Time-selective Fading Channels Research Articles

Secrecy Outage Probability and Secrecy Diversity Order of Alamouti STBC with Decision Feedback Detection over Time-Selective Fading Channels

Secrecy Outage Probability and Secrecy Diversity Order of Alamouti STBC with Decision Feedback Detection over Time-Selective Fading Channels

Secrecy outage probability and diversity order of Alamouti STBC over time-selective fading channels

Secrecy outage probability and diversity order of Alamouti STBC over time-selective fading channels

Indexed-channel estimation under frequency and time-selective fading channels in high-mobility systems

<p><span lang="EN-US">Index modulation (IM) techniques have been employed in different communication systems to improve bandwidth efficiency by carrying additional information bits. In high-mobility communication systems and under both time-selective and frequency-selective fading channels with Doppler spread, channel variations can be tracked by employing pilot-aided channel estimation with minimum mean-squared error estimation. However, inserting pilot symbols among information symbols reduces the system's spectral efficiency in pilot-aided channel estimation schemes. We propose pilot-aided channel estimation with zero-pilot symbols and an energy detection scheme to tackle this issue. Part of the information bit-stream is conveyed by the indices of zero-pilot symbols leading to an increase in the system's spectral efficiency. We used an energy detector at the receiver to detect the transmitted zero-pilot symbols. This paper examines the impacts of diversity order on the zero-pilot symbol detection error probability and the mean-squared of error estimation. The impacts of pilot symbols number and the zero-pilot symbol number on the mean-squared error of the minimum mean-squared error (MMSE) estimator and the system error performance are also investigated in this paper.</span></p>

Physical Layer Security Performance of NOMA-Aided Vehicular Communications Over Nakagami-$m$ Time-Selective Fading Channels With Channel Estimation Errors

This paper considers a single-input-multiple-output non-orthogonal multiple access enabled vehicular communication system, and investigates its secrecy performance over time-selective fading and channel estimation errors. We formulate two scenarios considering the decoding ability at eavesdropper, viz., 1) Scenario I: when it has sufficient decoding capability, and 2) Scenario II: when its decoding capability can be the same as the legitimate vehicles. Under such a realistic system setup, we derive the secrecy outage probability (SOP) and ergodic secrecy capacity expressions for both legitimate vehicles and the overall system under both Scenarios I and II over Nakagami- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> fading channels. We then analyze the asymptotic SOP limits of both legitimate vehicles under Scenarios I and II, by formulating cases based on the average transmit signal-to-noise ratio and average channel gains associated with the main links and wiretap link. From the asymptotic analysis under these cases, it is observed that both legitimate vehicles do not achieve any secrecy diversity order even after leveraging the benefits of fading severity parameters and multiple antennas. We also formulate several special cases of interest to reveal some more insightful information about the system's secrecy performance. Finally, we corroborate our analytical and theoretical findings via simulation results.

Analysis of an LSTM-based NOMA Detector Over Time Selective Nakagami-m Fading Channel Conditions

This work examines the efficacy of deep learning (DL) based non-orthogonal multiple access (NOMA) receivers in vehicular communications (VC). Analytical formulations for the outage probability (OP), symbol error rate (SER), and ergodic sum rate for the researched vehicle networks are established Rusing i.i.d. Nakagami-m fading links. Standard receivers, such as least square (LS) and minimum mean square error (MMSE), are outperformed by the stacked long-short term memory (S-LSTM) based DL-NOMA receiver. Under real time propagation circumstances, including the cyclic prefix (CP) and clipping distortion, the simulation curves compare the performance of MMSE and LS receivers with that of the DL-NOMA receiver. According to numerical statistics, NOMA outperforms conventional orthogonal multiple access (OMA) by roughly 20% and has a high sum rate when considering i.i.d. fading links.

End-to-End Learning for OFDM: From Neural Receivers to Pilotless Communication

The benefits of end-to-end learning has been demonstrated over AWGN channels but has not yet been quantified over realistic wireless channel models. This work aims to fill this gap by exploring the gains of end-to-end learning over a frequency- and time-selective fading channel using OFDM. With imperfect channel knowledge at the receiver, the shaping gains observed on AWGN channels vanish. Nonetheless, we identify two other sources of performance improvements. The first comes from a neural network-based receiver operating over a large number of subcarriers and OFDM symbols which allows to reduce the number of orthogonal pilots without loss of BER. The second comes from entirely eliminating orthogonal pilots by jointly learning a neural receiver together with either superimposed pilots (SIPs), combined with conventional QAM, or an optimized constellation. The learned constellation works for a wide range of signal-to-noise ratios, Doppler and delay spreads, has zero mean and does hence not contain any form of SIP. Both schemes achieve the same BER as the pilot-based baseline with 7% higher throughput. Thus, we believe that a jointly learned transmitter and receiver are a very interesting component for beyond-5G communication systems which could remove the need and associated overhead for demodulation reference signals.

