BER and BER performance of OFDM-IM under different 5G mainstream modulation modes and different detection, such as ML and LLR, are studied. Finally, the OFDM-IM system performance with (7, 4) hamming code is studied. Compared to the classical OFDM system, the OFDM-IM system can improve performance.

Orthogonal Frequency Division Multiplexing with Index Modulation (OFDM-IM) is an effective communication technology that utilizes subcarrier indices to transmit information in addition to the information traditionally modulated on active subcarriers. However, OFDM-IM suffers from high peak-to-average power ratio (PAPR), which can result in signal distortion and hence system performance degradation. One of the techniques used to alleviate the PAPR in OFDM-IM is the selected mapping (SLM) technique. The effectiveness of the SLM PAPR reduction technique is affected by OFDM-IM system parameters, such as the number of subcarriers and subcarrier activation ratio. This paper explores the impact of these OFDM-IM system parameters on the PAPR reduction performance of the SLM technique with different phase sequence settings. The obtained results provide an overview of the collective effect of both OFDM-IM and SLM parameters in different operation cases. The findings show that the amount of PAPR reduction is mainly affected by the number of subcarriers, and less affected by the subcarrier activation ratio. That is a greater PAPR reduction of up to 4.2 dB is achievable with relatively smaller number of subcarriers. The results show the limited effect of more complex phase vectors, hence suggesting simpler SLM parameter setting alternatives to achieve a target PAPR reduction performance. Finally, the results show the improvement in Bit Error Rate (BER) performance of OFDM-IM system employing the SLM with carefully selected PAPR reduction parameters.

In this paper, we propose a secure index and data symbol modulation scheme for orthogonal frequency division multiplexing with index modulation (OFDM-IM) systems. By exploiting the notion of the channel reciprocity in time division duplexing mode over wireless channels for shared channel state information as a secret key, we investigate randomized mapping rules for index modulation as well as data symbol modulation. Due to the randomized mapping rules for index and data symbol modulation in OFDM-IM, an eavesdropper is not able to correctly decide message bits even though active subcarriers and their symbols are correctly estimated. In particular, we exploit a characteristic of OFDM-IM which uses a fraction of subcarriers for transmissions to enhance security of data symbol modulation. In addition, to design a set of mapping rules for data symbol modulation, we investigate both a random-selection-based set and a bit-mismatch-based set. Through the analysis and simulation results, we demonstrate that the proposed scheme based on the randomized mapping rules for index modulation and data symbol modulation has a better performance than an existing scheme (modified for OFDM-IM) in terms of bit error rate (BER) and successful attack probability. In particular, we can show that the BER at an eavesdropper is much higher if the bit-mismatch-based set of mapping rules is used.

In this letter, performance analysis of orthogonal frequency division multiplexing with index modulation (OFDM-IM) is presented in term of bit error rate (BERs). The analysis considers its performance under two impairments, symbol time offset (STO) and carrier frequency offset (CFO) in frequency-selective fading channel. As orthogonal multicarrier system, OFDM-IM is subject to both inter-symbol interference (ISI) and inter-carrier interference (ICI) in a frequency-selective fading channel. OFDM-IM is a new multicarrier communication system, where the active subcarriers indices are used to carry additional bits of information. In general, in the previous existing works, OFDM-IM are evaluated only for near-ideal communication scenarios by only incorporating the CFO factor. In this work, the OFDM-IM performance is investigated and compared with conventional OFDM in the presence of two impairments, STO and CFO. Simulation results show that OFDM-IM outperforms the conventional OFDM with the presence of STO and CFO, especially at high SNR areas.

In this paper, we propose an orthogonal frequency division multiplexing with index modulation (OFDM-IM) based on high-dimensional spherical codes. Unlike the traditional OFDM-IM system, which conveys the traditional M-ary constellation symbol on the active subcarriers, we use the adjacent subcarriers in the sub-block as a whole to be modulated based on high dimensional spherical code. Since the Euclidean distances between the constellation points increase by using high dimensional modulation, the performance of the OFDM-IM system is improved. We derive the general form of symbol error rate (SER) for the proposed OFDM-IM system with the maximum likelihood (ML) detection. The simulation results show that the bit error rate (BER) and SER of our proposed scheme is better than that of the traditional OFDMIM system. We also investigate the effect of channel estimation error on our proposed scheme.

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.

Jamming is a simple but efficient attack method that challenges communications while traditional physical-layer defence solutions are limited. Recently, index modulation (IM) is proposed to enable index transmission in addition to data symbol transmission, which gains resistance against jamming but with limited efficacy. This letter proposes a coding split and adjustment (CSA) framework for low-density parity-check (LDPC) coding assisted orthogonal frequency division multiplexing with IM (OFDM-IM) to enhance its anti-jamming capability. CSA splits the traditionally single system code rate into an index code rate and a data code rate, each independently adjusted based on jamming impacts to error performance. Simulation results verify that OFDM-IM with CSA exhibits superior error performance over traditional OFDM-IM and OFDM in three investigated jamming scenarios. Moreover, this letter reveals that the CSA assisted OFDM-IM can still recover signals while OFDM-IM and OFDM completely fail under high jamming power.

