Frequency-selective Fading Channels Research Articles

Fast Convolution Filter-Bank Based Non-Orthogonal Multiplexed Cognitive Radio (NOMCR) Receiver Design Using Cyclostationarity Based FRESH Filtering

Non-orthogonal multiple access (NOMA) systems are being considered as candidates for 5G wireless systems due to their promise of improved spectral efficiency. NOMA schemes are being combined with popular multicarrier schemes such as orthogonal frequency division multiplexing (OFDM) to take advantage of the benefits of multicarrier signals. A variant of the power domain NOMA is Layer Division Multiplexing (LDM). The most commonly deployed power domain LDM scheme involves successive interference cancellation (SIC) based decoding at the receiver. Fast convolution based filtered-OFDM (FC-F-OFDM) systems are becoming popular among 5G wireless access technologies due to their ability to process 5G physical layer signals efficiently. In this work, firstly, a cognitive multicarrier non-orthogonal multiplexed system based on the concept of LDM is discussed, which uses FC-F-OFDM and conventional OFDM as its component layers. Secondly, cyclostationary FREquency SHift (FRESH) filter based SIC decoding is used at the receiver side, which also utilizes artificial neural network (ANN) processing. Computer simulations indicate that the system provides good bit error rate (BER) performance under frequency selective Rayleigh fading channels.

Distributed Space Time Block Transmission and QRD Based Diversity Detector in Asynchronous Cooperative Communications Systems

Based on the zero prefix (ZP) structure, we propose an efficient block transmission algorithm using distributed space-time codes in the asynchronous cooperative communication systems. To accomplish the new frame, ZP is inserted in front of the data block to combat the relative delay between the asynchronous relays, while zero suffix (ZS) is appended at the end of the block to remove the interblock interference caused by the time dispersing channel; so, we employ this ZP–ZS precoding form to make the coded frames synchronous. It should be noted that single carrier transmission and orthogonal frequency division multiplexing (OFDM) are both compatible with the frame. For single carrier transmission, conventional space-time equalizers can be investigated at the destination node to cancel the multipath interference. To further improve the performance, an improved QR decomposition-based recursive interference cancellation (QRD-RIC) detector is employed, where joint QRD is adopted on the combination of channel matrices from different relays, and then spatial-and-multipath diversity combinations are introduced and improved into the proposed system, where the spatial and multipath diversities are acquired. For OFDM, the fractionally spaced frequency-domain MMSE diversity receiver is employed, since the guard interval in our framing scheme implies oversampling in the frequency domain. Numerical simulations show that the proposed algorithms are effective for the asynchronous transmissions under frequency selective fading channels.

A MC-CDMA system based on orthogonal filter banks of wavelet transforms and partial combining

Multi Carrier Code Division Multiple Access (MC-CDMA) is an appropriate modulation technique for high data rates. In this modulation scheme, employing of combining techniques are unavoidable for restoring orthogonality between different user signals. In this paper, the combining techniques of a MC-CDMA based on wavelet transform are studied. The partial combining of the wavelet based MC-CDMA system is suggested and its performance is compared with the maximum ratio combining (MRC), equal gain combining (EGC), orthogonality restoring combining (ORC) and the minimum mean square error (MMSE) techniques. Orthogonal filter banks of the wavelet transforms provide orthogonality between the wavelets and scaling functions, subcarriers and spreading sequences. Therefore, they provide new dimensions in combatting multipath fading, inter carrier interference (ICI) and narrowband interference in MC-CDMA systems. Furthermore, analysis of partial combining shows that coefficients of this technique can be computed in a simple manner. Simulations results indicate that the bit error rate (BER) performance of the proposed scheme is almost comparable with the MMSE combining scheme and the proposed system has a proper performance in terms of BER versus the signal to noise ratio (SNR) in frequency selective fading channels.

