Conventional OFDM Research Articles

Spectrally Efficient Frequency Division Multiplexing With Index-Modulated Non-Orthogonal Subcarriers

In this letter, we propose a novel index-modulated non-orthogonal spectrally efficient frequency-division multiplexing (SEFDM) scheme, in order to attain a higher bandwidth efficiency than the conventional orthogonal frequency-division multiplexing (OFDM) and SEFDM counterparts. More specifically, the recent index modulation concept is amalgamated with SEFDM, for the sake of reducing the effects of intercarrier interference while benefiting from SEFDM's increased spectral efficiency. We also formulate a low-complexity log-likelihood ratio-based detection algorithm, which allows the proposed SEFDM to operate in the configuration of an arbitrarily high number of subcarriers. Our simulation results demonstrate that the proposed SEFDM scheme outperforms the conventional SEFDM and OFDM, especially in a low-rate scenario.

Reduction of Peak-to-Average Power Ratio in Orthogonal Frequency Division Multiplexing System Using Haar Wavelet Approach

In Orthogonal Frequency Division Multiplexing (OFDM) based systems, with the increasing demand for data rate and reliability in Wireless communications and devices, several issues become very important like bandwidth efficiency, quality of service and radio coverage. However, OFDM is very sensitive to nonlinear effects due to the high peak-to-average power ratio (PAPR) owned by the transmitted signals and does not show robustness to spectral null channels. This paper proposes a novel BPSK OFDM system based on Haar wavelet transformation. The PAPR reduced by 10 log10 2 ≈ 3 dB at most, compared with the conventional OFDM system and shows robustness to spectral null channels, improving BER performance 3 dB at most. Finally, theoretical and simulated results of the Cumulative Distribution Functions (CDFs) of both Conventional and Proposed OFDM are compared to show that the Proposed OFDM has better performance than the Conventional OFDM.

Joint SCMA and waveform optimisation for WPDM‐based logging cable telemetry systems under sampling clock offset

This study introduces sparse code multiple access (SCMA) and wavelet packet division multiplexing (WPDM) to logging cable telemetry systems (LCTSs). The authors study the feasibility of SCMA for LCTSs and provide a WPDM-based system model, as well as the associated optimal waveform design under sampling clock offset (SCO) via a genetic algorithm. The performance advantages of the designed scheme over the conventional WPDM and orthogonal frequency division multiplexing (OFDM) are demonstrated by both analytical and simulation methods. Numerical results show the performance of the SCMA-based mapping method over quadrature amplitude modulation in terms of improved spectral efficiency. Furthermore, the new scheme outperforms conventional WPDM and OFDM in bit error rate performance and transmission rate under SCO. Such advantages with the flexible design of basis functions make SCMA and WPDM a significant candidate for LCTSs.

Fractional wavelet transform based OFDM system with cancellation of ICI

The future of wireless communication systems stimulate researchers to improve bandwidth efficiency and data rate by carrying out several analyses. The reliability of multiple input multiple output (MIMO) combined with orthogonal frequency division multiplexing (OFDM) in wireless communication is reduced by frequency error. This leads to damage of orthogonality between sub-carriers and induces inter carrier interference (ICI). To overcome this problem in conventional OFDM system, cyclic prefix is used. In this work, a novel technique named fractional wavelet transform (FrWT) is proposed. The FrWT reduces the effect of ICI in conventional system without any usage of cyclic prefix. Also, the bandwidth efficiency is improved due to its orthogonal wavelets and absence of cyclic prefix. The proposed work investigates the performance of FrWT-OFDM model and tested against carrier frequency offsets. The efficiency of ICI self cancellation technique in mitigating the effect of frequency offsets is analysed. The efficiency of proposed model is highlighted by comparing with existing FFT and wavelet based OFDM system by means of bit error rate (BER). Simulation results show reduced BER of 10−4.8 at 8 dB SNR for the proposed model with a normalized carrier frequency offset value of 0.1. Thus the effect of ICI is reduced efficiently for proposed transform in OFDM.

