Block Fading Channels Research Articles

Covert Wireless Communications Under Quasi-Static Fading With Channel Uncertainty

Covert communications enable a transmitter to send information reliably in the presence of an adversary, who looks to detect whether the transmission took place or not. We consider covert communications over quasi-static block fading channels, where users suffer from channel uncertainty. We investigate the adversary Willie’s optimal detection performance in two extreme cases, i.e., the case of perfect channel state information (CSI) and the case of channel distribution information (CDI) only. It is shown that in the large detection error regime, Willie’s detection performances of these two cases are essentially indistinguishable, which implies that the quality of CSI does not help Willie in improving his detection performance. This result enables us to study the covert transmission design without the need to factor in the exact amount of channel uncertainty at Willie. We then obtain the optimal and suboptimal closed-form solution to the covert transmission design. Our result reveals fundamental difference in the design between the case of quasi-static fading channel and the previously studied case of non-fading AWGN channel.

Energy Optimization For Hybrid ARQ With Turbo Coding: Rate Adaptation and Allocation

We consider incremental redundancy (IR) hybrid automatic repeat request (HARQ) over independent block-fading channels with turbo coding. We consider different cases of channel state information (CSI) at the transmitter: the transmitter has no knowledge of any CSI, or knows the CSI in previous transmission rounds through a perfect feedback channel, or knows both current and previous CSI. The transmitter decides the forward error correction code rate based on the CSI it has. We minimize the energy consumption of turbo-coded HARQ, subject to a packet loss rate constraint. Numerical results show that the energy consumption of HARQ decreases when more CSI information is available at the transmitter. We also compare IR combining with Chase combining and the system without combining, and IR combining yields the least energy consumption.

Multimode Integer-Forcing Receivers for Block Fading Channels

Integer-forcing (IF) receivers are a class of linear multiple-input multiple-output (MIMO) receivers to achieve near-optimal performance under quasi-static channels, with almost the same complexity as that of conventional linear receivers. Instead of trying to recover the transmitted messages directly, an IF receiver recovers their integer-linear combinations corresponding to a chosen integer-valued effective channel matrix, and then obtains them. Recently, the IF strategy has been extended to a block fading scenario. In this paper, we propose a novel IF receiver for block fading channels, called a multimode IF (M-IF) receiver. Unlike conventional IF receivers, the proposed M-IF receiver employs several integer-valued effective channel matrices for IF decoding. We also propose an efficient search algorithm for selecting such matrices with the aid of Gaussian elimination. We then show that for any channel realizations, the proposed M-IF receiver has better performance in terms of the effective noise variance than conventional IF receivers. Simulation results demonstrate that it works really well in various environments.

Design and Practical Decoding of Full-Diversity Construction A Lattices for Block-Fading Channels

Block-fading channel (BF) is a useful model for various wireless communication channels in both indoor and outdoor environments. Frequency-hopping schemes and orthogonal frequency division multiplexing (OFDM) can conveniently be modelled as BF channels. Applying lattices in this type of channel entails dividing a lattice point into multiple blocks such that fading is constant within a block but changes, independently, across blocks. The design of lattices for BF channels offers a challenging problem, which differs greatly from its counterparts like AWGN channels. Recently, the original binary Construction A for lattices, due to Forney, has been generalized to a lattice construction from totally real and complex multiplication (CM) fields. This generalized algebraic Construction A of lattices provides signal space diversity, intrinsically, which is the main requirement for the signal sets designed for fading channels. In this paper, we construct full-diversity algebraic lattices for BF channels using Construction A over totally real number fields. We propose two new decoding methods for these family of lattices which have complexity that grows linearly in the dimension of the lattice. The first decoder is proposed for full-diversity algebraic LDPC lattices which are generalized Construction A lattices with a binary LDPC code as underlying code. This decoding method contains iterative and non-iterative phases. In order to implement the iterative phase of our decoding algorithm, we propose the definition of a parity-check matrix and Tanner graph for full-diversity algebraic Construction A lattices. We also prove that using an underlying LDPC code that achieves the outage probability limit over one-block-fading channel, the constructed algebraic LDPC lattices together with the proposed decoding method admit diversity order $n$ over an $n$ -block-fading channel. Then, we modify the proposed algorithm by removing its iterative phase which enables full-diversity practical decoding of all generalized Construction A lattices without any assumption about their underlying code. In contrast with the known results on AWGN channels in which non-binary Construction A lattices always outperform the binary ones, we provide some instances showing that algebraic Construction A lattices obtained from binary codes outperform the ones based on non-binary codes in block fading channels. Since available lattice construction methods from totally real and complex multiplication (CM) fields do not provide diversity in the binary case, we generalize algebraic Construction A lattices over a wider family of number fields namely monogenic number fields.

