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Communications, IEE Proceedings-

Issue 5 • Date 14 Oct. 2003

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Displaying Results 1 - 17 of 17
  • Application of three-branch polarisation diversity in the indoor environment

    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (271 KB)  

    The paper presents an analysis of the performance of the novel three-branch polarisation diversity scheme in an indoor environment. The results show that the antenna elements in the configuration have very low mutual coupling and cross-correlation coefficients, making it suitable for diversity. An analysis of the diversity gains proves that the system provides gains of more than 5 dB and 2 dB over single- and dual-antenna systems, respectively. Such a system can be deployed in indoor environments where users require high-speed data services and can enhance the performance of existing wireless local area networks (WLANs) View full abstract»

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  • Modified prime-hop codes for optical CDMA systems

    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (260 KB)  

    In the early stage of optical CDMA systems, the spreading codes, such as optical orthogonal codes (OOCs) and prime codes, are only coded in the time domain. However, the small code size becomes the major issue for one-dimensional codes. To deal with this limitation of one-dimensional codes, a new family of two-dimensional codes is proposed for optical CDMA systems. The new codes are modified from the well known prime-hop codes. Hence, the authors refer to the new codes as 'modified prime-hop codes'. The modified prime-hop codes preserve the same code correlation properties as the original prime-hop codes, i.e. the cross-correlating constraint is equal to one and the autocorrelation constraint is equal to zero, and can be generated with lower cost, lower complexity and lower power loss encoders/decoders. Meanwhile, the optical CDMA systems with the modified prime-hop codes perform better and can accommodate more users than those with the original prime-hop codes View full abstract»

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  • Necessary and sufficient conditions for optimal flow control in multirate multicast networks

    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (282 KB)  

    The authors consider the optimal flow control problem in multirate multicast networks where all receivers of the same multicast group can receive service at different rates with different QoS. The objective is to achieve the fairness transmission rates that maximise the total receiver utility under the capacity constraint of links. They first propose necessary and sufficient conditions for the optimal solution to the problem, and then derive a new optimal flow control strategy using the Lagrangian multiplier method. Like the unicast case, the basic algorithm consists of a link algorithm to update the link price, and a receiver algorithm to adapt the transmission rate according to the link prices along its path. In particular if some groups contain only one receiver and become unicast, the algorithm will degrade to their previously proposed unicast algorithm View full abstract»

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  • Local stability of random exponential marking

    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (244 KB)  

    Random exponential marking (REM) is an attractive adaptive queue management algorithm. It uses the quantity known as 'price' to measure the congestion in a network. REM can achieve high utilisation, small queue length, and low buffer overflow probability. Many works have used control theory to provide the stable condition of REM without considering the feedback delay. Sufficient conditions for local stability of REM have been provided when the sources have a uniform one- or two-step feedback delay. Nevertheless, no work has been done for the case of arbitrary uniform delay. The authors propose a continuous time model to generalise the local stable condition for REM in a multilink and multisource network with arbitrary uniform feedback delay View full abstract»

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  • Design of multidimensional signal constellations

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    An algorithmic technique intended for signal constellation design for an N-dimensional Euclidean signal space is presented. Such signals are used for reliable and efficient digital communications on an additive white Gaussian noise channel. The minimum Euclidean distance between signals and the constellation-constrained capacity are used as performance measures. The basic idea is to spread out the signal vectors using an iterative method modelling the behaviour of equally charged particles in free space. Initially, the vectors are randomly distributed. The vectors reach equilibrium after a few iterations. Numerical calculations for N=2, 3, 4 and 5 dimensions and M=N+1, N+2, ...,20 signals show that the generated codes are nearly as good as previously known codes, and in some cases even better View full abstract»

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  • Equal resource sharing allocation with QoS differentiation for conversational services in wireless communication networks

    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (375 KB)  

    Flexible resource allocation (FRA) strategies are used in cellular systems to dynamically adjust the quality of service (QoS) of active calls according to the prevailing traffic conditions. This is done by assigning a variable number of resources to active calls. When the offered load is high, a small number of resources is assigned to offer high capacity while providing a QoS lower than the maximum requested. When the offered load is low, the opposite occurs. A new flexible resource allocation strategy called equal resource sharing allocation with differentiated QoS (ERSAQoS) is presented. This strategy is suitable for operation in environments with integrated conversational services (i.e. voice, video, videophone) each with different QoS and priorities. ERSAQoS consists of a two-phase allocation process. Resources are first distributed among different active calls with the same service type, respecting priorities, and then equally distributed among active calls with the same service type. The proposed strategy allows fine or soft control of differentiated QoS provisioning. Results show that ERSAQoS meets the upper capacity bound for FRA strategies while improving the QoS of the prioritised types of service View full abstract»

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  • Impact of congestion on the stability of a multicast tree in cumulative layered multicast

