In this paper, formation of stable coalitions of users, each exploiting resources in a femto-cell, and the resource allocation in each femto-cell is investigated in a UMTS long term evolution (LTE) network. We study a downlink scenario where users collaborate to increase network throughput and, simultaneously, attempt to increase their own payoffs. Payoffs to the users are defined as the monetary equivalent of the individual users' achievable throughput in the specified coalition structure. A distributed game-theoretic resource allocation mechanism is developed whereby users autonomously decide which sub-channel in which coalition to join. If each user operates according to the proposed algorithm, the sum throughput of all links converges with probability one to its maximum feasible value.

The 17 papers in this special issue cover various traditional as well as emerging networking and communications problems in innovative and insightful ways.

In this survey, we present a comprehensive list of major known security threats within a cognitive radio network (CRN) framework. We classify attack techniques based on the type of attacker, namely exogenous (external) attackers, intruding malicious nodes and greedy cognitive radios (CRs). We further discuss threats related to infrastructure-based CRNs as well as infrastructure-less networks. Besides the short-term effects of attacks over CRN performance, we also discuss the often ignored longer term behavioral changes that are enforced by such attacks via the learning capability of CRN. After elaborating on various attack strategies, we discuss potential solutions to combat those attacks. An overview of robust CR communications is also presented. We finally elaborate on future research directions pertinent to CRN security. We hope this survey paper can provide the insight and the roadmap for future research efforts in the emerging field of CRN security.

In this paper, we propose a novel routing protocol, WLAN-WiMAX Double-Technology Routing (WWDTR), for routing packets in the heterogeneous vehicular network. Any given route in the protocol can be generally formed of a combination of WLAN and WiMAX hops. WWDTR uses the position-based routing approach over the parts of the route in which packets are forwarded via WLAN radios, in order to handle relatively fast changes of the topology with respect to the shorter transmission ranges of WLAN-enabled vehicles. On the other hand, topology-based routing is employed over the parts of the route in which packets are forwarded via WiMAX radios, given the more stable routes comprised of WiMAX-enabled vehicles, thus overall yielding a hybrid routing scheme. The route selection logic takes both operators' and subscribers' preferences such as QoS and network utilization into account. Furthermore, we propose network architecture to facilitate forwarding packets towards the access network of a given operator over routes that partly utilize relaying over other operators' links. To the best of our knowledge, WWDTR is the first multi-technology multi-operator routing solution for heterogeneous vehicular networks.

This paper addresses the problem of optimal vertical handoff (VHO) in a vehicular network setting. The VHO objective can be minimizing the data transfer time or alternatively minimizing the cost of transmitting traffic. As a framework for performance evaluations, we first analyze a heterogeneous network consisting of a wide-area cellular network interworking with wireless local area networks (WLAN) with fixed inter-distance between access points (APs) placed along roadsides. We further analyze a scenario with random inter-distance between WLAN APs. In both aforementioned cases, only Vehicle-to-Infrastructure (V2I) capability is assumed. We show that in order to minimize the cost of transmission or alternatively transmission time, performing VHOs is an appropriate choice at lower speeds, whereas it would be better to avoid VHO and stay in the cellular network at higher speeds. We further generalize our study, to investigate the VHO strategies in a random inter-distance scenario with both V2I and Vehicle-to-Vehicle (V2V) communication capabilities. We demonstrate that the combination of WLAN plus cellular plus ad hoc networking outperforms any other networking strategies considered in this work in terms of transmission times and transmission costs. The presented results provide insightful guidelines for optimal VHO decision making based on the characteristics of the network as well as the user mobility profile.

We study how truthfulness can be enforced as a dominant strategy when a number of secondary cognitive radios participate in an online expiring spectrum sharing auction, held by the primary to lease its spectrum holes. The types of secondary cognitive radios, announced to the primary, are composed of valuation and arrival-departure periods. We show how, by the suitable choice of channel allocation and pricing schemes, the collusion incentive among secondary cognitive radios can be reduced. The performance of the deployed dynamic auction is compared with that of the Vickrey-Clarke-Groves (VCG) offline auction as a benchmark.