Generalized OFDM-IM With Noncoherent Detection

This paper generalizes the conventional noncoherent OFDM-IM systems by allowing selection of different numbers of active subcarriers for message representation. We then present the maximum-likelihood (ML) detection rule for frequency-selective block fading channels. The performance of the ML receiver is analyzed in terms of pairwise-error probability as well as diversity order. Asymptotic analysis of the proposed system reveals the role of partial Hamming distance in designing diversity-exploiting activation patterns. Practical issues including low-complexity receiver, and code (activation pattern) design techniques are discussed. Furthermore, the effect of inter-carrier interference in fast fading scenarios, and generalization of the technique to multiple-antenna scenarios are studied. Finally, we present simulation results to illustrate performance benefits of the generalized OFDM-IM systems over the conventional noncoherent OFDM-IM systems.

Saddlepoint Approximations for Short-Packet Wireless Communications

In recent years, the derivation of nonasymptotic converse and achievability bounds on the maximum coding rate as a function of the error probability and blocklength has gained attention in the information theory literature. While these bounds are accurate for many scenarios of interest, they need to be evaluated numerically for most wireless channels of practical interest, and their evaluation is computationally demanding. This paper presents saddlepoint approximations of state-of-the-art converse and achievability bounds for noncoherent, single-antenna, Rayleigh block-fading channels. These approximations can be calculated efficiently and are shown to be accurate for SNR values as small as 0 dB and blocklengths of 168 channel uses or more.

Generalized Code Index Modulation and Spatial Modulation for High Rate and Energy-Efficient MIMO Systems on Rayleigh Block-Fading Channel

In this article, a high data rate and energy-efficient multiple-input multiple-output transmission scheme is considered by combining two popular and rational modulation techniques: spatial modulation (SM) and code index modulation-spread spectrum (CIM-SS). Since in the considered system, called generalized CIM-SM (GCIM-SM), incoming information bits determine modulated symbols, activated transmit antenna indices as well as their corresponding spreading code indices, data bits are conveyed not only by modulated symbols but also by the indices of the active antenna and spreading codes. Hence, a GCIM-SM scheme accommodates faster data rates while spending less transmission power and possessing better error performance compared to the conventional direct sequence spread spectrum (DS-SS), SM, quadrature SM (QSM), and CIM-SS systems. The mathematical expressions of the GCIM-SM system for bit error rate, throughput, energy efficiency, and the system complexity are derived to analyze the overall system performance. Besides, it has been shown via computer simulations that the GCIM-SM system has lower transmission energy, faster data transmission rate, and better error performance than DS-SS, SM, QSM, and CIM-SS systems. Performance analysis of the considered system was performed on Rayleigh block-fading channels for quadrature amplitude modulation technique.

Efficient Relay Selection Algorithm for Non-Orthogonal Amplify-and-Forward Cooperative Systems over Block-Fading Channels

Efficient Relay Selection Algorithm for Non-Orthogonal Amplify-and-Forward Cooperative Systems over Block-Fading Channels

A Note on Implementation Methodologies of Deep Learning-Based Signal Detection for Conventional MIMO Transmitters

Baek et al. proposed a deep learning-based signal detector for conventional MIMO systems, which is a pioneering work of applying artificial intelligence into wireless communications. Although this work works well under static fading channels, it is worth notable that it fails to work under block-fading channels. In particular, the detection BER under block-fading channels is around 0.5 even in the high regime of SNR. To explain this unexpected result, we provide some simple yet efficient theoretical analysis, which clearly verifies that the proposed detector cannot decouple the phases between the channel parameters and transmitted signals and hence it fails to detect the transmitted signals under block-fading channels. The results in this paper can help understand and improve the detector. In particular, the detector structure should incorporate the channel state information for the application under block-fading channels.

Delay-Optimal and Energy-Efficient Communications With Markovian Arrivals

In this paper, delay-optimal and energy-efficient communication is studied for a single link under Markov random arrivals. We present the optimal tradeoff between delay and power over Additive White Gaussian Noise (AWGN) channels and extend the optimal tradeoff for block fading channels. Under time-correlated traffic arrivals, we develop a cross-layer solution that jointly considers the arrival rate, the queue length, and the channel state in order to minimize the average delay subject to a power constraint. For this purpose, we formulate the average delay and power problem as a Constrained Markov Decision Process (CMDP). Based on steady-state analysis for the CMDP, a Linear Programming (LP) problem is formulated to obtain the optimal delay-power tradeoff. We further show the optimal transmission strategy using a Lagrangian relaxation technique. Specifically, the optimal adaptive transmission is shown to have a threshold type of structure, where the thresholds on the queue length are presented for different transmission rates under the given arrival rates and channel states. By exploiting the result, we develop a threshold-based algorithm to efficiently obtain the optimal delay-power tradeoff. We show how a trajectory-sampling version of the proposed algorithm can be developed without the prior need of arrival statistics.