Next generation of mobile networks is the key area of research with increasing demand for new applications and use cases. In order to satisfy the applications with high data rate, high reliability, and low latency, multicarrier transmission through index modulation (IM) has been considered as a promising candidate owing to its flexible structures. In general, existing IM schemes are only evaluated for ideal communication scenarios. In this work, error performance of three frequency domain IM schemes are investigated and compared in the presence of carrier frequency offset (CFO) and in-phase/quadrature (IQ) imbalance. Orthogonal frequency division multiplexing with IM (OFDM-IM), OFDM with generalized IM (OFDM-GIM), and OFDM with subcarrier number modulation (OFDM-SNM) are assessed. It is shown via computer based simulations that although OFDM-GIM and OFDM-SNM provide higher spectral efficiency compared with OFDM-IM, they are more sensitive against RF impairments. Among them, OFDM-IM provides the best error performance under CFO and IQ imbalance.

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.

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.

In this paper, we investigate the estimation of fast-fading LTE downlink channel in high-speed applications of LTE Advanced. A robust channel estimation and interpolation algorithm is essential in order to adequately track the fast time-varying channel response. Using Support Vector Machine Regression (SVR) and exploiting kernel features, we develop an extended algorithm to jointly estimate the complex-valued channel frequency response in time and frequency domains from the transmission of known pilot symbols. This paper integrates both channel estimation at pilot symbols and interpolation at data symbols into the complex SVR interpolation method under very high mobility condition. The Bit Error Rate (BER) and Mean Square Error (MSE) performances of our fast-fading channel estimation scheme is demonstrated via simulation for LTE Downlink with 64-QAM modulation and 500 Km/h velocity.

Orthogonal frequency division multiplexing with index modulation (OFDM-IM) is an extension of the traditional OFDM modulation scheme. Its key idea is to adopt the activated subcarrier index to carry extra information. In this paper, we propose a new optical multi-input multiple-output (MIMO) visible light communication (VLC) system based on OFDM-IM and generalized light-emitting diode (LED) index modulation (GLIM). The proposed scheme utilizes the subcarrier index selection to improve the system flexibility and adopts the LED index modulation to increase the spectral efficiency, which also avoids the Hermitian symmetry processing. Simulation results show that the proposed scheme achieves better bit error rate (BER) performance than the traditional GLIM scheme. Moreover, the proposed scheme can flexibly select the number of the active subcarriers according to the channel condition and ensure better system performance.

To achieve reasonable bit error rate (BER) perfor- mance, accurate channel state information (CSI) is required for linear minimum mean squared error turbo equalizer(LMMSE- TE), which is hard to be obtained under the circumstance that long training sequence is not available or channel is time-varying. Soft and hard iterative channel estimation (HICE) algorithms were proposed as fine channel estimation schemes after coarse estimation. In this paper, a new iterative channel correction (ICC) algorithm that belongs to the HICE based on the least mean square (LMS) algorithm is proposed. A cost function using hard decision variables on the output of TE is formed. We show that the gradient descent (GD) algorithm is effective to iteratively minimize the cost function. Furthermore, the iterative step-size to guarantee the convergence for the ICC is analyzed, the mean squared error (MSE) lower bound of the estimated channels for the ICC is derived and validated by the simulations. Our simulations show that, given an imperfect CSI with MSE below the upper bound, the LMMSE-TE incorporated with the ICC yields small performance degradation compared to that with perfect CSI, while the traditional LMMSE-TE suffers from severe error floor even with more iterations, thus showing the robustness of our proposed algorithm to the imperfect channel estimations.

The Channel estimation in Vehicle-to-Vehicle (V2V) communications is critical due to the time-varying characteristics of the V2V channel. Hence, the authors propose a hybrid approach towards channel estimation by combining Pilot Symbol Aided Channel Estimation (PSACE), Improved Crow Search Algorithm (ICSA), and Decision Directed Channel Estimation (DDCE) by employing Weight Regularized Convolutional Neural Network (WCNN). Hybrid channel estimation by combining the properties of PSACE and DDCE. PSACE suffers from pilot overhead issue due to the demand of huge number of pilots for channel estimation. On the other hand, DDCE suffers from error propagation in fast fading environments. The proposed system employs an ICSA and WCNN to overcome the issues faced by channel estimation methods. Further, the proposed strategy replaces the analytical modeling of the channel with the help of the neural network. Optimized pilots obtained from ICSA along with data symbols are fed to WCNN to estimate the channel in a non-stationary environment. Finally, the performance of the proposed method is evaluated in terms of Bit Error Rate, Mean Square Error, and Packet Error Rate and the simulations are carried out under various V2V scenarios for testing its sturdiness.

Recently, orthogonal frequency division multiplexing (OFDM) with index modulation (OFDM-IM) as well as frequency and quadrature amplitude modulation (FQAM) have been proven to be promising techniques for the multicarrier transmission over frequency selective fading channels by successfully mitigating the induced interference due to multipath, while at the same time boosting the overall spectral efficiency. In this paper, we present a thorough study of OFDM-IM and FQAM schemes and conduct a comparison between them and the classic OFDM system. Different configurations are considered and evaluated in terms of their spectral efficiency and error performance, capitalizing on recent results from the literature.