# Ioannis DimitriouUniversity of Ioannina | UOI · Department of Mathematics

Ioannis Dimitriou

PhD in Mathematics

## About

66

Publications

5,395

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523

Citations

Introduction

Since August 2022, I am an Associate Professor at the Dept. of Mathematics, University of Ioannina, Greece. I was appointed as an Assistant Professor (Sept. 2019-August 2022), and Lecturer (July 2014-Sept. 2019) at the Dept. of Mathematics, Un. of Patras, Greece. I held also post-doc positions:
1. Oct. 2011-July 2012, Dept. EEE, Imperial College, London, UK.
2. Sept. 2013-July 2014, FORTH, Greece.
3. Jan. 2013-Dec. 2013, Dept. of Mathematics, AUTH, Greece.
4. May 2014-July 2014, INRIA, France.

## Publications

Publications (66)

Motivated by queueing applications, we consider a certain class of two-dimensional random walks for which their invariant measure is written as a linear combination of a finite number of product-form terms. In this work, we investigate under which conditions such an elegant solution can be derived by applying a finite compensation procedure. The co...

We introduce a novel single-server queue with general retrial times and event-dependent arrivals. This is a versatile model for the study of service systems, in which the server needs a non-negligible time to retrieve waiting customers upon a service completion, while future arrivals depend on the last realized event. Such a model is motivated by t...

In this work, we focus on the stationary analysis of a specific class of continuous time Markov-modulated reflected random walks in the quarter plane with applications in the modelling of two-node Markov-modulated queueing networks with coupled queues. The transition rates of the two-dimensional process depend on the state of a finite state Markovi...

We introduce the generalized join the shortest queue model with retrials
and two infinite capacity orbit queues. Three independent Poisson
streams of jobs, namely a \textit{smart}, and two \textit{dedicated}
streams, flow into a single server
system, which can hold at most one job. Arriving jobs that find the
server occupied are routed to the orbit...

We consider a Markovian single server retrial system with two infinite capacity orbits and a finite capacity priority line. Arriving job are primarily routed to the priority line. If an arriving job finds the priority line fully occupied, it is forwarded to the least loaded orbit queue with ties broken randomly. Orbiting jobs of either type retry t...

This work deals with the stationary analysis of two-dimensional partially homogeneous nearest-neighbour random walks. Such type of random walks are characterized by the fact that the one-step
transition probabilities are functions of the state-space. We show that its stationary behaviour is
investigated by solving a finite system of linear equation...

We introduce the generalized join the shortest queue model with retrials and two infinite capacity orbit queues. Three independent Poisson streams of jobs, namely a \textit{smart}, and two \textit{dedicated} streams, flow into a single server system, which can hold at most one job. Arriving jobs that find the server occupied are routed to the orbit...

In this work, we consider a random access Internet of Things IoT wireless network
assisted by two aggregators collecting information from two disjoint groups of sensors.
The nodes and the aggregators are transmitting in a random access manner under
slotted time, the aggregators perform network-level cooperation for the data collection.
The aggregat...

The effect of signals on stability, stable throughput region, and delay in a two-user slotted ALOHA based random-access system with collisions is considered. This work gives rise to the development of random access G-networks, which can model security attacks , expiration of deadlines, or other malfunctions, and introduce load balancing among highl...

This work introduces the join the shortest queue policy in the retrial setting. We consider a
Markovian single server retrial system with two infinite capacity orbits. An arriving job finding
the server busy, it is forwarded to the least loaded orbit. Otherwise, it is forwarded to an orbit
randomly. Orbiting jobs of either type retry to access the...

This book constitutes the refereed proceedings of the 17th European Workshop on Computer Performance Engineering, EPEW 2021, and the 26th International Conference, on Analytical and Stochastic Modelling Techniques and Applications, ASMTA 2021, held in December 2021. The conference was held virtually due to COVID 19 pandemic.
The 29 papers presented...

This work deals with the stationary analysis of two-dimensional partially homogeneous nearest-neighbour random walks. Such type of random walks are characterized by the fact that the one-step transition probabilities are functions of the state-space. We show that its stationary behaviour is investigated by solving a finite system of linear equation...

This work introduces the join the shortest queue policy in the retrial setting. We consider a Markovian single server retrial system with two infinite capacity orbits. An arriving job finding the server busy, it is forwarded to the least loaded orbit. Otherwise, it is forwarded to an orbit randomly. Orbiting jobs of either type retry to access the...

