On Orthogonal Frequency Division Multiplexing Systems Research Articles

A Full-Duplex MIMO System Based on CP-Free OFDM in Sensor Network

Recently, results have shown that full-duplex (FD) system based on orthogonal frequency division multiplexing (OFDM) is potential for wireless sensor networks. However, extending FD to further improve spectral efficiency by removing cyclic prefix (CP) with optimal canceling self-interference (SI) remains a challenge. In this paper, we analytically study the conventional FD OFDM system and proposed a CP-free system on this basis. The key point of the proposed scheme is the symbols transmitted in a unit of two identical symbols without CP. To eliminate the intersymbol interference, the disturbed part are replaced by the repeated signal at the receiver, which has the same effect as CP. Finally, an SI cancellation is down to restore the transmitted symbols. As a key of analysis, a multiple-input multiple-output (MIMO) FD simulation model is carried to show that our proposed scheme is effectiveness in terms of the bit error ratio (BER).

Resource Allocation for Intelligent Reflecting Surface Aided Wireless Powered Mobile Edge Computing in OFDM Systems

Wireless powered mobile edge computing (WP-MEC) has been recognized as a promising technique to provide both enhanced computational capability and sustainable energy supply to massive low-power wireless devices. However, its energy consumption becomes substantial, when the transmission link used for wireless energy transfer (WET) and for computation offloading is hostile. To mitigate this hindrance, we propose to employ the emerging technique of intelligent reflecting surface (IRS) in WP-MEC systems, which is capable of providing an additional link both for WET and for computation offloading. Specifically, we consider a multi-user scenario where both the WET and the computation offloading are based on orthogonal frequency-division multiplexing (OFDM) systems. Built on this model, an innovative framework is developed to minimize the energy consumption of the IRS-aided WP-MEC network, by optimizing the power allocation of the WET signals, the local computing frequencies of wireless devices, both the sub-band-device association and the power allocation used for computation offloading, as well as the IRS reflection coefficients. The major challenges of this optimization lie in the strong coupling between the settings of WET and of computing as well as the unit-modules constraint on IRS reflection coefficients. To tackle these issues, the technique of alternating optimization is invoked for decoupling the WET and computing designs, while two sets of locally optimal IRS reflection coefficients are provided for WET and for computation offloading separately relying on the successive convex approximation method. The numerical results demonstrate that our proposed scheme is capable of monumentally outperforming the conventional WP-MEC network without IRSs. Quantitatively, about 80% energy consumption reduction is attained over the conventional MEC system in a single cell, where 3 wireless devices are served via 16 sub-bands, with the aid of an IRS comprising of 50 elements.

A BCS code in the f-OFDM system: a promising candidate for 5G wireless communication systems

Recently, a novel BCS code differing from conventional concatenated (RS/BCH) codes has been proposed for LTE system. It achieved a significant improvement in the LTE system performance better than single codes and familiar concatenated (RS/BCH) codes. However, LTE systems that primarily depend on orthogonal frequency-division multiplexing (OFDM), no longer meet the demand of wireless-communication systems and their high data-rate requirements because of the rapid growth in the service aspect of such systems. In addition to high out-of-band emission (OOBE) and big peak to average power ratio (PAPR) of OFDM, it doesn’t meet the demand of 5G diverse service scenarios. Meanwhile, filtered OFDM (f-OFDM) is considered a candidate technique for 5G wireless communication systems which have OFDM features as well low OOBE. However, a trade-off exists among reducing OOBE, bit error rate (BER) degradation, and high PAPR. In this work, the BCS code is proposed for f-OFDM to enhance its system performance, while a root-raised-cosine windowed finite impulse response (FIR) filter is selected in f-OFDM to reduce OOBE levels. Results show that in the BPSK and QPSK modulation schemes, f-OFDM achieves a low level of OOBE after using an FIR digital filter against a high level of OFDM. Meanwhile, the proposed system using the BCS code enables remarkable improvement in BER performance and helps reduce PAPR better than conventional OFDM systems. Therefore, considering these features, the proposed system using the BCS code can be presented as a candidate for 5G wireless-communication systems.

