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Communications, IEE Proceedings-

Issue 1 • Date 24 Feb. 2005

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Displaying Results 1 - 17 of 17
  • Packet-loss modelling in IP networks with state-duration constraints

    Page(s): 1 - 5
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (542 KB)  

    A Gilbert-gamma topology is proposed to model packet-loss processes in UDP connections. The proposed topology introduces state duration modelling with gamma distributions. When compared with the ordinary Gilbert model the proposed topology substantially improves the likelihood of observed packet-loss processes, and gives reductions as high as 70% in the subjective estimation of speech quality transmitted over IP networks. The results presented can be easily applied to other real-time applications such as audio and video streaming. View full abstract»

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  • Parallel LSPs for constraint-based routing and load balancing in MPLS networks

    Page(s): 6 - 12
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (609 KB)  

    Two features of multiprotocol label switching are very useful in network traffic engineering: the path-oriented nature, and the capability to support multiple paths between an ingress-egress node pair. The first feature makes it easy to adaptively route traffic through the network based on the load condition in different parts of the network, while the second feature is often used for load balancing. The role of parallel label-switched paths (LSP) in load balancing and constraint-based routing is investigated. An algorithm named parallel-path-based bandwidth scheme (PPBS) is proposed to make use of parallel LSPs in choosing a bandwidth constraint path. The improvement on flow blocking probability by using PPBS is given quantitatively with respect to the average traffic load on the link, the hops along the path, and the possible number of parallel paths. In conjunction with the PPBS scheme, a feedback-based load-balancing algorithm (FBLB) is proposed to properly distribute traffic onto the parallel LSPs determined by the PPBS. This FBLB algorithm relies on the signalling packets to convey network status information back to the source. Consequently the sources can adjust the traffic distribution into each LSP accurately and promptly. Simulation results show that the FBLB algorithm is simple and effective. View full abstract»

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  • Fair TCP congestion control in heterogeneous networks with explicit congestion notification

    Page(s): 13 - 21
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (920 KB)  

    The authors propose a window-based congestion control algorithm to achieve max-min fair sharing of the available bandwidth in ECN capable heterogeneous TCP networks. The proposed algorithm extracts the network status from successive binary congestion information provided by ECN. From the extracted network information, the authors estimate a fair window size proportional to the propagation delay. Through simulations, the effect on TCP fairness of the proposed algorithm is shown for the heterogeneous network. View full abstract»

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  • Fair admission control in QoS capable networks

    Page(s): 22 - 28
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (554 KB)  

    Current admission controls that seek to optimise network utilisation create a bias against large-bandwidth calls. A buffer without preemption model is proposed that holds a call in a buffer rather than directly rejecting it when the residual bandwidth is insufficient. No other call can be admitted until this queued call is served. Such a mechanism increases the throughput since the buffer can temporarily hold a call. It also reduces the bias for large-bandwidth calls since all calls are rejected when the buffer is not empty, even when the residual bandwidth is sufficient for small-bandwidth calls. The proposed approach is compared with two alternative models, namely no buffer and buffer with preemption models and the performance of the models is analysed by solving a multi-dimensional Markov chain. Analytical and numerical results indicate that the proposed model achieves perfect fairness. A modified model, in which a waiting threshold is added to prevent an overlong wait for the queued call is also presented. Also, different thresholds for various bandwidth calls are set to maintain the optimal fairness. Analytical and numerical results demonstrate that this latter model actually achieves optimal fairness, even when perfect fairness cannot be achieved. View full abstract»

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  • Robust iterative decoding of turbo codes in heavy-tailed noise

    Page(s): 29 - 38
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (652 KB)  