Optical Comb Assisted OFDM RF Receiver

Orthogonal frequency-division multiplexing (OFDM) is a widely used signaling format in wireless communication and ultrawide band radio-frequency (RF) applications. An OFDM transceiver relies on a discrete Fourier transform (DFT) processor that matches both the resolution and the bandwidth of wireless link. A conventional wideband OFDM receiver necessarily relies on high-speed analog-to-digital converters (ADC) followed by a Fourier processing core. Both fundamental and practical ADC considerations that include bandwidth-resolution tradeoff impose strict limits on high precision digitization of the wideband OFDM signal. In addition to the quantization barrier, computational complexity of the high-speed DFT processor leads to high dissipation levels in conventional receivers. This report demonstrates an alternative OFDM receiver architecture that eliminates the need for high-speed ADC and digital DFT processor. The new design is based on hybrid, optoelectronic DFT processor, capable of generating DFT coefficients of wideband RF signal in real time. We demonstrate demodulation of a 4-quadratic amplitude modulation OFDM signal with 60 subcarriers positioned within 4–15.8 GHz band and separated by 200 MHz. The error vector magnitude is measured for all channels to characterize the performance of the comb-assisted OFDM RF receiver.

Fast channel estimation and equalization scheme for offset-QAM OFDM systems.

We show that the orthogonality between signal and intrinsic imaginary interference (IMI) in offset quadrature amplitude modulation (offset-QAM) orthogonal frequency division multiplexing (OFDM) can still be maintained under a certain condition even when the timing slots of different subcarriers are misaligned. We show that the phase and velocity differences over subcarriers induced by fiber dispersion satisfy this condition. Based on this, we propose a fast channel estimation and equalization scheme without prior channel information including coarse dispersion. We investigate the proposed scheme in a 40-Gbit/s offset-16QAM OFDM experiment and 240-Gbit/s polarization-division-multiplexed offset-16QAM OFDM simulations, both over 1200-km single-mode fiber. It is shown that the proposed scheme gives better performance, reduced overhead for the training sequence, and/or lower complexity than other schemes. We also compare offset-QAM OFDM with the proposed scheme and conventional OFDM, and show that in addition to the elimination of cyclic prefix overhead, offset-QAM OFDM gives better performance and longer transmission reach for a moderate/small number of subcarriers.

Modeling and Analysis on Radio Interference of OFDM Waveforms for Coexistence Study

The fifth-generation (5G) cellular mobile communications look promising with features that can help improving consumer experience and satisfaction. To be able to provide these features, more spectrum is required according to the Shannon–Hartley theorem. Spectrum is, however, a finite and scarce resource, and it can be allocated to a new service only when the spectral coexistence with other incumbents is ensured. New waveforms for 5G that differ from the conventional orthogonal frequency-division multiplexing (OFDM) are required in order to have a superior performance in terms of out-of-band emissions and to be able to utilize the fragmented spectrum in different bands. We developed the analytical models for evaluating the out-of-band emissions of the conventional cyclic prefix (CP)-OFDM as well as its alternatives: windowed OFDM and filtered OFDM, using their signal spectral modeling. The resulting expressions for the power spectral density (PSD) and the frequency-dependent rejection (FDR) involve simple closed-form expressions or easily computable integrals. We applied the expressions to the advanced minimum coupling loss model for assessing the feasibility of the spectral coexistence between the potential 5G systems (with linearized or nonlinear power amplifier) and the incumbent radar systems. The numerical simulation results indicate that both the windowed OFDM and filtered OFDM guarantee the coexistence at the low expense of the spectrum utilization and their coexistence performance can be reduced and reversed with nonlinearity distortion of the power amplifier.

