Power Control in Cognitive Radios

IEEE 802.22 is the first standard to uitilize cognitive radio technology for wireless regional area networks (WRANs). It has adopted a cellular topology containing one base station (BS) and multiple customer premises equipments (CPEs) in a cell. It covers a very large area with radius ranging up to 100 km. Orthogonal frequency-division multiple access (OFDMA) system is employed and it is further slotted in time-domain. In one slot only one CPE can be allocated, and every intra-cell packet needs to be routed through the BS. This point-to-multi-point structure significantly limits the network capacity in the case of CPE-CPE communication, which is predicted to happen, and even the use of OFDMA does not help much. Therefore, peer-topeer WRAN (P2PWRAN) has been proposed to support direct intra-cell communication in order to extend the network capacity. In this paper, the IEEE 802.22 OFDMA system is adapted to P2PWRAN to support direct CPE-CPE communications. The downstream burst allocation in P2PWRAN is similar to the OFDMA system of IEEE 802.16, which has been widely studied in the literature. Therefore, in this paper we look at the unexplored upstream burst allocation where slots can be reused among CPEs and the BS with power control mechanisms. We propose a burst allocation algorithm that maximizes the network capacity greedily. The algorithm is examined under various conditions such as, number of CPEs, size of requests, length of US subframes and type of requests (CPE-BS or CPE-BS). We also show that our algorithm performs better than the existing solutions.

IEEE 802.22 is proposed for reusing the TV channels with cognitive radio technology to build Wireless Regional Area Networks (WRANs). The cellular topology of a WRAN makes the spectrum management easy. However, this topology reduces the network capacity significantly, because in one slot only one user can be allocated with a channel. Therefore, Peer-to-Peer WRAN (P2PWRAN) is proposed, in which direct CPE-(Consumer Premises Equipment)-to-CPE intra-cell communication is supported. While increasing the network capacity significantly, reducing the total energy consumption is a challenge.The energy efficiency of nodes in the standard for WRAN is difficult to improve, since the transmission distance of nodes to the BS is relatively static. P2PWRAN enables to achieve higher energy efficiency through better possibilities of channel allocation. Channel allocation influences energy consumption and network capacity significantly. Channel allocation also affects the upper layers, for example the routing protocol. Therefore, guaranteeing the network performance and fairness amongst CPEs, while achieving green networks, needs cross-layer design. In this paper, we define the channel allocation problem in P2PWRAN and we recognize it as a Multi-objective Quadratic Programming (MOQP) problem. An Energy-aware Channel Allocation (ECA) algorithm is proposed for exploiting the P2PWRAN paradigm to achieve the lower communication energy consumption and higher network energy efficiency and throughput than simple greedy policies. ECA jointly considers the network layer and Media Access Control (MAC) layer. By controlling the parameters in ECA, the fairness of channel allocation can also be guaranteed.

The rapid growth in wireless technologies has intensified the demand for the radio spectrum. On the other hand, the research studies reveal that the spectrum utilization is unevenly distributed, which leads to the conclusion that there is a problem with the spectrum management and allocation rather than the scarcity of the spectrum itself. This inefficiency in spectrum usage in addition to the escalating demand for the radio spectrum fostered the research studies that focus on new communication paradigms referred to as Dynamic Spectrum Access (DSA) and cognitive radio networks, which are based on opportunistically utilizing the radio spectrum. IEEE 802.22 is the first standard for cognitive radio networks, in which, however, network entry and initialization, as well as the hidden incumbent problem have not yet completely been addressed. On the other hand, mobility is also an unexplored issue in cognitive radio networks. In this paper, we propose a novel protocol that combats the hidden incumbent problem during network entry, initialization and handover, while at the same time taking the mobility pattern of the cognitive devices into consideration. Our proposed scheme is based on a satellite assisted cognitive radio architecture. Our model outperforms the current IEEE 802.22 scheme and other work in the literature in terms of connection setup delay 1 .

The growing popularity of Internet of Things (IoT) applications brings new challenges to the wireless communication community. Numerous smart devices and sensors within IoT will generate a massive amount of short data packets. Future wireless transmission systems need to support the reliable transmission of such small data with extremely high energy efficiency. In this article, we introduce a novel data-oriented approach for characterizing the energy efficiency of wireless transmission strategies for IoT applications. Specifically, we present new energy efficiency performance limits targeting at individual data transmission sessions. Through preliminary analysis on two channel-adaptive transmission strategies, we develop several important design guidelines on green transmission of small data. We also present several promising future applications of the proposed data-oriented energy efficiency characterization.

