Uncoded Orthogonal Frequency Division Multiplexing Research Articles

A Generalized Space-Frequency Index Modulation Scheme for Downlink MIMO Transmissions With Improved Diversity

Multidimensional Index Modulations (IM) are a novel alternative to conventional modulations which can bring considerable benefits for future wireless networks. Within this scope, in this paper we present a new scheme, named as Precoding-aided Transmitter side Generalized Space-Frequency Index Modulation (PT-GSFIM), where part of the information bits select the active antennas and subcarriers which then carry amplitude and phase modulated symbols. The proposed scheme is designed for multiuser multiple-input multiple-output (MU-MIMO) scenarios and incorporates a precoder which removes multiuser interference (MUI) at the receivers. Furthermore, the proposed PT-GSFIM also integrates signal space diversity (SSD) techniques for tackling the typical poor performance of uncoded orthogonal frequency division multiplexing (OFDM) based schemes. By combining complex rotation matrices (CRM) and subcarrier-level interleaving, PT-GSFIM can exploit the inherent diversity in frequency selective channels and improve the performance without additional power or bandwidth. To support reliable detection of the multidimensional PT-GSFIM we also propose three different detection algorithms which can provide different tradeoffs between performance and complexity. Simulation results shows that proposed PT-GSFIM scheme, can provide significant gains over conventional MU-MIMO and GSM schemes.

Performance analysis of uncoded and SFBC OFDM system over composite fading channels

This paper presents the performance of uncoded and space-frequency block coded (SFBC) orthogonal frequency division multiplexing (OFDM) system for two recently proposed composite fading model i.e. Fisher–Snedecor (F) and κ-μ/gamma. Both models are adequate for wearable communication system. In this paper, we derive the exact and approximate expressions of average bit error rate (ABER) for uncoded and SFBC OFDM system using fast Fourier transform (FFT) and fractional Fourier transform (FrFT). Further, these generalized expressions reduce to special cases of both composite fading models. Particularly, M-ary quadrature amplitude modulation (MQAM) and M-ary phase-shift keying (MPSK) modulation schemes are considered to derive ABER expressions. The numerical results are compared with the previous results of Rayleigh and Nakagami-m distribution. Also, the Monte Carlo simulation results are provided to crosscheck the validity of derived expressions.

Performance analysis of concatenated BCH and convolutional coded OFDM system

ABSTRACT Orthogonal Frequency Division Multiplexing (OFDM) is a widely used technique for wireless communications. But uncoded OFDM is not sufficient by itself, that is why channel coding is included to increase the system performance. In this study, concatenated Bose Chaudhuri Hocquenghem (BCH) and Convolutional Coded (CC) OFDM system is investigated for multipath fading channel with Additive White Gaussian Noise (AWGN). The simulation results show that the proposed concatenated code needs lower Signal-to-Noise Ratio (SNR) when compared with single BCH code, single convolutional code and even with other concatenated systems. Throughout the simulations BCH coding is performed with 128, 256, 512 Fast Fourier Transform (FFT) lengths; whereas convolutional coding is performed with 1/2, 1/3 coding rates. Furthermore, interleavers are added to the system to prevent the burst errors that occur over the channel. With the proposed system, the best result is obtained by using BCH(511,340) and CC(3,1,7) concatenation which is 8.2 dB SNR value for 10−3 Bit Error Rate (BER). This result is very close to ideal AWGN channel value, which is 8 dB for 10−3 BER.

Null Subcarriers Assisted Impulsive Noise Mitigation for In-Vehicle Power Line Communication in the Presence of Narrowband Interference

Impulsive noise (IN) is one of the crucial affecting factors of in-vehicle power line communication (PLC) systems, and therefore an effective IN mitigation scheme is required to enhance the performance significantly. Narrowband interference (NBI) from various nearby radios often has great impact on the IN reconstruction and mitigation. In this paper, IN mitigation over uncoded orthogonal frequency division multiplexing (OFDM) PLC systems is investigated considering the impact of NBI. A null subcarriers assisted IN mitigation scheme is proposed to mitigate IN in the scenarios of NBI absence and NBI presence. The IN vector is reconstructed at the proposed receiver first, and then canceled out from the received signal. Theoretical analysis shows that the proposed scheme outperforms the blanking method. Also, the implementation of pre-joint NBI/IN mitigation with the aid of null subcarriers in the proposed scheme can combat the impact of intensive NBI, and achieve a near-optimal bit error rate performance with no iterations. The effectiveness of the proposed mitigation scheme is validated by simulation results, which is applicable to the vehicular environments under the disturbance of joint IN and NBI.

