Statistical Analysis of a Posteriori Channel and Noise Distribution Based on HARQ Feedback

In response to a comment on one of our manuscript, this work studies the posterior channel and noise distributions conditioned on the NACKs and ACKs of all previous transmissions in HARQ system with statistical approaches. Our main result is that, unless the coherence interval (time or frequency) is large as in block-fading assumption, the posterior distribution of the channel and noise either remains almost identical to the prior distribution, or it mostly follows the same class of distribution as the prior one. In the latter case, the difference between the posterior and prior distribution can be modeled as some parameter mismatch, which has little impact on certain type of applications.Comment: 15 pages, 2 figures, 4 table

Feedback Allocation For OFDMA Systems With Slow Frequency-domain Scheduling

We study the problem of allocating limited feedback resources across multiple users in an orthogonal-frequency-division-multiple-access downlink system with slow frequency-domain scheduling. Many flavors of slow frequency-domain scheduling (e.g., persistent scheduling, semi-persistent scheduling), that adapt user-sub-band assignments on a slower time-scale, are being considered in standards such as 3GPP Long-Term Evolution. In this paper, we develop a feedback allocation algorithm that operates in conjunction with any arbitrary slow frequency-domain scheduler with the goal of improving the throughput of the system. Given a user-sub-band assignment chosen by the scheduler, the feedback allocation algorithm involves solving a weighted sum-rate maximization at each (slow) scheduling instant. We first develop an optimal dynamic-programming-based algorithm to solve the feedback allocation problem with pseudo-polynomial complexity in the number of users and in the total feedback bit budget. We then propose two approximation algorithms with complexity further reduced, for scenarios where the problem exhibits additional structure.Comment: Accepted to IEEE Transactions on Signal Processin

Downlink Resource Scheduling in an LTE System

The problem of allocating resources to multiple users on the downlink of a Long Term Evolution (LTE) cellular communication system is discussed. An optimal (maximum throughput) multiuser scheduler is proposed and its performance is evaluated. Numerical results show that the system performance improves with increasing correlation among OFDMA subcarriers. It is found that a limited amount of feedback information can provide a relatively good performance. A sub-optimal scheduler with a lower computational complexity is also proposed, and shown to provide good performance. The sub-optimal scheme is especially attractive when the number of users is large, as the complexity of the optimal scheme may then be unacceptably high in many practical situations. The performance of a scheduler which addresses fairness among users is also presented

Mobile and Wireless Communications

Mobile and Wireless Communications have been one of the major revolutions of the late twentieth century. We are witnessing a very fast growth in these technologies where mobile and wireless communications have become so ubiquitous in our society and indispensable for our daily lives. The relentless demand for higher data rates with better quality of services to comply with state-of-the art applications has revolutionized the wireless communication field and led to the emergence of new technologies such as Bluetooth, WiFi, Wimax, Ultra wideband, OFDMA. Moreover, the market tendency confirms that this revolution is not ready to stop in the foreseen future. Mobile and wireless communications applications cover diverse areas including entertainment, industrialist, biomedical, medicine, safety and security, and others, which definitely are improving our daily life. Wireless communication network is a multidisciplinary field addressing different aspects raging from theoretical analysis, system architecture design, and hardware and software implementations. While different new applications are requiring higher data rates and better quality of service and prolonging the mobile battery life, new development and advanced research studies and systems and circuits designs are necessary to keep pace with the market requirements. This book covers the most advanced research and development topics in mobile and wireless communication networks. It is divided into two parts with a total of thirty-four stand-alone chapters covering various areas of wireless communications of special topics including: physical layer and network layer, access methods and scheduling, techniques and technologies, antenna and amplifier design, integrated circuit design, applications and systems. These chapters present advanced novel and cutting-edge results and development related to wireless communication offering the readers the opportunity to enrich their knowledge in specific topics as well as to explore the whole field of rapidly emerging mobile and wireless networks. We hope that this book will be useful for students, researchers and practitioners in their research studies