Cognitive radio (CR) network is the footstep and essential need of the new wireless emerging technologies like the Wireless Sensor Network (WSN), Internet of Things (IoT), Bluetooth, and Vehicular Ad Hoc Network (VANET). Due to tremendous progress in the number of wireless devices and their traffic, large scale usage of these technologies may soon cause a shortage of spectrum as all these technologies use unlicensed bands. So, CR is a vital choice for their survival. In this paper, we address the Quality of Service (QoS) parameters related to spectrum sharing among multiple Customer Premises Equipments (CPEs) in the context of CR based Wireless Regional Area Network (WRAN) using trunking theory. For this purpose, an analytical framework is developed to obtain QoS parameters such as traffic intensity, the total number of users in the network, and time to associate with the base station (BS). For a progressive environment, particularly the increase in the population of WRAN, we also a...

In this paper a collaborative coexistence mechanism for white space base stations is proposed. We look at the case where these base stations operate in geographical areas where the density of used TV channels is such that only one channel is left for broadband access. We show how with cooperative closed loop control and a clustering strategy, it is possible to find feasible power assignments that provide a flexible and stable coverage solution. The framework under which we study our proposal is based on the IEEE 802.22 standard, which provides white space guidelines for applications in broadband access or machine-to-machine communications. We propose and evaluate a self-organized, collaborative power control and design strategy to enable effective coexistence of base stations under extreme bandwidth constraints. Finally, we also portray how proposed approach positively compares against others from different wireless access technologies.

In this paper we look at the downlink interference mitigation problem in 802.22 wireless regional area networks. We study the case where the density of TV bands is such that a group of white space base stations can obtain access to only one communications channel, a scenario likely to occur close to urban centers. We propose a self-organizing power control and design strategy to mitigate interference among base stations as well as an optimal approach for the problem.

In this paper we address the issue of spectrum sensing in cognitive radio based wireless networks. Spectrum sensing is the key enabler for dynamic spectrum access as it can allow secondary networks to reuse spectrum without causing harmful interference to primary users. Here we propose a set of integrated medium access control (MAC) and physical layer (PHY) spectrum sensing techniques that provide reliable access to television (TV) bands. At the MAC level, we propose a two-stage spectrum sensing that guarantees timely detection of incumbents while meeting the quality of service (QoS) requirements of secondary users. At the PHY level, we introduce FFT-based pilot energy and location detection schemes that can detect a TV signal on a TV channel at levels as low as-116 dBm. We have evaluated these schemes through simulation and prototyping and show their effectiveness, reliability, and efficiency. These mechanisms are also part of the current IEEE 802.22 draft standard which is based on cognitive radio technology.

Research in the physical (PHY) and medium access control (MAC) layers for dynamic spectrum access (DSA) and dynamic spectrum sharing (DSS) is still at its infancy. Aspects such as spectrum sensing, coexistence, measurement and spectrum management, network reliability and QoS support in face of the need to avoid causing harmful interference into incumbents, to name a few, are key to the success of future cognitive radio (CR) systems and have received little attention so far. In addition, it is critical to understand the interplay of these various cognitive radio concepts and how they impact the overall network performance. In this paper we address these questions by presenting the design and performance evaluation of a CR-based PHY and MAC for DSA and DSS of vacant television (TV) channels. This air interface described here forms the baseline of the current IEEE 802.22 draft standard, and features a number of key PHY and MAC CR-based components for use by license-exempt devices in the spectrum that is currently allocated primarily to TV services. Through simulations and prototyping, we analyze the performance of this first CR-based wireless network with respect to spectrum sensing, system capacity, QoS support, coexistence, and network reliability.

In November/2004, we witnessed the formation of the first worldwide effort to define a novel wireless air interface 1 standard based on Cognitive Radios (CRs): the IEEE 802.22 Working Group (WG). The IEEE 802.22 WG is chartered with the development of a CR-based Wireless Regional Area Network (WRAN) Physical (PHY) and Medium Access Control (MAC) layers for use by licenseexempt devices in the spectrum that is currently allocated to the Television (TV) service. Since 802.22 is required to reuse the fallow TV spectrum without causing any harmful interference to incumbents (i.e., the TV receivers), cognitive radio techniques are of primary importance in order to sense and measure the spectrum and detect the presence/absence of incumbent signals. On top of that, other advanced techniques that facilitate coexistence such as dynamic spectrum management and radio environment characterization could be designed. In this paper, we provide a detailed overview of the 802.22 architecture, its requirements, applications, and coexistence considerations that not only form the basis for the definition of this groundbreaking wireless air interface standard, but that will also serve as foundation for future research in the promising area of CRs.