The aim of this work concerns the performance analysis of systems with interacting queues under the join the shortest queue policy. The case of two interacting queues results in a two-dimensional random walk with bounded transitions to non-neighboring states, which in turn results in complicated boundary behavior. Although this system violates the...

In this work, we focus on the stationary analysis of a specific class of continuous time Markov modulated reflected random walk in the quarter plane with applications in the modelling of two-node Markov-modulated queueing networks with coupled queues. The transition rates of the two-dimensional process depend on the state of a finite state Markovia...

We consider a tandem queue with coupled processors, which
is subject to global breakdowns. When the network is in the operating
mode and both queues are non empty, the total service capacity is shared among the stations according to fixed proportions. When one of the stations becomes empty, the total service capacity is given to the nonempty statio...

We provide power series approximations for a structured batch arrival single
server retrial system with probabilistic priority and two infinite capacity
weighted fair orbit queues, under a queue-based constant retrial policy. We
consider both exponential and arbitrary distributed service times. In both cases
we obtain power series expansions of the...

This work deals with the stationary analysis of two-dimensional partially homogeneous nearest-neighbour random walks. Such type of random walks in the quarter plane are characterized by the fact that the one-step transition probabilities are functions of the state-space. We show that its stationary behavior is investigated by solving a finite syste...

We consider a tandem queue with coupled processors, which is subject to global breakdowns. When the network is in the operating mode and both queues are non empty, the total service capacity is shared among the stations according to fixed proportions. When one of the stations becomes empty, the total service capacity is assigned to the non-empty st...

In this work, we investigate a slotted-time relay assisted cooperative random access wireless network with multipacket (MPR) reception capabilities. MPR refers to the capability of a wireless node to successfully receive packets from more than two other modes that transmit simultaneously at the same slot. We consider a network of N saturated source...

In this work, we consider a relay assisted adaptive queue-aware cooperative random access wireless network with multipacket (MPR) reception capabilities. The network consists of $N$ sources that transmit packets to a common destination node with the cooperation of two relay nodes equipped with queues. The relays assist the sources by forwarding the...

We consider a tandem queue with coupled processors, which is subject to global breakdowns. When the network is in the operating mode and both queues are non empty, the total service capacity is shared among the stations according to fixed proportions. When one of the stations becomes empty, the total service capacity is given to the non-empty stati...

In this work, we investigate the stability conditions of a multiclass retrial system with coupled orbit queues and service interruptions. We consider a single server system accepting N classes of customers according to independent Poisson inputs and with class-dependent, arbitrarily distributed service times. An arriving customer who finds the serv...

The effect of signals on stability, throughput region, and delay in a two-user slotted ALOHA based random-access system with collisions is considered. This work gives rise to the development of random access G-networks, which can model virus attacks or other malfunctions and introduce load balancing in highly interacting networks. The users are equ...

A single server retrial queueing system with two-classes of orbiting customers, and general class dependent service times is considered. If an arriving customer finds the server unavailable, it enters a virtual queue, called the orbit, according to its type. The customers from the orbits retry independently to access the server according to the con...

The scope of this work is twofold: On the one hand, strongly motivated by emerging engineering issues in multiple access communication systems, we investigate the performance of a slotted-time relay-assisted cooperative random access wireless network with collisions and with join the shortest queue relay-routing protocol. For this model, we investi...

In this work, we verify by simulation some recent theoretical
results describing the dynamics of the the retrial system with coupled
orbits. In such a system, retransmission rate of customers blocked in a
virtual orbit depends in general on the binary state, busy or idle, of other
orbits. We consider a system with N classes of customers, where an a...

In this work, we consider a random access IoT wireless network assisted by two aggregators. The nodes and the aggregators are transmitting in a random access manner under slotted time, the aggregators use network-level cooperation. We assume that all the nodes are sharing the same wireless channel to transmit their data to a common destination. The...

Motivated by design and performance challenges stemming from emerging applications in random-access networks, we focus on the performance of a dynamic two-user slotted-time ALOHA network with a general queue-dependent transmission policy. At the beginning of each slot, each user transmits a packet with a probability that depends on the number of st...

Motivated by design and performance challenges stemming from emerging applications in random-access networks, we focus on the performance of a dynamic two-user slotted-time ALOHA network with a general queue-dependent transmission policy. At the beginning of each slot, each user transmits a packet with a probability that depends on the number of st...