    The advancements in channel coding theory over the past decades have been accomplished by considering of additive white Gaussian noise (AWGN) channels. Much less is known about the consequences when the standard AWGN assumption is not fulfilled in realistic environments and, more importantly, the appropriate countermeasures. The paper investigates the robustness of turbo codes decoded by existing quadratic-type algorithms in heavy-tailed, non-Gaussian noise channels. It illustrates that impulsive noise constitutes a major impairment in turbo decoding by studying the a posteriori probabilities computed by the constituent decoders, in addition to a formal account in terms of error probability performance. It is found that the performance of turbo codes is extremely sensitive to the shape of the underlying noise density function, being considerably degraded when this function departs from Gaussianity into a heavy-tailed distribution. A robust variant of existing decoders for reliable decoding of turbo codes in heavy-tailed noise is proposed and studied. The robustness is achieved by the enforcement of a non-quadratic soft metric into the decoder for good estimation of the transition probabilities and reliable extraction of extrinsic information. View full abstract»

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  • Velocity-assisted paging in PCS through stochastic modelling of terminal displacement

    Page(s): 39 - 44
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (524 KB)  

    The authors propose a novel framework for the exploitation of mobile velocity for the optimisation of paging performance in PCS. The focus is on the optimisation of the paging process based on the estimation of terminal displacement from its last known position. This estimation is performed through a stochastic formulation, making use of both the velocity category of the terminal and the velocity category residence time. Simulation results are provided and the proposed method is compared with other paging strategies, indicating certain advantages that together with its very low complexity could make it applicable for both 2G and 3G systems. View full abstract»

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  • Performance analysis of dynamic packet assignment in cellular systems with OFDMA

    Page(s): 45 - 52
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (580 KB)  

    The authors present a performance analysis of orthogonal frequency division multiple access (OFDMA) based cellular systems with dynamic packet assignment (DPA). Most approaches in the current literature for analysing wireless data networks do not take into account the interference conditions on the radio channel. An analytical model based on a two-dimensional continuous-time Markov chain (CTMC), that takes into account the interference conditions on the OFDM subcarriers in addition to the traffic conditions, is proposed. The authors derive expressions for the mean delay and the average system throughput on the uplink and the downlink in OFDMA based cellular systems with data traffic. View full abstract»

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  • Pre-emptive channel allocation model for heterogeneous cellular networks

    Page(s): 53 - 60
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (680 KB)  

    The authors present a pre-emptive channel allocation model for heterogeneous cellular networks. A heterogeneous cellular network is defined to operate in various communication protocols, and each protocol may suit for certain service classes with different traffic characteristics. For example, GSM is widely used for conveying real-time, high-quality voice traffic and is operating in circuit-switching mode. On the other hand, GPRS and IMT-2000, operating in packing-switching mode, are suitable for conveying burst video or non-real-time bulk data. One of the innovatory designs presented is correct in that a real-time handoff call, operating in circuit-switching mode, is allowed to pre-empt on-going and non-real-time calls, operating in packet-switching mode. For the purpose of evaluation, a mathematical model with 3-D Markov chains is built to analyse blocking probabilities of handoff and new calls with different traffic characteristics under pre-emption. The analyses have shown that with pre-emption the blocking probability of real-time handoff calls can be reduced significantly, as compared to that without pre-emption. Besides, the average number of active calls, the average call completion rate, and the channel utilisation are substantially improved through the pre-emptive channel allocation model. View full abstract»

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  • New forms of cumulative probability distribution and their applications in communications

    Page(s): 61 - 68
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1030 KB)  

    Cumulative probability distribution (CPD) is a statistical approach in the comparison of multi-path fading models. One problem encountered in this comparison method is its inability to distinguish between certain sets of data. This problem is covered thoroughly and a solution is discussed for overcoming it. The overall new method is named 'new forms of CPD' (NFCPD). These three new forms are: simple sigma-r (SSR), complex sigma-r (CSR) and the 3-D mesh method. Each method has been covered thoroughly and shown to correct CPD's incompleteness, and although all three methods are very similar in structure and algorithms, each has a unique use of its own. Also, new applications of SSR are proposed in three fields: communication channel recognition, channel distortion measurement, and MTI radars. Although all results presented are obtained from SSR, they could also be obtained by CSR and the 3-D mesh method. SSR has been chosen because of its simplicity and relative accuracy. View full abstract»

