Open-loop Communication Research Articles

Reliability-Guaranteed Uplink Resource Management in Proactive Mobile Network for Minimal Latency Communications

Proactive Mobile Network (PMN) has been proposed to support extremely low latency communications with multi-tier computing architectures, machine-centricity, and data-driven operation features. Nevertheless, the communication reliability of the PMN introduces new substantial technological challenges. As PMN employs unique proactive open-loop communication, any feedback-based control is avoided to enhance end-to-end latency. This paper focuses on machine-initiated uplink transmission in PMN and proposes a reliability-guaranteed resource management scheme. Without requiring feedback control information, our scheme uniquely decomposes conventional resource management into two collaborative decision processes: predictive resource allocation suggested by network anchor nodes (ANs) and proactive smart resource utilization by smart equipment (SE). These two decision-making processes are constructed as independent reinforcement learning (RL) problems, but implicitly share the states of radio resources according to operating environments. Different algorithms for different operating scenarios have been investigated for this dual-decision solution. Simulation results in various scenarios show that our scheme enables PMN’s reliability close to the theoretical optimal value and successfully serves radio resource utilization in the PMNs.

Machine Learning Enables Radio Resource Allocation in the Downlink of Ultra-Low Latency Vehicular Networks

Autonomous driving and intelligent transportation demand ultra-low latency and high reliability communication in future vehicular networks. Proactive wireless communication can facilitate minimal latency by open-loop communication, which discards traditional feedback control mechanisms. However, appropriate radio resource allocation in such proactive mobile networks has not been fully studied due to lacking channel state information (CSI) and the alleviation of multiple access interference (MAI) in multiple virtual cells. This paper aims to ensure the reliability of downlink communication by a novel radio resource allocation scheme in proactive vehicular networks with ultra-low latency. We regard data transmission success rate as the reliability indicator and propose a joint radio resource allocation model based on the “generalized closed-loop”, where anchor node (AN) uses the radio resource utilization information (RRUI) from the vehicle in the immediate past uplink as a guide to assist resource allocation. Subsequently, we study the radio resource allocation model solution on the vehicle side and the network side respectively. On the vehicle side, vehicles use the local or global data transmission experience to select the radio resource with the best quality as the RRUI. On the network side, according to the latest RRUI of vehicle and resource occupancy information, deep reinforcement learning is proposed to make appropriate radio resource allocation decisions. Simulations demonstrate the effectiveness of the intelligent joint radio resource allocation scheme under the cooperation between vehicles and AN. When the resource load rate reaches 40%, the joint radio resource allocation scheme can achieve a data transmission success rate of more than 98%.

Precise Performance Characterization of Precoded Integer Forcing Applied to Two Parallel Channels

Precoded integer-forcing equalization is a low-complexity transmission and reception scheme, primarily designed for open-loop communication. The data is encoded into independent streams, all using the same linear code, after which linear precoding is applied, while integer-forcing equalization is applied at the receiver side. Previous works have established that this architecture achieves channel capacity up to a finite gap for general multiple-input multiple-output Gaussian channels, as well as obtained tighter bounds for the special case of diagonal (parallel) channels. The present work provides a precise performance characterization when integer-forcing equalization is applied to two parallel channels and where precoding is done using the full-diversity rotation matrix cyclo <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . It is shown that this scheme achieves capacity up to a gap bounded by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\log _{2}({\scriptstyle ^{\scriptstyle 9}}\hspace {-0.224em}/\hspace {-0.112em}{\scriptstyle 5})$ </tex-math></inline-formula> bits per complex channel use when the standard integer-forcing receiver is used, and the gap is reduced to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\log _{2}({\scriptstyle ^{\scriptstyle 5}}\hspace {-0.224em}/\hspace {-0.112em}{\scriptstyle 4})$ </tex-math></inline-formula> bits per complex channel use when its successive decoding variant is applied. In addition, a full characterization of the basis transformation matrices used by the integer-forcing receiver is derived. These turn out to consist only of Fibonacci numbers and are explicitly determined as a function of the condition number. The results obtained are also highly relevant to lattice-reduction detection schemes.

Open-Loop Communications for Up-Link URLLC Under Clustered User Distribution

To achieve ultra-low latency for ultra-reliable and low latency communications (URLLC) in fifth-generation (5G) systems, open-loop communications (OLC), which removes the time-consuming feedback procedure, has been proposed as a promising technique, especially for uplink transmission (UL). The performance of the OLC has been commonly analysed under a uniform user distribution model. Since the users are commonly clustered around highly populated areas, a clustered or non-uniform user distribution model is more realistic. However, the performance analysis under a clustered user distribution model is not trivial. In this paper, we therefore aim to analyse the OLC network performance under a clustered user distribution. We also investigate how to achieve URLLC by jointly employing the OLC mode, short packet transmission, multi-cell association and diversity schemes such as selection combining (SC) and maximum ratio combining (MRC). We provide a tractable approach to analyse the UL reliability and its approximated closed form. The simulation results show that the derived closed form can provide a reasonable approximation for the UL reliability when the SC algorithm is employed. Moreover, for the case of MRC, the analytical results can provide a tight approximation on the uplink reliability when the number of the associated BSs is lower than 2, while serving as a corresponding upper bound when the number of the associated BSs is larger than 2.