Implementation of convolution coded OFDM through different channel models on SDR platform

With the tremendous growth in the demand for high-speed reliable wireless communication systems worldwide, orthogonal frequency division multiplexing (OFDM) technique holds the promise for achieving the spectral efficiency and high data capacity requirements and with further coding of OFDM using forward error correction (FEC) codes minimizes the error probability minimizes over the crowded noisy channel by systematic addition of redundant bits. Further incorporating coded OFDM with Software Defined Radio (SDR) platform provide solutions to implement multiple standards on same platform without any alterations in hardware because it provides the entire signal processing functions at the software. In this paper, the impact of FEC on OFDM performance over both AWGN and fading (Rayleigh and Rician) channels in term of Error Rate has been studied. The simulation of coded OFDM on SDR and analyses of its performance under different channel models has been performed in this work. In this paper via GNU Radio Companion simulations, it has been analyzed that the coded OFDM has outperformed uncoded OFDM under all the considered channel models. It has provided with flexible, low cost and reliable solutions and has ability to incorporate different wireless standards without hardware changes as compared to existing ones.

Power allocation for multiuser precoded OFDM cognitive radio

The high sidelobe leakage from the transmitted signals in orthogonal frequency division multiplexing (OFDM)-based cognitive radio (CR) has always been a subject of concern, particularly when the OFDM-based CR users and the licensed primary users (PUs) coexist in adjacent frequency bands. Orthogonal precoding in OFDM-based CR systems is an effective spectral shaping technique that can significantly reduce this sidelobe leakage in adjacent PU band(s). In this paper, we propose an optimal power allocation scheme which is augmented with orthogonal precoding-based spectral shaping for multiuser OFDM CR systems. The precoding is realized on a user basis such that the transmitted multiuser OFDM signal and its first few derivatives become time-continuous. The objective of the proposed (precoded OFDM) optimal power allocation scheme is to maximize the downlink sum data rate of all users in the OFDM-based CR system, while the interference introduced to PUs is maintained below predefined thresholds and the transmit power of CR base station transmitter is kept within its power budget. Through simulation results, we demonstrate that the proposed precoded OFDM optimal power allocation scheme can deliver much higher sum data rate of CR users in comparison to the conventional (uncoded OFDM) optimal power allocation scheme and the precoded OFDM uniform power allocation scheme. Further, whereas the computational complexity of the precoded OFDM uniform power allocation scheme is the least, the proposed precoded OFDM optimal power allocation scheme has computational advantage over the conventional uncoded OFDM optimal power allocation scheme.

Performance Assessment of OFDM System using Reed-Solomon Codes over Fading Channels

This paper study the performance of Orthogonal Frequency Division Multiplexing (OFDM) system over AWGN and Rayleigh fading channels. OFDM performs multi-carrier transmission where all the subcarriers are orthogonal to each other. The channel coding was applied to improve the OFDM system performance. Reed Solomon (RS) codes was chosen that an efficient channel coding algorithm and has intense error correction ability. Adding the benefits of multicarrier OFDM system and Reed Solomon codes to achieve the expected system performance. The coded OFDM system was modeled over and simulated using Matlab. The bit error rate (BER) and energy bit (Eb) per noise power density (No) was performed during system simulations. QPSK modulation was used to modulate the random bits. From the simulation results It can be seen that BER vs. EB/No parameter for Coded OFDM system lower than uncoded OFDM. This was because RS codes succeed to correcting the error generated along data transmission.