Detection and Equalization of Set-Partitioned Offset-QAM OFDM in IMDD Systems

We design the detection algorithm for orthogonal frequency division multiplexing (OFDM) based on set-partitioned offset quadrature amplitude modulation (SP-offset-QAM) in adaptively-loaded intensity modulation and direct detection (IMDD) systems. The algorithm mitigates signal-signal beating interference (SSBI), improves the equalization capability of conventional one-tap equalizers, and reduces the complexity and the required length of training sequence in the decoding of multi-dimensional SP-QAM signals. We experimentally demonstrate the adaptively-loaded SP-offset-QAM OFDM over 50-km single-mode fiber to verify the proposed algorithm. It is shown that the modified equalization significantly improves the performance while the SSBI mitigation brings benefits after transmission. The proposed decoding scheme reduces the complexity comparable to that in conventional QAM. It is also shown that the SP-offset-QAM OFDM outperforms the SP-based conventional QAM and conventional offset-QAM OFDM both at back-to-back and after 50-km transmission, when the algorithm is applied to all schemes.

A Comparison of CP-OFDM, PCC-OFDM and UFMC for 5G Uplink Communications

Polynomial-cancellation-coded orthogonal frequency division multiplexing (PCC-OFDM) is a form of OFDM that has waveforms which are very well localized in both the time and frequency domains and so it is ideally suited for use in the 5G network. This paper analyzes the performance of PCC-OFDM in the uplink of a multiuser system using orthogonal frequency division multiple access (OFDMA) and compares it with conventional cyclic prefix OFDM (CP-OFDM), and universal filtered multicarrier (UFMC). PCC-OFDM is shown to be very much less sensitive to time and frequency offsets than either CP-OFDM or UFMC. For a given constellation size, PCC-OFDM in additive white Gaussian noise (AWGN) requires 3 dB lower signal-to-noise ratio (SNR) for a given bit-error-rate, and the SNR advantage of PCC-OFDM increases rapidly when there are timing and/or frequency offsets. For PCC-OFDM no frequency guard-band is required between different OFDMA users. PCC-OFDM is completely compatible with CP-OFDM and adds negligible complexity and latency, because it uses a simple mapping of data onto pairs of subcarriers at the transmitter, and a simple weighting-and-adding of pairs of subcarriers at the receiver. The weighting-and-adding step, which has been omitted in some of the literature, is shown to contribute substantially to the SNR advantage of PCC-OFDM. A disadvantage of PCC-OFDM (without overlapping) is the potential reduction in spectral efficiency because subcarriers are modulated in pairs, but this reduction is more than regained because no guard band is required and because, for a given channel, larger constellations can be used.

Iterative Per-Vector Equalization for Orthogonal Signal-Division Multiplexing Over Time-Varying Underwater Acoustic Channels

Orthogonal signal-division multiplexing (OSDM) is a promising modulation scheme that provides a generalized framework to unify orthogonal frequency-division multiplexing (OFDM) and single-carrier frequency-domain equalization. By partitioning each data block into vectors, it allows for a flexible configuration to trade off resource management flexibility with peak-to-average power ratio. In this paper, an OSDM system is proposed for underwater acoustic communications. The channel Doppler effect after front-end resampling is modeled as a common time-varying phase on all propagation paths. It leads to a special signal distortion structure in the OSDM system, namely, intervector interference, which is analogous to the intercarrier interference in the conventional OFDM system. To counteract the related performance degradation, the OSDM receiver performs iterative detection, integrating joint channel impulse response and phase estimation, equalization, and decoding in a loop. Meanwhile, to avoid inversion of large matrices in channel equalization, frequency-domain per-vector equalization is designed, which can significantly reduce the computational complexity. Furthermore, the performance of the proposed OSDM system is evaluated through both numerical simulations and a field experiment, and its reliability over underwater acoustic channels is confirmed.

