Bandwidth‐Efficient Selective Retransmission for MIMO‐OFDM Systems

In this work, we propose an efficient selective retransmission method for multiple‐input and multiple‐output (MIMO) wireless systems under orthogonal frequency‐division multiplexing (OFDM) signaling. A typical received OFDM frame may have some symbols in error, which results in a retransmission of the entire frame. Such a retransmission is often unnecessary, and to avoid this, we propose a method to selectively retransmit symbols that correspond to poor‐quality subcarriers. We use the condition numbers of the subcarrier channel matrices of the MIMO‐OFDM system as a quality measure. The proposed scheme is embedded in the modulation layer and is independent of conventional hybrid automatic repeat request (HARQ) methods. The receiver integrates the original OFDM and the punctured retransmitted OFDM signals for more reliable detection. The targeted retransmission results in fewer negative acknowledgements from conventional HARQ algorithms, which results in increasing bandwidth and power efficiency. We investigate the efficacy of the proposed method for optimal and suboptimal receivers. The simulation results demonstrate the efficacy of the proposed method on throughput for MIMO‐OFDM systems.     

PAPR reduction for OFDM‐based visible light communication systems using proposed hybrid technique

Orthogonal Frequency Division Multiplexing (OFDM) is utilized with visible light communication (VLC) systems to decrease the impacts of inter‐symbol interference and to achieve communication with high speed of data transmission and huge bandwidth. In any case, the main problem in OFDM‐based VLC systems is high peak‐to‐average power ratios (PAPRs). This paper proposes a hybrid PAPR reduction technique based on signal transformation combined with clipping. The Hadamard transform is used in the proposed technique to reduce the PAPR without affecting the bit error rate (BER) of the VLC systems. The optimum clipping threshold at which the PAPR is reduced simultaneously with the improvement the BER of the VLC systems is also determined. The performance of the proposed system is assessed in terms of complementary cumulative distribution function and the BER. The obtained results demonstrate that the proposed procedure can simultaneously decrease the PAPR and achieve good BER performance compared to the OFDM‐based VLC system.     

ESIM‐OFDM‐based transceiver design of a visible light communication system

Visible light communication (VLC) system has been becoming a potential technology for short‐range data communication over wireless optical channel. In this paper, a real‐time state‐of‐the‐art VLC system prototype is implemented mainly based on enhanced subcarrier index modulation orthogonal frequency division multiplexing (ESIM‐OFDM) modulation scheme. The prototype has been designed using NI cDAQ hardware equipment programmed in matlab /labview . In other words, the developed VLC transceiver labview model is loaded into cDAQ to perform real‐time transmission of text data. The design mainly comprises VLC transmitter, VLC receiver, interfacing NI cDAQ with labview , and implementation of ESIM‐OFDM. It should be noted that the direct output of ESIM‐OFDM is not suitable to drive VLC transmitter circuit; therefore, the key contribution lies in making use of sigma–delta modulator to modify ESIM‐OFDM output to produce constant current signal which in turn could drive VLC transmitter. The second main contribution of the work targets the luminous properties of the designed transmitter that have experimentally been characterized using a LUX meter. Finally, the last contribution of the work is the simulation carried out to analyze the performance of the proposed system with other existing VLC modulation schemes. Copyright © 2016 John Wiley & Sons, Ltd.     

Adaptive subcarrier bandwidth and power in OFDM‐based cognitive radio systems for high mobility applications

OFDM‐based cognitive radio systems are spectrally flexible and efficient, but they are vulnerable to intercarrier interference (ICI), especially in high mobility environments. High mobility of the terminal causes large Doppler frequency spread resulting in serious ICI. Such ICI severely degrades the system performance, which is ignored in the existing resource allocation of OFDM‐based cognitive radio systems. In this paper, an adaptive subcarrier bandwidth along with power allocation problem in OFDM‐based cognitive radio systems for high mobility applications is investigated. This adaptive subcarrier bandwidth method should choose the suitable subcarrier bandwidth not only to balance the tradeoff between ICI and intersymbol interference but also to be large enough to tolerate an amount of Doppler frequency spread but less than the coherence bandwidth. The power budget and interference to primary users caused by cognitive radio users are imposed for primary users' protection. With these constraints, a joint optimization algorithm of subcarrier bandwidth and power allocation is proposed to maximize the bandwidth efficiency of OFDM‐based cognitive radio systems in such conditions. Numerical simulation results show that the proposed algorithm could maximize the system bandwidth efficiency and balance this tradeoff while satisfying the constraints. Copyright © 2012 John Wiley & Sons, Ltd.     