Multi‐level construction of polar‐coded single carrier‐FDMA based on MIMO antennas for coded cooperative wireless communication

A multi-level construction of polar-coded single carrier frequency division multiple access (SC-FDMA) with multiple input multiple output (MIMO) antennas scheme is proposed and its bit error rate performance over frequency selective Rayleigh fading channels has been evaluated. The existence of a parallel split in the design of multi-level construction of polar code has enabled the authors to effectively extend the proposed scheme in coded cooperative scenarios. As in coded cooperation communication, the relay plays a crucial role in the design of communication system; therefore, an efficient selection criterion for reinforcing the bad channels has been adopted at the relay node. Moreover, the joint successive cancellation decoding is employed at the destination node. To check the efficacy in the performance of the proposed coded cooperative scheme, the authors have also upgraded the traditional nested polar-coded cooperative scheme in the context of SC-FDMA with MIMO antennas as a useful benchmark. Monte Carlo simulated results revealed that the proposed coded cooperative scheme have triumphed the conventional nested polar coded cooperative scheme based on SC-FDMA system by gain of 0.5 dB under identical conditions. Moreover, the proposed cooperative schemes outperform their corresponding non-cooperative counterpart schemes by the gain of 1 dB under identical conditions.

Low‐complexity multiple‐signal joint decoding for overlapped x domain multiplexing signalling

By multiplexing waveform mapped with shifting and weighted symbols, overlapped x domain multiplexing (OVXDM) encoding scheme can obtain high spectral efficiency under a low signal-to-noise ratio. Though the maximum likelihood sequence detection decoders can obtain the optimal decoding performance, their computational complexities which increase exponentially with spectral efficiency in OVXDM are unbearable for practical implementations. In this study, a low-complexity multiple-signal joint decoding (MSJD) algorithm for OVXDM signalling is proposed over additive white Gaussian noise (AWGN) channel and extended to frequency selective fading (FSF) channel. Based on a novel joint information-based metric, the proposed algorithm utilises information of each transmitted symbol contained in multiple received signals, and selects the proper number of received signals for decoding to achieve performance maximisation. Moreover, theoretical analysis is provided for the proposed algorithm in both AWGN and FSF channels, which indicates the relationship between performance and number of received signals under different OVXDM encoding parameters including multiplexing waveform and overlapping fold. Simulation results confirm the superior performance of the MSJD algorithm in terms of decoding performance and computational complexity compared with the present approaches.

Spectral efficiency analysis and pilot reuse factor optimisation for multi‐cell massive SC‐SM MIMO

As a combination of spatial modulation (SM) system and massive multiple-input multiple-output (MIMO) system, massive SM aided MIMO (SM-MIMO) system is recently proposed. In broadband scenarios, single-carrier (SC) modulation is introduced to massive SM-MIMO system, thus massive SC-SM MIMO system is proposed for uplink multi-user transmission over frequency-selective fading channels. In this study, the uplink spectral efficiency (SE) of multi-cell massive SC-SM MIMO system is analysed, meanwhile the pilot contamination effect is taken into consideration. A tight SE lower bound of multi-cell massive SC-SM MIMO system is proposed with maximum ratio (MR) combining, which also takes into account the imperfect channel estimation, transmit antenna correlation and path loss. The tightness of the authors' proposed closed-form SE lower bound is shown via simulation results. The optimal pilot reuse factor can be determined with different system configurations by simulations, and the pilot reuse factor that is larger than one is more suitable for less TAs and user equipments.

Design and analysis of a DCSK-based cognitive radio system using hybrid underlay/overlay transmission

In this paper, we present a hybrid Underlay/Overlay DCSK-based Cognitive Radio (DCR) system using the Cognitive Radio techniques and abilities of a Multi-Carrier DCSK (MC-DCSK) scheme together, which can be extended to many other similar non-coherent scenarios. In this scheme, secondary user (SU) employs the spectrum holes as a dedicated frequency to transmit reference signals in overlay mode and uses remaining common frequencies to transmit data bearing signals by an underlying approach. We derived an analytical BER expression for the DCR scheme over a frequency selective fading channel and adopted it as our optimization criterion. To formulate the optimization problem, we proposed a sub-carrier grouping approach in order to implement our min–max fairness strategy, considering the power budget and interference temperature constraints. The proposed DCR scheme not only employed spectrum holes, but also improved BER performance of the conventional multi-user MC-DCSK system significantly considering the power allocation policy and robustness against frequency selective channel. Finally, simulation results validated the advantages of the proposed scheme.

