The exponential increase in mobile data traffic is creating a strong pressure on network providers to increase the data rate and network throughput. Efficient spectrum reutilization using heterogeneous networks is foreseen as a key solution for accommodating such increasing demand. In heterogeneous settings, different network nodes such as macro, pico, femto base stations and relay nodes should efficiently share and reuse the available network spectrum. Hence, limiting interference among these nodes, while supporting a diverse set of applications and users with different quality-of-service (QoS) requirements, represents a key challenge that system designers have to deal with in next generation systems. Simultaneously, network operators are interested in cutting down their operating costs using smart solutions such as energy-saving techniques. Such techniques further complicate interference management due to the expected changes in network topology in response to network traffic variations. In this project, we target developing novel solutions for interference management along four major research thrusts as detailed below.
First, we consider power control and access policies to minimize mutual interference between collocated radios, possibly with different QoS using state of the art technologies such as cognitive radio. A core focus in this thrust is exploiting the utilization of Automatic Repeat Request (ARQ) signals for designing transmission protocols. Specifically, making use of the time correlation in predicting the transmission patterns and channel states of the network is expected to lead to promising results from our previous research. In the same research line, we discuss some fundamental theoretical limits on utilizing the ARQ, and then we discuss maximization of the network throughput via designing power control algorithms and channel access protocols that optimize a certain measure of the network throughput. We also present some ideas for overlay networks where cooperation between the heterogeneous networks tiers is assumed.
The second thrust explores green interference-aware networks. Specifically, we study novel methodologies for network re-planning and optimization via infrastructure switch off, using optimization and queuing theoretic formulations, for the minimization of energy consumption subject to interference and QoS constraints. We will investigate if and how the energy constraints could modify the techniques that are currently adopted for optimal network re-planning based on spatio-temporal dynamics of the wireless network load. Second, we extend the model to explore novel ideas in the design of proactive wireless networking protocols leveraging user profiles and predictable mobility which are envisioned to create energy saving opportunities. Finally, we seek optimal sequential decision policies, using a Markov Decision Process (MDP) optimization framework, for traffic offloading in multi-tier networks, such as those composed of primary and secondary infrastructure and its impact on interference. Our objective is to find the optimal traffic load split between the different tiers from an energy and network traffic-carrying capacity perspective.
The third thrust focuses on interference alignment (IA) techniques in 4G++ networks. First, we plan to characterize the number of independent streams that can be sent in a K-user interference channel where each terminal is equipped with multiple antennas. We then explore the limits on the number of independent streams that can exist in cellular networks, and propose new IA techniques that can maximize the number of these streams. Throughout the research proposed here, our focus is on both the fundamental limits of performance as well as the application of these limits to future wireless networks, with the overarching objective being the design of more efficient networks that allows universal access and better services.
In the fourth thrust, we focus on investigating and characterizing the utilization of relays in cellular systems. Currently, relaying is mainly utilized for increasing the coverage and throughput of cellular networks. In this research thrust, we will focus on the interference management capability of two-way relays. In particular, we will start by exploring the degrees of freedom and the diversity-multiplexing tradeoff of two-way relaying systems with multiple communicating pairs. Next, we will focus on deriving capacity characterizations (deterministic and Gaussian) of two-way relaying networks. In addition to developing such theoretical foundations, we will also derive spatial multiplexing algorithms for both amplify and-forward and decode-and-forward two-way relaying cellular systems that can utilize the existence of multiple antennas at the relay to manage the interference resulting from the transmission of multiple nodes. We will also extends the above problems to relaying systems with multiple relays and explore optimal two-way relay selection techniques.
