Frequency Diversity Gain Research Articles

Turbo Detection Aided Autoencoder for Multicarrier Wireless Systems: Integrating Deep Learning Into Channel Coded Systems

A variety of deep learning schemes have endeavoured to integrate deep neural networks (DNNs) into channel coded systems by jointly designing DNN and the channel coding scheme in specific channels. However, this leads to limitations concerning the choice of both the channel coding scheme and the channel paramters. We circumvent these impediments and conceive a turbo-style multi-carrier auto-encoder (MC-AE) for orthogonal frequency-division multiplexing (OFDM) systems, which is the first one that achieves the flexible integration of DNN into any given channel coded systems while achieving an iteration gain. More explicitly, first of all, we design the MC-AE independently of both the channel coding arrangement and of the channel model, where the output layer of the MC-AE decoder is designed for both accepting and producing reliable soft-bit decisions. Owing to the fact that bit-dependency is imposed by the MC-AE mapping, our bespoke MC-AE decoder becomes capable of achieving a beneficial iteration gain, when the extrinsic information is exchanged between the soft-decision MC-AE decoder and the soft-decision channel decoder. Secondly, in order to be able to interpret the performance advantages of our MC-AE over the conventional OFDM, we map the MC-AE’s input-output relationship to an equivalent model-based representation. The associated theoretical analysis verifies the fact that during the process of data-driven signal reconstruction across OFDM’s subcarriers, a beneficial frequency diversity gain is achieved by the proposed MC-AE design. Finally, our simulation results demonstrate that the MC-AE is capable of achieving substantial performance advantages over both conventional OFDM and OFDM based index modulation (OFDM-IM) in channel coded systems.

Reduced-state maximum-likelihood detection for OFDM systems with insufficient cyclic prefix

Dispersive channels deteriorate the bit error rate (BER) performance of orthogonal frequency-division multiplexing (OFDM) systems with insufficient cyclic prefix (CP). In this paper, we first analyze the intensity of the intersymbol interference (ISI) and intercarrier interference (ICI). For performance enhancement, an effective maximum-likelihood (ML) detection method, named reduced-state maximum-likelihood (RSML), is proposed to detect all target subcarriers parallelly. For each target subcarrier, the proposed RSML detects a few nearby subcarriers jointly, while the remaining subcarriers are directly determined by using pre-detection values. Simulations results show that the proposed RSML achieves a significant performance improvement in terms of BER compared to the existing detection methods and is robust to various channel models and CP lengths. In particular, for certain channel conditions with insufficient CP, the detection performance of RSML is even better than conventional detection with sufficient CP because additional frequency diversity gain is obtained from joint detection at the expense of complexity.

Frequency Diversity Gain of a Wideband Radar Signal

Wideband radar has high-range directional resolution, which can effectively reduce the fluctuation of echo and improve the detection probability of a target under the same detection probability requirement. In this paper, a unified wideband radar χ2 distribution target model with more practical significance is innovatively established, on which the probability density function and detection probability function of Swerling 0, Swerling II and Swerling IV targets are analyzed, respectively. A generalized “frequency diversity gain” of wideband radar is proposed and defined based on the contradiction between suppression of fluctuation and accumulation loss, which represents the ratio of Signal-to-Noise Ratio (SNR) gain between broadband signal and reference bandwidth signal under the same condition (when the reference bandwidth is used, the radar target has only one range unit), and the mathematical relation equation of the target detection performance and signal bandwidth (equivalent to the number of distinguishable range elements of the target) is given. A Monte Carlo simulation experiment is designed. Based on the target model established in this paper, the optimal number of target range units corresponding to different detection probability requirements is obtained, which verifies the correctness of the concept proposed in this paper.