Spectral Efficiency Enhancement Based on Sparsely Indexed Modulation for Green Radio Communication

Jointly enhancing both energy efficiency (EE) and spectrum efficiency (SE) of modulation schemes becomes one of the main issues for 5G mobile communications. Recently, an indexed modulation (IM) technique provides an interesting tradeoff between EE and SE. Data can be conveyed through the combination of subcarriers pattern that can be divided between activated/non-activated subcarriers in the frequency domain. Maximum SE can be attained at half subcarrier activation, hence producing symbols with half energy of the conventional orthogonal frequency-division multiplexing (OFDM) system. In this paper, alternatively, the new concept of sparsely indexing modulation (SIM) on overall subcarrier space is clarified. Sparse (few) subcarrier activations provide much higher EE, while the combinatorial indexing of the sparse subcarriers on the overall subcarriers as a single group spans huge combinatorial space that provides approximately the same SE of the plain OFDM system. The fallacy of indexing difficulty on overall subcarrier space without grouping is resolved. Moreover, a further SE improvement is suggested by introducing permutation-based indexing and combinatorial indexing on over-complete dictionaries. Sparsely indexing represents the cornerstone, which enables compressive sensing tools to enforce IM gains. Based on the conducted simulations, the proposed SIM scheme outperforms the conventional OFDM system in terms of the error performance, the peak-to-average power ratio, and the EE with the same spectral efficiency without channel coding complexity. The proposed SIM scheme is considered one of the energy saving-oriented modulations.

Time-domain oversampled orthogonal signal-division multiplexing underwater acoustic communications.

Orthogonal signal-division multiplexing (OSDM) is a recently emerging modulation scheme which, compared to conventional orthogonal frequency-division multiplexing, can effectively lower the peak-to-average power ratio and introduce intra-vector frequency diversity. In this paper, a time-domain oversampled OSDM system for underwater acoustic (UWA) communications is designed, where each OSDM vector is equivalently transmitted over multiple virtual channels, and thus an enhanced frequency diversity gain can be achieved. Moreover, at the receiver, zero vectors and frequency-shifted Chu sequences are used for Doppler compensation and channel estimation, respectively, while low-complexity per-vector equalization is performed based on the composite channel matrix factorization. Finally, the performance of the proposed OSDM system is evaluated through both numerical simulations and a short-range field experiment, and its effectiveness over time-varying UWA channels is confirmed.

DFT-Based OFDMA System with Phase Modulation for Broadband Communication

This paper proposes, a DFT-based OFDMA with phase modulation (DFT-OFDMA-PM) system. One main advantage of the phase modulated system is the constant envelope (CE) resulted signal, i.e, 0 dB PAPR, the second advantage is the ability to improve the diversity of multipath channels. These advantages have been exploited in the proposed system of this paper. The performance of the proposed system in terms of the bit error rate (BER) is studied and investigated and compared to the previously proposed DCT-OFDMA-PM system and the conventional OFDMA system without PM via computer simulation. The key parameter that affects the performance of the PM systems, the modulation index, is also studied and the optimum value is selected using exhaustive simulation scenarios. Moreover, the PAPR is also simulated for the proposed system and compared to the conventional system. Simulation results show the significant improvement of the proposed system in terms of PAPR and BER when compared to the conventional system. The simulation results for the proposed system show the effectiveness of the proposed system for wireless broadband communications.

Joint channel estimation and turbo equalisation for MIMO-OFDM-IM systems

ABSTRACTMultiple-Input Multiple-Output (MIMO) communications are frequently employed to improve the transmitted data rate and the link quality. Index modulated orthogonal frequency division multiplexing (OFDM-IM) improves the error rate performance and the peak-to-average power ratio (PAPR) compared with those of the conventional OFDM system due to the activation of partial subcarriers. The MIMO OFDM-IM can transmit additional information bits via the indices of active subcarriers. Also, in order to reduce the transmission power of the OFDM system, the MIMO OFDM-IM scheme can be employed to approach the demanded data transmission rate and the error rate performance. Multiple-input multiple-output orthogonal frequency division multiplexing index modulation (MIMO-OFDM-IM) is an effective multicarrier transmission scheme and can be proposed as an alternative to conventional MIMO-OFDM system. In this scheme, OFDM-IM is combined with MIMO transmission to take the benefits of these two techniques. In this paper, we propose a joint channel estimation and turbo equalisation receiver for MIMO-OFDM-IM system. Some simulation examples are given to demonstrate the effectiveness of the proposed receiver.

