Green Cellular Networks: A Survey, Some Research Issues and Challenges

Energy efficiency in cellular networks is a growing concern for cellular operators to not only maintain profitability, but also to reduce the overall environment effects. This emerging trend of achieving energy efficiency in cellular networks is motivating the standardization authorities and network operators to continuously explore future technologies in order to bring improvements in the entire network infrastructure. In this article, we present a brief survey of methods to improve the power efficiency of cellular networks, explore some research issues and challenges and suggest some techniques to enable an energy efficient or "green" cellular network. Since base stations consume a maximum portion of the total energy used in a cellular system, we will first provide a comprehensive survey on techniques to obtain energy savings in base stations. Next, we discuss how heterogeneous network deployment based on micro, pico and femto-cells can be used to achieve this goal. Since cognitive radio and cooperative relaying are undisputed future technologies in this regard, we propose a research vision to make these technologies more energy efficient. Lastly, we explore some broader perspectives in realizing a "green" cellular network technologyComment: 16 pages, 5 figures, 2 table

Combined Soft Hard Cooperative Spectrum Sensing in Cognitive Radio Networks

Providing some techniques to enhance the performance of spectrum sensing in cognitive radio systems while accounting for the cost and bandwidth limitations in practical scenarios is the main objective of this thesis. We focus on an essential element of cooperative spectrum sensing (CSS) which is the data fusion that combines the sensing results to make the final decision. Exploiting the advantage of the superior performance of the soft schemes and the low bandwidth of the hard schemes by incorporating them in cluster based CSS networks is achieved in two different ways. First, a soft-hard combination is employed to propose a hierarchical cluster based spectrum sensing algorithm. The proposed algorithm maximizes the detection performances while satisfying the probability of false alarm constraint. Simulation results of the proposed algorithm are presented and compared with existing algorithms over the Nakagami fading channel. Moreover, the results show that the proposed algorithm outperforms the existing algorithms. In the second part, a low complexity soft-hard combination scheme is suggested by utilizing both one-bit and two-bit schemes to balance between the required bandwidth and the detection performance by taking into account that different clusters undergo different conditions. The scheme allocates a reliability factor proportional to the detection rate to each cluster to combine the results at the Fusion center (FC) by extracting the results of the reliable clusters. Numerical results obtained have shown that a superior detection performance and a minimum overhead can be achieved simultaneously by combining one bit and two schemes at the intra-cluster level while assigning a reliability factor at the inter-cluster level

