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Dr Seiamak Vahid


Senior Research Fellow
PhD in communications

My publications

Publications

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 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 many-to-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.
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, iii 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.
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.
Ioannis Selinis (2020)Improving spatial reuse and MAC efficiency for future WLANs. University of Surrey
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).
Haeyoung Lee, Seiamak Vahid, Klaus Moessner (2020)Many-to-Many Matching based Resource Allocation to support Multi-Connectivity, In: 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.
Wenjuan Yu, Atta ul Quddus, Seiamak Vahid, Rahim Tafazolli (2018)Opportunistic spectrum access in support of ultra-reliable and low-latency communications, In: 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.
R Bassoli, H Marques, A Radwan, J Rodriguez, S Vahid, R Tafazolli (2013)Energy analysis of network coding in hard vertical handovers, In: 2013 3rd International Conference on Communications and Information Technology, ICCIT 2013pp. 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.
V Sucasas, A Radwan, H Marques, J Rodriguez, S Vahid, R Tafazolli (2014)How much overhead is required for stable group formation in VANETs?, In: 2014 IEEE 19TH INTERNATIONAL WORKSHOP ON COMPUTER AIDED MODELING AND DESIGN OF COMMUNICATION LINKS AND NETWORKS (CAMAD)pp. 21-25
H Lee, S Vahid, K Moessner (2012)Utility-based Dynamic Spectrum Aggregation algorithm in Cognitive Radio Networks, In: 2012 IEEE VEHICULAR TECHNOLOGY CONFERENCE (VTC FALL)
Haeyoung Lee, Y Ko, Seiamak Vahid, Klaus Moessner (2015)Practical Spectrum Aggregation for Secondary Networks with Imperfect Sensing, In: IEEE Transactions on Vehicular Technology65(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.
V Sucasas, A Radwan, H Marques, J Rodriguez, S Vahid, R Tafazolli (2013)On the energy efficiency of low-mobility subsystems in hierarchical ad hoc networks., In: CAMADpp. 48-52
G Alnwaimi, S Vahid, K Moessner (2015)Distributed Learning Game based Spectrum Sharing and Resource Selection for Femtocells, In: 2015 22ND INTERNATIONAL CONFERENCE ON TELECOMMUNICATIONS (ICT)pp. 331-337
G Alnwaimi, S Vahid, K Moessner (2015)Dynamic Heterogeneous Learning Games for Opportunistic Access in LTE-Based Macro/Femtocell Deployments, In: IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS14(4)pp. 2294-2308 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
RH Tehrani, S Vahid, D Triantafyllopoulou, H Lee, K Moessner (2016)Licensed Spectrum Sharing Schemes for Mobile Operators: A Survey and Outlook, In: IEEE Communications Surveys and Tutorials18(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.
J Pérez-Romero, O Sallent, F Bouali, H Sarvanko, M Mustonen, M Matinmikko, H Lee, S Vahid, K Moessner (2012)A spectrum selection framework for Opportunistic Networks, In: 2012 Future Network and Mobile Summit, FutureNetw 2012pp. 1-9
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.
G Alnwaimi, T Zahir, S Vahid, K Moessner (2013)Machine Learning based Knowledge Acquisition on Spectrum Usage for LTE Femtocells, In: 2013 IEEE 78TH VEHICULAR TECHNOLOGY CONFERENCE (VTC FALL)
M Hasanpour, Shahin Shariat, Payam Barnaghi, SA Hoseinitabatabaei, Seiamak Vahid, Rahim Tafazolli (2017)Quantum Load Balancing in Ad Hoc Networks, In: Quantum Information Processing16(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.
V Sucasas, A Radwan, H Marques, J Rodriguez, S Vahid, R Tafazolli (2014)On the Efficiency of Merging Procedures in Hierarchical Mobile Cooperative Networks, In: 2014 IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS (ICC)pp. 30-35
K Moessner, S Vahid, R Tafazolli (1999)Extension of the PNNI to support mobility management and handoffs in WATM networks based on CORBA, In: Proc. IEEE 49th Vehicular Technology Conf3pp. 2515-2519
Marcin Filo, Chuan Foh, Seiamak Vahid, Rahim Tafazolli (2020)Performance Analysis of Ultra-Dense Networks with Regularly Deployed Base Stations, In: 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.