BER Performance of SFBC OFDM System Over TWDP Fading Channel

In this letter, the performance of uncoded and space frequency block coding (SFBC) orthogonal frequency division multiplexing (OFDM) system over two-wave with diffuse power (TWDP) fading environment is analyzed. A general closed-form expressions of average bit error rate for both $M$ -ary phase shift keying and $M$ -ary quadrature amplitude modulation are derived using moment generating function of TWDP fading distribution. Results are verified with the benchmark results available in the literature (i.e., Rayleigh and Rician). Numerical results with various TWDP fading channel parameter ( $K$ and $\Delta $ ) and with different modulation orders for uncoded and SFBC OFDM systems show excellent agreement with simulation results.

Performance Analysis of Coded OFDM for Optical Wireless Communication System

Background/Objectives: Orthogonal Frequency Division Multiplexing (OFDM) is an efficient multicarrier modulation technique suitable for optical communication due to high data rate, high spectrum efficiency. A serious problem of this system is the large dynamic range of the transmitted signal due to the nonlinear Power Amplifier (PA). To achieve high power efficiency, Peak to Average Power Ratio (PAPR) value has to be reduced. Methods/Statistical Analysis: In OFDM system, error correction code is an essential for transmission of high data rate. Recently, Low-Density Parity-Check (LDPC) is commonly used as error correcting code. Findings: Here, we present LDPC coded OFDM system to analyze the performance metrics such as Complementary Cumulative Distribution Function (CCDF), Bit Error Rate (BER), Error Vector Magnitude (EVM), Modulation Error Rate (MER) on AWGN channel with and without irregular LDPC codes. Simulation results show that the LDPC code on OFDM system to improve CCDF as well as BER performance than that of uncoded OFDM system. Simulation results also show that by modulating a signal at fixed subcarriers, the EVM decreases and MER increases as the function of Signal to Noise Ratio (SNR) increases. Application/Improvements: Hence, the advantage of LDPC, which makes it possible work in application with low BER, such as wire communication, deep space communication and disk storage industry, etc.

Combining super‐orthogonal space‐frequency trellis coding, constellation shaping by shell mapping, and OFDM for high data rate broadband mobile communications

AbstractIn this paper, we seamlessly combine three transmission techniques to design an improved performance broadband mobile communication system with two transmit antennas. In so doing, we took inspiration from the V.34 data modem toolbox designed for public switched telephone network. We modified the constellation shaping by shell mapping algorithm to apply it only to the orthogonal frequency division multiplexing (OFDM) frame sent by the first antenna, in such a way that the peak‐to‐average power ratio (PAPR) is also reduced for the second antenna. We use a 24‐point quadrature amplitude modulation (QAM) signal constellation on each subcarrier, while the reference system employs 16 QAM for the same data rate. The 24 QAM constellation is partitioned into four subsets and six rings of four two‐dimensional points each. Two adjacent OFDM frequencies form with the symbols transmitted on them the entries of a 2 × 2 matrix similar to Alamouti's scheme. Two rings corresponding to adjacent OFDM frequencies form a super‐ring. The constellation shaping algorithm is applied to shells made up of super‐rings instead of rings as usual. Super‐orthogonal space‐frequency trellis coding is used on pairs of subcarriers. The proposed system was simulated using two channel models, and the performance was evaluated against the uncoded OFDM reference system. It has both reduced PAPR, and better bit error rate and frame error rate performances. Copyright © 2014 John Wiley & Sons, Ltd.

Improving the Bit Error Rate of OFDM using Convolutional codes

In wireless applications high throughput and better transmission quality can be achieved by parallel transmission of data, one of such technique is Orthogonal Frequency Division Multiplexing (OFDM) by adding channel coding to the uncoded OFDM ,the performance can be improved. In this paper, the system throughput of a working OFDM system has been enhanced by adding convolution coding. Convolution codes are used extensively in numerous applications in order to achieve reliable data transfer, including digital audio and video, mobile, radio communication, satellite deep space network communication. They can correct even burst and random errors. Simulation is done over Additive white Gaussian Noise (AWGN) channel has 64- sub carriers each is individually modulated by 16-QAM (Quadrature Amplitude Modulation). The performance parameter used for evaluation of BER(Bit Error Rate) with AWGN channel.

