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Dr Seiamak Vahid


Senior Research Fellow
PhD in communications

My publications

Publications

Lee H, Vahid S, Moessner K (2013) A survey of radio resource management for spectrum aggregation in LTE-advanced,IEEE Communications Surveys and Tutorials 16 (2) pp. 745-760
In order to satisfy the requirements of future IMT-Advanced mobile systems, the concept of spectrum aggregation is introduced by 3GPP in its new LTE-Advanced (LTE Rel. 10) standards. While spectrum aggregation allows aggregation of carrier components (CCs) dispersed within and across different bands (intra/inter-band) as well as combination of CCs having different bandwidths, spectrum aggregation is expected to provide a powerful boost to the user throughput in LTE-Advanced (LTE-A). However, introduction of spectrum aggregation or carrier aggregation (CA) as referred to in LTE Rel. 10, has required some changes from the baseline LTE Rel. 8 although each CC in LTE-A remains backward compatible with LTE Rel. 8. This article provides a review of spectrum aggregation techniques, followed by requirements on radio resource management (RRM) functionality in support of CA. On-going research on the different RRM aspects and algorithms to support CA in LTE-Advanced are surveyed. Technical challenges for future research on aggregation in LTE-Advanced systems are also outlined. © 2014 IEEE.
Alnwaimi G, Vahid S, Moessner K (2015) Distributed Learning Game based Spectrum Sharing and Resource Selection for Femtocells, 2015 22ND INTERNATIONAL CONFERENCE ON TELECOMMUNICATIONS (ICT) pp. 331-337 IEEE
Pérez-Romero J, Sallent O, Bouali F, Sarvanko H, Mustonen M, Matinmikko M, Lee H, Vahid S, Moessner K (2012) A spectrum selection framework for Opportunistic Networks, 2012 Future Network and Mobile Summit, FutureNetw 2012
This paper presents a framework for including cognitive management functionalities in the spectrum selection process for Opportunistic Networks (ONs).The framework is based on a decision making functionality interacting with a knowledge management block that stores and processes information about the spectrum use. Different approaches for spectrum selection are discussed covering specific cases including the capability to aggregate different bands and the possibility to jointly select the spectrum and the network interface. Illustrative results of the proposed framework are presented. © 2012 IIMC Ltd.
Sucasas V, Radwan A, Marques H, Rodriguez J, Vahid S, Tafazolli R (2014) How much overhead is required for stable group formation in VANETs?, 2014 IEEE 19TH INTERNATIONAL WORKSHOP ON COMPUTER AIDED MODELING AND DESIGN OF COMMUNICATION LINKS AND NETWORKS (CAMAD) pp. 21-25 IEEE
Sucasas V, Radwan A, Marques H, Rodriguez J, Vahid S, Tafazolli R (2014) A cognitive approach for stable cooperative group formation in mobile environments, 2014 IEEE International Conference on Communications, ICC 2014 pp. 3241-3245
Cooperative communications have been gaining huge attention lately, due to the increase in the number of mobile devices and the advancement in their capabilities. In this framework, a commonly suggested approach to benefit from cooperation is the formation of virtual groups of mobile terminals. Mobility-aware algorithms are commonly proposed, where nodes form cooperative clusters according their basic mobile characteristics. However, more detailed mobility pattern information is not used to improve the performance of these systems. The reason is the complexity of current mobility pattern recognition algorithms that require previous set up and advanced knowledge on the scenario. This paper presents a novel cognitive algorithm for the identification of mobility patterns that can be used for mobile group formation and that requires no configuration. The proposed algorithm, based on data mining techniques and cooperative optimization, can evaluate the number of mobility patterns in the scenario and classify nodes according these patterns in a distributive fashion. The algorithm is mathematically presented and tested with extended simulations in both Matlab and ns-2.1 © 2014 IEEE.
Sucasas V, Althunibat S, Radwan A, Marques H, Rodriguez J, Vahid S, Tafazolli R, Granelli F (2015) Lightweight security against combined IE and SSDF attacks in cooperative spectrum sensing for cognitive radio networks,SECURITY AND COMMUNICATION NETWORKS 8 (18) pp. 3978-3994 WILEY-BLACKWELL
Bassoli R, Marques H, Radwan A, Rodriguez J, Vahid S, Tafazolli R (2013) Energy analysis of network coding in hard vertical handovers, 2013 3rd International Conference on Communications and Information Technology, ICCIT 2013 pp. 304-308
In future mobile networks different technologies will coexist and wireless devices with multiple interfaces will move in a heterogeneous scenario. The capability to connect to different access radio technologies opens the way to vertical handover mechanisms. Then, allowing vertical handovers with low losses and costs will be a main requirement of future mobile networks. In this paper, we apply batched sparse (BATS) codes on hard vertical handovers to avoid packet losses due to erasures. The theoretical analysis shows that energy consumption per bit increases while BATS codes are used. In particular, the energy consumption per bit inversely grows with the size of the finite field of the code. © 2013 IEEE.
Sucasas V, Radwan A, Marques H, Rodriguez J, Vahid S, Tafazolli R (2014) On the efficiency of merging procedures in hierarchical mobile cooperative networks, 2014 IEEE International Conference on Communications, ICC 2014 pp. 30-35
Hierarchical ad hoc networks have gained popularity due to the huge number of mobile devices, as well as the social content distribution which is frequently location based. One major issue of hierarchical networks due to mobility is the overlapping of clusters. The issue is often resolved by merging of overlapping clusters. Some efforts propose immediate merging of clusters, while others suggest contention intervals before merging without detailed analysis of the length of such intervals. The literature lacks a thorough study of the efficiency of merging procedures. This paper represents a comprehensive study of the merging procedures. The paper derives an analytical model to compare the merging procedure with and without applying contention intervals. The effect of varying the length of contention intervals on the cluster stability and energy efficiency is also studied. The results show that adopting contention intervals is not recommended. Instead, to achieve the best energy efficiency when two clusters overlap, the clusters should either merge immediately or avoid merging, depending on the characteristics of the scenario. The analytical results are supported with simulations.1 © 2014 IEEE.
Rai MR, Vahid S, Moessner K (2015) SINR based Topology Control for Multihop Wireless Networks with Fault Tolerance, 2015 IEEE 81ST VEHICULAR TECHNOLOGY CONFERENCE (VTC SPRING) IEEE
Lee H, Ko Y, Vahid S, Moessner K (2014) Opportunistic spectrum aggregation for cognitive communications under collision constraints, Proceedings of the 2014 9th International Conference on Cognitive Radio Oriented Wireless Networks and Communications, CROWNCOM 2014 pp. 7-12