As the spectral efficiency of wireless communications systems increases, and the frequency reuse pattern shifts towards universal frequency reuse, the capacity of future wireless and mobile systems will becomes interference limited. We explore the interference pattern of a shared channel in order to establish optimal distributed resource allocation techniques in that channel considering the mutual interference of coexisting links on each other. It is shown that the optimal usage of resources in such a shared channel can only be achieved via a multi-dimensional resource allocation strategy, taking into account not only the link's own channel quality, but also its channel states towards coexisting links. The improvement of capacity as a result of this approach can be attributed to the interference diversity of the channel. The interference diversity gain can be harnessed in time, frequency and space domains. To this end we study two approaches, namely maximizing the capacity of the primary and secondary links under received interference constraint and minimizing the transmitted power of primary and secondary links under minimum QoS guarantee constraint. We study the Ergodic capacity to verify the significant performance improvement which can be achieved by exploiting interference diversity in multi-channel systems such as UMTS Long Term Evolution (LTE). Further we will show that utilizing this diversity gain in QoS-guaranteed scenarios results in a considerable transmission power saving. The case of Outage capacity, which is an instantaneous measure of channel throughput, is shown to be different whereby using an instantaneous received interference threshold outperforms the usage of average received interference limit.

The performance of legacy WLAN systems will significantly degrade as the number of active stations (STA) in the network increases, rendering satisfactory coverage of busy hot spots challenging in practice. A novel centralized architecture that coordinates several WLAN access points is the exploitation of Radio over Fiber (RoF) technology. In this paper we focus on spatial diversity techniques to enhance the performance of the newly proposed Cognitive WLAN over Fiber (CWLANoF) compared to legacy WLAN Extended Service Set (ESS). More specifically we develop an optimal transmitter-side beamforming technique and investigate the TCP performance enhancement as a result of usage of this scheme. Furthermore, the issue of fairness will also be discussed, where we introduce a fairness measure based on the distribution of TCP throughput among existing STAs in the network. The numerical results verify the superior performance of CWLANoF compared to WLAN ESS.

Security issues associated with successful operation of cognitive radio networks (CRNs) recently are gaining a lot of attention. In this paper we focus on a specific class of Denial of Service attack that is executed through Channel Eviction Triggering (CET), whereby the adversary nodes unduly invoke mechanisms inherent in CRN operation to protect the licensed users and thus disrupt secondary access to the otherwise idle licensed bands. Skewing the spectrum sensing decision of a CRN through sensing mis-reports is a manifestation of CET attacks. Whereas most studies in the literature focus on making the cooperative sensing more robust against such sensing misreports, we tackle the problem from an incentive perspective and develop strategies to minimize the utility difference of truthful and cheating cognitive radios so as to alleviate the incentive of mis-reporting. Our numerical results verify the effectiveness of the proposed CET defense scheme.

The issue of efficiency and fairness in resource allocation will continue to be of significant importance in scheduler designs for future wireless systems. Of particular importance is the development of distributed techniques for achieving desired efficiency and fairness tradeoffs. We will focus on the the design of selfishly efficient and as well as different fair policies through the use of a virtual timer. This virtual timer unifies various previously considered fairness methods including Round-Robin, Max-Min and Proportional fairness, which are rigorously investigated. The performance of the presented techniques are compared using extensive simulations.

Cognitive radio networks (CRN) are promising to enrich the connectivity demand of users through exploiting the under-utilized licensed bands. In this paper, the load-balanced resource allocation problem is formulated as a non-superadditive coalition formation game in which the cognitive radios (CR) form collaborative groups to exploit the spectrum available to cognitive base stations (CBS) most efficiently. A distributed decision-making framework is developed for coalition formation among CRs that converges to the core of the defined game, corresponding to the maximum average payoff conditional on feasibility and subject to a fairness rule. Further, we elaborate on the load balancing properties of the proposed scheme and demonstrate its superior performance, when compared to opportunistic scheduling, through numerical examples.

Non-contiguous orthogonal frequency division multiplexing (NC-OFDM) is an effective transmission method for cognitive radio systems. It can provide high bandwidth utilization and robustness against time dispersive channels. However, NC-OFDM inherits the problems of high sidelobes and peak-to-average power ratio (PAPR) of the OFDM signal. If not accounted for, this introduces leakage into a primary user (PU) band and distorts the signal of the secondary user (SU). This paper proposes a joint sidelobe and PAPR reduction method that can simultaneously reduce the inherent OFDM out-of-band (OOB) radiation and the OOB radiation caused by a nonlinear power amplifier. In this method, selected mapping (SLM) sequences, which are generated from sequences with a low average sidelobe power, are used to improve the PAPR performance. The key idea is to generate multiple representations of the transmit signal and select a sequence with low OOB radiation and PAPR. We examine the performance of the proposed method taking into account the effects of non-ideal (i.e. non-linear) power amplification. The simulation results show that the proposed technique can significantly reduce OOB radiation while improving the PAPR and bit error rate (BER) for NC-OFDM signals.

