Papers by Hossein Shokri-Ghadikolaei
2015 IEEE 1st International Forum on Research and Technologies for Society and Industry Leveraging a better tomorrow (RTSI), 2015
2015 IEEE 13th International Conference on Industrial Informatics (INDIN), 2015
2015 IEEE International Conference on Communications (ICC), 2015
ABSTRACT Millimeter wave (mmW) wireless networks are capable to support multi-gigabit data rates,... more ABSTRACT Millimeter wave (mmW) wireless networks are capable to support multi-gigabit data rates, by using directional communications with narrow beams. However, existing mmW communications standards are hindered by two problems: deafness and single link scheduling. The deafness problem, that is, a misalignment between transmitter and receiver beams, demands a time consuming beam-searching operation, which leads to an alignment-throughput tradeoff. Moreover, the existing mmW standards schedule a single link in each time slot and hence do not fully exploit the potential of mmW communications, where directional communications allow multiple concurrent transmissions. These two problems are addressed in this paper, where a joint beamwidth selection and power allocation problem is formulated by an optimization problem for short range mmW networks with the objective of maximizing effective network throughput. This optimization problem allows establishing the fundamental alignment-throughput tradeoff, however it is computationally complex and requires exact knowledge of network topology, which may not be available in practice. Therefore, two standard-compliant approximation solution algorithms are developed, which rely on underestimation and overestimation of interference. The first one exploits directionality to maximize the reuse of available spectrum and thereby increases the network throughput, while imposing almost no computational complexity. The second one is a more conservative approach that protects all active links from harmful interference, yet enhances the network throughput by 100% compared to the existing standards. Extensive performance analysis provides useful insights on the directionality level and the number of concurrent transmissions that should be pursued. Interestingly, extremely narrow beams are in general not optimal.
ABSTRACT As the spectrum is becoming more scarce due to exponential demand of formidable data qua... more ABSTRACT As the spectrum is becoming more scarce due to exponential demand of formidable data quantities, the new millimiterwave (mmW) band is considered as an enabling player of 5G communications to provide multi-gigabits wireless acccess. MmW communications exhibit high attenuation and blockage, directionality due to massive beamforming, deafness, low-interference, and may need micro waves networks for coordination and fallback support. The current mmW standardizations are challenged by the overwhelming complexity given by such heterogeneous communication systems and mmW band characteristics. This demands new substantial protocol developments at all layers. In this paper, the medium access control issues for mmW communications are reviewed. It is discussed that while existing standards address some of these issues for personal and local area networks, little has been done for cellular networks. It is argued that the medium access control layer should be equipped with adaptation mechanisms that are aware of the special mmW characteristics. Recommendations for mmW medium access control design in 5G are provided. It is concluded that the design of efficient access control techniques for mmW is in its infancy and much work still has to be done.
IEEE Journal on Selected Areas in Communications, 2014
Developing an efficient spectrum access policy enables cognitive radios to dramatically increase ... more Developing an efficient spectrum access policy enables cognitive radios to dramatically increase spectrum utilization while ensuring predetermined quality of service levels for primary users. In this paper, modeling, performance analysis, and optimization of a distributed secondary network with random sensing order policy are studied. Specifically, the secondary users create a random order of available channels upon primary users return, and then find optimal transmission and handoff opportunities in a distributed manner. By a Markov chain analysis, the average throughputs of the secondary users and average interference level among the secondary and primary users are investigated. A maximization of the secondary network performance in terms of the throughput while keeping under control the average interference is proposed. It is shown that despite of traditional view, non-zero false alarm in the channel sensing can increase channel utilization, especially in a dense secondary network where the contention is too high. Then, two simple and practical adaptive algorithms are established to optimize the network. The second algorithm follows the variations of the wireless channels in non-stationary conditions and outperforms even static brute force optimization, while demanding few computations. The convergence of the distributed algorithms are theoretically investigated based on the analytical performance indicators established by the Markov chain analysis. Finally, numerical results validate the analytical derivations and demonstrate the efficiency of the proposed schemes. It is concluded that fully distributed sensing order algorithms can lead to substantial performance improvements in cognitive radio networks without the need of centralized management or message passing among the users.