Project Team Members
The National Telecom Regulatory Authority (NTRA)
Progress & Outcomes
1 – Networks with different Quality of Service
Stability Analysis of a Cognitive Radio System with a Dedicated Relay
Fixed spectrum allocation leads to highly inefficient resources utilization. Cognitive radio networks (CRNs) are considered a promising technology to deal with the underutilization of spectrum resources in wireless communications. The cognitive radio systems allow the coexistence of different users with different priorities on the same channel. The main idea behind CRNs is to let unlicensed secondary user(s) (SU) use the licensed frequency band owned by the primary user (PU) while satisfying the quality of service requirements of the PU. Cooperation in cognitive systems benefits both the PU and the SU. The PU delivers its own packets more reliably so the number of retransmissions decreases. Therefore, the number of time slots available for the SU increases. The motivation to our work is that the conventional cooperative system, where the SU acts as a relay, that is adopted in literature has some problems when it comes to security and privacy of the PU’s data. Moreover, cognitive radio standards consider the isolation of the PU and the SU networks. Thus, there are challenges in terms of standards allowing cooperation of the SU with the PU. To the best of our knowledge, this work is among the first work that studies a cognitive network with a dedicated relay to store the undelivered PU packets. This system is considered to overcome the challenges introduced by the conventional SU relaying system. Our major contributions in this paper is two-fold:
- We characterize the stability region of the dedicated relay system under two different MAC schemes. The adopted MAC schemes result in possible collisions between PU and SU packets or between SU and relay packets. These collisions result in an interacting system of queues. The main challenge in characterizing the stable throughput region is to decouple the interaction between the queues in the system. To this extent, we provide upper and lower bounds on the arrival and service rates of the queues. The bounded rates are used to attain upper and lower bounds on the stability region of the system.
- We compare the performance of the dedicated relay system to the no relaying system and the SU relaying system, in which the SU acts as a relay, in terms of the achievable stability region. Results show that the SU relaying system has performance gains in terms of throughput when compared to the dedicated relay system. On the other hand, the relation between the performance of the no relaying system and the dedicated relay system depends on the adopted MAC protocol.
Sparse Spectrum Sensing in Cognitive Radio Networks
In this work, we utilize the tool of Compressive Sensing for infrastructure-less based CRNs where each SU makes linear measurements to detect the occupancy of PUs. Diversity based consensus algorithm is applied in the CRN, we show that the detection performance can tend to the performance of majority-rule Fusion Center (FC) based CRNs depending on the number of algorithm iterations, links’ quality between SU nodes, and the number of measurements. Specifically the contributions are:
- Utilizing the tool of compressive sensing in infrastructure-less CRNs.
- Developing consensus schemes for the CR nodes.
- Comparing the simulation results with fusion based CRNs in terms of convergence of detection and false alarm probabilities
Cache-Aided Heterogeneous Networks: Coverage and Delay Analysis
- We characterize the performance of K-tier CHNs.
- We study a generic model for a K-tier CHN, in which the BSs across tiers are distinguished by their spatial densities, transmission powers, path loss exponents and activity probabilities conditioned on the serving link.
- Moreover, we consider the generic scenario in which the BSs across different tiers may employ different placement caching strategies. Modeling the locations of BSs in each tier as an independent HPPP, we derive closed-form expressions for the coverage probability and local delay experienced by a typical user while trying to obtain each cached file in the network.
- We further obtain a simplified expression for the special case of having identical path loss exponents across tiers, through which we show the consistence of our obtained results with prior works for the single tier heterogeneous network scenario.
Caching and Delivery in Wireless Networks
- Studying the decentralized caching problem for a Fog Radio Access Network architecture with caches equipped at both transmitters and receivers.
- Designing a decentralized content placement and a coded delivery scheme that exploits the network topology to minimize the end to end delivery time.
- Deriving an information-theoretic lower bound on the minimum delivery time to prove the approximate optimality of our proposed scheme.
2- Role and Trade-offs of Energy Efficiency.
Role and Trade-offs of Energy Efficiency. On the Role of Finite Queues in Cooperative Cognitive Radio Networks with Energy Harvesting
One of the prominent challenges in wireless communication networks is to efficiently utilize the spectrum. The cognitive radio technology is one approach to tackle the hurdle of spectrum scarcity. In cognitive radio networks, the unlicensed users (secondary users (SUs)) are allowed to exploit the unused spectrum by the licensed users (the primary users (PUs)) to improve the utilization of the spectrum. Nevertheless, the spectrum occupation by the SUs is tied with a minimum quality of service guaranteed for the PUs.
Cooperative cognitive radio networks have recently attracted considerable attention. In a cooperative strategy with probabilistic relaying, the SU is equipped with two infinite length queues; one is for storing its own packets and the other is for relaying the PU packets. If the PU’s packet is not successfully decoded by the destination, whereas it is successfully decoded by the SU, the SU admits the PU’s packet with probability a. On the other hand, when the PU is sensed idle, the SU serves its own data queue with probability b or the relaying queue with probability 1-b. In prior works, it was implicitly assumed that the SU is equipped with unlimited energy supply, i.e., the SU can access the channel whenever the PU is inactive without any energy limitations. Differently from prior works, in this work, we study the scenario when the SU is equipped with limited energy source. Furthermore, we investigate the effects caused by the finiteness of queue lengths for both the relaying queue as well as the battery queue. It can be contemplated that the proposed system model constitutes an important step towards real systems.