    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (346 KB)  

    The authors study the impact of congestion on the stability of a multicast (one-to-many) tree in the context of a cumulative layered multicast system. A stability factor is defined to evaluate and quantify this impact. To obtain the general expression of the stability factor, they develop a simple statistical model. They show that, even in the case of lower link-marking probability, a tree will become more stable when the dependency degree between different links increases. To the best of the authors' knowledge, this is the first work to quantify a multicast tree's stability with layered multicast congestion control. The modelling techniques they use are generic, and can be applied not only to analyse the stability of trees in layered multicast systems, but also to design general algorithms for both layered multicast and single-rate multicast congestion control View full abstract»

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  • Data sources for simulation of m-ary systems

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    It has been generally assumed that binary data from a maximum-length pseudo-random binary source can be grouped into m-ary symbols that will uniformly cover constellation points in a modulation scheme due to the central limit theorem. This is not the case. Sources based on the Galois field GF(16) are introduced and compared to binary, GF(2), ones in terms of the mean and variability of the resulting bit error rate (BER) estimate using a single carrier quadrature amplitude modulation (16-QAM) system as an example. The use of data sources over GF(16) for Monte Carlo simulation of the example system eliminates BER estimate variability with certain channels. Such variability is important when the channel is such that some of the constellation points cause higher probabilities of error than others. The approach is then extended to orthogonal frequency division multiplex (OFDM) m-ary systems. It is found that it is important to map separate Galois field data sources to each sub-carrier in the case of OFDM systems. There is found to be some residual BER estimate variability. The extension of the approach to systems employing forward error correction coding and interleaving is discussed View full abstract»

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  • Reliable multicast routing in mobile networks: a neural-network approach

    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (307 KB)  

    For the sophisticated organisation of multicast communications in mobile networks, reliable and secure point-to-point, point-to-multipoint specific group communication is required. A reliable multicast tree is an efficient connectivity between the source node and the group members through dependable hosts. When a mobile host changes its access point, multicast routes must be updated. This poses several challenges to providing efficient multicast routing. A neural-network-based multicast routing algorithm is proposed for constructing a reliable multicast tree to connect multicast group participants. The mobile network is divided into clusters of nodes (mobile support stations) based on their adjacency relation, by considering a suitable neighbourhood distance. The centre cluster, whose nodes are almost equidistant from the group members, is computed to construct the shortest multicast tree that passes through the centre cluster and reliable routers among all the group members. A Kohonen self-organising-map neural network is used for clustering. Hopfield neural networks are used to construct a multicast tree which has a minimum number of links and passes through the nodes of the centre cluster. The tree is constructed as and when members move. This scheme should construct a reliable multicast tree and minimise recomputation time of the tree when the route is updated as mobile hosts change their access point. The computational power of the proposed algorithm is demonstrated by simulation. It is also tested for the mobility of participating mobile hosts. The proposed work facilitates a possible multicast routing algorithm for future high-speed mobile networks View full abstract»

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  • Performance analysis of rate control with scaling QoS parameters for multimedia transmissions

    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (363 KB)  

    Advanced networking technologies have enabled the integration of multimedia transmissions, such as video, voice and data, in packet-switched networks. At the network level, it has been shown that stringent quality of service (QoS) guarantees are required for multimedia traffic flows through appropriate rate controls. The authors propose a novel rate control model to regulate the generating rate of each media traffic source by scaling the QoS parameters, including video quantisation parameter, voice packet-dropping rate and delayed data transmission. They also study the mapping function between the rate regulation and the rate scheduling corresponding to the QoS parameters so that not only QoS but rate smoothing can be guaranteed for multimedia transmissions. The race condition caused by single-level feedback control is also mitigated by applying multilevel queueing. The performance of the rate control model is analysed by employing the M/D/1/K queueing model, and the effectiveness is demonstrated by numerical results View full abstract»

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  • New lower bounds on symbol error probability for MLSD of CPM signals on AWGN channel

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    The symbol error probability bounds for maximum likelihood sequence detection (MLSD) of continuous phase modulation (CPM) signals are studied. The calculation of the upper bound is based on the transfer function technique, which has been generalised. A new method for constructing a lower bound for CPM systems is proposed. From numerical comparisons, it can be seen that the proposed algorithm can substantially improve the lower bound compared to previous approaches by considering the entire set of transmitted data sequences and not only the worst case. This generalised algorithm may be applied to any system that can be described as a finite-state machine View full abstract»

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  • Joint predistortion and nonlinear equalisation for high-capacity wireless links

    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (281 KB)  

    A nonlinearity compensation scheme for a high-capacity wireless link employing multilevel QAM as the signalling format is discussed. The distinctive feature of this scheme is that the linearisation task is split over a predistorter and a nonlinear equaliser, to be implemented at the transmit and receive sections, respectively. It is shown that this scheme considerably relieves the overall computational requirement for nonlinear channel compensation, as compared to the approach in which the task of linearisation is entirely allocated to a nonlinear equaliser without any signal predistortion View full abstract»