Refined Asymptotics for a Block Fading Channel Under Energy Harvesting and Fixed Power Constraints

We consider a block fading additive white Gaussian noise (AWGN) channel with perfect channel state information (CSI) at the transmitter (CSIT) and the receiver (CSIR). For a given codeword length and non-vanishing average probability of error, we obtain lower and upper bounds on the maximum transmission rate. First, we derive bounds when the transmitter has a time varying peak power constraint imposed by an energy harvesting device ; such devices power many modern wireless devices. Next, using the same techniques we obtain bounds for the canonical peak and average power constraint cases. The bounds characterize the finite blocklength back-off from the channel capacity, which is in turn achieved by water-filling power allocation across time. Finally, we derive bounds on the moderate deviations constant for the block fading model. We compare the bounds numerically to bring out the efficacy of our results.

Cube-Split: A Structured Grassmannian Constellation for Non-Coherent SIMO Communications

In this paper, we propose a practical structured constellation for non-coherent communication with a single transmit antenna over Rayleigh flat and block fading channel without instantaneous channel state information. The constellation symbols belong to the Grassmannian of lines and are defined up to a complex scaling. The constellation is generated by partitioning the Grassmannian of lines into a collection of bent hypercubes and defining a mapping onto each of these bent hypercubes such that the resulting symbols are approximately uniformly distributed on the Grassmannian. With a reasonable choice of parameters, this so-called cube-split constellation has higher packing efficiency, represented by the minimum distance, than the existing structured constellations. Furthermore, exploiting the constellation structure, we propose low-complexity greedy symbol decoder and log-likelihood ratio computation, as well as an efficient way to associate it to a multilevel code with multistage decoding. Numerical results show that the performance of the cube-split constellation is close to that of a numerically optimized constellation and better than other structured constellations. It also outperforms a coherent pilot-based scheme in terms of error probability and achievable data rate in the regime of short coherence time and large constellation size.

Rate Selection for Wireless Random Access Networks Over Block Fading Channels

We study uncoordinated random access over fading channels where each user independently decides whether to send a packet or not to a common receiver at any given time slot. Specifically, we develop an information theoretic formulation to characterize the overall system throughput. We consider two scenarios: classical slotted ALOHA, where no multiuser detection (MUD) capability is available and slotted ALOHA with MUD. In each case, in order to maximize the system throughput, we provide methods to obtain the optimal rates and channel activity probabilities using the user distances to the receiver (or, equivalently, their average signal to noise ratios) assuming a Rayleigh block fading channel. The results demonstrate that the newly proposed optimal rate selection solutions offer significant increase in the expected system throughputs compared to the “same rate to all users” approach commonly used in the literature. In addition to the overall throughput optimization, we also address the issue of fairness among users and propose approaches guaranteeing a minimum amount of individual throughput to each user, and design systems with limited individual outage probabilities for increased energy efficiency and reduced delay.

Rateless Coded Multi-User Uplink Transmission With Distributed Fronthaul Compression in Cloud RAN

In this paper, we design the rateless coded uplink transmission in cloud radio access network (C-RAN) with two users, two remote radio heads (RRHs), and one baseband processing unit (BBU) pool under block fading channel. Each user sends the Raptor coded signals continuously until receiving ACK from the BBU pool. A distributed fronthaul compression scheme with LDPC code is proposed for the RRHs which can reduce the compression loss without increasing the fronthaul traffic, through leveraging the correlation between the received signals at both RRHs. To guarantee the compression performance, the LDPC code profile is optimized based on extrinsic information transfer (EXIT) analysis. Furthermore, the Raptor code profiles applied at the two users are also jointly optimized to improve the average system throughput. Simulation results show that the proposed transmission scheme with the optimized LDPC compression code and Raptor code can achieve good BER and throughput performance.

On Single-Antenna Rayleigh Block-Fading Channels at Finite Blocklength

This paper concerns the maximum coding rate at which data can be transmitted over a noncoherent, single-antenna, Rayleigh block-fading channel using an error-correcting code of a given blocklength with a block-error probability not exceeding a given value. A high-SNR normal approximation of the maximum coding rate is presented that becomes accurate as the signal-to-noise ratio (SNR) and the number of coherence intervals $L$ over which we code tend to infinity. Numerical analyses suggest that the approximation is accurate at SNR values above 15dB and when the number of coherence intervals is 10 or more.