We analyze a novel Markovian single-server system with two queues for call-back option and feedback. If upon arrival the server is busy, the job registers to be called back later by the server. A job that completes its service may request another round of service. Registered jobs and feedback jobs are said to be in two separated queues from which t...

We provide power series approximations for a structured batch arrival single server retrial system with two infinite capacity weighted fair orbit queues, i.e., the re-transmission rate of an orbit depends on the state of the other orbit queue. We consider both exponential and arbitrary distributed service times. In both cases we obtain power series...

In this work, we consider a random access IoT wireless network assisted by two aggregators. The nodes and the aggregators are transmitting in a random access manner under slotted time, the aggregators use network-level cooperation. We assume that all the nodes are sharing the same wireless channel to transmit their data to a common destination. The...

In this work we consider a two-user and a three-user slotted ALOHA network with multi-packet reception (MPR) capabilities. The nodes can adapt their transmission probabilities and their transmission parameters based on the status of the other nodes. Each user has external bursty arrivals that are stored in their infinite capacity queues. For the tw...

In this paper, we investigate the effect of bursty traffic and random availability of caching helpers in a wireless caching system. More explicitly, we consider a general system consisting of a caching helper with its dedicated user in proximity and another non-dedicated user requesting for content. Both the non-dedicated user and the helper have l...

In this work we consider a single-server system accepting $N$ types of retrial customers, which arrive according to independent Poisson streams. In case of blocking, type $i$ customer, $i=1,2,...,N$ is routed to a separate type $i$ orbit queue of infinite capacity. Customers from the orbit queues try to access the server according to the constant r...

In this work, motivated by emerging applications in wireless concurrent access networks and manufacturing, we analyze for the first time a system of two coupled queues accepting three flows of jobs, say a central and two background. A job from the central flow is split in two sub-jobs, which are placed simultaneously at each queue, while each backg...

In this work, we consider an asymmetric two-user random
access wireless network with interacting nodes, time-varying links and
multipacket reception capabilities. The users are equipped with infinite
capacity buffers where they store arriving packets that will be transmitted
to a destination node. Moreover, each user employs a general
transmission...

We investigate a novel queueing system that can be used to
model relay-assisted cooperative cognitive networks with coupled relay
nodes. Consider a network of two saturated source users that transmit
packets towards a common destination node under the cooperation of two
relay nodes. The destination node forwards packets outside the network,
and eac...

In this work, we consider an asymmetric two-user random access wireless network with interacting nodes, time-varying links and multipacket reception capabilities. The users are equipped with infinite capacity buffers where they store arriving packets that will be transmitted to a destination node. Moreover, each user employs a general transmission...

We investigate a novel queueing system that can be used to model relay-assisted cooperative cognitive networks with coupled relay nodes. Consider a network of two saturated source users that transmit packets towards a common destination node under the cooperation of two relay nodes. The destination node forwards packets outside the network, and eac...

We analyze a novel multiclass two-station retrial system with processor sharing
to model the access of elastic traffic in modern mobile networks managed
by two service providers. We focus on a small region containing a single access
point per provider, which admits a limited number of users, say the handover
users, the subscribers and flex users. F...

In this work we analyze a novel queueing system for modeling cooperative wire
less networks. We consider a network of three saturated source users, say a
central and two background users, two relay nodes and a common destination
node. When the central user fails to transmit a packet directly to the destina
tion, it forwards a copy of the blocked pa...

We consider a single server system accepting two types of retrial customers, which arrive according to two independent Poisson streams. The service station can handle at most one customer, and in case of blocking, type i customer, i =1, 2, is routed to a separate type i orbit queue of infinite capacity. Customers from the orbits try to access the s...

Modeling wireless access and spectrum markets is challenging due to a plethora of technological and economic aspects that affect their evolution. This work develops a modeling framework for analysing such markets using network economics, game theory, and queueing networks. The framework models the service selection of users as well as the competiti...

In this work we analyze a novel queueing system to model cooperative wireless networks with two relay nodes and simultaneous packet reception. We consider a network of three saturated source users, say a central and two background source users, two relay nodes and a common destination node. Source users transmit packets to the destination node with...