Enhancing Throughput of Optimum Channel in Ofdm Based Full Duplex Communication

The efficient use of radio facilities in cellular networks is essential and being widely studied. This letter sees a cellular relay system where various user couples perform trans-directional interaction via various relays relying on Orthogonal Frequency-Division Multiplexing (OFDM) communication.. Joint implementation of route and transmit allocation, along with subcarrier coupling, subcarrier distribution as well as relay choice, for complete output redistribution is identified as a combination estimation issue.. Using a graph conceptual strategy, we can efficiently fix the problem in exponential time by converting it into a Optimum Adjusted Bipartite Tracking (MWBM) issue. Simulation experiments are conducted to assess the complete throughput of the network versus transmitting energy per node and the number of relay nodes. Carrier frequency offset causes a number of impairments including attenuation and rotation of each of the subcarriers and intercarrier interference (ICI) between carrier frequency offset causes a number of impairments including attenuation and rotation of each of the subcarriers and intercarrier interference (ICI) between subcarriers. In the mobile radio environment, the relative movement between transmitter and receiver causes doppler frequency shifts, in addition, the carriers can never be perfectly synchronized. These random frequency errors in OFDM system distort orthogonality between subcarriers and thus intercarrier interference (ICI) occurs. A Number of methods have been developed to reduce this sensitivity to frequency offset

Low-Complexity Superresolution Frequency Offset Estimation for High Data Rate Acoustic OFDM Systems

This paper addresses the problem of compensating for motion-induced Doppler frequency offset in multicarrier acoustic communication systems based on orthogonal frequency-division multiplexing (OFDM). In mobile acoustic systems, Doppler effect can be severe enough that the received OFDM signal experiences nonnegligible frequency offsets even after initial resampling. To target these offsets, a superresolution, yet low-complexity method based on a stochastic gradient approach is proposed. The method relies on differentially coherent detection that keeps the receiver complexity at a minimum and requires only a small pilot overhead. Differential encoding is applied across carriers, promoting the use of a large number of closely spaced carriers within a given bandwidth. This approach simultaneously supports frequency-domain coherence and efficient use of bandwidth for achieving high bit rates. While frequency synchronization capitalizes on differentially coherent detection, it can also be used as a preprocessing stage in coherent receivers without creating undue complexity. Using simulation, as well as the experimental data transmitted over a 3-7-km shallow-water channel in the 10.5-15.5-kHz acoustic band, we study the system performance in terms of data detection mean squared error and bit error rate, and show that the proposed method provides excellent performance at low computational cost. Such advantages are of paramount importance for practical implementation of high data rate acoustic OFDM systems.

Performance of HARQ-Assisted OFDM Systems Contaminated by Impulsive Noise: Finite-Length LDPC Code Analysis

Impulsive noise (IN) constitutes a limiting factor in power-line communication systems, especially in those relying on orthogonal frequency division multiplexing (OFDM). If a single time-domain sample is contaminated by an impulse, all subcarriers become contaminated after the spreading of the discrete Fourier transform-based demodulation. In order to alleviate this problem, hybrid automatic repeat-and-request (HARQ) is often invoked. Moreover, the powerful low-density parity-check (LDPC) codes have been increasingly employed in a variety of current and next-generation communication standards. Against this background, in this paper, we explicitly characterize the performance of LDPC-coded HARQ-assisted OFDM systems in the face of IN, where the performance metrics of the outage probability (OP) and the average number of retransmissions, as well as the effective throughput, are analyzed. First of all, we conceive a new algorithm for evaluating the OP in a realistic finite-length LDPC regime, by adapting both the so-called density evolution technique and the waterfall signal-to-noise ratio analysis method. Following this, both the average number of retransmission attempts and the effective throughput are investigated based on our analysis of the OP. The accuracy of the proposed analysis is confirmed by the simulation results, which also effectively quantify the impact of IN on HARQ-assisted OFDM systems in a finite-length LDPC regime.

Vector OFDM Transmission Over Non-Gaussian Power Line Communication Channels

Most of the recent power line communication (PLC) systems and standards, both narrow-band and broadband, are based on orthogonal frequency-division multiplexing (OFDM). This multiplexing scheme however suffers from the high peak-to-average power ratio (PAPR) which can considerably impact the energy efficiency, size and cost of PLC modems as well as cause electromagnetic compatibility (EMC) issues. This paper investigates the performance of vector OFDM (VOFDM), which has inherently better PAPR properties, over non-Gaussian broadband PLC channels equipped with two nonlinear preprocessors at the receiver. In addition, the low PAPR property of the VOFDM system is exploited to further enhance the efficiency of the nonlinear preprocessors. The achievable gains are studied in terms of the complementary cumulative distribution function of the PAPR, probability of noise detection error and the signal-to-noise ratio at the output of the nonlinear preprocessors. For comparison's sake, the performance of conventional OFDM systems is also presented throughout the paper. Results reveal that the proposed system is able to provide up to 2 dB saving in the transmit power relative to the conventional OFDM under same system conditions, which eventually also translates into a system that is more resilient to EMC limits, reduced cost and size of PLC modems. It is also shown that the achievable gains become more significant as the vector block (VB) size of the VOFDM system is increased.