We consider a collision-sensitive secondary system that intends to opportunistically aggregate and utilize spectrum of a primary system to achieve higher data rates. In such opportunistic spectrum access, secondary transmission can collide with primary transmission. When the secondary system aggregates more channels for data transmission, more frequent collisions may occur, limiting the performance obtained by the opportunistic spectrum aggregation. In this context, dynamic spectrum aggregation problem is formulated to maximize the ergodic channel capacity under the constraint of collision tolerable level. To solve the problem, we develop the optimal spectrum aggregation approach, deriving closed-form expressions for the collision probability in terms of primary user traffic load, secondary user transmission interval, and the random number of sub-channels aggregated. Our results show that aggregating only a subset of sub-channels will be a better choice, depending on the ratio of collision sensitivity requirement to the primary user traffic. © 2014 ICST.
Alnwaimi G, Vahid S, Moessner K (2015) Dynamic Heterogeneous Learning Games for Opportunistic Access in LTE-Based Macro/Femtocell Deployments,IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS 14 (4) pp. 2294-2308 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
© 2002-2012 IEEE.Interference is one of the most limiting factors when trying to achieve high spectral efficiency in the deployment of heterogeneous networks (HNs). In this paper, the HN is modeled as a layer of closed-access LTE femtocells (FCs) overlaid upon an LTE radio access network. Within the context of dynamic learning games, this work proposes a novel heterogeneous multiobjective fully distributed strategy based on a reinforcement learning (RL) model (CODIPAS-HRL) for FC self-configuration/optimization. The self-organization capability enables the FCs to autonomously and opportunistically sense the radio environment using different learning strategies and tune their parameters accordingly, in order to operate under restrictions of avoiding interference to both network tiers and satisfy certain quality-of-service requirements. The proposed model reduces the learning cost associated with each learning strategy. We also study the convergence behavior under different learning rates and derive a new accuracy metric in order to provide comparisons between the different learning strategies. The simulation results show the convergence of the learning model to a solution concept based on satisfaction equilibrium, under the uncertainty of the HN environment. We show that intra/inter-tier interference can be significantly reduced, thus resulting in higher cell throughputs.
Lee H, Vahid S, Moessner K (2012) Utility-based dynamic spectrum aggregation algorithm in cognitive radio networks, IEEE Vehicular Technology Conference
In this paper, we propose a utility-based spectrum aggregation algorithm to enhance the performance of a cognitive radio network considering multiple objectives: (i) maximization of overall throughput, (ii) reduction of channel switching, (iii) reducing the number of sub-channels comprising the aggregate channel, aimed at opportunistic spectrum use by secondary users (SUs). These three objectives are integrated into a weighted sum utility function. The weight associated with each objective can be set differently (typically done manually) depending on the metric to be optimized. In this article however, we propose and evaluate an automatic mechanism for setting weights. The proposed algorithm including the learning module allows for automated (no manual intervention) adaptable setting of objective- function weights depending on the environment changes and its performance is also shown via the simulation results. © 2012 IEEE.
Vahid S (2015) SON Evolution for 5G Mobile Networks, In: Fundamentals of 5G Mobile Networks 10 John Wiley & Sons
Jonathan Rodriguez. all data flowing between the targeted end user and the
network. This attack exploits the unprotected user traffic in some part of the HeNB
. For that reason, unprotected user data should never leave a secure domain
inside ...
Vahid S (2015) Small Cells for 5G Mobile Networks, In: Fundamentals of 5G Mobile Networks 3 John Wiley & Sons
Jonathan Rodriguez. all data flowing between the targeted end user and the
network. This attack exploits the unprotected user traffic in some part of the HeNB
. For that reason, unprotected user data should never leave a secure domain
inside ...
Tehrani RH, Vahid S, Triantafyllopoulou D, Lee H, Moessner K (2016) Licensed Spectrum Sharing Schemes for Mobile Operators: A Survey and Outlook,IEEE Communications Surveys and Tutorials 18 (4)
The ongoing development of mobile communication networks to support a wide range of superfast broadband services has led to massive capacity demand. This problem is expected to be a significant concern during the deployment of the 5G wireless networks. The demand for additional spectrum to accommodate mobile services supporting higher data rates and having lower latency requirements, as well as the need to provide ubiquitous connectivity with the advent of the Internet of Things (IoT) sector, is likely to considerably exceed the supply, based on the current policy of exclusive spectrum allocation to mobile cellular systems. Hence, the imminent spectrum shortage has introduced a new impetus to identify practical solutions to make the most efficient use of the scarce licensed bands in a shared manner. Recently, the concept of dynamic spectrum sharing has received considerable attention from regulatory bodies and governments globally, as it could potentially open new opportunities for mobile operators to exploit spectrum bands whenever they are underutilised by their owners, subject to service level agreements. Although various sharing paradigms have been proposed and discussed, the impact and performance gains of different schemes can be scenario-specific and vary depending on the nature of the sharing parties, the level of sharing and spectrum access scheme. In this survey, we describe the main concepts of dynamic spectrum sharing, different sharing scenarios, as well as the major challenges associated with sharing licensed bands. Finally, we conclude this survey paper with open research challenges and suggest some future research directions.
Lee H, Vahid S, Moessner K (2012) Utility-based dynamic spectrum aggregation algorithm in cognitive radio networks, IEEE Vehicular Technology Conference
In this paper, we propose a utility-based spectrum aggregation algorithm to enhance the performance of a cognitive radio network considering multiple objectives: (i) maximization of overall throughput, (ii) reduction of channel switching, (iii) reducing the number of sub-channels comprising the aggregate channel, aimed at opportunistic spectrum use by secondary users (SUs). These three objectives are integrated into a weighted sum utility function. The weight associated with each objective can be set differently (typically done manually) depending on the metric to be optimized. In this article however, we propose and evaluate an automatic mechanism for setting weights. The proposed algorithm including the learning module allows for automated (no manual intervention) adaptable setting of objective- function weights depending on the environment changes and its performance is also shown via the simulation results. © 2012 IEEE.
Sucasas V, Radwan A, Marques H, Rodriguez J, Vahid S, Tafazolli R (2013) On the energy efficiency of low-mobility subsystems in hierarchical ad hoc networks, 2013 IEEE 18th International Workshop on Computer Aided Modeling and Design of Communication Links and Networks, CAMAD 2013 pp. 48-52