Joint Waveform and Beamforming Optimization for MIMO Wireless Power Transfer

In this paper, we study a multi-sine multiple-input multiple-output (MIMO) wireless power transfer (WPT) system with the objective to increase the output DC power. We jointly optimize the multi-sine waveform and beamforming accounting for the rectenna nonlinearity, and consider two combining schemes for the rectennas at the receiver, namely DC and RF combinings. For DC combining, the waveform and transmit beamforming are optimized, as a function of the channel state information (CSI). For RF combining, the optimal transmit and receive beamformings are provided in closed form and the waveform is optimized. We also consider a practical RF combining circuit using phase shifter and RF power combiner and optimize the waveform, transmit beamforming, and analog receive beamforming adaptive to the CSI. Two types of performance evaluations, based on the nonlinear rectenna model and accurate and realistic circuit simulations, are provided. The evaluations demonstrate that the joint waveform and beamforming design can increase the output DC power by leveraging the beamforming gain, the frequency diversity gain, and the rectenna nonlinearity. It also shows that the joint waveform and beamforming design provides a higher output DC power than the beamforming-only design with a relative gain of 180% in a two-transmit antenna sixteen-sinewave two-receive antenna setup.

Space-, Time- and Frequency-Domain Index Modulation for Next-Generation Wireless: A Unified Single-/Multi-Carrier and Single-/Multi-RF MIMO Framework

As the enabling technologies move up to the mmWave and even to the TeraHertz bands for the next-generation wireless systems, the signal processing of high-bandwidth orthogonal frequency division multiplexing (OFDM) becomes increasingly power-thirsty, owing to the following OFDM deficiencies: (1) the high peak-to-average power ratio (PAPR); (2) the bandwidth efficiency loss due to the cyclic prefix (CP) overhead; (3) the sensitivity to carrier frequency offset; (4) the complex out-of-band (OOB) filtering. Over the past six decades, a variety of waveforms have been developed in order to mitigate these deficiencies, which are generally achieved at the cost of compromising some of OFDM’s beneficial properties, such as its subcarrier (SC) orthogonality, its high throughput and its straighforward adoption to multiple-input multiple-output (MIMO) systems. Against this background, we propose a new waveform termed as multi-band discrete Fourier transform spread-OFDM with index modulation (MB-DFT-S-OFDM-IM), where the component multi-carrier techniques are conceived to constructively function together in order to mitigate the OFDM deficiencies <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">without compromising the beneficial OFDM properties</i> . More explicitly, first of all, the PAPR is reduced by the DFT-precoding. Secondly, thanks to the IM design, MB-DFT-S-OFDM-IM is capable of achieving a high throughput that is strictly equal to or higher than the OFDM throughput. Thirdly, MB-DFT-S-OFDM-IM achieves a beneficial frequency diversity gain, which leads to a higher tolerance to carrier frequency offset. Fourthly, the OOB filters are placed in each sub-band before DFT, so that the SC orthogonality remains intact, which is unique to the proposed MB-DFT-S-OFDM-IM structure. Last but not least, we extend the proposed MB-DFT-S-OFDM-IM to support a variety of MIMO schemes, where the IM philosophy is integrated with the space-, time- and frequency-domains within a single <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">unified</i> platform.

Split Channel Two-Tone On-Off Keying for Internet of Things Communication in Fading Channel

In medical treatment, healthcare and home appliances, the demand for Internet of Things (IoT) services is increasing in urban areas. However, in fading environments of urban area, the existing IoT communication system can degrade severely. The purpose of this paper is to develop a new IoT communication technology that improves communication performance in fading channels. IoT communication systems are aiming at low power long distances. In addition, the amount of data transmission is small due to the service requirements. To achieve this goal, existing IoT communication modems are designed with narrowband signals. However, narrowband signals may cause performance degradation in a frequency selective fading channel environments in urban area. In this paper, we propose a split channel two tone OOK (SC-TT-OOK) that implements on-off keying (OOK) using two tones. Unlike the existing OOK by using one tone, SC-TT-OOK uses spaced two tones to improve communication performance in fading channels. The conventional OOK method has a similar characteristic to that of the existing IoT modems with a narrow bandwidth spectrum. The proposed SC-TT-OOK is OOK using two separated tones. Since the proposed modulation uses two tones, it requires twice the frequency resources of the conventional OOK. However, if the frequencies of the two tones are sufficiently spaced apart, frequency diversity gain can be achieved in the frequency selective fading channels, and communication reliability can be improved in urban environments. The proposed SC-TT-OOK is examined through computer simulation. The simulation results show that the proposed scheme outperforms the conventional scheme in the frequency selective fading channel. Therefore, the proposed method can be used as an alternative technique for IoT modem in urban environments.