Relay-Assisted OFDM for Ultraviolet Communications: Performance Analysis and Optimization

Ultraviolet (UV) communication enables non-line-of-sight (NLOS) outdoor wireless connectivity and is particularly desirable to relax or eliminate pointing and tracking requirements of infrared links. In comparison to infrared counterparts, UV links are subject to relatively higher path loss as well as intersymbol interference resulting from frequency selective nature of the channel. In this paper, we propose the powerful combination of relay-assisted transmission and multi-carrier architecture based on orthogonal frequency division multiplexing (OFDM). Specifically, we consider a cooperative diversity system with orthogonal cooperation protocol and use DC-biased optical OFDM as the underlying physical layer. We consider both amplify-and-forward (AF) and detect-and-forward (DF) relaying. We investigate the error rate performance of the proposed relay-assisted OFDM UV system under consideration and demonstrate performance gains over point-to-point OFDM UV systems. We further determine optimal power allocation schemes to improve the performance. We also propose a variable-rate UV OFDM system and improve system throughput via bit loading.

On the efficiency of PAPR reduction schemes deployed for DRM systems

Digital Radio Mondiale (DRM) is the universally, openly standardized digital broadcasting system for all frequencies including LW, MW, and SW as well as VHF bands. Alongside providing high audio quality to listeners, DRM satisfies technological requirements posed by broadcasters, manufacturers and regulatory authorities and thus bears a great potential for the future of global radio. One of the key issues here concerns green broadcasting. Facing the need for high-power transmitters to cover wide areas, there is room for improvement concerning the power efficiency of DRM-transmitters. A major drawback of DRM is its high peak-to-average power ratio (PAPR) due to the applied transmission technology based on Orthogonal Frequency Division Multiplexing (OFDM), which results in non-linearities in the emitted signal, low power efficiency, and high costs of transmitters. To overcome this, numerous schemes have been investigated for reducing PAPR in OFDM systems. In this paper, we review and analyze various technologies to reduce PAPR providing that the technical feasibility and DRM-specific system architecture and edge conditions regarding the system performance in terms of modulation error rate, compliance with frequency mask, and synchronization efficiency are ensured. All evaluations are carried out with I/Q signals which are monitored in real operation to present the actual performance of proposed PAPR techniques. Subsequently, the capability of the best approach is evaluated via measurements on a DRM test platform, where achieved transmit power gain of 10 dB is shown. According to our evaluation results, PAPR reduction schemes based on active constellation extension followed by a filter prove to be promising towards practical realization of power-efficient transmitters.

Be Cautious When Analyzing the Sampling Clock Offset in OFDM Systems

Many studies have analyzed the effects of sampling clock offset (SCO) on orthogonal frequency-division multiplexing (OFDM) systems. The results of these studies are based on one of two discrete representations of continuous time-domain OFDM signals: (1) a subcarrier set consisting of both negative and positive frequencies, or (2) a subcarrier set comprising only positive frequencies. This paper discusses whether there is any difference between the two representations in the presence of SCO. When SCO is not considered, both representations are equivalent because of aliasing. In the presence of SCO, we derive a closed form of signal-to-interference-plus-noise ratio (SINR) expressions for both cases and compare their performances in terms of SINR by numerical analysis. The latter representation shows worse SINR performance compared to the former representation because the effect of SCO is more relevant for high frequencies and the latter representation has complex exponentials with high frequencies. Therefore, for fair comparisons of performance, caution is needed when analyzing SCO in OFDM systems.

Hybrid time-frequency domain equalization for LED nonlinearity mitigation in OFDM-based VLC systems.