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  • Error analysis of M-QAM with equal-gain diversity over generalised fading channels

    Page(s): 69 - 74
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1270 KB)  

    M-ary quadrature amplitude modulation (M-QAM) is a spectrally efficient modulation scheme, used for video transmission applications in third and forthcoming generations of wireless networks. In the paper, the authors present a unified framework for the error performance of M-QAM, employing L-branch equal-gain combining over generalised fading channels, such as Nakagami-m, Rice, Hoyt or Weibull. For each channel model an exact closed-form expression is derived for the moments of the EGC output signal-to-noise ratio (SNR). Using these expressions, the symbol error performance of M-QAM is studied, with the aid of the moment-generating function approach and the Pade´ approximants theory. The proposed mathematical analysis is illustrated by selected numerical results, pointing out the effect of the input SNRs unbalancing, as well as the fading severity on the system's performance. Simulations are also performed to check the validity and the accuracy of the proposed analysis. View full abstract»

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  • MIMO OFDM for broadband fixed wireless access

    Page(s): 75 - 81
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (846 KB)  

    A broadband multiple-input multiple-output (MIMO) OFDM system has been designed and evaluated for a fixed wireless link between tall buildings in an urban area. Channel measurements for a typical installation location are analysed to determine the relevant spatial, temporal and frequency characteristics. The small number of multipath components leads to a high spatial correlation and limited potential for spatial diversity to provide spectral efficiency gains. However, the slowly-varying characteristics support the use of a closed-loop diversity scheme, such as spatial multiplexing along the eigenmodes of the channel matrix, to maximise the achievable throughput. This scheme is evaluated using the measured channel data, and it is shown that the self-interference caused by channel estimation errors is the limiting factor on the system performance. Taking into account the estimation errors and feedback delay, it is demonstrated that a three-fold increase in spectral efficiency relative to a single-element antenna system is achievable using eight-element antennas, even in this highly correlated environment. View full abstract»

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  • Empirical analysis of the correlation of MIMO channels in indoor scenarios at 2 GHz

    Page(s): 82 - 88
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (708 KB)  

    An experimental analysis is given of the spatial correlation between MIMO subchannels in indoor scenarios, analysing the various factors which affect the correlation, such as the type of scenario, presence or not of line of sight and the relative orientation between the transmitter and receiver array. The results show a great variability in the correlation coefficient as a function of the distance between the elements in the receiver array. They also show that the correlation coefficient for the same distance between elements in the array changes in response to different relative positions and orientations between the transmitter and receiver arrays. Thus a statistical characterisation of the correlation coefficient is required. High values are obtained for the correlation between various subchannels, even for separations between receiving elements greater than 0.5 λ. View full abstract»

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  • Joint spatial and temporal channel-shortening techniques for frequency selective fading MIMO channels

    Page(s): 89 - 94
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (546 KB)  

    It is well understood that the maximum likelihood estimator is a powerful equalisation technique for frequency selective fading channels, and in particular for MIMO systems. The complexity of this estimator, however, grows exponentially with the number of users and multipath taps, hence limiting the use of this algorithm in MIMO systems. In the paper, the authors propose a joint spatial and temporal channel-shortening filter as a pre-processor to reduce significantly the complexity of a maximum likelihood estimator. Their design is based on both minimum mean square error and maximum signal-to-noise ratio criteria. They compare the performance of their proposed schemes with the existing methods that employ channel-shortening techniques only in the spatial or temporal dimension, and thereby demonstrate the advantage of using joint spatial and temporal channel shortening in terms of complexity and bit error rate performance. View full abstract»

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  • Block spectral precoding for blind multiuser detection of DS/CDMA signals