Space Time Regularized Zero Forcing in Downlink Code Division Multiple Access Systems with Complementary Codes

Complementary coded code-division multiple access (CC-CDMA) has been originated as one of the extremely robust multiuser access technique in designing high data rate systems with frequency diversity for futuristic wireless communication. The integration of multiple-input multiple-output (MIMO) systems with CC-CDMA offers space diversity gain in addition to the achievable frequency diversity in CC-CDMA systems. However, the performance of conventional chip level space–time receiver in MIMO CC-CDMA is deteriorated due to multiple access interference (MAI) and spatial interference under frequency selective fading channels. A zero-forcing receiver used for minimizing MAI provides unsatisfactory performance in interference limited environments due to amplification of noise and hence the use of regularized zero-forcing (RZF) receiver is proposed and investigated in this study to achieve a space–time interference cancellation (STIC) system. Simulation results are performed to reveal the significance of RZF-STIC in achieving performance gain than conventional equalization schemes, and at the same time enhancing frequency diversity and spatial diversity gain with less complexity at all system loads.

Successive interference cancellation in asynchronous CC-CDMA systems under Rician fading channels

The unprecedented frequency diversity gain offered by complementary coded code-division multiple access (CC-CDMA) makes it more attractive than conventional CDMA systems in frequency selective fading channels. CC-CDMA loses its interference-resilient capability due to non-ideal correlation characteristics of complementary codes (CC) under uplink asynchronous communications. The existence of multiple access interference (MAI) in asynchronous systems results in drastic reduction in error rate performance in CC-CDMA systems. In this paper, the analysis of successive interference cancellation (SIC) is proposed for CC-CDMA systems as an alternative to existing non-interference cancellation schemes in uplink communication. The average bit error probability expressions for single branch and M-branch CC-CDMA systems operating under Rician fading channels are derived using alternate integral expression of Q-function. The accuracy of obtained closed form error probability expressions are confirmed through the proposed joint SIC with maximal ratio combining and minimum mean square error combining techniques in uplink CC-CDMA systems. The significance of soft-decision compared to hard decision SIC for CC-CDMA systems is also analyzed for both equal power and extreme near-far conditions. Furthermore, the simulation results exposed that significant performance improvement is obtained under near-far situations in uplink asynchronous CC-CDMA systems overcoming the effects of MAI and achieving improved system capacity.

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.

Interference Cancellation in CDMA Systems Employing Complementary Codes Under Rician Fading Channels

The ideal correlation properties of single user code division multiple access (CDMA) systems with complementary codes (CC) are lost by multiple access interference and near–far effects in downlink frequency selective fading channels. This severity in interference has motivated to develop ways to overcome the threats with suitable suboptimal interference cancellation schemes. In this paper, we propose successive interference cancellation (SIC) for downlink CDMA systems with CC to improve the system capacity and reduce error rate under near–far situations. Theoretical study and extensive simulations were conducted to verify the effectiveness of proposed SIC under frequency selective Rician fading channels in achieving frequency diversity gain, close to theoretical lower bound in complementary coded CDMA (CC-CDMA) systems.