Ioannis Selinis, Konstantinos Katsaros, Seiamak Vahid, Rahim Tafazolli (2017)Exploiting the Capture Effect on DSC and BSS Color in Dense IEEE 802.11ax Deployments, In: Proceedings of the Workshop on ns-3 - 2017 WNS3pp. 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.
Ioannis Selinis, Marcin Filo, Seiamak Vahid, Jonathan Rodriguez, Rahim Tafazolli (2017)Evaluation of the DSC algorithm and the BSS color scheme in dense cellular-like IEEE 802.11ax deployments, In: 2016 IEEE 27th Annual International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC)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.
Ioannis Selinis, Konstantinos Katsaros, Seiamak Vahid, Rahim Tafazolli (2018)Control OBSS/PD Sensitivity Threshold for IEEE 802.11ax BSS Color, In: 2018 IEEE 29th Annual International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC)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.
H Lee, Y Ko, S Vahid, K Moessner (2014)Opportunistic Spectrum Aggregation for Cognitive Communications Under Collision Constraints, In: 2014 9TH INTERNATIONAL CONFERENCE ON COGNITIVE RADIO ORIENTED WIRELESS NETWORKS AND COMMUNICATIONS (CROWNCOM)pp. 7-12
H Lee, S Vahid, K Moessner (2014)A Sub-Optimal Channel Switching-Aware Spectrum Aggregation Approach for CRNs, In: 2014 9TH INTERNATIONAL CONFERENCE ON COGNITIVE RADIO ORIENTED WIRELESS NETWORKS AND COMMUNICATIONS (CROWNCOM)pp. 114-119
Ioannis Selinis, Konstantinos Katsaros, Seiamak Vahid, Rahim Tafazolli (2019)Damysus: A Practical IEEE 802.11ax BSS Color Aware Rate Control Algorithm, In: International Journal of Wireless Information Networkspp. 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.
Haeyoung Lee, Seiamak Vahid, Klaus Moessner (2019)Machine Learning based RATs Selection supporting Multi-Connectivity for Reliability, In: 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.
V Sucasas, A Radwan, H Marques, J Rodriguez, S Vahid, R Tafazolli (2014)A Cognitive Approach for Stable Cooperative Group formation in Mobile Environments, In: 2014 IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS (ICC)pp. 3241-3245
Ioannis Selinis, Konstantinos Katsaros, Seiamak Vahid, Rahim Tafazolli (2019)Eliminating the Use of IEEE 802.11 Acknowledgements; A Network Coding Approach, In: Proceedings of the 2019 IEEE Wireless Communications and Networking Conference (WCNC 2019) Institute of Electrical and Electronics Engineers (IEEE)
Haeyoung Lee, Seiamak Vahid, Klaus Moessner (2017)Traffic-aware Carrier Allocation with Aggregation for Load Balancing, In: 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.
Dionysia Triantafyllopoulou, Seiamak Vahid, Klaus Moessner (2018)Co-Primary Spectrum Sharing in Uplink SC-FDMA Networks, In: 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.
S Vahid (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 ...
K Moessner, S Vahid, R Tafazolli (2001)Terminal reconfigureability-the OPTiMA framework, In: Proc. Second Int 3G Mobile Communication Technologies Conf. (Conf. Publ. No. 477)pp. 241-246
V Sucasas, A Radwan, H Marques, J Rodriguez, S Vahid, R Tafazolli (2015)Can We Apply Clustering in Fast Moving Objects?, In: WIRELESS INTERNET (WICON 2014)146pp. 76-84
R Bassoli, VN Talooki, H Marques, J Rodriguez, S Vahid, R Tafazolli (2014)LT codes for video streaming in burst erasure channels: An energy analysis., In: ISCCpp. 1-4
V Sucasas, S Althunibat, A Radwan, H Marques, J Rodriguez, S Vahid, R Tafazolli, F Granelli (2015)Lightweight security against combined IE and SSDF attacks in cooperative spectrum sensing for cognitive radio networks, In: SECURITY AND COMMUNICATION NETWORKS8(18)pp. 3978-3994 WILEY-BLACKWELL
H Lee, S Vahid, K Moessner (2014)The Study on Spectrum/Channel Fragmentation from Dynamic Spectrum Aggregation in CRNs, In: 2014 EUROPEAN CONFERENCE ON NETWORKS AND COMMUNICATIONS (EUCNC)
MR Rai, S Vahid, K Moessner (2015)SINR based Topology Control for Multihop Wireless Networks with Fault Tolerance, In: 2015 IEEE 81ST VEHICULAR TECHNOLOGY CONFERENCE (VTC SPRING)
H Lee, S Vahid, K Moessner A survey of radio resource management for spectrum aggregation in LTE-advanced, In: IEEE Communications Surveys and Tutorials16(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.