    Page(s): 95 - 102
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (571 KB)  

    A new approach for blind multiuser detection of DS/CDMA signals is proposed. This approach is based on modifying the data power spectrum, uniquely for each user, through a two-stage colouring-whitening precoding scheme. This enables detection of a desired user's signal in an unknown multipath channel, regardless of the power of the interferers. The proposed precoding method does not alter the data rate or system bandwidth. Detection does not require spreading code knowledge, and consequently avoids channel estimation and the possible associated errors. The method is seen to yield reasonable performance besides being able to operate properly in nearly saturated systems, in contrast to some existing methods. Besides this, it can tolerate severe carrier frequency offsets. View full abstract»

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  • Effect of spatial fading correlation on CDMA code-acquisition performance

    Page(s): 103 - 112
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (878 KB)  

    The effect of correlations among the fading signals at the multiple antennas on the code-acquisition performance for preamble search in a code-division multiple-access (CDMA) uplink is investigated. Previous results for frequency-selective fading assumed independent fading at each antenna. A general case of spatial fading correlation as a function of antenna spacing and azimuth spread is presented for the performance analysis of code-acquisition systems. The correlated antennas are exploited to derive the probability density function of a decision variable. The performance of a maximum-likelihood (ML) code-acquisition technique based on spatially correlated antennas is analysed by considering the detection, miss, and false-alarm probabilities. The result of performance analysis of a code-acquisition scheme utilising uncorrelated diversity antennas is also described. The detection performance and mean acquisition time given by partial correlation in spatial fading are presented and compared with those offered by antenna decorrelation via numerical evaluation. It is shown that without exploiting time diversity, the code-acquisition approach with multiple antennas provides much better detection performance than the corresponding scheme with a single antenna in a frequency-selective fading channel and that the performance enhancement depends on the degree of spatial fading correlation. View full abstract»

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  • Implementation of Costas loop using CORDIC algorithm for software radio applications

    Page(s): 113 - 118
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (805 KB)  

    A novel method is presented for implementing the Costas carrier recovery loop for software radio application. This method is fully digital, FPGA-friendly, and process- and resource-efficient. In order to validate the scheme, a Matlab simulation and a hardware description code have been developed. Also, a new procedure has been introduced to generate dynamic test vectors for hardware simulation through Matlab. By application of this method, system and hardware simulators can be coupled. The output of hardware simulation, which includes the effect of quantisation and logic delays, can be evaluated by Matlab. This leads to a realistic performance prediction of the loop. In addition, Verilog code has been synthesised on a specific FPGA to calculate practical resource requirement and maximum achievable frequency. View full abstract»

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  • Dynamic routing, wavelength, and fibre selection algorithms for multifibre WDM grooming networks

    Page(s): 119 - 127
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (858 KB)  

    The problem of dynamic routing in a multifibre time-slotted wavelength routed WDM network is considered. Requests to establish a connection are dynamic and each connection requests an integer multiple of some basic unit. Each wavelength has the capacity of carrying a multiple of these channels, where each channel is represented by a time-slot as in TDM networks. The network is represented as a layered graph model with multiple layers, where each layer represents a specific wavelength. Each link in the layered graph has one or more fibres and an associated cost. The cost of layered graph links could be a function of the loading state of the respective wavelength across the available fibres. Algorithms are proposed for fibre selection and for setting link costs, and a modified Dijkstra algorithm on the layered graph is used to select a route on the layered graph for a new connection request. The selected route on the layered graph represents the physical path and the selected wavelength. The performance of these algorithms is evaluated for a realistic mesh network topology for various combinations of the number of wavelengths, the number of fibres per link, the number of time-slots per wavelength, and the offered load for both uniform and non-uniform traffic loads. The best performing fibre selection and link cost update methods are identified. The proposed algorithm for routing and wavelength assignment in mesh optical WDM grooming networks outperforms previously proposed work in the literature. View full abstract»

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