The process of admitting secondary spectrum users via spectrum auctions provides a convenient framework for the primary license holder to control utilization of its idle resources. While most existing solutions in the literature are static, offline auction mechanisms, the dynamics of arrival and departure of secondary bidders, as well as the uncertainty of availability of a given sub-channel for leasing in future, calls for novel auction strategies. In this paper we propose a dynamic, online auction design that addresses two key issues. First, we determine optimal auction instances so as to maximize the profitability of this auction model. Further, by utilizing critical pricing strategy, the incentive of bidding secondaries to misreport their price, time of arrival, and departure is alleviated and hence a truthful, profitable auction results. Our numerical results verify the profitability of this dynamic auction as compared with static Vickrey-Clarke-Groves auctions with fixed auction time.

We propose a multi-user medium access control (MAC) protocol to simultaneously support multi-packet transmission (MPT) and multi-packet reception (MPR) in wireless local area networks. By assuming perfect physical layer functions, we evaluate the maximum number of active stations that can be supported under different traffic arrival rates. Our analysis show that when the number of transmitting antennas is K=2, simultaneous MPT and MPR supports 40~50% more stations than previous multi-user MAC that uses only MPT. The gain increases to 60% when short packets (40 bytes) are used. When K=4, the advantage of MPT+MPR over MPT-only enlarges to 130%. We also investigated the impacts of imperfect physical layer functions and the effects of enabling or disabling RTS/CTS in uplink.

We explore the interference pattern of a shared channel in order to establish optimal distributed resource allocation techniques in that channel. The interference diversity gain can be achieved in time, frequency and space domain. In particular we develop resource allocation techniques to harness interference diversity gain in the frequency domain. To this end we study two approaches, namely maximizing the capacity of the primary and secondary links under received interference constraint and minimizing the transmitted power of primary and secondary links under minimum QoS guarantee constraint. We will also elaborate on the application of interference diversity in UMTS Long Term Evolution (LTE) systems.

Cognitive radio is presently being researched as a means to greatly improve spectrum usage efficiency, connectivity and QoS among wireless devices and systems. In parallel, efficient and fair resource allocation strategies are being studied in order to address the requirements of future wireless technologies. In this paper, we assess a novel resource allocation scheme for OFDM systems which aims to maximize performance while limiting received interference at each user to below a defined threshold. We show that our scheme is relevant to cognitive radio, and demonstrate that through our approach, significant power savings can be achieved in wireless systems.

A cognitive radio (CR) network is a secondary network operating in the frequency band and geographical area of a primary network to which the spectrum is licensed. In this paper, we consider a primary broadcasting network consisting of a transmitter and a number of primary users (PUs). The secondary users (SUs) make opportunistic unlicensed spectrum access to communicate with their base station (BS). With this system model, a joint power allocation and beamforming scheme is proposed in the uplink of the CR network to minimize the maximum interference to the PUs in two different scenarios. In the first scenario, the primary transmitter does not cooperate with the CR network and it transmits with a fixed power, whereas it cooperates in the joint power allocation and beamforming scheme and adjusts its transmission power accordingly, in the second scenario. In the former case, our proposed joint power allocation and beamforming scheme finds the beamforming weights and transmission power values under quality of service (QoS) constraints of all SUs, taking into account their maximum transmission power constraints. In the latter case, the proposed scheme finds the beamforming weights of the BS and transmission powers of primary transmitter and SUs under their target signal-to-interference-plus-noise ratios (SINRs) and maximum power constraints. Our approach employs Genetic Algorithm (GA) to determine optimum transmission power values and beamforming weight vectors. Simulation results show the performance of the proposed scheme.

In this paper we develop a framework for resource allocation in a secondary spectrum access scenario, where a group of cognitive radios (CR) access the resources of a primary system. We assume the primary system is a cellular OFDM-based network operating in uplink. We develop an optimum resource allocation strategy, using cooperative game theory, which guarantees the primary's required QoS and allocates an achievable rate at a given bit error rate for the secondary, when possible. The proposed cognitive radio game (CRG) is a network-assisted resource management method, where users (both primary and secondary) inform the primary system's BS of their channel state information and power limitation and the base station calculates the optimum sub-channel and power allocation for all users. Using Game theoretic axiom of fairness, i.e., Nash Bargaining Solutions (NBS), we develop an alternative efficient and fair resource allocation and compare its performance with the proposed CRG method. We use Sequential Quadratic Programming (SQP) to solve the proposed non-linearly constrained CRG optimization problem.

In this paper we develop a framework for resource allocation in a secondary spectrum access scenario where a group of cognitive radios (CR) access the resources of a primary system. We assume the primary system is a cellular OFDM-based network. We develop the optimum resource allocation strategies which guarantee a level of QoS, defined by minimum rate and the target bit error rate (BER), for the primary system. Using the game theoretic axiom of fairness, i.e., Nash bargaining solutions (NBS), we show that by allocating a priority factor to all players an efficient and fair resource allocation can be achieved. We show how the priority factors are assigned in this scheme and outline a method to select the users who are allowed to share a specific sub-channel.