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  • Parallel matching algorithm for CIOQ switches with multiple traffic classes

    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (452 KB)  

    It has been shown that a combined input and output queued (CIOQ) switch with a speed-up factor of 2 can exactly emulate an output-queued (OQ) switch. The main benefit of using a CIOQ switch is to reduce memory bandwidth requirement while providing quality of service (QoS) guarantee. The key component for exact emulation is a matching algorithm for bipartite graphs. For example, a CIOQ switch with a speed-up factor of 2, which adopts the least cushion first/most urgent first (LCF/MUF) matching algorithm, can exactly emulate an OQ switch with any arbitrary service discipline. However, the complexities of cushion calculation and cell reordering required at the output ports make the algorithm very difficult to be realised in a high-speed switch. The authors propose an approximate LCF/MUF algorithm and evaluate its performance for the weighted round-robin service discipline. For ease of implementation, the proposed algorithm calculates approximate cushions and does not perform reordering at the output ports. The trade-off is that it loses the property of exact emulation. It was found, via computer simulations, that the performance of a CIOQ switch with the proposed single-iteration matching algorithm is close to that of an OQ switch under uniform, nonuniform, and correlated input traffic models for offered load up to 0.9. In addition, the cell departure order can be maintained under the single-iteration algorithm View full abstract»

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  • Comprehensive performance model of differentiated service with token bucket marker

    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (309 KB)  

    Differentiated services (DiffServ) networks use two type of routers: edge routers mark the packet headers according to their service profile, while the core routers implement queue management to provide service differentiation between packets according to information in their packet headers. Most previous studies on the performance of DiffServ have considered either the effect of the edge marking scheme or the effect of core queue management on the throughput of TCP applications. To study the combined effect of marking at the edge router and queue management at the core router on the performance of DiffServ, the authors propose a simple model that takes into consideration the committed information rate and maximum allowed burst size of the marker, in addition to the number of flows in an aggregate, round trip time, and the probability of a packet dropping at the core network. Observations from the model are presented, and simulations were performed to validate the accuracy of the model. The model shows that the performance of DiffServ is largely flow-based rather than desired-aggregate-based, and the DiffServ network suffers from unfairness issues View full abstract»

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  • Analytical technique for accelerating the simulation of packet networks

    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (276 KB)  

    Conventional simulation of packet-switched networks models each individual packet through the network, such that each packet's arrival at, or departure from, a network element forms an event. However, statistical considerations mean that very large numbers of packets have to be simulated to guarantee the accuracy of these results, and this tends to result in very long simulation times, often amounting to many hours of 'real' time just to simulate a few minutes of 'simulated' time. The authors present a technique for the acceleration of simulation studies of packet-based networks. This uses a mathematical technique for forcing equivalence in queueing systems; i.e. forcing them to have the same statistics even when one will require far fewer events to simulate. The results indicate that this technique shows great promise as a method of achieving simulation acceleration, providing accuracy as well as speed-up. Furthermore, it can be used in conjunction with a number of pre-existing acceleration methodologies View full abstract»

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  • Efficient encoding and decoding algorithms for variable-length entropy codes

    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (268 KB)  

    Successive versions of the MPEG and JPEG coding standards have been widely adopted for compression of moving pictures and still images. Variable-length codes (VLC) such as Huffman codes are generally employed within the frameworks of these standards to achieve optimal coding efficiency in the entropy coding stage. Conventional table look-up encoding/decoding techniques for VLC are inefficient in memory usage. This is a particularly important consideration for hand-held devices, such as cellular phones with image reception and processing capabilities. The authors present a series of efficient and simple algorithms for VLC encoding and decoding based on a pointer look-up approach. They present both software and hardware implementations of these algorithms, and a comparison of memory requirements associated with the pointer look-up and conventional techniques View full abstract»

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  • Performance estimation of M-ary PSK in cochannel interference using fast simulation

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    Fast simulation techniques are applied to estimate accurately the error rates of coherent M-ary phase shift keying in the presence of cochannel interference (CCI) and additive white Gaussian noise in nonfading channels. This type of interference often occurs in mobile wireless systems that employ frequency reuse. In much of the literature, error rates are calculated based on the assumption that CCI can be modelled as Gaussian. It is shown that this assumption is valid only in noise-dominated environments. When interference is dominant, the assumption yields excessively high error rates. Simulation techniques based on adaptive importance sampling (IS) are developed for this classical problem. However, they are of sufficient generality to be applicable to any form of signalling in interference situations. Several new results are presented, and these can be used to calculate the cellular capacities of wireless systems. Some of the IS results have been validated using numerically computed analytical formulas View full abstract»

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