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Communications, IET

Issue 11 • Date July 22 2011

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Displaying Results 1 - 17 of 17
  • Novel approach to adjust the step size for closed-loop power control in wireless cellular code division multiple access systems under flat fading

    Publication Year: 2011 , Page(s): 1469 - 1483
    Cited by:  Papers (1)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1574 KB)  

    In this article, we study the power control (PC) process in wireless cellular code division-multiple access systems under flat fading and propose a novel approach to find an optimum step size for closed-loop power control algorithms. In this approach, an optimum step size will be computed from a proposed function. This function depends on system parameters such as, the number of co-channel users, processing gain, the period of PC, Doppler frequency, channel attenuation and the order of diversity. Based on this computation, the mobile station (MS) adjusts its transmit power optimally to decrease interference for other co-channel users. Simulation results for different sets of system parameters show that the proposed algorithm decreases the bit error rate, the outage probability at the base station (BS), and increases the battery life of the MS compared with other values of the step size. The performance of the proposed algorithm is compared with the fixed-step-size power control algorithm and superiority of its performance is confirmed by simulation results. Moreover, the upper and lower bounds of the outage probability and the received signal-to-interference ratio for the proposed algorithm at the BS will be calculated. View full abstract»

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  • Error-rate performance of uplink cooperative communications with decode-and-forward protocol

    Publication Year: 2011 , Page(s): 1484 - 1490
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (515 KB)  

    For cooperative communications, the decode-and-forward (DF) protocol can give promising error-rate performance provided that the relay node has the decoding capability. In this study, the authors consider the DF cooperation for cellular uplink communications, in which the source node is equipped with a single antenna, and both the relay and destination nodes are equipped with multiple antennas. For this case, the authors analyse the exact symbol-error-rate (SER) for M-quadrature amplitude modulation (QAM) signals over Nakagami-m fading channels. To avoid the cumbersome integral calculation, the authors derive an asymptotic approximation for the SER, which lets them not only gain fast understanding of the improvement from cooperative diversity, but also have more insight into how the channel characteristics and the number of antennas affect the SER performance. Specifically, to reduce the SER, the authors find that: first, good condition of the relay-destination channels is more critical than good condition of the source-relay channels; second, large number of antennas at the relay node is more critical than large number of antennas at the destination node. All these results are not exploited and discussed before. Simulations are finally presented to demonstrate the correctness of the authors- analysis and perception for the considered cooperation scenario. View full abstract»

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  • Successive crosstalk cancellation facing the dynamic situation in very-high-speed digital subscriber line 2 systems

    Publication Year: 2011 , Page(s): 1491 - 1496
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (238 KB)  

    Broadband digital transmission over cable wires, such as in very-high-speed digital subscriber line 2 systems, applies the discrete multitone technique in combination with a bit-loading procedure. The crosstalk interference among the different cables inside a binder is the major performance degradation in the systems. Different crosstalk cancellation schemes considering computation complexity as well as the quality-of-service (QoS) aspect have been investigated to improve the system performance. Nowadays, driven by the newest high-layer applications the QoS requirements are more critical and a dynamic situation happening quite often in reality has to be considered. In this situation, new active cable lines might significantly influence the existing running lines in terms of achieved data rates. Therefore a successive crosstalk cancellation scheme is presented in this study to deal with the dynamic situation. The performance degradation because of the new active lines is predicted. Based on the prediction a line-independent partial cancellation with successive procedure is performed until the QoS requirements are fulfilled again. View full abstract»

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  • Optimal sub-graph selection over coded networks with delay and limited-size buffering the authors consider

    Publication Year: 2011 , Page(s): 1497 - 1505
    Cited by:  Papers (2)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (369 KB)  

    The authors consider the problem of finding a minimum cost multicast sub-graph based on network coding, where delay values associated with each link, limited buffer-size of the intermediate nodes and link capacity variations over time are taken into account. The authors consider static multicast (where membership of the multicast group remains constant for the duration of the connection) in lossless wireline networks. For such networks, first the continuous-time (asynchronous packet transmissions) model is formulated. Subsequently, the discrete-time model for the synchronous packet transmissions scenario is derived. Then, by using an auxiliary time-expanded network, a decentralised algorithm in the presence of link delays, limited buffer-size and time-varying link capacity is proposed. The proposed algorithm when coupled with decentralised code construction schemes results in a fully decentralised approach for achieving minimum-cost in such multicast networks. Also, how adding buffering capability at intermediate nodes reduces the overall cost of the optimal sub-graph is discussed in this study. In addition, as will be shown, inclusion of buffering capacity at intermediate nodes makes it possible to find minimum multicast solution in scenarios that such solutions do not exist otherwise. View full abstract»

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  • Increasing the local girth of irregular low-density parity-check codes based on degree-spectrum analysis

