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Communications, IET

Issue 6 • Date July 2008

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Displaying Results 1 - 15 of 15
  • Editorial - Cognitive Spectrum Access

    Page(s): 721 - 723
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  • Outage-constrained capacity of spectrum-sharing channels in fading environments

    Page(s): 724 - 732
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    Cognitive radio technology has been recently proposed for sharing and utilising the spectrum in order to satisfy the increasing demands for spectrum access. In this radio technology, secondary users may be granted access to the spectrum bands occupied by a primary user as long as the interference power, inflicted on the primary receiver as an effect of the transmission of the secondary user, is deemed unharmful. In this paper the authors assume that the successful operation of the primary user requires a minimum rate to be guaranteed by its channel for a certain percentage of time and obtain the interference-power constraint that is required to be fulfilled by the secondary user. Considering the input transmit-power constraint, on average or peak power, for the secondary user, the authors investigate the capacity gains offered by this spectrum-sharing approach when only partial channel information of the link between the secondary's transmitter and primary's receiver is available to the former. In particular, the lower bounds on the capacity of a Rayleigh flat-fading channel with two different transmission techniques, namely channel inversion and optimum rate allocation with constant power transmission, are derived. Closed-form expressions for these capacity metrics are provided, and numerical simulations are conducted to corroborate the theoretical results. View full abstract»

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  • Spectral efficiency of spectrum-pooling systems

    Page(s): 733 - 741
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (462 KB)  

    The authors investigate the idea of using cognitive radio to reuse locally unused spectrum to increase the total system capacity. The authors consider a multiband/wideband system in which the primary and cognitive users wish to communicate to different receivers, subject to mutual interference and assume that each user knows only his/her channel and the unused spectrum through adequate sensing. The basic idea under the proposed scheme is based on the notion of spectrum pooling. The idea is quite simple; a cognitive radio will listen to the channel and, if sensed idle, will transmit during the voids. It turns out that, although its simplicity, the proposed scheme showed very interesting features with respect to the spectral efficiency and the maximum number of possible pairwise cognitive communications. We impose the constraint that users successively transmit over available bands through selfish water filling. For the first time, our study has quantified the asymptotic (with respect to the band) achievable gain of using spectrum pooling in terms of spectral efficiency compared with classical radio systems. The authors then derive the total spectral efficiency as well as the maximum number of possible pairwise communications of such a spectrum-pooling system. View full abstract»

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  • Linear precoding for the downlink of multiple input single output coexisting wireless systems

    Page(s): 742 - 752
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (322 KB)  

    Coexisting radio systems, often called cognitive radio (CR), have attracted much attention because of the lack of spectrum resources and the low usage statistics of existing spectrum allocations. Interference suppression and cancellation are seen as key technologies for enabling coexisting systems, and the application of multiple antennas might be one solution to tackle interference. Linear vector precoding for downlink of multiple input single output CR systems is addressed. The maximum ratio transmission, zero forcing, optimal interference-free, and optimal interference-constrained (IC) precoding algorithms in the sense of minimum mean squared error (MMSE) are presented. Then the authors compare and analyse these algorithms under different channel assumptions. The simulation results show that the proposed IC precoding algorithm can maximise the utilisation of multiple antennas and greatly improve the system performance. View full abstract»

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  • Information theoretic criterion-based spectrum sensing for cognitive radio

    Page(s): 753 - 762
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    Energy detection is a promising candidate for spectrum sensing of cognitive radio systems due to its merits of simplicity and easy implementation. However, energy detection needs to subjectively pre-configure a threshold and requires the knowledge of noise power. Furthermore, energy detection is vulnerable to noise uncertainty, which inevitably occurs in practical implementations. To solve the above problems, two spectrum sensing methods requiring no knowledge of noise power and no subjective threshold settings are proposed based on the information theoretic criterion. Additionally, the most important advantage of these methods is that their performance is totally unaffected by noise uncertainty. Finally, simulation results are provided to demonstrate the benefits of these methods. View full abstract»

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  • Wideband spectrum sensing in unknown white Gaussian noise

    Page(s): 763 - 771
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (272 KB)  