Comparative Performance Analysis of Uncoded OFDM and SC-FDE Systems

In this study, we carry out theoretical analysis combined with computer simulations for performance comparisons between uncoded orthogonal frequency division multiplexing (OFDM) and single-carrier frequency-domain equalization (SC-FDE) systems on frequency-selective static channels as well as frequency-selective fading channels. We consider both zero-forcing and minimum mean square error equalizations for data detection. Theoretical evaluations for output signal-to-noise ratio and error rate performance are presented for both OFDM and SC-FDE systems. Then, error rate bounds for both systems are derived. Combining all the analytical results with computer simulations, we reach certain important conclusions on the superiority between OFDM and SC-FDE systems.

Performance Comparison of Uncoded OFDM Systems with Trellis Shaping over Nonlinear Channels

Orthogonal frequency-division multiplexing (OFDM) can achieve remarkable performance in terms of spectral efficiency, but its power amplifier efficiency becomes poor because of its high peak-to-average power ratio (PAPR) nature of transmitted signals. Trellis shaping is an effective method to reduce peak and average power without signal distortion at the cost of transmitter complexity and its performance depends on the metric used for the trellis decoder. Several metrics, defined in either time or frequency domain, have been proposed in the literature. In many practical OFDM systems, however, slight nonlinearity is tolerable as long as the resulting distortion meets the system requirements such as bit error rate (BER) and spurious level of transmitted signals. In this paper, we make a thorough performance comparison of trellis-shaped uncoded OFDM systems with different metrics in terms of complexity, achievable PAPR, and resulting BER as well as power spectrum in a practical channel where the signal envelope is distorted by a power amplifier (PA). Our study shows that the metric designed in frequency domain is mostly favorable in such a scenario.

Mask Compliant Precoder for OFDM Spectrum Shaping

In this letter, a precoder for shaping the spectrum of orthogonal frequency division multiplexing (OFDM) is presented. Unlike existing methods that focus on minimizing or forcing the sidelobes to zero, the proposed precoder is designed with the goal of controlling the out-of-band emissions beneath a particular power level without impacting the bit error rate (BER) performance. A small distortion is added to the information symbols forcing the emitted spectrum under a prescribed radio frequency mask. Obtained results show that the BER performance of the proposed precoder is similar to that of uncoded OFDM under relaxed masks. Moreover, the effect of the proposed precoder on edge subcarriers is investigated. Error vector magnitude (EVM) per subcarrier shows the designed precoder introduces more distortion on edge subcarriers. Consequently, the proposed precoder when combined with adaptive modulation provides further improvement in BER performance under strict masks.

Performance evaluation of variable transmission rate OFDM systems via network source coding

Studies related to the network source coding have addressed rate-distortion analysis in both noiseless and noisy channels. However, to the best of authors’ knowledge, no prior work has studied network source coding in the context of orthogonal frequency division multiplexing (OFDM) systems. In addition, the system performance using network source coding schemes also remains unknown. In this article, we first propose two variable transmission rate (VTR) OFDM systems by utilizing network source coding schemes, and then evaluate the system performance of these two proposed VTR-OFDM systems. For the proposed VTR single-input single-output OFDM system, we employ the concept of a network with intermediate nodes to develop a 3-stage encoder/decoder, and the proposed encoder provides three different coding rates from 0.5 to 0.8. As for the proposed VTR multi-band OFDM system, two correlated sources are simultaneously transmitted using the multiterminal source coding schemes, and two sources are encoded by different coding rates from 0.25 to 0.5. Finally, compared with the traditional uncoded OFDM system, the proposed VTR-OFDM systems have at least 1 to 4 dB gain in signal-to-noise ratio to achieve the same symbol error rate in the additive white Gaussian noise channel.