Nonlinear EH-Based UAV-Assisted FD IoT Networks: Infinite and Finite Blocklength Analysis

In this article, we investigate the nonlinear energy harvesting (EH)-based unmanned aerial vehicle (UAV)-assisted full-duplex (FD) Internet of Things (IoT) network with infinite and finite blocklength (FBL) codes. The reliability performance of the considered network, having two half-duplex UAVs and an FD IoT device, is analyzed in terms of the block error rate (BLER) with given ultrareliable and low-latency communication constraints. With the assumption of the combined effect of fading and shadowing, the closed-form expressions for BLER and network goodput are obtained over the Rician shadowed fading channels considering various shadowing scenarios, EH receiver architecture, IoT device mobility, inter-UAV interference, and self-interference (SI) cancelation capabilities at FD IoT device. The obtained results over the Rician shadowed fading for the nonlinear EH receiver architecture are also compared with the linear EH and over the Rician fading channels. The numerical results reveal important observations related to the impact of time-selective fading channels with imperfect channel state information, shadowing severity in the suburban areas, SI cancelation capabilities, blocklength, and the number of channel uses on the reliability performance of the UAV-assisted FD IoT network. Furthermore, the tightness of the approximation presented is verified through the Monte-Carlo simulations.

Non-Coherent Detection and Bit Error Rate for an Ambient Backscatter Link in Time-Selective Fading

This paper focuses on the non-coherent detection in ambient backscatter communication, which is highly appealing for systems where the trade-off between signaling overhead and the actual data transmission is very critical. Modeling the time-selective fading channel as a first-order autoregressive (AR) process, we propose a new receiver architecture based on the direct averaging of the received signal samples for detection, which departs significantly from the energy averaging-based receivers considered in the literature. For the proposed setup, we characterize the exact asymptotic bit error rate (BER) for both single-antenna (SA) and multi-antenna (MA) receivers, and demonstrate the robustness of the new architecture to timing errors. Our results demonstrate that while the direct-link (DL) interference from the ambient power source leads to a BER floor in the SA receiver, the MA receiver can remove this interference by estimating the angle of arrival (AoA) of the DL. The analysis further quantifies the effect of improved angular resolution on the BER as a function of the number of receive antennas. A key intermediate result of our analysis is the derivation of a new concentration result for a general sum sequence that is central to the derivation of the conditional distributions of the received signal.

PEP and OP examination of relaying network over time-selective fading channel

In this paper, we consider the selective decode and forward cooperative communication system and investigate its performance over time-selective fading in the presence of nodes’ mobility and channel estimation errors. Closed-form expressions for the outage probability (OP) and per-block average pairwise error probability (PEP) are derived, considering constraints at the relay, source, and destination nodes. We assume that both dual and multiple hops are subject to independent but non-identically distributed time-selective Nakagami-m fading channel. It has been observed that with an increase in the value of shape parameter and link strength of the relay-to-destination (RD) fading link gain, the per-block average PEP performance improves. Also increasing RD link gain is more significant than increasing the shape parameter of the RD link. The OP performance for optimal power allocation factors is better than its performance for equal power allocation factors. It is demonstrated that the OP decreases with increasing the value of distance-dependent power transfer factors (PTFs). Further, it has been observed that due to the mobility of nodes and imperfect channel state information, the PEP performance of the system experiences degradation, especially for high values of signal-to-noise ratio. Furthermore, when all the nodes are static and the estimation processes are perfect, the asymptotic error floors vanish, i.e., the nodes' mobility impact is removed. Monte Carlo simulation results confirm the accuracy of the analytical results.

ICI analysis of hyperbolic FRFT‐FBMC based on optimal order of transform for Internet of Things applications

This study aimed at analysing intercarrier interference (ICI) of filter bank multicarrier signals which are made by hyperbolic fractional Fourier transform, depend on optimum order of transform for the Internet of Things applications. Calculation of ICI power based on joint time and frequency selective fading channel has been derived. The upper bound of ICI power has been found as an applicable equation for the system design, which has been mathematically calculated. In order to minimise the amount of ICI power which is relative to the order of transform, the optimum order of transform with almost accurate formula has been computed. Simulations demonstrate the proposed signal is so robust to the frequency offset and by selecting the optimum order of transform, the acceptable amount of ICI is generated.