2014 IEEE International Conference on Communications (ICC), 2014
ABSTRACT Developing an efficient spectrum access policy enables cognitive radios to dramatically ... more ABSTRACT Developing an efficient spectrum access policy enables cognitive radios to dramatically increase spectrum utilization while assuring predetermined quality of service levels for the primary users. In this paper, modeling, performance analysis, and optimization of a distributed secondary network with random sensing order policy are studied. Specifically, the secondary users create a random order of the available channels and then find a transmission opportunity in a distributed manner. By a Markov chain analysis, the average throughputs of the secondary users and average interference level between the secondary and primary users are evaluated. Then, a maximization of the performance of the secondary network in terms of throughput while keeping under control the average interference is proposed. A simple and practical adaptive algorithm is established to optimize the network. Finally, numerical results are provided to validate the analytical derivations and demonstrate the performance of the proposed schemes. It is shown that distributed algorithms can achieve substantial performance improvements in cognitive radio networks without the need of centralized operations or management.
Powerful spectrum handover schemes enable cognitive radios (CRs) to use transmission opportunitie... more Powerful spectrum handover schemes enable cognitive radios (CRs) to use transmission opportunities in primary users' channels appropriately. In this paper, we consider the cognitive access of primary channels by a secondary user (SU). We evaluate the average detection time and the maximum achievable average throughput of the SU when the sequential method for hand-over (SMHO) is used. We assume that a prior knowledge of the primary users' presence and absence probabilities are available. In order to investigate the maximum achievable throughput of the SU, we end into an optimization problem, in which the optimum value of sensing time must be selected. In our optimization problem, we take into account the spectrum hand over due to false detection of the primary user. We also propose a weighted based hand-over (WBHO) scheme in which the impacts of channels conditions and primary users' presence probability are considered. This spectrum handover scheme provides higher average throughput for the SU compared to the SMHO method. The tradeoff between the maximum achievable throughput and consumed energy is discussed, and finally an energy efficient optimization formulation for finding a proper sensing time is provided.
Increased density of wireless devices, ever growing demands for extremely high data rate, and spe... more Increased density of wireless devices, ever growing demands for extremely high data rate, and spectrum scarcity at microwave bands make the millimeter wave (mmWave) frequencies an important player in future wireless networks. However, mmWave communication systems exhibit severe attenuation, blockage, deafness, and may need microwave networks for coordination and fall-back support. To compensate for high attenuation, mmWave systems exploit highly directional operation, which in turn substantially reduces the interference footprint. The significant differences between mmWave networks and legacy communication technologies challenge the classical design approaches, especially at the medium access control (MAC) layer, which has received comparatively less attention than PHY and propagation issues in the literature so far. In this paper, the MAC layer design aspects of short range mmWave networks are discussed. In particular, we explain why current mmWave standards fail to fully exploit the potential advantages of short range mmWave technology, and argue for the necessity of new collision-aware hybrid resource allocation frameworks with on-demand control messages, the advantages
of a collision notification message, and the potential of multihop communication to provide reliable mmWave connections.
IEEE Transactions on Communications
The millimeter wave (mmWave) frequency band is seen as a key enabler of multi-gigabit wireless ac... more The millimeter wave (mmWave) frequency band is seen as a key enabler of multi-gigabit wireless access in future cellular networks. In order to overcome the propagation challenges, mmWave systems use a large number of antenna elements both at the base station and at the user equipment, which lead to high directivity gains, fully-directional communications, and possible noise-limited operations. The fundamental differences between mmWave networks and traditional ones challenge the classical design constraints, objectives, and available degrees of freedom. This paper addresses the implications that highly directional communication has on the design of an efficient medium access control (MAC) layer. The paper discusses key MAC layer issues, such as synchronization, random access, handover, channelization, interference management, scheduling, and association. The paper provides an integrated view on MAC layer issues for cellular networks, identifies new challenges and tradeoffs, and provides novel insights and solution approaches.