Ultra-Reliable and Low-Latency Communications Using Proactive Multi-Cell Association

Attaining reliable communications traditionally relies on a closed-loop methodology but inevitably incurs a good amount of networking latency thanks to complicated feedback mechanism and signaling storm. Such a closed-loop methodology thus shackles the current cellular network with a tradeoff between high reliability and low latency. To completely avoid the latency induced by closed-loop communication, this article aims to study how to jointly employ open-loop communication and multi-cell association in a heterogeneous network (HetNet) so as to achieve ultra-reliable and low-latency communications. We first introduce how mobile users in a HetNet adopt the proposed proactive multi-cell association (PMCA) scheme to form their virtual cell that consists of multiple access points (APs) and then analyze the communication reliability and latency performances. We show that the communication reliability can be significantly improved by the PMCA scheme and maximized by optimizing the densities of the users and the APs. The analyses of the uplink and downlink delays are also accomplished, which show that extremely low latency can be fulfilled in the virtual cell of a single user if the PMCA scheme is adopted and the radio resources of each AP are appropriately allocated.

Resource Allocation for Open-Loop Ultra-Reliable and Low-Latency Uplink Communications in Vehicular Networks

To support ultra-reliable and low-latency communication (URLLC) in vehicular networks, the virtual cells, where multiple access points (APs) cooperatively serve one mobile node, have been proposed to reduce the end-to-end latency in the downlink. The latency can be further reduced by eliminating the need for retransmission and feedback control, i.e., open-loop communications. However, it is difficult to achieve a high reliability of the uplink via virtual cells, because of multiple access interference and collisions from other virtual cells nearby. In this paper, we formulate the proactive radio resource allocation in the open-loop uplink of vehicular networks as a stochastic optimization problem with the objective to maximize the uplink reliability while ensuring the network stability. The optimal resource allocation policies are obtained solving the optimization problem using the Lyapunov optimization technique in a distributed manner. To reduce the computational and to improve the challenges of the Lyapunov optimization, we propose a virtual resource slicing algorithm that maps radio resource units to virtual resource blocks. Simulation results exhibit that both ultra-low latency and ultra high reliability are guaranteed in the open-loop uplink of vehicular networks. Based on the theoretical performance analysis and simulations, the uplink radio access procedure is summarized for the URLLC in vehicular networks.

Worst-Case Guarantees for Remote Estimation of an Uncertain Source

Consider a remote estimation problem where a sensor wants to communicate the state of an uncertain source to a remote estimator over a finite-time horizon. The uncertain source is modeled as an autoregressive process with bounded noise. Given that the sensor has a limited communication budget, the sensor must decide when to transmit the state to the estimator who has to produce real-time estimates of the source state. In this article, we consider the problem of finding a scheduling strategy for the sensor and an estimation strategy for the estimator to jointly minimize the worst-case maximum instantaneous estimation error over the time horizon. This leads to a decentralized minimax decision-making problem. We obtain a complete characterization of optimal strategies for this decentralized minimax problem. In particular, we show that an open-loop communication scheduling strategy is optimal and the optimal estimate depends only on the most recently received sensor observation.

Two-Stage ICI Suppression in the Downlink of Asynchronous URLLC

Applying the concept of virtual cell together with open-loop communications has been recently shown to achieve ultra-reliable and low-latency communication (uRLLC) in vehicular networks. Nevertheless, losing perfect synchronization due to proactive communications creates difficulty in mitigating multiple access interference (MAI). Multiple carrier frequency offsets (CFOs) resulting from different oscillators at different access points (APs) incur serious inter-carrier interference (ICI) to further complicate downlink MAI. Asynchronous multiuser detection (MUD) with ICI-Whitening was shown leading to satisfactory performance, but the whitening scheme needs the covariance matrix of ICI that is practically hard to obtain for downlink receivers. We develop a two-stage ICI suppression method to resolve this challenge. The first-stage processing is Pseudo-ICI-Whitening (P-ICI-W) or its simplified version Pseudo-Truncate ICI-W (PT-ICI-W), which does not rely on the estimation of ICI covariance and is suitable for asynchronous downlink. In terms of post-processing signal-to-interference-plus-noise ratio (SINR) and bit-error rate (BER), our proposed mechanism can approach ICI-Whitening. The second-stage processing is based on the recently proposed generalized linear minimum mean square error (LMMSE) projection to further cancel some ICI terms. Moreover, our proposed mechanism is compatible with space-time-block-coded signals, namely Alamouti coding and Complex Interleaved Orthogonal Design (CIOD), to yield more reliable proactive wireless communications.