Improved Signal Detection Techniques for QOSTBC System in Fast Fading Channel

Most existing quasi-orthogonal space time Block coding (QO-STBC) schemes have been developed relying on the assumption that the channel is at or remains static during the length of the code word symbol periods to achieve an optimal antenna diversity gain. However, in time-selective fading channels, this assumption does not hold and causes intertransmit-antenna-interferences (ITAI). Therefore, the simple pairwise maximum likelihood decoding scheme is not sufficient to recover original transmitted signals at the receiver side. To avoid the interferences, we have analyzed several signal detection schemes, namely zero forcing (ZF), two-step zero forcing (TS-ZF), minimum mean square error (MMSE), zero forcing - interference cancelation - decision feedback equalizer (ZF-IC-DFE) and minimum mean square error - interference cancelation { decision feedback equalizer (MMSE-IC-DFE). We have proposed two efficient iterative signal detection schemes, namely zero forcing - iterative interference cancelation - zero forcing { decision feedback equalization (ZF-IIC-ZF-DFE) and minimum mean square error - parallel interference cancelation - zero forcing – decision feedback equalization (MMSE-IIC-ZF-DFE). The simulation results show that these two proposed detection schemes significantly outperform all conventional methods for QOSTBC system over time selective channel.

A Novel LMMSE-EM Channel Estimator for High Mobility STBC-OFDM System

In time-selective fading channel, the Alamouti orthogonality principle is lost due to the variation of channel from symbol-to-symbol in space–time block-coded orthogonal frequency division multiplexing (STBC-OFDM) system and causes co-channel interference (CCI) effects. To combat the CCI effects, various signal detection schemes have been proposed earlier by assuming that a priori channel state information (CSI) is known to the receiver. However, in practice, the CSI is unknown and therefore accurate estimation of channel is required for efficient signal detection. In this paper, by exploiting circulant properties of the channel frequency response (CFR) autocorrelation matrix [Formula: see text], we propose an efficient low complexity linear-minimum-mean-square-error (LMMSE) estimator. This estimator applies an expectation–maximization (EM) iterative process to reduce the computational complexity significantly. Finally, we compare the proposed LMMSE-EM estimator with conventional least square (LS) and LMMSE estimator in terms of performance and computational complexity. The simulation results show that the proposed LMMSE-EM estimator achieves exactly the same performance as the optimal LMMSE estimator with much lower computational complexity.

Reducing the Codeword Search Complexity of FDD Moderately Large MIMO Beamforming Systems

It is of great interest to develop larger scale multiple-input multiple-output (MIMO) technology based on frequency division duplexing (FDD) for backward compatibility with existing standards and already deployed cellular networks. One hindrance to the FDD approach is the complexity of the codeword search performed by the receiver, which is known to scale with the number of transmit antennas $N_{t}$ in a system. The codeword search complexity does not scale linearly and soon becomes infeasible even for the not-so-massive MIMO systems expected in the very near future. We address this problem by proposing a natural sequence-based orthogonal codebook and an intra-array interference magnitude (IIM) framework for MIMO beamforming based on quantized equal gain transmission that altogether tremendously simplifies the codeword search problem. We also present an assessment on the quality of the codeword selected by the proposed framework that explains it’s near optimal performance, and, provide detailed codeword search complexity quantification. Finally, we provide simulation analyses of beamforming gain under time selective fading channels and an FDD MIMO system transmit delay model. Our analyses indicate that despite significantly reducing complexity, the proposed IIM framework executed by already commercialized processors is capable of approaching the ideal performance of an static system even in the presence of time-selectivity inherent to high-speed mobile communications. Our analyses also quantify the impact of feedback delay on average beamforming gain of moderately large transmit arrays.