Millimeter wave (mmWave) communication systems use large number of antenna elements that can pote... more Millimeter wave (mmWave) communication systems use large number of antenna elements that can potentially overcome severe channel attenuation by narrow beamforming. Narrow-beam operations in mmWave networks also reduce multiuser interference, introducing the concept of noise-limited wireless networks as opposed to interference-limited ones. The noise-limited or interference-limited regime heavily reflects on the medium access control (MAC) layer throughput and on proper resource allocation and interference management strategies. Yet, these regimes are ignored in current approaches to mmWave MAC layer design, with the potential disastrous consequences on the communication performance. In this paper, these regimes are investigated in terms of collision probability and throughput. Tractable closed-form expressions for collision probability and MAC layer throughput of mmWave ad hoc networks, operating under slotted ALOHA, are derived. The new analysis reveals that mmWave networks may exhibit a non-negligible transitional behavior from a noise-limited regime to an interference-limited one, depending on the density of the transmitters, density and size of obstacles, transmission probability, operating beamwidth, and transmission power. Such transitional behavior necessitates a new framework of adaptive hybrid resource allocation procedure, containing both contention-based and contention-free phases with on-demand realization of the contention-free phase. Moreover, the conventional collision avoidance procedure in the contention-based phase should be revisited, due to the transitional behavior of interference, to maximize throughput/delay performance of mmWave networks. It is concluded that, unless proper hybrid schemes are investigated, the severity of the transitional behavior may significantly reduce throughput/delay performance of mmWave networks.
IEEE Communications Magazine , 2015
Limited spectrum resources and dramatic growth of high data rate applications have motivated oppo... more Limited spectrum resources and dramatic growth of high data rate applications have motivated opportunistic spectrum access utilizing the promising concept of cognitive networks. Although this concept has emerged primarily to enhance spectrum utilization and to allow the coexistence of heterogeneous network technologies, the importance of energy consumption imposes additional challenges, because energy consumption and communication performance can be at odds. In this paper, the approaches for energy efficient spectrum sensing and spectrum handoff, fundamental building blocks of cognitive networks is investigated. The tradeoff between energy consumption and throughput, under local as well as under cooperative sensing are characterized, and what further aspects need to be investigated to achieve energy efficient cognitive operation under various application requirements are discussed.
IEEE Global Communications (GLOBECOM) Workshop, 2015
In millimeter wave (mmWave) communication systems,
narrow beam operations overcome severe channe... more In millimeter wave (mmWave) communication systems,
narrow beam operations overcome severe channel attenuations, reduce multiuser interference, and thus introduce the new concept of noise-limited mmWave wireless networks. The regime of the network, whether noise-limited or interference-limited, heavily reflects on the medium access control (MAC) layer throughput and on proper resource allocation and interference management strategies. Yet, alternating presence of these regimes and, more importantly, their dependence on the mmWave design parameters are ignored in the current approaches to mmWave MAC layer design, with the potential disastrous consequences
on the throughput/delay performance. In this paper,
tractable closed-form expressions for collision probability and MAC layer throughput of mmWave networks, operating under slotted ALOHA and TDMA, are derived. The new analysis reveals that mmWave networks may exhibit a non-negligible transitional behavior from a noise-limited regime to an interference-limited regime, depending on the density of the transmitters, density and size of obstacles, transmission probability, beamwidth, and transmit power. It is concluded that a new framework of adaptive hybrid resource allocation procedure, containing a proactive contention-based phase followed by a reactive contention-free one with dynamic phase durations, is necessary to cope with such transitional behavior.
IEEE Conference, European Wireless (EW) 2014, 2014
Effective spectrum sensing strategies enable cognitive radios to enhance the spectrum efficiency.... more Effective spectrum sensing strategies enable cognitive radios to enhance the spectrum efficiency. In this paper, modeling, performance analysis, and optimization of spectrum handoff in a centralized cognitive radio network are studied. More specifically, for a given sensing order, the average throughput of secondary users and average interference level among the secondary and primary users are evaluated for a cognitive radio network with only one secondary user. By a Markov chain analysis, a network with multiple secondary users performing cooperative spectrum sensing is modeled, and the above performance metrics are derived. Then, a maximization of the secondary network performance in terms of throughput while keeping under control the average interference is formulated. Finally, numerical results validate the analytical derivations and show that optimally tuning sensing time significantly enhances the performance of the spectrum handoff. Also, we observe that exploiting OR rule for cooperative spectrum sensing provides a higher average throughput compared to AND rule.