Ensuring self-coexistence among IEEE 802.22 networks is a challenging problem owing to opportunistic access of incumbent-free radio resources by users in co-located networks. In this study, we propose a fully-distributed non-cooperative approach to ensure self-coexistence in downlink channels of IEEE 802.22 networks. We formulate the self-coexistence problem as a mixed-integer non-linear optimization problem for maximizing the network data rate, which is an NP-hard one. This work explores a sub-optimal solution by dividing the optimization problem into downlink channel allocation and power assignment sub-problems. Considering fairness, quality of service and minimum interference for customer-premises-equipment, we also develop a greedy algorithm for channel allocation and a non-cooperative game-theoretic framework for near-optimal power allocation. The base stations of networks are treated as players in a game, where they try to increase spectrum utilization by controlling power and...

This paper analyses the performance of spectrum sensing in terms of the throughput of a cognitive radio (CR) system. Dealing with the optimization problem of spectrum sensing, this paper evaluates the throughput of a CR system by considering such situations as the penalty time of a channel search and incumbent user (IU) detection delay caused by a missed detection of an incumbent signal. Also, this paper suggests a serial channel search scheme as the search method for a vacant channel, and derives its mean channel search time by considering the penalty time due to a vacant channel search error. The numerical results suggest the optimum sensing time of the channel search process using the derived mean channel search time of a serial channel search in the case of a sensing hardware structure with single radio frequency (RF) path. It also demonstrates that the average throughput is improved by two separate RF paths in spite of the hardware complexity of an RF receiver.

This paper proposes the scheme of the out-of-band cooperative spectrum sensing in the cognitive radio (CR) base station to be operated by the multiple spectrum bands. And it suggests signal detection results for the ATSC TV signal as an incumbent signal, and derives signal detection probability and false-alarm probability for the out-of-band cooperative spectrum sensing in the frequency selective Rayleigh fading channel. Numerical results demonstrate that sensing performance for the incumbent signal is improved by the out-of-band cooperative spectrum sensing in case the power strengths of the incumbent signal to be measured by the CR users of the multiple channels are almost similar.

Cognitive radio (CR) has been recognized as future prospect for efficient and dynamic allocation of bandwidth among users of which dynamic spectrum access is an important aspect focusing on identification and opportunistic utilization of vacant spectrum in television broadcasting licensed bands, known as television white spaces (TVWS). TVWS has been selected by numerous IEEE standards spanning diverse operating zones for implementing CR technology. Specifically, we focus our attention to IEEE 802.22, IEEE 802.11af and IEEE 802.15.4m standards operating in TVWS pertaining to regional, local and personal area networks respectively. The PHY layer in each of these standards is depending on orthogonal frequency division multiplexing (OFDM) for spectrum-wise efficient communication as well as dynamic frequency allocation. Pertinent OFDM design challenges corresponding to IEEE standards in TVWS are revealed. PHY layer structure and cognitive techniques employed in cognition-aware IEEE standards in TVWS are reviewed in detail. Lastly, open research issues and implementation challenges for TVWS IEEE standards are highlighted.

The previous and most popular broadband wireless technology i.e. WiMAX which is limited about to 10 miles, there are power and line of sight issues yet to be resolved for a broader coverage area. WiMAX deployment is therefore limited to densely populate metropolitan areas. What about rural and sparsely populated, geographically dispersed regional areas? Here is the upcoming solution for that which is implemented by IEEE .The IEEE 802.22 standard defines a system for a Wireless Regional Area Network i.e. WRAN that uses unused or white spaces within the television bands between 54 and 862 MHz, especially within rural areas .The 802.22 standard is the first standard to adopt cognitive radio spectrum sensing as a means of gaining greater use of the radio spectrum. Cognitive radios are to provide broadband wireless access (BWA) in the licensed TV bands on a secondary access basis. This concept is examined to see under what conditions BWA could be viable. Rural areas require long range co...