Iterative Receivers for Downlink MIMO-SCMA: Message Passing and Distributed Cooperative Detection

The rapid development of the mobile communications requires ever higher spectral efficiency. The non-orthogonal multiple access (NOMA) has emerged as a promising technology to further increase the access efficiency of wireless networks. Amongst several NOMA schemes, the sparse code multiple access (SCMA) has been shown to be able to achieve better performance. In this paper, we consider a downlink MIMO-SCMA system over frequency selective fading channels. For optimal detection, the complexity increases exponentially with the product of the number of users, the number of antennas and the channel length. To tackle this challenge, we propose near optimal low-complexity iterative receivers based on factor graph. By introducing auxiliary variables, a stretched factor graph is constructed and a hybrid belief propagation (BP) and expectation propagation (EP) receiver, named as `Stretch-BP-EP', is proposed. Considering the convergence problem of BP algorithm on loopy factor graph, we convexify the Bethe free energy and propose a convergence-guaranteed BP-EP receiver, named as `Conv-BP-EP'. We further consider cooperative network and propose two distributed cooperative detection schemes to exploit the diversity gain, namely, belief consensus-based algorithm and Bregman alternative direction method of multipliers (ADMM)-based method. Simulation results verify the superior performance of the proposed Conv-BP-EP receiver compared with other methods. The two proposed distributed cooperative detection schemes can improve the bit error rate performance by exploiting the diversity gain. Moreover, Bregman ADMM method outperforms the belief consensus-based algorithm in noisy inter-user links.

Bit Error Probability for MMSE Receiver in GFDM Systems

In this letter, we consider the minimum mean-square error receiver for the generalized frequency division multiplexing system (GFDM) over frequency selective fading channels. We derive an approximate probability density function for the signal-to-interference-plus-noise ratio. This expression allows us to obtain a new approximate, but rather accurate formulation for the bit error probability for a $\mathcal {M}$ -quadrature amplitude modulation scheme. Our results resort on the pivotal properties exhibited by eigenvalues of a circulant matrix. Since the entries of the channel matrix $\text {H}_{\text {ch}}$ are complex Gaussian distributed, and the eigenvalues are given as a weighted sum of its entries, the joint eigenvalue distribution is also Gaussian. Comparisons of the simulated and analytical results validate our formulation and allow a quick and efficient tool to compute the bit error rate for the GFDM system.

Enhanced channel estimation in OFDM systems with neural network technologies

Orthogonal frequency division multiplexing (OFDM) provides an effective and low complexity means of eliminating inter-symbol interference for transmission over frequency selective fading channels. In OFDM systems, channel state information (CSI) is required for the OFDM receiver to perform coherent detection or diversity combining, if multiple transmit and receive antennas are deployed. In practice, CSI can be reliably estimated at the receiver by transmitting pilots along with data symbols. In this paper, we investigate and compare various efficient pilot-based channel estimation schemes by neural network technologies for OFDM systems. We present further the application of functional link neural fuzzy network (FLNFN) for channel estimation in the investigated OFDM systems. We compared bit error rates of the proposed neural network with that of the other neural network technologies, the least square (LS) algorithm, and the minimum mean square error (MMSE) algorithm. Our results demonstrate that the proposed FLNFN algorithm can enhance the performance of channel estimation in existing OFDM channel environments.

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.

Achieving Full Diversity on a Single-Carrier Distributed QOSFBC Transmission Scheme Utilizing PAPR Reduction

In this paper, a cooperative single-carrier transmission scheme considering a distributed quasi-orthogonal space-frequency block code (QOSFBC) that provides improved peak-to-average power ratio (PAPR) performance is constructed for uplink communications. Focused on reducing the size and computational complexity, the source node, i.e., the mobile devices, is equipped with two transmit antennas, while only one antenna is required at the relay node in order to collaboratively generate a distributed QOSFBC signal using four transmit antennas. To achieve full diversity, a phase rotation strategy is proposed at the relay node. In addition, since single-carrier schemes considering SFBCs re-permute the spectral components of the transmitted signal and induce high PAPR value, a modified SFBC coding technique is proposed that allows the high-PAPR problem to be mitigated. Consequently, the designed distributed single-carrier frequency-division multiple access (SC-FDMA) QOSFBC scheme is able to provide full transmit diversity over frequency-selective fading channels, with a lower PAPR value on mobile devices when compared with a conventional SC-FDMA QOSFBC scheme.