MULTI USER COOPERATION SPECTRUM SENSING IN WIRELESS COGNITIVE RADIO NETWORKS

With the rapid proliferation of new wireless communication devices and services, the demand for the radio spectrum is increasing at a rapid rate, which leads to making the spectrum more and more crowded. The limited available spectrum and the inefficiency in the spectrum usage have led to the emergence of cognitive radio (CR) and dynamic spectrum access (DSA) technologies, which enable future wireless communication systems to exploit the empty spectrum in an opportunistic manner. To do so, future wireless devices should be aware of their surrounding radio environment in order to adapt their operating parameters according to the real-time conditions of the radio environment. From this viewpoint, spectrum sensing is becoming increasingly important to new and future wireless communication systems, which is designed to monitor the usage of the radio spectrum and reliably identify the unused bands to enable wireless devices to switch from one vacant band to another, thereby achieving flexible, reliable, and efficient spectrum utilisation. This thesis focuses on issues related to local and cooperative spectrum sensing for CR networks, which need to be resolved. These include the problems of noise uncertainty and detection in low signal to noise ratio (SNR) environments in individual spectrum sensing. In addition to issues of energy consumption, sensing delay and reporting error in cooperative spectrum sensing. In this thesis, we investigate how to improve spectrum sensing algorithms to increase their detection performance and achieving energy efficiency. To this end, first, we propose a new spectrum sensing algorithm based on energy detection that increases the reliability of individual spectrum sensing. In spite of the fact that the energy detection is still the most common detection mechanism for spectrum sensing due to its simplicity. Energy detection does not require any prior knowledge of primary signals, but has the drawbacks of threshold selection, and poor performance due to noise uncertainty especially at low SNR. Therefore, a new adaptive optimal energy detection algorithm (AOED) is presented in this thesis. In comparison with the existing energy detection schemes the detection performance achieved through AOED algorithm is higher. Secondly, as cooperative spectrum sensing (CSS) can give further improvement in the detection reliability, the AOED algorithm is extended to cooperative sensing; in which multiple cognitive users collaborate to detect the primary transmission. The new combined approach (AOED and CSS) is shown to be more reliable detection than the individual detection scheme, where the hidden terminal problem can be mitigated. Furthermore, an optimal fusion strategy for hard-fusion based cognitive radio networks is presented, which optimises sensing performance. Thirdly, the need for denser deployment of base stations to satisfy the estimated high traffic demand in future wireless networks leads to a significant increase in energy consumption. Moreover, in large-scale cognitive radio networks some of cooperative devices may be located far away from the fusion centre, which causes an increase in the error rate of reporting channel, and thus deteriorating the performance of cooperative spectrum sensing. To overcome these problems, a new multi-hop cluster based cooperative spectrum sensing (MHCCSS) scheme is proposed, where only cluster heads are allowed to send their cluster results to the fusion centre via successive cluster heads, based on higher SNR of communication channel between cluster heads. Furthermore, in decentralised CSS as in cognitive radio Ad Hoc networks (CRAHNs), where there is no fusion centre, each cognitive user performs the local spectrum sensing and shares the sensing information with its neighbours and then makes its decision on the spectrum availability based on its own sensing information and the neighbours’ information. However, cooperation between cognitive users consumes significant energy due to heavy communications. In addition to this, each CR user has asynchronous sensing and transmission schedules which add new challenges in implementing CSS in CRAHNs. In this thesis, a new multi-hop cluster based CSS scheme has been proposed for CRAHNs, which can enhance the cooperative sensing performance and reduce the energy consumption compared with other conventional decentralised cooperative spectrum sensing modes

Contributions to the Performance Analysis of Intervehicular Communications Systems and Schemes