In this paper we develop a cellular cognitive radio network based on MC-CDMA, which is able to coexist with a number of legacy RANs in a shared band. Using the simple-to-obtain knowledge of RF environment, namely the bandwidth and frequency location of each legacy system, the cognitive MC-CDMA transmitter mitigates the interference in the corresponding receivers by adaptive transmission. We will develop an algorithm for calculating the adaptive transmission parameters and elaborate on error performance of this approach analytically and numerically.

Spectrum sharing methods, such as secondary spectrum access, are powerful candidates to improve spectrum utilization and efficiency. In this paper, we develop a novel resource allocation scheme for WiMAX networks, which is designed to maximize performance while limiting the received interference at each user. By defining different interference tolerances for different sets of users, the proposed allocation scheme can be exploited in a secondary spectrum access scenario where two WiMAX operators, one as the primary operator and the other one as the secondary operator, are using a shared band. The primary system benefits either by charging the secondary system for the use of its resources, or by some form of reciprocal arrangement allowing it to use the secondary system's licensed bands when needed. Numerical results show our resource allocation approach to achieve an excellent fairness, while incurring only a slight reduction in system throughput compared with the theoretical upper limit of opportunistic scheduling.

In this survey paper, we investigate some of the challenges to be addressed before a practical real-time secondary market for spectrum can be developed. We differentiate the methods of dynamic spectrum allocation as coordinated usage of resources, interworking solutions, integration solutions and secondary access of spectrum. Secondary spectrum access approaches are generally classified as real-time and non-real-time secondary access. However, the focus of this work is on real-time secondary spectrum utilization which can follow a negotiated or opportunistic access strategy. While different solutions for increasing spectrum utilization and efficiency are under investigation, many aspects of spectrum sharing technologies are still open questions. After an extensive literature survey, the major challenges and open questions of real-time secondary usage of spectrum are addressed here. We also provide some insights on potentially important design considerations and requirements when developing successful spectrum management schemes to realize real-time secondary spectrum usage.

Efficient and fair resource allocation strategies are being extensively studied in current research in order to address the requirements of future wireless applications. A novel resource allocation scheme is developed for orthogonal frequency-division multiplexing (OFDM) networks designed to maximise performance while limiting the received interference at each user. This received interference is in essence used as a fairness metric; moreover, by defining different interference tolerances for different sets of users, the proposed allocation scheme can be exploited in various cognitive radio scenarios. As applied to the scheme, the authors investigate a scenario where two cellular OFDM-based networks operate as primary and secondary systems in the same band, and the secondary system benefits by accessing the unused resources of the primary system if additional capacity is required. The primary system benefits either by charging the secondary system for the use of its resources or by some form of reciprocal arrangement allowing it to use the secondary system's licenced bands in a similar manner, when needed. Numerical results show our interference-limited scheduling approach to achieve excellent levels of efficiency and fairness by allocating resources more intelligently than proportional fair scheduling. A further important contribution is the application of sequential quadratic programming to solve the non-convex optimisation problems which arise in such scenarios.

In this paper, we propose a cognitive cellular system based on DS MC-CDMA to coexist with a number of narrow-band legacy systems in the same frequency range. If the bandwidth and the frequency location of each narrow-band system are known to this cognitive MC-CDMA, it is possible to mitigate their interfering effect in the cognitive receivers by adaptive transmission. We will develop an algorithm for calculating the adaptive transmission parameters and show that it combats interference in the channel effectively.

Cognitive radio is certain to transform our traditional radio resource allocation regimes. Through cognitive radio, better spectrum efficiency can be leveraged by radios intelligently altering their radio resource usages dependent on locally available information about radio conditions. A significant challenge for cognitive radio, however, is awareness of these local radio conditions. One crude solution is for the radio to periodically scan the complete spectrum band over which cognition in radio resource usages can apply. Such an operation would be time-consuming and wasteful of power, and ultimately would be entirely impractical. In this paper, we therefore propose the concept of a resource awareness channel (RAC)-a single universally available channel on a fixed frequency, on which information about resource usage is conveyed by all radios using the cognitive radio spectrum band. This allows cognitive radios that want to alter their transmission resources to make an informed decision about the practicality or these prospective changes in terms or interference for the system as a whole, simply by listening to the RAC. We analyse the performance or this proposed RAC concept, and show it to be practical even in areas that are highly populated by radios