Soliton content in the standard optical OFDM signal.

The nonlinear Schrödinger equation (NLSE) is often used as a master path-average model for fiber-optic transmission lines. In general, the NLSE describes the co-existence of dispersive waves and soliton pulses. The propagation of a signal in such a nonlinear channel is conceptually different from linear systems. We demonstrate here that the conventional orthogonal frequency-division multiplexing (OFDM) input optical signal at powers typical for modern communication systems might have soliton components statistically created by the random process corresponding to the information content. Applying the Zakharov-Shabat spectral problem to a single OFDM symbol with multiple subcarriers, we quantify the effect of the statistical soliton occurrence in such an information-bearing optical signal. Moreover, we observe that at signal powers optimal for transmission, an OFDM symbol incorporates multiple solitons with high probability. The considered optical communication example is relevant to a more general physical problem of the generation of coherent structures from noise.

Performance analysis of AO‐OFDM‐CDMA with advanced 2D‐hybrid coding for amplifier‐free LR‐PONs

All-optical orthogonal frequency division multiplexing (AO-OFDM) and optical code-division multiple-access system (OCDMA) are combined in the first analytical model, which considers subcarrier hopping by means of advanced two-dimensional (2D) hybrid-coded (2D-HC) signature. The model incorporates probabilistic subcarrier overlapping, multiple-access interferences and is tested, for the first time, over amplifier-free long-reach passive optical networks (LR-PONs) using cost-effective intensity modulations and direct detection. For the upstream direction at 40 Gb/s, AO-OFDM-OCDMA outperforms a ‘classical’ multi-channel OCDMA system for low received powers and any number of simultaneous users. In comparison to conventional 1D Walsh–Hadamard, 1D prime code and 2D prime hop system, coding with 2D-HC can improve the performance of AO-OFDM-CDMA, thus allowing a higher number of simultaneous users in LR-PON without optical amplification. From numerical simulations, the authors show that 16-quadrature amplitude modulation (16-QAM) AO-OFDM-CDMA with 45 users has comparable performance to conventional multi-channel 16-QAM coherent optical OFDM in the downstream direction and up to 58 km with 1:45 split ratio, without employing complex coherent technology. Similarly, based on the feasibility of physical implementation configuration, a budget power calculation is performed showing 108 km as maximum reachability distance for 40 Gb/s QAM signal, 1:64 split ratio when considering standard forward-error-correction.

Energy-Efficient and Secure Resource Allocation for Multiple-Antenna NOMA With Wireless Power Transfer

Non-orthogonal multiple access (NOMA) is considered as one of the promising techniques for providing high data rates in the fifth generation mobile communication. By applying successive interference cancellation schemes and superposition coding at the NOMA receiver, multiple users can be multiplexed on the same subchannel. In this paper, we investigate resource allocation algorithm design for an OFDM-based NOMA system empowered by wireless power transfer (WPT). In the considered system, users who need to transmit data can only be powered by the WPT. With the consideration of an existing eavesdropper, the objective is to obtain secure and energy efficient transmission among multiple users by optimizing time, power and subchannel allocation. Moreover, we also take into consideration for the practical case that the statistics of the channel state information of the eavesdropper is not available. In order to address the optimization problem and its high computational complexity, we propose an iterative algorithm with guaranteed convergence to deliver an upper bound and a suboptimal solution in more general cases. For some special cases, we identify the optimality condition that ensures the global optimum in our algorithm. Extensive simulation studies demonstrate the competitiveness and effectiveness of the proposed algorithmic solution over conventional OFDMA systems as well as over other existing NOMA resource allocation schemes.