RÉSUMÉ Le but des systèmes de communication intervéhicule (Inter-Vehicle Communication – IVC) est d'améliorer la sécurité de conduite en utilisant des capteurs et des techniques de communication sans fil pour être en mesure de communiquer mutuellement sans aucune intervention extérieure. Avec l'utilisation de ces systèmes, les communications véhicule à véhicule (V2V) peuvent être plus efficaces dans la prévention des accidents et la décongestion de la circulation que si chaque véhicule travaillait individuellement. Une des solutions proposées pour les systèmes IVC est l’utilisation des systèmes de communication coopérative, qui en principe, augmentent l'efficacité spectrale et énergétique, la couverture du réseau, et réduit la probabilité de défaillance. La diversité d'antenne (entrées multiples sorties multiples « Multiple-Input Multiple-Output » ou MIMO) peut également être une alternative pour les systèmes IVC pour améliorer la capacité du canal et la diversité (fiabilité), mais en échange d’une complexité accrue. Toutefois, l'application de telles solutions est difficile, car les communications sans fil entre les véhicules sont soumises à d’importants effets d'évanouissements des canaux appelés (canaux sujets aux évanouissements de n*Rayleigh, « n*Rayleigh fading channels»), ce qui conduit à la dégradation des performances. Par conséquent, dans cette thèse, nous proposons une analyse de la performance globale des systèmes de transmission coopératifs et MIMO sur des canaux sujets aux évanouissements de n*Rayleigh. Cette analyse permettra d’aider les chercheurs pour la conception et la mise en œuvre de systèmes de communication V2V avec une complexité moindre. En particulier, nous étudions d'abord la performance de la sélection du relais de coopération avec les systèmes IVC, on suppose que la transmission via « Amplify-and-Forward» (AF) ou bien «Decode-and-Forward» (DF) est assurée par N relais pour transférer le message de la source à la destination. La performance du système est analysée en termes de probabilité de défaillance, la probabilité d'erreur de symbole, et la capacité moyenne du canal. Les résultats numériques démontrent que la sélection de relais réalise une diversité de l'ordre de (d≈mN/n) pour les deux types de relais, où m est un paramètre évanouissement de Rayleigh en cascade. Nous étudions ensuite la performance des systèmes IVC à sauts multiples avec et sans relais régénératifs. Dans cette étude, nous dérivons des expressions approximatives pour la probabilité de défaillance et le niveau d’évanouissement lorsque la diversité en réception basée sur le ratio maximum de combinaison (MRC) est employée. En outre, nous analysons la répartition de puissance pour le système sous-jacent afin de minimiser la probabilité globale de défaillance. Nous montrons que la performance des systèmes régénératifs est meilleure que celle des systèmes non régénératifs lorsque l’ordre de cascade n est faible, tandis qu’ils ont des performances similaires lorsque n est élevé. Ensuite, nous considérons le problème de la détection de puissance des signaux inconnus aux n* canaux de Rayleigh. Dans ce travail, de nouvelles expressions approximatives sont dérivées de la probabilité de détection moyenne avec et sans diversité en réception MRC. En outre, la performance du système est analysée lorsque la détection de spectre coopérative (CSS) est considérée sous diverses contraintes de canaux (par exemple, les canaux de communication parfaits et imparfaits). Les résultats numériques ont montré que la fiabilité de détection diminue à mesure que l'ordre n augmente et s’améliore sensiblement lorsque CSS emploie le schéma MRC. Il est démontré que CSS avec le schéma MRC maintient la probabilité de fausse alarme minimale dans les canaux d’information imparfaite plutôt que d'augmenter le nombre d'utilisateurs en coopération. Enfin, nous présentons une nouvelle approche pour l'analyse des performances des systèmes IVC sur n*canaux de Rayleigh, en utilisant n_T antennes d'émission et n_R antennes de réception pour lutter contre l'effet d’évanouissement. Dans ce contexte, nous évaluons la performance des systèmes MIMO-V2V basés sur la sélection des antennes d'émission avec un ratio maximum de combinaison (TAS/MRC) et la sélection combinant (TAS/SC). Dans cette étude, nous dérivons des expressions analytiques plus précises pour la probabilité de défaillance, la probabilité d'erreur de symbole, et l’évanouissement sur n*canaux Rayleigh. Il est montré que les deux régimes ont le même ordre de diversité maximale équivalent à (d≈mn_T n_R /n) . En outre, TAS / MRC offre un gain de performance mieux que TAS/ SC lorsque le nombre d'antennes de réception est plus que celle des antennes d’émission, mais l’amélioration de la performance est limitée lorsque n augmente.----------Abstract The purpose of intervehicular communication (IVC) systems is to enhance driving safety, in which vehicles use sensors and wireless communication techniques to talk to each other without any roadside intervention. Using these systems, vehicle-to-vehicle (V2V) communications can be more effective in avoiding accidents and traffic congestion than if each vehicle works individually. A potential solution can be implemented in this research area using cooperative communications systems which, in principle, increase spectral and power efficiency, network coverage, and reduce the outage probability. Antenna diversity (i.e., multiple-input multiple output (MIMO) systems) can also be an alternative solution for IVC systems to enhance channel capacity and diversity (reliability) but in exchange of an increased complexity. However, applying such solutions is challenging since wireless communications among vehicles is subject to harsh fading channels called ‘n*Rayleigh fading channels’, which leads to performance degradation. Therefore, in this thesis we provide a comprehensive performance analysis of cooperative transmission and MIMO systems over n*Rayleigh fading channels that help researchers for the design and implementation of V2V communication systems with lower complexity. Specifically, we first investigate the performance of cooperative IVC systems with relay selection over n*Rayleigh fading channels, assuming that both the decode-and-forward and the amplify-and-forward relaying protocols are achieved by N relays to transfer the source message to the destination. System performance is analyzed in terms of outage probability, symbol error probability, and average channel capacity. The numerical results have shown that the best relay selection approach achieves the diversity order of (d≈mN/n) where m is a cascaded Rayleigh fading parameter. Second, we investigate the performance of multihop-IVC systems with regenerative and non-regenerative relays. In this study, we derive approximate closed-form expressions for the outage probability and amount of fading when the maximum ratio combining (MRC) diversity reception is employed. Further, we analyze the power allocation for the underlying scheme in order to minimize the overall outage probability. We show that the performance of regenerative systems is better than that of non-regenerative systems when the cascading order n is low and they have similar performance when n is high. Third, we consider the problem of energy detection of unknown signals over n*Rayleigh fading channels. In this work, novel approximate expressions are derived for the average probability of detection with and without MRC diversity reception. Moreover, the system performance is analyzed when cooperative spectrum sensing (CSS) is considered under various channel constraints (e.g, perfect and imperfect reporting channels). The numerical results show that the detection reliability decreases as the cascading order n increases and substantially improves when CSS employs MRC schemes. It is demonstrated that CSS with MRC scheme keeps the probability of false alarm minimal under imperfect reporting channels rather than increasing the number of cooperative users. Finally, we present a new approach for the performance analysis of IVC systems over n*Rayleigh fading channels, using n_T transmit and n_R receive antennas to combat fading influence. In this context, we evaluate the performance of MIMO-V2V systems based on the transmit antenna selection with maximum ratio combining (TAS/MRC) and selection combining (TAS/SC) schemes. In this study, we derive tight analytical expressions for the outage probability, the symbol error probability, and the amount of fading over n*Rayleigh fading channels. It is shown that both schemes have the same maximum diversity order equivalent to (d≈mn_T n_R /n). In addition, TAS/MRC offers a better performance gain than TAS/SC scheme when the number of receive antennas is more than that of transmit antennas, but the performance improvement is limited as n increases