Design of NS3 VLC module and performance analysis of ad hoc network under VLC and Wi‐Fi layers

Visible light communication (VLC) is the branch of optical wireless communications that uses light‐emitting diodes for the dual purpose of illumination and very‐high‐speed data communication. The main motivation behind the current work is finding alternatives to the saturated radio frequency spectrum, radio frequency security issue, and vulnerability to interferences. The current work is aimed at developing a module for the physical layer of a VLC‐based ad hoc network in network simulation 3. The VLC physical layer module is developed by using the optical signal modules available in the network simulator. The work also includes modelling of VLC‐based transmitter, wireless optical channel, and the optical receiver. Furthermore, the implementation and evaluation of the VLC‐based physical layer is carried out over a typical ad hoc network under different performance metrics. The designed ad hoc network is also tested under Wi‐Fi module followed by its comparison with corresponding ad hoc network under VLC module. The comparison is based on bit error rate curves, system throughput, and gain in received signal‐to‐noise ratio mainly. Finally, the suitability of different modulation schemes is also investigated in the current work for both Wi‐Fi– and VLC‐based ad hoc networks.     

Frequency shift filter‐based multi‐carrier transceiver

It is well known that orthogonal frequency division multiplexing (OFDM) is sensitive to carrier frequency offset (CFO) and suffers from a high peak‐to‐average ratio. In addition, the performance of OFDM is severely affected by strong co‐channel interference and strong narrowband interference. To mitigate the limitations of OFDM, we propose a new multi‐carrier transceiver based on frequency‐shift filter. A frequency‐shift filter can separate spectrally overlapping sub‐carrier signals by exploiting the spectral correlation inherent in the cyclostationary modulated signals. To increase spectral efficiency, we increase the percentage of spectral overlap between two adjacent sub‐channels. We derive an upper bound and a lower bound on the bit error rate performance of the proposed multi‐carrier transceiver in additive white Gaussian noise channel and frequency‐nonselective Rayleigh fading channel, respectively. Compared with OFDM, our simulation results show that the proposed multi‐carrier transceiver is much less sensitive to CFO and has a lower peak‐to‐average ratio; moreover, without any additional interference suppression technique, the proposed transceiver has the advantage of being able to mitigate strong co‐channel interference with CFO from the intended multi‐carrier signal and mitigate strong narrowband interference in additive white Gaussian noise channel and in Rayleigh fading channel in which a large CFO between the transmitted signal and the received signal often occurs. Copyright © 2011 John Wiley & Sons, Ltd.     

16‐QAM OFDM‐Based W‐Band Polarization‐Division Duplex Communication System with Multi‐gigabit Performance

This paper presents a novel 90 GHz band 16‐quadrature amplitude modulation (16‐QAM) orthogonal frequency‐division multiplexing (OFDM) communication system. The system can deliver 6 Gbps through six channels with a bandwidth of 3 GHz. Each channel occupies 500 MHz and delivers 1 Gbps using 16‐QAM OFDM. To implement the system, a low‐noise amplifier and an RF up/down conversion fourth‐harmonically pumped mixer are implemented using a 0.1‐μm gallium arsenide pseudomorphic high‐electron‐mobility transistor process. A polarization‐division duplex architecture is used for full‐duplex communication. In a digital modem, OFDM with 256‐point fast Fourier transform and (255, 239) Reed‐Solomon forward error correction codecs are used. The modem can compensate for a carrier‐frequency offset of up to 50 ppm and a symbol rate offset of up to 1 ppm. Experiment results show that the system can achieve a bit error rate of 10^–5 at a signal‐to‐noise ratio of about 19.8 dB.     

Peak‐to‐average power ratio reduction of orthogonal frequency division multiplexing systems using modified tone reservation techniques

This paper proposed a modified tone reservation (TR) technique that can reduce the peak‐to‐average power ratio (PAPR) of the orthogonal frequency division multiplexing (OFDM) system and is able to correct errors to avoid channel interference. The TR technique is a widely used PAPR reduction technique, which divides subcarriers of the OFDM system into two sets to generate peak‐canceling signals and transmit modulated data. The subcarriers used to reduce the PAPR are called the peak reduction tone sets. The mechanism of peak‐canceling signal generation is a primary factor in determining the quality of the PAPR reduction performance of the TR technique. Currently, two signal generation mechanisms exist: TR‐gradient‐based and TR‐clipping‐based techniques. Although TR techniques can effectively reduce the high PAPR in the OFDM system, TR techniques lack the ability to correct errors. Therefore, this paper combined block coded modulation codes and TR techniques to provide the modified TR techniques with error correction abilities. From the simulation results, the modified TR techniques had a superior effect on PAPR reduction performance compared with the conventional TR technique. The modified TR technique also possessed the ability to correct errors during signal transmission. Copyright © 2015 John Wiley & Sons, Ltd.     