Hierarchical topologies have been suggested to increase network stability in mobile ad hoc networks (MANETs). In a hierarchical ad hoc network nodes are clustered in more stable virtual groups. Ideally, the members of these groups can establish links with longer lifetimes with the leader of the group, c1usterhead. However, in highly mobile ad hoc networks, hierarchical topologies face the problem of continuous reorganization. The overhead required to maintain the hierarchical topology is increased and average link lifetime is greatly reduced, producing an increase in the packet loss rate. This two-fold effect lowers the energy efficiency of the system. In this framework, this paper explores the effectiveness of the division of a highly mobile network in smaller and low mobile subsystems where clustering operations can form and maintain more stable hierarchies. We analytically prove that the division of a high mobility ad hoc network in different mobility patterns can produce low mobility subsystems that foster clustering stability. The impact of the increased stability on the energy efficiency is then evaluated by simulations. © 2013 IEEE.
Bassoli R, Talooki VN, Marques H, Rodriguez J, Vahid S, Tafazolli R (2014) LT codes for video streaming in burst erasure channels: An energy analysis, Proceedings - International Symposium on Computers and Communications Workshops
© 2014 IEEE.The diffusion of mobile devices, that are capable to play videos, opened new challenges for video streaming applications. Since wireless communications are prone to erasures, an important goal is to research possible ways to reliably provide video content. Fountain codes are an efficient way to protect video streaming against erasures: in particular, burst erasures are the ones that can significantly reduce the quality of the transmitted video. In this work, Luby codes for burst erasure channels are investigated. Our main focus is on the design of Luby codes to make the receiver able to recover all the lost information without any retransmission. Furthermore, we analytically demonstrate that lower energy consumption is achievable by using LT codes instead of a general retransmission scheme.
Bassoli R, Marques H, Rodriguez J, Vahid S, Tafazolli R (2013) Network Coding for Vertical Handoffs Between LTE and IEEE 802.11n: An Energy Perspective., EW VDE-Verlag
Alnwaimi G, Zahir T, Vahid S, Moessner K (2013) Machine learning based knowledge acquisition on spectrum usage for LTE femtocells, IEEE Vehicular Technology Conference
The decentralised and ad hoc nature of femtocell deployments calls for distributed learning strategies to mitigate interference. We propose a distributed spectrum awareness scheme for femtocell networks, based on combined payoff and strategy reinforcement learning (RL) models. We present two different learning strategies, based on modifications to the Bush Mosteller (BM) RL and the Roth-Erev RL algorithms. The simulation results show the convergence behaviour of the learning strategies under a dynamic robust game. As compared to the Bush Mosteller (BM) RL, our modified BM (MBM) converges smoothly to a stable satisfactory solution. Moreover, the MBM significantly reduces the interference collision cost during the learning process. Both the MBM and the modified Roth-Erev (MRE) algorithms are stochastic-based learning strategies which require less computation than the gradient follower (GF) learning strategy and have the capability to escape from suboptimal solution. Copyright © 2013 by the Institute of Electrical and Electronic Engineers, Inc.
Lee H, Vahid S, Moessner K (2013) A Survey of Radio Resource Management for Spectrum Aggregation in LTE-Advanced, IEEE Communications Surveys and Tutorials
In order to satisfy the requirements of future IMT-Advanced mobile systems, the concept of spectrum aggregation is introduced by 3GPP in its new LTE-Advanced (LTE Rel. 10) standards. While spectrum aggregation allows aggregation of carrier components (CCs) dispersed within and across different bands (intra/inter-band) as well as combination of CCs having different bandwidths, spectrum aggregation is expected to provide a powerful boost to the user throughput in LTE-Advanced (LTE-A). However, introduction of spectrum aggregation or carrier aggregation (CA) as referred to in LTE Rel. 10, has required some changes from the baseline LTE Rel. 8 although each CC in LTE-A remains backward compatible with LTE Rel. 8. This article provides a review of spectrum aggregation techniques, followed by requirements on radio resource management (RRM) functionality in support of CA. On-going research on the different RRM aspects and algorithms to support CA in LTE-Advanced are surveyed. Technical challenges for future research on aggregation in LTE-Advanced systems are also outlined.
Lee H, Vahid S, Moessner K (2014) A sub-optimal channel switching-aware spectrum aggregation approach for CRNs, Proceedings of the 2014 9th International Conference on Cognitive Radio Oriented Wireless Networks and Communications, CROWNCOM 2014 pp. 114-119
We consider a cognitive radio network (CRN) that intends to opportunistically aggregate and utilize spectrum of a primary network to achieve higher data rates. In such an opportunistic spectrum access, primary user can reclaim a channel used by a secondary transmission. When the secondary transmission is interrupted by a primary transmission, the secondary network needs to switch the channel of the interrupted transmission, resulting in additional delay. When a secondary user accesses more spectrum to increase its data rate, channel switching could be more frequent. In this context, we formulate a dynamic spectrum aggregation optimisation problem to minimize channel switching delay. While considering multiple users, the problem is formulated as a sum of fractional programming problems. We propose a sub-optimal algorithm that simplifies the fractional programming to linear programming first and solves each linear programming (for each user) using Dinkelbach's algorithm. Simulation results demonstrate that the proposed algorithm can reduce the channel switching delay. When multiple users are served in the network, the proposed algorithm also shows good performance in terms of fairness and total data transmission time. © 2014 ICST.
Sucasas V, Radwan A, Marques H, Rodriguez J, Vahid S, Tafazolli R (2015) Can We Apply Clustering in Fast Moving Objects?, WIRELESS INTERNET (WICON 2014) 146 pp. 76-84 SPRINGER-VERLAG BERLIN
Hasanpour M, Shariat S, Barnaghi P, Hoseinitabatabaei S, Vahid S, Tafazolli R (2017) Quantum Load Balancing in Ad Hoc Networks,Quantum Information Processing 16 (148) Springer Verlag
this paper presents a novel approach in targeting load balancing in ad hoc networks utilizing the properties of quantum game theory. This approach benefits from the instantaneous and information-less capability of entangled particles to synchronize the load balancing strategies in ad hoc networks. The Quantum Load Balancing (QLB) algorithm proposed by this work is implemented on top of OLSR as the baseline routing protocol; its performance is analyzed against the baseline OLSR, and considerable gain is reported regarding some of the main QoS metrics such as delay and jitter. Furthermore, it is shown that QLB algorithm supports a solid stability gain in terms of throughput which stands a proof of concept for the load-balancing properties of the proposed theory.
Lee Haeyoung, Vahid Seiamak, Moessner Klaus (2017) Traffic-aware Carrier Allocation with Aggregation for Load Balancing,EuCNC 2017 Conference Proceedings
We consider the resource allocation with aggregation of multiple bands including unlicensed band for heterogeneous traffic. While the mobile data traffic including high volume of video traffic is expected to increase significantly, an efficient management of radio resources from multiple bands is required to guarantee the quality of service (QoS) of different traffic types. In this context, we formulate an optimal resource allocation by using different utility functions for heterogeneous traffic and the two-step resource allocation algorithm including resource grouping has been proposed. Simulation results demonstrate that the proposed algorithm enhances the connection robustness and shows good performance in terms of higher utility value of inelastic traffic even at high traffic loads by steering elastic traffic to unlicensed band.