    Publication Year: 2011 , Page(s): 1506 - 1511
    Cited by:  Papers (1)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (327 KB)  

    A low-density parity-check (LDPC) code can be described by a Tanner graph composed of symbol nodes and check nodes. Short cycles in the Tanner graph should be avoided because such cycles will degrade the performance of the decoder and hence increase the error rate. In most of the circumstances, cycles of length 4 can be totally eliminated in the Tanner graph when the LDPC code has been designed properly. Locally, for each variable node in the Tanner graph, the girth may take the value of 6 or 8, or even higher. In this study, the authors attempt to increase the local girths of a Tanner graph with an aim to improving the error performance of the code. The authors propose and analyse the `degree spectrum` of the symbol nodes connecting to each of the check nodes. Based on the analysis, the authors can determine if the local girths can be further increased. The authors also propose simple ways to construct codes with higher local girths. Finally, the authors show the characteristics and the error performance of the codes constructed with the proposed methods. View full abstract»

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  • Error probability-based optimal training for linearly decoded orthogonal space-time block coded wireless systems

    Publication Year: 2011 , Page(s): 1512 - 1529
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1337 KB)  

    An optimal training strategy is devised for the linearly decoded orthogonal space-time block coded (OSTBC) wireless systems in quasi-static fading channel, based on the performance analysis using pairwise error probability (PEP) and symbol error probability (SEP). The PEP/SEP analyses allow us to find a generic expression for the performance improvement due to optimal training compared to the conventional case for OSTBC system equipped with any number of transmit and receive antennas and any linear modulation scheme. It is observed that the linear processing in the receiver, the most attractive feature of OSTBC, although destroys the orthogonality in the presence of channel estimation error, does not reduce diversity, but causes performance penalty as a loss of signal-to-noise ratio (LoSNR) due to training. This loss is quantified analytically and minimised by optimal allocation of power between training and data symbols. The performance of optimal power allocation improves with the higher number of space-time blocks in a frame. Furthermore, the LoSNR depends only on the OSTBC and is independent of any modulation scheme and the full rate Alamouti and other high rate OSTBCs suffer more in terms of performance due to training compared to the lower rate OSTBC. View full abstract»

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  • Joint power rate allocation for minimum mean transmission delay in cellular cdma downlink

    Publication Year: 2011 , Page(s): 1530 - 1539
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (660 KB)  

    An optimum power and rate allocation rule for cellular code division multiple access (CDMA) downlink networks is derived; it minimises mean transmission delay of the active users with different channel qualities under the constraints of total transmission power and the required signal-to-interference ratio. The proposed rule states that the power assigned to a user is inversely proportional to the square root of the user's channel quality factor, whereas the rate assigned to a user is proportional to the square root of the user's channel quality factor. The amount of reduction in average mean transmission delay by the proposed optimum allocation rule against the equal rate allocation with power control and the equal power allocation with rate control is also derived for the case when each user undergoes shadowing, path loss and inter-cell interference. It is shown that the proposed allocation rule not only achieves the minimum mean transmission delay, but also minimises the probability of the data rate being less than a tolerable threshold, against the equal rate or equal power allocation schemes. View full abstract»

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  • Bypassing internet service provider traffic shaping with peer-to-peer file sharing through deliberate false positives

    Publication Year: 2011 , Page(s): 1540 - 1543
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (172 KB)  

    QoS applications rely on accurate detection of protocols in order to effectively manage traffic passing across networks. Peer-to-peer developers already use encryption and network overlays to bypass ISP traffic shaping but their methods only obfuscate telltale signatures. Unidentifiable or encrypted traffic can still be classified as such and therefore can still be managed. The author addresses the feasibility of using protocol mimicry to invoke deliberate false positives in order to bypass existing traffic analysis systems by masquerading as web browsing and VoIP traffic. Statistical analysis is undertaken to determine the costs associated with such modifications. It is found that peer-to-peer protocols can easily be modified to be incorrectly identified as genuine web and voice traffic without impractical increases in bandwidth consumption. The incorrect classification of such traffic can cause havoc with regard to priority-based queuing mechanisms, whereas allowing users to use throttled applications without restrictions. It is certainly feasible for file-sharing protocols such as BitTorrent being further developed to mimic the traits of less throttled protocols to bypass traffic shaping. This poses a huge risk to future ISP and corporate traffic management. View full abstract»

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  • Blind frequency-offset tracking scheme for multiband orthogonal frequency division multiplexing using time-domain spreading

    Publication Year: 2011 , Page(s): 1544 - 1549
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (427 KB)  