In this paper, we propose a handover mechanism for supporting of IEEE 802.22 WRAN system. Generally, every CPE (Consumer Premise Equipment) in IEEE 802.22 WRAN (Wireless Regional Area Network) is considered as a fixed user device. For this reason, IEEE 802.22 WRAN system is not suitable for adapting to portable and mobile user devices. Mobility is one of the most important factors for mobile devices. In order to support mobility of CPEs and continue to provide broad band network service in IEEE 802.22 WRAN system, an efficient handover mechanism is essential. In this paper we propose an efficient handover mechanism using characteristics of IEEE 802.22 WRAN system. We discuss the how handover procedure can be achieved with IEEE 802.22 control frames and then we evaluate L2 handover IEEE 802.22 WRAN system. We define a handover process with a decision making algorithm to reduce the packet loss during handover operation. The handover decision is made at the BS(Base Station) based on collected information by CPEs which are located on the boundary region located near the edge of BS coverage. Also the performance results show that our proposed mechanism not only achieves low handover latency but also does so with low packet loss rate.

This work focuses on a passive coherent location (PCL) system that exploits the signals emitted by IEEE 802.22 devices, referred hereafter as a white-space PCL (WS-PCL) system. To cope with the very low transmitted equivalent isotropically radiated power of the IEEE 802.22 emitters, we focus on the design of a WS-PCL system that exploits all the useful signals, emitted from both the base station and customer-premises equipment, received in each frame. In this work, we study the feasibility of a WS-PCL system by deriving the receiver operating characteristic and defining the multistatic velocity profiling algorithm for the estimation of the target velocity vector. The performance of the proposed receiver is compared to that of a WS-PCL system that exploits only the signal emitted by the base station.

IEEE 802.22 networks consist of base stations and consumer premise equipments (CPEs) where the base station in each cell opportunistically accesses and allocates (uplink and downlink) channels to all the CPEs in its cell. Information on white space (unused primary channels) availability is reported by the CPEs to the base stations. Thus, a base station's effectiveness to allocate channels are based on its ability to gauge the spectrum usage at various locations. In this paper, we propose a channel usage estimation framework where a base station uses the spectrum reports from other neighboring base stations to estimate the spectrum usage scenario at any arbitrary location within its cell. Our estimation framework is based on Shepard's interpolation technique for irregular points. We propose a channel allocation scheme that minimizes interference among CPEs and maximizes white space utilization. Through simulation experiments, we demonstrate the accuracy of the estimation technique, utilization of the available spectrum, and efficiency of allocation scheme. We also show that our scheme achieves very low false positives and no false negatives. Finally, we show that the optimal number of base stations that need to be consulted is in accordance with Shepard's bounds 1 .

Cognitive radio (CR) network is the footstep and essential need of the new wireless emerging technologies like the Wireless Sensor Network (WSN), Internet of Things (IoT), Bluetooth, and Vehicular Ad Hoc Network (VANET). Due to tremendous progress in the number of wireless devices and their traffic, large scale usage of these technologies may soon cause a shortage of spectrum as all these technologies use unlicensed bands. So, CR is a vital choice for their survival. In this paper, we address the Quality of Service (QoS) parameters related to spectrum sharing among multiple Customer Premises Equipments (CPEs) in the context of CR based Wireless Regional Area Network (WRAN) using trunking theory. For this purpose, an analytical framework is developed to obtain QoS parameters such as traffic intensity, the total number of users in the network, and time to associate with the base station (BS). For a progressive environment, particularly the increase in the population of WRAN, we also a...

Cognitive radio (CR) network is the footstep and essential need of the new wireless emerging technologies like the Wireless Sensor Network (WSN), Internet of Things (IoT), Bluetooth, and Vehicular Ad Hoc Network (VANET). Due to tremendous progress in the number of wireless devices and their traffic, large scale usage of these technologies may soon cause a shortage of spectrum as all these technologies use unlicensed bands. So, CR is a vital choice for their survival. In this paper, we address the Quality of Service (QoS) parameters related to spectrum sharing among multiple Customer Premises Equipments (CPEs) in the context of CR based Wireless Regional Area Network (WRAN) using trunking theory. For this purpose, an analytical framework is developed to obtain QoS parameters such as traffic intensity, the total number of users in the network, and time to associate with the base station (BS). For a progressive environment, particularly the increase in the population of WRAN, we also a...