Filo Marcin, Foh Chuan, Vahid Seiamak, Tafazolli Rahim (2020) Performance Analysis of Ultra-Dense Networks with Regularly Deployed Base Stations,IEEE Transactions on Wireless Communications Institute of Electrical and Electronics Engineers
The concept of Ultra Dense Networks (UDNs) is
often seen as a key enabler of the next generation mobile
networks. The massive number of BSs in UDNs represents a
challenge in deployment, and there is a need to understand the performance behaviour and benefit of a network when BS locations are carefully selected. This can be of particular importance to the network operators who deploy their networks
in large indoor open spaces such as exhibition halls, airports or train stations where locations of BSs often follow a regular pattern. In this paper we study performance of UDNs in downlink
for regular network produced by careful BS site selection and compare to the irregular network with random BS placement. We first develop an analytical model to describe the performance of regular networks showing many similar performance behaviour
to that of the irregular network widely studied in the literature. We also show the potential performance gain resulting from proper site selection. Our analysis further shows an interesting finding that even for over-densified regular networks, a nonnegligible
system performance could be achieved.
Lee Haeyoung, Vahid Seiamak, Moessner Klaus (2018) Traffic-aware Resource allocation with aggregation in Heterogeneous Networks with WLANs,Proceedings of the 27th European Conference on Networks and Communications 2018 (EUCNC 2018) Institute of Electrical and Electronics Engineers (IEEE)
We consider resource allocation with aggregation for
different types of traffic in heterogeneous networks, including
WLANs. While mobile data traffic is expected to increase, efficient
management of multiple bands including unlicensed band
becomes increasingly important. In this context, we formulate a
resource allocation problem using utility functions for heterogeneous
traffic and propose a novel algorithm that considers the
estimated UE speed, traffic types and channel quality. Simulation
results illustrate performance of the proposed algorithm in
terms of higher utility value and fairness, even at high traffic
loads. Additional improvements in resource utilization through
estimating UE speed and allocating low-mobility UEs to Wi-Fi
are shown.
Lee H, Ko Y, Vahid S, Moessner K (2015) Practical Spectrum Aggregation for Secondary Networks with Imperfect Sensing,IEEE Transactions on Vehicular Technology 65 (7) pp. 5474-5484 Institute of Electrical and Electronics Engineers (IEEE)
We investigate a collision-sensitive secondary network that intends to opportunistically aggregate and utilize spectrum of a primary network to achieve higher data rates. In opportunistic spectrum access with imperfect sensing of idle primary spectrum, secondary transmission can collide with primary transmission. When the secondary network aggregates more channels in the presence of the imperfect sensing, collisions could occur more often, limiting the performance obtained by spectrum aggregation. In this context, we aim to address a fundamental query, that is, how much spectrum aggregation is worthy with imperfect sensing. For collision occurrence, we focus on two different types of collision: one is imposed by asynchronous transmission; and the other by imperfect spectrum sensing. The collision probability expression has been derived in closed-form with various secondary network parameters: primary traffic load, secondary user transmission parameters, spectrum sensing errors, and the number of aggregated sub-channels. In addition, the impact of spectrum aggregation on data rate is analysed under the constraint of collision probability. Then, we solve an optimal spectrum aggregation problem and propose the dynamic spectrum aggregation approach to increase the data rate subject to practical collision constraints. Our simulation results show clearly that the proposed approach outperforms the benchmark that passively aggregates sub-channels with lack of collision awareness.
Triantafyllopoulou Dionysia, Vahid Seiamak, Moessner Klaus (2018) Co-Primary Spectrum Sharing in Uplink
SC-FDMA Networks
,
IEEE PIMRC 2018 Proceedings IEEE
In this paper we present a co-primary spectrum
sharing algorithm for the Quality of Service (QoS)
enhancement of uplink Single-Carrier Frequency Division
Multiple Access (SC-FDMA) systems. We consider the
limitations that are resulting from the fact that each
user can only be provided with only contiguous sets of
resource blocks (following the constraints of the localized
SC-FDMA physical layer), and the effect of the limited, or
even lack of, knowledge of each user?s buffer status and
packet delays in the uplink. The sharing of available resources
is based on the operator spectrum access priority,
an estimation of the packet delays in the uplink direction,
the average delay and data rate of earlier allocations, and
the power per resource block. Simulation results show
that the proposed algorithm considerably improves the
performance in terms of packet loss rate, goodput, and
fairness.
Yu Wenjuan, ul Quddus Atta, Vahid Seiamak, Tafazolli Rahim (2018) Opportunistic spectrum access in support of ultra-reliable and low-latency communications,Proceedings of 2018 IEEE Global Communications Conference (GLOBECOM) Institute of Electrical and Electronics Engineers (IEEE)
This paper addresses the problem of opportunistic
spectrum access in support of mission-critical ultra-reliable and
low latency communications (URLLC). Considering the ability
of supporting short packet transmissions in URLLC scenarios,
a new capacity metric in finite blocklength regime is introduced
as the traditional performance metrics such as ergodic capacity
and outage capacity are no longer applicable. We focus on an
opportunistic spectrum access system in which the secondary
user (SU) opportunistically occupies the frequency resources of
the primary user (PU) and transmits reliable short packets to
its destination. An achievable rate maximization problem is then
formulated for the SU in supporting URLLC services, subject to
a probabilistic received-power constraint at the PU receiver and
imperfect channel knowledge of the SU-PU link. To tackle this
problem, an optimal power allocation policy is proposed. Closedform
expressions are then derived for the maximum achievable
rate in finite blocklength regime, the approximate transmission
rate at high signal-to-noise ratios (SNRs) and the optimal average
power. Numerical results validate the accuracy of the proposed
closed-form expressions and further reveal the impact of channel
estimation error, block error probability, finite blocklength and
received-power constraint.
With the massive deployment of broadband access to the end-users, the continuous improvement of the hardware capabilities of end devices and better video compression techniques, acceptable conditions have been met to unleash over-the-top bandwidth demanding and time-stringent P2P applications, as multiview real-time media distribution. Such applications enable the transmission of multiple views of the same scene, providing consumers with a more immersive visual experience.