    A blind scheme for estimating the residual carrier-frequency offset of multiband orthogonal frequency division multiplexing (MB-OFDM)-based ultra-wideband (UWB) systems is proposed. In the MB-OFDM UWB system, time-domain spreading (TDS) is used by transmitting the same information across two adjacent OFDM symbols. By using the TDS structure, the proposed frequency estimation scheme does not require the use of pilot symbols. To demonstrate the usefulness of the proposed estimator, analytical expression of the mean square error is derived and the performance is compared with a conventional pilot-assisted estimator. View full abstract»

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  • Spatial-temporal fractionally spaced decision-feedback equalisation for fading channels with dispersive interference

    Publication Year: 2011 , Page(s): 1550 - 1559
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (585 KB)  

    The authors study the spatial-temporal minimum mean square error decision-feedback equaliser (ST-DFE) to combat dispersive interference over fading channels. The proposed ST-DFE consists of an anticausal fractionally spaced feedforward filter (FFF) on each antenna branch to perform co-channel interference (CCI) and precursor intersymbol interference (ISI) cancellation, followed by a symbol-spaced feedback filter to remove postcursor ISI. An optimum ST-DFE with a double-sided symbol-spaced FFF demonstrates excellent CCI/ISI cancellation ability, but occupies wider time span and is too complicated to implement in practice. Several ST-DFE schemes using the anticausal symbol-spaced FFF were proposed, but the simulation reveals that such an approach is unable to eliminate dispersive CCI efficiently and even performs substantially worse than spatial-temporal linear equalisers, when the continuous transmitted pulse shape is considered. The performance might become worse owing to the error propagation. The proposed ST-DFE is capable of improving the performance significantly, and even outperforms the double-sided symbol-spaced ST-DFE. The algorithm for computing the fractionally spaced coefficients is presented and then applied to the fast error probability calculation using the Gauss quadrature rule which can approximate the probability of the density function of the residual ISI and CCI. Such an approach avoids the interference characteristic of the inaccurate Gaussian method. View full abstract»

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  • Optimisation of low-density parity-check codes with deterministic unequal error protection properties

    Publication Year: 2011 , Page(s): 1560 - 1565
    Cited by:  Papers (1)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (260 KB)  

    In this study, the authors propose a systematic method of designing low-density parity-check codes with deterministic unequal-error-protection characteristics over an additive white Gaussian noise channel. The proposed method optimises the code rate or the noise threshold while at the same time guaranteeing that different classes of code bits can achieve their respective target error probabilities within a specified number of decoding iterations. The authors also derive the necessary and sufficient stability condition for the optimisation method. View full abstract»

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  • High reliable disjoint path set selection in mobile ad-hoc network using hopfield neural network

    Publication Year: 2011 , Page(s): 1566 - 1576
    Cited by:  Papers (1)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (539 KB)  

    Mobile ad-hoc network (MANET) is a collection of wireless mobile computers forming a temporary network without any fixed infrastructure or wired backbone. Topological changes in MANET frequently render routing paths unusable. A suitable technique for addressing this problem is to enhance the diversity of paths between the source and destination. However, multipath routing is a challenging task. In particular, the correlation between the failures of the paths in a path set should be as small as possible. Shared nodes and links between the paths are usual failure points. Disjointed path sets require the multiple paths to be link-disjoint or node-disjoint. However, selecting an optimal path set is an NP-complete problem. Artificial neural networks have been proposed as computational tools to solve constrained optimisation problems. The use of Hopfield neural network as a path set selection algorithm is explored. Since this algorithm produces a set of backup paths with much higher reliability, it is beneficial for MANETs. We use link expiration time (LET) between two nodes to estimate link reliability. In this approach, node-disjoint and link-disjoint path sets can be found simultaneously with route discovery algorithm. So, if someone wants to find both node-disjoint and link-disjoint path sets, there is no need to submit extra control messages, as overhead, to the MANET. Simulation results show that the proposed protocol can find path sets with higher reliability in comparison to the other recent proposed algorithms. View full abstract»

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  • Blind channel estimation for equalisation in dispersive fading channel

    Publication Year: 2011 , Page(s): 1577 - 1586
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (692 KB)  

    A new blind channel estimation technique is presented for non-linear/linear equalisation in a frequency-selective Rayleigh fading channel. At each symbol interval, a decision algorithm first makes a primary data estimate based on constrained linear minimum mean square error criterion, and then this is applied to subsequent channel estimation. Channel estimates are obtained in the form of two alternative, related methods: directly from the Wiener solution or Kalman-based recursion. The former performs better but requires more values from the normalised time-correlation function. The performance is evaluated by simulation, allowing fair comparison with the benchmark of equalised coherent detection; optimal uncoded orthogonal frequency division multiplexing with perfect channel state information at the receiver; the conventional decision-directed Kalman filtering which employs channel tracking with delay; and for the special case of flat fading, an optimised, pilot symbol-assisted modulation system. View full abstract»

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  • Performance evaluation of irregular low-density parity-check codes at high signal-to-noise ratio