Throughput Maximization by Adaptive Switching with Modulation Coding Scheme and Frequency Symbol Spreading

It is required to realize higher transmission rate and higher reliability for mobile communication due to the increase in Internet use. However, wireless channel capacity can not be used with maximum efficiency due to fluctuating channels affected by shadowing, multipath fading and mobility. Adaptive modulation and coding (AMC) scheme is now commonly implemented to maximize the throughput performance under the given link qualities. Forward Error Correction (FEC) based link adaptation is effective to improve throughput in a lower SNR regime, however, it immolates maximal throughput in good channel condition. Frequency symbol spreading (FSS) has been proposed that can improve BER even without FEC. It fully exploits the frequency diversity gain by spreading symbol per subcarrier to all frequency components. This paper proposes a new adaptation control scheme for OFDM by switching FSS and legacy AMC. Simulation result verifies its maximized throughput performance harvesting both of frequency diversity gain and coding gain.

Wirelessly Powered Backscatter Communications: Waveform Design and SNR-Energy Tradeoff

This paper shows that wirelessly powered backscatter communications is subject to a fundamental tradeoff between the harvested energy at the tag and the reliability of the backscatter communication, measured in terms of SNR at the reader. Assuming the RF transmit signal is a multisine waveform adaptive to the channel state information, we derive a systematic approach to optimize the transmit waveform weights (amplitudes and phases) in order to enlarge as much as possible the SNRenergy region. Performance evaluations confirm the significant benefits of using multiple frequency components in the adaptive transmit multisine waveform to exploit the nonlinearity of the rectifier and a frequency diversity gain.

Backtracking Codebook Matching Algorithm Based on Codebook Correlation for Uplink SCMA System

In this letter, we have investigated the frequency diversity gain of a sparse code multiple access (SCMA) system to improve spectrum utilization by exploiting the codebook correlation in the SCMA system. To this end, a backtracking codebook matching (BCM) algorithm is proposed, whereby there are more successfully scheduled users, which are allocated the low-correlated codebooks. Simulation results exhibit the superiority of the BCM algorithm and the impact of codebook correlation on scheduling.

Open-Loop Correlation Reduction Precoding in Overloaded MIMO-OFDM Systems

This paper proposes an open-loop correlation reduction precoding scheme for overloaded multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. In overloaded MIMO-OFDM systems, frequency diversity through joint maximum likelihood (ML) decoding suppresses performance degradation owing to spatial signal multiplexing. However, on a line-of-sight (LOS) channel, a channel matrix may have a large correlation between coded symbols transmitted on separate subcarriers. The correlation reduces the frequency diversity gain and deteriorates the signal separation capability. Thus, in the proposed scheme, open-loop precoding in order to reduce the correlation between codewords transmitted on different signal streams is employed at the transmitter of an overloaded MIMO system. The proposed precoding scheme changes the amplitude as well as the phase of the coded symbols transmitted on different subcarriers. Numerical results obtained through computer simulation show that the proposed scheme improves the bit error rate performance on Rician channels.

Precoded generalised frequency division multiplexing system to combat inter‐carrier interference: performance analysis

The expected operating scenarios of fifth-generation (5G) pose a great challenge to orthogonal frequency division multiplexing which has poor out of band spectral properties, stringent synchronisation requirements and large symbol duration. Generalised frequency division multiplexing (GFDM) which is the focus of this work has been suggested in the literature as one of the possible solutions to meet 5G requirements. In this study, the analytical performance evaluation of minimum mean square error (MMSE) receiver for GFDM is presented. The authors also proposed precoding techniques to enhance the performance of GFDM. A simplified expression of signal-to-interference and noise ratio (SINR) for MMSE receiver of GFDM is derived using special properties related to the modulation matrix of GFDM, which are described in this study. This SINR is used to evaluate the bit error rate performance. Precoding schemes are proposed to reduce complexity of GFDM–MMSE receiver without compromising on the performance. Block inverse discrete Fourier transform (BIDFT) and discrete Fourier transform (DFT)-based precoding schemes are found to outperform GFDM–MMSE receiver due to frequency diversity gain while having complexity similar to zero-forcing receiver of GFDM. It is shown that both BIDFT- and DFT-based precoding schemes reduce peak-to-average power ratio significantly. Computational complexities of different transmitters and receivers of precoded and uncoded GFDM are also presented.