Capacity Scaling in a Non-Coherent Wideband Massive SIMO Block Fading Channel

The scaling of coherent and non-coherent channel capacity is studied in a single-input multiple-output (SIMO) block Rayleigh fading channel as both the bandwidth and the number of receiver antennas go to infinity jointly with the transmit power fixed. The transmitter has no channel state information (CSI), while the receiver may have genie-provided CSI (coherent receiver), or the channel statistics only (non-coherent receiver). Our results show that if the available bandwidth is smaller than a threshold bandwidth which is proportional (up to leading order terms) to the square root of the number of antennas, there is no gap between the coherent capacity and the non-coherent capacity in terms of capacity scaling behavior. On the other hand, when the bandwidth is larger than this threshold, there is a capacity scaling gap. Since achievable rates using pilot symbols for channel estimation are subject to the non-coherent capacity bound, this work reveals that pilot-assisted coherent receivers in systems with a large number of receive antennas are unable to exploit excess spectrum above a given threshold for capacity gain.

Spectral efficiency analysis for massive MIMO systems in Ricean fading channels

This study focuses on the spectral efficiency of massive multiple-input–multiple-output (MIMO) systems over Ricean fading channels. Assuming the channel reciprocity of uplink and downlink transmissions, the channels are estimated through uplink pilot sequence with least minimum mean square error (MMSE) estimator, which facilitates the investigation of equivalent channel model. With the equivalent channel model, adopting joint MMSE receiver, the uplilnk achievable sum-rate is studied, and finally, the asymptotic expression of spectral efficiency is obtained. Based on the proposed asymptotic spectral efficiency, numerical results give some guidelines about system parameters design: the authors should choose a proper length of the block fading channels to reduce the effect of pilot overhead, and an optimal length of pilot sequence always exists to guarantee a satisfying system performance in terms of spectral efficiency. Furthermore, the simulation results show that when fixing the number of users, shrinking the cell serving area and increasing the number of antennas can improve the system performance.

Age-Optimal Power Control for Status Update Systems With Packet-Based Transmissions

This letter investigates the average age of information (AoI) minimization in status update systems in which the update packets are transmitted with fixed rate. It is assumed that the source avoids queue-induced delay by generating and sending a new status update packet in each time slot. Assuming that the transmit power can be adapted based on the number of successive negative acknowledgments (NACKs) received, a closed-form expression for the average AoI is derived considering transmissions over block fading channel models. An optimization problem for minimizing the average AoI under average power constraints is formulated and a stochastic optimization algorithm for obtaining a feasible solution is proposed. Numerical results on the proposed power control policy show that more power is preferred to be allocated to packets after a certain number of failure packets in the low-power regime and the proposed power control reduces the average AoI compared to the constant power policy. An alternative on–off power control policy is also proposed and shown to achieve satisfactory performance.

Comprehensive Effects of Imperfect Synchronization and Channel Estimation on Known Interference Cancellation

Known interference (KI) is commonly used in many scenarios, where the original interference data is known to the receiver in advance. Unfortunately, due to the impacts of hardware imperfections and propagation environment, the received KI may suffer frequency offset, phase noise, and multi-path propagation with different time delays and channel gains. However, perfect time-frequency synchronization and channel estimation are hard to implement in practice, making it challenging to cancel the KI completely. In this paper, the comprehensive effects of synchronization and channel impairments on KI cancellation are investigated. First, the normalized time synchronization error is modeled as the inter-symbol interference (ISI), and the frequency synchronization error and phase noise are modeled as the inter-frequency interference (IFI). Subsequently, the closed-form expression of KI cancellation ratio (KICR) is derived for the multi-path block fading channel, by jointly considering imperfect time-frequency synchronization and channel estimation, as well as phase noise. Furthermore, assuming the channel and synchronization parameters estimation reach their corresponding Cramer-Rao bounds (CRBs), the upper bound of KICR and the lower bound of channel capacity loss are analyzed for the single-path channel. Simulation results confirm the theoretical analysis that the KICR is more sensitive to phase noise compared to time-frequency synchronization and channel estimation errors. In addition, it is also proved that lower KI power is less sensitive to the implementation imperfections.

Cross-Packet Coding for Delay-Constrained Streaming Applications

Delay-constrained streaming from a moving platform is of paramount importance for applications such as the remote control of drones, where a low-delay video stream is required in order to provide visual feedback to the pilot while providing high quality video at playback. In this letter we propose a scheme based on cross-packet coding for delay-constrained streaming over block fading channels with channel state information at the receiver only. The proposed scheme largely enhances upon a memoryless transmission approach in terms of average decoded rate and provides increased protection to packets transmitted earlier within a block, which is useful for successively compressed sources such as IPPPP video streams. The proposed scheme approaches the asymptotic upper bound over a wide range of SNR already for blocks with size of practical relevance and has a comparatively low complexity.