IEEE Infocom 2014, 2014
Developing an efficient spectrum access policy enables cognitive radios to dramatically increase ... more Developing an efficient spectrum access policy enables cognitive radios to dramatically increase spectrum utilization while assuring predetermined quality of service levels for the primary users. In this abstract, modeling, performance evaluation, and optimization of a distributed secondary network with random sensing order policy are studied. Specifically, the secondary users create a random order of the available channels upon primary users return, and then find an optimal transmission opportunity in a distributed manner. After modeling the behavior of the SUs by a Markov chain, the average throughputs of the secondary users and interference level among the secondary and primary users are evaluated. Then, a maximization of the secondary network performance in terms of throughput while keeping under control the average interference is proposed. A simple and practical adaptive algorithm is developed to optimize the network. Interestingly, the proposed algorithm follows the variations of the wireless channels in non-stationary conditions and outperforms even static brute force optimization, while demanding few computations. Finally, numerical results are provided to demonstrate the efficiencies of the proposed schemes. It is shown that fully distributed algorithms can achieve substantial performance improvements in cognitive radio networks without the need of centralized management or message passing among the users.
The limited spectrum resources and dramatic growth of high data rate communications have motivate... more The limited spectrum resources and dramatic growth of high data rate communications have motivated opportunistic spectrum access using the promising concept of cognitive radio networks. Spectrum sensing and handoff are a fundamental mechanism in cognitive networks, by which dynamic channel access and interference avoidance mechanisms are realized, allowing for a smooth coexistence between primary and secondary systems. The secondary users, however, might be able to sense one channel at a time, due to hardware and processing constraints, which introduces new design parameter; sensing order. That is, a secondary users should sequentially sense the channels based on an appropriate order. Due to limited understanding about mutual impact of sensing order and potential quality-of-service (QoS) parameters of secondary and primary users, many open and challenging issues can be found in recent literatures that reveal the lack of appropriate design frameworks for spectrum handoff mechanism. In this letter, investigation of these issues from a QoS provisioning view-point is introduced as a comprehensive research proposal.
Effective spectrum sensing strategies enable cognitive radios (CRs) to identify and opportunistic... more Effective spectrum sensing strategies enable cognitive radios (CRs) to identify and opportunistically transmit on under-utilized spectral resources. In this paper, sequential channel sensing problems for single and multiple secondary users (SUs) networks are effectively modeled through finite state Markovian processes. More specifically, a model for single user case is introduced and its performance is validated through analytical analysis. Then, in order to address multiple SUs case, this model is extended to include the modified p-persistent access (MPPA) protocol. Since the scheme utilized experiences a high level of collision among the SUs, to mitigate the problem appropriately, p-persistent random access (PPRA) protocol is considered, which offers higher average throughput for SUs by statistically distributing their loads among all channels. The structure and performance of the proposed schemes are discussed in detail, and a set of illustrative numerical results is presented to validate and compare the performance of the proposed sense-access strategies.
Powerful spectrum sensing schemes enable cognitive radios (CRs) to find transmission opportunitie... more Powerful spectrum sensing schemes enable cognitive radios (CRs) to find transmission opportunities in spectral resources allocated exclusively to primary users. In this paper, the problem of maximizing the average throughput of a cognitive radio system through optimizing its spectrum sensing time is investigated, and a systematic neural network-based optimization approach is proposed which avoids challenges associated with the conventional analytical solutions. The proposed method exploits a novel learning and optimization cycle to enable an effective cooperation between two kinds of well-known artificial neural networks and finds the optimum value of the channel sensing time without any prior knowledge or assumption about the wireless environment. The structure and algorithm of the proposed sensing time optimization scheme are discussed in detail, and a set of illustrative numerical results is presented to validate its performance.