Haeyoung Lee, Seiamak Vahid, Klaus Moessner (2018)Traffic-aware Resource allocation with aggregation in Heterogeneous Networks with WLANs, In: 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.
R Bassoli, H Marques, J Rodriguez, S Vahid, R Tafazolli (2013)Network Coding for Vertical Handoffs Between LTE and IEEE 802.11n: An Energy Perspective., In: EW
S Vahid (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 ...
Ioannis Selinis, Konstantinos Katsaros, Marion Allayioti, Seiamak Vahid, Rahim Tafazolli (2018)The Race to 5G Era; LTE and Wi-Fi, In: IEEE Access6pp. 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.
Small cells are becoming a promising solution for providing enhanced coverage and increasing system capacity in a large-scale small cell network. In such a network, the large number of small cells may cause mobility signalling overload on the core network (CN) due to frequent handovers, which impact the users Quality of Experience (QoE). This is one of the major challenges in dense small cell networks. Such a challenge has been considered, this thesis addresses this challenging task to design an effective signalling architecture in dense small cell networks. First, in order to reduce the signalling overhead incurred by path switching operations in the small cell network, a new mobility control function, termed the Small Cell Controller (SCC) is introduced to the existing base station (BS) on the Radio-Access-Network(RAN)-side. Based on the signalling architecture, a clustering optimisation algorithm is proposed in order to select the optimal SCC in a highly user density environment. Specifically, this algorithm is designed to select multiple optimal SCCs due to the growth in number of small cells in the large-scale environment. Finally, a scalable architecture for handling the control plane failures in heterogeneous networks is proposed. In that architecture, the proposed SCC scheme controls and manages the affected small cells in a clustered fashion during the macro cell fail-over period. Particularly, the proposed SCC scheme can be flexibly configured into a hybrid scenario. For operational reduction (reducing a number of direct S1 connections to the CN), better scalability (reducing a number of S1 bearers on the CN) and reduction of signalling load on the CN, the proposed radio access network (RAN) signalling architecture is a viable and preferable option for dense small cell networks. Besides, the proposed signalling architecture is evaluated through realistic simulation studies.
In 2008, Institute of Electrical and Electronics Engineers (IEEE) published its standard IEEE 802.21 for media-independent handover services. The main scope of this work was to design a technology agnostic mobility platform to perform vertical handovers between heterogeneous networks. Regarding vertical handover procedures, a key issue to address is the control of packet loss, which is responsible for high handover latency and low communication quality. The solution proposed by the standard IEEE 802.21 guarantees reliability by exploiting Automatic Repeat Request (ARQ). However, the use of an acknowledgement service has been demonstrated not to be the best way to handle frame loss. In this thesis, we propose a novel architecture and protocol to efficiently perform vertical handovers. This protocol is called Enhanced-Coded MIH (EC-MIH) and exploits Forward Error Correction (FEC) instead of ARQ. In fact, it performs built-in coding operations to handle erasures of MIH frames. Moreover, we designed a novel hybrid concatenated coding scheme called Hybrid Serial Concatenated Network Code (HSCNC), composed of the serial concatenation of a classical erasure code and systematic Random Linear Network Coding (RLNC). We show via theoretical analysis as well as MATLAB simulations that the concatenation approach can outperform RLNC alone in terms of decoding error probability. Moreover, this work analyses the frame loss of Media-Independent Handover (MIH) protocol during vertical handovers via system level simulations. The proposed HSCNC design is then integrated into the new EC-MIH protocol and evaluated. We then discuss how the new protocol outperforms the legacy protocol in terms of throughput (at TCP layer, above MIH) and handover delay.