An investigation of wireless S-DF hybrid satellite terrestrial relaying network over time selective fading channel

An investigation of wireless S-DF hybrid satellite terrestrial relaying network over time selective fading channel

An investigation of wireless S-DF hybrid satellite terrestrial relaying network over time selective fading channel

An investigation of wireless S-DF hybrid satellite terrestrial relaying network over time selective fading channel

Alamouti-OSTBC Wireless Cooperative Networks With Mobile Nodes and Imperfect CSI Estimation

This paper is concerned with investigating the impact of both nodes mobility and imperfect channel-state-information (CSI) estimation on the symbol-error-probability (SEP) performance of a multiple-relay amplify-and-forward cooperative diversity system employing Alamouti-type orthogonal-space-time-block-code transmission at the source and classical Alamouti's space-time-decoder (ALD) at the destination. Due to nodes mobility, the system's links are characterized by time-selective fading channels, which are modeled by the first-order-autoregressive (AR1) process, and due to imperfect CSI estimation, the estimated channel gains are assumed to be corrupted by Gaussian errors. For such a system model, an approach is proposed to derive a tight approximate expression for the system's conditional SEP. This approach is based on utilizing the AR1 model to derive exact expressions for the signal-to-interference-plus-noise ratios of the ALD decoder's decision variables, and on benefiting from the central limit theorem to approximate some of the non-Gaussian interference and noise terms. The obtained conditional SEP expression is function of both the fading channel correlation parameters and the estimation error variances, and thus, it is valid for mobile as well as static nodes for imperfect as well as perfect CSI estimation processes. The system's average SEP is evaluated semianalytically using the obtained conditional SEP expression. Numerical results along with Monte Carlo link-level simulations are provided to validate the accuracy of the presented analyses and to demonstrate the system performance under several realistic scenarios.

A Differential ML Combiner for Differential Amplify-and-Forward System in Time-Selective Fading Channels

We propose a new differential maximum-likelihood (DML) combiner for noncoherent detection of the differential amplify-and-forward (D-AF) relaying system in the time-selective channel. The weights are computed based on both the average channel quality and the correlation coefficient of the direct and relay channels. Moreover, we derive a closed-form approximate expression for the average bit error rate (BER), which is applicable to any single-relay D-AF system with fixed weights. Both theoretical and simulated results are presented to show that the time-selective nature of the underlying channels tends to reduce the diversity gains at the low-signal-to-noise-ratio (SNR) region, resulting in an asymptotic BER floor at the high-SNR region. Moreover, the proposed DML combiner is capable of providing significant BER improvements compared with the conventional differential detection (CDD) and selection-combining (SC) schemes.

Performance analysis of spatial multiplexing MIMO-OFDM system with different equalisers and guard intervals

Multiple-input multiple-output MIMO system is a very popular solution in the field of wireless communications, in order to improve the capacity and reliability of radio link. Nevertheless, during any transmission over fading channel, the performance of MIMO system is degraded due to the inter symbol interference ISI. The combination of orthogonal frequency division multiplexing MIMO-OFDM and the use of equalisation at the reception have become very effective to eliminate the ISI and reduce the imperfection of the channel. The optimal maximum likelihood ML equaliser suffers from high computational complexity. In this context, we propose the combination of vertical bell labs space time VBLAST multiplexing and ordered successive interference cancellation OSIC equaliser in MIMO-OFDM system, whose performance is compared with different equalisers, in particular zero forcing ZF, minimum mean square error MMSE, and ML using cyclic prefix CP and zero padding ZP intervals in Rayleigh time-selective fading channel. The simulation results show that the performance of ZF/MMSE-OSIC equaliser with ZP-ODFM is close to the ML with a low complexity and it works better as compared to other algorithms.

Asymptotic Diversity Analysis of Alamouti Transmit Diversity with Quasi-ML Decoding Algorithm in Time-Selective Fading Channels

This paper addresses the asymptotic diversity performance of Alamouti transmit diversity technique especially in time-selective fading channels. In particular, a linear quasi-maximum-likelihood (QML) decoding method is employed for the Alamouti transmit diversity system to solve the error floor problem induced by the conventional linear ML decoding method. By judiciously utilizing the derived asymptotic closed-form formula of symbol pairwise error rate (SPER), it is theoretically verified that the asymptotic diversity orders achieved by the QML decoding algorithm become 2 and 1 in quasi-static and time-selective fading channels, respectively.

A Novel Pilot Expansion Approach for MIMO Channel Estimation

A training-based MIMO channel estimation scheme is presented to operate in severe frequency and time selective fading channels. Besides the new pilot bits designed from the ‘Paley-Hadamard’ matrix to exploit its orthogonal and ‘Toeplitz-like’ structures and minimising its pilot length, a novel pilot expansion technique is proposed to estimate the length of the channel impulse response, by flexibly extending its pilot length as required in order to capture the number of multipath existed within the MIMO channel. The pilot expansion can also help to deduce the initial channel variation and its Doppler rate which can be subsequently applied for MIMO channel tracking using decision feedback Kalman filter during the data payload.