A novel hybrid time-frequency domain equalization scheme is proposed and experimentally demonstrated to mitigate the white light emitting diode (LED) nonlinearity in visible light communication (VLC) systems based on orthogonal frequency division multiplexing (OFDM). We handle the linear and nonlinear distortion separately in a nonlinear OFDM system. The linear part is equalized in frequency domain and the nonlinear part is compensated by an adaptive nonlinear time domain equalizer (N-TDE). The experimental results show that with only a small number of parameters the nonlinear equalizer can efficiently mitigate the LED nonlinearity. With the N-TDE the modulation index (MI) and BER performance can be significantly enhanced.

Energy and delay constrained scheduling of real time traffic over OFDM communication system with non-causal channel information

A point to point wireless communication system based on Orthogonal Frequency Division Multiplexing (OFDM) is considered, wherein R bits of information have to be scheduled within a hard delay deadline of T time slots over a frequency selective slow fading channel. The scheduling objective is to minimize the energy expenditure with non-causal knowledge of channel state over a limited time horizon, number of bits to be served and delay constraints. We assume a convex monomial energy-bit cost function and propose novel efficient two stage heuristic algorithm that allocate the R bits on the time slots, followed by greedy optimal bit loading over the OFDM subcarriers. An analytical model is developed and numerical simulations are performed to validate the proposed approach as well as the results show that our algorithms outperform existing approaches in published literature by up to 10 dB in power margin on an average in high load conditions while meeting the deadline of T timeslots.

SIR Analysis for OFDM Transmission in the Presence of CFO, Phase Noise and Doubly Selective Fading

This paper presents an analysis of the detrimental effects of carrier frequency offset (CFO), phase noise (PHN) and doubly-selective fading on orthogonal frequency division multiplexing (OFDM) transmission performance. In particular, we derive a closed-form expression for the signal-to-interference ratio (SIR) at an OFDM receiver in the presence of CFO, PHN and doubly-selective fading. Simulation and analytical results under various OFDM system settings are in good agreement and reveal the contributions of these channel impairments in the SIR degradation.

Estimation of Fast Time-Varying Channels in OFDM Systems Using Two-Dimensional Prolate

Modern communication systems are based on orthogonal frequency division multiplexing (OFDM). They are designed for dealing with frequency selective channels considering invariance within the time-span of an OFDM symbol. However, this assumption is no longer valid when the transceivers operate in higher mobility scenarios or higher carrier frequencies. This condition provokes inter-carrier interference (ICI) that greatly degrades system performance. State-of-the-art approaches that satisfactorily mitigate this problem have a complexity of O(N3), which makes them infeasible with current technology. In this paper, a novel channel estimation algorithm to cope with this problem is presented. It is based on a subspace approach using two-dimensional Prolate functions, achieving a complexity of only O(N2). It depends only on the maximum delay spread and maximum Doppler spread while being robust in the sense that it is independent of the particular channel scattering function. Performance analysis of the proposed algorithm is presented. Simulation results under the WiMAX standard show that this algorithm improves previous results, achieving a bit error rate (BER) close to the one obtained with perfect channel state information (CSI) in very-fast transceiver mobility, as high as 874 Km/h over a 2.4 Ghz carrier frequency.

LED Nonlinearity Mitigation Techniques in Optical Wireless OFDM Communication Systems

Light emitting diode (LED) nonlinearities have a significant impact on the performance of optical wireless (OW) communication systems based on orthogonal frequency division multiplexing (OFDM) modulation. In this paper, different techniques to compensate for the induced nonlinearity distortions are presented and analyzed. First, single-carrier frequency division multiple access (SC-FDMA) is proposed instead of OFDM for OW systems. In SC-FDMA, some of the major OFDM problems such as high peak-to-average power ratio are inherently resolved. It is shown that the performance of OW OFDM systems and their counterpart SC-FDMA systems are traded off for certain input signal powers and modulation orders. Second, OW OFDM systems with hard, soft, and turbo channel coding techniques are considered. It is reported that significant performance gains can be achieved through channel coding. Finally, a novel method to reduce signal clipping distortion through the use of iterative signal clipping (ISC) and multiple transmit LEDs is proposed. The ISC technique is based on iterative clipping of the time domain OFDM signal and transmission from a multiple LED transmitter. Transmit LEDs are synchronized, located close to each other, and placed to emit light in the same direction. Hence, the channel path gains from each LED to the receiver photodiode are similar. The received signals from the different LEDs add coherently at the receiver. Reported results demonstrate that the effect of distortion is eliminated or significantly reduced based on the considered number of transmit LEDs.