Optimal power allocation scheme for distributed antenna system with OFDM in frequency‐selective fading channels

Optimal power allocation (PA) scheme for distributed antenna systems with orthogonal frequency division multiplexing (DAS-OFDM) over frequency-selective fading channels is presented. The PA design is to optimise the transmit powers of subcarriers to maximise the capacity subject to the total power constraint. Based on the Karush–Kuhn–Tucker optimality conditions, the optimal PA to the distributed nodes for a subcarrier can be simplified to allocating the power to the node of the largest channel gain. With this result, an efficient algorithm is derived to obtain the optimal PA to the nodes for all the subcarriers of the DAS-OFDM. Compared with an existing suboptimal scheme, the developed scheme can obtain higher capacity and has lower complexity. By computer simulations, the effectiveness of the proposed scheme is verified.

Cooperative Ambient Backscatter Communications for Green Internet-of-Things

Ambient backscatter communication (AmBC) enables a passive backscatter device to transmit information to a reader using ambient RF signals, and has emerged as a promising solution to green Internet-of-Things (IoT). Conventional AmBC receivers are interested in recovering the information from the ambient backscatter device (A-BD) only. In this paper, we propose a cooperative AmBC (CABC) system in which the reader recovers information not only from the A-BD, but also from the RF source. We first establish the system model for the CABC system from spread spectrum and spectrum sharing perspectives. Then, for flat fading channels, we derive the optimal maximum-likelihood (ML) detector, suboptimal linear detectors as well as successive interference-cancellation (SIC) based detectors. For frequency-selective fading channels, the system model for the CABC system over ambient orthogonal frequency division multiplexing carriers is proposed, upon which a low-complexity optimal ML detector is derived. For both kinds of channels, the bit-error-rate expressions for the proposed detectors are derived in closed forms. Finally, extensive numerical results have shown that, when the A-BD signal and the RF-source signal have equal symbol period, the proposed SIC-based detectors can achieve near-ML detection performance for typical application scenarios, and when the A-BD symbol period is longer than the RF-source symbol period, the existence of backscattered signal in the CABC system can enhance the ML detection performance of the RF-source signal, thanks to the beneficial effect of the backscatter link when the A-BD transmits at a lower rate than the RF source.

Multiple-Threshold Estimators for Impulsive Noise Suppression in Multicarrier Communications

Multicarrier wireless communication systems, which usually operate over frequency-selective fading channels, are practically also impaired by environmental impulsive noise. In order to boost the signal-to-noise power ratio (SNR) at the receiver, this paper proposes and analyzes nonlinear estimators based on the multiple thresholding, with associated piecewise attenuation, or clipping, of the received signal amplitude. The proposed approach exploits a new heuristic criterion to set the thresholds. Although the obtained thresholds are slightly suboptimal in output SNR, the proposed approach has the nice and useful property to allow closed-form analytical derivations for both the threshold(s) and the associated attenuating/clipping parameters, when a Gaussian source is impaired by an impulsive noise. Such a quite general class of estimators, which could be applied also in other scenarios, is particularly attractive for orthogonal frequency division multiplexing communication systems in the presence of additive multicomponent impulsive noise. We are also letting to derive closed-form expressions for the output SNR in frequency-selective multipath fading channels. The SNR performance and the associated symbol error rate have been compared with those of more traditional blanking, clipping, and clipping-blanking processors. Specifically, analytical and simulation results, carried out for Class-A and symmetrical alpha-stable (S $\alpha$ S) impulsive noise, show that the proposed threshold-based suppressors are superior to the traditional ones. Furthermore, as the number of thresholds increases, the proposed estimators closely approach the performance of the optimal minimum mean square error Bayesian estimator. However, as it is shown, in practical conditions only few thresholds are necessary.

Doppler Spread Estimation in MIMO Frequency-Selective Fading Channels

One of the main challenges in high-speed mobile communications is the presence of large Doppler spreads. Thus, accurate estimation of maximum Doppler spread (MDS) plays an important role in improving the performance of the communication link. In this paper, we derive the data-aided (DA) and non-data-aided (NDA) Cramer–Rao lower bounds (CRLBs) and maximum likelihood estimators (MLEs) for the MDS in multiple-input multiple-output (MIMO) frequency-selective fading channel. Moreover, a low-complexity NDA-moment-based estimator (MBE) is proposed. The proposed NDA-MBE relies on the second- and fourth-order moments of the received signal, which are employed to estimate the normalized squared autocorrelation function of the fading channel. Then, the problem of MDS estimation is formulated as a non-linear regression problem, and the least-squares curve-fitting optimization technique is applied to determine the estimate of the MDS. This is the first time in the literature, when DA- and NDA-MDS estimation is investigated for MIMO frequency-selective fading channel. Simulation results show that there is no significant performance gap between the derived NDA-MLE and NDA-CRLB, even when the observation window is relatively small. Furthermore, the significant reduced-complexity in the NDA-MBE leads to low root-mean-square error over a wide range of MDSs, when the observation window is selected large enough.