Frequency Diversity-Aware Wi-Fi Using OFDM-Based Bloom Filters

With the increasing move towards wide band operation in recent Wi-Fi networks, the frequency diversity awareness has become critical for throughput optimization. To exploit frequency diversity in Wi-Fi channels, the access point should measure the channel quality and coordinate the channel contention for all the stations. However, there is a tradeoff between achieving the frequency diversity gain and sustaining protocol efficiency because the channel estimation and coordination consume time and frequency resource that ideally should be used for data transfer. In this paper, we present Diversity-aware Wi-Fi (D-Fi), a novel PHY/MAC protocol, that efficiently exploits frequency diversity. In particular, D-Fi leverages an OFDM-based Bloom filter that synergistically integrates two operations: (i) the channel quality estimation and (ii) the contention based channel allocation. D-Fi also employs a machine learning (ML) method to resolve the false-positive ambiguity caused by the Bloom filter. Furthermore, we develop a decentralized algorithm, called $K\epsilon$ -greedy , based on the Multi-Armed Bandit (MAB) framework, so that it achieves sub-optimal performance by studying the gain for exploring new channel quality information. We implement the prototype of D-Fi on the USRP/GNURadio to validate the feasibility of our work. The experiments and trace-driven simulations show that D-Fi provides up to 3 $\times$ throughput improvement compared to the existing solutions.

PB/MC-CDMA 시스템에서 결정 귀한 채널 추정 기법을 이용한 성능 향상 방법

Abstract In this paper, to improve the performance of PB/MC-CDMA system, we have researched about the decision feedback channel estimation method using the pilot symbol with data symbol. The PB/MC-CDMA system is able to obtain the improved BER by frequency diversity gain and frequency domain equalizer in the frequency selective fading channel. However, when it is not the estimating of channel exactly, it is degrading the performance of BER in the system for occurred the interference among users. To improve the performane system in the multi-user environment the proposed system is using the decision feedback to estimate channel using the channel estimated value of the first stage and second stage. The proposed system is evaluated by computer simulation. The proposed system is not only able to improve the performance of BER by decreasing the interference to each user, but also the proposed system is possible to reduce number of pilot symbol to estimate the channel. Therefore, it confirmed the proposed system improves the performance than the conventional system.

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

SummaryWe 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 &amp; Sons, Ltd.