This thesis proposes an architecture to distribute multiview real-time media content using a hybrid DVB-T2, client-server and P2P paradigms, supported by an also novel QoS solution. The approach minimizes packet delay, inter-ISP traffic and traffic at the ISP core network, which are some of the main drawbacks of P2P networks, whilst still meeting stringent QoS demands. The proposed architecture uses DVB-T2 to distribute a self-contained and fully decodable base-layer video signal, assumed to be always available to the end-user, and an IP network to distribute in parallel - with increased delay - additional IP video streams. The result is a decoded video quality that adapts to individual end-user conditions and maximizes viewing experience.

To achieve its target goal this architecture: defines new services for the ISP?s services network and new roles for the ISP core, edge and border routers; makes use of pure IP multicast transmission at the ISP?s core network, greatly minimizing bandwidth consumption; constructs a geographically contained P2P network that uses P2P application-level multicast trees to assist the distribution of the IP video streams at the ISP access networks, greatly reducing inter-ISP traffic, and; describes a novel QoS control architecture that takes advantage of the Internet resource over-provisioning techniques to meet stringent QoS demands in a scalable manner.

The proposed architecture has been implemented in both real testbed implementation and ns-2 simulations. Results have shown a highly scalable P2P overlay construction algorithm with very fast computation of application-level multicast trees (in the order of milliseconds) and efficient reaction to peer-churn, with no perceptually annoying impairments noticed. Furthermore, huge bandwidth savings are achieved at the ISP core network, which considerably lower the management and investment costs in infrastructure. The QoS based results have also shown that the proposed approach effectively deploys a fast and scalable resource and admission control mechanism, greatly minimizing QoS related signalling events by using a per-class over-provisioning approach and thus preventing per-flow QoS reservation signalling messages. Moreover, the QoS control architecture is aware of network link resources in real-time and supports for service differentiation and network convergence by guaranteeing that each admitted traffic flow receives the contracted QoS.

Finally, the proposed Scalable Architecture for Multiview Real-Time Media Distribution for Next Generation Networks, as a component for a large project demonstrator, has been evaluated by an independent panel of experts following ITU recommendations, obtaining an excellent evaluation as computed by Mean Opinion Score.