Our main contribution in this work is three-fold;
- First, we show the challenges of fully characterizing the stable throughput region when having finite relaying and battery queues.
- Second, we characterize the stable throughput region for two energy constrained systems, namely, finite battery queue with infinite relay queue and finite relay queue with infinite battery queue.
- Third, we formulate two optimization problems to investigate the maximum achievable throughput of the SU, subject to queue stability conditions, for the two simpler systems. Despite the complexity of the formulated optimization problems caused by their non-convexity, we exploit the problems’ structure to cast them as linear programs. This, in turn, leads to efficiently solve the formulated optimization problems using standard optimization tools. Our numerical results reveal interesting insights about the effects of finite relay and energy queues as well as the energy limitations on the achievable stable throughput region.
COOPERATIVE D2D COMMUNICATIONS IN THE DOWNLINK OF CELLULAR NETWORKS WITH TIME AND POWER DIVISION CONSTRAINTS
- We consider a cooperative device-to-device (D2D) transmission in a cellular network where the D2D transmitter (DT) acts as a relay for the undelivered packets from the base station (BS).
- We formulate an optimization problem that maximizes the service rate of the DT by dividing the time and power at the DT between its own transmission and the relayed cellular packets while satisfying the stability conditions for all queues in the system.
Cache Aware Source Coding
- We show that Huffman coding method is not optimal for cache-aided networks.
- We propose an optimal cache-aware source coding algorithm
Non-Orthogonal Multiple Access Schemes in Wireless Powered Communication Networks
- We propose Low Complexity Decoding (LCD) and Successive Interference Cancellation Decoding (SICD), two schemes that aim at optimizing the sum throughput of a WPCN with and without interference cancellation.
- Since LCD leads to a non-convex problem, we solve a sub-problem via casting it as a series of geometric programs.
- On the contrary, we formally establish the convexity of the sum throughput maximization problem with SICD and propose an algorithm to find the optimal transmission durations and powers. Our numerical results show the superiority of the interference cancellation scheme over the simpler LCD.
Optimal Resource Allocation for Full-duplex Wireless Powered Communication Networks (WPCNs).
Wireless Energy Transfer (WET) has emerged in the past few years as a promising paradigm for RF energy harvesting, whereby an energy rich source, e.g., a base station, transfers energy wirelessly to the devices on the downlink, so that the devices utilize it for communication on the uplink. Most of the prior work on WPCNs has considered half-duplex radios where the uplink and downlink communications cannot proceed at the same time. Our major contributions in this work are:
- Study and model wireless powered communication networks with full-duplex radios at the base stations.
- Unlike prior work, largely focusing on slot-oriented optimization, we study the long-term weighted throughput maximization problem, in order to capture the CSI variations over future slots. This significantly increases the complexity of the optimization problem since we need to consider both CSI variations over future slots and the evolution of the batteries state when deciding the optimal resource allocation.
- Formulate the problem as a sequential decision problem, for a system of one base station and two wireless devices, based on an infinite time horizon dynamic programming framework and show how to solve it.
- Our numerical results show the superiority, in terms of the long-term reward (throughput), of our proposed full-duplex WPCN compared to half-duplex WPCNs and reveal interesting insights about the effect of perfect as well as imperfect self-interference cancellation techniques on the network performance
RF Energy Harvesting in Wireless Networks with Hybrid ARQ (HARQ).
Hybrid automatic repeat request (HARQ) has emerged recently as a promising technology for reliable communications over interference-limited wireless networks. Unlike traditional ARQ, HARQ with incremental redundancy uses forward error correcting codes (FEC) and adapts its code redundancy according to channel variations. In this work, we study the problem of RF energy (interference) harvesting in a wireless network using a HARQ protocol. To the best of the authors’ knowledge, this is one of the early works in this direction. Our major contributions in this work are:
- Study and model RF energy harvesting networks with the HARQ protocol.
- Formulate and solve a finite time horizon dynamic programming problem to determine the optimal time switching policy.