Energy efficiency and high reliability are major aspects of modern mobile wireless networks. Therefore, it is a critical step to use power management methods along with fault-tolerant techniques. In this paper, we mathematically investigate the Discontinuous Reception, a power saving mechanism in 3GPP LTE wireless networks, with checkpointing and r...

We mathematically investigate a single server system accepting two types of retrial customers and paired service. If upon arrival a customer ﬁnds the server busy, it is routed to an inﬁnite capacity orbit queue according to each type. Upon a service completion epoch, if at least one orbit queue is non-empty, the server seeks to ﬁnd customers from t...

We investigate a model to assess the performance of a base station (BS) fully powered by renewable energy sources. The BS is modeled as a three-queue system where two of them are coupled. One represents accumulated energy, the second is the data queue and the third one serves as a reserve energy queue. This smart BS is able to dynamically adjust it...

We investigate a two-station retrial queueing system to model the access in modern cellular networks managed by two service providers. Each provider owns a single access point, which operates under processor sharing discipline, and accepts three types of users: the handover users and, the originating subscribers and the originating flex users. At t...

In this paper we analyze a retrial queue that can be used to model fault-tolerant systems with checkpointing and rollback recovery. We assume that the service time of each job is decomposed into N modules, at the end of each of which a checkpoint is established. Checkpointing and rollback recovery consists, basically, of saving periodically the sta...

In this paper, we investigate the power management of mobile devices using a variant of an M/G/1 queue with probabilistic inhomogeneous multiple vacations and generalized service process. Under the vacation scheme, at the end of a vacation the server goes on another vacation, with a different probability distribution, if during the previous vacatio...

In this paper, Discontinuous Reception, a power saving mechanism in Long Term Evolution wireless networks, is analytically investigated in the presence of transient faults by using an unreliable M/G/1 queue with multiple vacations. System failures may occur either due to transient faults in the mobile device or due to wireless link errors. When a f...

We consider an unreliable single server retrial queue with batch arrivals, accepting two
types of customers, say ordinary and transit. In addition, there is a fixed number of recur-
rent customers in a retrial box. Ordinary and transit customers arrive in batches according
to a compound Poisson process while in addition, each individual customer is...

A retrial queue accepting two types of positive customers and negative arrivals, mixed priorities, unreliable server and multiple vacations is considered. In case of blocking the first type customers can be queued whereas the second type customers leave the system and try their luck again after a random time period. When a first type customer arriv...

In this paper, we analyse a priority retrial queue with repeated inhomogeneous vacations. Two types of customers arrive at the system and if they find the server unavailable, the first type join an ordinary queue, while the second have to reattempt after a random period. The server departs for a vacation when the ordinary queue is empty upon a serv...

In this paper we consider an unreliable single server retrial queue accepting two types of customers, with negative arrivals,
preemptive resume priorities and vacations. A distinguishing feature of the model is that the rates of the Poisson arrival
process depends on the server state. For this model we investigate the stability conditions and the j...

A queueing system with a single server providing n-phases of service in succession is considered. Every customer receives service in all phases. Arriving customers join a single ordinary queue, waiting to start the service procedure. When a customer completes his service in the ith phase he decides either to proceed to the next phase or to join the...

A repairable queueing model with a two-phase service in succession, provided by a single server, is investigated. Customers arrive in a single ordinary queue and after the completion of the first phase service, either proceed to the second phase or join a retrial box from where they retry, after a random amount of time and independently of the othe...

We consider a queueing system with Poisson arrivals and arbitrarily distributed service times, vacation times, and start-up and close-down times. The model accepts two types of customers—the ordinary and the retrial customers—and the server takes a single vacation each time he becomes free. For such a model the stability conditions and the system s...

A queueing system with a single server providing two stages of service in succession is considered. Every customer receives
service in the first stage and in the sequel he decides whether to proceed to the second phase of service or to depart and
join a retrial box from where he repeats the demand for a special second stage service after a random a...

## Projects

Project (1)

The goal is to evaluate the energy saving at base stations. First we evaluate the energy saving achieved in a base station when the terminals attached to it have the power saving mode enabled. The evaluation was carried out for a Poisson traffic and for web traffic. Going further, we reviewed the strategies adopted by the operators and those under consideration to “green” the base stations. We analyze the energy saving obtained with the powering off of power amplifiers
when the base station is inactive. Last, we study base stations fed only with renewable energy sources, assuming the base station, by adjusting its transmission power, can control the number of terminals attached to it and consequently the traffic intensity.