Selinis Ioannis, Katsaros Konstantinos, Allayioti Marion, Vahid Seiamak, Tafazolli Rahim (2018) The Race to 5G Era; LTE and Wi-Fi,IEEE Access 6 pp. 56598-56636 IEEE
We are on the brink of a new era for the wireless telecommunications, an era that will
change the way that business is done. The fifth generation (5G) systems will be the first realization in this
new digital era where various networks will be interconnected forming a unified system. With support
for higher capacity as well as low-delay and machine-type communication services, the 5G networks
will significantly improve performance over the current fourth generation (4G) systems and will also
offer seamless connectivity to numerous devices by integrating different technologies, intelligence, and
flexibility. In addition to ongoing 5G standardization activities and technologies under consideration in the
Third Generation Partnership Project (3GPP), the Institute of Electrical and Electronic Engineers (IEEE)
based technologies operating on unlicensed bands, will also be an integral part of a 5G eco-system. Along
with the 3GPP-based cellular technology, IEEE standards and technologies are also evolving to keep pace
with the user demands and new 5G services. In this article, we provide an overview of the evolution of
the cellular and Wi-Fi standards over the last decade with particular focus on Medium Access Control
(MAC) and Physical (PHY) layers, and highlight the ongoing activities in both camps driven by the 5G
requirements and use-cases.
Lee Haeyoung, Vahid Seiamak, Moessner Klaus (2019) Machine Learning based RATs Selection supporting Multi-Connectivity for Reliability,Proceedings of the 14th EAI International Conference on Cognitive Radio Oriented Wireless Networks (CrownCOM 2019) European Alliance for Innovation / Springer
While ultra-reliable and low latency communication (uRLLC)
is expected to cater to emerging services requiring real-time control, such
as factory automation and autonomous driving, the design of uRLLC of
stringent requirements would be very challenging. Among novel solutions
to satisfy uRLLC's requirements, interface diversity is widely regarded
as an efficient enabler of ultra-reliable connectivity. When mobile de-
vices are connected to multiple base stations (BSs) of different radio
access technologies (RATs) and same data is transmitted via multiple
links simultaneously, the transmission reliability can be improved. How-
ever, duplicate transmission of same data causes an increase in the traffic
loads, leading to radio resource shortage. Considering it, efficient config-
uration of multi-connectivity (MC) for mobile devices is important. In
this paper, the RAT selection scheme including efficient MC configura-
tion is proposed. By adopting distributed reinforcement learning (RL),
each device could learn the policy for efficient MC configuration and
select appropriate RATs. Simulation results show that 20.8% reliabil-
ity improvements over the single connectivity scheme is observed. Com-
paring to the method to configure MC for devices all the time, 37.6%
improvement is achieved at high traffic loads.
Selinis Ioannis, Katsaros Konstantinos, Vahid Seiamak, Tafazolli Rahim (2018) Control OBSS/PD Sensitivity Threshold for IEEE 802.11ax BSS Color,Proceedings of the 2018 IEEE 29th Annual International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC 2018) pp. 1-7 Institute of Electrical and Electronics Engineers (IEEE)
IEEE 802.11ax Spatial Reuse (SR) is a new category in the IEEE 802.11 family, aiming at improving the spectrum efficiency and the network performance in dense deployments. The main and perhaps the only SR technique in that amendment is the Basic Service Set (BSS) Color. It aims at increasing the number of concurrent transmissions in a specific area, based on a newly defined Overlapping BSS/Preamble-Detection (OBSS/PD) threshold and the Received Signal Strength Indication (RSSI) from Overlapping BSSs (OBSSs). In this paper, we propose a Control OBSS/PD Sensitivity Threshold (COST) algorithm for adjusting OBSS/PD threshold based on the interference level and RSSI from the associated recipient(s). In contrast to the Dynamic Sensitivity Control (DSC) algorithm that was proposed for setting OBSS/PD, COST is fully aware of any changes in OBSSs and can be applied to any IEEE 802.11ax node. Simulation results in various scenarios, show a clear performance improvement of up to 57% gain in throughput over a conservative fixed OBSS/PD for the legacy BSS Color and DSC.
Selinis Ioannis, Katsaros Konstantinos, Vahid Seiamak, Tafazolli Rahim (2017) Exploiting the Capture Effect on DSC and BSS Color in Dense IEEE 802.11ax Deployments,Proceedings of the 2017 Workshop on ns-3, Porto, Portugal, June 2017 (WNS3 2017) pp. 47-54 Association for Computing Machinery (ACM)
Future wireless local area networks (WLANs) are expected to serve thousands of users in diverse environments. To address the new challenges that WLANs will face, and to overcome the limitations that previous IEEE standards introduced, a new IEEE 802.11 amendment is under development. IEEE 802.11ax aims to enhance spectrum efficiency in a dense deployment; hence system throughput improves. Dynamic Sensitivity Control (DSC) and BSS Color are the main schemes under consideration in IEEE 802.11ax for improving spectrum efficiency In this paper, we evaluate DSC and BSS Color schemes when physical layer capture (PLC) is modelled. PLC refers to the case that a receiver successfully decodes the stronger frame when collision occurs. It is shown, that PLC could potentially lead to fairness issues and higher throughput in specific cases. We study PLC in a small and large scale scenario, and show that PLC could also improve fairness in specific scenarios.
Selinis Ioannis, Katsaros Konstantinos, Vahid Seiamak, Tafazolli Rahim (2019) Eliminating the Use of IEEE 802.11 Acknowledgements; A Network Coding Approach,Proceedings of the 2019 IEEE Wireless Communications and Networking Conference (WCNC 2019) Institute of Electrical and Electronics Engineers (IEEE)
Selinis Ioannis, Katsaros Konstantinos, Vahid Seiamak, Tafazolli Rahim (2019) Damysus: A Practical IEEE 802.11ax BSS Color Aware Rate Control Algorithm,International Journal of Wireless Information Networks pp. 1-23 Springer Verlag
Institute of Electrical and Electronics Engineers (IEEE) 802.11ax Spatial Reuse (SR) is a new feature in the IEEE 802.11 family, aiming at improving the spectrum efficiency and the network performance in dense deployments. The main and perhaps the only SR technique in that amendment is the Basic Service Set (BSS) Color. It aims at increasing the number of concurrent transmissions in a specific area, based on a newly defined Overlapping BSS/Preamble-Detection threshold. In this paper, we overview the latest developments introduced in the IEEE 802.11ax for the SR and propose a rate control algorithm developed to exploit the BSS Color scheme. Our proposed algorithm, Damysus is specifically designed to function in dense environments where other off-the-shelf algorithms show poor performance. Simulation results in various dense scenarios, show a clear performance improvement of up to 113% gain in throughput over the well known MinstrelHT algorithm.
Selinis Ioannis, Filo Marcin, Vahid Seiamak, Rodriguez Jonathan, Tafazolli Rahim (2017) Evaluation of the DSC algorithm and the BSS color scheme in dense cellular-like IEEE 802.11ax deployments,Proceedings of the 2016 IEEE 27th Annual International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC 2016) pp. 1641-1647 Institute of Electrical and Electronics Engineers (IEEE)
Coping with the extreme growth of the number of users is one of the main challenges for the future IEEE 802.11 networks. The high interference level, along with the conventional standardized carrier sensing approaches, will degrade the network performance. To tackle these challenges, the Dynamic Sensitivity Control (DSC) and the BSS Color scheme are considered in IEEE 802.11ax and IEEE 802.11ah, respectively. The main purpose of these schemes is to enhance the network throughput and improve the spectrum efficiency in dense networks. In this paper, we evaluate the DSC and the BSS Color scheme along with the PARTIAL-AID (PAID) feature introduced in IEEE 802.11ac, in terms of throughput and fairness. We also, exploit the performance when the aforementioned techniques are combined. The simulations show a significant gain in total throughput when these techniques are applied.
The concept of Ultra Dense Networks (UDNs) is often seen as a key enabler of the next generation mobile networks. In contrast to the traditional cellular networks, it is foreseen that the UDNs will be in many cases installed without traditional RF planning and proper site selection. One of the main characteristics (and at the same time challenge) of the UDNs is therefore excessive inter-cell interference. Additionally, small cells as one of the main components on UDNs are foreseen to operate in licensed as well as license-exempt frequency bands, and therefore inter-system and inter-tier interference become major concerns.