Designing proper spectrum decision schemes for cognitive radios (CRs) improves spectral usage eff... more Designing proper spectrum decision schemes for cognitive radios (CRs) improves spectral usage efficiency as well as CR network throughput. One of the key effecting factors of the CR network throughput is the spectrum sensing sequences of the secondary users (SUs). In this paper, the SUs throughput maximization through appropriately selecting the sensing sequences of the SUs, called sensing matrix (SM), is investigated. More specifically, first the average throughput of the CR network is evaluated for a given SM in the case of error-free channel sensing. Then, an optimization problem is formulated with the aim to maximize the network throughput by finding the optimal SM. In order to avoid major challenges associated with the optimal solution, a novel sub optimal solution is proposed. Despite of having less computational complexity as well as lower consumed energy, the proposed solution performs quite well compared to the optimal solution (the optimal SM). Required considerations for the extension of the proposed approach to the case of non-perfect sensing are discussed, and then a set of illustrative numerical results is presented to validate its efficiencies.
Powerful spectrum decision schemes enable cognitive radios (CRs) to find transmission opportuniti... more Powerful spectrum decision schemes enable cognitive radios (CRs) to find transmission opportunities in spectral resources allocated exclusively to the primary users. One of the key effecting factors on the CR network throughput is the spectrum sensing sequence used by each secondary user (SU). In this study, SUs' throughput maximisation through finding an appropriate sensing matrix (SM) is investigated. First, the average throughput of the CR network is evaluated for a given SM. Then, an optimisation problem based on the maximisation of the network throughput is formulated in order to find the optimal SM. As the optimum solution is very complicated, three novel suboptimal solutions are proposed for various cases including perfect and non-perfect sensing. Despite having less computational complexities as well as lower consumed energies, the proposed solutions perform quite well compared to the optimum solution. The structure and performance of the proposed SM setting schemes are discussed in detail and a set of illustrative numerical results is presented to validate their efficiencies.
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Papers by Hossein Shokri-Ghadikolaei
of a collision notification message, and the potential of multihop communication to provide reliable mmWave connections.
narrow beam operations overcome severe channel attenuations, reduce multiuser interference, and thus introduce the new concept of noise-limited mmWave wireless networks. The regime of the network, whether noise-limited or interference-limited, heavily reflects on the medium access control (MAC) layer throughput and on proper resource allocation and interference management strategies. Yet, alternating presence of these regimes and, more importantly, their dependence on the mmWave design parameters are ignored in the current approaches to mmWave MAC layer design, with the potential disastrous consequences
on the throughput/delay performance. In this paper,
tractable closed-form expressions for collision probability and MAC layer throughput of mmWave networks, operating under slotted ALOHA and TDMA, are derived. The new analysis reveals that mmWave networks may exhibit a non-negligible transitional behavior from a noise-limited regime to an interference-limited regime, depending on the density of the transmitters, density and size of obstacles, transmission probability, beamwidth, and transmit power. It is concluded that a new framework of adaptive hybrid resource allocation procedure, containing a proactive contention-based phase followed by a reactive contention-free one with dynamic phase durations, is necessary to cope with such transitional behavior.
of a collision notification message, and the potential of multihop communication to provide reliable mmWave connections.
narrow beam operations overcome severe channel attenuations, reduce multiuser interference, and thus introduce the new concept of noise-limited mmWave wireless networks. The regime of the network, whether noise-limited or interference-limited, heavily reflects on the medium access control (MAC) layer throughput and on proper resource allocation and interference management strategies. Yet, alternating presence of these regimes and, more importantly, their dependence on the mmWave design parameters are ignored in the current approaches to mmWave MAC layer design, with the potential disastrous consequences
on the throughput/delay performance. In this paper,
tractable closed-form expressions for collision probability and MAC layer throughput of mmWave networks, operating under slotted ALOHA and TDMA, are derived. The new analysis reveals that mmWave networks may exhibit a non-negligible transitional behavior from a noise-limited regime to an interference-limited regime, depending on the density of the transmitters, density and size of obstacles, transmission probability, beamwidth, and transmit power. It is concluded that a new framework of adaptive hybrid resource allocation procedure, containing a proactive contention-based phase followed by a reactive contention-free one with dynamic phase durations, is necessary to cope with such transitional behavior.