An Efficient Scheme to Achieve Differential Unitary Space‐Time Modulation on MIMO‐OFDM Systems

Differential unitary space‐time modulation (DUSTM) has emerged as a promising technique to obtain spatial diversity without intractable channel estimation. This paper presents a study of the application of DUSTM on multiple‐input multiple‐output orthogonal frequency division multiplexing (MIMO‐OFDM) systems with frequency‐selective fading channels. From the view of a correlation analysis between subcarriers of OFDM, we obtain the maximum achievable diversity of DUSTM on MIMO‐OFDM systems. Moreover, an efficient implementation strategy based on subcarrier reconstruction is proposed, which transmits all the signals of one signal matrix in one OFDM transmission and performs differential processing between two adjacent OFDM blocks. The proposed method is capable of obtaining both spatial and multipath diversity while reducing the effect of time variation of channels to a minimum. The performance improvement is confirmed by simulation results.     

Blind channel estimation for single‐input multiple‐output OFDM systems: zero padding based or cyclic prefix based?

Orthogonal frequency division multiplexing (OFDM) transmission equipped with multiple receive antennas constitutes a single‐input multiple‐output (SIMO) OFDM system. SIMO‐OFDM systems have been widely used in wireless communications. Compared to those approaches using training sequences, blind channel estimation methods for SIMO‐OFDM systems have the advantage of saving bandwidth and improving energy efficiency and system throughput. As far as blind channel identification is concerned, it is known that zero padding (ZP)‐based single‐input single‐output (SISO)‐OFDM systems have desirable features compared to conventional cyclic prefix (CP)‐based SISO‐OFDM systems. However, it is yet unknown whether ZP‐ or CP‐based SIMO‐OFDM systems are favourable for blind channel estimation. To investigate this problem, we first propose a short‐data effective method for blind channel estimation for ZP‐based SIMO‐OFDM systems. Then we analyse a number of issues surrounding blind channel estimation for ZP‐ and CP‐based SIMO‐OFDM systems. The issues brought up in the paper have not been discussed in the existing research. The significance of our investigation is that it provides a deep insight into blind channel estimation for ZP‐ and CP‐based SIMO‐OFDM systems. Copyright © 2011 John Wiley & Sons, Ltd.     

Peak‐to‐average power ratio reduction of a hidden training sequence‐aided precoding scheme for MIMO‐OFDM

We analyze a peak‐to‐average power ratio (PAPR) reduction property based on a hidden training sequence‐aided precoding scheme for MIMO‐OFDM systems. In addition to the benefits of a hidden training sequence‐aided precoding scheme such as improvement in bandwidth efficiency and frequency diversity gain, we address that power amplifier efficiency can be improved without any additional complexity burden. By mathematically analyzing PAPR of the precoded MIMO‐OFDM signal with a hidden training sequence, we demonstrate that PAPR reduction can be obtained by varying the allocated power to the hidden training sequence. Because of the low PAPR property of this scheme, it is possible to utilize a low‐cost power amplifier, resulting in the reduction in the total cost for hardware implementation. Copyright © 2014 John Wiley & Sons, Ltd.     

A Novel Channel Estimation Scheme for OFDM/OQAM‐IOTA System

An OFDM/offset QAM (OQAM)‐IOTA system uses the isotropic orthogonal transform algorithm (IOTA) function, which has good localization properties in the time and frequency domains. This is employed instead of the guard interval used in a conventional OFDM/QAM system in order to be robust for multi‐path channels. However, the conventional channel estimation scheme is not valid for an OFDM/OQAM‐IOTA system due to the intrinsic inter‐symbol interference of the IOTA function. In this paper, a condition is derived to reduce the intrinsic interference of the IOTA function. This condition is obtained with the proposed pilot structure used for perfect channel estimation. We also derive the preamble structure appropriate for practical channel estimation of the OFDM/OQAM‐IOTA system. Simulation results show that the OFDM/OQAM‐IOTA system with the proposed preamble structure performs better than the conventional OFDM system, and it has the additional advantage of an increased data transmission rate which corresponds to the guard interval retrieval.     