Following are the answers to the comments of reviewers. The answers are in blue text. All queries have been addressed and author(s) have tried their best to make modifications as suggested. Reviewer #1: The work provides an innovative mathematical methodology regarding the selection process and association process for IEEE 802.22 networks. The approach is largely evaluated for several cases and is robust enough to serve as a knowledge base for the Base Station. Since the discussed environment is related to TV white spaces I would suggest to add to the introduction a well established review work on the topic, namely,

Initial ranging is the primary and important process in wireless networks for the customer premise equipments (CPEs) to access the network and establish their connections with the base station. Contention may occur during the initial ranging process. To avoid contention, the mandatory solution defined in the standards is based on a truncated binary exponential random backoff (TBERB) algorithm with a fixed initial contention window size. However, the TBERB algorithm does not take into account the possibility that the number of contended CPEs may change dynamically over time, leading to a dynamically changing collision probability. To the best of our knowledge, this is the first attempt to address this issue. There are three major contributions presented in this paper. First, a comprehensive analysis of initial ranging mechanisms in wireless networks is provided and initial ranging request success probability is derived based on number of contending CPEs and the initial contention window size. Second, the average ranging success delay is derived for the maximum backoff stages. It is found that the collision probability is highly dependent on the size of the initial contention window and the number of contending CPEs. To achieve the higher success probability or to reduce the collision probability among CPEs, the BS needs to adjust the initial contention window size. To keep the collision probability at a specific value for the particular number of contending CPEs, it is necessary for the BS to schedule the required size of the initial contention window to facilitate the maximum number of CPEs to establish their connections with reasonable delay. In our third contribution, the initial window size is optimized to provide the least upper bound that meets the collision probability constraint for a particular number of contending CPEs. The numerical results validate our analysis.

In this paper, we propose novel spectrum sensing algorithms to detect multi-channel CR signal in TV white space. Most of CR systems in TV white space are considering multi-channel operation to support enhanced system throughput. Therefore, spectrum sensing algorithm to detect multi-channel CR system is essentially required for coexistence in TV white space. The proposed algorithms can detect various types of multi-channel CR signal with reasonable detection performance. By employing the proposed algorithms, we can contribute to efficient implementation of spectrum sensing process as well as intelligent coexistence mechanisms in TV white space.

In this paper, the performance of an underlay multiple-input multiple-output (MIMO) cognitive radio system is analytically studied. In particular, the secondary transmitter operates in a spatial multiplexing transmission mode, while a zero-forcing detector is employed at the secondary receiver. Additionally, the secondary system is interfered by single-antenna primary users (PUs). To enhance the performance of secondary transmission, optimal power allocation is performed at the secondary transmitter with a constraint on the maximum allowable outage threshold specified by the PUs. Further, the effective number of secondary transmit antennas is specified based on the optimal power allocation for an arbitrary MIMO scale. Also, a lower bound on the ergodic channel capacity of the secondary system is derived in a closed-form expression. Afterwards, the scenario of a massive MIMO secondary system is thoroughly analyzed and evaluated, where the harvesting-enabled secondary transmission is studied. The optimal power allocation, the effective number of secondary transmit antennas, the efficient tradeoff between transmit-and-harvest secondary antennas, and the average channel capacity of the secondary system are analytically presented. Finally, extensive numerical and simulation results corroborate the effectiveness of our analysis, while some useful engineering insights are provided.

Measurements performed at several locations clearly show that frequency spectrum is under-utilized. Cognitive radio (CR) is a strong candidate to ensure better spectrum utilization by providing access in an opportunistic manner, i.e., when the spectrum is temporary or spatially unutilized (a.k.a. white-space). The key enabling technology for realizing better frequency spectrum utilization in CR system is spectrum sensing. In particular, spectrum sensing in television (TV) bands is of great interest due to the potential availability of spectrum as a result of the planned migration of analog TV broadcasting to digital TV broadcasting. In this paper, we present the TV white-space prototype that we developed to detect vacant spectrum in TV bands. The prototype is implemented on a real-time platform and tested in practical environments. Results show that our prototype is able to detect vacant channels up to sensitivities from −116 to −120 dBm under realistic conditions. This confirmation will give the much needed confidence in the capability of CR systems to detect the operation of primary users and protect their use of spectrum.