Widely used and well established systems such as LTE, or WiFi are often proposed by the industry and academia to be reused in the context of UDNs. However, as these systems were not designed to deal with problems caused by the UDN deployment, a significant effort is currently being undertaken to adapt them and enable their operation in dense and ultra-dense environments. Despite this tremendous effort, due to the need for backward compatibility, the proposed updates and patches usually provide sub-optimal gains and often lead to significant signaling overheads.

In this thesis we highlight some of the main challenges and requirements related to UDNs and then provide an extensive review of state-of-art UDN performance analysis and approaches to medium access control (MAC) design for UDNs. Then we investigate performance limits of regular and irregural UDNs. More specifically, we examine the impact of the relative antenna height between BS and UE antennas on the performance of UDNs. Based on our study, we found that regular networks share many of the same performance behaviour as irregular network. In partivular, we showed that by decreasing the relative antenna heights across the network we can counter the decay of per cell average achievable rate. We explicitly derived the relationship between BS density and relative antenna height and confirmed that both regular and irregular networks share this property. Despite the pessimistic conclusion related to the per cell performance found in the literature, in this work we also show that area spectral efficiency does not necessarily decay to zero as BS density approaches infinity. In terms of the benefit of proper BS site selection, we compare the average per cell rate of regular networks and that of the irregular networks, and we find that proper BS deployment may improve network performance to some extent. Finally, based on the lessons learned, we present and discuss a novel MAC protocol designed for 5G UDN deployments. In contrast to other candidates considered by the industry for UDN deployment, the proposed MAC provides a number of built-in features which improve its efficiency in dense and ultra dense deployments. The multi-channel operation along with the dynamic channel selection constitutes the core of the proposed MAC, limiting performance degradation resulting from high level of inter-cell interference and simplifying network planning. The proposed MAC design is further evaluated through simulations for outdoor deployments in non-coexistence and coexistence scenarios. Our results reveal that the proposed MAC is capable of operating effectively in highly dense deployment scenarios when tuned appropriately. In case of the coexistence capabilities of the investigated design, we show that coexistence with LBT-based systems such as WiFi is also possible, but requires additional tuning to maintain fair channel access for all systems. Lastly,we show that the proposed MAC design outperforms WiFi and LTE (which are commonly considered for UDN deployment) in all considered scenarios. More specifically, our results indicate that area spectral efficiency for the proposed MAC is approximately 500% higher compared to WiFi (IEEE 802.11ac), and 40% higher compared to LTE (excl. CA and MIMO), with improved performance for cell-edge users.