Novel Digital Cancelation Method in Presence of Harmonic Self‐Interference

In‐band full‐duplex (IFD) communication has recently attracted a great deal of interest because it potentially provides a two‐fold spectral efficiency increase over half‐duplex communications. In this paper, we propose a novel digital self‐interference cancelation (DSIC) algorithm for an IFD communication system in which two nodes exchange orthogonal frequency‐division multiplexing (OFDM) symbols. The proposed DSIC algorithm is based on the least‐squares estimation of a self‐interference (SI) channel with block processing of multiple OFDM symbols, in order to eliminate the fundamental and harmonic components of SI induced through the practical radio frequency devices of an IFD transceiver. In addition, the proposed DSIC algorithm adopts discrete Fourier transform processing of the estimated SI channel to further enhance its cancelation performance. We provide a minimum number of training symbols to estimate the SI channel effectively. The evaluation results show that our proposed DSIC algorithm outperforms a conventional algorithm.     

A semi‐definite programming approach to spatial decorrelation of independently polarized signals

A MIMO channel spatial decorrelation scheme based on semi‐definite programming is introduced. As a particular application example, the paper addresses the potential gain of using multiple antennas and MIMO–OFDM techniques in order to increase the bandwidth efficiency in satellite communication systems. In particular, we consider the increase in channel capacity that is possible by exploiting satellite and polarization diversity. A fundamental case is studied with three satellite branches, and where each transmit/receive antenna unit consists of six elemental electric and magnetic dipoles yielding six distinguishable parallel polarization channels per frequency. The numerical examples show that capacity increases linearly on a logarithmic signal‐to‐noise ratio scale where the constant of proportionality is the number of active parallel channels. In this respect, the simultaneous use of triple electric and triple magnetic dipoles has the potential to triple the capacity of an antenna system based on antenna units of single dipoles. Copyright © 2006 John Wiley & Sons, Ltd.     

Cramer–Rao bound based mean‐squared error and throughput analysis of superimposed pilots for semi‐blind multiple‐input multiple‐output wireless channel estimation

This work presents a study of the mean‐squared error (MSE) and throughput performance of superimposed pilots (SP) for the estimation of a multiple‐input multiple‐output (MIMO) wireless channel. The Cramer–Rao bound (CRB) is derived for SP based estimation of the MIMO channel matrix. Employing the CRB analysis, it is proved that the asymptotic MSE bound is potentially 3 dB lower than the MSE performance of the existing SP mean based estimation (SPME) schemes. Motivated by this observation, a novel SP semi‐blind scheme is presented for MIMO channel estimation. This scheme asymptotically achieves the CRB and hence has a lower MSE of estimation when compared with SPME schemes. We also derive closed form expressions for the optimal source‐pilot power allocation in SP by maximizing the post‐processing signal‐to‐noise power ratio at the receiver. In the final part, a new result is presented for the worst‐case capacity of a communication channel with correlated information symbols and noise. This framework is employed to quantify the throughput performance of SP and also to demonstrate the bandwidth efficiency of SP compared with that of a conventional pilot based system. Copyright © 2012 John Wiley & Sons, Ltd.     

Linear precoded wavelet OFDM‐based PLC system with overlap FDE for impulse noise mitigation

In recent years, power lines have gained significant interest for their use in high‐speed communications because of the already deployed power distribution infrastructure. However, to achieve high data rates with reliability in power line communication (PLC), robust signal processing techniques are required to mitigate channel distortion and noise. Orthogonal frequency division multiplexing (OFDM) as a multicarrier modulation technique has been standardized for PLC; however, to further enhance the quality of communication, wavelet OFDM (WOFDM) has been proposed as a suitable choice. In this article, OFDM‐based and WOFDM‐based PLC systems are studied, and overlap frequency domain equalization (OFDE) as a robust and efficient equalization technique is presented. Moreover, to enhance the efficiency of the OFDE, linear precoding (LP) is also suggested for fast Fourier transform and wavelet transform–based filter bank transceivers over the PLC channel. Performance of the proposed LP‐based OFDM‐OFDE and LP‐based WOFDM‐OFDE systems is compared with previously proposed equalization structures in terms of bit error rate, peak‐to‐average power ratio, and computational complexity via computer simulations. Furthermore, the performance of proposed architectures is also compared with classical equalization techniques under impulse noise with different intensities. Results show that not only the proposed LP‐based WOFDM‐OFDE transceiver performs better than the previous equalization models but also the LP‐based OFDM‐OFDE at the expense of slight increment in computational complexity.     