    The spectrum sensing of a wideband frequency range is studied by dividing it into multiple subbands. It is assumed that in each subband either a primary user (PU) is active or absent in a additive white Gaussian noise environment with an unknown variance. It is also assumed that at least a minimum given number of subbands are vacant of PUs. In this multiple interrelated hypothesis testing problem, the noise variance is estimated and a generalised likelihood ratio detector is proposed to identify possible spectrum holes at a secondary user (SU). Provided that it is known that a specific PU can occupy a subset of subbands simultaneously, a grouping algorithm which allows faster spectrum sensing is proposed. The collaboration of multiple SUs can also be considered in order to enhance the detection performance. The collaborative algorithms are compared in terms of the required exchange information among SUs in some collaboration methods. The simulation results show that the proposed detector outperforms the energy detector in the presence of noise variance mismatch above 2.3 dB. Some involved trade-offs in the spectrum sensing using the proposed detector are discussed. View full abstract»

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  • Performance of dynamic spectrum access based on spectrum occupancy statistics

    Page(s): 772 - 782
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (436 KB)  

    At present multiband wireless devices are able to select their working frequency only to a limited extent because of the strict, current spectrum regulation. Dynamic spectrum access is a promising approach that might solve this inefficiency. The authors focus on spectrum sensing, one of the main tasks involved. First, three strategies are compared to efficiently sense the current spectrum based on the spectrum occupancy information statistics. In contrast to the simulation-based studies, the authors evaluate the performance of those strategies on real spectrum occupancy data gathered during an extensive measurement campaign. The authors show that the usage of historical information considerably improves the spectrum sensing process and also that the modelling of the periodic behaviour of the licensed signals leads to negligible performance enhancements because only very few periods shorter than several minutes can be found within 20 MHz 6 GHz. Secondly, the authors unveil the fundamental tradeoff between the required bandwidth for the transmission and the total bandwidth that has to be sensed in order to guarantee that the required bandwidth is available. All the results are provided in terms of outage probability that can be viewed as an approximation of the packet loss rate. View full abstract»

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  • Frequency domain interpretation of power ratio metric for cognitive radio systems

    Page(s): 783 - 793
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (843 KB)  

    Software radio (SWR) is an enabling technology for cognitive radio (CR) systems which promises to (de) modulate any signal, at any frequency. SWR signal therefore is composed of different standard's signals, and each standard's signal is either multicarrier or multiplex of single carriers. This combination leads to high temporal fluctuations and thus SWR signal inherits high peak to average power ratio (PAPR) or simply high power ratio (PR). Nonlinear analogue components (amplifiers, converters etc.) cause distortions (in and out of band distortion) for high PR signals which result in system performance degradation. Usually PR problem is addressed in time domain, and here frequency domain interpretation of PR which is more appropriate in SWR context is presented. Gaussian equivalence between SWR signal and orthogonal frequency division multiplexing (OFDM) signal is proved first to accentuate high PR issue in SWR as OFDM suffers the same problem. Then frequency domain interpretation of PR metric is discussed which results in a PR upper bound. This PR upper bound depends only upon spectral values of the signal thus associates spectrum with PR. As a result this bound assists in spectrum access for CR systems by providing PR metric information related to any available bandwidth. Thus bandwidth allocation in a spectrum access scenario under PR constraint is simplified. View full abstract»

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  • Advanced spectrum management in wideband code division multiple access systems enabling cognitive radio usage

    Page(s): 794 - 805
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (607 KB)  

    The authors propose a new advanced spectrum management (ASM) methodology for wideband code division multiple access systems based on the concept of coupling matrix, which is able to capture inter-cell interactions. The proposed methodology takes into account the fact that each cell can be associated to more than one carrier and aims at liberating some carriers in large geographical zones, so that they could eventually be used by, for example, secondary cognitive radio users that exploit the flexible frequency allocation and opportunistic spectrum access. Simulation results show that the proposed methodology increases spectrum efficiency while guaranteeing the requested QoS levels. Moreover, a new metric has been introduced to reflect the capability of the ASM methodology to liberate some carriers in large geographic zones. This metric has been used to compare the different approaches presented here. View full abstract»

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  • Interference-limited resource allocation for cognitive radio in orthogonal frequency-division multiplexing networks

    Page(s): 806 - 814
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (508 KB)  

    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. View full abstract»

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  • Optimal distributed joint frequency, rate and power allocation in cognitive OFDMA systems