ICI Mitigation Schemes for Uncoded OFDM over Channels with Doppler Spreads and Frequency Offsets – Part I: Review

Orthogonal frequency-division multiplexing (OFDM) suffers from inter-carrier interference (ICI) if the channel has the Doppler spread and/or frequency offset. Polynomial cancellation coding (PCC), symmetric cancellation coding (SCC), and windowing are simple, but effective, schemes to mitigate the ICI in the OFDM system without error-correcting coding. In this two-part paper, we derive approximations to the signal-to-interference plus noise power ratio (SINR) attainable by PCC, SCC, and windowing and compare their performances in terms of SINR. To this end, in this first part, we review these known schemes and discuss some of interrelationships between them.

OFDM With Iterative Blind Channel Estimation

A new blind channel estimation technique is presented for uncoded orthogonal frequency-division multiplexing (OFDM) systems. Instead of using pilots to sound the channel, a decision algorithm first makes primary estimates of the data symbol for each subcarrier based on a constrained linear minimum mean square error (MMSE) criterion. Then, these estimates are applied to optimal MMSE channel estimation. The technique requires only one value from the time–frequency correlation of the channel transfer function. Performance is evaluated by simulation so that comparison can be made with known optimal coherent/differential detection. Compared with known decision-directed Kalman-based estimation and two pilot-aided OFDM schemes (block pilots and comb pilots), the presented technique performs better for regions with mid to high signal-to-noise ratios (SNRs). Its robustness to the time variation of the channel is also quantified by simulation, showing only small degradation in performance relative to the quasistatic case of wireless local area network (WLAN) systems. Finally, the impact of covariance assumptions in the channel modeling is quantified using simulation, offering a feel for the performance with mismatch between the channel model and the receiver assumptions.

Convergence of the Complex Envelope of Bandlimited OFDM Signals

Orthogonal frequency division multiplexing (OFDM) systems have been used extensively in wireless communications in recent years; thus, there is significant interest in analyzing the properties of the transmitted signal in such systems. In particular, a large amount of work has focused on analyzing the variation of the complex envelope of the transmitted signal and on designing methods to minimize this variation. In this paper, it is established that the complex envelope of a bandlimited uncoded OFDM signal converges weakly to a Gaussian random process as the number of subcarriers goes to infinity. This shows that the properties of the OFDM signal will asymptotically approach those of a Gaussian random process over any finite time interval. The convergence proof is then extended to two important cases, namely, coded OFDM systems and systems with an unequal power allocation across subcarriers.

Symbol Error Performance Analysis of OFDM Relaying System with Subcarrier Mapping Scheme

This letter presents a study on statistical and symbol error performance analysis of the two-hop orthogonal frequency division multiplexing (OFDM) relay link with amplify-and-forward (AF) protocol and subcarrier mapping (SCM) scheme. Closed-form expressions for probability density function (PDF) and symbol error probability (SEP) of the signal-to-noise ratio (SNR) for the kth mapped subcarrier pair link (k-MSPL) are provided. Our asymptotic SEP analysis reveals that the traditional SCM which can maximize the capacity can not improve the average SEP for uncoded OFDM system, however the average SEP can be decreased significantly by dropping several worst links.

Coded cooperation diversity for uncoded oversampled OFDM systems

Recently, user-cooperation diversity has been introduced as an effective scheme that can bring about antenna diversity in wireless networks. In this paper, we introduce a coded cooperation diversity technique for single antenna uncoded orthogonal frequency division multiplexing (OFDM) systems, in which implementation of coded cooperation is provided by the oversampling potential in OFDM, instead of using extra channel coding. In fact, zero-padding followed by IFFT in OFDM is similar to oversampling and could be an alternative to applying correction codes. Furthermore, we use this oversampling to implement an iterative receiver at the partner terminal. This receiver works based on the nonuniform sampling theorem for reconstructing of lost symbols. The lost symbols appear at the partner terminal of cooperative network because of dividing each OFDM block into two segments through the puncturing at the user terminal of the cooperative network. We provide simulation results for our proposed scenario, and observe significant gains over the non-cooperative oversampled OFDM systems without any need whatever for using extra channel coding.