Me falta tanto. No tengo la humildad de aquel mendigo, ni canto como el trovador del pueblo, ni escribo como el vate que bendigo, ni bailo como ese artista, en pleno vuelo. No rezo como la abuela persistente, ni leo como aprendiz tan absorbente, ni sueno como los ninos de mi frente, ni sirvo como la madre permanente. No avanzo como ese parvulo sonriente, ni creo como quisiera, inmensamente, ni pienso como el filosofo valiente, ni enseno como maestro, mansamente. No soy lo que quisiera, internamente, ni comprendo a las personas, santamente, ni disculpo el mal causado injustamente, ni amo a los demas, completamente. Me hace falta hablar menos, tantas veces, y dejar que fluya mas la vida verdadera, me hace falta hablar mas, en ocasiones, en que la paz se prostituye, cual quimera. Me falta tanto, para ser mi propio ideal de persona, y estoy lleno de las maldades que critico, y estoy harto de mis incoherencias, y me cansa ir por donde yo no quiero. Sin embargo, estoy aqui Senor, para apr...

Cognitive Radio Network is the effective solution to the spectrum scarcity.  Dynamic spectrum access is a paradigm used to access the spectrum dynamically. A Markov Chain is a stochastic model describing a sequence of possible events in which probability of each event depends only on the state attained in the previous event. We model the dynamics of cognitive user with 2-D Markov chain. The resource distribution probability (RDP) verses addition/ elimination rate of the channels in the network is also plotted. The RDP verses utilization factor of the queue, which is the secondary user in the network, is also plotted. This plot helps to maintain the total arrival and departure rate based on the RDP. The base station of the network will use this relation to maintain the proper RDP for the devices. The dynamics of each cognitive user and its correlation with Markov Chain is an interesting approach. Here we considered the DSA at Base Station as a Markov chain and analyzed it. This analy...

In this paper, the performance of an underlay multiple-input multiple-output (MIMO) cognitive radio system is analytically studied. In particular, the secondary transmitter operates in a spatial multiplexing transmission mode, while a zero-forcing detector is employed at the secondary receiver. Additionally, the secondary system is interfered by single-antenna primary users (PUs). To enhance the performance of secondary transmission, optimal power allocation is performed at the secondary transmitter with a constraint on the maximum allowable outage threshold specified by the PUs. Further, the effective number of secondary transmit antennas is specified based on the optimal power allocation for an arbitrary MIMO scale. Also, a lower bound on the ergodic channel capacity of the secondary system is derived in a closed-form expression. Afterwards, the scenario of a massive MIMO secondary system is thoroughly analyzed and evaluated, where the harvesting-enabled secondary transmission is studied. The optimal power allocation, the effective number of secondary transmit antennas, the efficient tradeoff between transmit-and-harvest secondary antennas, and the average channel capacity of the secondary system are analytically presented. Finally, extensive numerical and simulation results corroborate the effectiveness of our analysis, while some useful engineering insights are provided.

White spaces promise to revolutionize the way wireless connectivity is delivered over wide areas. However, large-scale white space networks face the problem of allocating channels to multiple contending users in the wide white space band. To tackle the issue, we first examine wireless propagation in a long-distance outdoor white space testbed and find that a complex combination of free-space loss and antenna effects impacts transmission in white spaces. Thus, a need arises for a strategy that goes beyond simple channel utilization balancing, and uses frequency probing to profile channels according to their propagation properties. We devise VillageLink, a Gibbs sampling-based method that optimizes channel allocation in a distributed manner with a minimum number of channel switching events. Through extensive simulations we demonstrate that VillageLink results in a significant capacity improvement over alternative solutions.

Efficient use of the scarce radio spectrum is important for accommodating the rapid growth of wireless communications. This thesis investigates the feasibility of Cognitive Radio (CR) to exploit unoccupied frequencies for an enhanced spectral utilisation. CR refers to a wireless architecture that enables dynamic spectrum access, where unlicensed devices are allowed to operate in temporally/spatially unused licensed channels. Extensive spectral opportunities have been identified from an intensive measurement study, indicating that a significant gain in spectral utilisation may be brought by a CR technology. The main challenge for CR is a robust protection mechanism to guarantee an adequate Licensed Users (LU) system performance at all times. This requires a reliable spectrum sensing technique to accurately identify frequency opportunities; and an autonomous transmit power control algorithm that remains effective in severe fading environments. Cooperation among CR devices to assist th...