可见光通信中融合VOOK和分层OFDM的高效频谱混合调制方法

为了满足可见光通信(VLC)的照明和高速率数据传输的双重需求,该文提出一种融合可变开关键控(VOOK)和分层正交频分复用(OFDM)的高效频谱混合调制。首先针对分层OFDM调制方法设计了相应的VOOK信号,避免了VOOK对分层OFDM信息传输的干扰。然后,为了保证混合调制信号工作在发光二极管(LED)的线性区间,提出一种全新的重构分层光OFDM(RLO-OFDM),并构建VOOK-RLO-OFDM混合调制方法,实现了调光控制和高效频谱数据传输的双重功能。混合调制能够并行地进行VOOK和RLO-OFDM的信号检测,同时,RLO-OFDM的信号检测可以采用标准的OFDM接收机,无须依赖串行干扰消除方式,显著地降低了接收复杂度和处理时延。仿真结果表明,所提方法能够实现线性调光控制,且具有高效的频谱效率。     

Joint Processing of Zero‐Forcing Detection and MAP Decoding for a MIMO‐OFDM System

We propose a new bandwidth‐efficient technique that achieves high data rates over a wideband wireless channel. This new scheme is targeted for a multiple‐input multiple‐output orthogonal frequency‐division multiplexing (MIMO‐OFDM) system that achieves transmit diversity through a space frequency block code and capacity enhancement through the iterative joint processing of zero‐forcing detection and maximum a posteriori (MAP) decoding. Furthermore, the proposed scheme is compared to the coded Bell Labs Layered Space‐Time OFDM (BLAST‐OFDM) scheme.     

16‐QAM OFDM‐Based K‐Band LoS MIMO Communication System with Alignment Mismatch Compensation

This paper presents a novel K‐band (18 GHz) 16‐quadrature amplitude modulation (16‐QAM) orthogonal frequency‐division multiplexing (OFDM)‐based 2 × 2 line‐of‐sight multi‐input multi‐output communication system. The system can deliver 356 Mbps on a 56 MHz channel. Alignment mismatches, such as amplitude and/or phase mismatches, between the transmitter and receiver antennas were examined through hardware experiments. Hardware experimental results revealed that amplitude mismatch is related to antenna size, antenna beam width, and link distance. The proposed system employs an alignment mismatch compensation method. The open‐loop architecture of the proposed compensation method is simple and enables facile construction of communication systems. In a digital modem, 16‐QAM OFDM with a 512‐point fast Fourier transform and (255, 239) Reed‐Solomon forward error correction codecs is used. Experimental results show that a bit error rate of 10^?5 is achieved at a signal‐to‐noise ratio of approximately 18.0 dB.     

Performance analysis of Doppler shift effects on OFDM‐based and MC‐CDMA‐based cognitive radios

The present development of high data rate wireless applications has led to extra bandwidth demands. However, finding a new spectrum bandwidth to accommodate these applications and services is a challenging task because of the scarcity of spectrum resources. In fact, the spectrum is utilized inefficiently for conventional spectrum allocation, so Federal Communications Commission has proposed dynamic spectrum access mechanism in cognitive radio, where unlicensed users can opportunistically borrow unused licensed spectrum, which is a challenge to obtain contiguous frequency spectrum block. This also has a significant impact on multicarrier transmission systems such as orthogonal frequency division multiplexing (OFDM) and multicarrier code division multiple access (MC‐CDMA). As a solution, this paper develops non‐contiguous OFDM (NC‐OFDM) and non‐contiguous MC‐CDMA (NC‐MC‐CDMA) cognitive system. The implementation of NC‐OFDM and NC‐MC‐CDMA systems provides high data rate via a large number of non‐contiguous subcarriers without interfering with the existing transmissions. The system performance evaluates NC‐OFDM and NC‐MC‐CDMA for mobile scenario where each propagation path will experience Doppler frequency shift because of the relative motion between the transmitter and receiver. The simulation results of this paper proved that NC‐OFDM system is a superior candidate than NC‐MC‐CDMA system considering the mobility for cognitive users. Copyright © 2013 John Wiley & Sons, Ltd.