Open Loop and Closed Loop Power Control Analysis on LTE

LTE downlink is using Orthogonal Frequency Division Multiple Access (OFDMA) multiple access system which have high invulnerability from multipath problem. One of the weakness of OFDM system is the high level from Peak to Average Power Ratio (PAPR) that was required higher level transmit power for maintaining the Bit Error Rate (BER) requirement. Using uplink scheme with Single Carrier FDMA (SC-FDMA) which is OFDMA modification, will be offered better level of PAPR than its conventional OFDM. The main problem of using OFDMA is the high level of PAPR, while using SC-FDMA the problem is intra-cell interference. Intra-cell or inter-cell interference is the common problem that can reduce the LTE performance. Minimizing received power for each users (UE) which is still at acceptable tolerance parameter, can be used for reducing the interference problem to another UE. Power control is the appropriate solution for minimizing the interference level. In this paper will be analyzed the power control using open loop and closed loop scheme at LTE network. The simulation result show that without power control schemes, the transmit power of UE is 23 dBm. While, after applying power control scheme, the transmit power is 18.8 dBm at ?=0.4 of open loop condition and 9.05 dBm at closed loop condition. Using this transmit power value as the UE power can improve the SINR performance. The SINR average value without power control scheme is only 20.38 dB which is lower than using open loop scheme is achieved 22.44 dB and 24.02 dB at closed loop scheme.

Low-Complexity PTS Scheme for PAPR Reduction in FBMC-OQAM Systems

Most of the peak-to-average power ration (PAPR) reduction techniques, such as partial transmit sequence (PTS) algorithm and selective mapping algorithm, for conventional OFDM systems are not directly applied to filter bank multi-carrier with offset quadrature amplitude modulation (FBMC-OQAM) systems because of the overlapping structure of FBMC-OQAM signals. Furthermore, the complexity of the existing algorithms based on PTS is very high for the FBMC-OQAM systems. In this letter, we propose a low-complexity PTS based on hybrid processing algorithm (H-PTS) to reduce the PAPR values of FBMC-OQAM systems. Firstly, the proposed algorithm processes phase factors of the PTS by double-layer search. As a result, the complexity of searching the optimal phase factors is simplified. Moreover, an efficient algorithm of estimating the PAPR values of the FBMC-OQAM signals is employed to further reduce computational complexity. Simulation results show that, compared with the conventional PTS algorithm and segmental PTS algorithm, the proposed H-PTS algorithm could achieve a better PAPR reduction as well as lower computational complexity.

Performance Evaluation of SFBC MIMO–OFDM Using FrFT Under TWDP Fading Channel

Multiple-input–multiple-output orthogonal frequency division multiplexing (MIMO–OFDM) is a promising 4G technology to increase data rate and capacity of a system. Further, the space–frequency-block-coding (SFBC) technique in MIMO–OFDM system has increased reliability of system in high mobility channels. Fourier transform in conventional OFDM system can obtain either in time or frequency domain, hence it can be acceptable to reduce inter-carrier-interference while it fails for doubly dispersive channels where both the domain are changing simultaneously. Another mathematical tool named as fractional Fourier transform (FrFT) cancels the effect of rapidly varying time–frequency-distortions by considering new rotating axis \(\left( {u,v} \right)\) at an angle \(\alpha\) from conventional time–frequency plane. In this paper, the performance of SFBC encoded MIMO OFDM system using FrFT in place of FFT has been analyzed under two-wave-diffused-power (TWDP) frequency selective fading channel. In this paper, the novel closed form expression for average bit-error-rate (BER) for coded and un-coded FrFT based MIMO–OFDM system under TWDP has also been presented in this paper. The results carried out using simulation shows that the proposed model achieves BER of 10−2 at 5 dB SNR whereas conventional achieves the same at 14 dB hence, concluded that the novel system performance improved by nearly 9 dB.