    Page(s): 815 - 826
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    The problem of wireless resource management in broadband cognitive OFDMA networks is addressed. The objective is to maximise the multiple cognitive users' weighted rate sum by jointly adjusting their rate, frequency and power resource, under the constraints of multiple primary users' interference temperatures. First, based on two interpretations of the interference temperatures, the problem studied is formulated as two nonlinear and non-convex optimisation problems. Secondly, these two problems are analysed, and a centralised greedy algorithm is proposed to solve one problem, as well as a centralised algorithm based on Lagrangian duality theory for the other. The two centralised algorithms are shown to be optimal and both have polynomial time complexities. Finally, it is shown that the two centralized algorithms can be distributively implemented by introducing the idea of virtual clock. And the distributed algorithms can be interpreted as an interesting distributed negotiated secondary market approach. It is believed that the work will provide a good reference for the emerging cognitive network protocol design. View full abstract»

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  • Darwinian approach for dynamic spectrum allocation in next generation systems

    Page(s): 827 - 836
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (402 KB)  

    The authors present the use of a genetic algorithm (GA) model as a solution approach to the dynamic spectrum allocation (DSA) problem considered as a difficult combinatorial optimisation problem. The proposed multi-objective GA model enhances overall spectral efficiency of the network, while optimising its own spectrum utilisation to generate accessible spectrum opportunities for other radio technologies. A novel two-dimensional encoding technique is defined to represent solutions in the problem domain and the technique enables significantly shorter convergence times. A simulation tool has been developed to model the GA-based DSA and to compare the new scheme with the conventional fixed spectrum allocation (FSA) scheme under both uniform and non-uniform traffic distributions. The proposed scheme significantly outperformed the FSA scheme both in terms of spectral efficiency gain and spectral utilisation. View full abstract»

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  • Opportunistic WiMAX-GSM coexistence

    Page(s): 837 - 845
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (364 KB)  

    In future multi-media applications dynamic access to spectral resources will play an important role. However, regulation of spectrum will only change very slowly and spectrum rights holders might not be willing to share their resources without gaining something out of it. Therefore the authors analyse the suitability of WiMAX as an overlay system on GSM where available resources will be auctioned among several competitors. The authors design a cross-layer approach to facilitate dynamic resource allocation. It is demonstrated that, with minor changes of the original WiMAX frame and a small signalling overhead which is currently unused, GSM resources can be allocated efficiently without decreasing system performance. Thus, this adapted WiMAX system used in opportunistic coexistence to GSM offers a suitable alternative to broadband service access. View full abstract»

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  • Spectrum etiquettes for terrestrial and high-altitude platform-based cognitive radio systems

    Page(s): 846 - 855
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (481 KB)  

    Two spectrum etiquettes are developed for the use on the downlink of coexisting high-altitude platform (HAP) and terrestrial fixed broadband systems that are intended for future application with cognitive radio-based user terminals with directional antennas. The spectrum etiquettes are based on the interference to noise ratio and carrier to interference plus noise ratio levels at the receiver of an incumbent user. Antenna beamwidths and multiple modulation scheme levels determine the parameter settings for coexistence performance. It is shown that coexistence performance can be improved by exploiting the surplus transmitter power of the terrestrial system, thereby enabling the incumbent terrestrial system to accommodate additional interference arising from a newly activated HAP system. View full abstract»

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  • Application of cooperative sensing in radar-communications coexistence

    Page(s): 856 - 868
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1182 KB)  

    The feasibility of coexistence between radar and a communication system is investigated under log-normal shadowing conditions. A key element that enables coexistence is a sensing receiver to detect the presence of a radar system and prevent transmissions when interference may occur. Initial studies where each communication terminal does its own sensing and decision-making shows that while bandsharing is possible, under stringent interference requirements, communication efficiency can be limited. Analytical and simulation results illustrate that cooperative sensing leads to an improved detection range, higher detection probability and lower complexity of sensing nodes than that of single terminal sensing. It is concluded that cooperative spectrum sensing is able to enhance detection capabilities to maximise radar spectrum-sharing usage efficiency. View full abstract»

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IET Communications covers the theory and practice of systems, networks and applications involving line, mobile radio, satellite and optical technologies for telecommunications, and Internet and multimedia communications.

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