상향링크 SIMO 시스템에서 공간 및 주파수 다이버시티 이득에 따른 SC-FDMA의 BER 성능 분석

상향링크 SIMO(Single Input Multiple Output) 시스템의 SC-FDMA 기법에서 공간 및 주파수 다이버시티 이득에 따른 BER(Bit Error Ratio) 성능 변화를 분석한다. 본 논문에서 분석한 주요내용은 다음과 같다. 첫째, 공간 다이버시티 컴바이닝과 주파수 다이버시티 컴바이닝을 동시에 수행할 수 있는 통합된 시스템과 공간 다이버시티 컴바이닝과 주파수 다이버시티 컴바이닝을 순서대로 수행하는 단계별 시스템이 동등한 성능을 가지는 것을 확인한다. 단계별 시스템의 주파수 다이버시티 컴바이닝 기법과 통합된 시스템의 다이버시티 컴바이닝 기법이 동일할 때, 단계별 시스템에서 주파수 다이버시티 컴바이닝보다 공간 다이버시티 컴바이닝을 선행하면서 공간 다이버시티 컴바이닝 기법을 MRC(Maximal Ratio Combining)로 하면 두 시스템의 성능이 동일함을 신호 모형화 결과를 통해 증명한다. 둘째, 신호 모형화 결과와 BER 실험 결과를 통해 공간 다이버시티 이득과 주파수 다이버시티 이득이 각각 성능에 어떤 영향을 미치는지 분석한다. 부반송파 개수가 증가함에 따라 주파수 다이버시티 이득이 증가함을 알 수 있고 이는 주파수 다이버시티 기법이 ZF(Zero Forcing)일 때의 성능과 MMSE(Minimum Mean Square Error)일 때의 성능 차이는 유지하면서 높은 SNR(Signal to Noise Ratio) 영역의 성능 향상에 영향을 미치는 것을 보인다. 그리고 수신안테나 개수의 증가는 공간 다이버시티 이득을 증가시키며 공간 다이버시티 이득의 증가는 모든 SNR 영역의 성능을 향상시키면서 주파수 다이버시티 컴바이닝이 ZF일 때와 MMSE일 때의 성능 차이를 줄이는데 영향을 미침을 보여준다. 마지막으로, 공간 다이버시티 이득이 신호 모형화 유도과정에서 어떤 영향을 미치는지 분석하여 수신안테나 개수가 6개 이상이면 주파수 다이버시티 컴바이닝을 ZF으로 했을 때의 성능이 MMSE로 했을 때의 성능을 대체할 수 있음을 확인할 수 있다. We investigate BER (Bit Error Ratio) performance according to the gain of spatial and frequency diversities in uplink SC-FDMA of SIMO (Single Input Multiple Output) systems. The main results of the analysis in this paper are as follows. First, we prove that performance of integrated system for considering spatial and frequency diversity combining in parallel is equivalent with the performance of sequential system for performing diversity combining in sequence. By signal modeling, it is demonstrated that the performances of both systems are the same when the frequency diversity combining technique of the sequential system is equal to diversity combining technique of the integrated system, and spatial diversity combining technique of the sequential system is performed as MRC in advance of frequency diversity combining. Secondly, it is found that effect on the BER performance is different according to the gain of spatial and frequency diversities, respectively. The frequency diversity gain increases by increasing the number of subcarrier. It might affect the performance improvement of high SNR(Signal to Noise Ratio) while it maintains gap between performances of ZF(Zero Forcing) and MMSE(Minimum Mean Square Error) in frequency diversity combining schemes. Also, spatial diversity gain increases as the number of receiving antennas increases. It means that it can reduce performance gap between ZF and MMSE in frequency diversity combining schemes by increasing the number of receiving antennas. In addition, it might affect the performance improvement of the whole SNR. Finally, through the analysis of performance according to the spatial diversity gain, the performance of ZF in frequency diversity combining is equal to the MMSE if the number of receiving antennas is 6 or more.

Invariant Target Detection in Multiband FM-Based Passive Bistatic Radar

FM-based passive bistatic radars (PBRs) exploiting a single broadcasted channel suffer from time-varying detection performance due to the time-varying program content of the transmitted signal as well as the propagation condition of that channel. To circumvent this matter, it is a viable idea to exploit multiple broadcasted channels from a single transmitter for detection. In this case, we formulate the problem of detecting a target in the presence of interference signals such as receiver noise, direct signal, multipath/clutter echoes and interfering targets as a composite hypothesis test. To address this, we derive a multiband uniformly most powerful invariant (UMPI) test as an optimal invariant detector. Closed-form expressions for calculating false alarm probability and detection probability for the target models of Swerling 0 and Swerling 1 are derived. A thorough performance assessment is also given, and the results show the robustness of the proposed multiband detector against the time-varying program content of FM radio channels. In addition to the reliable detection performance, a multiband PBR system offers advantages in terms of combined diversity gain and frequency diversity gain as compared with a single band PBR system. The former happens due to the joint exploitation of multiple broadcasted channels for detection, while the latter occurs when independent returns are received from one target over different frequency channels.