In light of recent interests and activities by cellular networks operators to exploit the unlicensed bands to boost network capacity, never has the issue of fair coexistence and spectrum sharing been at the heart of most feasibility and performance studies of radio access technologies coexisting in unlicensed bands. This has been the case because overall system performance by incumbent radio access technologies is not expected to be compromised in the spirit of fairness. Therefore, fair coexistence or spectrum sharing has become a key performance metric in evaluating the performance of most solutions proposed to permit radio access technology coexistence in the unlicensed bands. Time-
based fairness, the focus of this thesis, refers to a mechanism which can by adopted to evaluate fairness performance among coexisting radio access technologies in the time domain, however, limited studies have been conducted and practical implementation solutions are still out of reach. In this thesis, the objectives are to address existing gaps in accessing practical time-based fairness solutions.
Firstly, a review of the state of the art on fairness issues, metrics and approaches are discussed, providing an overview of current approaches and solutions and identifies their shortcomings to practical implementation.
Secondly, the estimation of number of nodes contending over the unlicensed spectrum, which is a requirement for many fairness oriented schemes proposed for radio access technologies coexisting in unlicensed bands, is addressed. A novel technology-neutral estimation method for node numbers is proposed. The transmission interval observed over the unlicensed channel represents a probabilistic distribution, which can be obtained via the uniform difference distribution. The characteristic features of the uniform difference distribution are exploited to aid estimation of node numbers in scenarios
where nodes contend for the channel within the same contention window and under multiple contention windows. The benefit of the proposed method over existing methods is the level of accuracy and its ability to provide a tighter estimate to small increase in numbers of contenders on the channel.
Thirdly, two approaches to achieving time-based fairness are proposed. The first being a deterministic approach and the second a probabilistic one. The deterministic approach aims to study the upper bound performance of time-based fairness utilizing the estimation method for node numbers proposed in this thesis. An optimal value is computed as a backoff value for each transmission cycle while keeping the transmission opportunity of all coexisting radio access technologies the same. The results show time-based fairness improves spectrum utilization and overall throughput performance. The probabilistic approach seeks for a practical and implementable solution to achieving time-based fairness based on the proven performance benefits shown from the deterministic approach. The transmission interval distribution obtained from observation of the channel activity during the node number estimation serves as the building block towards a practical solution. The distribution is continually monitored and mapping of the occurrence of transmission intervals to its probability density functions are performed. The mean of the distribution is then optimized to provide a balance between the channel access probabilities in order to achieve approximately equal transmission
time by all nodes contending over the channel. Two parameters are adjusted to attain time-based fairness, which are the contention window sizes and transmission opportunity. Simulation results show that time-based fairness under the proposed scheme can improve spectrum utilization, reduce the disparity in throughput performance and guarantee fairness among coexisting radio access technologies.
Spectrum sensing has received abundant attention from the research community. However, with sensing scenarios becoming increasingly complex, existing spectrum sensing schemes can hardly meet the demand for fast, accurate spectrum sensing, particularly in the very low signal-to-noise ratio (SNR) range, without dramatically increasing system complexity and the need for precise information about signal and noise. Furthermore, the widespread adoption of multicarrier modulation in various existing and evolving standards is driving efforts to develop a robust and practical solution for multicarrier signal spectrum sensing. The main challenges identified in lieu of changing spectrum scenarios are detection in low SNR, high accuracy, low computational and sample complexity, and possible operation without the knowledge of the primary user (PU) signal and channel.
Stochastic Resonance (SR) is a phenomenon in which a signal too weak to cross the detection threshold becomes detectable in a nonlinear system with the addition of noise. This research considers the application of SR to sense multi-frequency/multi-carrier signals. The effect of the SR receiver has never been verified for multi-carrier signals, which is a popular modulation system in various existing and evolving standards. The thesis provides an understanding of how and why the SR effect for multi-frequency signals differs from the SR effect for single frequency signals. Special features such as ghost resonance, multi-resonance and doubly SR are demonstrated with simulations. A novel method to identify ghost resonance is proposed and the relationship of noise intensity and frequencies of the driving signal is derived. SR for multi-frequency signals is quantified in terms of the SNR.
In the presented research, SR is used as a pre-processing technique to enhance SNR prior to the detector, which significantly improves spectrum sensing performance. The two main contributions in this part of the research are (i) A novel algorithm for dynamic determination of SR system parameters and noise intensity, which results in maximum SNR; (ii) An SR-based sensing method particularly tailored to give near-optimal performance for multi-carrier signals by using the multi-taper spectral estimate (MTSE) method. A simple Fourier transform-based method is also evaluated as a computationally light alternative to MTSE. The FFT-based method combined with (i) provides near-optimal performance for single-carrier modulated schemes. The performance is evaluated in low SNR, flat/frequency selective fading,
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shadowing, interference and time/frequency offset. The results show that by using SR pre-processing, the performance of energy-based detection (ED) can be significantly improved. The proposed method is also evaluated in cooperative sensing scenarios. The results show that the proposed SR-ED with a basic cooperative mechanism can match the performance of ED-based cooperative sensing with optimal fusion. The proposed method has several distinctive features including low latency, high accuracy, reasonable computational complexity, robustness to low SNR, robustness to flat/frequency selective fading, robustness to noise/channel uncertainty. It also requires no prior knowledge of the PU signal.
One of the ways to provide greater coverage and capacity for future wireless networks is through network densification. This is also one of the drivers for future IEEE 802.11 deployments, aiming not only to improve throughput per link, but the overall network performance in dense deployments. That said, the IEEE 802.11ax amendment is currently focusing on addressing the challenges and improving the spectrum efficiency in dense deployments with hundreds of Access Points (APs) and Stations (STAs). This
work strives to shed some light in the area of spectrum efficiency by trying to understand (i) the operation and the impact of the newly introduced Spatial Reuse feature of
the IEEE 802.11ax amendment and (ii) if it is possible to realise multicast/broadcast transmissions over Wi-Fi while preserving reliability.

Although the IEEE 802.11ax Spatial Reuse feature, namely BSS Color, offers several advantages and good potential for improving spectrum efficiency, it also imposes several
challenges. Towards filling the aforementioned gaps and address challenges, particular contributions were made in this thesis. First, this work presents a performance evaluation of the BSS Color scheme in various scenarios, where its shortcomings are identified. Second, this work proposes a generic framework to obtain throughput for dense cellular-like (small-cell) deployments, based on a mathematical model. Third, this work introduces COST, a novel Spatial Reuse technique for improving BSS Color performance by exploiting the information provided by this scheme and providing throughput gain of up to 57% while preserving fairness between BSSs. Fourth, this thesis proposes the design of a rate control algorithm that leverages the BSS Color and COST, providing up to 113% throughput gain in dense deployments when compared to the traditional off-the-shelf MinstrelHT. Finally, this thesis elaborates a network coding approach to enable multicast/broadcast transmissions over Wi-Fi, that could enhance throughput performance by 20% when compared with the legacy MAC feedback mechanism. The main goal for this contribution is to provide a means for realising reliable multicast/broadcast communications by reducing the use of the Wi-Fi feedback mechanism.

The above contributions were evaluated through system-level simulations, emulating real-world deployments. This work showed that advanced techniques, that exploit all available information by monitoring the inter-BSS and intra-BSS frames, are required to support the IEEE 802.11ax Spatial Reuse feature operation and provide throughput gain while preserve fairness among users. Furthermore, it was shown that the network coding should carefully be designed and enabled only when it is required, otherwise throughput loss could be observed due to the transmitted overhead. The scenario and application?s requirements should also be taken into account (e.g. latency).

Lee Haeyoung, Vahid Seiamak, Moessner Klaus (2020) Many-to-Many Matching based Resource Allocation to support Multi-Connectivity,Proceedings of the 11th International Conference on ICT Convergence (ICTC 2020), Institute of Electrical and Electronics Engineers (IEEE)
In the fifth and beyond (5G/B5G) communication,
wireless networks are evolved towards offering various services
of different use cases and, therefore, need to span a wide range
of requirements. While different services will be supported at
the same time, radio resource management needs to consider
the different requirements. In addition, as wireless systems are
capable to support multi-connectivity, radio resource allocation
becomes more challenging. In this context, we introduce a manyto-
many matching game, and develop a distributed radio resource
allocation algorithm supporting multi-connectivity. Simulation
results demonstrate that the proposed approach improves the
QoS levels of UEs by up to 14.9% considering their service
requirements.