A hybrid double-threshold based cooperative spectrum sensing over fading channels

This paper investigates double-threshold based energy detector for cooperative spectrum sensing mechanisms in cognitive wireless radio networks. We first propose a hybrid double-threshold based energy detector (HDTED) to improve the sensing performance at secondary users (SUs) by exploiting both the local binary/energy decisions and global binary decisions feedback from the fusion centre (FC). Significantly, we derive closed-form expressions and bounds for the probabilities of missed detection and false alarm considering a practical scenario where all channel links suffer from Rayleigh fading and background noise. The derived expressions not only show the improved performance achieved with the HDTED scheme but also enable us to analyse the impacts of the number of the SUs and the fading channels on the cooperative spectrum sensing performance. Furthermore, based on the derived bounds, we propose an optimal SU selection algorithm for forwarding the local decisions to the FC, which helps reduce the number of forwarding bits for a lower-complexity signaling. Finally, numerical results are provided to demonstrate the validity of the analytical findings

Cognitive Radio Systems

Cognitive radio is a hot research area for future wireless communications in the recent years. In order to increase the spectrum utilization, cognitive radio makes it possible for unlicensed users to access the spectrum unoccupied by licensed users. Cognitive radio let the equipments more intelligent to communicate with each other in a spectrum-aware manner and provide a new approach for the co-existence of multiple wireless systems. The goal of this book is to provide highlights of the current research topics in the field of cognitive radio systems. The book consists of 17 chapters, addressing various problems in cognitive radio systems

A hybrid double-threshold based cooperative spectrum sensing over fading channels

This paper investigates double-threshold based energy detector for cooperative spectrum sensing mechanisms in cognitive wireless radio networks. We first propose a hybrid double-threshold based energy detector (HDTED) to improve the sensing performance at secondary users (SUs) by exploiting both the local binary/energy decisions and global binary decisions feedback from the fusion centre (FC). Significantly, we derive closed-form expressions and bounds for the probabilities of missed detection and false alarm considering a practical scenario where all channel links suffer from Rayleigh fading and background noise. The derived expressions not only show the improved performance achieved with the HDTED scheme but also enable us to analyse the impacts of the number of the SUs and the fading channels on the cooperative spectrum sensing performance. Furthermore, based on the derived bounds, we propose an optimal SU selection algorithm for forwarding the local decisions to the FC, which helps reduce the number of forwarding bits for a lower-complexity signaling. Finally, numerical results are provided to demonstrate the validity of the analytical findings

MAC-PHY Frameworks For LTE And WiFi Networks\u27 Coexistence Over The Unlicensed Band

The main focus of this dissertation is to address these issues and to analyze the interactions between LTE and WiFi coexisting on the unlicensed spectrum. This can be done by providing some improvements in the first two communication layers in both technologies. Regarding the physical (PHY) layer, efficient spectrum sensing and data fusion techniques that consider correlated spectrum sensing readings at the LTE/WiFi users (sensors) are needed. Failure to consider such correlation has been a major shortcoming of the literature. This resulted in poorly performing spectrum sensing systems if such correlation is not considered in correlated-measurements environments