Downlink Multiuser Detection in the Virtual Cell-Based Ultra-Low Latency Vehicular Networks

To achieve ultra-low latency mobile networking, recent efforts to integrate virtual cell with open-loop communications and proactive network association suggest the facilitation of new technological paradigm, but the interference from different co-locating virtual cells is hard to handle. Open-loop transmissions make beam-forming/interference alignment infeasible due to the need of channel state information feedback. Multiuser detection (MUD) is therefore employed to address the downlink interference. We note that the bit error rate (BER) of maximum-likelihood MUD (ML-MUD) is sensitive to the modulation of the interference. As the interferer uses low-order modulation, the BER of desired signal can approach the ideal case without interference. But if the interferer adopts high-order modulation, the resultant BER is significantly degraded. Our study shows that, such modulation sensitivity can be eased by the multi-antenna technique. We also propose two methods to reduce the notorious computational complexity of MUD, particularly involving higher order modulations. The first scheme is termed as the reduced-computation ML-MUD (R-ML-MUD) that exploits the characteristic of downlink to shrink the ML solution space, consequently leading to lower detection complexity. The second scheme is a new projection receiver, called generalized linear minimum mean square error equalizer, resulting in notable signal-to-noise ratio gain over the conventional projection method. The simulation results indicate that the proposed schemes and their integration can achieve satisfactory BER performance.

On the Performance of Millimeter Wave-Based RF-FSO Multi-Hop and Mesh Networks

This paper studies the performance of multi-hop and mesh networks composed of millimeter wave-based radio frequency (RF) and free-space optical (FSO) links. The results are obtained in cases with and without hybrid automatic repeat request (HARQ). Using the central limit theorem as well as other state-of-the-art approximation schemes, we derive closed-form expressions for the networks’ outage probability and ergodic achievable rates. We also evaluate the effect of various parameters such as power amplifiers efficiency, number of antennas as well as different coherence times of the RF and the FSO links on the system performance. Finally, we determine the minimum number of the transmit antennas in the RF link such that the same rate is supported in the RF- and the FSO-based hops. The results show the efficiency of the RF-FSO setups in different conditions. Moreover, HARQ can effectively improve the outage probability/energy efficiency, and compensate for the effect of hardware impairments in RF-FSO networks. For common parameter settings of the RF-FSO dual-hop networks, outage probability of 10−4 and code rate of 3 nats-per-channel-use, the implementation of HARQ with a maximum of 2 and 3 retransmissions reduces the required power, compared to cases with open-loop communication, by 13 and 17 dB, respectively.

Wireless Energy and Information Transmission Using Feedback: Infinite and Finite Block-Length Analysis

In this paper, we propose and analyze a wireless energy and information transfer system. To reduce the outage probability, compared with open-loop communication, we implement retransmission protocols both in the energy and in the information transmission phases. We use some recent results on finite block-length codes to analyze the effect of the energy and information signals lengths on the system outage probability/throughput. Finally, under a packet transmission delay constraint, we derive the optimal power allocation and time sharing between the energy and information signals, such that the energy-constrained outage probability is minimized. The simulation and analytical results demonstrate that the retransmission-based protocols are efficient techniques to reduce the energy-limited outage probability of wireless energy and information transmission systems.

Ultra-low-latency ubiquitous connections in heterogeneous cloud radio access networks

Although heterogeneous cloud radio access networks (H-CRAN) have emerged for efficient network management, resource management, and throughput enhancement, new technical challenges remain to support the urgent need to achieve full automation for ultra-low-latency connections. To simultaneously accommodate such diverse technical requirements in the HCRAN, in this article, a new paradigm of HCRAN design to significantly reduce latency is presented. From the refined radio access in the air interface, new methodology of radio resource optimization in the system architecture, and intelligent routing/paging in the backhaul, the provided foundation suggests open-loop communications and other new directions to facilitate state-of-the-art practices of the H-CRAN.

Pulse state interval modulation and open loop communication experiment in the hostile environments

Based on several atmospheric channel field tests and analyses, the modulation model of wireless laser communication in hostile weather is investigated, for enhancing the link rate of wireless laser communication. Firstly, the conversion relationship between SER and BER under different conditions is studied. Afterwards, by analyzing atmospheric channel tests result, the importance of atmospheric channel bandwidth was featured during test. On the basis of this, an efficiency pulse state-interval coded modulation is present for the first time and its characteristics are analyzed. In order to verify whether this modulation model is feasible and effective as well as its robustness, a wireless laser communication feasibility experiment in the hostile environments was launched. According to the results, a prospect is pointed out that atmospheric channel properties feedback technique in real time may be a key and precondition in all-weather wireless optical communication.

Information theoretic aspects of feedback control systems

The theory of choice information has evolved through the study of open-loop communication systems. Objective information is termed choice information to distinguish it from the semantic variety. Names of outstanding contributors to the start of choice information theory are Lord Kelvin, Nyquist, Küpfmüller, Hartley, Wiener, Gabor and Shannon. Choice information concepts have been rewardingly applied to computers during the last several years. In the present paper these information theoretic concepts are applied to servo systems. This approach to servos was announced at the Second IFAC Congress at Basel in the summer of 1963.