Physical layer security of MIMO–OFDM systems by beamforming and artificial noise generation

In this paper we address physical layer security in multiple-input-multiple-output (MIMO) frequency selective wireless channels in the presence of a passive eavesdropper, i.e., the associated channel is unknown to the transmitter. Signalling is based on orthogonal frequency division multiplexing (OFDM). Spatial beamforming and artificial noise broadcasting are chosen as the strategy for secure transmission. The contribution of channel frequency selectivity to improve secrecy is presented by performance and probabilistic analysis. Moreover, we investigate the capability of the eavesdropper to jeopardize the security of the system (defined as the SNR difference between the intended receiver and the eavesdropper) by mitigating the interfering effect of the artificial noise using zero forcing as a receive beamforming strategy. The results show that although zero forcing is not the optimal strategy to maximize the SNR, it offers (from the eavesdropper’s perspective) a better performance than MMSE for MIMO frequency selective channels and thus threatens the overall security of the system.

Analysis of performance degradation due to non-linearity and phase noise in orthogonal frequency division multiplexing systems

This study presents an exact analysis of circuit non-linearity and phase noise effects on orthogonal frequency division multiplexing (OFDM) communication systems. Using an analytical approach, the degradation effects of these impairments on the performance of OFDM systems are studied. A truncated power series is used to represent the third-order non-linear behaviour of a radio frequency circuit. The phase noise is modelled with Lorentzian power density function. Then, by use of an expression for interferences and intermodulation terms, a closed-form signal-to-noise degradation relation is theoretically extracted. The analytical results are compared with simulation outcomes. The comparison shows that bit error rate of the analytical results closely matches with simulation results of the OFDM system.

Performance analysis of turbo decoding algorithms in wireless OFDM systems

Turbo codes are well known to be one of the error correction techniques which achieve closer results to the Shannon limit. Nevertheless, the specific performance of the code highly depends on the particular decoding algorithm used at the receiver. In this sense, the election of the decoding algorithm involves a trade off between the gain introduced by the code and the complexity of the decoding process. In this work we perform a thorough analysis of the different iterative decoding techniques and analyze their suitability for being implemented in the user terminals of new cellular and broadcast systems which are based on orthogonal frequency division multiplexing (OFDM). The analyzed iterative decoding algorithms are the max-log-MAP and the soft output Viterbi algorithm (SOVA), since both of them have a relative low computational complexity, simplifying their implementation in cost efficient terminals. Simulation results have been obtained for different encoder structures, block sizes and considering realistic channel conditions (an OFDM transmission over a wireless channel).

New Papr Reductionin Ofdm Systems By Hybrid Algorithm of Ptsand Appr Methods

In this paper, we propose a new peak to average power ratio (PAPR) reduction technique by a hybrid algorithm based on a partial transmit sequence (PTS) and adaptive peak power reduction (APPR ) methods. This technique is used in a system based on orthogonal frequency division multiplexing (OFDM). OFDM has orthogonally modulated sub-carriers which unexpectedly give large PAPR and tends to reduce the power efficiency of a RF amplifier. To reduce PAPR, the sequence of input data is rearranged by PTS for the reduction of PAPR and then fed to APPR process in the proposed system. The APPR method controls the peak level of the modulation signal by an adaptive algorithm. It reduces modulation signals over a predefined range. A proposed reduction method consists of these two methods and realizes both advantages at the same time. In simulation results, the proposed method shows the improvement on PAPR, PSD, and also the high performance on bit error rate of an OFDM system.

Design of an OFDM Cooperative Space-Time Diversity System

In this paper, we propose a wireless system that realizes theoretical benefits of space-time cooperation. Specifically, we design a space-time cooperative system based on orthogonal frequency division multiplexing (OFDM), which we refer to as a cooperative (CO)-OFDM system. Our design includes a two-phase space-time cooperation protocol, as well as a transmitter and receiver architecture that facilitates cooperation. Furthermore, we devise a frame structure, on which we build practical timing and frequency synchronization algorithms and a channel estimation algorithm. In particular, the proposed frequency synchronization algorithm utilizes the underlying structure of the cooperation protocol, and the proposed channel estimation algorithm is based on a pairwise orthogonal construction of two sequences. We validate the performance of the proposed synchronization and channel estimation algorithms through simulations. We then present simulation results that demonstrate the overall performance advantage of the CO-OFDM system over an OFDM system without cooperation, not only under idealistic assumptions but also under realistic situations where the proposed algorithms are employed.