The path towards ultra-reliable low-latency communications via HARQ

Ultra-reliable Low-latency Communications (URLLC) is potentially one of the most disruptive communication paradigms offered by the next generation of wireless networks, 5G. This is easily demonstrated by the diverse set of applications it enables, such as autonomous driving; remote surgery; wireless networked control systems; mission-critical machine type communication; and many more. Basically, URLLC consists of the almost 100% guarantee of message delivery within a very short time interval. Furthermore, the pressure from climate change coupled with the massive growth of cellular networks expected to occur in the near future means that URLLC must also be energy efficient. On its own, achieving low-latency with high reliability is already a stringent requirement, but when that is coupled with the need for resource efficiency, it becomes even more challenging. That is the motivation behind this thesis: to study URLLC in the context of resource efficiency. Thus, a study of the counterintuitive use of retransmissions, more specifically Hybrid Automatic Repeat Request (HARQ), in the scenario of URLLC is proposed and carried out. HARQ is very attractive in terms of resource efficiency, and that is the motivation behind using it even when stringent time constraints are imposed. Four contributions are made by the present work. Firstly, a mathematical problem is presented and solved for optimizing the number of allowed retransmission rounds considering HARQ in URLLC, considering both energy efficiency as well as electromagnetic irradiation. This representation relies on a few assumptions in order to be realizable in practical scenarios. Namely, these assumptions are regarding the possibility of early error detection for sending the feedback signals and on not having to consider medium access control introduced delays. Secondly, we consider one important aspect of wireless systems, which is that they can be greatly optimized if they are designed with a specific application in mind. Based on this, a study of the use of HARQ specifically tuned for Networked Control Systems is presented, taking into account the particular characteristics of these applications. Results here show that fine-tuning for the specific characteristics of these applications yields better results when compared to using the results from the previous contribution, which are more application-agnostic. These improved results are possible thanks to the exploitation of application-specific characteristics, more specifically the use of a packetized predictive control strategy jointly designed with the communication protocol. Next, the concept of HARQ for URLLC is extended to a larger scale in an effort to relax the aforementioned assumptions. This is studied within the framework of self-organizing networks and leverages machine learning algorithms in order to overcome those strict assumptions from the first contribution. This is demonstrated by developing a digital twin simulation of the city of Glasgow and generating a large dataset of users in the cellular network, which is a third contribution of this thesis. Then, machine learning (more specifically long short-term convolutional neural networks) is applied for predicting message failures. Lastly, a protocol to exploit such predictions in combination with HARQ to deliver downlink URLLC is applied, resulting in a fourth contribution. In summary, this thesis presents a latency aware HARQ technique which is shown to be very efficient. We show that it uses up as much as 18 times less energy than a frequency diversity strategy and that it can emit more than 10 times less energy electromagnetic field radiation when compared to the same strategy. We also propose joint design techniques, where communication and control parameters are tweaked at the same time, enabling wireless control systems with a three-fold reduction in required bandwidth to achieve URLLC requirements. Lastly, we present a digital twin of the city of Glasgow which enables us to create a prediction algorithm for predicting channel quality with very high accuracy—root mean square error on the order of 10−2. This ties into the rest of the contributions as it can be used to enable early feedback detection, which in turn can be used to make sure the latency aware protocol can be employed

Effective Capacity Analysis of H-ARQ Assisted Cooperative Communication Systems

In this paper, the effective capacity of cooperative communication (CC) systems with hybrid Automatic repeat request (HARQ) is derived. The derived expressions are valid for any channel distribution and with any arbitrary number of retransmissions by the source and relay for both HARQ-repetition redundancy (RR) and HARQ-incremental redundancy (IR) over asymmetric channels. As an example, we use the derived EC expression over Rayleigh fading channels. Several results are obtained for a low rate and signal-to-noise ratio (SNR). We can see that the EC attends its maximum value with a small number of retransmissions. As expected when the relay-destination channel has low SNR, it is better than the relay does not participate especially when we assign a large number of transitions at the relay. For high data rates and strict quality of service (QoS) constraints, it is better to increase the number of relay transmissions. Finally, when we increase the number of source retransmissions, the effective capacity improves even for low values.Comment: 30 pages, 6 figure

Achieving reliable and enhanced communication in vehicular ad hoc networks (VANETs)

A thesis submitted to the University of Bedfordshire in partial fulfilment of the requirement for the degree of Doctor of PhilosophyWith the envisioned age of Internet of Things (IoTs), different aspects of Intelligent Transportation System (ITS) will be linked so as to advance road transportation safety, ease congestion of road traffic, lessen air pollution, improve passenger transportation comfort and significantly reduce road accidents. In vehicular networks, regular exchange of current position, direction, speed, etc., enable mobile vehicle to foresee an imminent vehicle accident and notify the driver early enough in order to take appropriate action(s) or the vehicle on its own may take adequate preventive measures to avert the looming accident. Actualizing this concept requires use of shared media access protocol that is capable of guaranteeing reliable and timely broadcast of safety messages. This dissertation investigates the use of Network Coding (NC) techniques to enrich the content of each transmission and ensure improved high reliability of the broadcasted safety messages with less number of retransmissions. A Code Aided Retransmission-based Error Recovery (CARER) protocol is proposed. In order to avoid broadcast storm problem, a rebroadcasting vehicle selection metric η, is developed, which is used to select a vehicle that will rebroadcast the received encoded message. Although the proposed CARER protocol demonstrates an impressive performance, the level of incurred overhead is fairly high due to the use of complex rebroadcasting vehicle selection metric. To resolve this issue, a Random Network Coding (RNC) and vehicle clustering based vehicular communication scheme with low algorithmic complexity, named Reliable and Enhanced Cooperative Cross-layer MAC (RECMAC) scheme, is proposed. The use of this clustering technique enables RECMAC to subdivide the vehicular network into small manageable, coordinated clusters which further improve transmission reliability and minimise negative impact of network overhead. Similarly, a Cluster Head (CH) selection metric ℱ(\u1d457) is designed, which is used to determine and select the most suitably qualified candidate to become the CH of a particular cluster. Finally, in order to investigate the impact of available radio spectral resource, an in-depth study of the required amount of spectrum sufficient to support high transmission reliability and minimum latency requirements of critical road safety messages in vehicular networks was carried out. The performance of the proposed schemes was clearly shown with detailed theoretical analysis and was further validated with simulation experiments