- Characterize the optimal time switching (TS) policy at the receiver side, assumed to depend on wireless energy harvesting (WEH) as an exclusive power source, to maximize the probability of successful message decoding over a fixed number of re-transmissions.
- Motivated by the sheer complexity of the problem, we propose two low complexity greedy algorithms for the two cases, with and without available CSI. We compare their performance to a lower bound on the average number of re-transmissions, as well as, the probability of decoding failure.
3- Interference Alignment in Cellular Relay Networks
Cooperative D2D in Downlink Cellular Networks with Energy Harvesting Capability
- Proposed a superposition coding transmission scheme.
- Formulated optimization problems to maximize the cellular rate subject to a target rate for D2D link.
- Utilizing the capability of employing successive interference cancellation decoding schemes
Degrees of Freedom in Cached MIMO Relay Networks With Multiple Base Stations
- We considered a cached relay network with multiple base stations.
- We proposed a cache-assisted relaying protocol using maximum distance separable coding which improves the cooperation opportunity between the base stations and the relay.
- Then, we propose a global optimal cache content placement solution.
- And, we proposed a low complexity self-learning algorithm to find a near optimal cache content placement solution without explicit knowledge of the content files popularity.
- Numerical results have been provided to show the performance improvement of the proposed scheme over various baseline schemes.
On the Degrees of Freedom of SISO X-networks with Synergistic Alternating Channel State Information at Transmitters
- Analyzing the achievable degrees of freedom (DoF) for the K _ K, K _ 2 and 2 _ K X-networks with alternating channel state information at the transmitters (CSIT).
- Proving the optimality of achieving 2K/(K+1) for any transmission scheme with two phases for the KxK X-networks.
- Characterizing the DoF region as the function of CSIT distributions.
The wireless caching networks with one-hop device-to-device (D2D) communications
- We are studying caching and D2D link establishment in a cell divided into clusters
- We consider the physical model of interference as opposed to the protocol model used in the literature
- Physical model of interference results in inter-cluster interference which complicates the D2D link establishment problem. Besides the cached files availability, the problem now depends on the inter-cluster interference and the node positions inside each cluster.
- The first step in our analysis is to characterize the interference level at each node, then investigate the impact of interference on the D2D link establishment and on the optimal cluster size.
Stability Analysis of a Cognitive Radio System with a Dedicated Relay.
Cognitive radio is considered as a promising technology to deal with the underutilization of spectrum resources in wireless systems. The key idea behind cognitive radio is to allow secondary users (SUs) to exploit the licensed spectrum of the primary users (PUs) under the condition of guaranteeing the PUs quality of service. Cooperative communication through the use of relays has been introduced and studied extensively with the target of improving the reliability of wireless systems.
In this work, we consider a primary system aided by a cognitive relay and sharing its spectrum with a secondary system. The modeling and analysis of this system poses a challenge due to the interaction between the primary system, its relay, and the secondary system.
Major contributions in this work are:
- Build a queuing theoretic model of the primary relay-aided system and the secondary system.
- Provide inner and outer bounds on the stable throughput region of the system.
4-Interference Management Using Two-way Relays
The Degrees of Freedom Region of Device Relaying Cellular Network
- Devices outside the coverage region of a base station (BS) can establish their communication with the BS through other sets of devices forming ad-hoc network and we will have device relaying cellular network (DRCN).
- We study the degrees of freedom (DoF) region of such DRCN that consists of three users and one BS where each user exchanges its messages with the BS.
- We assume that one of the users has no direct link to the BS and utilizes device-relaying to exchange data with the BS via an intermediate user device that has a direct link to the BS and device to device (D2D) link to the device.
- We provide an outer bound on the DoF region for this DRCN by using the notion of genie-aided in addition to cut-set bounds.
- We provide general schemes that provide inner bound on the DoF region that coincides with the outer bound.
- We provide the remaining two inequalities of the upper bound
- We provide a new beam forming design based on signal space alignment to satisfy the inequality: d12+d13+ d41<=N.
- We provide the achievability scheme when N>=2M
- Finally, we have presented achievable schemes that provide an inner bound which coincides with the outer bound on the DoF region and overcome the existed gap in the previous report.
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 M. Zohdy, T. ElBatt, M. Nafie and O. Ercetin, “RF Energy Harvesting in Wireless Networks with HARQ,” 2016 IEEE Globecom Workshops (GC Wkshps), Washington, DC, 2016.
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