    Publication Year: 2011 , Page(s): 1587 - 1596
    Cited by:  Papers (2)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (490 KB)  

    Elementary trapping sets (ETSs) have been found to be the main cause of error floor in the decoding of low-density parity-check (LDPC) codes. Moreover, irregular LDPC codes that avoid harmful [w; u; e] ETSs have been constructed and have shown to possess superior error performance compared with other LDPC codes. In this paper, we attempt to evaluate irregular LDPC code performance under an additive white Gaussian noise channel within the high SNR region using the importance sampling (IS) approach in conjunction with the identification of [w; u; e] ETSs. For any given irregular LDPC code, we will first apply a three-step method that aims to search as many ETSs within the code as possible. Then, we will classify these ETSs into different groups based on their labels, i.e., [w; u; e]'s. Further, by dividing the error region into various sub-regions centered by ETSs, we apply the IS simulator to evaluate the error rate of each of the sub-regions. Based on the error rates of all the sub-regions, we can estimate the overall error rate of the LDPC code. Results have indicated that our proposed IS scheme can produce speed-up gains up to 3.9'109 times compared to Monte Carlo simulations. View full abstract»

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  • Multi-scale direct sequence ultra-wideband communications over time-dispersive channels

    Publication Year: 2011 , Page(s): 1597 - 1606
    Cited by:  Papers (1)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (553 KB)  

    In this study, the authors introduce a new multiple access technique for ultra-wideband (UWB) communications called multi-scale (MS) direct sequence (DS)-UWB (MS/DS-UWB). In the MS/DS-UWB design, the dyadic-scaled orthogonal wavelets spread by DS codes create a dyadic-scaled multi-band structure which exhibits the scalability of the spectrum. Hence, it can be incorporated with cognitive radio by intelligent spectrum adaptation when part of the UWB spectrum cannot be used. MS/DS-UWB is a multi-rate system, where services are assigned to different orthogonal sub-bands with respect to their data rates. Compared with the conventional single-band DS-UWB, the increased pulse durations (or scaled sub-bands) relax the system design. Besides, it eliminates the disadvantages associated with orthogonal frequency division multiplexing-based UWB systems. The authors analyse the probability of error performance of the MS/DS-UWB system and compare it with a multi-processing gain (MPG) DS-UWB system over UWB channels. Numerical results show that the MS/DS-UWB performs much better than that of MPG/DS-UWB due to the low cross correlation among the scaled wavelets. View full abstract»

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  • Fair subcarrier and power allocation for multiuser orthogonal frequency-division multiple access cognitive radio networks using a colonel blotto game

    Publication Year: 2011 , Page(s): 1607 - 1618
    Cited by:  Papers (3)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (876 KB)  

    The problem of subcarrier allocation (SA) and power allocation (PA) for both the downlink and uplink of cognitive radio networks (CRNs) is studied. Two joint SA and PA schemes based on Blotto games are presented for orthogonal frequency-division multiple access (OFDMA)-based CRNs. In this work, the authors consider a more practical scenario by taking into account the correlation between adjacent subcarriers. In the proposed games, secondary users (SUs) simultaneously compete for subcarriers using a limited budget. In order to win as many good subcarriers as possible, the SUs are required to wisely allocate their budget subject to the transmit power, budget and interference temperature constraints. Two PA and budget allocation strategies are derived to enable fair sharing of spectrum among the SUs. It is shown that by manipulating the total budget available for each SU, competitive fairness can be enforced. In addition, the conditions to ensure the existence and uniqueness of Nash equilibrium (NE) in the proposed methods are established and algorithms which ensure convergence to NE are proposed. Simulation results show that the proposed methods can converge rapidly and allocate resources fairly and efficiently in correlated fading OFDMA channels. View full abstract»

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  • Survey of media access control protocols for vehicular ad hoc networks

    Publication Year: 2011 , Page(s): 1619 - 1631
    Cited by:  Papers (20)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (555 KB)  

    Recent advances in various wireless communication technologies and the emergence of computationally rich vehicles are pushing vehicular ad hoc network (VANET) research to the forefront in academia and industry. A lot of research results have been published in various areas (such as routing, broadcasting, security and others) of VANET in the last decade covering both vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) scenarios. One specific area of VANET that still faces significant challenges is the design of reliable and robust media access control (MAC) protocols for V2V communications. The authors present a survey of V2V MAC methods (including various VANET standards) that have been proposed for VANETs over the last few years. The authors also focus on the benefits and limitations of the proposed MAC techniques as well as their ease of implementation in practice and future deployment. In addition some of the challenges that still need to be addressed to enable the implementation of highly efficient and high performance MAC protocols for V2V communications are discussed. Finally, some innovative solutions that can be developed to address these challenges are proposed. View full abstract»

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