Non-data aided joint estimation of symbol timing offset and carrier frequency offset for OFDM/OQAM systems

This paper deals with the problem of blind synchronization for orthogonal frequency division multiplexing based on offset quadrature amplitude (OFDM/OQAM) systems without the prior information of the channel response and transmitting signals power. A novel non-data aided (NDA) joint estimation method of symbol timing offset (STO) and carrier frequency offset (CFO) for OFDM/OQAM is conceived in multipath fading channels condition. The multipath fading channels are equivalent to multiple parallel sub-channels with flat Rayleigh fading characteristics firstly, and then STO and CFO are estimated by the second-order cyclic-cumulant with different cycle frequency values and time delay values. Moreover, the modified Cramer-Rao Bound (MCRB) of NDA joint STO and CFO estimation is presented. The simulation results demonstrate that the proposed method is effective and feasible over frequency selective fading channels.

Reverse-link Power Allocation with Distributed Dynamic Channel Assignment in Mobile Cellular Systems

In this paper, assuming Direct Stage Code Division Multiple Access (DS-CDMA) [1], the author evaluate the throughput performance of turbo coded ARQ schemes over frequency-selective Rayleigh fading channel with antenna diversity reception and Rake combining. The theoretical expression for the turbo code in the presence of multiple channels was derived and the throughput performance evaluated by computer simulations firstly for ideal channel estimation and later compared with a practical channel estimation scheme called weighted multi-slot averaging (WMSA) channel estimation [2]. Studying the effect of various system parameters like the frame length, the fading rate, the spreading factor, the number of diversity antennas, the number of transmissions, number of users and propagation parameters like the fading rate and the power delay profile of the multipath channel (number of paths and profile shape) is the major contribution of this work.

An enhanced channel estimation technique for CDD OFDM system over time varying channels

Cyclic delay diversity (CDD), employing multiple antennas provides increased frequency-selectivity and thereby achieves frequency diversity using forward error correction in orthogonal frequency division multiplexing (OFDM). However, channel estimation of CDD OFDM becomes crucial over frequency-selective fading channels. In this paper, we propose a low-complexity channel estimation technique for CDD OFDM systems over time-varying channels using basis expansion model. We first analyze the frequency correlation of CDD OFDM channels and then design scattered pilot structure for estimating the channel coefficients. In the proposed channel estimator, Slepian sequences are employed estimate the channel impulse response by exploiting the frequency correlation of CDD OFDM over time-varying channel. Simulation results demonstrated that the proposed channel estimator achieves better mean square error and bit-error-rate performance compared with that of existing methods over time-varying channels.

Enhanced Widely Linear Filtering to Make Quasi-Rectilinear Signals Almost Equivalent to Rectilinear Ones for SAIC/MAIC

Widely linear (WL) receivers have the capability to perform single antenna interference cancellation (SAIC) of one rectilinear (R) or quasi-rectilinear (QR) co-channel interference (CCI), a function which is operational in global system for mobile communications (GSM) handsets in particular. Moreover, SAIC technology for QR signals is still required for voice services over adaptive multi-user channels on one slot (VAMOS) standard, a recent evolution of GSM/EDGE standard, to mitigate legacy GSM CCI in particular. It is also required for filter bank multi-carrier offset quadrature amplitude modulation (FBMCOQAM) networks, which are candidate for 5G mobile networks, to mitigate inter-carrier interference (ICI) at reception for frequency selective propagation channels in particular. In this context, the purpose of this paper is twofold. The first one is to get more insights into the existing SAIC technology, and its extension to multiple antenna called MAIC, by showing analytically that, contrary to what is accepted as true in the literature, SAIC/MAIC implemented from standard WL filtering may be less efficient for QR signals than for R ones. From this result, the second purpose of the paper is to propose and to analyze, for QR signals and frequency selective fading channels, a SAIC/MAIC enhancement based on a three-input WL frequency shift (FRESH) receiver, making QR signals always almost equivalent to R ones for WL filtering in the presence of CCI. The results of the paper, completely new, may contribute to develop elsewhere new powerful WL receivers for QR signals and for both VAMOS and FBMC-OQAM networks in particular.