The rapid growth of wireless services and the breakneck proliferation of wireless devices continue to strain limited spectrum resource. While the need for efficient spectrum sharing mechanisms has been emphasized, opportunistic spectrum access has been considered as a promising mechanism for dynamic spectrum sharing. However, although the idle spectrum could exist, it is usually rather fragmented and distributed, and hence the secondary network users would face the difficulty in finding required contiguous spectrum. Spectrum aggregation can be exploited to provide effective wide bandwidth communication but at the cost of complexity and overhead. When a primary network uses spectrum dynamically, from the nature of opportunistic spectrum access, collisions can occur between primary and secondary transmissions and spectrum handoff can be utilised to provide reliable communication. However, collision occurrence results in spectrum handoff delay in a secondary network user (SU) along with short-term interference to a primary network user (PU). As a SU accesses more spectrum for higher data rates by spectrum aggregation, collisions can occur more frequently and frequent spectrum handoff will be required. While spectrum aggregation will allow the SU to have high flexibility in spectrum use and spectrum handoff can help improve the reliability of secondary transmissions, the SU faces a new spectrum allocation problem: How wide and which parts of spectrum opportunities should be aggregated while considering the complexity and the overhead for aggregation and for spectrum handoff? This thesis addresses the key challenge of opportunistic spectrum access, focusing on efficient spectrum sharing considering the fragmentation of spectrum opportunities in frequency and time domains. First, considering complexity and overhead for aggregation, the spectrum aggregation approach is investigated and guidelines are derived how to reduce spectrum fragmentation for the efficient spectrum utilisation based on simulation results. Second, the relationship between collision occurrence and spectrum aggregation is analysed. Collision probabilities between primary and secondary transmissions are derived and the impacts of spectrum aggregation on data rates and spectrum handoff are investigated. Then, a spectrum aggregation algorithm is proposed to maximise data rates for a given collision probability threshold. Third, when considering spectrum handoff, the impacts of spectrum aggregation on spectrum handoff and short-term interference to PUs are analysed. Then, the spectrum aggregation algorithm is designed with the aim to minimise collision. Finally, the results of this study are summarised, conclusions are presented and a number of future research topics are proposed.
Nowadays there is a plethora of wireless handsets in the market such as smartphones, tablets, laptops and wearable devices, that together with the the future emerging scenarios on vehicle to vehicle communications and smart city infrastructure will populate urban environments with a broad diversity of multi-standard wireless devices. This increase in the density and diversity of mobile devices have been the driver for collaborative protocols that can deliver effective communications. Cooperation is a technology that has the potential to provide energy efficient and scalable communications, where nodes play an important role to coordinate local traffic and act as gateways to the core network. Despite the diverse power requirements of multi-standard wireless interfaces and the different channel characteristics, support for energy efficient communications where relay nodes can be selected with lower energy requirements or with higher order modulation opportunities, is still expected. In this framework, clustering is a widely accepted technique that allows nodes create and join virtual cooperative groups, and to select a clusterhead that can provide a high speed and energy efficient backhaul link to the mobile network. The vast majority of existing clustering techniques assume that the collection of nodes that form a cluster are either static or have very low relative velocity. However, in practice nodes or devices are constantly on the move providing the impetus for mobility aware clustering techniques that elect a subset of nodes with a common mobility pattern. In this context, mobility-aware clustering, based on geolocation, is an active field of research due to the increasing interest of vehicular communication technology. However, clustering has a wide range of applications where GPS information is not always available. This requires a new design of clustering algorithms that do not depend on GPS coordinates. This challenge has fostered a new vision of clustering based on cognition, where nodes form mobile clusters that can adapt on-demand to the scenario characteristics. This thesis investigates cluster formation exploiting the notion of wisdom of crowds, where the nodes are aware of the surrounding mobility patterns and can adapt the cluster formation strategy to suit the current mobility trends. Moreover, this thesis also caters for a novel analytical model for cluster lifetime that is used to validate our simulation results. Another dimension to the clustering problem is how to exploit available spectral opportunities for cluster formation in a secure manner. Cognitive radio, and more concretely cooperative spectrum sensing is evaluated in this thesis as a solution for data channel assignment in mobile clusters. In this scenario, we focus on the security concerns of cooperative spectrum sensing. Namely, we address spectrum sensing data falsification and incumbent emulation attacks, and propose an energy efficient security mechanisms based on lightweight cryptography to address these threats.
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.